Conveyance and Storage of Biosolids

Biosolids land application always requires the biosolids to be conveyed to the ultimate location. Successful biosolids land application programs should also have provisions to deal with daily biosolids production at wastewater treatment plants in the event that biosolids cannot be land-applied immediately. This contingency plan generally involves storage.

Transportation Biosolids may be transported by pipeline, truck, barge, rail, or a combination of these. The method of transportation chosen and it cost depend on the consistency and quantity of biosolids to be transported and the distance from origin to destination. Small to medium-sized treatment plants use trucks as the mode of transportation, as the distance to the destination is usually less than 100 km (62 miles). To minimize the danger of spills, odors, and dissemination of pathogens to the air, liquid biosolids should be transported in closed vessels such as tank trucks. Dewatered biosolids can be transported in open trucks, but the trucks should be covered and should have rubber-sealed rear gates to minimize nuisance odors and spills. One way to reduce public exposure to odors is to choose a hauling route that avoids densely populated residential areas. Making sure that trucks used to haul biosolids are clean and well maintained is another effective way to keep the road surfaces clean and to control odors during biosolids transport.

Storage Storage is necessary during inclement weather when land application sites are not accessible and during winter months when land application to snow-covered and frozen soil is prohibited or restricted. Storage may also be needed to accommodate seasonal restrictions on land availability due to crop rotations. For small biosolids generators, storage allows accumulation of enough material to complete land application efficiently in a single spreading operation.

The term storage refers to temporary or seasonal storage. Storage operations involve an area of land or facilities to hold biosolids until material is land applied on designated and approved sites. More permanently constructed storage facilities can involve state or locally permitted areas of land or facilities used to store biosolids. These facilities may be used to store any given batch of biosolids up to two years. They are usually located at or near land application sites and are managed so that biosolids come and go on a relatively short cycle, based on weather conditions, crop rotations, and land and equipment availability. Storage options include field stockpiling for short-term storage, and constructed facilities that include concrete, asphalt, clay, or compacted earth pads; basins and lagoons; tanks; or other structures that can be used continually to store liquid, semisolid, or solid biosolids.

Field Stockpiling Field stockpiling is used for short-term storage of dewa-tered cake, dried, or composted class A or B biosolids at the land application site. It is generally limited to the amount of biosolids needed to meet agronomic or reclamation requirements at a field or site. Field stockpiles should be placed in the best physical location possible in or adjacent to the fields that will receive the biosolids. For sites with a significant slope, provisions need to be made to manage up- and down-slope water. Forming windrows across slopes should be avoided to reduce the potential for piles to become anaerobic at the base where overland flow accumulates.

To the extent possible, piles should be shaped to shed water. Stockpiled biosolids form an air-dried crust that sheds precipitation and prevents significant percolation of water through the pile. Nonetheless, some states require that stockpiles be covered. For composted or dried (at least 50% solids) bio-solids, tarps, wind barriers, or periodic wetting may be necessary to minimize blowing of dust, particularly in arid, windy climates when stockpiles are in close proximity to sensitive downwind areas. There have been some instances of tarps catching fire when used on compost materials. Incompletely composted materials have the potential to self-heat because microbial growth can still occur on the remaining nutrients. Heat-dried products that are rewetted or have not been sufficiently dried and cooled (<90% solids, >30°C) also can self-heat. In the presence of enough available water, microbes will utilize the nutrients in the biosolids and generate heat that cannot dissipate because of the mass of the stockpiles. Therefore, composted and heat-dried biosolids piles should be monitored so that a fire hazard does not develop. A noticeable increase in odor is a reliable indicator of microbial activity and the potential development of hot spots.

Storage Basins and Lagoons Storage basins for liquid or semisolid biosolids need to be large enough to provide adequate storage volume during worst-case weather conditions (long periods of inclement weather when field application is restricted and the basin cannot be emptied). Depth of storage basins may vary from 3 to 5 m (10 to 16 ft). The design volume must also include space for accumulation of precipitation expected over the storage period, plus the capacity to hold severe storm events. An impermeable liner is recommended to ensure against loss of biosolids constituents to groundwater by leaching. This type of design may negate the need for groundwater monitoring wells. Biosolids can be removed from the basins using a mud pump mounted on a floating platform or a mobile crane using a dragline.

Lagoons are used for long-term storage of sludge. They are usually part of the wastewater treatment plant. They are simple and economical if the treatment plant is located in a remote location. A lagoon is an earthen basin into which untreated sludge or digested biosolids are deposited. In lagoons with untreated sludge, the organic matter is stabilized by anaerobic and aerobic decomposition, which may give rise to objectionable odors. If the lagoons are used only for digested biosolids, nuisance odors are usually not a problem. The stabilized solids settle to the bottom of the lagoon and accumulate. Excess liquid from the lagoon may be decanted and returned to the plant for treatment. Solids concentrations as high as 35% have been reported in the bottom layer of lagoons used for treatment and long-term storage.

Storage Tanks Storage tanks for class A or B liquid biosolids are above- or belowground structures that are permanent and part of the sludge-processing facilities in a wastewater treatment plant. They are watertight and are generally concrete or steel structures which may be prefabricated or constructed entirely on site. Due to their impervious nature, these facilities do not warrant groundwater-monitoring wells.

Storage tanks may be open-topped or enclosed. The tank volumes should be large enough to contain daily biosolids produced during worst-case periods of inclement weather, or backup options must be part of the planning process. Enclosed tanks should be ventilated through passive vents or mechanical fans. Depending on the type of biosolids, tank design, climatic conditions, and airflow rates, a gas detection meter and alarm system tied to ventilation fans may be advisable to eliminate buildup of explosive levels of methane that might result from anaerobic biological activity in the tank.

Dewatered Biosolids Storage Dewatered biosolids storage facilities can be covered or uncovered and are designed to provide up to two years of storage for class A or B dewatered, air-dried, heat-dried, dry-lime stabilized, or composted biosolids. These facilities include open-sided or enclosed buildings and open-topped bunkers or pads. Storage facilities need to be large enough to provide adequate storage volumes during long periods of inclement weather when field application is restricted.

Unroofed facilities should have a durable hard pad with push walls and stormwater curbs, containment walls, and sumps. An impermeable floor is recommended to help control runoff, protect against loss of biosolids constituents to groundwater by leaching, and to accommodate vehicle traffic. Recommended materials include concrete or asphalt. For storing class A biosolids in arid areas, compacted soil or gravel with appropriate runoff controls may be satisfactory.

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