Sludge thickening and dewatering
Chapter 17: Sludge Thickening and Dewatering
This chapter describes dewatering of solids (sludge and biosolids) prior to disposal or prior to further treatment and stabilization. This includes sludge/biosolids conditioning, gravity and mechanical thickening, mechanical dewatering and drying beds for dewatering.
A summary of the thickening and dewatering performance expectations for conventional processes is provided in Appendix V, which should be used in conjunction with the details in this chapter.
The sludge solids concentrations that are listed in Section 16.1.1 - Sludge Quantities and Characteristics and Table 16-1 are the concentrations which can generally be achieved without the use of separate thickening or dewatering facilities. To achieve any significantly higher concentrations, sludge thickening and/or dewatering facilities will be required. The designer needs to evaluate the dewatering requirements for any planned sludge utilization/disposal operation and the end-use.
Sludge thickening normally refers to the process of reducing the free water content of sludges; whereas, dewatering refers to the reduction of floc-bound and capillary water content of sludges. See Tables 17-1 and 17-2 for typical performance expectations for various thickening and dewatering processes, respectively.
The benefits which can be derived from reductions in sludge water content include:
- Reduction in digester sizing requirements to achieve the same solids retention time;
- Reduction in heat exchange capacity requirements for anaerobic digestion;
- Reduction in sludge pumpage and transportation costs;
- Decrease in ultimate disposal costs;
- Reduction of handling problems and leachate production during sludge landfilling operations;
- Sizing for subsequent treatment or stabilization options [e.g. sludge drying, autothermal aerobic digestion (ATAD)]; and
- Reduction in storage volume is required to comply with the General Regulation (O. Reg. 267/03) made under the Nutrient Management Act. 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.
There may be some disadvantages to excessive reduction in water content which should also be taken into consideration:
- Sludge mixing and blending facilities may be required to combine sludges of differing water content for subsequent treatment operations;
- Sludge at 12 to 15 percent total solids (TS) is not free flowing and may require special sludge handling equipment;
- Dewatered sludges because of their significant loss in plant available nitrogen content may not be as acceptable or desirable for spreading on agricultural lands as liquid sludges are; and
- Contaminant loading in recycle streams requiring treatment.
Wherever possible, pilot plant and/or bench-scale data is recommended for the design of sludge thickening and dewatering facilities. With new plants, this may not always be possible and, in such cases, empirical design parameters should be used. The following subsections outline the normal ranges for the design parameters of such equipment.
In considering the need for sludge thickening and dewatering facilities, the designer should evaluate the economics of the overall treatment processes, with and without facilities for sludge water content reduction. This evaluation should consider both capital and operating costs of the various plant components and sludge disposal operations affected.
Recycle streams (i.e., centrate and filtrate) need to be carefully considered and their impact on the liquid train of the sewage treatment plant (STP) accounted for. The impact of these recycle streams need to be related to their schedule of operation, as thickening and dewatering processes are often operated only during the day shift and on weekdays. With or without equalization, the loading of these streams will increase the loading to the liquid train. These recycle streams may be small in terms of their volumetric loading, but can be concentrated in terms of organic loading, especially total ammonia-nitrogen loading from dewatering of biosolids from anaerobic digestion processes. Also the operation of the thickening and dewatering processes should be coordinated with the operation of the sludge feed processes, as these processes may be continuous and the thickening/dewatering processes may be operated periodically (e.g. 7 to 8 hours per day, 5 days per week). The impact should be assessed based on the actual flow and biological load from the dewatering facility on the liquid train performance, calculated over the actual time when the sidestream is returned rather than averaged over a 24-hour period, unless equalization of the recycle flows is provided.
Instrumentation should be considered for all thickening and dewatering processes to monitor influent and effluent flows, and sampling should confirm influent, thickened or cake and recycle stream solids. Additional sampling of the recycle stream is recommended to confirm the loading to the liquid train.
17.2 Sludge Conditioning
Sludge thickening and sludge dewatering operations (depending on the process used), are highly dependent upon sludge conditioning for their effective operation. Sludge conditioning affects the solids concentration of the thickened or dewatered sludge and the solids capture efficiency.
There are several characteristics of sludges, including industrial-derived constituents which may adversely affect attempts to achieve solid-liquid separation. The presence of colloidal particles increases the specific resistance of the sludge and adversely affects sedimentation processes. The net negative charge exhibited by most sewage sludges tends to make the particles repulse each other and thus resist agglomeration into larger particles. Sludge particles have a bound water content which, if retained, results in low cake solids after solid-liquid separation. Sludge conditioning operations attempt to alter one or more of the above sludge characteristics so as to improve the efficiency of the solid-liquid separation processes.
There are two sludge conditioning approaches that can be used. Sludge can be conditioned by physical methods, such as heat treatment or addition of fly ash or by chemical methods, involving the addition of either coagulants and/or polymers.
The method selected will not only differ in its effect on the thickening or dewatering process, but will have different effects on subsequent sludge handling operations and on the STP itself, due to recycle streams.
17.2.1 Chemical Methods
Chemical conditioning methods involve the use of organic or inorganic flocculants to promote the formation of a porous, free draining cake structure. Chemical conditioning for thickening operations attempts to promote more rapid phase separation, higher solids concentration and a greater degree of solids capture. With dewatering operations, chemical conditioning is used in an attempt to enhance the degree of solids capture by destabilization and agglomeration of fine particles. This promotes the formation of a cake, which then becomes the true filter media in the dewatering process.
