MATERIALS: 7,000, LABOR: 9,000; SLUDGE BULKING CONTROL: PRICELESS

The City of Billings Montana Wastewater Treatment Plant (WWTP) is a 26 million gallon per day (MGD) activated sludge process with current average flows around 16 MGD. The plant has had a historic filamentous sludge bulking problem that in recent years necessitated chlorinating return activated sludge (RAS) to keep the sludge volume index (SVI) under control.The aeration basin configuration, shown in Figure 1, included a selector (Zone 1) which would act either anoxic or anaerobic depending on whether the plant was nitrifying or not. The selector was not effective. Zone 1 is mixed with a vertical turbine mixer. In an attempt to better control filamentous bulking and improve sludge thickening properties it was decided to experiment with an additional anaerobic selector at full scale in one of the aeration basin treatment trains.Converting Zone 2A to an anaerobic reactor, in addition to Zone 1, resulted in an average two zone selector HRT (hydraulic retention time) of 37 minutes with two secondary treatment trains operating. To cost effectively convert Zone 2A to an anaerobic reactor plant, staff constructed the modifications using nonpermanent materials. A summary of the conversion to an anaerobic selector mode included:An aluminum plate to block off one overflow weir in the existing Zone 1 concrete baffle wallAluminum plates to block off submerged openings in the Zone 2A baffle wallsWood baffle walls attached to the walls and the old mechanical aerator supports to promote plug flow in the Zone 2ARemoval of fine bubble diffusers in Zone 2A to facilitate using low air flow for mixingAdding membranes to spare diffusers in Zone 2B to handle the higher anticipated loadings in that zone.Air piping modifications to enable the diffuser grid changes.The initial operation of the anaerobic selector was somewhat successful with two aeration basin treatment trains operating. However, when a third aeration basin treatment train was added to experiment increasing the HRT, the SVI measurements increased to the 500-600 range. The theory was that the air being added in Zone 2A for mixing, while reduced to a minimum, was still too high and was resulting in localized aerated zones. To diminish this potential effect the operators decreased the air flow to Zone 2A incrementally while visually checking to see if the solids were staying in suspension. Eventually the air was shut-off completely with no detrimental effects.With the Zone 2A air shut off, the SVIs began to decrease and phosphorous release in the selector and uptake in the aeration basin increased. Since the air was shut off, no chlorine has been used for filamentous bulking control in the aeration basin treatment train with the new selector and SVIs have consistently been around 150-200.The project success was not only measured by decreased filamentous sludge bulking and improved sludge thickening, but also by the cost effective construction which was approximately 7,000 in materials and 9,000 in labor.