Chapter 11: Residuals management

This chapter provides guidance for the handling, treatment and environmentally acceptable disposal of water treatment process waste residuals. A discussion of all types of residuals and treatment technologies is beyond the scope of this guideline, however, a brief overview of the types of plant waste residuals and treatment options commonly used in Ontario is provided.

11.1 General

Provisions should be made for proper treatment and disposal of water treatment plant process residuals such as clarification/sedimentation sludge, softening sludge, iron/manganese removal sludge, filter backwash water, filter-to-waste, brine, wastes from fluoride or arsenic removal processes, slow sand or diatomaceous earth filtration processes, membrane reject water, spent carbon and ion exchanger regenerants, pump seal water waste, laboratory wastes and on-line instrument wastes.

Most residuals produced by a water treatment plant will require treatment. The degree of treatment necessary will be dictated by the disposal method or, in the case of a discharge to the environment, the assimilative capacity of the receiving water body (surface or ground water). The effluent quality criteria for discharge to the environment should be established through consultation with the appropriate ministry Regional Office. For wastewater flow streams that are regulated by discharge quality limits [as specified in requirements such as a Certificate of Approval (C of A) Drinking Water Works Permit (DWWP) / Municipal Drinking Water Licence (Licence) or Sewer Use By-Law] it is recommended that appropriate process monitoring devices be provided to allow operators to react to wastewater treatment process upsets. Designers should review any relevant wastewater effluent criteria pertaining to the facility and design wastewater treatment processes to meet such requirements.

The impact of and issues related to residuals management should be considered at the time of water treatment process evaluation and selection. Due to likely heavy wear, provisions should be made to ensure reliability and redundancy of residuals handling equipment.

For information on residuals management, the designer may refer to the AwwaRF report Water Treatment Plant Residuals Engineering (Project #2934), 2006.

11.2 Specific water treatment process residuals

11.2.1 Sedimentation/Clarification

Treatments for sludges from the sedimentation/clarification process should be designed to reduce the volume for disposal. For large plants, mechanical dewatering methods such as centrifuges, rotary drum thickeners or filter presses may be appropriate. For medium and small plants with sufficient land area available, the natural freeze-thaw method may be considered.

Sludge may be directed to a sanitary sewer where the sewer and the sewage treatment plant have sufficient hydraulic and treatment capacity and with the agreement of the municipality/owner.

11.2.2 Chemically-Assisted Granular Media Filtration

This process produces high volume, short duration wastewater flows (backwash) that require handling in a suitably sized surge/equalization tank (Section 11.4.1 Flow Equalization). Surge tank discharges may be directed to a sanitary sewer where the sewer and the sewage treatment plant have sufficient hydraulic and treatment capacity and with the agreement of the municipality/owner.

Where limited sewage treatment capacity exists, the waste flow may be directed to a holding tank and allowed to settle before the supernatant is discharged to a sanitary sewer and the sludge removed for further treatment. Where sewer discharge is not possible and appropriate effluent quality control measures are provided, direct discharge to a receiving water body may be acceptable.

When these options are not available or are limited, the surge tank discharges should be directed for further treatment.

11.2.2.1 Recycling Treated Backwash Water

Recycling of effluent (supernatant) from backwash treatment facilities involves consideration of special hazards due to the potential for increased concentration of pathogens in the water. This may be especially significant for pathogen impacted raw water sources. Where this process is incorporated into the design of the treatment facility, the designer should ensure that the recycle stream after treatment is directed to the head of the water treatment plant. Backwash water treatment should be designed to provide highly efficient particulate removal by incorporating such measures as low clarifier overflow rates [3.0 m/h (1.2 USgpm/ft2)] with polymer addition, or membrane filtration so that the increased concentration of viable cysts and contaminants as a result of the recycling process is minimized. The use of a disinfecting agent effective for the inactivation of cysts, typically ultraviolet light, is recommended in the recycling line. In addition, filter backwash water should not be recycled when the raw water contains excessive algae or when disinfection by-product levels in the distribution system may exceed allowable levels. Monitoring of the recycle stream quality may also be necessary.

11.2.3 Membrane Filtration

Chemically unaltered membrane reject water may be discharged without treatment provided it meets the effluent quality criteria for discharge to the environment established through consultation with the appropriate ministry Regional Office. Where this option is not available, the concentrate or reject should be directed to a treatment facility as described in Section 11.2.1 Sedimentation/Clarification.

Disposal options for membrane backwash residuals are similar to those for conventional water treatment plants, and typically include the following:

  • Discharge to the sanitary sewer;
  • Treatment with supernatant recycle and solids disposal; and
  • Discharge to a suitable surface water body.

