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4.5 Disposal (Topic e)

4.5.1 Land disposal of liquid effluent

Land based disposal is practiced for individual homes not connected to central treatment systems, and for some small communities with sufficient available land. In more agricultural areas, most homes will use soil absorption systems. In 1986, 20,900,000 soil absorption systems were operating in the U.S. (WEF Manual of Practice No. 8).

Conventional gravel trench systems are typically used for wastewater flows of 190 m3/d or less.

Where site conditions mandate, a variety of technology modifications have been developed. Deep absorption trenches are used where a shallow impermeable layer exists in the soil with a more permeable layer below, and the groundwater level is low. Elevated drain fields are used where the permeable layer or groundwater table is less than the depth required by local regulation. The "at grade" system is the upper extent of elevated drain fields, where the bottom of the trench aggregate is located on tilled soil surface. Sand lined beds and fill systems, referred to as built-up mound systems (see Figure 4.8) are used above shallow fractured bedrock or high permeability soil. Mound systems were developed at the University of Wisconsin (Wisconsin Mounds) and have found wide use across North America where there is high groundwater, a shallow impervious layer, low permeability soil, or a shallow soil profile. This is essentially a variation on conventional gravel trench systems where a sand mound is built above grade and trenches are installed in the sand. Careful design and construction practices must be followed if the system is designed for more than 24 homes.

Figure 4.8: Mound System

Contour trenching systems, developed in Nova Scotia, Canada, are used where slopes are steep (i.e. ski hills), and where soils have low permeability. A single trench is run along a slope contour, stretching out the drain field over one long run so that the hydraulic flux to the soil is minimized and spread over a large cross sectional area of soil. Although smaller systems may dose by gravity, larger systems are typically pressure dosed.

For all trench style systems, pressure dosing is being encouraged as prolonging the life of the drainfield. Gravity flow, using distribution boxes, is still used for smaller systems as it is less expensive.

Chamber leaching systems are plastic arched chambers installed underground, which provide a large storage volume in the trenches to handle peak flows. This technology replaces conventional gravel filled trench technology. Wastewater percolates through the trench bottom, which is open, and through openings in the chamber sidewalls. Storage volume in the trenches is not lost over time, as can happen with gravel filled trenches. Several areas in North America (ie: Maine, Texas) allow leachfields using chambers to be sized at 50 to 60 percent of the effective area required for gravel leaching systems. Other areas are testing the systems and considering changes in local regulations. Over 350,000 chamber leaching systems are in use.

Irrigation using wastewater as a disposal method is mainly practiced in the more arid regions of North America, as freezing is a concern with these systems. Sufficient storage for wet and cold seasons must be provided. Pasture, forest, row and field crops have been irrigated using wastewater. The type of crop grown, location, and public access affect the level of upstream treatment and disinfection applied.
Upstream secondary treatment and disinfection is generally required for above ground systems; flood irrigation, spray irrigation, and furrow irrigation (also known as slow rate land application). Only where the application location is isolated with restricted public access and the intended crop is not for human consumption is primary treatment only recommended. Sprinkle irrigation is not recommended for above-ground food crops. Frequently a buffer zone is provided around a spray irrigation site to allow for drifts of aerosols.

Shallow irrigation systems were developed in North Carolina, and consist of a buried distribution system shallow enough that water is available to plants. No more than primary treatment is required upstream, but root crops grown for human consumption should not be the irrigated crop.

Drip irrigation systems consist of a pressure dosing pump, backwashable filter, flow meter, Process Logic Controller (PLC), drip irrigation lines, and valves on each lateral to control flow. Drip emitters may be pressure compensating or non-pressure compensating. Some designers consider that upstream disinfection is also necessary to prevent plugging of such systems (Perkins, 1989). Sites with slopes up to 45% have been serviced in this manner.

Rapid infiltration beds consist of shallow spreading basins in permeable soils to which wastewater is intermittently applied and discharges either to groundwater or to underdrains. Deep permeable soils are required. Upstream aerobic and facultative lagoons are not recommended unless effluent suspended solids are controlled, as algae from the lagoons can clog the infiltration surfaces.

