Improving Wet Weather Treatment Capacity by Converting a Two-Stage Activated Sludge Process to a Single-Stage Process

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ABSTRACT
The Western Virginia Water Authority (WVWA) operates a 42 mgd wastewater treatment facility which treats flows from the City of Roanoke and adjoining municipalities. In recent years, the WVWA WPCP has experienced peak wet weather flows exceeding 140 mgd, well above the plant’s existing peak flow capacity. Under these conditions, the plant is forced to rely heavily on flow equalization facilities, as hydraulic and process constraints limit the flow to the secondary treatment facilities. Equalization facilities include two basins in series. Influent flows which exceed the secondary process capacity are pumped to the first equalization basin. When the first basin is full, it overflows into the second basin. The overflow into the second basin is disinfected with sodium hypochlorite. When the second basin is full, the disinfected effluent discharges to the river. In order to minimize wet weather overflows, the current expansion addresses several unit process improvements targeted at increasing wet weather flow capacity including activated sludge process improvements.

The existing activated sludge process is a two stage process consisting ten aeration basins, ten secondary clarifiers, six nitrification basins, and six nitrification clarifiers. The treatment capacity of the activated sludge process is limited primarily by the performance of the secondary and nitrification clarifiers. Field testing indicated that the maximum capacity of the existing firststage clarifiers and second stage clarifiers was approximately 40 mgd (1000gpd/sf) and 48 mgd (800 gpd/sf) respectively. A bypass line provides the ability to bypass flows in excess of the first stage clarifier capacity directly to the second stage aeration tanks. The hydraulic capacity of this bypass line is approximately 6 mgd. The existing clarifiers also suffer from several deficiencies including poor flow distribution, limited settled sludge flow capacity, and the inability to control the settled sludge flow from individual clarifiers. These deficiencies resulted in frequent loss of solids in both first and second stage clarifiers under high flow conditions.

Two alternatives were considered to improve peak flow capacity of the activated sludge facilities: (1) maintain the two stage biological process configuration, increase first stage bypass capabilities, and construct additional second stage clarifiers or (2) convert the activated sludge process to a single stage biological process configuration, allowing the second stage tankage to function in parallel with the first stage tankage. These two process approaches were evaluated
based on peak flow capacity, ease of operation, and capital and operating costs. After reviewing the results of this evaluation, the WVWA determined that the activated sludge process should be converted to the single-stage, configuration.

INTRODUCTION
The Western Virginia Water Authority (WVWA) operates a 42 mgd wastewater treatment facility which treats flows from the City of Roanoke and adjoining municipalities. Major unit processes include conventional preliminary and primary treatment followed by a two-stage activated sludge process, chemical phosphorus removal facilities, tertiary settling, filtration and chlorine based disinfection. Solids treatment includes gravity and DAF thickening prior to anaerobic digestion. The treatment facility is currently being expanded to 55 mgd and upgraded to enhance process operation. The effluent permit limits for the WPCP are shown in Table 1.

The WVWA WPCP has consistently met permit limits under normal flow conditions. However, in recent years, the WPCP has experienced peak wet weather flows exceeding 140 mgd, well above the plant’s existing peak flow capacity. Under these conditions, the plant is forced to rely heavily on flow equalization facilities, as hydraulic and process constraints limit the flow to the secondary and tertiary treatment facilities. Figure 1 shows a simple process schematic for the Roanoke Regional WPCP. Equalization facilities include two basins in series. Influent flows which exceed the secondary process capacity are pumped to the first equalization basin. When the first basin is full, it overflows into the second basin. The overflow into the second basin is disinfected with sodium hypochlorite. When the second basin is full, the disinfected effluent discharges to the river. In order to minimize wet weather overflows, the current expansion addresses several unit process improvements targeted at increasing wet weather flow capacity including a new 160 mgd influent pump station and preliminary treatment facilities, additional primary clarifiers, expanded equalization pumping capabilities, activated sludge process improvements, and effluent filter improvements. This paper addresses the proposed improvements to the activated sludge process.

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