Fate of Escherichia coli, Enterococci, and Campylobacter in the Bluebloqs Biofilter

Abstract

Biofilters are increasingly being implemented in urban environments to treat stormwater but knowledge on their pathogen removal capacity at field scale is limited. Adequate pathogen removal is, however, needed if treated water is reused. This research evaluates the fate of Escherichia coli, enterococci and Campylobacter in a field scale biofilter in Rotterdam. This biofilter is part of the Urban Waterbuffer Spangen that combines biofiltration with Aquifer Storage and Recovery (ASR) to provide irrigation water for Sparta’s sports field. Microbiological water quality was analysed to assess the removal capacity of the biofilter. To identify factors causing adverse treatment performance, system construction, and operation were investigated, water flows in the biofilter were assessed, and hydraulic conductivity was measured. Lastly, model simulations were used to investigate how short-circuiting and event frequency and duration affect microbiological outflow concentrations. Results showed leaching of enterococci and Campylobacter and minimal retention of Escherichia coli. Leaching is likely caused by secondary contamination with bird faeces and multiple design and operational choices lead to low microbial retention. Coarse filter media results in high hydraulic conductivity, and consequently, short hydraulic retention times. Electrical conductivity measurements revealed short-circuiting pathways between the inlet and outlet. Lastly, uneven distribution of feed water presumably reduced the effective reactor volume. It is recommended to upgrade biofilter design by incorporating small particles in the filter media and maximizing the distance between the inlet and outlet. Additionally, reintroduce a ponding zone and rearranging event frequency and duration to lower microbial outflow loads is advised. Model simulations suggest that short, frequent events are beneficial to control microbial loads. However, further work to clarify microbial decay rates is recommended. To conclude, results imply that the biofilter is currently unable to improve the microbiological water quality but can become a treatment barrier if design and operation are upgraded to tackle encountered problems.