Sludge stabilization

Chapter 16: Sludge Stabilization

This chapter describes treatment of waste solids (sludge and biosolids) from sewage treatment plants, including information on sludge quantity and concentration, biological anaerobic and aerobic stabilization and storage. Alkaline stabilization, thermal drying, solar drying, composting and incineration, are also discussed. Enhanced biosolids processing is used to provide either increased stabilization, especially for pathogen reduction, for reuse or land application, or maximum volume reduction by incineration and diversion away from land application.

The treatment of sewage results in the production of solids commonly referred to as sludge. Sludge that has undergone dewatering or treatment is generally referred to as treated sludge. When the treated sludge is suitable for land application, it is referred to as biosolids.

A summary of design loading and sludge quantity data for conventional sludge stabilization processes is provided in Appendix V, which should be used in conjunction with the details in this chapter.

16.1 General

Sludge stabilization is generally achieved by digestion and these guidelines deal with two types of sludge digestion processes that are commonly used in Ontario - anaerobic and aerobic. Alkaline stabilization is discussed, although it is not in broad use in Ontario at this time. However, proprietary alkaline stabilization systems are available and used in Ontario.

Anaerobic mesophilic digestion is the most commonly used process for the digestion of primary and mixtures of primary and waste secondary treatment sludges (e.g. waste activated sludge), particularly at larger plants. In anaerobic digestion systems, methane and other hazardous gases are created. Access to any enclosed spaces and ventilation of these spaces should follow all required safety codes. It is essential to review the Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993 and the National Fire Protection Association’s (2003) Fire Protection in Wastewater Treatment and Collection Facilities, US NFPA 820. The latest editions of these codes should be applied in designing digestion systems, as they provide comprehensive details concerning gas handling and safety.

Aerobic digestion has normally been used in Ontario for the stabilization of waste activated sludges from sewage treatment plants which do not have primary sedimentation tanks. Aerobic digestion is most common at smaller plants. Although there are some aerobic systems treating mixtures of raw, primary and waste activated sludges, due to the higher oxygen requirements and associated higher energy costs, it is recommended that the aerobic digestion process generally not be used for such sludges in new plants. Autothermal aerobic digestion systems (ATAD) that operate at thermophilic condition have been constructed in Ontario and elsewhere.

Facilities for processing sludge should be provided at all sewage treatment plants, unless facilities are available and can be used at another facility, in which case only storage may be needed. Handling equipment should be capable of processing sludge to a form suitable for ultimate disposal unless provisions acceptable to the ministry are made for processing the sludge at an alternate location.

Designers are cautioned to give thorough consideration to what type of digestion will best suit a particular sewage treatment plant (STP) and what type of overall system, including plant type and digestion type will produce the desired results economically. A process control narrative (including level of automation) should be developed and agreed on by all parties (owner, operator and consultant) during the pre-design stage.

In certain plants, separate processing of the waste activated sludge (WAS) in aerobic digesters and primary sludge in anaerobic digesters may be economically justifiable, but this is not common in Ontario or elsewhere.

Sludge handling, digestion and disposal represent a large portion of the overall sewage treatment operation and may impact other unit processes at the STP. No one unit operation can be considered in isolation from the other plant components. If sludge unit processes not described in this chapter are being considered or are necessary to meet provincial or federal sludge disposal requirements, details of the process need to be evaluated and included in the design brief.

The General Regulation (O. Reg. 267/03) made under the Nutrient Management Act should be referred to for criteria for land application of biosolids, e.g. seasonal storage requirement. For STP which are not phased in under the Nutrient Management Act, requirements are set out in the Certificate of Approval (C of A), based on the MOE and the Ministry of Agriculture, Food and Rural Affairs' Guidelines for the Utilization of Biosolids and Other Wastes on Agricultural Land, 1996.

16.1.1 Sludge Quantities and Characteristics

An overall STP mass balance should be provided to account for sludge production from each treatment unit process, including the liquid and solids trains and the influence of recycle streams. Steady state or dynamic simulation models can be used to assist the designer with accurately estimating a plantwide solids and nutrients mass balance. At minimum the total and volatile solids loading rate to the sludge processing units should be estimated.

Wherever possible, such as in the case of plant expansions, actual sludge quantity data should be considered for digester or other sludge treatment process design. Often, due to errors introduced by poor sampling techniques, inaccurate flow measurements or unmeasured sludge flow streams, the sludge data from existing plants may be unsuitable for use in design. A good check of the data is to conduct a sludge accountability analysis based on the primary or raw, biological and chemical sludge expected for a plant. The quantity of primary or raw sludge generated can be based on field data from a similar or actual installation taking into account removal rates. The biological sludge production can be calculated using the current BOD₅ loading and typical unit sludge values. Unit biological sludge values vary from 0.70 kg TSS/kg BOD₅ (0.70 lb TSS/lb BOD₅) removed for activated sludge plants with primary clarifiers to 0.85 kg TSS/kg BOD₅ for activated sludge plants without primary clarifiers. The amount of chemical sludge is calculated from the stoichiometry. Details for this method are available in ministry publication The Ontario Composite Correction Program Manual for Optimization of Sewage Treatment Plants (1996).

Before sludge data are used for design, they should be assessed for their accuracy. When reliable data are not available, the sludge generation rates and characteristics given in Table 16-1 may be used to initiate the estimation of sludge quantity. However, a comprehensive plant mass balance is recommended and should be provided.

The designer should refer to Chapter 17 - Sludge Thickening and Dewatering for methods for sludge thickening and dewatering that may be applicable to digestion practices. Mechanical thickening prior to digestion can reduce digester hydraulic loading and thus increase digester hydraulic and solids retention time and reduce heating requirements. Mechanical thickening can also reduce digester supernatant return loads to the liquid train of the STP.

In addition to sludge thickening prior to digestion, wherever possible, separate removal of grit, oils and greases and other debris such as rags is recommended to prevent entry into the digestion process.

Pretreatment of WAS or thickened WAS by a process that induces cell lysis could be considered, to increase VS destruction and gas generation in the anaerobic digestion system.

16.1.2 Process Selection

The selection of sludge handling and treatment processes should be based upon at least the following considerations:

• End use of sludge or biosolids;
• Plant size and economy-of-scale;
• Sludge characteristics (e.g. quality and proportion of various types of solids);
• Odour control;
• Local land use;
• System energy requirements;

16.2 Anaerobic Sludge Digestion

16.2.1 General

Anaerobic digestion may be provided using mesophilic (35°C or 95oF) or thermophilic (55°C or 131oF) temperatures. Active digestion results in volatile solids reduction and gas production. The basis of design should be supported by sewage analyses to determine the presence of undesirable materials, such as high concentrations of sulphates and inhibitory concentrations of heavy metals.

16.2.2 Mesophilic Anaerobic Digestion

Two-stage mesophilic anaerobic digestion has typically been practiced at larger STP in Ontario. This arrangement with primary and secondary digesters is considered to be high-rate digestion, consisting of a heated and mixed primary digester and an unheated and unmixed secondary digester. Multiple units in each stage may be required, depending on plant size.

Two stages with a minimum of one digester in each stage should be provided in all plants. Facilities for treated sludge or biosolids storage and supernatant separation in an additional unit may be required, depending on raw sludge concentration and disposal methods for biosolids and supernatant. In the case where only one digester is provided in each stage, piping should be designed to allow either stage to receive raw sludge with measures to allow for mixing and heating. The number of digesters in each stage of larger plants should be dictated by economics.

16.2.2.1 Process Variables

Anaerobic digester geometry has largely been based on low profile cylindrical vessels. Sidewall depth-to-diameter ratio typically ranges from 0.3 to 0.7. Shallower tanks are not conducive to good mixing. A freeboard allowance for scum and foam should be provided. In addition, bottom sediments can reduce active digestion volume.

If process design provides for supernatant withdrawal, the proportion of depth to diameter should be designed to allow for the formation of a reasonable depth of supernatant liquor. A minimum sidewater depth of 6.1 m (20 ft) is recommended.

New geometrical high rate anaerobic digester design includes “egg-shaped” digesters. These digesters do not normally provide for internal gas storage, but are claimed to provide better control of scum and bottom deposit formation and better active zone mixing. Higher ratios of height to diameter in cylindrical digesters may provide similar benefits; however, heating losses will need to be verified in design.

In addition to two-stage digestion, two-phase digestion is an innovative alternative that separates the acid forming and methane forming reactions of the primary digestion process in separate vessels. This has not been practiced to-date in Ontario. Other innovative staging options exist including temperature-phased anaerobic or mixed aerobic/anaerobic digestion process.

Provision for digester clean-out should be provided in design together with redundancy of tankage or convertibility of secondary digesters to primary digestion. Fine grit and other debris have a tendency to accumulate in digesters over time, reducing the effective digestion volume. To facilitate emptying, cleaning and maintenance, the features outlined in the subsection below are desirable for cylindrical digestion tanks.