With most thickening operations and with belt filter press dewatering operations, the most commonly used conditioning chemicals are polymers. For dewatering by vacuum filtration, ferric salts, often in conjunction with lime, are most commonly used. Chemical conditioning using polymers is most prevalent with centrifuge dewatering, with metal salts being avoided mainly due to corrosion problems. For dewatering by filter presses, the use of high molecular weight polymers for sludge conditioning has been successfully employed in lieu of lime and ferric chloride. The ultimate disposal methods may also have an effect on the choice of conditioning chemicals. For instance, lime and ferric compounds should be avoided with incineration options.
The selection of the most suitable chemical(s) and the appropriate dosage requirements for sludge conditioning can best be determined by pilot and fullscale testing and optimization. Pilot- or full-scale testing should assess the impact of residual polymer on the liquid train processes and consider the additives increase in metal content of the sludge.
Laboratory testing should be used to narrow down the selection process and to arrive at approximate dosage requirements. Generally, laboratory testing will yield dosage requirements within 15 percent of full-scale needs.
Mixing of the chemicals may be accomplished by either diffused air or mechanical mixers. If diffused aeration is used, an air supply of 0.85 L/(m3·s) (51 cfm/1000 ft3) of mixing tank volume should be provided. 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 need to be designed to minimize fouling with debris in the sludge and consideration should be made to provide continuity of service during freezing conditions.
17.2.2 Physical Methods
Heat conditioning of sludge consists of subjecting the sludge to high levels of heat and pressure. Heat conditioning can be accomplished by either a nonoxidative or oxidative system. With this process, the sludge is treated at temperatures of 175 to 204°C (347 to 399°F), pressures of 1,700 to 2,800 kPa (247 to 406 psi) and for detention times of 15 to 40 minutes. The high temperatures cause hydrolysis of the encapsulated water solids matrix and lysing of the biological cells. The hydrolysis of the water matrix destroys the gelatinous components of the organic solids and thereby improves the solidliquid separation characteristics.
This process may result in a significant organic loading to the biological treatment process of the sewage treatment plant, if the supernatant is returned to the bioreactor, due to the solubilization of organic matter during sludge hydrolysis. This liquor can represent 25 to 50 percent of the total loading on the secondary treatment process and allowances should be made in the STP design to accommodate this loading increase.
Heat conditioning results in the production of extremely corrosive liquids requiring the use of corrosion resistant materials such as stainless steel. Scale formation in the heat exchangers, pipes and reactor will require acid washing equipment to be provided.
Heat conditioning, particularly the non-oxidative process, can also result in the production of odorous gases. If ultimate sludge disposal is via incineration, these gases can be incinerated in the upper portion of the furnace [760°C (1,400°F) or higher]. If incineration is not a part of the sludge handling process, a catalytic or other type of oxidation unit should be used.
The design requirements for a heat conditioning system should be determined by either batch or small-scale continuous pilot-plants. Through such methods, the necessary level of hydrolysis to produce the desired reduction in the specific resistance of the sludge and the liquor characteristics can be determined. Tests can also be made at different temperatures and detention times to determine the most effective full-scale operating conditions.
Another common form of physical conditioning is the addition of admixtures such as fly ash, incinerator ash, diatomaceous earth or waste paper. These conditioning techniques are most commonly used with filter presses or vacuum filters. The admixtures when added in sufficient quantities produce a porous lattice structure in the sludge which results in decreased compressibility and improved filtering characteristics. When considering such conditioning techniques, the beneficial and detrimental effects of the admixture on such parameters as overall sludge mass, calorific value, should be evaluated along with the effects on improved solids content.
Although once widely use as a conditioning technique, elutriation is no longer a popular process and is not be covered in these guidelines.
Freezing of sludges has been used successfully in Ontario for water treatment plant sludges, but there are no known systems intentionally using slow freezing as a conditioning method for STP sludges. Thawed sludge releases its moisture more rapidly than sludge that has not been frozen and the sludge is left in a light, fluffy condition. The process reportedly produces good results for subsequent gravity dewatering of the thawed sludge (up to 16 percent solids for waste activated sludge (WAS) and up to 25 percent solids for digested sludge). The disadvantages of the system are the high BOD5 of the effluent and the high cost of the process unless natural means of freezing can be used.
17.3 Sludge Thickening
Sludge thickening can be employed in the following locations in an STP:
- Prior to digestion for raw primary, secondary sludge or mixed primary and secondary sludges;
- Prior to dewatering facilities;
- Following digestion for sludges or supernatant; and
- Following dewatering facilities for concentration of filtrate, decant, or centrate.
The commonly used methods of sludge thickening and their suitability for the various types of sludge are shown in Table 17.1. In selecting a design figure for the thickened sludge concentration, the designer should recognize that thickening devices are adversely affected by high sludge volume index (SVI) and benefit by low SVI in the activated sludge. The ranges of thickened sludge concentrations given in the table below assume an SVI of approximately 100 mL/g. Thickening targets should also consider digestion needs. Pre-thickening sludge to greater than 4 percent TS prior to aerobic digestion can lead to autothermal digestion and issues associated with this process such as odours and foaming problems.