All membrane plants require periodic chemical cleaning. Where possible, chemical cleaning residuals should be treated on-site and discharged to either a sanitary sewer or holding tank for further disposal. Oxidants such as chlorine used in the chemical cleaning process should be quenched prior to discharge, and acids and bases should be neutralized. The use of other chemicals, such as surfactants or proprietary cleaning agents, may require additional treatment.

The rinse water applied to the membranes after the cleaning process may also represent a chemical waste and thus may require treatment prior to discharge. Although the rinse water increases the volume of the chemical cleaning residuals, this increase can be balanced somewhat by the recovery and reuse of a significant portion of the cleaning solutions.

11.2.4 Iron & Manganese Removal

Backwash waste and sludge from iron and manganese removal systems treating groundwater can be handled by discharge to a sanitary sewer, or to a

holding tank for decanting, recycling of supernatant to the head of the plant, and trucking sludge for off-site disposal. Discharge to a sanitary sewer should conform to the requirements of Section 11.3.1 Disposal to Sanitary Sewer. The designer should be aware that biological activity in the aquifer close to the well over time may cause a change in the incoming water characteristics and force the abandonment of the recycling option.

11.2.5 Ion Exchange Processes

Waste from ion exchange plants, ion exchange softening, demineralization plants or other plants which produce a brine may be disposed of through the sanitary sewer, where permitted, and where the impact on sewage treatment processes is negligible. A surge/equalization tank with discharge control may be needed.

Where discharging to a sanitary sewer is not available, a holding tank should be provided to facilitate off-site disposal.

11.2.6 Precipitative Softening

Sludge from precipitative softening processes varies in quantity and in chemical characteristics depending on the softening process and the chemical characteristics of the water being softened. The designer should consider that the quantity of sludge produced may be much larger than indicated by stoichiometric calculations. Methods of treatment and disposal include lagoons, land application, mechanical dewatering and landfilling. Lime sludge drying beds are not recommended.

Discharge of lime sludge to sanitary sewers should be avoided and may only be used in situations where the sewage system capacity is adequate to accommodate the lime sludge.

11.3 Disposal options

11.3.1 Disposal to Sanitary Sewer

Discharges may be directed to a sanitary sewer where the sewer and the sewage treatment plant have adequate hydraulic and treatment capacity and subject to the agreement of the municipality/owner. Disposal of plant process wastewater and sludges to municipal sewers may be limited by municipal by-laws, and consultation with the sewage system operating authority is needed.

The capacity of the sanitary sewer system and sewage treatment plant may be such that surge/holding tanks are needed.

11.3.2 Land Application

The Nutrient Management Act, 2002 (NMA) addresses materials containing nutrients applied to agricultural land. It includes provisions for the development of strong standards for all land-applied materials containing nutrients and a registry system for all land applications. Land applied materials include sewage biosolids and other non-agricultural source materials. The General (O.Reg. 267/03) regulation under the NMA sets out the application rates and other conditions relating to the spreading of biosolids e.g., separation distances, groundwater and surface water protective measures, and biosolids handling and spreading practices.

Sludges from municipal drinking-water systems may be utilized as an organic soil conditioner. When considering this option, the designer should refer to Ministry publications Guidelines for Utilization of Biosolids and Other Wastes on Agricultural Lands (PIBS 3425e), developed in conjunction with the Ontario Ministry of Agriculture, Food and Rural Affairs, and Guide to Applying for a Certificate of Approval to Spread Sewage and Other Biosolids on Agricultural Lands (Organic Soil Conditioning) (PIBS 3681e). The local Ministry District Office is responsible for issuing a C of A under Part V, Section 27 of the Environmental Protection Act (EPA). Applications may require review by the Biosolids Utilization Committee.

11.3.3 Disposal in a Landfill

Drinking-water system residuals disposed of in a sanitary landfill must first have solids concentrated to a semisolid or cake form. Transportation of screenings, dewatered sludge or other final residue solids to a municipal landfill site for ultimate disposal requires Director approval under Part V, Section 27 of the EPA and Director approval of the Ministry Environmental Assessment and Approvals Branch (EAAB). All sites (whether or not requiring public hearings or subject to the EPA) are processed by the EAAB Waste Unit.

11.4 Treatment options

The choice of residuals treatment process will depend on the raw water quality, the treatment plant processes as well as the discharge and ultimate disposal requirements. In cases where satisfactory operating data to confirm the suitability of a particular treatment process does not exist for a given residuals stream, pilot testing may be needed.

