Water Quality Overview of Indirect Potable Reuse System in Northern Virginia

Wastewater reuse benefit water shortage and a variety purposes but effect to water quality of receiving body is a concern, especially when effluent of sewage treatment plant is introduced to source of drinking water facility. Indirect potable reclamation characterized as mixing effluent into either surface or groundwater source, subsequently flowing into drinking water treatment plant, presents advantage for maintaining downward water quality through dilution mechanism or natural remediation.

The Occoquan Watershed Monitoring Laboratory (OWML) has been conducting water quality monitoring of upstream Upper Occoquan Service Authority (UOSA) discharge, and downstream Bull Run creek, before it joins Occoquan Reservoir and serves as source water by Griffith Drinking Water Treatment facility. For year 2020, data at Bull Run station showing chemical oxygen demand (COD) ranges 8.2-33.4ppm, oxidized nitrogen (OX_N) 0.8-8.71ppm, total nitrogen (TN) 1.53-9.70ppm, total phosphorus (TP) 0.03-0.23ppm, total suspended solid (TSS) 1.5-205ppm, and turbidity 1.23-9.26NTU. While at UOSA discharging spillway COD ranges 7.8-11.9ppm, OX_N 6.41-13.0ppm, TN 6.97-14.6ppm, TP constantly 0.06-0.08ppm, and TSS 3.5-13.2ppm. Plots using multi-year data also support readiness of stream to accommodate OX_N and TN; however COD is clearly not a welcomed parameter generally.

The snapshot is primarily focused on most recent ten years, although the monitoring program has been ongoing since 1970s. Water quality data at immediate receiving stream ratify importance of advanced treatment before discharge especially to reservoirs for public water supply use. Continuing efforts still required to investigate physical, chemical, and microbiological processes underneath the numbers.