17.3.1 Design Considerations
Sludge thickeners to reduce the volume of sludge should be considered to reduce the required digester capacity. The design of thickeners (gravity, dissolved-air flotation, centrifuge, gravity belt thickeners, rotary drum screens and others) should consider the type and concentration of sludge, the downstream sludge stabilization processes, dewatering and storage requirements, the method of ultimate sludge disposal, chemical needs and the cost of operation.
Particular attention should be given to the pumping and piping of the concentrated sludge and possible onset of anaerobic conditions and impact of corrosion. Provision should be made for draining and flushing of discharge lines.
The designer should consider odour and aerosol/humidity control for all thickening technologies. Wherever thickening devices are being installed, special consideration should be given to the need for sludge pretreatment in the form of sludge grinding to avoid plugging pumps, lines and thickening equipment. Also, where thickeners are to be housed, adequate ventilation and odour control will be required, meeting all applicable codes.
17.3.2 Gravity Thickening
Gravity thickening is principally used for primary sludge and mixtures of primary and waste activated sludges. Due to the better performance of other thickening methods for WAS, gravity thickening has limited application for such sludges.
Gravity thickeners should be designed in accordance with the following parameters:
- Tank shape - circular;
- Tank sidewater depth - 3 to 3.7 m (9.8 to 12.1 ft);
- Tank diameter - up to 21 to 24 m (69 to 79 ft); and
- Floor slope - acceptable range of 2:12 to 3:12.
- Primary sludge - 96 to 120 kg/(m2·d) [20 to 25 lb/(ft2·d)];
- Waste activated sludge - 12 to 36 kg/(m2·d) [2.5 to 7.4 lb/(ft2·d)];
- Combination of primary and waste activated sludges based on weighted average of above loading rates; and
- Use of metal salts for phosphorus removal will increase solids loading rates by at least the stoichiometric amount.
- To prevent septic conditions, an overflow rate of 0.19 to 0.38 L/(m2·min) (0.28 to 0.56 USgpm/ft2) is recommended.
- Rake should have a tip speed of 50 to 100 mm/s (9.8 to 19.7 ft/min);
- To be equipped with hinged lift mechanisms when handling heavy sludge such as lime-treated primary sludge, otherwise optional; and
- Surface skimmer is recommended.
Sludge Underflow Piping
- Keep length of suction lines as short as possible; and
- Dual sludge withdrawal lines should be considered.
- Provision should be made for the addition of conditioning chemicals into the sludge influent lines (polymers, ferric chloride or lime are the most likely chemicals to be used to improve solids capture).
17.3.3 Dissolved Air Flotation
Unlike heavy sludges, such as primary and mixtures of primary and waste bioreactor sludges, which are generally most effectively thickened in gravity thickeners, light waste bioreactor sludges can be successfully thickened by dissolved air flotation (DAF).
The advantages of DAF compared with gravity thickeners for excess secondary treatment sludges include its reliability, production of higher sludge concentrations and better solids capture. Its disadvantages include the need for greater operating skill and higher operating costs.
Experience has shown that DAF operations cannot be designed on the basis of purely mathematical computations or by the use of generalized design parameters. Some bench- and/or pilot-scale testing will be necessary. The following, design parameters are given only as a guide to indicate the normal range of values experienced in full-scale operation:
- Air buoyancy systems - vary with suppliers;
- Air-to-solids weight ratio - 0.02 to 0.05; and
- Recycle ratios - vary with suppliers (0 to 500 percent).
Solids Loadings (With WAS to achieve 4 percent Float Solids)
- 48 kg/(m2·d) [10 lb/(ft2·d)] (without flocculating chemicals); and
- Up to 240 kg/(m2·d) [49 lb/(ft2·d)] (with flocculating chemicals).
- Feed chemical to mixing zone of sludge and recycled flow;
- Most installations now use chemical conditioning with polymers to achieve more economical operation; and
- Polymer feed range 0 to 25 g/kg (0 to 50 lb/ton) of dry solids.
- Up to 1.74 L/(m2·min) (2.56 USgpm/ft2) (based on total flow including recycle, when polymers used);
- Without chemicals, lower rate should be used; and
- Feed rate should be continuous rather than intermittent.
- Not critical providing that particle rise rate is sufficient and horizontal velocity in the unit does not produce scouring of the sludge blanket.
Thickened Sludge Withdrawal
- Surface skimmer moves thickened sludge over dewatering beach into sludge hopper;
- Either positive displacement or centrifugal pumps that will not air bind should be used to transfer sludge from hopper to the next phase of process; and
- In selecting pumps, maximum possible sludge concentrations should be taken into consideration.
- A bottom collector to move settled sludge into a hopper should be provided; and
- Sludge removal from the hopper may be by gravity or pumping.
Centrifuges are commonly used for sludge dewatering and are increasingly being considered for sludge thickening. As thickening devices, their use has been generally restricted to waste activated sludges. Three types of centrifuges have been used with such sludges - the solid-bowl decanter, disc-nozzle and basket types.