11.4.1 Flow Equalization

The purpose of a flow equalization or surge tank is to contain large volumes of wastewater which accumulate in a short period of time and to allow it to be introduced to a subsequent treatment process at a constant rate.

The surge tank should be designed to receive backwash water, filter-to-waste flows and sedimentation blowdown. The tank should be sized to accept the anticipated backwash plus filter-to-waste volume from a minimum of two filters and should be increased according to the number of filters. The surge tank should be equipped with an air or mechanical mixing system to keep solids in suspension.

The surge tank should also be equipped with transfer pumps continuously discharging at constant rate to a clarifier. The pumps should be sized to meet the designed surface overflow rate for the applicable type of clarifier (Section 5.5 Clarification and Chapters 8 and/or 10 of the Design Guidelines for Sewage Works).

The surge tank may also allow the wastewater to be directed to the sanitary sewer at a uniform rate, if this option is available.

11.4.2 Sedimentation/ Clarification

Increasing the concentration of solids in the waste stream may be accomplished by gravity settling, with or without plate or tube settlers, dissolved air flotation, ballasted clarification or other sedimentation/clarification processes. Refer to Section 5.5 Clarification for more information regarding the design of these treatment processes.

The supernatant from the sedimentation tank may be returned to the source, upon consultation with the appropriate ministry Regional Office, or be directed to the sanitary sewer (Section 11.3.1 Disposal to Sanitary Sewer). The settled sludge may be transferred to a thickener, a holding tank for off-site disposal, or directed to a sanitary sewer.

11.4.3 Thickening

Sludge thickening is performed primarily for reduction in the volume of sludge which will require subsequent treatment. This step may be combined in a single tank with the sedimentation/clarification process. Thickening tanks may also serve as equalization facilities to provide a uniform feed to the dewatering step. A liquid polymer(s) feed system may be provided to chemically condition the sludge and assist sludge thickening.

Historically, gravity thickening has been the process most often used in the water industry. Gravity thickening is typically accomplished in a circular tank designed and operated similarly to a solids-contact clarifier or sedimentation tank (Section 5.5 Clarification). Solids loading rates are typically between 20 and 80 kg/(m2·d) [4 to 16 lb/(ft2·d)] for coagulant sludges. Where mechanical scraper units are used for sludge removal, the velocity of the scraper should not exceed 18.0 m/h (60 ft/h) to prevent resuspension of the settled sludge.

The thickened sludge may be transferred for further treatment or disposal.

Sludge thickening ability can be highly variable and pilot testing may be needed for establishing design criteria for satisfactory performance. Seasonal changes in the effectiveness of these processes should also be considered.

11.4.4 Dewatering

Methods of dewatering include the following:

  • Air/ gravity drying processes:
    • Sand drying beds;
    • Freeze-thaw beds;
    • Solar drying beds; and
    • Vacuum assisted drying beds.
  • Mechanical dewatering processes:
    • Belt filter presses;
    • Centrifuges; and
    • Pressure filters.

A complete discussion on each of these dewatering processes is beyond the scope of these guidelines. Some general recommendations, however, are provided in the following sections.

11.4.4.1 Air/ Gravity Drying Beds

Decanting and drainage systems should be provided. Climate, drainage discharge location, required solids concentration and slump characteristics requirements of the final pre-selected solid waste disposal site should be considered. Sludge layers should be kept thin to maximize drying rates.

Sand drying beds dewater primarily by gravity drainage of water from the sludge by placing the sludge on a sand medium. They are more effective for lime sludges than for coagulant sludges. Loading rates are typically between 1.0 and 2.4 kg/m2 (0.2 and 0.5 lbs/ft2). Draining time is typically 3 to 4 days. Applied sludge depth should be 200 to 750 mm (8 to 30 in) for coagulant sludges and 300 to 1200 mm (12 to 48 in) for lime sludges.

Freeze-assisted drying beds use a freeze-thaw cycle to break the molecular bonds between the water and the sludge which greatly enhances the dewatering rate. These systems are more suitable for dewatering coagulant sludges in cold climates. Freeze-thaw systems should be designed with two (2) drying beds, each sized to accommodate one (1) year of sludge storage.

Solar drying beds use asphalt or concrete as a sub-base for dewatering of sludge. The heat promotes faster drying. This process does not have widespread applicability in Ontario due to the local climate.

In vacuum-assisted drying beds, suction draws water from the underside of rigid, porous medial plates upon which the residuals are placed. Frequent plate cleaning and chemical sludge conditioning is typically required for this type of process.

