b'shaft to further facilitate the metabolization ofradiation, which can disinfect without leaving any organics in the wastewater. residual in the euent, is becoming more competitive with chlorine as a wastewater disinfectant.Tertiary treatment is only used in special cases where more than 85 percent of total solids and BOD must beThe processes described above, to varying degrees, removed, or when dissolved nitrate and phosphateare essential to public sewer systems. However, many levels must be reduced. The typical tertiary processesIdahoan homes and buildings are not served by sewer used includes euent polishing, land treatment, and/ systems and thus require on-site wastewater systems, or removal of plant nutrients. The tertiary processcommonly called septic systems, to treat and dispose can remove more than 99 percent of all impuritiesof wastewater. Septic systems discharge wastewater from sewage; however it can be very expensive, ofteninto an underground tank located on the property, doubling the cost of secondary treatment. where solids and water are then separated. Solids settle to the bottom of the tank where bacteria break down Prior to discharge of the sewage euent into a bodyorganic matter. Euent then ows from the tank into a of surface water, the last step is disinfection, whichdrain eld of underground pipes surrounded by gravel destroys any remaining pathogens in the euentand soil. The pipes slowly release the water, while and protects public health. Disinfection is usuallythe gravel and soil lter out remaining contaminants accomplished by mixing the euent with chlorinebefore it reaches ground water. Residual sludge in the gas or with liquid solutions of hypochlorite chemicalstanks requires periodic pumping.5in a contact tank for at least 15 minutes. ChlorineCLEAN WATER ACT PERMITSresiduals in the euent may have adverse eects on aquatic life, so an additional chemical may be addedWastewater treatment plants must receive permits to dechlorinate the euent. Alternatively, ultravioletprior to discharging wastewater. These permits, Wastewater TreatmentFIGURE 3. Primary and Secondary Wastewater Treatment 4Wastewater 125'