Sludge storage and disposal
Chapter 18: Sludge Storage and Disposal
Sludge produced by treatment of sewage may require temporary storage prior to processing, disposal or utilization. This chapter provides guidance for designing sludge or biosolids storage systems. Disposal options, including land application and landfilling, are also described.
18.1 General
Sludge storage facilities should be provided for all mechanical sewage treatment plants. Storage facilities may consist of any combination of drying beds, lagoons, separate tanks and pad areas to store liquid, dewatered or dried sludge. Additional volume in biosolids stabilization units may be used, but this is usually limited in capacity. The design should provide for odour control in sludge storage tanks and sludge lagoons. Refer to Chapter 17 - Sludge Thickening and Dewatering for information on sludge volume reduction and drying.
Calculations to establish the number of days and volume of storage should be carried out and should be based on the total sludge handling and disposal system. Refer to Chapter 16 - Sludge Stabilization for sludge characteristics and generation rates of various sewage treatment processes.
The General Regulation (O. Reg. 267/03) made under the Nutrient Management Act should be consulted for storage capacity requirements for biosolids destined to agricultural land application. If the land application method of sludge disposal is the only means of disposal used at a sewage treatment plant (STP), storage should be provided in accordance with O. Reg. 267/03 based on considerations of the following items:
- Inclement weather effects on access to the application land;
- Temperatures including frozen ground and stored sludge cake condition, (land application of biosolids during the period beginning on December 1 of one year and ending on March 31 of the following year or at any other time when the soil is snow-covered or frozen is not allowed by O. Reg. 267/03);
- Haul road restrictions including spring thawing conditions;
- Seasonal rainfall patterns of the area;
- Cropping practices on available land;
- Potential for increased sludge volumes from industrial sources during the design life of the plant;
- Available area for expanding sludge storage; and
- Pathogen reduction and vector attraction reduction requirements.
A minimum of 240 days storage should be provided for the ultimate design capacity of the STP unless a different period is justified on the basis of the site specific conditions. The minimum 240 days storage requirements under O. Reg. 267/03 can be a combination of a permanent biosolids nutrient storage facility, a temporary field nutrient storage site (dewatered municipal sewage biosolids only) or a combination of such facilities and sites that is capable of storing generated sewage biosolids during a period of at least 240 days.
18.2 Sludge Storage Lagoons
Fully treated (i.e., stabilized) sludge may be stored in lagoons prior to further processing or disposal. Planning and design should address design elements such as dike engineering and liners and consider factors including long-term planning issues.
Sludge should be stabilized in such a way that offensive odours do not result from a lagoon site. Where sludge lagoons are used, adequate provisions should be made for alternative acceptable sludge handling methods in the event of upset or failure of the sludge stabilization process.
Sludge lagoons should be located as far as practicable from inhabited areas or areas likely to be inhabited during the lifetime of the structures.
Adequate provisions should be made to seal the sludge lagoon bottoms and embankments to prevent leaching into adjacent soils or groundwater. The seal should be protected to prevent damage from sludge removal activities. Exfiltration of water from the lagoon may not be acceptable and, if occurring, should meet the requirements of the ministry Guideline B-7, Incorporation of the Reasonable Use Concept into Groundwater Management which provides the framework for determining acceptable off-property impacts on groundwater resources. The designer is referred to Section 12.3.4 - Lagoon Construction for additional information.
Provisions should be made for pumping or heavy equipment access for sludge removal from the sludge lagoon on a routine basis.
Lagoon supernatant should be returned to the STP at locations and rates that minimize the effect on the treatment processes.
Two types of sludge lagoon systems have been most commonly constructed in Ontario - thickening lagoons and sludge transfer site lagoons.
18.2.1 Thickening Lagoons
Thickening lagoons have generally been built at or near the site of the sewage treatment plants so that the sludge can be conveyed to the lagoons by pumping or gravity flow and that supernatant can be returned to the STP for further treatment.