11.4.4.2 Mechanical Dewatering Processes

Belt and diaphragm filter presses dewater residuals by sandwiching sludge between two porous belts and are suitable for dewatering coagulant sludges to 15% to 20% and lime sludges to 50% to 60%. The applied pressure is typically in the 600 to 1,500 kPa (87 to 218 psi) range. Roller bearings should be designed to have a service life (L10) of approximately 300,000 hrs. A polymer conditioning system should be provided for all belt filter presses. Consideration should also be given to desired cake solids content, conditioning requirements, pressure requirements, belt speed, belt tension, belt type and belt mesh size.

Centrifuges dewater residuals by forcing water from solids under high centrifugal forces. Both co-current and counter-current designs are available. Design criteria will be proprietary in nature and the manufacturer should be consulted in cases where a centrifuge is being considered. A polymer conditioning system should be provided for all centrifuge systems.

Similar to air/gravity dewatering systems, decanting and drainage systems should be provided and the required solids concentration and slump characteristics required for the final pre-selected solid waste disposal site should be considered.

11.4.5 Lagoons

Where residuals treatment through lagoons is proposed, a minimum of two cells should be provided, each capable of independent operation. Each cell should be sufficiently large to hold twelve months' sludge production plus a minimum of one day liquid waste volume. Typically, the lagoon design should allow for sludge depths of 0.5 to 0.75 m (1.6 to 2.5 ft), supernatant depths of 0.5 to 0.75 m (1.6 to 2.5 ft) and allow for ice cover as appropriate to local conditions. Inlet piping should be designed to distribute the incoming waste uniformly and minimize disruption of the settled sludges. The piping should be designed to be free draining to reduce the possibility of frost or ice damage in winter. Outlet piping should be designed to permit displacement operation during winter, and should be free draining. Each cell should have a supernatant decant system which is adjustable. The required effluent quality criteria for lagoon discharge to the environment (including subsurface infiltration) should be established through consultation with the appropriate ministry Regional Office. The design for lagoons should provide for:

  • Location free from flooding;
  • Where necessary, dikes, deflecting gutters or other means of diverting surface water so that it does not flow into the lagoon;
  • A minimum usable depth of 1.5 m (5 ft);
  • Adequate free board of a least 0.6 m (2 ft);
  • Adjustable decanting device;
  • Effluent sampling point;
  • Adequate safety provisions;
  • Parallel operation; and
  • A minimum of two cells, each with appropriate inlet/outlet structures to facilitate independent filling/dewatering operations.

11.5 Residuals piping design

The designer should consider the provision of an emergency filter backwash residuals by-pass overflow to allow treatment plant operations to continue at all times.

Piping for process residual streams should not pass through treated water retaining structures. Appropriate backflow protection should be provided on residuals piping as needed to protect the quality of the treated water.

Residuals piping should be provided with adequate cleanouts and provision for flushing. A minimum velocity of 0.5 m/s (1.6 ft/s) is recommended for all wastewater lines.

11.6 Radioactive materials

Where radioactivity has been detected during source water characterization, the treatment process may accumulate radioactive materials in waste residual streams to a level that requires special handling provisions. The designer should refer to the Guidelines for the Management of Naturally Occurring Radioactive Materials (NORM) available from the Health Canada website.

Chapter 12: Challenging conditions

This chapter presents design guidelines, suggestions and ideas which may assist in the application and design of alternate technologies for underground servicing of areas that are affected by challenging conditions. For more detailed information on this subject the designer should refer to the American Society of Civil Engineers/ Canadian Society for Civil Engineers (ASCE/CSCE) Cold Regions Utilities Monograph, 1996 (formerly Cold Climate Utilities Deliver Design Manual, Environment Canada) available from the ASCE bookstore website.

12.1 General

Challenging conditions may be a result of: climate, geology, hydrogeology, location (remoteness) of the area, topography or any combination of these factors. These challenging conditions are often associated with northern communities, but can also occur in urban areas where above ground piping is necessary (i.e., bridge crossings or over permafrost) and/or shallow buried obstructions such as culverts that cause pipe to be placed in the frost zone.

12.2 Climatic factors

The main climatic elements that can affect low ground temperatures are cold air temperatures and the amount of snow cover. With below freezing temperatures, the designer must determine whether the conditions are such that the proposed water service will freeze or be otherwise negatively impacted. Historical information on Ontario climate is available from the Environment Canada website. Design temperature data are also provided in the Supplementary Standard SB-1 of the Building Code (O.Reg. 350/06) made under the Building Code Act, 1992.

The main indicator utilized to determine the relative "air coldness" of an area is the "Freezing Index". The "Freezing Index" is defined as the number of degree days above and below 0°C (32°F) between the highest point in autumn and the lowest point the next spring on the cumulative degree-day time curve for one freezing season. It is recommended that the designer consider the coldest month.

