Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
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
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
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
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.
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 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
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