The following general design considerations are provided:
- Centrifugal thickening operations can have substantial maintenance and operating costs;
- Where space limitations or sludge characteristics make other methods unsuitable or where high capacity mobile units are needed, centrifuges have been used;
- Thickening capacity, thickened sludge concentration and solids capture of a centrifuge is greatly dependent on the SVI of the sludge;
- 85 to 95 percent solids recovery will generally be the most suitable operating range;
- Polymer feed range 0 to 6.0 g/kg (0 to 128.0 lb/ton) of dry solids;
- Early experience with disc nozzle-type centrifuges found clogging of the sludge discharge nozzles to require frequent maintenance; recent use of rotary screens and cyclones for pretreatment have helped alleviate these problems; and
- Basket type centrifuges have seen limited use; due to their low capacities and batch operations, their use has been generally restricted to small plants.
17.3.5 Gravity Belt Thickener
The gravity belt thickener (GBT) uses a slow moving fabric belt to separate sludge solids and free water. Polymer is required to precondition the sludge and is prepared and aged in a small tank upstream of the thickening process. Sludge thickening on the device is aided by multiple rows of plows and drainage elements which slow the flow of sludge and provide additional retention time over the horizontal gravity belt. GBTs generally require a smaller footprint than other sludge thickening processes, are cost-effective and use less energy than other mechanical thickening devices (i.e., DAF and centrifuge). However, GBTs require preconditioning with chemicals and are sensitive to the quality of the sludge being thickened.
The following are general design considerations:
- Performance of the GBTs is subject to upstream conditions in the STP. The better the settling of solids in the plant, the better the GBT will function and potentially at lower chemical dosages;
- Adequate attention should be given to transporting the thickened solids, in particular for handling the maximum solids content expected;
- Prior to digestion, adequate mixing or blending of thickened solids with other solids is required;
- Plows on the gravity belt turn and distribute the thickened solids to allow for water to drain through the belt fabric. The number and location should be adjustable for each type of sludge being thickened;
- Chemical addition and mixing equipment are important, as are multiple injection points;
- GBTs should have an air handling system to maintain a safe working environment; this could include a complete enclosure with exhaust, odour control, inspection door, and access for cleaning;
- GBTs should have a curb around them and floors sloped to drains so that operators can properly clean the equipment;
- Metering of solids into and out of the equipment is important;
- Thickened solids need to be designed to move all expected material and avoid accumulation and overload;
- Due to height of equipment, an elevated walkway will probably be needed to operate and maintain the equipment; and
- Scum (grease) should not be placed on the GBT because blinding of the fabric can create problems.
17.3.6 Rotary Drum Thickener
Rotary drum screen thickeners are internally fed with sludge from a headbox or flocculation tank after conditioning with polymer. The suspension is distributed onto the internal surface of the rotating drum and physically strained for the separation of free water. The cylinder can be fitted with interchangeable screening panels and is slowly rotated (e.g. 2-10 rpm) with a variable speed drive electric gear motor. Separated solids are retained on the surface of the screen and are conveyed to the discharge end of the unit where they drop out through a chute. The rotary drum thickener (RDT) process has a built-in spray backwashing system, controlled with programmable timers that can be optimized for each application. The rotational speed of the drum can be adjusted and optimized based on site-specific operating requirements to achieve the desired levels of thickened sludge concentration, solids capture efficiency and polymer consumption. The system can be supplied as an enclosed unit with a vent stack for containment and minimization of odour and vapour releases.
As with GBTs, chemical addition is required. Similar design considerations to the GBT should be considered, although as this process is enclosed, odour and environmental issues are reduced.
|Thickening Method||Sludge Type||Expected Performance|
|Centrifugation||Waste Activated with Polymer1||8-10% TS and 80-90% Solids|
Capture with Basket Centrifuges;
4-6% TS and 80-90% Solids
Capture with Disc-nozzle Centrifuges;
5-8% TS and 70-90% Solids
Capture with Solid Bowl Centrifuges.
|Gravity Belt Thickener (GBT)||Waste Activated with Polymer||4-8% TS and ³ 95% Solids Capture|
|Rotary Drum Thickener (RDT)||Waste Activated with Polymer||4-8% TS and ³ 95% Solids Capture|
|Gravity||Raw Primary||8-10% TS|
|Gravity||Raw Primary and Waste Activated||5-8% TS|
|Gravity||Waste Activated||2-3% TS (Better results reported for oxygen rich activated sludge)|
|Gravity||Digested Primary Digested Primary and Waste Activated||8-14% TS|
|Dissolved Air Flotation (DAF)||Waste Activated (Not Generally Used for Other Sludge Types)||4-6% TS and ≥ 95% Solids Capture With Flotation Aids|
Solids concentrations for centrifuges without polymer will be reduced.
17.4 Sludge Dewatering
Sludge dewatering will often be required at STP prior to ultimate disposal of sludge/biosolids or as a prelude to further treatment or stabilization. Since dewatering processes differ significantly in their ability to reduce the water content of sludges, the ultimate sludge disposal method will generally have a major influence on the dewatering method most suitable for a particular STP. Also of influence will be the characteristics of the sludge requiring dewatering; that is, whether the sludge is raw or digested, whether the sludge contains WAS or whether the sludge has been previously thickened. With raw sludge, the freshness of the sludge will have a significant effect on dewatering performance (septic sludge will be more difficult to dewater than fresh raw sludge).
A rational basis of design for sludge production values should be developed. In lieu of actual sludge production data, an overall plant 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 on the main liquid train treatment processes.