The climatic factor most seriously impacting the design, cost and operation of piped water services is frost. The depth to which it penetrates depends upon the Freezing Index, the frost susceptibility of the soil and the thermal conductivity of the soil. The designer should refer to the ASCE/CSCE Cold Regions Utilities Monograph, 1996.

Another factor to be considered in any design is frost heave. As the water in the pores of the soil freezes, there is an associated increase in the volume of the soil of up to 5%. If ice lenses form in the soil, much greater increases may occur. Any service system that is to be constructed within the frost zone must be designed with consideration given to the rise of the ground surface due to frost action.

For more information regarding frost and freezing, the designer should refer to the National Research Council of Canada (NRC), Institute for Research in Construction and the Ontario Ministry of Transportation.

12.3 Geological factors

The predominant geological factor which can have an effect on service design conditions is the presence of rock and its proximity to the surface. This phenomenon is common in many areas of the Province and predominant in Northern Ontario where the main geological feature is the Precambrian formation of the Canadian Shield. Other factors of concern respecting the geology of the northern parts of the Province, and hence the design of water services, are:

  • The presence of muskeg which can be found in depths varying from less than 0.3 m (1 ft) to in excess of 3.0 m (10 ft);
  • Soil classification and frost susceptibility;
  • Soil thermal conductivity;
  • Soil chemistry (i.e., acidic and alkali soils); and
  • The presence of a high water table.

12.4 Location (Remoteness)

In certain northern regions of the province, the location of the community to be serviced may be a factor in the design. Access to the site may be difficult, limited and/or expensive due to the lack of adequate road or rail transportation.

These access problems affect the supply of materials such as chemicals, construction equipment, and replacement parts, as well as servicing. In these areas, the designer should ensure that the servicing methods are adapted as simply as possible to suit local conditions. If special fittings and accessories are required that may be difficult to obtain, replace and service, this should be considered at the design stage and spares purchased during construction.

12.5 Permafrost

Permafrost is defined as soil, bedrock or other material that has remained below 0°C (32°F) for two or more years. Continuous Permafrost occurs in areas that are underlaid by permafrost with no thawed areas. Discontinuous Permafrost occurs in an area underlain mostly by permafrost but containing small areas of unfrozen ground.

In Ontario, a state of discontinuous permafrost exists north of the line drawn from the southern tip of Hudson Bay, westerly to the point where the 53°N parallel intercepts the Ontario western boundary, to the 55°N parallel. More information on the distribution of permafrost in Ontario is available from Natural Resources Canada.

Passive construction is usually used in permafrost conditions. This maintains the state of frozen permafrost by constructing insulated water services. Permafrost conditions will not likely be met in any but the most remote northern areas of the Province.

12.6 Difficulties associated with conventional practices

12.6.1 General

This section contains conventional design practices and some of the design and installation problems associated with these practices. Alternative design practices are outlined in Section 12.8 Alternative Design Practices.

12.6.2 Watermains & Water Distribution

The governing factor affecting the design depth of watermain and service connection installations in Ontario is the necessity to protect the pipe and its contents from the effects of frost.

Accepted practice has been to locate the watermain or service connection at such a depth that it is either below the frost line or the incidence of frost-related failures is at an acceptable level. The required burial depth (i.e., frost penetration depth) varies across the province from approximately 1.2 m (4 ft) to greater than 3.0 m (10 ft). The designer should refer to the National Research Council of Canada (NRC), Institute for Research in Construction for information used to determine frost penetration.

The designer should be specifically aware that problems can arise when frost depth penetration values are applied without an adequate factor of safety in an effort to minimize the high cost of excavation and hence watermain installation costs where watermains are installed in trenches blasted in rock or areas with high water level conditions.

The water service connection is the most common place where freezing will occur due to the small size of pipe and the fact that the water does not flow at all times. It is not uncommon practice to alleviate this problem by bleeding individual services by leaving a tap running in the individual home with flows of 2,600 to 3,100 Litres (685 to 820 USgal) per home per day discharging to the sewer. The practice of bleeding places an extraordinary burden on water supply and sewage facilities.

12.7 Retrofitting of existing systems

12.7.1 General

There are several improvements which may be applied to the upgrading of existing services in areas subject to challenging conditions. These recommendations are supplementary to the appropriate sections of Chapter 8 Treated Water Storage and Chapter 10 Distribution Systems.

