ITT's Place in the Cycle of Water - Water Solutions

The Cycle of Water: Wastewatersecondary treatmentPrimary treatment alone is seldom sufficient to meetthe demands for environmental protection. The primarypurpose of secondary treatment is to remove thesoluble organic material, (biochemical oxygen demand- BOD) that escapes primary treatment and to providefurther removal of suspended solids.Secondary treatment typically utilizes biologicaltreatment processes whereby microorganisms convertnon-settling solids into a form in which they canbe removed. A proportion of the resulting “sludge”is returned to the aeration stage to maintain highconcentrations of organisms. Recently, membranefilters have been used in place of clarification tanksto separate solids from the wastewater flow. Thesesystems, known as Membrane Bioreactors (MBR) areconsidered to provide various advantages comparedto conventional activated sludge with clarification,including smaller footprints, better effluent qualityand easier operation.The microorganisms are sustained by the oxygenlevels found in the wastewater stream. Oxygen maybe transferred from air into water by pressurizeddiffused aerators or self-aspirating mechanical devices.The proliferation of the microorganisms drives thebiodegradation of the organic compounds found in thewater, and subsequently forms harmless by-products,which include water, carbon dioxide and solids thatsettle out of the wastewater. The activated sludgeprocess is typically continuous; however, a batch typevariation (known as a Sequential Batch Reactor system– SBR) can be suitable where influent loads vary and/orwhere enhanced control/treatment flexibility is requiredto ensure high effluent quality. To meet ever-increasingstandards, removal of TSS and BOD from wastewaterlagoons can be accomplished with a Dissolved AirFlotation system.Although the activated sludge process is nowcommonplace, secondary treatment is also undertakenby trickling filters, where microorganisms areencouraged to grow on beds of granular media (ormodern engineered alternatives that provide a highsurface area). Air currents are passively induced in thebeds providing the microorganisms with the oxygenrequired to metabolize the pollutant load. Wastewateris trickled over the surface of the beds using rotatingarms, with excess microorganisms being scoured offby the flow and subsequently allowed to settle out ina sedimentation stage (referred to as humus tanks).Although trickling filters have low operating costs,their considerable land requirements often make themunattractive compared to the more intense activatedsludge process. A fairly recent development insecondary treatment techniques, known as BiologicalAerated Filters (BAF), combines elements of theactivated sludge and trickling filter processes. The BAFprocess employs submerged media within an activatedsludge-type aeration vessel, which gives the benefitof a surface upon which microorganisms can reside.BAF processes are claimed to be more efficient thanalternative techniques.CASE STORY:ITT SBR Provides Large Cleanupon Small Dublin Bay SiteOne of the largest capacity wastewatertreatment plants in Europe is on a small plotof land at Ringsend on Dublin Bay in Ireland.Sequencing Batch Reactor (SBR) technologyand 100,000 diffusers from ITT’s Sanitairebrand were selected to meet the challengingwaste treatment needs for this site.43