slow rate (SR) land treatment is the controlled application of wastewater to vegetated land surface at a rate typically measured in terms of a few centimeters of liquid per week (see Figure 1).The design flow path depends on infiltration, percolation, and usually lateral flow within the boundaries of the treatment site.
Treatment occurs at the soil surface and as the wastewater percolates through the plant root-soil matrix. Depending on the specific system design, some to most of the water may be used by the vegetation, some may reach the groundwater, and some may be recovered for other beneficial uses. Off-site runoff of any of the applied wastewater is specifically avoided by the system design4.
The hydraulic pathways of the applied water can include:
Vegetation irrigation with incremental percolation for salt leaching
Some vegetative uptake with percolation the major pathway
Percolation to under drains or wells for water recovery and reuse
Percolation to groundwater and/or lateral subsurface flow to adjacent surface waters
Wastewater applications can be via ridge and furrow or border strip flood irrigation or with sprinklers using fixed nozzles or moving sprinkler systems. The selection of the application method is dependent on site conditions. The surface vegetation is an essential component in all SR systems.
Slow rate land treatment can be operated to achieve a number of objectives including:
Treatment of the applied wastewater
Economic return from the use of water and nutrients to produce marketable crops
Exchange of wastewater for potable water for irrigation purposes in arid climates to achieve overall water conservation
Development and preservation of open space and greenbelts
These goals are not mutually exclusive, but it is unlikely that all can be brought to an optimum level within the same system. In general, maximum cost-effectiveness for both municipal and industrial systems will be achieved by applying the maximum possible amount of wastewater to the smallest possible land area. That will in turn limit the choice of suitable vegetation and possibly the market value of the harvested crop. In the more humid optimization of treatment is usually the major objective for land treatment systems5.
Optimization of a system for wastewater treatment usually results in the selection of perennial grasses because a longer application season, higher hydraulic loadings, and greater nitrogen removals are possible compared to other agricultural crops.