Table 17-2 gives the solids capture, solids concentrations normally achieved, energy requirements and suitable ultimate disposal options for various dewatering methods. The solids concentrations shown in the table assume that the sludges have been properly conditioned. Designers should be aware that phosphorus removal chemicals (i.e., alum or ferric chloride) will reduce allowable solids loading rates for dewatering equipment and produce a lower cake solids concentration than would be expected without phosphorus removal. This is expected due to the additional sludge loading due to the chemical sludge and its potential lower initial concentration.
|Dewatering Method||Solids Capture (%)||Solids Concentrations Typically Acheived1||Median Energy Required (MJ/dry t)2 [Btu/lb]|
|Belt Filter Press||85-95||Raw or Digested Primary + WAS (14-25%)|
|Raw or Digested Primary + WAS (14-25%)|
|Centrifuge (Solid Bowl)||95-99||Raw or Digested Primary + WAS (15-30%)|
WAS (12-15%, with polymer)
|Filter Press||90-95||Raw Primary + WAS (30-50%) Digested Primary + WAS (35-50%) WAS (25-50%)||360 |
|Vacuum Filter||90-95||Raw Primary + WAS (10-25%) Digested Primary + WAS (15-20%) WAS (8-12%)||1080 |
Including conditioning chemicals (i.e., polymer).
MJ/dry t tonne denotes megajoules per dry tonne of sludge throughput.
The required solids concentration for sludges which are to be landfilled at municipal sanitary landfill sites are normally specified by the landfill authority. With small quantities of sludge for co-disposal land filling with garbage, liquid sludge at solids concentrations as low as 3 percent TS may be acceptable. For sludge-only landfill operations, a minimum of 15 percent solids concentration is generally required to support cover material. If sludge is to be disposed of in sludge lagoons, dewatering may not be necessary unless it is justifiable for economic reasons relating to haulage costs.
For ultimate disposal by incineration, sludges should ideally be concentrated to a solids concentration where they will burn autogenously or will be self-generating. This solids concentration will vary somewhat with sludge type, volatile solids percentage and the chemical composition of the solids, but a minimum concentration in the order of 30 percent total solids will generally be required. With conditioning by heat treatment, sludge dewatering methods such as filter presses, belt filter presses, centrifuges and perhaps even vacuum filters will be capable of producing autogenous sludge solids concentrations.
As with thickening systems, dewatering facilities may require sludge pretreatment in the form of sludge grinding to avoid plugging pumps and lines and plugging or damaging dewatering equipment. Also, adequate ventilation equipment and odour control will be required in buildings housing dewatering equipment, meeting all applicable codes.
In evaluating dewatering system alternatives, the designer should consider the capital and operating costs, including labour, parts, chemicals and energy, for each alternative as well as for the effects that each alternative will have on the sewage treatment and subsequent sludge handling and ultimate disposal operations. Since labour and especially energy costs are escalating at a rapid rate, it is suggested that these annual costs be converted to capital cost equivalents for evaluation purposes (i.e., life-cycle costing approach should be used).
17.4.1 Mechanical Dewatering Facilities
Provision should be made to maintain sufficient continuity of service so that sludge or biosolids may be dewatered without accumulation beyond storage capacity. The number of vacuum filters, centrifuges, filter presses, belt filters, other mechanical dewatering facilities or combinations thereof should be sufficient to dewater the sludge produced with the largest unit out of service (i.e., firm capacity). Unless other standby wet sludge facilities are available, adequate storage facilities of at least 4-days production volume prior to dewatering in addition to any other sludge storage needs should be provided.
Back-up vacuum and filtrate pumps should be provided. Overall, the design of dewatering facilities should facilitate the removal and replacement of all equipment.
Adequate facilities should be provided for ventilation of the dewatering area. The exhaust air should be properly conditioned to avoid odour nuisance.
Lime-mixing facilities should be completely enclosed to prevent the escape of lime dust. Chemical handling equipment should be automated to eliminate manual lifting requirements.
The centrifuge types which have been used for sewage sludge dewatering include the solid bowl, basket and disc centrifuges. The most frequently used is the continuous countercurrent solid bowl centrifuge. Due to their infrequent use for dewatering and their inherent plugging problems, disc centrifuges, although capable of reaching the lower end of dewatering solids concentrations, are generally only used for thickening operations and will not be discussed further in this dewatering section.
17.4.2. Solid Bowl Centrifuges
The designer should consult the manufacturer to obtain the equipment design parameters.
The machine variables of importance for dewatering centrifuges include bowl length/diameter ratio, bowl angle, bowl flow pattern, bowl speed, pool volume, internal conveyor design and relative conveyor speed.
Bowl length/diameter ratios of 2.5 to 4.0 are usually provided to ensure adequate settling time and surface area. Bowl angles should be kept shallow.
The bowl flow pattern can be either countercurrent or co-current. By using co-current flow, the settled sludge is not disturbed by the incoming feed and turbulence is reduced. The disadvantages of co-current flow are the need for a long feed tube and the long travel distance needed to remove the sludge. Other proprietary feed inlets have also been developed to minimize the disturbance to the previously settled solids.
Increased bowl speed increases the centrifugal forces available for clarification, but the settled solids become more difficult to remove due to the higher gravitational (G) forces. Increased bowl speed, however, will also increase abrasion damage within the centrifuge, noise and vibration. Lower speed machines have been developed to achieve high solids capture. Sludge inlet conditions with these low-speed machines have also been improved to minimize acceleration and turbulence. These low-speed machines have lower noise levels, minimized internal wear and have lower power requirements, but may have increased conditioning chemical requirements.