18.2.2 Sludge Transfer Site Lagoons
Sludge transfer site storage lagoons have usually been built as temporary sludge storage facilities. These sites may be required as part of a program for biosolids utilization on agricultural land to hold biosolids during times of the year when land spreading cannot be carried out. With true transfer site lagoons, no particular attempt is usually made to withdraw supernatant or to otherwise thicken the sludge beyond the natural thickening that occurs due to evaporation. Where suitable land surrounds the lagoon, a combination of thickening and transfer lagoon may be used where supernatant may be spray irrigated onto the surrounding land and the thickened biosolids can be hauled away for spreading on farmland. Withdrawal of supernatant may result in increases in sludge concentration to the extent that sludge removal by pumping may become difficult or impossible, particularly above a total solids (TS) concentration of 7 to 8 percent.
These lagoons require the ministry approval in accordance with Ontario Regulation 347, General – Waste Management, made under the Environmental Protection Act.
The designer should refer to the ministry document “Guide on Applying for Approval of Waste Disposal Sites” for additional information.
18.2.3 Anaerobically Digested Sludge Storage
Anaerobically digested solids may be stored in covered basins or facultative solids basins. The anaerobically digested solids storage facility should be designed to abate vector attraction and odour conditions. The facultative solids storage basin should be designed to maintain an aerobic surface layer free of scum accumulation. The organic loading rate for a facultative solids storage basin should not exceed 0.1 kg VS/m2 (20 lb/100 ft2) of surface area per day. Surface aerators should be used to maintain the aerobic zone and break up surface film. The surface aerators should be designed to minimize the mixing action between the aerobic and anaerobic zones. The facultative solids basin should have a minimum sidewater depth of 3.7 m (12 ft). The top 0.9 m (3 ft) should be kept aerobic.
18.2.4 Aerobically Digested Sludge Storage
Aerobically digested solids may be stored for extended periods of time and the basin contents should be kept thoroughly mixed using diffused air or mechanical aeration. A minimum air requirement of 30 m3/(1000 m3·min) (30 cfm/1000 ft3) should be provided. If mechanical surface aerators are used, a minimum power requirement of 2.6 kW/1000 m3 (0.1 hp/1000 ft3) should be provided.
18.2.5 Alkaline Stabilized Sludge Storage
Liquid alkaline stabilized high pH (>12) sludge should not be stored in a lagoon, but should be stored in a tank or vessel equipped with rapid sludge withdrawal mechanisms for sludge disposal or re-treatment. Provisions should be made for adding alkaline material in the storage tank. Mixing equipment should be provided in all storage tanks.
On-site storage of dewatered alkaline stabilized sludge should be limited to 30 days. Provisions for rapid re-treatment or disposal of dewatered sludge stored on-site should be made in case of sludge pH reduction.
18.3 Storage for Sludge or Biosolids
18.3.1 Dewatered Sludge or Biosolids
Dewatered sludge with a solids content of less than 35 percent may be stored on-site up to 7 days. An excess capacity should be provided due to inclement weather or other factors that do not allow transport or disposal. The dewatered sludge may be stored in steel or concrete containers and should be located and stored to preclude re-wetting by rainfall.
Dewatered sludge with solids content greater than or equal to 35 percent may be stored on-site for up to 90 days. The dewatered sludge may be stored in containers or in stockpiles. The storage facility should be located to preclude groundwater contamination. Open stockpiles should include provisions for collecting rainfall runoff. All rainfall runoff should be collected and returned to the head of a treatment facility.
18.3.2 Dried Sludge and Biosolids
Dried sludge with a solids content of greater than or equal to 50 percent may be stored on-site in bins or covered facilities. Enclosed structures may produce explosive gaseous byproducts or dust. The enclosed area of the storage structure should be sufficiently ventilated to eliminate the accumulation of dangerous gas mixtures. The enclosed storage structure should be mechanically ventilated with approximately 20 to 30 air changes per hour. All exhaust air should pass through an odour control system.
18.3.3 Sludge Storage Tanks and Basins Prior to Dewatering
Holding tanks and basins are commonly provided as an integral part of most conditioning processes and many stabilization processes. Tanks and basins may be used for blending materials such as sewage sludge from primary and secondary clarifiers.
Large storage tanks are generally constructed of concrete. Smaller tanks are often constructed of carbon steel with a suitable coating or liner. Tank and basin equipment often includes an aeration system, mechanical mixers or a recycling system for mixing. All equipment within the tank should be constructed of a corrosion-resistant material such as polyvinyl chloride (PVC), polyethylene (PE), stainless or glass-lined steel.