16.2.2.2 Design Considerations

The following design considerations should be assessed:

• The tank bottom should slope to drain toward the withdrawal pipe. For tanks equipped with a suction mechanism for sludge withdrawal, a bottom slope not less than 1 to 12 is recommended. Where the sludge is to be removed by gravity alone, a 1 to 4 slope is recommended;
• The Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993 specifies access manhole numbers and dimensions and the most up-to-date version of this code should be reviewed for details. There should be stairways and catwalks to reach the access manholes;
• A separate side wall manhole should be provided that is large enough to permit the use of mechanical equipment to remove grit and sand. The side wall access manhole should be low enough to facilitate heavy equipment handling and may be buried in the earthen bank insulation;
• Access to any tanks or enclosed spaces should be provided in accordance with the Confined Spaces Regulation (O. Reg. 632/05) made under the Occupational Health and Safety Act). These enclosed spaces may contain gases that result in respiratory failure unless proper approved methods for entry are followed;
• Non-sparking tools, rubber-soled shoes, safety harness, gas detectors for flammable and toxic gases and at least two self-contained breathing units should be provided for emergency use;
• Multiple sludge inlets and draw-offs and, where used, multiple recirculation suction and discharge points to facilitate flexible operation and effective mixing of the digester contents should be provided;
• Maximum flexibility should be provided in terms of sludge transfer from primary and secondary sewage treatment units to the digesters, between primary and secondary digesters and from the digesters to subsequent treated sludge or biosolids handling operations. The minimum diameter of sludge pipes should be 150 mm (NPS-6). Provision should be made for flushing and cleaning of sludge piping. Sampling points should be provided on all sludge lines. Main sludge lines should be from the bottom of the primary digester to the midpoint of the secondary digester, if pumped. Additional transfer lines should be from intermediate points in the primary digester (these can be dual purpose supernatant and sludge withdrawal lines);
• One inlet should discharge above the liquid level and be located at approximately the center of the tank to assist in scum breakup. The second inlet should be opposite to the suction line at approximately the 2/3 diameter point across the digester;
• Raw sludge inlet points should be located to minimize short-circuiting to the digested sludge or supernatant draw-offs;
• Treated sludge withdrawal to disposal should be from the bottom of the tank. The bottom withdrawal pipe should be interconnected with the necessary valving to the recirculation piping to increase operational flexibility in mixing the tank contents; and
• An unvalved vented overflow should be provided to prevent damage to the digestion tank and cover in case of accidental overfilling. This emergency overflow should be piped to an appropriate point and at an appropriate rate to the STP liquid train or sidestream treatment facilities to minimize the impact on process units.

16.2.2.3 Tank Design Capacity

The total digestion tank capacity should be determined by rational calculations based upon such factors as: sewage characterization; sewage treatment processes; volume of sludge added, percent solids and character; the temperature to be maintained in the digesters; the degree or extent of mixing to be obtained; degree of volatile solids reduction required; solids retention time at peak loadings; method of sludge disposal and size of the installation with appropriate allowances for gas, scum, supernatant and digested sludge storage.

The nominal minimum hydraulic retention time (HRT) in the primary digester should be at least 15 days [the theoretical solids retention time (SRT) requirement of slowest methane producers is approximately 10 days]. Secondary digesters of two-stage series digestion systems, which are used for digested sludge storage and concentration, should not be credited in the calculations for volumes required for sludge digestion.

Calculations should be prepared to justify the basis of design. The minimum digestion tank capacity outlined below is required. Such requirements assume that the raw sludge is derived from ordinary domestic sewage, a digestion temperature is to be maintained at 35°C (95°F), 40 to 50 percent volatile matter in the digested sludge, and that the digested sludge will be removed frequently from the process.

The secondary digester should be sized to permit solids settling for decanting and solids thickening operations and, in conjunction with off-site facilities, to provide necessary digested sludge storage. The storage volume should be based on the highest and lowest level in the digester without losing the gas seal. The necessary total storage time will depend on the means of ultimate sludge disposal, with the greatest time required with soil conditioning operations and with less storage required with landfilling or incineration as ultimate disposal methods.

16.2.3 Digested Sludge Storage

Onsite or offsite storage in sludge lagoons, sludge storage tanks, or other facilities may be used. If high rate primary digesters are used and efficient thickening within the secondary digester is required, the secondary digester should be conservatively sized to allow adequate solids separation (secondary to primary volume ratios of 2:1 to 4:1 are recommended). The secondary digester provides relatively little storage capacity, particularly if a land application program is practiced.

The designer should refer to General Regulation (O. Reg. 267/03) made under the Nutrient Management Act for minimum biosolids storage requirement for a biosolids land application program.

16.2.4 Digester Design Loadings

The volatile solids loading rates to mesophilic anaerobic digesters should be designed as outlined below:

Completely Mixed Systems – High Rate Digestion

For digestion systems providing for intimate and effective mixing of the digester contents, the system may be loaded up to 1.6 kg/(m³·d) of volatile solids (100 pounds per 1000 cubic ft of volume per day) in the active primary digestion units. Higher digester loading rates have been proposed for wellmixed systems.

Moderately Mixed Systems – Low Rate Digestion

For digestion systems where mixing is accomplished only by circulating sludge through an external heat exchanger, the system may be loaded up to 0.65 kg/(m3·d) of volatile solids (40 pounds per 1000 cubic feet of volume per day) in the active digestion units. This loading may be modified upward or downward depending upon the degree of mixing provided.

16.2.5 Digester Mixing

Primary anaerobic digesters can potentially experience severe losses of active mixing volume to dead space and short-circuiting of raw sludge past the mixing zone and appearing in the digester outlet. Tracer tests are recommended to determine the state of digester mixing as temperature or solids profiles do not alone provide evidence of mixing degree.

There are three main mixing techniques:

• Gas - confined and unconfined;
• Mechanical; and
• Pump and in situ mixers.

Gas mixing systems recirculate compressed digester gas in either unconfined or confined mixing. Both create upward mixing actions.

Mechanical mixing uses axial flow propellers with roof- or external-mounted draft tubes. The roof mounted draft tubes limit the digester size to less than 24 m (80 ft) in diameter; whereas, the external mounted tubes can accommodate diameters of 24 m (80 ft) and greater. Mechanical mixers using vertical mixing action can also be considered.

Pump mixing uses axial flow patterns, and screw-type centrifugal or choppertype features. This draws sludge from the bottom and pumps it back into the top. In situ mixers include the internal plunger cam design.

Where sludge recirculation pumps are used for mixing they should meet design recommendations contained in Section 16.7 - Sludge Pumps and Piping.

Typical power requirement for primary stage mixing ranges between 5 to 8 W/m³ [0.2 to 0.3 hp/(1000 ft³)] for compressed gas mixing and 6.6 W/m³ [0.25 hp/(1000 ft³)] for mechanical mixing.

The designer’s calculations of the actual power requirements should be based on tank size, sludge rheology, types of mixers, mixer performance and mixing energy or shear rate required.

One concern with mixing is the formation of foam and grease on the digester surface. Means of foam/grease removal or suppression should be defined. Digesters treating thickened WAS may have significantly different mixing requirements than digesters not treating such a feed stream.

16.2.5.1 Digestion Tank Heating

If digestion tanks are constructed above grade level they need to be suitably insulated to minimize heat loss. Maximum utilization of earthen bank insulation should be used.

Sludge may be heated by circulating the sludge through external heaters or by units located inside the digestion tank. The system may be designed to provide for the preheating of feed sludge before introduction into the digesters. Grinder-type pumps are recommended to avoid heat exchanger clogging. Provisions should be made in the layout of the piping and valving to facilitate heat exchanger tube removal and cleaning of the lines. Heat exchanger sludge piping should be sized for peak heat transfer requirements. Heat exchangers should have a heating capacity of 130 percent of the calculated peak heating requirement to account for the occurrence of sludge tube fouling.

The use of hot water heating coils affixed to the walls of the digester, or other types of internal heating equipment that require emptying the digester contents for repair, are not acceptable.

Other systems and devices have been developed to provide both mixing and heating of anaerobic digester contents. These systems should be reviewed on their own merits. Operating data detailing their reliability and operation and maintenance characteristics should be reviewed. Process calculations should be documented to demonstrate that sufficient heating and mixing will be obtained.

Sufficient heating capacity should be provided to consistently maintain the design sludge temperature considering insulation provisions and ambient cold weather temperature conditions. Where digester gas is used for other purposes, an auxiliary fuel may be required. The design operating temperature should be 35°C (95°F) for optimum mesophilic digestion. Operating temperature may have to be elevated to 38oC to 40oC to achieve the E. coli standards required for agricultural land application of sewage biosolids especially for digesters that may have short-circuiting or dead volume problems.

