Wet Weather Capacity Can't be Built in a Day, But Improved Settleability Can Shift the Paradigm Quickly

Introduction Johnson County Wastewater (JCW) desires to achieve treatment of 3Q (three times dry weather flow) during wet weather at all of its six WRRFs. JCW has presented at prior conferences how it pioneered the use of dual-purpose cloth filters for auxiliary treatment of wet weather flows at its Tomahawk Creek Treatment Facility (Szerwinski, 2023). This paper highlights an alternate approach taken by JCW at its Middle Basin Treatment Facility (Middle Basin) to improve peak flow capacity over the past 15 years, including results from a successful pilot of InDENSE hydrocyclones in 2023. Full-scale implementation of hydrocyclones will allow JCW to defer further into the future a costly upgrade to cloth filters for auxiliary treatment. Middle Basin WWTF Middle Basin has a permitted capacity of 55 million liters per day (MLD) and it currently treats an annual average flow of about 45 MLD. Figure 1 is a process flow diagram of liquid treatment facilities at Middle Basin, which has four independent trains each consisting of a primary clarifier, aeration basin, and secondary clarifier. The secondary effluent is then disinfected with ultraviolet (UV) lamps. These facilities are referred to as the Mechanical Plant. The fourth train was constructed from 2008-2010 to reduce the frequency of discharges from the facility's Equalization Lagoons to Indian Creek during wet weather. Previously, discharge from the Lagoons was allowed in the facility's permit if sodium hypochlorite was dosed and if numerical permit limits were met. However, numerical limits were not consistently met and thus the fourth train was built to improve performance. This expansion nominally designed the upgraded Mechanical Plant for a peak flow capacity of 87 MLD, which is only a 1.6 peaking factor (1.6Q) relative to the facility's permitted annual average capacity. Even though not required by permit limits, JCW also voluntarily converted the aeration basins into biological nutrient removal (BNR) basins configured in a modified Johannesburg process. Peak Flow Optimization Table 1 provides a timeline of the measures implemented over the past 15 years to incrementally increase the peak flow treated through its Mechanical Plant BNR trains. Addition of the fourth train substantially increased peak flow capacity of Mechanical Plant from only 1.2-1.5Q up to about 2Q. The quantity of discharges from the Lagoons was reduced by 87% on average. The number of days requiring lagoon discharge reduced from an average of 45 to 12 days per year. JCW experimented with additional operational changes -- some of which were successful and others which were not -- and both will be discussed further in our paper. The 2020-2022 improvements shown in Table 1 yielded a Mechanical Plant capacity regularly exceeding 2Q capacity up to about 98 MLD. The discharges from the lagoons reduced to an average of about 43 million liters (11 million gallons) and only 4 days per year. The reductions achieved in lagoon discharges from 2010-2022 were worth celebrating, but JCW has sought to find ways to continue increasing capacity while also reducing stress on operations staff by seeking improved sludge settleability to gain more secondary clarification capacity. Worse SVI30 often coincides with the rainiest months from March to May (Figure 2), and so sludge settleability has been the greatest limit to further reductions in lagoon discharge. This led JCW to initiate a pilot of InDENSE hydrocyclones in January 2023 to test how improvements in sludge settleability could enhance peak wet weather capacity. InDENSE Pilot Results Selectively wasting poorer settling particles and filaments from an activated sludge basin has been shown to improve sludge settleability. This can be accomplished with surface wasting or with external selectors. The use of hydrocyclones as an external selector to accomplish selective wasting is seeing increased adoption at municipal facilities (Sturm, 2020; Avila, 2021; Regmi 2022). The Middle Basin layout offered a unique opportunity to pilot InDENSE hydrocyclones for sludge wasting on one of the two worst performing BNR trains, while having the identical BNR train adjacent to it for comparison as a control train. Three weeks after startup of the pilot, JCW observed that the 5-minute settleability of mixed liquor in the pilot train decreased significantly below the independent control train (Figure 3). Six months into the pilot, benchtop settling velocity tests (Figure 4) were performed to characterize the capacity of the clarifiers, which is shown in Figure 5. These results correlate to the JCW staff's full-scale experience during a major storm event in February 2023, where the sludge blankets rose rapidly in the control train while less than 2-foot blankets were retained in the InDENSE train. Conclusion This paper will present what has and has not worked to increase peak flow capacity at Middle Basin WWTF, including a successful pilot in 2023 using InDense hydrocyclones for sludge wasting. Although a hydraulic bottleneck capital projects will be needed to further increase peak flow capacity, the implementation of InDENSE with these hydraulic improvements will help JCW to defer further into the future a costly upgrade to dual-purpose cloth filters to attain 3Q.