Eugene/Springfield Water Pollution Control Facility 1984-1991

New Search

Page 22 Section D: PLANT OPBRATION5 cesses. Support equipment includes channel aera- tion blowers, secondary diversion flow control gates, andheavysolids/secondaryscum/drainpump stations. Most of the secondary treatment equip- mentand functions are monitored and controlled by the plant process computer, including the re- turn activated sludge flow rate, waste activated sludge flow, secondary diversion flow, and the aeration blower operation. The activated sludge process is one of the most critical and sensitive plant processes. Effectively removing suspended solids and BOD to meet al- lowable discharge levels depends almost entirely on successfully operating the activated sludge pro- cess. The Operations staff monitors and controls the process through inventory, tracking, and control of solids in the system. The amount of solids is determined by calculating sludge units. Sludge detention time is usually figured by sludge age and total sludge units. Inventorying solids through the sludge unit system is faster than using the pound system. PERFORMANCE: Secondary treat- mentfacilities thatusethe activatedsludge process typically achieve at least 90 per- centremoval of BOD andTSS.The plant's operation has achieved an annual aver- age BOD removal of 94 percent and an average TSS removal of 95 percent. • Aeration Basins The activated sludge process uses eight, 2.2-MG aeration basins. Permanently mounted sensors continuously monitor dissolved oxygen in the ba- sins and relay readings to the computer. Originally, the basins were equipped with coarse airbubblediffusers.In 1987,fourof theeightbasins were retrofitted with fine air bubble diffusers to improve oxygen transfer efficiency and reduce energy costs. Thebasins are configured toprovidefouractivated sludge treatment modes: complete mix, plug flow, step feed, andcontact stabilization. In the firstyear of plant operation, operators experimented with each mode to determine feasibility and efficiency. Although all modes provided satisfactory treat- ment results, plug flow has been the most stable. The plug flow mode is usually used during dry weather flows and contact stabilization during wet weather flows. In the past, contact stabilization was preferred during peak flows because of its ability to maintain solids that would be washed through the secondary system. Operations have since found plug flow satisfactory in the winter. Peak secondary flows in the winter were expanded to 104 MGD. • Secondary Clarifiers The plant has eight, 130-foot-diameter circular secondary clarifiers with doubled-welted launders to handle low-surface overflow rates and draft tubes to withdraw return sludge. • Return. Activated Sludge Pumps Return activated sludge is pumped from a wet well at the Secondary Treatment Complex by four vertically mounted, mixed-axial-flow centrifugal pumps. Two of the pumps have a 13 MGD capacity each and are driven by 125-HP electric motors. The other two pumps each have 23 MGD capaci- tiesand are driven by 200-HP motors with variable speed controls. To date, the smaller return sludge pumps have provided all the pumping capacities needed. The larger pumps are used in winter months because of their higher energy demands and larger flow capa- bility. • Channel Aeration Blowers The large mixed liquor channels in the aeration basin and secondary clarifier areas have a com- bined capacity of over 500,000 gallons. This vol- ume requires that channels be aerated continu- ously to keep solids in suspension and to maintain aerobic conditions until solids reach the secondary clarifiers. Originally, the channels were aerated by three 150-HP centrifugal airblowers. In 1987, the mixed liquor channel aeration system was tied into the aerationbasin air supply line. Using the additional available capacity of the aeration blowers to aerate the mixed liquor channels allowed the channel aeration blowers to be shut down, reducing energy consumption and expense.