The provision of standby heating capacity or the use of multiple units sized to provide the heating requirements should be considered unless acceptable alternative means of handling raw sludge are provided for the extended period that digestion process outage is experienced due to heat loss.

A suitable automatic mixing valve should be provided to temper the boiler water with return water so that the inlet water to the removable heat jacket or coil in the digester can be held below a temperature at which caking will be accentuated. Manual control should be provided by suitable bypass valves.

The boiler should be provided with suitable automatic controls to maintain the boiler temperature at a minimum 82°C (180°F) to minimize corrosion and to shut off the main gas supply in the event of pilot burner or electrical failure, low boiler water level, low gas pressure, or excessive boiler water temperature or pressure. The boiler water chemical quality should be checked for suitability for this use. Refer to Section 8.7.2 - Water Supply for required break tank for indirect water supply connections.

Boiler water pumps should be sealed and sized to meet the operating conditions of temperature, operating head and flow rate. Duplicate units should be provided.

Thermometers should be provided to indicate inlet and outlet temperatures of the sludge, hot water feed, hot water return and boiler water.

Controls necessary to ensure effective and safe operation are required. Provision for duplicate units in critical elements should be considered.

16.2.5.2 Gas Collection and Handling

All portions of the gas system including the space above the tank liquid level, storage facilities and piping should be so designed that under all normal operating conditions, including sludge withdrawal, the gas will be maintained under pressure. All enclosed areas where any gas leakage might occur should be adequately ventilated.

Anaerobic digestion systems produce digester gas which has methane as its main constituent. To safeguard anaerobic digester and gas handling system design, the designer should follow the requirements of the Canadian Gas Association (CGA) Code for Digester Gas (and Landfill Gas) System Installation, CAN/CGA-B105-M93 (1993).

The Ontario Technical Standards and Safety Authority (TSSA), if requested, will carry out a review of anaerobic digester gas systems design on a fee-forservice basis prior to construction. Certification of these systems will only be granted following inspection of the constructed works by the TSSA.

To provide gas storage volume and to maintain uniform gas pressures, a separate gas storage sphere should be provided or at least one digester cover should be of the gas holder floating type. If only one floating cover is provided it should be on the secondary digester. Pressure and vacuum relief valves and flame arrestors, adequately protected from the elements, should be provided. Access manholes and sampling wells should be provided on the digester covers.

Steel is the most commonly used material for digester covers. However, other properly designed and constructed materials are used such as fiberglass and concrete.

All necessary safety facilities should be included where digester gas is produced. Pressure and vacuum relief valves and flame arrestors together with automatic safety shut off valves should be provided and protected from freezing. Water seal equipment should not be installed. Safety equipment and gas compressors should be housed in a separate room with at least one exterior door.

16.2.5.3 Gas Piping

Gas piping should have a minimum diameter of 100 mm (4 in). A smaller diameter pipe at the gas production meter is acceptable. Gas piping should slope to condensation traps at low points. The use of float-controlled condensate traps is not permitted. Condensation traps should be protected from freezing. The Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993 or its amendments need to be reviewed and the applicable code requirements met.

16.2.5.4 Gas Appurtenances

Tightly fitted self-closing doors should be provided at connecting passageways and tunnels, which connect digestion facilities to other facilities to minimize the spread of gas. Piping galleries should be ventilated in accordance with the Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993.

Gas burning boilers and engines should be located in well-ventilated rooms. Such rooms would not ordinarily be classified as a hazardous location if isolated from the digestion gallery. Gas lines to these units should be provided with suitable flame traps.

Electrical equipment, fixtures and controls in places enclosing and adjacent to anaerobic digestion appurtenances, where hazardous gases may accumulate, should comply with the Electrical Safety Code for Class I, Zone 1 (old Division 1), Group D locations (O. Reg. 164/99 made under the Electricity Act, 1998), Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993 and the National Fire Protection Association’s (2003) Fire Protection in Wastewater Treatment and Collection Facilities, US NFPA 820.

Waste gas burners should be readily accessible and should be located at least 15 m (50 ft) away from the digester perimeter and any plant structure. Waste gas burners should be of sufficient height and so located to prevent injury to personnel due to wind or downdraft conditions. All waste gas burners should be equipped with automatic ignition such as a pilot light or a device using a photoelectric cell sensor. Pilot light should be either natural or propane gas type.

Gas piping should be sloped at a minimum of 2 percent up to the waste gas burner with a condensate trap provided in a location not subject to freezing. The ventilation rate for enclosed areas without a gastight partition from the digestion tank or areas containing gas compressors, sediment traps, drip traps, gas scrubbers, or pressure regulating and control valves, if continuous, should be at least 12 complete air changes per hour.

Any underground enclosures connecting with digestion tanks or containing sludge or gas piping or equipment should be provided with forced ventilation in accordance with the Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993 and the National Fire Protection Association’s (2003) Fire Protection in Wastewater Treatment and Collection Facilities, US NFPA 820.

16.2.5.5 Gas Metering

A gas meter with bypass should be provided to meter total gas production for each active digestion unit. Total gas production for two-stage digestion systems operated in series may be measured by a single gas meter with proper interconnected gas piping.

Where multiple primary digestion units are used with a single secondary digestion unit, a gas meter should be provided for each primary digestion unit. The secondary digestion unit may be interconnected with the gas measurement unit of one of the primary units. Interconnected gas piping should be properly valved with gastight valves to allow measurement of gas production from either digestion unit or maintenance of either digestion unit.

Gas meters are generally of the orifice plate, turbine, thermal dispersion, or vortex type. Positive displacement meters should not be used. The meter should be designed specifically for contact with corrosive, moist and dirty gases.

16.2.5.6 Supernatant Withdrawal

Where supernatant separation is to be used to concentrate sludge in the digester units and increase digester solids retention time, the design should provide for ease of operation and positive control of supernatant quality.

Supernatant piping should not be less than 150 mm (NPS-6) in diameter.

Piping should be arranged so that withdrawal can be made from three or more levels in the tank. An unvalved vented overflow should be provided. The emergency overflow should be piped to an appropriate point and at an appropriate rate to the STP liquid train or sidestream treatment units to minimize the impact on process units. The design of the overflow should prevent digester gas migration to other process areas. Supernatant flow measurement should be provided.

On fixed-cover tanks the supernatant withdrawal level should preferably be selected by means of interchangeable extensions at the discharge end of the piping.

A fixed-screen supernatant selector or similar type device should be limited for use in an unmixed secondary digester unit. If such supernatant selector is provided, provision should be made for at least one other draw-off level located in the supernatant zone of the tank, in addition to the unvalved emergency supernatant draw-off pipe. High-pressure backwash facilities should be provided.

Provision should be made for sampling at each supernatant draw-off level. Sampling pipes should be at least 40 mm (1½ in) in diameter and should terminate at a suitably sized sampling sink or basin meeting the Code requirements. Special designed sampling valves that ensure a representative sample, can be used in lieu of a sampling sink.

Supernatant return facilities should be designed to alleviate adverse hydraulic and organic loadings effects on sewage treatment plant liquid train operations. If nutrient removal (e.g. phosphorus) is to be accomplished at an STP, then a separate supernatant sidestream treatment system may be provided.

16.2.5.7 Sludge Production

For calculating design sludge handling and disposal needs, sludge production values from a two-stage anaerobic digestion process should be based on a maximum solids concentration of 4 percent without additional thickening. Facility sizing using population equivalent (P.E.) should not be used as the primary basis for design. Process calculation based on raw sewage treated and process type should form the basis for designing of needed facilities where P.E. is used for rough estimation of needed capacities. Estimates should also consider other solids accepted by the STP including (but not limited to) septage and water treatment plant residuals. Where P.E. is used for rough estimation, the approximate sludge production values on a dry weight basis should be at least 0.07 kg/(P.E.·d) (0.15lb/P.E./d) for a conventional activated sludge plant with phosphorus removal.

16.2.6 Thermophilic Anaerobic Digestion

Thermophilic anaerobic digestion is in principle similar to anaerobic mesophilic digestion, except that thermophilic digestion occurs between 50 to 60°C (122 to 140°F). Because biochemical reaction rates increase with increased temperatures, thermophilic digestion is faster than mesophilic digestion for the same volatile solids reduction. The higher temperature at which thermophilic digestion takes place allows for increased pathogen destruction.

16.2.6.1 Process Variables

Similar to mesophilic digestion, the following are the four main process variables to be considered in the design and operation of thermophilic anaerobic digestion:

• Solids loading rate;
• Solids retention time (SRT);
• Hydraulic retention time (HRT);
• Temperature; and
• pH.