It is recommended that conditions of no snow cover be assumed for purposes of determining the maximum depth of frost. In many cases the system may be located partially in the frost zone, however, as long as the water is kept moving the system will not freeze. At dead ends, fire hydrants, and some service connections, where the freezing water may not be replaced, additional frost protection should be provided.

An additional concern exists regarding the increased loading on the pipe due to frost. Accordingly, the design of watermains in areas where the frost is experienced should include an allowance for this frost loading.

12.7.2 Water Distribution

12.7.2.1 System Layout

Where water services are located either partially or totally within the frost zone, and where it is not possible to eliminate dead end watermains, there are two possible solutions utilizing conventional design practices. The first is to insulate the watermain and the other is the replacement of the freezing water within the system by ensuring a constant flow of water through a recirculation pipe or a municipally controlled bleeder.

12.7.2.2 Service Connections

Service connections are the most common point where freezing will occur within any distribution system. There are several contributing factors, such as:

  • The water service is subject to prolonged periods (e.g., overnight) of no-flow, thereby resulting in excessive cooling of the water; and
  • Inadequate installation depths because of the cost of excavating in rock and/or in a zone of excessive frost depth.

In instances where repeated problems with freezing services have occurred or uncontrolled bleeders are in use to prevent freezing, the following alternatives may be considered:

  1. Re-laying of the entire service connection at an adequate depth of cover;
  2. Replacement of the entire service connection with a new service connection installed in a pre-insulated service duct, with thermostatically controlled heat tracing if required;
  3. Provision of insulation over the existing service connection, with heat tracing if required; or,
  4. Installation of a municipally controlled bleeder.

The preferred alternative in areas with deep frost penetration and rock trenches is (b) above. Alternative (d) is the least desirable, particularly when widespread problems are being encountered in the system. The designer may consider this alternative where occasional problems recur. When such a device is used, the bleeder installation should be accompanied by a water meter.

12.7.2.3 Insulation

Rigid slab insulation placed above the water pipe has been used with some success as an alternative to burying the pipe below the frost zone. Its main application has been in attempting to correct existing freezing problems at hydrants and other dead ends and in situations where the cover over an existing watermain is reduced due to the reconstruction of the road or regrading.

Caution must be exercised in the use of flat slab insulation and it should only be used when there is a reasonable heat source from high groundwater from below the pipeline.

When rigid slab insulation is to be used to provide frost protection, the thickness of the slab must be carefully determined. The thickness is, in effect, replacing natural cover, thereby permitting reduced construction depths or providing additional protection.

Based upon average conditions, the thickness of slab provided in an installation should be the equivalent of 25 mm (1 in) for every 300 mm (12 in) reduction in the depth of cover [i.e., approximately 83 mm (3 in) for every 1.0 m (3 ft) reduction].

It is recommended that the flat slab insulation be laid in 50 mm (2 in) thicknesses with joints offset by half the width of board in progressive layers.

As the frost penetration depth increases, the width of insulation required also increases. It may become more economical to provide an inverted "U" or box type of slab insulation or use insulated pipe. The former two methods are extremely labour intensive and close field supervision is required to ensure the structural soundness of the "U" or the box.

12.7.2.4 Bleeders

Bleeders should only be utilized as a last resort on existing systems, and should be under the strict control of the municipality or operating authority, regardless of their location.

Bleeders associated with dead end watermains should be designed with the discharge of bleeder flow to a ditch, if this is feasible without creating nuisance icing conditions. If such a point of discharge is unavailable, the bleeder may be discharged to an adjacent sewer, through an air gap backflow preventer.

Individual bleeders installed on private service connections should be located downstream of the water meter with discharge to a sewer through an air gap backflow preventer. This bleeder should be equipped with a corporation seal and activated by authorized personnel only. Another alternative is a factory manufactured automatic flow control or balancing valve.

12.8 Alternative design practices

12.8.1 General

The cost of installing water services increases as the depth to bury increases. As discussed in Section 12.6 Difficulties Associated with Conventional Practices, in areas that are subject to the effects of challenging conditions (such as the presence of rock, extreme frost or a high water table) the costs would be much greater as the depth to which these services need to be installed increases.

12.8.2 Thermal Considerations

When dealing with services and/or mains that are located in the active frost zone, it is possible to reduce heat loss and increase time before freezing by using pre-insulated piping with or without electric heat tracing.

12.8.3 Shallow Buried Pre-insulated Servicing Systems

"Shallow buried" means a system that is partially or totally within the frost zone (i.e., cover only for physical protection) and "insulated" means reducing the heat loss from the pipe by applying various amounts of insulation to the buried pipe. In addition to the insulation, varying amounts of heat can be added to the service by inducing circulation or adding supplementary heat (e.g., heat tracing or hot water). The preferred system is the "factory fabricated, pre-insulated, flexible piping system".

