Full-scale pilot test of a Hydrogen Peroxide Treatment for Dewatering Improvement

Background and Objectives The San Antonio Water Systems' (SAWS) Steven M. Clouse Water Recycling Center (SMCWRC) operates as a centralized sewage sludge treatment facility for the sludge produced at Leon Creek Water Recycling Center (LCWRC), Medio Creek Water Recycling Center (MCWRC) and by its own liquid water treatment process. All plants combined have a permitted treatment capacity of 187 MGD and disposes of approximately 450 wet tons of biosolids per month. The sewage sludge treatment consists of mechanical thickening (gravity belt thickeners and centrifuges), anaerobic digestions and dewatering (belt filter presses and sand drying beds). The filtrate from both thickening and dewatering is sent back to the head of the SMCWRC liquid treatment process. Historically, SMCWRC has alternated periods of high and low polymer usage with an average around 19 lbs/dry-ton from 2018 to 2020 (see Figure 1). The year 2021 has been the exception for SAWS. The polymer use has averaged 23 lbs/dry-ton from January to September 2021. Initially it was believed the high polymer demand was due to recovery needed from challenges during the cold weather during the week-long snowstorm of February 2021. However, as the weather stabilized, the polymer usage did not decrease. The SMCWRC operation staff was approached by their chemical vendor and recommended to utilize hydrogen peroxide to improve dewatering. The hydrogen peroxide is intended to oxidize ferrous iron present in the solids to ferric iron which combines with the soluble orthophosphate. High concentrations of soluble phosphorus have been shown to reduce dewatering performance. According to previous iron measurements, SAWS operations would not need to add supplemental iron since enough ferrous sulfate (residual from odor control at the collection system) is available in the wasted solids. A full-scale hydrogen peroxide dosing unit was installed in July 2021 to pilot the recommended treatment. The objective of the pilot is to demonstrate that hydrogen peroxide treatment can increase solids concentration in the BFP cake, decrease dewatering polymer usage, decrease soluble phosphorus concentrations and is economically viable as compared to other commercially available processes. The economic viability of this method will be evaluated depending on several items that include: 1 Polymer usage reduction 2) Iron available for regeneration 3) Need for supplemental iron 4) Reduced solids disposal costs 5) Reduced solids transportation costs 6)Reduced maintenance costs due to struvite reduction post digestion The proposed presentation described in this abstract will summarize the results obtained after months of operation. Methods Before the installation of the pilot, a bench scale jar test was performed to determine the recommended peroxide dose. Post-digestion sludge samples were collected and dosed with increasing hydrogen peroxide concentrations while measuring dissolved orthophosphate concentrations. The optimum dose was determined to be around 400 mg/L hydrogen peroxide reducing the orthophosphate concentration from 230 mg/L as P to 50 mg/L as P. A full-scale hydrogen peroxide dosing unit was installed at SMCWRC and doses approximately 300 gallons per day of Hydrogen Peroxide into the belt filter press feed pumps header. Digested sludge (pre-H2O2 dosing) and belt filter press filtrate samples are collected twice a week. The dissolved orthophosphate concentration is measured on both samples. Other data regularly collected by plant staff include sludge flow, polymer use, digested solids concentrations and cake dryness. Preliminary Results As shown in Figure 2, there is an immediate measurable reduction in soluble orthophosphate concentration in the belt filter press filtrate after hydrogen peroxide dosing starts. The concentrations of soluble orthophosphate have continued to decrease slightly but so has the digested sludge soluble orthophosphate concentration. These results seem to indicate that hydrogen peroxide is successful at oxidizing iron and thus removing some phosphorus from solution and into the cake solids. The removed phosphorus is not recycled to the headworks and contributes to an overall phosphorus decrease in the system. The polymer use data and cake dryness was also reviewed to determine the effectiveness of the peroxide treatment. Figure 3 shows the belt filter press cake concentration in % total dry solids. The data shows a lower than typical cake dryness between January and April of 2021 and a quick improvement and sustained dryness between 17.5 and 18% TS afterwards. The installation of the peroxide feed did not seem to significantly increase or decrease the performance of the of the belt filter presses. Moreover, the cake dryness seems to be related more closely to the volatile solids fed into the BFPs. The higher the input volatile solids concentration, the higher the volatile solids loading rate and thus a decreased performance. Biosolids with high volatile solids concentrations are known to be more difficult to dewater and thus produce wetter cakes. Subsequently, the polymer use was analyzed to determine if the sustained performance of the BFPs was achieved while allowing operators to reduce their use of dewatering polymer. Figure 4 shows the 30-day running average of the polymer use during 2021. There was no perceived reduction on polymer use after the implementation of the peroxide pilot. The polymer use is at their highest this year after the dosing of peroxide. Sensitivity test will be conducted in the upcoming weeks by deliberately lowering polymer concentration and measuring cake TS concentration. It is possible that time is required for operators to develop a sensitivity to the new system and feel comfortable lowering polymer doses. On September 30th, the bench scale jar test was performed again to confirm the recommended peroxide dose. Figure 5 shows the ortho-P reduction at different hydrogen peroxide dosages. The current set-point is still valid for the full-scale pilot. Periodic jar test will be performed in the upcoming months to adjust hydrogen peroxide dosing as the soluble orthophosphate concentrations reduce in the system. Current Status and Future Work As of October 2021, phosphorus, solids and peroxide dosing data is continued to be collected. Polymer jar testing is being scheduled to determine if a change in coagulation polymer formula is required. Peroxide dosing will be reevaluated if polymer is replaced with a different formula. Trend analysis will be applied to study the changes over time in measured variables including dissolved orthophosphate concentration, soluble iron, polymer use, digested solids concentrations and cake dryness. Kendall's tau test will be used in this work to identify any trends in the variables of interest by testing for significance at a P value of 0.05, and Sen Slope estimator will be used to determine the magnitude of change over time. In addition, the dataset for each measured variable will be plotted versus time on a quarterly time period. If any pattern was identified, Seasonal Kendall test will be used in trend analysis. The Seasonal Kendall test is used to explore seasonal variations by comparing data from different years within the same season. In a Seasonal Kendall test, the dataset for each measured parameter will be plotted separately using a quarterly schedule, such as comparing May data only with May for different years. The data limitation in trend analysis will be 5 years (about 20 data points). Based on the data obtained thus far, a life cycle analysis will be performed to determine if hydrogen peroxide dosing is beneficial from the standpoint of reduced maintenance costs due to lower phosphorus concentrations. These results will be compared to other commercially available phosphorus recovery technologies that advertise improve dewatering performance. These results will be available for presentation at the Residuals and Biosolids Conference 2022.