16.2.6.2 Design Considerations Thermophilic anaerobic digestion generally has the same design considerations as anaerobic mesophilic digestion, specifically:

• Digester shape;
• Digester cover and bottom;
• Digester volume;
• Mixing system;
• Heating system; and
• Gas collection, storage and use.

16.2.6.3 Operational Considerations The same operational considerations for mesophilic digestion generally apply to thermophilic digestion, such as:

• Feeding and withdrawal;
• Temperature control;
• pH and alkalinity monitoring; and
• Odour control.

Because of the higher sensitivity of thermophilic bacteria to temperature changes and potential process upsets, temperature control, pH and alkalinity monitoring are important. Thermophilic anaerobic digestion produces a higher concentration of volatile fatty acids (1,000 to 2,000 mg/L) than mesophilic digestion. The higher operating temperature in thermophilic digesters tends to suppress scum and foam formation, so that scum and foam control is often less problematic than in mesophilic digesters.

16.3 Aerobic Sludge Digestion

16.3.1 General

The aerobic sludge digestion system should include provisions for digestion, supernatant separation, sludge concentration and sludge storage. These provisions may be accomplished by separate tanks or processes, or within the digestion tanks.

Multiple digestion units capable of independent operation are desirable and should be provided in STP where the design average daily flow exceeds 380 m³/d (0.1 mUSgd). Two stages with a minimum of one digester in each stage should be provided. If economics permit or plant size dictates, more than one digester can be used. Plants not having multiple units should provide alternate sludge handling and disposal methods. A loading rate of 1.6 kg/(m³·d) [100 lb/(1000 ft³·d)] volatile solids based upon first stage volume only should be provided.

Tank design is generally open and may be common wall or earthen bermed to minimize heat loss. Aerobic digesters may be covered to minimize heat loss for colder temperature applications. Tank depths of 3.6 to 4.6 m (11.8 to 15.1 ft) are suggested. The tanks and piping should be designed to permit sludge addition, withdrawal and supernatant decanting from various depths to and from both the primary and secondary digesters.

16.3.2 Tank Design Capacity

Digestion tank capacities are based on a solids concentration of 2 percent with supernatant separation performed in a separate tank and providing sufficient tank volume based on process engineering calculations using sewage and treatment characteristics. Volumes are based on digester temperatures of 10°C (50°F).

Sizing should be designed to achieve a minimum SRT of 45 days, including both digester stages and the SRT of the activated sludge treatment process. It is recommended that 2/3 of the total digester volume be in the first stage and 1/3 be in the second stage.

If supernatant separation is performed in the digestion tank, a minimum of 25 percent additional tank volume is required. These capacities should be provided unless sludge thickening facilities are used to thicken the feed solids concentration to greater than 2 percent. If such thickening is provided, the digestion volumes may be decreased proportionally; however, excess thickening may result in the occurrence of unintended autothermal digestion resulting in foam, scum and odour that requires control.

If primary sludge is to be included for digestion, minimum SRT and air requirements may have to be increased based on specific process calculations.

Actual storage requirements will depend upon the ultimate disposal operation. Minor additional storage requirements may be made up in the second stage digester, but if major additional storage volumes are required, separate on-site or off-site storage facilities should be considered to avoid the power requirements associated with operating greatly oversized aerobic digesters. The General Regulation (O. Reg. 267/03) made under the Nutrient Management Act should be reviewed for specific quality and seasonal storage requirements for a land application program.

Designers should be cautioned that aerobically digested sludges have a greater odour producing potential then anaerobically digested sludge, if septicity occurs.

16.3.3 Mixing

Aerobic digesters should be provided with mixing equipment that can maintain solids in suspension and ensure complete mixing of the digester contents.

16.3.4 Air Requirements

Sufficient air should be provided to keep the solids in suspension and maintain dissolved oxygen between 1 mg/L and 2 mg/L. For minimum mixing and oxygen requirements, an air supply of 0.5 L/(m³·s) (30 cfm/1000 ft³) should be provided with the largest blower out of service. If diffusers are used, the nonclog type is recommended and they should be designed to permit continuity of service. Air supply to each tank should be separately valved to allow aeration shut-down in either tank. The diffuser type should not be susceptible to plugging during frequent shutdown periods. All diffuser drop pipes should be able to withstand impact of ice masses that may form in the tankage in winter and should allow for easy removal for diffuser maintenance.

If mechanical turbine aerators are used, at least two turbine aerators per tank should be provided to permit continuity of service. A minimum bottom velocity of 0.25 m/s (0.82 ft/s) should be maintained while aerating. Mechanical aerators are not recommended for use in aerobic digesters where freezing conditions will cause ice build-up on the aerator and support structures.

16.3.5 Supernatant Separation and Scum and Grease Removal

Facilities should be provided for effective separation or decanting of supernatant. Separate facilities are recommended; however, supernatant separation may be accomplished in the digestion tank provided additional volume is provided (Section 16.3.2 - Tank Design Capacity).

The supernatant draw off unit should be designed to prevent recycle of scum and grease back to the STP liquid train process units. Facilities should be provided for the effective collection of scum and grease from the aerobic digester for final disposal to prevent long-term accumulation and potential discharge in the effluent. Provision should be made to withdraw supernatant from multiple levels of the supernatant withdrawal zone.

16.3.6 High Level Emergency Overflow

An unvalved high level overflow and necessary piping should be provided to return digester overflow to the head of the sewage treatment plant or to the secondary treatment process in case of accidental overfilling. Design considerations related to the digester overflow should include waste sludge rate and duration during the period that the sewage treatment plant is unattended, potential effect on plant process units, discharge location of the emergency overflow and potential discharge of suspended solids in the STP effluent.

16.3.7 Sludge Production

For calculating design sludge handling and disposal needs, sludge production values from aerobic digesters should be based on a maximum solids concentration of 2 percent without additional thickening. Facility sizing using population equivalent should not be used as the primary basis for design. Process calculations based on raw sewage treated, process type, chemical addition, septage solids and water treatment plant waste solids impacts, where applicable, should form the basis for design of needed facilities where P.E. is used for rough estimation of needed capacities.

Where P.E. is used for rough estimation, the approximate sludge production values on a dry weight basis should be at least 0.05 kg/(P.E.·d) (0.11 lb/ P.E./d) for an extended aeration STP with phosphorus removal.

16.3.8 Digested Sludge Storage Volume

Sludge storage should be provided in accordance with the General Regulation (O. Reg. 267/03) made under the Nutrient Management Act to accommodate sludge production volumes and as an operational buffer for unit outage and adverse weather conditions for biosolids destined for land application. Designs utilizing increased SRT in the activated sludge treatment process as a means of storage should be avoided.

The designer should refer to Chapter 18 - Sludge Storage and Disposal for more detailed information.

16.4 Autothermal Thermophilic Aerobic Digestion

16.4.1 General

Thermophilic aerobic digestion is in principle similar to mesophilic aerobic digestion, except that thermophilic digestion occurs between 50 to 70°C (122 to 158°F). Because biochemical reaction rates increase with increased temperatures, thermophilic digestion is faster than mesophilic digestion for the same, or greater volatile solids reduction. The higher temperature at which thermophilic digestion takes place allows for increased pathogen destruction. Higher solids loading and lower air flows allow for the excess energy from the exothermic reaction to raise the temperature of the solids to required levels without an external heat source. Mechanical energy is supplied for mixing in lieu of utilizing air mixing.

Retaining the heat from the biological process is vital to this process. The tank design should consider insulation, below grade construction or other means to control heat loss from the system. The effects of heat and corrosion on the tank (and cover) structures should be considered in the design.

The total solids concentration to the digesters should be 4 to 6 percent, and the volatile solids should be at least 2.5 percent. Volatile solids (VS) could range from 2.5 to 5 percent. Other loading rates are possible and should be established based on detailed calculations and on a site-specific basis, provided those rates can maintain adequate process temperatures.

Tanks should provide a freeboard of 1.0 to 2.0 m (3 to 6 ft). Provisions for draining and cleaning the tanks should be considered.

Single-Stage Systems

Single-stage Autothermal thermophilic aerobic digestion (ATAD) units operate with the entire process being completely contained within one tank. Multiple tanks may be provided but are often operated as parallel units and may be loaded on alternate days to allow for the appropriate isolation period. Continuous or semi-batch feeding is more common in these systems and allows for the distribution of the aeration demands throughout the entire process cycle. Process temperatures for single-stage units should be maintained at 50 to 70oC (122 to 158oF). Detention time should be a minimum of 10 days. The foam blanket should be controlled and optimized but not eliminated. Energy requirements for mechanical mixing may be in the range of 50 to 150 kW/1000 m³ (2 to 6 hp/1000 ft³) of active digester volume. Air requirements may range from 10 to 50 m³/min/1000 m³ (10 to 50 cfm/1000 ft³) of active digester volume to achieve 30 to 45 percent VS destruction and are often operated on variable frequency drives. Air requirements should be based on loading rate and degree of VS destruction. Adjustments to the air requirements for altitude and loading rates should be considered.

