Wastewater Treatment Process

Treatment Processes:

  • Preliminary Screening – Influent flow passes through screens which are intended to remove large material before they enter the treatment plant; this would include rags, branches, Styrofoam, tennis balls, plastic articles and miscellaneous debris. The material is removed to protect downstream equipment from being plugged or damaged. The turbulence in the collection sewer breaks up fecal matter and other biodegradable organics (wastewater solids) sufficiently that they are not retained on the screens. Screened material is dewatered before being stored in waste containers for offsite hauling.
  • Grit Removal – The grit removal system is provided to remove inorganic material such as sand and gravel from the wastewater, to protect the downstream equipment from accumulating sediment and to reduce damage that can be caused by the abrasive action of the inorganic material. Fecal matter and other biodegradable organics (wastewater solids) are not intended to be removed in the grit removal process. If excess organics are removed, the grit waste containers will be contaminated with this material and become odorous.
  • Primary Clarification – The purpose of primary clarifiers is to continuously remove nearly all settleable solids from the wastewater. Primary clarification is a cost effective removal method for solids in wastewater held in suspension by flow velocity. Particles in raw wastewater will form heavier particles that will settle by gravity under quiescent conditions. In addition, any grease and scum will float to the surface of the clarifier. Settled and floatable solids are conveyed to thickening for further treatment. Primary clarified effluent is conveyed to the aeration basins for further treatment.
  • Secondary Treatment – the secondary treatment system is a biological nutrient removal system that includes aeration basins and secondary clarification. The purpose of the aeration basin is to perform the following functions: (1) create biological conditions to further consume carbonaceous waste from the influent stream, (2) convert influent ammonia to nitrate through nitrification and, (3) convert the nitrate to nitrogen gas through denitrification, (4) nitrogen gas is released to the atmosphere, and (5) reduce pathogenic organism counts in the influent stream. The system performs this task by biologically converting the dissolved and finely divided organic solids into settleable solids called Mixed Liquor Suspended Solids (MLSS) within the aeration basin. The resulting biological solids flow to secondary clarification, where it is allowed to flocculate into large solids, settle and subsequently returned to the Aeration Basins. This is referred to as Return Activated Sludge (RAS). The MLSS includes microorganisms that continually multiply and therefore, they must be removed from the secondary treatment system to maintain a balanced ratio with the influent organic matter for proper treatment. This is accomplished by means of wasting RAS or MLSS from the treatment system. In addition any grease, and other light microorganisms float to the surface of the clarifier are removed as scum.
  • Tertiary Treatment – The tertiary treatment system includes filtration and disinfection. Filtration removes the minor amount of suspended solids that escape settling in the secondary clarification. Water is separated from these suspended impurities by passing the water throughorous filter media. The water may also receive preparatory treatment prior to its application to the filters, including chemical addition, rapid mix, and flocculation. Disinfection of the filtered effluent is accomplished by injecting a chlorine solution in the form of sodium hypochlorite (bleach) to filtered effluent. The disinfection system continuously injects bleach for elimination of remaining microbial pathogens in the filtered effluent. Pathogenic microbes are exposed to chlorine at an elevated concentration for a significant time period. The chlorine solution disrupts microbial cell membranes, destroying the microorganism by chemical oxidation of molecular bonds in enzymes and proteins used for cell metabolism. When enzymes do not function properly, a cell or bacterium will die. This process is rapid but not instantaneous. Therefore in order to ensure that all microorganisms are exposed or contacted with chlorine, the chlorination reaction occurs in a long residence time contact basin.
  • Recycled Water and Dechlorination – From the effluent of the contact basins, disinfected tertiary recycled water is pumped into the distribution pipeline network to deliver recycled water to the facility for internal use, industrial users, irrigation, and groundwater recharge. Water not pumped into the recycled water distribution pipeline network is dechlorinated with sodium bisulfite prior to being discharged to an outfall point. The facility outfalls may discharge to creeks, rivers, or lakes.
  • Solids Thickening – Solids removed from primary clarification and secondary treatment is thickened prior to anaerobic digestion. Settled primary solids are conveyed to the gravity thickening process. The solids concentration is increased from approximately 1% total solids to at least 4% total solids. Thickened solids are pumped to the anaerobic digestion process and removed liquid from both processes are reintroduced towards the beginning of the treatment plant for liquids treatment. The gravity thickening resembles and operates much like a primary or secondary clarifier in which the solids are settled by the force of gravity to the bottom of the tank, forming a concentrated solids blanket. Gravity thickening is one of the most common sludge concentration processes, due to the relatively inexpensive operation and maintenance of the system. Floatable primary solids and secondary biological solids are conveyed to the dissolved air flotation thickening process. Solids concentration within the dissolved air flotation thickener is accomplished by forcing the solids to float to the top of the thickening unit and removing the relatively clear water from below the solids. Floating the solids is achieved by injection of air, which lifts the sludge particles as it bubbles to the surface of the liquid. Secondary sludge solids are only a little denser than water, and therefore they float easily with attached air bubbles.
  • Anaerobic Digestion – After being thickened by the gravity thickening process and the dissolved air flotation thickening process, the solids removed from the primary and secondary treatment are subjected to the process of anaerobic digestion. This process has three major purposes: sludge stabilization, pathogen destruction, and sludge volume reduction. Anaerobic digestion is accomplished by retaining the thickened solids in the absence of oxygen within the large, airtight digestion tanks for an average detention time of 20 to 25 days. Within these tanks, microorganisms digest the sludge and convert the solids into cells and gaseous byproducts, resulting in a stabilized sludge and a significant volume reduction. To increase the rate at which he biological digestion process takes place, the digester contents are heated to 100 to 127 degrees Fahrenheit. The gases that are produced are collected and used as a fuel source in the energy recovery process. The stabilized solids are conveyed to the dewatering process.
  • Dewatering – The dewatering process is the final step of wastewater solids treatment. Following the anaerobic digestion process, solids are dewatered by belt filter presses or centrifuges. The purpose of the dewatering process is to remove a substantial portion of the water from the stabilized solids. Furthermore, by decreasing the volume, sludge transportation and disposal costs are reduced. The liquid removed is pumped outside of the water recycling treatment plant, due the liquid’s high concentration of ammonia which would negatively affect the biology in the secondary treatment process.