Spray irrigation of pretreated wastewater for residential scale systems is a final treatment and disposal method which is often called slow rate land treatment for larger scale systems. Depending upon local regulatory policies and requirements, it may be an option where conventional or other types of onsite subsurface disposal methods are not feasible due to site conditions. This type of system must be designed by a licensed professional engineer.
Prior to surface application of the wastewater through a sprinkler system, both secondary pretreatment and disinfection must be achieved. The treated wastewater is applied evenly to a vegetated area on the residential lot. A means for preventing surface runoff of the sprayed effluent, and for diverting rainwater runoff away from the spray field area, may be required for these systems.
Spray irrigation allows for the greatest amount of evaporation as compared with other on-site effluent disposal methods. Some volatilization of ammonia nitrogen is also usually achieved, which contributes to total nitrogen removal. Treatment is achieved through a combination of infiltration and soil treatment processes, plant uptake of nutrients, and "attached growth" treatment from the vegetated ground cover where the wastewater is applied. Moisture is removed by both evaporation and infiltration. Spray systems are usually time-dosed based systems. Spray application occurs during early morning hours so that use of the spray area is not affected.