Faecal microorganism removal in stormwater biofilters

Stormwater has emerged as an alternative water resource. However, a wide range of pathogens present at varying concentrations in stormwater pose a significant human health risk if exposure to untreated stormwater occurs, either via recreational activities or when harvested for reuse. Hence, the adequate treatment of stormwater is essential before discharging it into recreational water bodies or harvesting for reuse. Stormwater biofilters are a promising water sensitive urban design technology. The capacity of this technology to sequester and/or remove indicator bacteria has been explored previously with biofilters being identified as potential candidates for indicator bacterial removal. Limited knowledge on the governing microbial removal mechanisms, and major influential design characteristics and operational conditions, are hindering the improvement of biofilters as a reliable treatment strategy for faecal microorganisms. Paucity of data on actual pathogen removal capabilities has also led to the current lack of credit given to stormwater biofilters. Moreover, the absence of an adequate model predicting microbial removal in stormwater biofilters to aid current decision making in stormwater management was identified as an area warranting further investigation. Hence, the current research was undertaken to address these main knowledge gaps in faecal microbial removal in stormwater biofilters. The research employed a combination of laboratory experiments, field monitoring and modelling work using Escherichia coli as the main faecal microorganism of interest. It was found that faecal microbial removal in stormwater biofilters is governed by a set of wet weather and dry weather processes. Adsorption, straining and desorption were the governing wet weather processes responsible for the retention of faecal microorganisms in biofilters. Die-off was the governing dry weather process controlling survival kinetics of retained faecal microorganisms, between wet weather events. It was found that desorption of previously retained microorganisms within the biofilter, during subsequent wet weather events, decreases the overall microbial removal performance, in some cases can even leading to net leaching. Faecal microorganism removal in stormwater biofilters was affected by a range of biofilter design modifications and operational conditions. Filter media composition, infiltration rate, vegetation and inclusion of a submerged zone (SZ) were found to be the most influential biofilter design parameters. Antecedent dry weather periods, inflow concentration, clogging and length of operation were found to be the most influential biofilter operational conditions. In fact, these main biofilter design and operational conditions were often found to have interactive effects on the abovementioned major microbial removal processes. Consequently, the overall removal performance increased or decreased depending on the dominant effects prevailing at any given time. The findings from this research demonstrated that, a biofilter which consists of a filter media with a high fines fraction (such as loamy sand), a vegetation species with an extensive root structure (such as Leptospermum continentale, Melaleuca incana and Carex appressa) and a steady SZ volume can achieve a higher faecal microbial removal performance. The overall biofilter performance was found to increase with the length of operation. However, both extremely short and extended antecedent dry periods were found to reduce faecal microbial removal in stormwater biofilters. Desorption of previously attached viable microorganisms, due to inadequate die-off, caused decreased performance in wet weather events which follow short, antecedent, dry periods. Further, increased infiltration rates, due to the formation of cracks and macropores during extended antecedent dry weather periods, caused decreased performance in subsequent wet weather events. Also, it was found that net leaching of microorganisms normally occurs in wet weather events have a relatively low inflow microbial concentration as well as a relatively short antecedent dry weather period, preceded by other event(s) with very high inflow microbial concentrations. As mentioned previously, desorption of previously retained microorganisms in the subsequent event was found to be the main driving force behind this poor performance. The current study is one of the first studies to suggest that stormwater biofilters can act as a treatment device against different reference pathogens for bacteria, viruses and protozoa and their indicator microorganisms. Cryptosporidium oocysts were the most effectively removed reference pathogen, followed by adenoviruses. The lowest removal performance was observed in the reference bacterial pathogen, Campylobacter spp. with a potential risk of leaching under very challenging operational conditions. It appears to be that, on average, C. perfringens and FRNA coliphages were the two indicators whose removal performances can be comparable to the corresponding reference pathogen removal (Cryptosporidium oocysts and adenoviruses respectively) in stormwater biofilters. Conversely, E. coli seemed to generally overestimate the removal of the selected reference pathogen for bacteria, Campylobacter spp. A preliminary predictive model was developed using the experimental data and knowledge gained from the existing literature. It was found that the microbial removal performance in stormwater biofilters can be adequately predicted using a model incorporating a set of governing retention and survival processes as well as their influential biofilter design and operational conditions. However, since this is only a preliminary, predictive model, it requires improvement and further testing in the future. In summary, the findings of this research expand our current knowledge on faecal microbial removal in stormwater biofilters. The knowledge gained from this research on governing microbial removal mechanisms major influential design components and operational conditions have the potential to optimise stormwater biofilters for faecal microorganism removal. Subsequent to further research, it suggests biofilters are a credible technology to augment safe stormwater harvesting and improve water quality in surface waters