Real time control of biofilters delivers stormwater suitable for harvesting and reuse

Abstract

Stormwater biofilters have great potential to treat stormwater for harvesting and reuse, but their variable performance in pathogen removal requires further optimisation prior to widespread uptake. This paper provides the first evidence that real time control (RTC) of stormwater biofilters can mitigate the impact of operational characteristics that result in poor microbial removal. We developed two RTC strategies and validated them using long-term laboratory experiments, utilising biofilters with a raised outlet pipe that creates a submerged zone. The first RTC strategy focuses on delivering the best water quality for harvesting and reuse or for recreational waterways. It has two components which ensure adequate treatment (microbial die-off): (1) it retains water in the biofilter for at least two days before allowing any further inputs into the system, and (2) the input volume is restricted to the submerged zone’s pore volume. This strategy was effective and significantly improved water quality in the biofilter effluent. However, since the system favours bypassing influent to ensure good quality effluent, only 28.4% of the stormwater was treated. This still resulted in a 62.3% reduction in the influent E. coli load because the system was effective at removing E. coli under controlled conditions. The second RTC strategy builds upon the first strategy, and focuses on delivering a balance between good water quality for harvesting and protecting the environment (i.e., lower bypass). Three hours before the next rainfall event begins, the water that has remained in the biofilter’s submerged zone for at least two days is drained and collected for harvesting through a bottom pipe. When stormwater inflow begins, the bottom pipe is closed and the biofilter operates without control, with water leaving the biofilter to the environment via the raised outlet pipe. The harvested effluent of this RTC strategy met the Australian stormwater harvesting guideline requirements for dual reticulation with indoor and outdoor use and irrigation of commercial food crops. Although only 5.4% of stormwater was collected for harvesting in this strategy, the environment was better protected because of a significantly reduced bypass volume. Our experiments also showed that the nutrient and sediment removal was high for both RTC strategies. This study presents the first stepping stone toward RTC of stormwater biofilters, demonstrating that these systems can deliver safe stormwater for harvesting and reuse, and for active recreational uses.