Multiple-Stage Systems

The multiple-stage ATAD process consists of two or more stages. It is good practice to provide tankage, piping and pumping facilities so that at least two tanks are available. The piping should allow transfer of biosolids from one tank to another to allow for maintenance and continuous digester operation. Process temperatures for the first stage should be maintained at 35 to 50°C (95 to 122°F). Second stage temperatures should be maintained at 50 to 65°C (122 to 149°F). Detention time should be a minimum of 10 days. The foam blanket should be controlled and optimized but not eliminated. The dissolved oxygen (DO) level in the liquid should be maintained at 2 mg/L and should not be less than 1 mg/L.

Energy requirements for mechanical mixing should be in the range of 79 to 105 kW/1000 m³ (3 to 4 hp/1000 ft³) of active digester volume. Air requirements should be at least 70 m³/min/1000 m³ (70 cfm/1000 ft³) of active digester volume to achieve 30 to 45 percent VS destruction. Adjustments to the air requirements for altitude and loading rates should be considered.

16.4.1.1 Operational Considerations

The same operational considerations for mesophilic aerobic digestion generally apply to thermophilic digestion, including:

• Feeding and withdrawal;
• Temperature control;
• pH;
• DO;
• Oxidation-reduction potential;
• Foam control; and
• Odour control.

The thermophilic bacteria are less sensitive to process changes and are not prone to process upsets. Temperature control and solids monitoring are important. The higher operating temperature in the thermophilic aerobic digester in combination with aeration tends to produce more foam, so foam control is an important design consideration. Additionally, given the high temperature and the increased ammonia from the volatile solids destruction, ammonia and other compounds may be given off in the off gas. Odour collection and control is therefore an important consideration in the design.

16.5 Other Sludge Treatment Methods

The following sludge treatment methods are generally employed to further stabilize biosolids and/or reduce volume.

16.5.1 Alkaline Stabilization

Alkaline material may be added to liquid primary and/or secondary sludges for sludge stabilization in lieu of digestion facilities, to supplement existing digestion facilities, or for interim sludge handling. There is no direct reduction of organic matter or sludge solids with the high pH alkaline stabilization process. There is actually an increase in the mass of dry sludge solids. Without supplemental dewatering, additional volumes of sludge will be generated. The design should account for the increased sludge quantities for storage, handling, transportation, disposal methods and associated costs.

Alkaline material should be added to liquid sludge to produce a homogeneous mixture with a minimum pH of 12 after 2 hours of vigorous mixing. To achieve stabilization sufficient alkaline material should be added to the liquid sludge to retain a homogeneous mixture with a minimum pH of 12 after 72 hours. Material should be provided to maintain the pH of the sludge during interim sludge storage periods.

Other proprietary alkaline stabilization systems using various alkaline reagents which include a pasteurization process are available to produce biosolids from sludge.

16.5.1.1 Odour Control and Ventilation

Odour control facilities should be provided for sludge mixing and treated sludge storage tanks when located within 0.8 km (0.5 mile) of residential or commercial areas. Air pollution control design objectives should be met for various types of air scrubber units. Ventilation is required for indoor sludge mixing, storage or processing facilities. (Section 7.2.10 - Safety Ventilation).

16.5.1.2 Mixing Tanks and Equipment

Mixing tanks may be designed to operate as either a batch or continuous flow process. A minimum of two tanks should be provided, of adequate size to provide a minimum of 2 hours contact time in each tank.

The following items should be considered in determining the number and size of tanks:

• Peak sludge flow rates;
• Storage between batches;
• Dewatering or thickening performed in tanks;
• Repeating sludge treatment due to pH decay of stored sludge;
• Sludge thickening prior to sludge treatment; and
• Type of mixing device used and associated maintenance or repair requirements.

Mixing equipment should be designed to provide vigorous agitation within the mixing tank, maintain solids in suspension and provide for a homogeneous mixture of the sludge solids and alkaline material. Mixing may be accomplished either by diffused air or mechanical mixers. If diffused aeration is used, an air supply of 0.5 L/(m³·s) (30 cfm/1000 ft³) of mixing tank volume should be provided with the largest blower out of service. When diffusers are used, the non-clog type is recommended and they should be designed to permit continuity of service. If mechanical mixers are used, the impellers should be designed to minimize fouling with debris in the sludge. Manufacturers’ specifications should be reviewed for mixing power required for mechanical mixers. An approximate minimum requirement for liquid sludge and lime slurries is to provide a bulk fluid velocity of 0.13 m/s (0.42 ft/s) and an impeller Reynolds number greater than 1,000.

Consideration should be made to provide continuity of service during freezing weather conditions.

16.5.1.3 Chemical Feed and Storage Equipment

Alkaline material is caustic in nature and can cause eye and tissue injury. Equipment for handling or storing alkaline material should be designed for adequate operator safety. Storage and feed equipment should be sealed as airtight as practical to prevent contact of alkaline material with atmospheric carbon dioxide and water vapor and to prevent the escape of dust material. All equipment and associated transfer lines or piping should be accessible for cleaning.

16.5.1.4 Feed and Slaking Equipment

The design of the feeding equipment should be determined by the STP size, type of alkaline material used, slaking required and operator requirements. Equipment may be either of manual batch or automated type. Automated feeders may be of the volumetric or gravimetric type depending on accuracy, reliability and maintenance requirements. Manually operated batch slaking of quicklime (CaO) should be avoided unless adequate protective clothing and equipment are provided. At small plants, use of hydrated lime [Ca(OH)₂] is recommended over quicklime due to safety and labour-saving reasons. Feed and slaking equipment should be sized to handle a minimum of 150 percent of the peak sludge flow rate including sludge that may need to be retreated due to pH decay. Duplicate units should be provided.

16.5.1.5 Chemical Storage Facilities

Alkaline materials may be delivered either in bag or bulk form depending upon the amount of material used. Material delivered in bags should be stored indoors and elevated above floor level. Bags should be of the multi-wall moisture-proof type. Dry bulk storage containers should be as airtight as practical and should contain a mechanical agitation mechanism. Storage facilities should be sized to provide a minimum 30-day supply.

16.5.1.6 Sludge Storage and Disposal

Refer to Chapter 18 - Sludge Storage and Disposal for general design considerations for sludge storage facilities. In addition, the design should incorporate the following considerations for the storage of high pH stabilized sludge:

Liquid Sludge

High pH stabilized liquid sludge should not be stored in a lagoon. The sludge should be stored in a tank or vessel equipped with rapid sludge withdrawal mechanisms for sludge disposal or treatment. Provisions should be made for adding alkaline material in the storage tank. Mixing equipment should be provided in all storage tanks.

Dewatered Sludge

On-site storage of dewatered high pH stabilized sludge should be limited to 30 days. Provisions for rapid treatment or disposal of dewatered sludge cake stored on-site should be made in case of sludge pH decay. Weather protection for dewatered sludge cake should be provided for long-term storage.

Off-Site Storage

There should be no off-site storage of high pH stabilized sludge unless specifically designed for such storage.

Disposal

Immediate sludge disposal methods and options are recommended to reduce the sludge inventory on the STP site and the amount of sludge that may need to be retreated to prevent odours if sludge pH reduction occurs. If the land application disposal option is utilized for high pH stabilized sludge, the sludge should be incorporated into the soil during the same day of delivery to the site.

16.5.2 Thermal Drying

Thermal drying is the process of evaporating water from sludge or biosolids by the addition of heat. Complete drying typically results in a product with 5 to 10 percent moisture content, corresponding, from a typical liquid sludge with 95 percent moisture content, to an approximate 20-fold volume reduction. Dewatering is usually a prerequisite intermediate step in the drying process to reduce energy costs for evaporation of moisture (refer to Chapter 17).

During drying, sludge or biosolids undergo several structural changes as the moisture content decreases. The most critical stage is called the plastic stage when the moisture content is between 40 to 60 percent dry solids (DS). In this stage, the dried product becomes sticky and difficult to manipulate. The power input required to move the product through this phase to higher concentrations is high.

Dryers are classified on the basis of:

• The predominant method of transferring heat to the solids (convection, conduction, radiation or a combination of these);
• The method of transition through the plastic phase;
• Whether drying and pelletization occurs in one or two steps; and
• Whether the biosolids are partially (i.e., <90 percent DS) or completely (i.e., >90 percent DS) dried.