12.8.4 Water System Design

12.8.4.1 General

The fundamental concepts of water distribution system design and appurtenances should follow the recommendations contained in Chapter 8 Treated Water Storage and Chapter 10 Distribution Systems.

12.8.4.2 System Layout

While straight line grid systems are preferred, the designer should consider routing flexibility to avoid a challenging condition such as exposed rock by utilizing unconventional alignments. It is advisable to consider an alternative to the road allowance alignment such as the ditch line of the road where it is not snow-ploughed, or the use of parallel front yard or backyard easements. In areas where the water service will be located either partially or totally within the frost zone and where it is not possible to eliminate dead end watermains, there are several alternatives available to reduce the probability of frost problems, as follows:

  • Construction of the facility with a pre-insulated pipe package with heat tracing or an external thaw tube;
  • Insulation of the water service with slab type insulation with or without a circulation line;
  • Design of the system to include an induced flow via either a recirculation system or as a last resort, a municipally controlled bleeder; or,
  • A combination of the above.

As has been noted in Section 12.7.2.4 Bleeders, bleeders should only be utilized as a last resort, and be under the control of the municipality. Where a bleeder is the only alternative, the designer may consider the use of the last private connection as the bleeder in a controlled, heated environment.

12.8.4.3 Insulation of Watermains & Services

The preferred alternative for a shallow buried, thermally insulated water distribution system is to employ a factory-fabricated, pre-insulated package type piping system.

There are three basic options for a factory-fabricated, pre-insulated flexible piping system:

  • Pre-insulated piping system without heat tracing or an external thaw tube;
  • Pre-insulated piping system with heat tracing; and
  • Pre-insulated piping system with an external thaw tube.

Similarly, there are two basic types of factory-fabricated pre-insulated service connection pipe:

  • Individually insulated service connection (either with or without heat tracing); and
  • Pre-insulated duct with a single service connection inserted, without unions, within the duct (either with or without heat tracing).

The heat tracing cable for either of the above pipe systems can have two functions, depending upon the design requirements: it can thaw the pipe once it has frozen (passive tracing no thermostat), or it can be used to prevent the water in the pipe from freezing (active tracing thermostatic control).

Since the thermal resistance of plastic is significant (125 times that of steel), the heat tracing density for plastic pipes should be considered carefully. The designer should consult the manufacturer of the factory insulated pipe system for guidance.

The insulation and its jacket material should have a higher density than the surrounding soil to be able to withstand the trench and service loadings without subjecting the service or duct pipe to excessive deflections or compression. Rigid polyurethane foam is recommended.

12.8.4.4 Pipe Materials

The designer should specifically consider the ability of the pipe joint to maintain "zero" leakage over the long term when subjected to frost action and the ability of the pipe material itself to resist structural damage or failure when subjected to inadvertent freezing and/or frost action such as frost heave or differential loading.

It is recommended that the purchase of a pre-insulated pipe package should be from a single source. This single source manufacturer should supply and guarantee the total package including carrier pipe, insulation, jacketing, heat trace cable and thermostatic controls. In addition, the manufacturer should be required to provide in-field service and technical expertise.

The provision of active heat tracing is generally not needed on large diameter watermains and forcemains [i.e., diameter above 150 mm (6 in)] and may not be needed on smaller diameters within the public right-of-way, provided the water is kept moving. A minimum acceptable time-to-freeze of approximately 96 hours should be considered an acceptable level of risk. The final decision in this regard rests with the designer and the municipality/owner.

Where the time-to-freeze is less than 96 hours or a dead end watermain or water service connection is involved, it is recommended that thermostatically controlled (solid state) heat trace cable be provided (active heat tracing).

Where it is determined that it is necessary or advisable to provide active heat tracing on main lines, as opposed to service connections, the designer should consider:

  • Available voltage and "power points";
  • Maximum heat cable circuit length; and
  • Power consumption (if electricity is extremely costly, consider constant wattage cable which only draws power when absolutely required).

When the time to freeze is greater than 96 hours, it is recommended that a manually activated heat trace cable or an external thaw tube (passive thaw system) be provided. A passive system would only be activated when the freezing had occurred.

Thawing equipment is available from a number of suppliers.

12.8.4.5 Service Connections

All new water service connections which are installed within the frost zone should be constructed in a pre-insulated HDPE duct with active heat tracing and a thermostatic control. Bleeders should not be employed as a method of freeze protection on new services.

Where heat tracing is to be provided, factors such as type of heat trace cable, cable jacketing and thermostats should be considered.

