Navigating the Stillaguamish: Expediting Wwtp Expansion with Ongoing Tmdl Cleanup Plans on a Western Washington River

The City of Arlington is a community of approximately15,000 in northern Snohomish County, approximately 50 miles north of Seattle. A vicinity map is offered in Figure 1. Through both development and annexation to its service area, the City projects that its population will double within the next 20 years. The City's current wastewater treatment plant (WWTP), which discharges through a single outfall to the Stillaguamish River, is already near capacity. In order to remain in compliance with its current National Pollutant Discharge Elimination System (NPDES) permit, prepare for anticipated limits to be added to the permit in 2008, and maintain control of development standards within the annexation areas that will add appreciable influent flows and loadings at the WWTP within the next 5 years, the City is attempting to expedite plant expansion plans. However, impaired water quality conditions along various reaches of the Stillaguamish have resulted in river total maximum daily loads (TMDLs) for parameters including temperature, fecal coliform, dissolved oxygen (DO), pH, mercury, and arsenic. Although the TMDL reports have been finalized with the U. S. Environmental Protection Agency (USEPA), a detailed implementation plan for river cleanup has not been executed, and restoration of the impaired parameters will involve ongoing analysis of an implementation plan that includes adaptive management measures for both point and non-point sources. Waste load allocations (WLAs) specific to Arlington for temperature and fecal coliform were established for the WWTP point source. Additionally, a target DO of 7.0 milligrams per liter (mg/l) has been established for the reach of the Stillaguamish River immediately downstream of the WWTP (modeling indicated that merely removing anthropogenic sources during critical flow periods could not attain the 8 mg/l State criterion for this river classification). The Arlington WWTP was identified as a likely contributor to the DO depletion in the downstream reach due to the nutrient loads inherent within the treated effluent discharged through the outfall. The WWTP is one of the few point sources upstream of the river segments experiencing DO depletion, and thus can be controlled under regulatory authority. Because potential future WLAs for nitrogen and phosphorus may be unachievable with current economically feasible treatment technologies, the City was reluctant to commit to a costly plant expansion project. No immediate viable alternatives are available for full diversion of the treated effluent from the WWTP during critical low flow river months. Therefore, this paper focuses on the approach the City will take to negotiate methods to comply with future TMDL limits. The presenter will also provide background information regarding the river characterization provided by the Washington State Department of Ecology (Ecology) through its watershed research and field data collection. The City is currently concluding a sensitive negotiation phase with Ecology on the TMDL approach and is optimistic that they will reach a formal understanding that allows the facility expansion design to proceed before this paper is presented. A comprehensive assessment of the City's water resources was conducted while crafting the WWTP TMDL approach. This assessment included the following concepts, which will be presented in the paper. •Incorporation of new treatment techniques into the WWTP expansion design to reduce nutrient-related waste loads that correlate to DO depletion, including biological nutrient removal; tertiary filtration for biochemical oxygen demand (BOD) and total suspended solids (TSS) reduction and potential diversion of reclaimed water (although no large potential users have been identified that would justify the cost of building transmission pipelines); chemical addition to enhance nutrient removal; reverse osmosis to meet stringent waste load allocations; and super-saturation of the outfall effluent via the mechanical addition of oxygen. •Incorporation of new treatment techniques and/or facilities into the WWTP expansion design to reduce effluent temperature discharged to the river, including unit process tank covers to minimize temperature rise from solar radiation and blower control to minimize heat transfer to the effluent from air addition. Passive effluent cooling (via cooling towers or heat exchangers) and mechanical cooling (using chillers) were also evaluated. • Diversion opportunities for either plant influent or treated plant effluent to limit the discharge flow (and associated nutrient and temperature loads) through the outfall into the river, including fully diverting the plant effluent through an approximate 5-mile extension of the plant outfall to a river discharge point downstream of the critical DO depletion zone; diverting plant effluent to an adjacent undeveloped parcel owned by the City along the river bank immediately downstream of the WWTP site, with rapid infiltration discharge to realize further nutrient removal and effluent cooling before reaching the river through the hyporheic zone; diverting Class A reclaimed water produced by the WWTP to beneficial end uses, such as golf courses and turf farms within City limits; and diverting a portion of the collection system flows by either constructing a scalping facility that could alternatively divert solid and liquid streams from the WWTP, or offloading the flows by pumping to the adjacent municipal wastewater collection system. •Trading portions of the expanded WWTP future waste load allocations for nutrients with non- point sources, primarily agricultural, by implementing best management practices, riparian improvements, and incentives to eliminate failed septic systems. Of the concepts listed above, Ecology's most recent directive has been to pursue a combination of (1) implementing feasible treatment technologies at the WWTP to reduce nutrient loading to the receiving waters and (2) identifying additional measures that might be used to further reduce nutrient loading during the critical low-flow summer period.