Operational Strategies to Prepare the Wastewater Treatment Facilities for Extreme Weather Events

Background Wastewater infrastructure plays a critical role in urban communities, providing for the safe and efficient conveyance and treatment of sewage to protect human health and the environment. In the United States, over 16,000 centralized wastewater treatment plants serve approximately 74% of the population (Environmental Protection Agency, 2004). The majority of these plants provide at least secondary treatment of wastewater before discharging to local water bodies. Some plants may also provide advanced treatment to remove additional constituents that could impair the quality of receiving waters (EPA, 2004). Wastewater treatment plants (WWTPs) are strategic infrastructures that remove contaminants from wastewater, thus supporting human health and environmental protection (Panico et al. 2013). Majority of the wastewater treatment plants are generally located in low elevations to facilitate the conveyance of wastewater flows to the plant by gravity to minimize the number of pumping stations and the cost of wastewater treatment. Due to its location, wastewater treatment plants are more vulnerable to the extreme weather events such as floods, sea level rise and hurricanes. These extreme weather events could increase or decrease the wastewater flows to the plant, increase or decrease the pollutant concentration and increase or decrease the wastewater temperature depending on the type and severity of the events whether it is high precipitation and flooding or drought conditions or snow melt. During these events, the performance of the wastewater treatment plants can deteriorate or fail completely when it is stressed beyond the design or operation conditions, consequently impacting the human health and environment. The objective of this paper is to focus on the operational strategies that could be implemented at the wastewater treatment plants to face the challenges of extreme weather events to be more reliable and resilient wastewater treatment system. Objectives: During the extreme weather events, the operational failure and deterioration of effluent quality can occur due to high influent flows or pollutant concentrations. Some typical reasons for plant performance degradations are (NYCDEC 2000): loss of biomass from the aeration tanks and secondary clarifiers, overloading of the aeration system from high biochemical oxygen demand (BOD) loadings caused by solids washout, electrical overload of mechanical surface aerators, and decreased BOD removal efficiency due to shortened hydraulic retention time in the aeration tanks. The objective of this work is to illustrate how the operational strategies such as blending of the partially treated effluent with biologically treated effluent, increased aeration, increased chemical use, increased return sludge recirculation, controlling sludge retention time (SRT), use of unused or redundant tanks for aeration can be utilized to handle the increased influents flows and pollutant loads during the high precipitation (rainfall) associated with the extreme weather events. Methodology, Results and Discussions Influent flows and pollutant concentrations significantly influence the performance of the wastewater treatment plants in maintaining effective treatment to provide the desired effluent quality. High precipitation rates will result in increase in influent flows into WWTPs due to flow from combined systems and I&I. While the volume or 'flow' of the wastewater increases due to increased stormwater infiltration, the mass of pollutants in the wastewater may remain the same, resulting in a dilution of the influent to the WWTP which can affect biological treatment processes and solids separation process. Typically, the wastewater treatment plants are designed to handle the maximum month average daily flows while the plant is designed hydraulically to convey the peak hourly flows which is expected last few hours in a day. During the extreme weather events, the plant inflows may be expected to receive the peak flows for several hours or even few days depending on the severity of the weather event. In this work, a hypothetical conventional activated sludge plant of 5 MGD was created using the process modeling software BioWin 6.2 (Figure 1) to determine the performance and maximum treatment capacity of the wastewater treatment plant in response to the increased influent flows and pollutant concentrations (BOD, Ammonia-nitrogen, Total Suspended Solids (TSS) and Total Phosphorus (TP)) for the extended period of time under the following operational strategies: Diverting and blending partially treated wastewater: Flows that exceed the maximum biological treatment capacity of the aeration basin is diverted after the preliminary treatment system, treated at the primary clarifier, and blended with the biologically treated effluent from the main process. This strategy will be effective on facing the challenge of high flows as the pollutant concentration will be more diluted due to the extraneous stormwater mixed with the wastewater. Increasing Return Activated Sludge Recirculation: During the high flows, the sludge blanket at the secondary clarifier is likely to increase resulting high TSS leaving the effluent and impacting both the effluent quality and the biological treatment process in maintaining mixed liquor suspended solids. Increasing the RAS recirculation rate will alleviate this problem in maintaining the effluent quality in a short run. Increased aeration: During the high flows, although the pollutant concentration is likely to be low, the mass loading still could be relatively high resulting increase in high aeration demands. Increasing the airflow rate can remove BOD and increase the nitrification to remove the ammonia-nitrogen. Controlling Solids Retention Time: Solids Retention Time (SRT) is one of important operational parameter of the wastewater treatment plant operation in accomplishing desired BOD and nitrogen removal. SRT is dependent on several other operational parameters such as mixed liquor suspended solids in the aeration basins, excess waste activated sludge wasted from the secondary clarifier, influent flow rate, influent wastewater characteristics and RAS recirculation rate. Maintaining the SRT at desirable range by controlling other operational parameters could be an effective short-term strategy to deal with the increased flows from the extreme weather event. Increased chemical use: During the high influent flows, the chemical treatment can be used for improving solids separation and also for removing phosphorus by addition of the iron or alum along with polymers as a short-term strategy to improve the effluent quality. Use of unused or redundant tanks for use as aeration basins: Unused tanks, if any, at the wastewater treatment plant or back up tanks can be added at the wastewater plants to increase the hydraulic retention time (HRT) and SRT to provide the effective treatment. Conclusions This work will demonstrate how the operational strategies discussed in the above section can be used to determine the maximum handling capacity of a hypothetical 5 MGD wastewater treatment plant to prepare for the plant for the high influent inflows during the extreme weather events. The illustrative case study will provide the quantitative examples with limiting treatment capacities for each of the operational strategies discussed above using the process simulation results from the BioWin 6.2 Software that was used for creating the process model for the 5 MGD plant. The methods and operational strategies demonstrated in this work can be extended to the advanced wastewater treatment (AWT) facilities.