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Glycogen accumulating organisms (GAOs) are thought to compete with polyphosphate accumulating organisms (PAOs) for the often-limiting carbon sources available in wastewater, deteriorating enhanced biological phosphorus removal (EBPR) performance at high temperatures. Fermentation liquids are often used to provide an additional carbon source supply in EBPR processes, where butyrate is known to be an important volatile fatty acid (VFA) produced in sludge fermentation. Nevertheless, the impact of butyrate on the PAO-GAO competition is not well understood especially at high temperature. The results of this study demonstrate that butyrate, as a supplemental or sole carbon source, could be promising for EBPR in tropical climates. When the carbon source was gradually changed from acetate to butyrate, a substantial PAO population was found under both conditions, despite a substantial shift in the abundance of Candidatus Accumulibacter phosphatis (decreased from 37.4% to 13.9%) to Rhodocyclaceae (increased from 2.0% to 14.5%), where both organisms likely played an important role in P-removal. Thus, a relatively stable P removal performance was realized throughout the whole operation period. Nevertheless, butyrate had a negative impact on GAOs. The biomass concentration and microbial diversity continually decreased in the GAO reactor, and Candidatus Competibacter phosphatis reduced from 27.3% to 6.2%, where the dominant population was replaced by Zoogloea. With the addition of butyrate as carbon source, the total amount of synthesized PHAs reduced in both PAO and GAO cultures and the composition of PHA was greatly changed. The presence of a novel PHA fraction (PHH) may disturb the microbial activity in the aerobic phase, where the GAO culture was more severely affected. Glycogen cycling also seemed to be limited in both reactors. This could reduce the GAO metabolism in both cultures and favor PAOs and P removal. Furthermore, the biomass growth rate of the PAO culture was higher than that of GAO when fed with butyrate, which also provides PAO a competitive advantage. All the above results indicate that butyrate could not be well metabolized by GAOs, but could provide PAOs a competitive advantage. Thus, mixed VFAs (i.e. acetate, propionate and butyrate) are likely to favor PAOs over GAOs in EBPR processes operated in warm climates, where the impact of substantial butyrate fractions represents an advantage towards successful process operation. Nanyang Technological University The authors are grateful for the financial support provided by the Advanced Environmental Biotechnology Centre (AEBC) of Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University, Singapore, and the Research Fund for the Doctoral Program of Singapore and Interdisciplinary Graduate School of Nanyang Technological University.

A new sludge pretreatment approach was developed to disintegrate waste activated sludge by mechanical cutting in this study, and the effect of mechanical cutting pretreatment on sludge fermentation and anaerobic digestion was analysed. The study results showed that a sludge disintegration degree of 6.7 % was achieved when the sludge was mechanically cut for 8 min. Moreover, the sludge fermentation was promoted by mechanical pretreatment. After the pretreatment, the soluble COD (SCOD) production in the sludge fermentation system was as high as 3052 mg/L, which was 2.28 times higher than that of the control group. More short-chain fatty acids (SCFAs), especially acetic acid, were observed in the pretreatment group. The mechanical pretreatment accelerated the sludge digestion process and increased biogas production, resulted in a maximum yield of 539.30 mL and 3.06 times higher than that of the control group. High-throughput sequencing results indicated that the microbial community composition in the sludge fermentation system was affected by the mechanical pretreatment, and these changes were conducive to sludge fermentation. The composition of methanogens in the anaerobic digestion reactors was changed by mechanical pretreatment, the relative abundance of Methanosaeta in the control group was 52.70 %, while that in the pretreatment group was as high as 94.05 %. The methanogens in the sludge anaerobic digestion system involving and without involving pretreatment was 5.47 × 107 and 5.12 × 107 copies/g, respectively. Overall, the mechanical pretreatment was beneficial for the enrichment of methanogens, especially acetotrophic Methanosaeta, and it increased methane production.

Sludge fermentation liquid (FL) is a promising and sustainable carbon source for biological denitrification. In this study, four types of FLs were prepared using different chemical treatments to enhance denitrification. The profile of effluent organic matter (EfOM) were evaluated. Results indicate that the FLNaOH (NaOH treatment at pH 10) contained the maximum amount of soluble organics, but led to the worst nitrate removal performance. Nitrate removal efficiency with FLcontrol was much higher than that of FLCaO2, FLCa(OH)2, and FLNaOH. On the other hand, it was found some organics in FL was hard to be utilized by denitrifiers. In particular, EfOM with FLNaOH presented the highest building blocks, high molecular weight (HMW) protein and HMW polysaccharide. It also contained much higher C–(C/H) group. This study provided a new insight on the FL driven denitrification. The necessarity of pursuing the alkali pretreatment to produce carbon source were discussed and evaluated.