Thickening Impacts and Optimization when Transitioning to BNR -- Salt Lake Case Study

Background: The paper will present a case study on thickening impacts when implementing biological nutrient removal (BNR) at the Salt Lake City Water Reclamation Facility (SLCWRF) as part of a $750+ million-dollar plant upgrade. The 48 MGD trickling filter / activated sludge plant is being upgraded to a BNR facility using the Westbank process. The new facility is being constructed in the area that previously housed the sludge drying beds (Figure 1). The upgrade will include new primary sludge thickeners that will be operated as fermenters to provide volatile fatty acids (VFA's) to the BNR facility. The existing rotary drum thickeners (RDT) will continue to be used for waste activated sludge (WAS) thickening. Primary Sludge Thickening: SLCWRF currently uses gravity thickening for primary sludge, but the existing thickeners do not meet current seismic codes and are not sized appropriately to be operated as fermenters. The existing gravity thickeners are also not located in an ideal location at the site in comparison to where the new BNR facility will be located. For initial construction two 67-foot-diameter primary sludge thickeners will be used, designed for the maximum month total suspended solids load. This is based on requiring one in service for the annual average condition, and two for the maximum month condition. Under average conditions, the solids retention time (SRT) will be 6 days with two primary sludge thickeners in operation and 3 days with one out of service. These SRTs are considered adequate for volatile fatty acid generation for the biological phosphorus removal process. The site layout also accounts for space for two future gravity thickeners to allow for future plant expansion. The new gravity thickeners are under construction as shown in Figure 2. WAS Thickening Four existing RDTs, as shown in Figure 3, were installed in the last ten years and will continue to be used to thicken the WAS from the new BNR process. Each RDT is rated for 400 gpm and the building includes space to add a fifth RDT to allow for future expansion. The RDTs are used to increase the WAS concentration from an average of 0.5 percent total solids (TS) to approximately 5 percent TS. RDT filtrate will be pumped to the new WAS gravity thickener supernatant pump station, using the existing filtrate pumps. After being combined, the two supernatant streams will be pumped to the primary clarifiers. The RDT sludge will be pumped to the existing digesters for stabilization, using the four existing sludge pumps. Reusing the existing WAS thickening, however, resulted in several challenges for the new BNR facility. These challenges were due to existing issues with the rotary drum thickeners as well as challenges integrating it with the new BNR facility. A first challenge related to the original design intent to implement surface wasting from the BNR which would result in very thin WAS and a high volume that would exceed the capacity of the RDTs. The low TS in the WAS (less than 0.5%) would also derate the capacity of the RDT further exacerbating the capacity issue. Options such as gravity pre-thickening of the WAS were considered initially but was ultimately ruled out since WAS doesn't settle well; this would add additional cost and the gravity pre-thickening may also promote phosphorus release. A second challenge related to the first challenge was meeting design throughput of the existing RDTs (400 gpm) with the current dry polymer set up. When using dry polymer, the RDT's could only run at less than half of the design capacity. Testing with emulsion polymer, however, showed that the design capacity of 400 gpm could be achieved, but this would result in much higher polymer costs. At current WAS loads, the capacity of the RDTs operating with the lower cost dry polymer at less than half design capacity was sufficient for operations, but this would not provide sufficient capacity for design future loadings. A third challenge was maintenance issues with the existing rotary drum thickeners. This included uneven wear on trunnion ultra-high molecular weight polyethylene roller bearings that caused pre-mature failures. After initially replacing only the failed bearings it was found that this would result in other bearing failures. Now when one bearing fails, all are replaced. Several RDTs experienced catastrophic failures involving horizontal cracking in drums at trunnion rollers. The repairs to the RDTs included upgraded materials for stabilizer rollers, sprockets, chain, and base tensioner. Because of these challenges, it is desired to have future unit(s) with bearings outside of wetted parts. Because of the capacity and maintenance challenges, several additional thickening alternatives were considered. This included replacing all of the existing RDTs with a different manufacturer, adding a 5th RDT of a different manufacturer, replacing RDTs with Volute Presses and replacing RDTs with DAFs. The results of the evaluation found that adding a fifth RDT would be the lowest cost option, and that design future capacity could be met with the RDTs if polymer was switched from dry to emulsion polymer. To better evaluate alternatives, SLCWRF pilot tested an alternative RDT and a volute thickener. This pilot testing showed similar performance results as existing RDTs with emulsion polymer. The pilot test also found that capacity was derated when operating with dry polymer but data with dry polymer was limited. After completing the study and pilot testing, several recommendations were developed for future WAS thickening. First, SLCWRF will integrate use of emulsion polymer to increase capacity when needed. Second, a fifth RDT will be added for capacity, when needed, likely with an alternative RDT manufacturer. Finally, SLCWRF will re-evaluate thickening technology when the existing RDTs reach the end of their useful life. Conclusion The experience from Salt Lake City shows the importance of thickening when considering advanced treatment such as BNR. The impacts on technology selection or using existing assets will be important for other utilities which may be considering similar process upgrades.