The Perfect Trifecta of Wastewater Collection: Odor Control, Prevent GHG Emissions, and Reduce BOD

Three compounds in wastewater collection systems stand above the rest regarding the degradation of the environment. Hydrogen sulfide (H2S) is commonly known and has had multiple technologies developed for its treatment, methane (CH4) emissions have been overlooked and hard to quantify until recent years and BOD which requires significate investment at WWTP to remove. Excess BOD in Long untreated force mains consume all of the available Dissolved Oxygen (D.O.) and when gone, the bacteria under anerobic conditions generate H2S, causing severe odor and corrosion problems, and emit the potent greenhouse gas methane. These two components form only under conditions with zero D.O.concentration. Inversely, if there is a positive D.O. the Oxidation Reduction Potential (ORP) will remain positive and prevent these environmentally adverse components from forming due to their very low ORP generation requirement of -300 millivolts. Municipal wastewater collection and treatment systems are critical infrastructures, and the reduction of anthropogenic methane emissions that contribute to climate change as well as obnoxious H2S. Fugitive emissions of methane across the sewer systems remains a major challenge. Research suggests that over half of the US centralized wastewater industries Scope 1 greenhouse gas emissions are from sewer methane, as depicted in Table 1. To mathematically quantify the amount of methane generation within a foreman, we can implement Equation 1 to all existing force mains for an understanding of their methane emission potential. A case study will be provided on a three-mile, 24-inch force main, which left untreated would result in approximately 35 lbs per day of methane generated. This translates to nearly 400,000 lbs per year of CO2 equivalent-- comparable to thirty-two cars worth of CO2 pollutant year-round. Differentiating between the treatment of the two compounds, Hydrogen sulfide once formed under anaerobic conditions is readily biodegradable within ~30 minutes of maintained aerobic conditions. As shown in Figure 1, the vapor phase hydrogen sulfide (H2S) concentration is completely reduced due to the biodegradation of dissolved sulfides in oxic wastewater. However, methane once formed under anaerobic conditions is for all practical purposes not biodegradable in the presence of D.O. or nitrate as demonstrated by the fact that methane cannot serve as the carbon source for denitrification. Focusing on biofilms and bacterial floc, if the bulk D.O. is not sufficiently high to make the entire biofilm depth oxic, H2S formed deep in the biofilm can be re-oxidized completely as it diffuses back out of the aerobic surface biofilm into the bulk liquid and thus does not re-enter the environment. This is not true with dissolved CH4 due to its exceptionally low rate of aerobic biodegradability; essentially, all the CH4 generated will diffuse through the aerobic surface biofilm into the bulk liquid and will eventually escape to the environmental atmosphere at manholes, odor control exhaust, and wastewater treatment process at the facility. Methanotrophs are a class of bacteria that can oxidize methane under oxic conditions. Although this class of bacteria is significant globally over a long-time span, they metabolize so slowly that their contribution over a matter of hours in the collection and treatment system is negligible. Therefore, any methane formed in collection and treatment is not significantly metabolized in the time span of a few hours. Maintaining oxic conditions throughout a collection system has traditionally posed a challenge due to the extended hydraulic retention times experienced during nighttime pumping cycles. These elongated retention times coupled with the biofilms oxygen uptake rate (OUR) requires more dissolved oxygen than physically possible when only utilizing atmospheric air. Superoxygenation is a process designed to use pure oxygen and force main pressure to achieve almost 100% oxygen absorption efficiency with essentially no stripping of volatile components from the wastewater. Conventional aeration absorbs ~5% of the oxygen from air and acts as an efficient stripper for present volatile components, e.g. H2S and CH4. Wastewater treatment plants are being stressed today to treat higher BOD loading than ever before. Large investments are being made at the wastewater treatment plants to increase the plant's capacity to treat the higher BOD loads. An underutilized tool to help municipalities increase WWTP capacity is to do pretreatment in the collection system to reduce BOD. Modern wastewater treatment is based upon converting organic matter (measured as BOD) into biomass under aerobic conditions (in aeration tanks). The same principle can be applied to wastewater collection system where D.O. can be utilized to reduce BOD in the wastewater as it travels through the collection system to the WWTP. In the case study referenced, 180 mg/L of D.O. is added to a force main reducing the BOD by nearly 160 mg/L in the wastewater. That is a reduction of over 80% in typical municipal wastewater of 200 mg/L. SuperOxygenation technology allows for the efficient absorption of pure oxygen by providing an intense captive bubble swarm and utilizing existing line pressure within a Speece Cone. This provides D.O. saturation concentrations of over 100 mg/L, while staying below the saturation threshold and preventing effervescence.