The main benefits of drying sludge thermally can be summarized as follows:

• Increased pathogen destruction is achieved;
• Storage of dried sludge requires less volume and is easier to handle;
• Transportation costs are reduced;
• The final product can be marketed more easily as a fertilizer or soil conditioner;
• Dried sludge has a higher fuel value and can be incinerated or thermally converted; and
• Sludge drying increases the number of final disposal or utilization options.

Two general alternatives for thermal drying are direct and indirect systems. Thermal drying equipment is proprietary; the designer should consult the manufacturer to obtain specific design parameters.

16.5.2.1 Direct (Convection) Dryer Process Alternatives

In convection dryers the wet sludge is in direct contact with the heat transfer medium, which is usually a hot gas. Direct dryer designs typically include:

• Rotary drum dryers; and
• Fluidized bed dryers.

Rotary Dryers

Rotary dryers have found successful application in municipal biosolids facilities. Rotary dryers consist of a horizontal cylindrical steel drum, rotating at 5 to 25 rpm. The wet sludge/biosolids are mixed with an amount of dried product at the feed point. Flue gases from a burner flow co-currently and in direct contact with the biosolids. The mixture of biosolids and hot gases is conveyed to the discharge end of the drier where the dry product is separated from the gas and vapour mixture.

The temperature of the hot gas at the inlet of the drum is typically between 450 to 500°C (842 to 932°F) and the temperature of the product is approximately 100 to 140°C (212 to 284°F). The oxygen content at the dryer outlet is between 15 and 17 percent. The flue gas and vapour mixture is sent to a condenser and the flue gases and non-condensables should be treated in an odour control unit.

There are three main disadvantages with these types of dryers: the high oxygen content in the drum which presents fire and explosion risks, the large volume of gas that needs to be treated in an odour control unit and the energy losses from the large stack required. To address these disadvantages, some manufacturers implement air/vapour recirculation systems with heat exchangers.

Fluidized Bed Dryers

In fluidized bed systems, the biosolids are fluidized when brought into contact with hot gases moving upward. These are vertically mounted systems and in recent designs the hot gases are recirculated in a closed loop. Wet biosolids are mixed with dried product, enter at the top of the chamber and sink to the bottom. As the product dries its density decreases and as a result it occupies the highest part of the dryer. The dried product is discharged through an overflow and the gases are directed to a cyclone separator and an odour control unit. The cyclone captures the dust created by the attrition of the particles caused by fluidization. Fluidized bed dryers tend to be sensitive to variations in sludge composition because of its effect on the fluidization process. The heat exchangers incorporated in the chamber suffer from abrasion. The system is considered to have high power requirements.

16.5.2.2 Indirect (Conduction) Dryer Process Alternatives

In conduction dryers a solid retaining wall separates the wet sludge/biosolids from the heat transfer medium, which is usually steam or another hot fluid. Indirect (conduction) dryers include:

• Thin film dryers;
• Disc dryer;
• Paddle dryers;
• Vertical dryers;
• Pelletizers; and
• Multiple-effect evaporation dryers.

A brief description of the most common indirect dryer systems employed for municipal sludge/biosolids follows.

Thin Film Dryers

This is an example of a drying system that dries biosolids through the plastic phase without dry product recirculation. It is a horizontal system where biosolids are introduced in a fixed shell containing a rotating shaft. The material is spread onto the wall where it forms a thin film on a steam or thermal oil heated jacket. Blades mounted on the shaft scrape the product and force it across the dryer to the discharge end.

The main disadvantage of this type of drying system is the large amount of mechanical wear exerted by the dried product when it is above 80 percent DS.

Disc Dryers

Disc dryers are composed of heated hollow discs set one after the other in parallel along a rotor. The discs and rotor are enclosed in a fixed shell. Biosolids fill the shell and submerge the discs and rotor. There are scrapers attached to the encasing shell extending inward until just above the rotor shaft and the discs are equipped with large paddles, which control the residence time of the product. Disc dryers can be used for partial or complete drying. If used for complete drying, dried product is mixed with wet feed before entering the dryer. This configuration is subject to heavy mechanical wear.

Paddle Dryers

Paddle dryers have a similar configuration to disc dryers. Hollow wedgeshaped, self-cleaning blades take the place of the discs and casing. The rotor speed is low and the residence time is high. Paddle dryers are subject to similar wear problems as disc dryers when used for complete drying.

16.5.2.3 Radiation Dryer Process Alternatives

In radiation dryers, infrared lamps, electric resistance elements or gas-heated incandescent refractories supply the energy required to heat the wet sludge/biosolids and evaporate moisture.

16.5.2.4 Solar Thermal Drying

Solar thermal drying within a greenhouse is a thermal dewatering and drying process that uses solar energy as the primary energy source. This process is applicable to municipal sludge/biosolids. Proprietary systems are available that utilize this technology.

In solar drying, vapour pressure differences result in evaporation of moisture to the atmosphere. The transparent cover provides insulation and transmission of solar radiation and is constructed to withstand wind and snow loads. Liquid or dewatered sludges are spread out on the floor of the drying pens and are mixed and passively aerated through the mixing action. Automated ventilation and optional heating system should be provided.

Solar drying can provide a more controlled environment, however the technology requires a significant amount of land. It is modular and can be decentralized. Solar drying facilities can handle liquid sludge from 3 to 10 percent dry solids (DS) and dewatered sludges from 10 to 40 percent DS. The treatment process provides dewatering and drying. The process can achieve a substantial volatile solids and pathogen reduction depending on the drying time. Drying times of three months during winter operation without external energy, and shorter periods during summer conditions, are typical.

16.5.3 Pelletization of Dried Sludge

Pelletization of dried sludge can be used, especially if the dry product is to be used as a fertilizer or soil supplement. Pelletization may be accomplished in one step with drying or it may follow as a separate step. In the former case, dewatered sludge is first sent through a pelletizer where it is transformed into a pellet. This procedure gives cohesion to the sludge and creates a large external surface area that accelerates the drying process.

16.5.4 Indirect Vertical Dryer

Another process is an indirect vertical dryer with a number of heated trays constructed one above the other inside a cylindrical shell. The sludge/biosolids dry as they contact the heated trays. Recycled pellets are coated with a thin layer of incoming wet material and introduced to the dryer at the top. As they move from the top to the bottom trays they dry out and are finally transported to a separation hopper where they are sorted for size. Pellets are recycled 5 to 7 times, growing in size with each pass through the dryer, until they reach the desired diameter at which stage they are separated from the recycling stream and sent to the storage facility.

Pelletization is provided for the following reasons:

• Storage of dried sludge/biosolids pellets reduces the risk of fire and glowing, which is higher for dried sludge dust; and
• Handling of dried sludge pellets is easier and poses less danger to the environment and the personnel in contact with it.

16.5.4.1 Technical Considerations

For each of the dryer categories described above, there are specific manufacturer technical and design considerations. However, the following factors and considerations apply to all types of dryers and play the most important role in the dryer selection and sizing.

• The desired moisture content of the wet and dried sludge/biosolids will affect dryer selection;
• The amount of flexibility required in the design to accommodate varying sludge/biosolids characteristics;
• Mechanical dewatering is a requirement prior to drying;
• Continuous or batch drying operations affect dryer size;
• Storage requirements for wet and dried sludge/biosolids are an important consideration;
• Condensate from air recycle streams should be considered;
• Dust may be a hazard during processing or if the dried biosolids are stored in large volumes; dust creation should be prevented to avoid ignition and explosive conditions;
• Energy sources for the dryer may be natural gas or fuel oil; because of the large amounts of energy required, recovery of heat from the exhaust gases should be considered; in addition, future energy costs should be considered;
• Safety requirements, especially prevention of risk of fire or explosion; sludge dried and stored in reducing conditions may dry exothermically and potentially lead to autogenous combustion (the National Fire Protection Association’s (2003) Fire Protection in Wastewater Treatment and Collection Facilities, US NFPA 820 requirements should be reviewed); and
• Consideration should be given to odour control especially if unstabilized sludge/biosolids are dried (if rewetted, odours will be emitted). (Section 4.4 - Odour Control and Abatement Measures).

16.5.5 Composting

Composting is a biological process in which organic material undergoes biological degradation to a stable end product called humus. Composting has received attention as an alternative for enhanced stabilization and utilization of biosolids for a number of potential beneficial uses.

The Canadian Food Inspection Agency (CFIA) regulates the use of compost in accordance with the Fertilizers Regulations (C.R.C., c. 666) made under the Fertilizers Act (R.S.C. 1985, c. F-10). Canadian Council of Ministers of the Environment (CCME) Guidelines for Compost Quality (2005) specifies criteria for product safety and quality: foreign matter, maturity, pathogens and trace elements. The Standards Council of Canada (SCC) provides voluntary National Standard of Canada - Organic Soil Conditioners - Composts.

The ministry guidelines Interim Guidelines for the Production and Use of Aerobic Compost in Ontario provides guidance applicable to the establishment and operation of composting facilities in Ontario.

