New Solids Processing Facility Helps WRRF Achieve Nutrient Reduction and Solids End/Use Disposal Goals

INTRODUCTION The City of Edmond owns and operates the Coffee Creek Water Resource Recovery Facility (CCWRRF) that treats domestic sewage and waste flows from its service area population. The treatment facility includes preliminary treatment (screening/grit removal), secondary treatment (aeration basins and clarifiers), tertiary treatment (filtration/disinfection), and solids stabilization (sludge lagoons). CCWRRF is currently permitted for an annual average flow (AAF) of 9 million gallons per day (mgd) but needed expansion based on population projections. Furthermore, plant modifications were required to meet stricter discharge limits, including a new regulatory limit on nitrate. The CCWRRF expansion includes improvements to the liquid treatment and solids processing facilities that will be divided into two phases. The Phase 1 Expansion is currently under construction and includes improvements to increase the AAF capacity to 10 mgd and meet the new regulatory limit on nitrate. Modifications include new external anoxic zones and a mixed liquor recycle system to operate the secondary treatment facility with a Modified Ludzack-Ettinger process. The Phase 2 Expansion will include improvements to increase the AAF capacity to 12 mgd and meet new effluent limits as they are enforced. The Phase 1 Expansion is anticipated to be completed in early 2024 but several elements are already in operation. DISCUSSION The Phase 1 Expansion modifications to the solids processing facilities were key to meet the new regulatory limits because the recycle streams from the sludge lagoons returned a large ammonia load to the liquid treatment train. The Phase 1 Expansion improvements will decommission the sludge lagoons and replace them with a new train that includes sludge screening, sludge thickening, aerobic digestion, sludge dewatering, and cake storage. Figure 1 presents a process flow diagram of the new solids processing facility. Overall, these modifications will increase the biosolids quality by removing trash and debris, decrease the nitrogen load in the recycle streams, and provide end use/disposal operational flexibility. WAS is pumped from the secondary clarifiers to two in-line screens that remove debris upstream of the aerobic digesters. The screened sludge flows to two digester trains. Figure 2 presents a photograph of the new aerobic digestion facility. The digesters incorporate features to overcome existing and anticipated future operational challenges: - Two-stage configuration that reduces the required solids retention time (SRT) required to meet Class B by minimizing the potential for short-circuiting. - Flexible tank configuration to provide redundancy and reliability by allowing for any basin to be taken out of service while the remaining basins remain in operation. - Each tank is equipped with coarse-bubble diffusers and a vertical shaft mechanical mixer to decouple aeration and mixing, allowing operation with simultaneous nitrification and denitrification (SND). This process reduces ammonia loads in the recycle streams while preventing process failure associated with pH depletion. - Recuperative thickening system to decrease the required aerobic digester tanks volume and prevent overheating of the digesters due to exothermic reactions during warm weather. - Foam abatement spray bars to provide relief during digester foaming episodes. Sludge from the second stage is pumped to a thickening facility that includes two membrane thickening tanks. Each tank houses six flat-sheet submerged membrane units and an air scour system. The thickening system also includes a clean-in-place system and three progressive cavity permeate pumps (2 duty and 1 standby). Air scouring is provided by three (2 duty and 1 standby) hybrid positive displacement blowers, each with a capacity of 4,400 scfm. The thickened sludge flows to the first stage aerobic digester basins and the permeate is pumped to the stormwater ponds. Membrane thickening presented a lower lifecycle cost than mechanical thickening technologies. Unlike most mechanical thickening technologies, membrane thickening does not require polymer, which represented a major operation and maintenance cost. The digested sludge is pumped from the second state aerobic digesters to a new dewatering facility that includes two 3-meter belt width belt filter presses. Figure 3 presents a photograph of the new dewatering building. The dewatered cake is conveyed to a biosolids storage pad using a belt conveyor. The biosolids are stored during non-growing season and then a hauled to Class B land application sites. The secondary treatment improvements were commissioned in October 2021 and have been performing well with average discharge concentrations of 0.08 mg/L ammonia nitrogen (NH3-N) and 6.6 mg/L nitrate nitrogen (NO3-N). The solids screening, thickening, and digestion units were commissioned in August 2023. The anaerobic digesters have been operating with SND throughout the startup period. The facility startup presented challenges including aeration turndown before the digester total solids concentration reached the target concentration of 2.0%. Initially, the air flow was too high and needed to be reduced and balanced as the total solids concentration increased. In addition, there were challenges optimizing the membrane thickening operational parameters. The dewatering facility will be commissioned in December 2023. This presentation will cover the highlight the operational challenges and the startup and commissioning of the membrane thickening, aerobic digesters, and dewatering facilities. Process and operations data collected during the commissioning period will also be presented.