Integrating Harvested Stormwater with Wastewater Reuse for Nutrient Load Reduction
- Matthew P. Jones PhD, PE - Hazen and Sawyer
As the need to reduce stormwater pollutant loads in separately sewered areas continues to develop and the most cost-effective opportunities for water quality improvement are exhausted, new stormwater management strategies and innovative adaptations of existing tools are increasingly important. One of these tools, stormwater harvesting, has existed for thousands of years as a means of water supply, but is a relatively new stormwater management and water quality improvement tool. Stormwater and rainwater harvesting systems are being incorporated into site designs with increasing regularity; however, access to a substantial, consistent, and verifiable demand for non-potable water can limit the ability of these systems to effectively manage stormwater runoff and reduce pollutant loads, particularly at large scales.
Integration of harvested stormwater with a wastewater reuse system presents an opportunity to benefit from the non-potable water demands of a large-scale reuse system and relative differences in nutrient content of harvested stormwater and treated wastewater effluent. At the same time, there are some key considerations affecting the feasibility and cost-effectiveness of this approach to stormwater management. A feasibility analysis examined the components of an integrated stormwater and wastewater reuse system, conditions affecting the viability of this stormwater management approach, and factors affecting cost-effectiveness.
The basic components required to integrate stormwater with a wastewater reuse system are a stormwater collection system, storage basin, and treatment and distribution systems. These components could be implemented in a decentralized fashion along the reuse distribution system or in close proximity to a wastewater treatment plant. With a distributed approach, it may be possible to manage stormwater from a larger overall drainage area and make use of existing stormwater basins; however, distributed treatment systems will be needed to ensure captured stormwater meets water quality standards for reuse. Although reuse standards vary by jurisdiction, stormwater commonly violates reuse standards for pathogens and turbidity. Treatment prior to integration with the reuse system can be accomplished with filtration and UV or chlorine disinfection systems. Integrating stormwater with a reuse system in close proximity to a wastewater treatment plant may allow the use of existing treatment facilities to satisfy filtration and disinfection needs, improving cost effectiveness. At the same time, the water quality benefit of a centralized system may be limited to management of a catchment area in the immediate vicinity of the WWTP, unless stormwater pump stations or other conveyance infrastructure modifications are implemented.
A key factor impacting the feasibility of stormwater harvesting integration is the relative difference in nitrogen and phosphorus concentrations between treated wastewater effluent and captured stormwater. Due to the inconsistent nature of stormwater supply, harvesting stormwater is not expected to significantly augment the total amount of water provided by the reuse system. As such, harvested stormwater will replace the demand of wastewater effluent, typically resulting in that effluent being discharged to a receiving water. Since the combined volume of stormwater and wastewater discharged to the receiving water will remain the same, water quality
benefits are only realized when the nutrient content of stormwater supplied to the reuse system exceeds that of wastewater effluent discharged to the receiving water. An analysis of monitored stormwater nutrient concentrations reported in the International BMP Database (WERF, 2015) in relation to wastewater effluent permit limits across the United States reveals that substantial water quality benefits may be realized in some circumstances, but only for WWTPs with a high level of nutrient control.
An assessment of site-specific system configurations was performed to further evaluate the feasibility and cost-effectiveness of integrating stormwater with a wastewater reuse system. This analysis included a 30-year simulation of stormwater capture and utilization to size storage, pumping, and treatment facilities, quantification of nutrient load reductions, and cost opinions for system components. For a centralized facility at a wastewater treatment plant in Virginia, management of a 50 acre area supplied 38 million gallons annually to the wastewater reuse system, reducing stormwater phosphorus loads by 84 pounds per year. When accounting for the discharge of treated wastewater effluent that would otherwise be utilized by the reuse system, the net reduction in total phosphorus load to the receiving water was 51 pounds per year. For this example, the cost of nutrient load reduction via reuse exceeded the typical cost of nutrient credits exchanged between non-point sources. Catchment area, concentration differential, and the magnitude of conveyance, storage, and treatment needs were found to have a substantial impact on cost-effectiveness.
Feasibility analysis results have demonstrated that harvested stormwater can be integrated into a wastewater reuse system and provide substantial pollutant load reductions in some circumstances. While cost-effective opportunities for this stormwater management approach may currently be limited, the practicality of an integrated system is expected to increase over time as low-cost nutrient reduction options are exhausted. Consequently, there are benefits to considering the integration of stormwater with wastewater reuse for long-term stormwater and water quality improvement planning efforts.
Purpose and Conclusions
Integration of harvested stormwater with a wastewater reuse system presents an opportunity to substantially reduce nutrient loads delivered to receiving waters in an unconventional manner. A feasibility analysis reveals the circumstances required for such a system to be practical and cost-effective, as well as insight into potential water quality benefits.
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