Mitigating Volatile Sulfur Compound Emission From Primary and Secondary Activated Sludge Systems Using New Low-Cost Operational Strategies

INTRODUCTION
Since the early twentieth century, treatment plants have typically been located at remote locations on the outskirts of the cities they serve. Due to an increase in population size, urbanization, life standard and irrigation needs, the demand for water and wastewater treatment plants (WWTP) has been rising rapidly (Estrada et al., 2011). As a result, large WWTPs are often found near main metropolitan areas, so the need for odor prevention increased accordingly. A critical factor in facilitating the integration of intensified WWTP within urbanized regions is cost-effective and efficient odor emissions prevention, control, and treatment. For decades, research and technologies have been focused on the treatment of foul air from treatment processes providing end-of-pipe solutions for volatile sulfur compound (VSC) removal that results in higher capital and operational costs to utilities (Giri et al., 2014). Technologies developed include physical-chemical approaches that can remove H2S as well as biological approaches (i.e. biofilters and trickling filters) that can also remove volatile organic sulfur compounds (VOSCs) (Camp Dresser & McKee, 2003). Treatment and cost-effectiveness of these end-of-pipe approaches are mainly dependent on the ease to capture the foul air and, therefore mostly applied for sewer networks, screening and grit removal facilities, and biosolids storage facilities. However, one of the significant sources of nuisance odor emission within WWTP is the primary and secondary treatment system, which is mainly caused by VOSC emission rather than H2S (Sekyiamah et al., 2008). Due to the low odor threshold of VOSCs such as methyl mercaptan (MM), dimethyl sulfide (DMS), dimethyl disulfide (DMDS) compared to H2S (Estrada et al., 2011), the smaller fluxes observed within WWTP and in particular within secondary systems can have a significant impact on the overall nuisance odor emission. Due to its large surface area, the end-of-pipe odor treatment technologies often require significant capital investment and are often impractical.
In this project, we proposed new operational strategies for low-cost odor mitigation in both primary treatment systems and secondary activated sludge systems based on a better understanding of odor production kinetics. For the primary treatment system, a new sludge pumping mode was proposed to minimize compression and retention of sludge blankets. For the secondary system, a switch was made from a step feed mode with anaerobic selectors to a high-rate contact stabilization mode with aerobic stabilizer and contactor with anaerobic selectors. The achieved odor reduction through these approaches is presented in this paper.
MATERIALS AND METHODS
Sampling campaign: An AC'SCENT surface Emission Isolation Flux Chamber (Bazemo et al., 2021) was used for gas phase sampling at the surface of primary clarifiers and the different passes and zones in secondary reactor system. The VOSCs off-gas samples were sent to ALS Environmental Laboratory to be analyzed for MM, DMS, and DMDS (ALS, USA). H2S measurements were carried out using an electrochemical sensor (Odalog, USA). The influent and sludge characterizations were captured. Historical measurements using the same sampling methods were used as a baseline for the secondary system, while different operational modes were compared side by side for the primary system. Developing and evaluating new operational mode for primary clarifier: Three modes were tested at different underflow rates detailed in Figure 1. It should be noted that mode 1 and mode 3A & 3B alternated every two cycles on one clarifier for short-term testing and were tested side by side during steady-state period. As both modes ran in parallel on the same day, the impacts of wastewater characteristics and temperature on odor production outcomes between modes were minimized. Full-scale high-rate CS implementation in secondary system: West and East secondary full-scale activated sludge systems at Blue Plains were switched from stepfeed to CS configuration on April 7th, 2020 and August 1st, 2020 respectively (Figure 1B & 1C). More details on how this switch improved secondary performance can be found in Ngo et al., 2021.
RESULTS & DISCCUSIONS
Primary system odor production decreased by more than a half using a new pumping strategy: The proposed strategy was based on the reports from Bazemo et al. (2021), showing 64% of H2S and 89% VOCS emissions were produced in the sludge blankets. Figure 2A & 2B indicated that mode 3A and 3B resulted in significantly lower H2S and VOSC emissions compared to mode 1. Minimizing the sludge buildup time was more important than increasing the sludge pump flow rate, as no clear differentiation between mode 3A and 3B was observed. Thus, the total primary sludge pump flow rates from each house could be maintained while minimizing potential odor generation from further thickening. The steady-state operation showed that with this low-cost operation change, a reduction of 57% of H2S and 63% of VOSC was feasible (Figure 2C and 2D). In addition, this new mode maintains H2S levels under 1.6 ppm (2.2 ppm baseline) and 46 ppbv VOSC (111 ppbv baseline). Bioaugmentation and Contact stabilization (CS) mode decreased VSC emission by 23% and 81%, respectively: In secondary treatment, VOSC is the main contributor of odor and the only H2S emitted comes from primary effluent stripping (Bazemo et al., 2021). In addition, off-gas testing during baseline testing showed that 78% of odor emission in secondary was related to the first pass and production of odor compounds in sludge blankets and anaerobic zones. In addition, kinetic experiments using batch testing showed an evident inhibition of VOSC production due to nitrate (Figure 3A) or oxygen (Figure 3B) presence. Bioaugmentation using biological nutrient removal (BNR) sludge into secondary systems has been demonstrated to improve nutrient removal, oxygen transfer efficiencies, and settleability (Ngo et al., 2021). Due to the nitrate production of 0.43 mg/L-N, when one of the secondary systems receives all BNR sludge during the summer months, VOCSs emission was reduced by 23% with double bioaugmentation scenario (Fig. 3C). When both secondary systems receive each half of the BNR sludge, such levels of nitrate cannot be achieved and VOSCs emission increased again, while benefits of oxygen transfer and settleability were maintained (Fig.3C). When CS mode was applied, and bioaugmentation was maintained at 50%, VOSC reduced by 83% (Figure 3C) and H2S production decreased by 75%, despite the lack of net nitrate production. As larger part of the sludge inventory is stored under aerobic conditions rather than under anaerobic conditions, overall emission was clearly reduced.
CONCLUSION
This study developed new operational strategies in primary and secondary systems to mitigate the VSC emissions by minimizing the anaerobic sludge inventory without impacting performance in the primary or even improving overall system performance in the secondary Ngo et al. (2021). By understanding odor production kinetics, low-cost operational strategies can offer an alternative to end-of-pipe odor treatment technologies with significant capital investment costs.