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MWMC FACILITIES PLAN the influent flow requirements. A biofilter is used to remove air odors at 13,000 ft3/min e(City of Eugene, 2004). The current capacity of the preliminary treatment facilities ranges from 175 to 210 mgd, and is limited by the influent screening capacity (see "Peak Flow Management Alternatives" technical memorandum in the appendix). Primary Treatment. During primary treatment, solids and scum are removed through settling and skimming (City of Eugene, 2004). Primary clarifiers thicken the sludge to 4 to 6 percent solids. The thickened sludge is pushed by rake arms into a hopper, where the sludge is transferred by air-operated diaphragm pumps to the primary (anaerobic) digesters (Management Plan for a Dedicated Biosolids Land Application Site [CH2M HILL, 2003]). The effective peak flow primary treatment capacity is approximately 90 mgd with all four primary clarifiers in service, although the facility moves in excess of 200 mgd through the clarifiers during peak flow events. Secondary Treatment. Secondary treatment at the WPCF consists of aerating and recycling bacteria, and converting fine particles and dissolved organic matter into settleable solids (City of Eugene, 2004). Secondary treatment facilities consist of eight aeration basins and eight secondary clarifiers. During secondary treatment, sludge can be thickened in one of two ways, co-thickened by mixing WAS with primary influent and settled in primary clarifiers or thickened directly at the GBT. The activated sludge flow can be routed to the grit channels to be mixed with primary influent or the WAS is combined with polymer and applied to the gravity filter belts, then thickened to 4 percent dry solids. After thickening, the waste activated sludge is pumped to the anaerobic digestion process. During the anaerobic digestion process, sludge is inoculated with bacteria and heated to approximately 36°C. Bacteria reduce the organic material to water, carbon dioxide, and methane gas. The process takes 15 to 35 days, depending on the amount of sludge pumped into the digesters. The methane gas is used to produce heat and electrical energy, which is used in the operation of the Regional Treatment Facihty. The current peak flow secondary treatment capacity is estimated at 103 mgd and is limited by secondary clarification. PE flows over 103 mgd are diverted around secondary treatment through a diversion conduit and recombined with the SE prior to disinfection. Disinfection. Disinfection, the final wastewater treatment stage at the WPCF, consists of disinfection by chlorine, followed by addition of sulfur dioxide to dechlorinate. Disinfection facilities consist of gaseous chlorine, four chlorine contact basins, and gaseous sulfur dioxide. The peak flow disinfection capacity, estimated at 215 mgd, typically is limited by the chlorinators, although hydraulic restrictions based on river level may limit capacity to 175 mgd. 2 The biofilter has a peak capacity of 18,000 cfm. 9-2 MWMC_9.0_REV14.DOC