Materials and methods 5.2 Introduction

and diversity, displaying it is a resilient and competitive genus. Tetrasphaera were present in 69% of samples, showing that it is a prolific genus resistant to Fe dosing. Terrimonas and Tetrasphaera displayed opposite behaviours in thirty-two parameters measured. Little is known of the environmental significance of Terrimonas, this analysis may help illuminate the behaviour of Terrimonas in EBPR. Results of this study can be used as a benchmark to measure EBPR performance and certain microorganisms can be isolated to improve EBPR performance.

Keywords: biological phosphorus removal; chemical phosphorus removal; EBPR upset

5.2 Introduction

Waste water treatment directives set stringent phosphorus (P) final effluent targets to be achieved by wastewater treatment plants (WWTP) which aim to reduce the eutrophication risk of water bodies. Meeting these low discharge targets forced the development of improved biological and chemical methods of P removal. Among them, enhanced biological phosphorus removal (EBPR) is an effective method of reducing effluent P by the activity of a microbial community (Liu et al., 2008). In EBPR, P removal is achieved by the release and uptake of P by polyphosphate accumulative microorganisms

(PAO) in an anaerobic-aerobic sequence (Wentzel et al., 2008). In the anaerobic phase PO4-P is released

and in the aerobic phase luxury PO4-P uptake occurs, resulting in a net removal of PO4-P and an

activated sludge rich in PO4-P (Wentzel et al., 2008). More P is taken up during the aerobic phase than is

released in the anaerobic phase resulting in the net removal of P (Henze et al., 2008). EBPR performance is reliant on the metabolism and interactions of microbial communities present (Hashimoto et al., 2014). In the process, PAO populations are subjected to cycles of anaerobic (feast) and aerobic (famine) phases (Kristiansen et al., 2013).

WWTP operating EBPR processes frequently suffer from struvite (MgNH4PO4.6H2O) precipitation which

blocks pumps and pipes; FeCl2 solution dosing is used to suppress this struvite formation (Doyle &

Parsons, 2002; de Haas et al., 2000). PO4-P measures the soluble P fraction which can be taken up

directly by plant cells, whereas total P measures all forms of P, i.e. dissolved and particulate (Murphy,

2007). Fe ions react with PO4-P present in the system, producing an insoluble phosphate that

precipitates into the sludge (Crutchik & Garrido 2012). Chemical dosing allows P to be removed into sludge, achieving discharge consents more easily, reducing P concentrations, and decreasing struvite precipitation. However, chemical dosing is expensive and increases sludge volume by 37-97% compared to EBPR sludge (Ofverstrom et al., 2011). Excess sludge production increases loads on other WWTP processes and cost of transporting sludge to land. Surplus coagulant use can adversely affect EBPR performance (De Gregorio et al., 2010). A more expedient method to reduce struvite precipitation potential is to recover P as struvite fertiliser. P recovery reduces the amount of P recycling through the

site and diminishes the potential for struvite to clog pumps and pipes (Jeanmaire & Evans 2001). Struvite can be used as a slow-release P, N, and Mg fertiliser, the sale of which provides revenue for the WWTP.

To increase struvite recovery rates up to an efficient and economical >80% recovery rate, FeCl2 solution

dosing must be reduced to increase PO4-P; the form of P recovered by the Ostara process. While

reducing chemical dosing to improve one process onsite, the effects of this on other processes (i.e. EBPR) must be monitored to ensure the consistent and efficient operation of the WWTP to meet environmental regulatory discharge targets.

While microbial communities present in EBPR plants and their functions have been studied to some detail, the response of those communities to chemical dosing remains unclear. In recent years, powerful high resolution molecular techniques have been developed that allow for detailed analysis of the composition, structure and activity of microbial communities. The analysis of the microbial communities generated in EBPR wastewater processes shows a high dominance of α-Proteobacteria and β- Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes (Beer et al., 2004; Liu et al., 2008; Wan et al., 2011; Zhang et al., 2012). Some of those are reported to improve the performance and stability of EBPR, while others have been shown to have a negative effect on the process.

Interactions between chemical dosing and biological processes are complex and not well understood, and full-scale studies are rare. To the best of the authors’ knowledge, only one article has reported the influence of chemical dosing on biological P removal at a full-scale WWTP (Liu et al. 2011). In that study, it is shown that Al significantly inhibits biological P release and uptake, whereas Fe salts exert a weak effect on EBPR and is inhibitory only at high Fe doses (Liu et al., 2011). The effects of chemical dosing on biological processes were examined, but the effects on microbial communities were not monitored. Knowledge on interactions between chemical nutrient removal and biological nutrient removal must be expanded to improve the performance of biological processes. Full-scale studies of EBPR plants are required, as lab-scale research does not correlate well with full-scale WWTP operations. In this work, we analysed the effects of chemical dosing on an EBPR process at a full-scale WWTP over a one year

sampling period. We studied how FeCl2 solution dosing affects the composition of the microbial

populations, their diversity, and EBPR performance, and identified the microbial species that are influential in the process. This research is based on the hypothesis that reducing chemical dosing will increase microorganism diversity by reducing stress conditions and PAO death (De Gregorio et al., 2010;

Motlagh et al., 2015). An increase in EBPR performance (i.e. specific PO4-P release and specific PO4-P

uptake) is expected through reducing the inhibitory effects of chemical coagulant dosing (Liu et al., 2011).