The installation of proprietary domestic systems which heat and/or recirculate water within the service line may also be considered for individual connections that may be prone to freezing. Adequate backflow prevention should be provided.

12.8.4.6 Installation Details

The fundamentals of pre-insulated pipe installation are similar for this type of pipe as for a conventional pipe installation. However, several additional features should be included on shallow buried, pre-insulated systems as follows:

  • Pre-insulated pipe should not be buried with less than 0.6 m (2 ft) cover when it will be subjected to vehicular loadings (i.e., driveways and highway crossings) without the provision of additional protection for the pipe. This protection can take the form of either a metal or plastic jacketed pipe installed within a culvert section(s);
  • A nominal 50 mm × 200 mm (2 in × 8 in) warning board should be placed 150 mm (6 in) above any pre-insulated pipe with less than 1.2 m (4 ft) cover in order to afford protection to the pipe and warn excavators;
  • Sand bags should be placed between the pipe and the trench wall, when the pipe is being installed in a rock trench on a long radius deflection or curve;
  • The pipe design should include a check for buoyancy in areas where the pipe will be located at or below the groundwater table; and
  • The pipe should be snaked in the trench in order to allow for expansion and contraction.
12.8.4.7 Fittings & Appurtenances

When a shallow buried, thermally insulated type system is proposed, special consideration must be given to both the type of fitting proposed and its proper installation.

Valves

Low thermal conductivity valve box materials with valve system extension pieces should be used.

Chambers

Where it is necessary to provide valve or meter chambers, the chambers should be adequately insulated, provided with frost covers in the access hatches, the pipe proper should be insulated and, when necessary, heat traced. The chamber design should incorporate all safety features for access/egress and emergency evacuation situations.

Bends & Tees

Where ductile iron fittings are used with mechanical joint fittings, restraining type glands should be utilized as opposed to concrete thrust blocks to better facilitate either factory applied or field applied insulation. The insulation should be coated with a suitable moisture barrier.

High density polyethylene pipe can be adapted to this type of system through the use of thermally butt fused end flanges connected to flanged to plain end ductile iron pipe filler pieces.

Hydrants

Hydrants and their leads are essentially a dead end watermain and must, therefore, be accorded special attention. The importance of this special attention is reinforced by the necessity of maintaining the hydrant operational year round.

The simplest and most effective way of protecting the hydrant assembly is to locate the watermains off the traveled road allowance (i.e., within the ditch line or easements), and close couple the hydrant assembly to the tee and shut-off valve assembly and insulate the barrel.

12.8.4.8 Single-Pipe Recirculation System

The single-pipe recirculation system consists of one or more uninterrupted loops or sub-loops originating at a recirculation facility and returning to that facility.

The design of such a system should minimize the length of pipe required and, in turn, minimize energy losses.

This system allows for positive simple control of the distribution system via the installation of flow, pressure and temperature monitoring on the return line(s) at the recirculation facility. The rate of recirculation is controlled by the supply and return temperatures. The actual variation in rate can be accomplished via either the use of a pressure reducing, pressure sustaining type valve, turning additional pumps on or a combination of these.

Supply temperatures should be in the order of 4 to 7°C (39 to 45°F) with return temperatures between 1 to 2°C (34 to 36°F). In some instances, pretempering (i.e., pre-heating) of the supply water may be required. This can be accomplished, when necessary, via the use of a supplementary water heater.

As the length of the loop increases, the risk of service loss also increases in case of a shutdown due to a problem along the line. The designer should consider short loop links installed at strategic locations to break the main loop. These links should be valved off to allow the pipe to be left empty. In case of an emergency, these links can be opened to reroute the flow of water and possibly isolate the break.

The single-pipe recirculation system can be designed to supply water in the normal return line as well as the supply line under fire conditions. For this reason, the return line should not decrease drastically in size.

The recirculating facility can be located at the source or in a separate pumping facility, or a combination of the two.

By planning community growth in a dense circular pattern, maximum efficiency can be made of this method of servicing.

Back-of-lot mains are preferred if possible. If the mains are placed in the street, the appurtenances (e.g., valve boxes) are subject to physical damage.

Placing the mains at the rear lot line reduces service line connections and permits service lines of equal length on both sides of the main. With mains in the road allowances, usually to one side, services are of unequal length.

A further advantage of mains located along the rear lot line is that the manholes containing water line valves and hydrants and freeze protection controls can be elevated in cylindrical shape approximately 1.0 m (3 ft) above grade. This allows easier access during the winter as the immediate area around the elevated manhole is often blown clear of snow.