The designer should be familiar with the current requirements of these regulations and guidelines and consult with the ministry regarding site-specific design and operating criteria related to buffer zones, storage of composting material, runoff or leachate control, odour control and other process issues.

Composting is accomplished under aerobic conditions. The self-heating aerobic process attains temperatures in the pasteurization range of 50 to 70°C (122 to 158°F). This results in the inactivation of pathogens and the production of well-stabilized compost that can be stored and has minimal odour. The high quality biosolids product can be used beneficially as a soil conditioner or organic fertilizer supplement.

Maintenance of a minimum temperature of 55°C (131°F) for at least three days can achieve virtually complete inactivation of pathogens within in-vessel and aerated static pile systems. Composting is a sludge processing technology that, depending on process design, can treat dewatered undigested and/or digested sludge and potentially produce a “pathogen-free” biosolids product. Dewatering is to provide at least 18 percent solids concentration prior to addition of bulking agents. This could defer or eliminate the need for future digester upgrades and expansions and can represent a flexible option as part of a diversified biosolids management program. Additional volatile solids destruction and degradation of persistent organic substances in digested biosolids may be possible.

The mass of compost product is typically about one-half of the mass of wet dewatered sludge that is added to the process. However, there is little change in the volume, as the product is less dense than the wet sludge.

Methods for composting include in-vessel, windrow and static aerated pile composting. Table 16- 2 summarizes key requirements.

Three separate stages of microbial activity occur during the composting process:

• Initial mesophilic stage, during which temperatures within the pile increase from ambient to about 40°C (104°F);
• Thermophilic stage, caused by the heat generated through conversion of organic matter to carbon dioxide and water vapour, where temperatures can range from 40 to 70°C (104 to 158°F); and
• Cooling stage associated with reduced microbial activity as composting approaches completion (i.e., curing).

Composting under aerobic conditions, depending on the system design, involves the following steps:

• Mixing of dewatered sludge with a bulking agent or amendment to ensure an adequate mixture porosity for proper aeration, structural integrity, acceptable mixture density, reduced bulk moisture content and to provide supplemental carbon to adjust the energy balance and carbon-to-nitrogen ratio;
• Aeration and/or agitation of the mixture to promote the aerobic microbiological decomposition reactions (i.e., active composting); and
• Curing of the compost to complete the stabilization process.

In addition to providing the required oxygen for organics degradation, aeration and agitation facilitate the removal of exhaust gases, water vapour and heat. The rate of aeration may be used to control process temperature and the rate of drying. Drying during the composting process can produce solids concentrations of 50 to 55 percent.

Product curing, which follows active composting, may be preceded or followed by screening. The overall detention time for composting and curing is typically between 50 to 80 days. If feasible, the bulking agent is recovered by screening for reuse. An area for temporary storage of the final stabilized product is usually provided at the composting site.

16.5.5.1 Process Alternatives

The following are the major available process options for sludge composting:

• Aerated static pile;
• Windrow; and
• In-vessel systems.

Aerated Static Pile

In the aerated static pile process, the mixture of dewatered cake and coarse bulking agent is placed over a porous bed (i.e., a grid of closed and perforated piping). Air is supplied to each pile by a dedicated blower and piping and is drawn downward or forced upward through the mixture. The pile is covered with an insulating blanket of wood chips or screened compost. The active composting period is 21 to 28 days.

Small applications can consist of a number of individual piles whereas larger applications can involve a continuous pile that is divided into sections representing the contribution of each day. New facilities are typically covered and some are fully enclosed for reduced odour and for improved process control.

Windrow

Windrows consist of long narrow parallel piles of the mixture through which aeration is achieved by natural convection and diffusion. In the aerated windrow process, supplemental forced aeration through underlying air channels is used. The windrow is remixed periodically by a turning mechanism to facilitate air movement and moisture release. Windrow operations are covered or enclosed systems. The active composting period is 21 to 28 days.

In-Vessel Systems

In-vessel systems for active composting are enclosed and mechanized processes, comprising a reactor(s) and conveyors that offer an increased degree of process and odour control. The systems are compact and can be highly automated, including programmable logic controller (PLC)-based automatic control systems. The control of environmental conditions such as air flow, temperature, moisture and oxygen concentration permits shorter composting times.

The mixture of dewatered sludge, amendment and recycled compost is fed into one end of a tunnel, silo, or channel of the in-vessel process and moves continuously towards the discharge end. Air supplied by blowers is forced through this mixture which may be periodically agitated.

The three typical in-vessel reactor designs include:

• Vertical plug-flow;
• Horizontal plug-flow (i.e., tunnel reactor); and
• Agitated bin reactors.

The plug-flow systems involve periodic feeding (e.g. daily) and discharge of “finished” compost from the opposite end. Unlike the plug-flow designs, the dynamic agitated bed process uses mechanical mixing during composting. Depending on the particular process or system supplier, the detention time in the reactor can vary between 10 to 21 days for active composting. Compared with static pile and windrow composting, in-vessel processes can produce a more consistent product, require less space and provide an enhanced degree of odour containment and control. Modular system designs can facilitate future expansion.

Bulking agent may be removed after completion of composting by screening to reduce mass and to recycle the bulking agent.

16.5.5.2 Technical Considerations

Process variables that can affect composting operations and performance include temperature, bed porosity, moisture content, ratio of organics to nutrients, pH, aeration levels and detention time. Parameters that can be monitored and used to control in-vessel composting processes include:

• Mixture temperature;
• Blower static pressure;
• Relative humidity of the fresh air supply;
• Relative humidity of process headspace;
• Volume of fresh air;
• Blower speed; and
• Oxygen concentration in process headspace. A number of factors can influence selection of the most appropriate composting process for a given application. These can include:
• Characteristics of the sludge supply (e.g. solids content, degree of stabilization, if any, and loading rates);
• Type of equipment and chemicals used in upstream sludge dewatering and the consistency of the resultant cake; and
• Land availability.

Dewatered sludge cake of 18 to 25 percent solids can be mixed with bulking agent or amendment to produce the desired solids content of the feed supply.

The uniformity of the mixture with respect to porosity is critical in static pile systems and less so in windrow and in-vessel systems.

Sludge supply that is stabilized by aerobic or anaerobic digestion prior to composting can reduce the size of the composting facility due to the reduced organic solids content. In the event that the composting system is overloaded or out-of-service (e.g. for maintenance), the stabilization of sludge, prior to composting, offers the ability to directly apply biosolids to land on an interim basis. The application of biosolids to agricultural land should be designed into the program, including relevant permitting requirements (e.g. Organic Conditioning Site Certificate of Approval). Other considerations include:

• Composting of undigested sludge results in higher reaction rates, oxygen demand, heat generation and odour potential;
• Material being composted should be regularly mixed or turned, depending on the compost process, to prevent drying, caking and air channeling;
• Process temperature should be kept at between 55 to 65°C (131 to 149°F) for a defined period of time until pathogen control requirements are met. For the first few days, temperature should be maintained at optimum levels of between 50 and 55°C (122 and 131°F) to promote maximum rates of organics degradation and stabilization;
• New composting facilities typically include odour control systems for the containment and treatment of exhausts. Odour control systems can include biofilters, wet scrubbers and/or thermal oxidation for the removal of ammonia and other odour compounds; measures for odour control after winter freezing should be provided;
• Depending on process design, it may be possible to co-compost municipal STP sludge with other organic solid wastes. The latter solid wastes require pre-sorting and pulverizing prior to mixing with sludge;
• Site considerations include land availability, access, proximity to the STP, site drainage and runoff control, proximity to end users of the finished product, climatic conditions and availability of buffer zone; and
• The market for compost varies regionally based on local conditions such as land use, availability of competing soil amendment and fertilizer products, guidelines for biosolids compost and public acceptance of biosolids products.

16.5.5.3 Metals Content in Finished Compost

Metals content of finished compost affects the usability of the product and should be carefully considered during the process design to ensure a market for the final product. The ministry guidelines Interim Guidelines for the Production and Use of Aerobic Compost in Ontario provides the metals concentrations criteria which finished compost needs to comply with, and should be referenced.

16.5.6 Incineration

Incineration is a unit process for sludge management that “burns” the organic matter present in the sludge. Combustion releases the heat value of the organic matter in the sludge through rapid high temperature chemical oxidation reactions and reduces the volume and weight of solid residuals (ash) for ultimate disposal. Depending upon temperature, this process can destroy, transform or reduce trace organic materials.

