The impact of temperature on the metabolism of volatile fatty acids by polyphosphate accumulating organisms (PAOs)

Highlights

• High temperature affects PAO’s kinetics but not its anaerobic stoichiometry.

• PAO could utilize butyrate and iso-butyrate, hardly used valerate and

• iso-valerate.

• Propionate as sole carbon source causes high aerobic glycogen replenishment.

• A combination of acetate, propionate and perhaps butyrate may be a better for

• EBPR.

• Multi-cycle operation may be crucial for PAOs outcompete GAOs at high

• temperature.

Abstract

This study investigated the effects of different carbon sources on enriched Accumulibacter PAO cultures at high temperature (30 °C) and compared the carbon transformation with low temperature (20 °C) cases reported in literature, revealing several key metabolic differences. While PAOs seemed to prefer propionate anaerobically as compared to other VFAs at high temperature, high aerobic glycogen replenishment was realized with propionate as the anaerobic carbon source, a trait not previously observed at low temperatures. Therefore, it was found that propionate is not correlated with high P removal by Accumulibacter PAO at high temperatures. A combined substrate of acetate, propionate and perhaps butyrate seemed to be a better carbon source combination, since the total VFA uptake rate increased by up to 46%, and this increased the aerobic P-removal efficiency by up to 38.4% and reduced the glycogen recovery by more than 63% compared to the use of only propionate as substrate. This study improves our understanding of how to stimulate successful EBPR operation in warm climates by augmenting the P removal performance of PAOs.

Introduction

The enhanced biological phosphorus removal (EBPR) process can be achieved by exposing polyphosphate accumulating organisms (PAOs) to alternating anaerobic and aerobic conditions (Liu et al., 1994; Mino, 2000; Oehmen et al., 2005b; Zhou et al., 2012). During anaerobic condition, PAOs can uptake carbon sources, mainly volatile fatty acids (VFAs) and convert them into polyhydroxyalkanoates (PHAs) where the required energy is derived from hydrolysis of intracellular polyphosphate (poly-P) and degradation of glycogen. Meanwhile, orthophosphate is released from polyphosphate degradation into the bulk liquid. During aerobic condition, PAOs oxidize intracellular PHA for excessive phosphorus uptake, biomass growth and replenishment of glycogen.

It is known that carbon source and temperature are two highly crucial factors that influence the EBPR performance and stability (Lopez-Vazquez et al., 2009; Lu et al., 2006; Whang and Park, 2006). Previous full-scale studies reported Accumulibacter as the main PAOs in the process consuming volatile fatty acids (VFAs) as carbon source (Lanham et al., 2014; Law et al., 2016; Qiu et al., 2019; Yun et al., 2013). Besides, Pijuan et al. (2004a) and Carvalheira et al. (2014) demonstrated that propionate was the preferred carbon source to support EBPR systems at a temperature of 20 °C. Oehmen et al. (2004) also proved that propionate provided Accumulibacter PAOs with an advantage over glycogen accumulating organisms (GAOs) because Accumulibacter PAOs were able to utilize propionate while certain GAOs could not do so under similar conditions. Other studies showed that butyrate could also be taken up by Accumulibacter PAOs, though usually at lower rates as compared to acetate and propionate (Begum and Batista, 2014; Oehmen et al., 2004; Pijuan et al., 2004a), and valerate was generally less preferred than butyrate (Cai et al., 2019).

VFAs are always limited in municipal wastewater, and it is often necessary to add additional carbon source to improve the nutrient removal performance of wastewater treatment plants. Compared to commercial carbon sources, such as methanol and acetate, VFAs produced from waste activated sludge (WAS) fermentation would be a more economical and sustainable alternative (Cao et al., 2019; Li et al., 2011; Tong and Chen, 2007; Zheng et al., 2010). In the fermentation liquid, acetate and propionate are the two most dominant VFAs, while butyrate, valerate and their isomers are also commonly present. It is not clear how such a VFA profile would affect the metabolic activities and behavior of Accumulibacter PAOs.

Many studies have reported that EBPR performance would fluctuate or even be deteriorated under high temperature conditions (Whang and Park, 2002; Oehmen et al., 2007; Panswad et al., 2003). This is because high temperature (30 °C) favors the growth of GAOs over PAOs (Lopez‐Vazquez et al., 2008; Panswad et al., 2003). In our previous work, a stable EBPR performance was realized with a multi-cycle feeding strategy under high temperature (30 °C). Surprisingly, acetate was identified as the preferred carbon source to maintain system stability over propionate (Shen et al., 2017), unlike what is typically observed at low temperature (20 °C). Oehmen et al. (2005b) indicated that propionate could be used to minimize Competibacter which could provide a selective advantage for PAOs, and Thomas (2008) even showed that propionate was the key factor providing competitive advantage for PAOs at low temperature. Nevertheless, our previous study was unable to establish why acetate might have led to a superior P removal performance as compared to propionate. We hypothesized that the behavior of Accumulibacter PAOs is different under low and high temperatures. The objective of the present work is to determine how the metabolism of VFA differs for Accumulibacter PAOs at high temperature as compared to low temperature, which has not previously been assessed. The impact of acetate, propionate and other VFA on Accumulibacter PAOs at high temperature is needed in order to understand if their metabolism is different under these conditions and how it is different. This will improve our capacity to optimize the performance and better control EBPR systems operated in warm climates.

In this study, acetate, propionate, butyrate, valerate and their isomers were used as sole substrate or in mixtures to provide carbon source to two enriched Accumulibacter PAO cultures operated at 30 °C. The aims of this study were to (1) understand the response of different Accumulibacter PAO cultures to different carbon sources at high temperature (30 °C); (2) evaluate its impact on PAO metabolism and performance under anaerobic/aerobic conditions; (3) compare the performance of Accumulibacter PAO at high and low temperatures (previously established in literature) with the various carbon sources.