Detention time in the centrifuge will increase with increases in pool volume. With longer detention times achieved by greater pool depth, solids capture increases, but cake solids concentrations will decrease due to reduced detention time on the drying deck and also due to the capture of finer solids with higher moisture content. Pool depth can be varied by adjustable weirs.
Conveyor design and speed will affect the efficiency of solids removal. Differential speed should be kept low enough to minimize turbulence and internal wear yet high enough to provide sufficient solids handling capacity. The most suitable internal conveyor pitch will be affected by the characteristics of the sludge to be handled. With high solids concentration, conveyors with high pitch angles can be used, but with lower solids, low pitch angles should be used. Conveyor differential speed can often be optimized following installation.
Important process variables affecting the centrifuge efficiency are feed rate, feed consistency, temperature and the chemical coagulants used.
As hydraulic flow rate increases through a centrifuge, solids capture decreases and cake solids concentrations increase due to the loss of fines in the centrate. If the feed solids are too high, solids buildup within the bowl can take place, reducing clarification volume. Therefore, both solids and hydraulic overloading can occur.
For most sewage sludges, the capacity of the centrifuge will be limited by the clarification capacity (hydraulic capacity) and therefore the solids concentration. Increasing the feed solids will increase the solids handling capacity. Thickening should, therefore, be considered as a pretreatment operation.
Since temperature affects the viscosity of sludges, if temperatures vary appreciably (as with aerobic digestion), the required centrifuge capacity should be determined for the lowest temperature expected.
The chemical conditioning agents most commonly used with centrifuges are polymers. Flocculating agents are generally injected directly into the interior of the centrifuge to avoid shearing the floc. Maximum effectiveness is generally achieved by diluting the flocculant to concentrations of 0.1 percent or less.
Other general design guidelines for solid bowl centrifuges are as follows:
- Feed pump - sludge feed should be continuous; pumps should be variable flow type; one pump should be provided per centrifuge for multiple centrifuge systems; chemical dosage should vary with pumpage rate;
- Return line – should be included for wet cake during start-up;
- Sludge pretreatment - depending upon the sewage treatment process, grit removal, screening or maceration may be required for the feed sludge stream;
- Solids capture - 85 to 95 percent generally desirable;
- Machine materials - generally carbon steel or stainless steel; parts subject to wear should be protected with hard facing materials such as a tungsten carbide material;
- Machine foundations - foundations should be capable of absorbing the vibratory loads; and
- Provision for maintenance - sufficient space should be provided around the machine(s) to permit disassembly; an overhead hoist should be provided; hot and cold water supplies will be needed to permit flushing out of the machine; drainage facilities will be necessary to handle wash water.
17.4.3 Belt Filter Presses
Although variations in the process exist, a belt filter press (BFP) basically consists of two continuous, separate belts: a press belt and a filter belt. Sludge is confined between the two belts with the press belt exerting pressure on the filter belt, thereby continuously dewatering the sludge.
There are generally three distinct dewatering zones through the process. The first zone is a gravity drainage zone, the second is a pressure zone and the third is a shear zone. Pressure is exerted by the rollers, conveying belts or other external devices. In the shear zone, the sludge cake is further dewatered by deforming the sludge cake by passing the belts around rolls and/or between vertically offset rollers causing a serpentine configuration in the sludge cake movement.
Most types of sewage sludges can be dewatered with BFPs and the results achieved are generally superior to those with vacuum filters. Belt filter presses generally use only one-third the power requirements of vacuum filters and do not experience sludge pickup problems often encountered with vacuum filters. BFPs have reportedly been used to further dewater the sludge cake from vacuum filters with excellent results. Such a method should be considered for upgrading existing dewatering systems. Chemical conditioning is generally with polymers.
Solids handling capabilities are likely to range from 50 g/(m2.s) [0.61 lb/(ft2·min)] of dry solids (based on belt width) for waste activated sludge to 330 g/(m2.s) [4.1 lb/(ft2·min)] for primary sludge. Expected solids concentration results are shown in Table 17-2. Solids capture is usually in excess of 85 percent and often as high as 95 percent.
An enclosure should be considered for each BFP to contain odours and splashing, including air exhaust with odour control.
17.4.4 Vacuum Filters
Rotary vacuum filters were commonly used mechanical systems for sludge dewatering in Ontario. With the recent developments of new, lower cost and more effective dewatering systems the use of vacuum filters is beginning to decline.
Rotary drum, rotary belt and spring coil variations of the rotary vacuum filter are available for use. The primary machine variables which affect dewatering are vacuum pressure, drum submergence, drum speed, degree of sludge agitation and filter medium. The operation variables which affect dewatering performance are sludge type, sludge conditioning and sludge characteristics including initial solids concentration, nature of sludge solids, chemical composition, sludge compressibility, sludge age, temperature and filtrate viscosity.
Of primary importance with vacuum filters is the solids concentration of sludge feed. With all other operating variables remaining constant, increases in filtration rates vary in direct proportion to feed solids. Sludge thickening prior to vacuum filters is therefore extremely important. Higher concentrations in the sludge feed also result in lower filtrate solids.
