Innovative Baffle System Tripling Primary Sedimentation Capacity and Saving Millions

Learning Objective - To provide insight and share the experience of using an innovative baffle system in the primary sedimentation process at Dublin San Ramon Services District (DSRSD) to enhance the total suspended solids (TSS) and biochemical oxygen demand (BOD) removal efficiencies and increase capacity. Results of the stress testing indicated that PSTs equipped with baffle system achieved a total suspended solids removal of 46 percent at overflow rate of 8000 gpd/ft2 which is more than the triple the original capacity of the PSTs. Background The DSRSD operates a 17 MGD wastewater treatment plant in Dublin, California. The wastewater treatment plant had four primary sedimentation tanks (PSTs). Additional BOD5 removal in the primaries reduces loading to the secondary process thus reducing aeration demand as well as capital costs of new infrastructure for potential future nutrient removal requirements. Therefore, DSRSD initiated a project to increase the capacity and improve performance of the primary process. Feld testing and preliminary modeling was conducted to identify potential structural improvements. The study indicated that by optimizing the existing four tanks (with internal baffles) and adding one new tank, DSRSD would be able to achieve the goal. Objectives The objectives are threefold: 1.To introduce an innovative baffle system to increase the capacity and improve suspended solids and biochemical oxygen demand capture in conventional primary treatment. Such baffle systems could be applied at many conventional primary treatment plants leading to significant savings in downstream secondary treatment capital and operational costs. 2.To present the baffle systems as a potential replacement of chemically enhanced primary treatment as means of increasing solids capture. 3.To present DSRSD experience and lessons learned during the design, testing and construction of the baffle system. Status In 2017, DSRSD initiated an optimization study as part of the design project to investigate the potential of optimizing the existing PSTs. The study indicated that optimizing existing tanks and adding one more tank will achieve DSRSD goal. Since finishing the bulk of construction (2021), the primaries have exceeded the anticipated primary solids capture (upwards of 70 percent) under normal flow conditions. Stress testing indicated that the PSTs equipped with the baffle system was capable of achieving TSS removal efficiency of 46 percent at overflow rates as high as 8000 gpd/ft2. This overflow rate is almost triple the original flow rate that could have been sustained in the past (before installing the baffle system) by the PSTs while maintaining reasonable performance. A co-benefit of increasing the solids capture across the primaries is an increase in biogas production. Biogas production has increased approximately 20 percent. This increase is attributed to a combination of higher solids/organics capture across the primaries and primary solids having higher BTU content than waste activated sludge. Methodology Using CFD, an influent flocculation baffle, two mid-tank baffles , and a sludge protector canopy were modeled. Modeling results predicted that the preferred design would increase TSS and BOD5 removal efficiencies by increasing flocculation, improving the flow regime inside the tanks, decreasing short-circuiting, and decreasing sludge re-suspension. Field Testing and Data Analysis: Table 1 shows the sequence of the testing activities. PST#4 is one of the old tanks with deeper side water depth than PSts # 1, 2 and 3. As part of this project, internal baffles and transversal launders were installed into tank #4. PST # 5 is a completely new tank. For the stress testing, the old PSTs were taken offline to increase the loadings on the new PSTs. Effluent suspended solids and sludge density samples were collected as part of the stress testing. Figure 1 shows the results of the stress testing with both PSTs # 4 and 5 online. Table 2 summarizes the results of the testing. It is clear from the data that PSTs were able to achieve TSS removal efficiency as high as 59 percent at over flow rate of approximately 4100 gpd/ft2. On the second day of stress testing, PST# 4 was taken offline and the whole plant flow of approximately 15.75 mgd (average) was diverted to PSTs #5, during the testing, the effluent launders of PSTs # 5 were flooded indicating exceeding the hydraulic capacity of the tank. However, TSS removal efficiency was still satisfactory exceeding 46 percent. Conclusions 1.The new baffle system improved PSTs performance by increasing the TSS removal rate from 38 to 65 percent. 2.The new baffle system increased the process capacity of the PSTs and enables the PSTs of achieving TSS removal efficiency of 46 percent at over flow rate higher than 8000 gdp/ft2. 3.The new baffle system increased the TSS captured in the PST leading to increase ion biogas production. Significance: The baffle systems described herein improved TSS capture and allowed the PSTs to operate at much higher overflow rate than typically applied at conventional primary treatment. Such baffle systems could be applied at many primary treatment in many plants leading to significant savings in downstream secondary treatment capital and operational costs. Also, the baffle systems could be used in lieu of chemically enhanced primary treatment as means of increasing solid capture