Supporting Regulatory Compliance Strategies through Water Quality Modeling

摘要

This manuscript summarizes the value Citizens Energy Group has realized by using water quality modeling to develop compliance strategies for its combined sewer overflow (CSO) Consent Decree and National Pollutant Discharge Elimination System (NPDES) permits. Citizens is presently implementing a 20-year LTCP with required completion in 2025 and has an NPDES permit for two advanced wastewater treatment (AWT) plants. Both the Consent Decree and permit are derived from instream water quality standards (WQS). Citizens utilizes an integrated collection system and water quality model to evaluate the impacts of CSO and AWT plant discharges to instream water quality for bacteria and dissolved oxygen (DO). The dissolved oxygen model includes the effects of nitrogen, phosphorus, sediment oxygen demand, and algae (Rubchinskaya et. al, 2015). With the majority of the Consent Decree projects completed or in construction, Citizens evaluated multiple strategies for regulatory compliance, and identified preference for a demonstrative approach of attainment of current Indiana WQS or the Federal Recreational Water Quality Criteria (RWQC), with an emphasis on the impairment attributable to Citizens CSOs (Hawbaker et. al, 2016). A five-year continuous simulation for representative precipitation was simulated, with emphasis on the April to October recreation season. The water quality model was also utilized by Citizens for the recent NPDES permit renewal. Citizens' AWT Plants had an effluent dissolved oxygen limit of 8 mg/L, which is supersaturated in summer conditions. Through use of the water quality model, Citizens was able to demonstrate that ambient water quality standards can be achieved at a lower effluent limit. The permit was renewed for 7 mg/L effluent DO in summer conditions (IDEM, 2018), which provides operational cost savings for Citizens in the summer months. Previously Citizens would apply liquid oxygen or peroxides to supersaturate the AWT effluent.