The incineration of wastewater sludges employs high temperature chemical reactions, typically at 700 to 870°C (1,300 to 1,600°F), which convert the organic carbon and hydrogen in the sludge and the oxygen in the combustion air into carbon dioxide and water vapour. Supplemental fuel, usually natural gas or fuel oil, is burned if required, which would use additional combustion air and produce additional carbon dioxide and water vapour. Inorganic matter in the sludge remains as a solid “ash” residue. Metals (except for relatively volatile mercury and, to a lesser extent, cadmium) are converted to stable and generally insoluble oxides within the ash.

The exhaust gases contain, in addition to carbon dioxide and water vapour, nitrogen and unconsumed oxygen from the combustion air plus a smaller amount of air pollutant emission of suspended particulates, carbon monoxide, oxides of sulphur (SO_X) and oxides of nitrogen (NO_X). These usually result when the compounds are volatilized before they are exposed to high enough temperatures to be properly oxidized. Therefore, afterburning often has to be used to destroy these volatilized compounds, especially in multiple hearth incinerators.

The heating value of the wet sludge cake is a major variable governing the design and operation of an incinerator. Sludge heating values vary depending on the types of sludges and on the form and performance of the conditioning and dewatering processes used prior to incineration. The heating values of primary and secondary sludges, both raw and digested, average about 23,300 to 25,600 kJ/kg (10,000 to 11,000 Btu/lb) of volatile solids.

Supplemental fuel is needed for heating up an incinerator until it reaches combustion temperatures. Supplemental fuel may be required during routine operation, depending on the overall heat balance for the incinerator. Normally, either natural gas (37,300 kJ/m³ or 1,000 Btu/ft³) or heavy fuel oils (3.9x107 kJ/m³ or 1.05x106 Btu/ft³ for #2 grades and 4.0x107 kJ/m³ or 1.07x106 Btu/ft³ for #4 grades) are selected. In the case of incineration of anaerobically digested sludge, the ability to use recovered excess digester gas (about half the thermal value of natural gas) is an option for supplemental fuel.

Incinerator ash should be disposed of in a properly designed ash pond or approved landfill.

Sludges should ideally be concentrated to a solids concentration where they will burn autogenously. This solids concentration will vary somewhat with sludge type, volatile solids percentage and the chemical composition of the solids, but a concentration in the order of 30 percent TS will generally be required. A detailed mass and energy balance should be provided by the designer to calculate energy requirements and allowable solids loading rate. The design calculations should define the potential for heat recovery from the exhaust gases based on the overall mass and energy balance calculations.

Two types of incinerators are generally used for wastewater sludge: multiplehearth and fluidized bed. Fluidized Bed Incineration (FBI) is considered the type of incineration which is the state-of-the-art system and is usually the type that is provided for a new installation. The multiple hearth incinerator technology has largely been replaced by the FBI for sludge incineration.

16.5.6.1 Fluidized Bed Incinerator

The Fluidized Bed Incinerator (FBI) is a vertical, cylindrical, refractory-lined steel shell that contains no internal moving mechanical parts, only a sand bed and fluidizing air diffusers. It is normally available in sizes from 2.7 to 7.6 m (9 to 25 ft) in diameter. Units with diameters up to 10.7 m (35 ft) have been built. The sand bed is approximately 0.76 m (2.5 ft) thick at rest and sits on a refractory-lined grid. This grid contains tuyeres through which air is injected into the furnace at a gauge pressure of 20 to 34 kPa (3 to 5 psi) to fluidize the bed, which expands to approximately 200 percent of its at-rest volume.

Sludge is fed directly into the fluidized sand bed. Due to the fluidization, there is violent mixing in the bed which provides rapid and uniform distribution of fuel and air and consequently, good heat transfer and combustion. Heat transfer and reaction between the gases and the solids are rapid because of the large surface area available. The bed provides substantial heat storage capacity, which helps to reduce short-term temperature fluctuations that may result from varying feed sludge heating values or moisture contents. Organic particles remain in the sandbed until they are reduced to mineral ash. The violent motion of the bed comminutes the ash material, preventing the buildup of clinkers (agglomerated material). The resulting fine ash is stripped from the bed by the upflowing gases and carried out the top of the furnace and removed by air pollution control devices.

The temperature in the bed is maintained between 760 and 815°C (1,400 and 1,500°F) by addition, if needed, of supplemental fuel directly into the bed. For short periods of time during start up of the fluidized bed, an auxiliary burner(s) located either above or below the sand bed is utilized. In installations with autogenous combustion, a water spray (cooling system) above the bed is used to regulate the furnace temperature. In essence, the reactor is a single chamber unit in which both drying and combustion occur in the sandbed. All of the combustion gases pass through the combustion zone, with residence times of several seconds at 760 to 815°C (1,400 and 1,500°F).

The required air flow into the furnace is determined by several factors. Fluidizing and combustion air should be sufficient to expand the bed to a proper density, yet low enough to prevent the sludge from rising to and floating on top of the bed. Too much air can blow sand and products of incomplete combustion into the off-gases, while too little air can cause oxygen levels to fall below stoichiometric requirements for complete oxidation of all volatile solids in the sludge feed. Once the air flow rate has been determined in design, only minor adjustments are required during operation. Temperatures should be sufficiently high to assure complete deodorizing, but low enough to protect the refractory, heat exchanger and flue gas ducting. Optimum thermal economy is generally obtained with 20 to 45 percent excess air, a figure which varies as a function of the feed sludge composition.

The combustion process should be followed by an air emission control system to meet the requirements of Air Pollution-Local Air Quality, (O. Reg. 419/05), made under the Environmental Protection Act. The designer should refer to Section 3.11 - Emissions of Contaminants to Air.

16.6 Odour Control

Odour control facilities should be provided for sludge mixing and treated sludge storage tanks when located within 0.8 km (0.5 miles) of residential or commercial areas. The designer should refer to Section 4.4 - Odour Control and Abatement Measures.

Ventilation is required for indoor sludge mixing, storage or processing facilities (Section 7.2.10 - Safety Ventilation). Ventilation may be either continuous or intermittent. Ventilation, if continuous, should provide at least 12 complete air changes per hour; if intermittent, at least 30 complete air changes per hour. Air should be forced into the area by mechanical means rather than solely exhausted from the area. The air change requirements should be based on 100 percent fresh air. Portable ventilation equipment should be provided if there is no permanently installed ventilation equipment.

Anaerobic digester pressure relief valves should be designed and checked to prevent unnecessary release of digester gas. Similarly, compressor systems should be designed, checked and maintained to prevent gas leakage that could be either a safety problem or odour source.

16.7 Sludge Pumps and Piping

16.7.1 Sludge Pumps

Pump capacities should be adequate but not excessive. Provision for varying pump capacity is desirable. Duplicate units, or sufficient units with at least the largest unit out-of-service, should be provided at all installations.

Pumps with demonstrated solids handling capability should be provided for handling raw and digested sludge. Where centrifugal pumps are used, a parallel positive displacement pump may be provided as an alternate to pump heavy sludge concentrations, such as primary or thickened sludge, that may exceed the pumping head of the centrifugal pump. Some pumps require upstream grinders for successful operation.

A minimum positive head of 610 mm (24 in) should be provided at the suction side of centrifugal pumps and is desirable for all types of sludge pumps. Maximum suction lifts should not exceed 3 m (10 ft) for plunger pumps. Unless sludge sampling facilities are otherwise provided, quick-closing sampling valves or piston valves should be installed at the sludge pumps. The size of valve and piping should be at least 40 mm (1.5 in) and terminate at a suitable location.

16.7.2 Sludge Piping

Digested sludge withdrawal piping should have a minimum diameter of 200 mm (NPS-8) for gravity withdrawal and 150 mm (NPS-6) for pump suction and discharge lines. Where withdrawal is by gravity, the available head on the discharge pipe should be at least 1.2 m (4 ft) and preferably more. Undigested sludge withdrawal piping should be sized in accordance with Section 11.3.2.3 - Sludge Removal Pipeline.

Gravity piping should be laid on uniform grade and alignment. Slopes on gravity discharge piping should not be less than 3 percent for primary sludges and all sludges thickened to greater than 2 percent solids. Slopes on gravity discharge piping should not be less than 2 percent for aerobically digested sludge or waste activated sludge with less than 2 percent solids. Where gravity sludge transfer is proposed, provision should be made for a pumped transfer on a regular basis to remove deposits and clean out the lines. The pumped operation may be necessary after a few years of gravity operation when the gravity operation is not possible due to sludge deposits. Valving should be provided to allow for both gravity and pumped transfer.

Cleanouts should be provided for all gravity sludge piping. Provisions should be made for draining and flushing discharge lines. All sludge piping should be suitably located or otherwise adequately protected to prevent freezing. The section of piping between isolation valves should have a drain and vent valves or other means to relieve built-up pressure, due to gas formation, prior to dismantling the piping for cleaning or repairs.

Special consideration should be given to the corrosion resistance and permanence of supporting systems for piping located inside the digestion tank.