Vacuum filtration systems should be designed in accordance with the following parameters:
- Sludge feed pumps - variable capacity;
- Vacuum pumps - generally one per machine with a capacity of 10 L/(m2·min) (15 USgpm/ft2) at 65 kPa (9.4 psi) vacuum or more;
- Vacuum receiver - generally one per machine; maximum air velocity 0.8 to 1.5 m/s (2.6 to 4.9 ft/s); air retention time 2 to 3 minutes; filtrate retention time 4 to 5 minutes; all lines to slope downward to receiver from vacuum filter;
- Filtrate pumps - generally self-priming centrifugal; suction capacity greater than vacuum pump [65 to 85 kPa vacuum (9.4 to 12.3 psi)]; with flooded pump suctions; with check valve on discharge side to minimize air leakage into the system; pumps should be sized for the maximum expected sludge drainage rates (usually produced by polymers);
- Sludge flocculation tank - constructed of corrosion-resistant materials; with slow speed variable drive mixer, detention time 2 to 4 minutes with ferric and lime; with polymers, shorter time may be used;
- Wash water - filtered final effluent generally used;
- Sludge measurement - should be provided unless measured elsewhere in plant; and
- Solids loading rate - 7-14 g/(m2.s) [0.086 - 0.17 lb/(ft2·min)] for raw primary; 2.8-7 g/(m2.s) [0.034 - 0.86 lb/(ft2·min)] for raw primary + WAS; 4-7 g/(m2.s) [0.049 - 0.86 lb/(ft2·min)] for digested primary + WAS; not considered practical for use with WAS alone.
17.4.5 Filter Presses
Recent changes in the design of filter presses, including elimination of leakage problems, more automation, improved filter media, greater unit capacities and development of high molecular weight polymers and compatible polymer feed systems, have resulted in renewed interest in this method of sludge dewatering.
Variations in the filter press process which are now available on the market include units with recessed plates or plates with frames, top or central sludge feed, air pressure assisted sludge cake release, automatic washing of filter media, sequential or simultaneous release of sludge cake and final compression stage using flexible diaphragm behind filter media.
The main advantage offered by filter presses is the ability to concentrate all types of sewage sludges to very high concentrations. Concentrations as high as 45 to 50 percent TS, can generally be achieved with properly conditioned sludges. Filter presses are also able to effect high efficiencies in solids capture and as a result produce relatively clear filtrate. Their primary disadvantages are that the process is batch rather than continuous and cake removal still requires some manual assistance and large quantities of conditioning agents are generally necessary.
As with vacuum filters, the capacity of filter presses are greatly affected by the initial solids concentration. With lower feed solids, chemical requirements increase significantly.
Sludge thickening should therefore be considered as a pretreatment step. The sludge is generally conditioned with a physical conditioning agent such as fly ash or with chemicals such as ferric chloride, lime and alum, although use of polymer conditioning agents is becoming more common with the development of compatible polymer feed systems. In some instances, precoats are applied to the filter media prior to the addition of sludges to prevent premature media blinding. Various materials can be used for precoat including diatomaceous earth, fly ash, incinerator ash and various types of industrial waste by-products.
Filter press systems should be designed in accordance with the following guidelines:
- Sludge conditioning tank - detention time maximum of 20 minutes at peak pumpage rate;
- Feed pumps - variable capacity to allow pressures to be increased gradually, without underfeeding or overfeeding sludge; pumps should be of a type to minimize floc shear; pumps should deliver high volume at low head initially and low volume at high head during latter part of cycle; ram or piston pumps, progressing cavity pumps or double diaphragm pumps are generally used;
- Cake handling - filter press should be elevated above cake conveyance system to allow free fall; cake can be discharged directly to trucks, into dumper boxes or onto conveyors (usually belt or drag chain type); conveyors should be able to withstand impact of sludge cakes; cable cake breakers may be needed;
- Cycle times - 1.5 to 6.0 h; and
- Operating Pressures - usually 700 to 1400 kPa (102 to 203 psi), but may be as high as 1750 kPa (254 psi).
Operating pressures depend on the types of presses and the chemical agents used for sludge conditioning. These pressures may be developed either hydraulically or by a combination of hydraulic and pneumatic means. For example, recessed plate filter presses with diaphragm membranes for dewatering polymer-conditioned sludges are first brought to approximately 700 kPa (102 psi) pressure hydraulically (pumping) and then the membranes are inflated pneumatically to provide a final squeezing pressure of approximately 1050 kPa (152 psi).
While the magnitude of pressure applied does not adversely affect the dewatering process, if lime and ferric chloride are used as sludge conditioning, it is very important that the generating pressure should not exceed 1050 kPa (152 psi) if polymer is applied as the conditioning agent, due to the interaction of the conditioners.
17.4.6 Filtrate and Drainage/Centrate Management
Filtrate or drainage from sludge drying beds and centrate from other dewatering units should be returned to the liquid train of the STP at appropriate points and rates. Appropriate monitoring and sampling of these streams should be provided.
17.4.7 Other Dewatering Processes
If it is proposed to dewater sludge by other alternative or innovative methods, a detailed description of the process and design data (including field or pilot data) should accompany the design report.
17.5 Sludge Drying Beds
Sludge drying beds may be used for dewatering stabilized sludge from either the anaerobic or aerobic process. Drying beds are confined, underdrained and shallow layers of sand over gravel on which wet sludge is distributed for draining and air drying. Drying beds have proved satisfactory at most small- and medium-sized sewage treatment plants located in warm, dry climates.