Tanks and basins may be sized to retain still-to-be dewatered sewage solids (liquid sludge/biosolids) for a period of several hours to a few days. If sewage solids are stored longer than two or three days, the product may deteriorate and can become difficult to dewater.
If the tank or basin is a closed vessel, the designer should ensure that there are access portholes for inspection and maintenance. All access portholes need to meet the Canadian Gas Association (2005) Code for Digester Gas and Landfill Gas Installation, CAN/CGA-B105-M93, 1993 requirements.
Short storage periods of unstabilized primary and secondary sewage sludges in a holding tank or basin can produce nuisance odours. Decanting tanks following thermal conditioning can often create odour problems. The design should include assessment of odour potential and provide for sufficient odour control equipment to minimize odour emissions.
18.3.4 Bulk Storage
Design considerations for a bulk storage area are as follows:
- The size of the biosolids storage area depends on the quantity of biosolids produced, when it can be used and its moisture content;
- Drier solids can be stacked higher with less tendency to slump;
- Additional space should be provided for scheduled process cleaning (lagoon dewatering or digester cleaning) and emergency situations; and
- Materials should not be stored in a manner that will likely result in contamination of ground or surface waters, air or land in case of flood or fire.
The storage area should be constructed and sited to prevent run-on and runoff of liquids. A solids storage area needs a water collection system and a way to treat the leachate produced from the pile. Care should be taken not to contaminate the solids with oil, grease, gas, rocks and litter. The area needs to be secure to prevent access by the public, domestic animals or wildlife.
Depending on the population proximity and density of the area, quality of the biosolids (stability) and prevailing winds, odour control should be provided to minimize the impact on the surrounding neighborhood. In addition, mechanical ventilation should be provided for enclosed storage sites.
18.4 Land Application
Biosolids may be used as a soil conditioner for agricultural, horticultural or reclamation purposes depending on the degree of stabilization provided. The quantity of solids generated by the selected treatment process should be calculated or estimated from similar full-scale facilities or pilot facilities. Pathogen levels and concentrations of metals in sludge should be determined using standard laboratory test procedures. Pathogen levels and concentration of metals should be less than the levels specified in O. Reg. 267/03 if land application is used. A mass balance approach should be used to determine the quantity of biosolids produced at the facility.
Important design considerations include but are not necessarily limited to:
- Type of sludge/biosolids stabilization process;
- Pathogen and vector attraction reduction to levels specified in O. Reg. 267/03;
- Biosolids characteristics including the presence of inorganic and organic chemicals;
- Application site characteristics (e.g. soils, groundwater elevations, setback distance requirements);
- Local topography and hydrology;
- Type of crop and land to which biosolids can be applied in accordance with O. Reg. 267/03. 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.
- Cropping practices, spreading and incorporation techniques;
- Population density and odour control; and
- Sampling, health and safety requirements in accordance with O. Reg. 267/03.
A contingency plan should be provided for flexibility in the event of equipment failure or conditions that prevent the primary use or disposal method. The design should account for weather factors such as rainfall, wind conditions and humidity in the selection of the use or disposal of sewage sludge. Due to inclement weather and cropping practices, alternative storage or disposal options are recommended to ensure the biosolids are properly managed. Mixing equipment or provisions to assist in the monitoring of land-applied biosolids should be considered in the design of biosolids handling and storage facilities.
Municipal sewage treatment plants that apply sewage biosolids on agricultural land are required under O. Reg. 267/03 to have prepared a nutrient management strategy (NMS) and have the strategy approved by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
18.5 Disposal
Immediate sludge disposal may be used to reduce the sludge inventory at the STP site and amount of sludge that may need to be retreated to prevent odours if sludge pH decay occurs.
18.5.1 Landfilling
Sludge may be disposed of in approved municipal sanitary landfills. The required solids concentration for sludges that are to be landfilled at sanitary landfill sites are normally specified by the landfill authority. With small quantities of sludge for co-disposal landfilling with municipal solid waste, liquid sludge at solids concentrations as low as 3 percent (TS) may be acceptable. For sludge-only landfill operations, a minimum of 18 percent TS concentration or more, or a slump of 150 mm (6 in) or less, is generally required to support cover material. The “Test Method for Determination of Liquid Waste (Slump Test)” is set out in Schedule 5 of Ontario Regulation 347 General – Waste Management, made under the Environmental Protection Act.