Evapotranspiration beds can be installed where there is net positive annual evaporation. They are used where there is high ground water, a low permeability soil, shallow fractured bedrock, or percolation is disallowed by a regulatory agency.

Liquid effluent disposal to rivers and ocean

Disposal to natural wetlands is generally only used after secondary treatment and possibly also tertiary treatment such as polishing in a constructed wetland. A high level of treatment is required to avoid damaging the natural ecosystem. The Northwest Regional Wastewater Treatment Facility in Seminole County, Florida uses natural wetland effluent disposal, as well as reclaimed water irrigation and rapid infiltration basins.

Outfall discharges are the most common method of disposal for communities located near a surface water source such as a river, lake or ocean. Design of new outfalls generally requires detailed current analysis, water temperature and salinity stratigraphy, as well as hydraulic and plume modelling. Most main outfalls in lakes or ocean environments are installed below a minimum depth to prevent surfacing of effluent.

Overflows from older combined sewer systems, originally intended to operate intermittently under extreme storm conditions, have proven to be a problem in maintaining receiving body water quality throughout North America. Most of the combined sewer overflows (CSOs) are located in large communities: for example of the 13,000 CSOs along the Ohio River, more than 75% of discharges are from 10 large cities. (WET Mar. 1998, Shamsi, U.M. "Thinking Small")
Overflows from the systems have become more common as communities have grown, and have become less acceptable as standards of environmental protection have risen. Combined sewer overflow control is now required in the United States, and communities are required by the US EPA to produce plans to reduce CSOs and remove floatable and suspended solids from such flows before discharge. Most older communities in North America are presently dealing with this problem, largely through the elimination of CSOs through sewer separation.

Sludge disposal

Increased cost of traditional sludge disposal options, especially landfilling, plus increased encouragement from the US EPA have substantially increased beneficial reuse of treated sewage sludge (biosolids) in the U.S. Regulation changes, such as prohibition of ocean dumping of sludge, has provoked cities such as New York to search for other options. The dumping of sludge into the ocean falls under provincial jurisdiction in Canada, and is not allowed in most provinces. Although beneficial reuse is encouraged, if lower cost options present themselves and are allowable by local regulation, they are generally taken. For example, dramatic declines in landfill transport and tipping fees in Philadelphia, Pennsylvania caused that city to change from nearly 100% beneficial use (land application and composting), to almost 50% landfilling.

Of the over 6.8 million dry tons/y of biosolids produced in the U.S., 55% is land applied, 19% is surface disposed including co-landfilling with solid waste, monofilling and permanent disposal in piles or lagoons, 17 % is incinerated, and 9% is either in long term storage such as in wastewater treatment ponds, or is hauled to other states for mostly land application and surface disposal. (derived from data in Bastian, 1997)

Land application

Land application includes application of liquid, dewatered cake, dried, composted, alkaline stabilized or otherwise processed product to cropland, forests, reclamation sites, and lands as organic fertilizer or soil amendments, or used in potting mixes and the production of topsoil, including daily or final landfill cover. Prevention of aesthetic problems, mainly odours, truck traffic and dust, is important for acceptability of a land application program.

Distribution and marketing of biosolids is now a big issue. Compost has been used for potting and horticultural mixes for green houses, nurseries and retail, soil replacement at field nurseries and sod farms, blending to produce topsoil, turf establishment at new developments, top dressing for golf courses and other institutions, land reclamation at landfills, mine and gravel pit operations, and landfill cover. Forest application of biosolids is practiced in Washington State and British Columbia.

Where sludge has been applied in excess of agronomic rates and no crop is grown, for example at reclamation sites, the method is called "dedicated land disposal".

Landfilling

Dewatered and dried sewage sludge is disposed of to both dedicated municipal sewage sludge landfills, and co-disposed with municipal solid waste. Sewage sludge monofills in the U.S. must meet strict siting criteria to prevent environmental impact, and must be provided with leachate and gas migration control.

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