Sludge drying bed area should be calculated based on:
- The volume of wet sludge produced by existing and proposed processes; and
- The time required on the bed to produce a removable cake. Adequate provision should be made for sludge dewatering and/or sludge disposal facilities for those periods of time during which outside drying of sludge on beds is hindered by weather.
Owing to simple operation, capability of producing high solids concentrations (greater than 40 percent TS) and low capital cost, conventional sand drying beds should be considered as a sludge dewatering alternative, especially for small to medium-sized STP. Due to the presence of sludge drying beds at a large number of existing Ontario STP, their use as an emergency sludge dewatering technique to backup mechanical dewatering processes should also be considered.
Since sludge conditioning can reduce the required drying time to one-third or less of the unconditioned drying time, provision should be made for the addition of conditioning chemicals, usually polymers.
The usual design parameters for conventional sand drying beds are as follows:
- Drainage tile 100 mm (4 in) diameter or more, spaced 2.4 to 3.0 m (8 to 10 ft) apart, with slope of one percent or more;
- Bottom of cell should be of impervious material such as clay or asphalt;
- Drainage tile bedded in gravel layer usually 200 mm to 500 mm (8 to 20 in) thick, graded from 25 mm (1.0 in) on the bottom to 3 mm (0.12 in) on the top;
- Sand layer above gravel usually 250 to 450 mm (10 to 18 in) thick with an effective size of 0.3 to 1.2 mm (0.012 to 0.047 in) and a uniformity coefficient of less than 5.0;
- Bed size 4.5 to 7.5 m (15 to 25 ft) wide with length selected to satisfy desired bed loading volume;
- Dosing depth 200 to 300 mm (8 to 12 in) for warm weather operating modes; for winter freeze drying cumulative depths of 1 to 3 m (3.3 to 10.0 ft) can be used depending upon the number of degree days in winter;
- One inlet pipe per cell, with inlet 300 mm (11.8 in) above bed surface and with splash pad to prevent bed disruption and to promote even distribution of sludge; provision for flushing inlet lines should be provided;
- Usually a minimum of 3 beds are desirable for flexibility of operation;
- Sludge removal can either be manual or mechanical; if mechanical, concrete vehicle tracks are generally required with clay tiles, but may not be necessary with perforated plastic pipe and/or flotation tire equipped front end loader;
- Outer walls and partition walls should be watertight; walls should extend 460 mm (18 in) above and at least 230 mm (9 in) below the surface of the bed. Outer walls should be watertight down to the bottom of the bed and extend at least 100 mm (4 in) above the outside grade elevation to prevent soil from washing into the beds;
- Each bed should be constructed so as to be readily and completely accessible to mechanical equipment for cleaning and sand replacement. Concrete runways spaced to accommodate mechanical equipment should be provided. Special attention should be given to assure adequate access to the areas adjacent to the sidewalls. Entrance ramps down to the level of the sand bed should be provided. These ramps should be high enough to eliminate the need for an entrance end wall for the sludge bed;
- Underdrains should discharge back to the secondary treatment section of the STP; and
- Recommended sizing for uncovered beds between latitudes 40 to 45°N is 0.16 m2/cap (1.72 ft2/cap) and for north of 45°N, 0.20 m2/cap (2.15 ft2/cap) (for primary plus waste activated sludge following anaerobic digestion); the recommended sizing for covered beds with the same sludge type is 0.13 and 0.16 m2/cap (1.40 and 1.72 ft2/cap), respectively. Equivalent mass loading rates can be calculated using Table 16-1; and
- Consideration should be given for providing a means of decanting the supernatant of sludge placed on the sludge drying beds. More effective decanting of supernatant may be accomplished with polymer treatment of sludge.
Other types of sludge drying beds that have been used include the following:
- Paved rectangular beds with a centre sand drainage strip, with or without heating and with or without covering;
- “Wedge-wire” drying beds with a wedge-wire system; provision for an initial flood with a water layer, followed by sludge introduction on top of water layer, controlled cake formation and provision for controlled underdrainage and mechanical sludge removal; and
- Rectangular vacuum assisted sand beds.
17.6 Sludge Thickening Lagoons
Thickening lagoons have generally been built at or near the site of the STP so that the sludge can be conveyed to the lagoons by pumping or gravity and so that the supernatant can be returned to the STP for further treatment.
In some circumstances where suitable land surrounds the lagoon, a combination thickening and transfer lagoon can be built where supernatant can be spray irrigated onto the surrounding land and the thickened sludge can then be hauled away for spreading on farmland. Withdrawal of supernatant will result in increases in sludge concentration to the extent that sludge removal by pumping may become difficult or impossible; above a solids concentration of 7 to 8 percent TS, pumping can become difficult.
The design and location of sludge thickening lagoons should take into consideration many factors, including the following:
- Possible nuisances: odours, appearance, mosquitoes (Chapter 4 - Odour Control);
- Design: number, size, shape and depth;
- Loading factors: solids concentration of digested sludge, loading rates;
- Soil conditions: permeability of soil, need for liner and stability of berm slopes;
- Groundwater conditions: elevation of maximum groundwater level, direction of groundwater movement, location of wells in the area;
- Sludge and supernatant removal: volumes, concentrations, methods of removal, method of supernatant treatment and final sludge disposal; and
- Climatic effects: evaporation, rainfall, freezing, snowfall, temperature, solar radiation.