b'Boise Wastewater Treatment FacilityFIGURE 2. An advanced wastewater treatment facility in Boise (left) and a lagoon system in Driggs (right).There is also a growing trend of smaller communitiesof screening, comminution, grit removal, and using euent (treated) wastewater for agriculturalsedimentation. These actions remove most solids in purposes as a low-cost and environmentally friendlythe water, preparing it for secondary treatment.4strategy. A large variety of treatment plant capabilities exist throughout the state. Large cities, like Boise,Secondary treatment removes the soluble organic have facilities capable of treating large daily inowsmatter and suspended solids that escape primary of sewage with advanced mechanical and biologicaltreatment. This is usually done by biological processes treatment methods. Smaller cities use less complex,in which microbes consume organic impurities and scaled-down facilities, such as simple lagoon systems,convert them into carbon dioxide, water, and energy to treat sewage (See Figure 2 for comparison).3 for their own growth and reproduction. These are the primary biological treatment methods:The size and capacity of wastewater treatment1. Trickling lter tanks lled with a deep bed of systems are primarily determined by the estimatedstones, which collects bacteria and settling tanks volume of the total sewage generated as well as thethat remove additional microbesanticipated inows and inltration; other factors2. The activated sludge process, which involves an include the number of customers, geographicalaeration tank followed by a secondary clarier; scenarios, site constraints, sewer connections, averagethis stage forms a suspension of biological solids and peak ows, technological feasibility, energycalled activated sludge, which absorbs dissolved consumption, and operation and maintenance costsorganics from the sewage, reducing BODinvolved. The three levels of wastewater treatment3. The oxidation ponds, also called lagoons or are primary, secondary, and tertiary treatmentstabilization ponds, are large, shallow ponds (see Figure3). Primary treatment removes aboutdesigned to treat wastewater through the 60 percent of total suspended solids and aboutinteraction of sunlight, bacteria, and algae35 percent of biochemical oxygen demand (BOD), though dissolved impurities are not removed at thisThough less common, biological contactors utilize stage. This rst stage includes the physical processesa rotating series of discs mounted on a horizontal 124 I D A H O I N F R A S T R U C T U R E G U I D E B O O K'