Accumulation and isolation of simultaneous denitrifying polyphosphate-accumulating organisms in an improved sequencing batch reactor system at low temperature
Introduction
Nitrogen (N) and phosphorus (P) are both key nutrients that stimulate the growth of algae and toxiccyano bacteria in natural water bodies, threatening drinking water safety and human health (Latif et al., 2011, Zhang et al., 2011, Bassin et al., 2012, Kim et al., 2013, Li et al., 2014). Since they are main pollution sources, removing N and P from wastewater has attracted extensive attention around the world (Hale et al., 2005, Chiu et al., 2007, Liu et al., 2010). According to conventional theory, N and P removals are function of different microorganisms in a wastewater treatment system (Liu et al., 1996, van Veldhuizen et al., 1999, Lee et al., 2003). Thus anaerobic denitrification unit and aerobic compartment of P removal are included in most wastewater treatment processes. Usually the residual sludge discharge is the only way for P removal. It suggests that the more P will be removed when the more P is absorbed by activated sludge. To enhance the capacity of P absorption, the aerobic sludge has to be recycled into the anaerobic unit for releasing more P, where denitrification also occurs simultaneously. Both denitrification process and P release of polyphosphate accumulating organisms (PAOs) require appropriate chemical oxygen demand (COD). Nitrate nitrogen (-N) or nitrite nitrogen (-N) had adverse effect on P release (Akin and Ugurlu, 2004, Oehmen et al., 2007, Wu et al., 2010, Park et al., 2011). So contradictions between denitrification and P release make it difficult to achieve N and P removals simultaneously in the same reaction fraction only by means of operation control and condition maintenance in treatment plants.
In biological N and P removal system, microorganism groups that are mainly responsible for P removal are PAOs. Its metabolism process includes two steps. Under anaerobic conditions, PAOs would take up carbon sources such as preferably simple and short volatile fatty acids and store them as carbon polymers called poly-β-hydroxybutyrates (PHBs or PHAs) (Jenkins and Tandoi, 1991, Merzouki et al., 2005). In the process, intracellular P was released by PAOs. Under aerobic or anoxic conditions, oxygen is used as only electron acceptors in respiratory chain (Mino et al., 1998, Blackall et al., 2002, Seviour et al., 2003, Zeng et al., 2003, McIlroy and Seviour, 2009, Tayà et al., 2011). At the same time, PAOs absorbed P from wastewater. -N can decrease P absorption of PAOs by impeding P release. Moreover, PAOs is one COD competitor of denitrifying bacteria that was responsible for removing -N. So in order to accumulate PAOs and reach up to the standards of effluent N and P, it is necessary to add carbon resource into the system.
However the discovery of denitrifying polyphosphate-accumulating organisms (DNPAOs) in enhanced biological phosphorus removal (EBPR) process offered a proper solution to solve the problems associated with COD limitation (Seviour et al., 2003, Wang et al., 2009). DNPAOs have parrallel metabolic mechanism with PAOs except for their performance to use -N or -N instead of oxygen as electron acceptors. This physiological characteristic alleviate the -N suppression of P release and the competition for COD between P release and denitrification. And N and P removals simultaneously occurred in the same cell (Kerrn-Jespersen and Henze, 1993, Seviour et al., 2003, Wang et al., 2009).
So the studies on isolation, identification of DNPAOs and their characteristics of N, P removals have been focused in wastewater treatment fields due to their advantages over traditional PAOs (Seviour et al., 2003, Zeng et al., 2003, Wang et al., 2009). Hiraishi et al., 1998, Wagner et al., 1994 and Ahn et al. (2007) have reported that some DNPAOs such as Acinetobacter, Betaproteobacteria and Actinobacteria obtained high enrichment. G.W. FUHS has isolated DNPAOs successfully (Fuhs and Chen, 1975). Moreover, M. Sarioglu applied the pure culture of DNPAOs on biological P removal in a sequencing batch reactor (SBR) (Sarioglu, 2005).
But most of studies were performed at room temperature (Fuhs and Chen, 1975, Sarioglu, 2005), few studies on DNPAOs isolation at low temperature were reported. As an important index of assessing the overall efficiency of a biological treatment process, temperature not only influences metabolic activities of microbial population, but also it has a profound effect on gas-transfer rates and settling characteristics of biological solids. Every functional organism has its proper temperature range, and temperatures below its optimum range have more significant effects on growth rate than those above its optimum range, because activities of microorganisms will sharply decrease with decreasing temperature. The reports showed room temperature is suitable for N and P removal microorganisms (Mulkerrins et al., 2004, Li et al., 2010, Gabarró et al., 2012). So it is more difficult to accumulate and isolate DNPAOs for simultaneous N and P removals. And there are limited DNPAOs resources at low temperature.
Thus, this study aimed to accumulate DNPAOs in a SBR reactor operated by two stage performance (anaerobic-aerobic-anoxic (A-O-A) and anaerobic-anoxic (A–A)) at 15 °C. To isolate and identify efficient DNPAOs strain with high N and P removals simultaneously from accumulated DNPAOs. To determine optimal conditions of isolated strain with favourable N and P removal efficiency. Important factors (temperature, pH, total phosphate (TP) concentration and rotating speed) affecting N and P removal efficiency of isolated strain were investigated in the present paper.
Section snippets
Inoculum and domestic wastewater
The seed sludge applied for DNPAOs accumulation was taken from the secondary clarifier sludge in a local sewage treatment plant in Harbin, China. Total suspended solids (TSS), volatile suspended solids (VSS), sludge volume index (SVI) and pH of the sludge were 7.27 g L−1, 5.17 g L−1, 93.48 and 7.3, respectively. Synthetic wastewater was prepared for DNPAOs accumulation in different nutrients loadings (COD: 200, 350 mg L−1, COD: ammonium (-N): TP = 200: 15: 2.5). The composition of
A-O-A SBR start-up and its efficiency analysis at low temperature
In order to prove PAOs accumulation in an improved A-O-A SBR, -N, -N, COD and TP concentrations were measured and the results were shown in Fig. 2. To eliminate the adverse effect of residual -N on P release in next cycle, the improved system was operated as described in original A-O-A SBR with adding an anoxic phase after the aerobic phase (Fig. 2-stage 2) and Table 2. It indicated the reverse effect of the residual -N on P release in next cycle was eliminated. The average
Conclusions
Efficient N and P removal system was established successfully by the operation of improved A-O-A SBR and A–A SBR even at 15 °C, which was deemed as lower critical threshold of temperature for denitrifying and dephosphatation. The system had stable N and P removals for treating domestic sewage. DNPAOs were successfully accumulated in the system and one DNPAOs strain (J6) of high N and P removal efficiency was isolated and identified. Strain J6 had the potential of efficient, simultaneous N and P
Acknowledgements
This study was conducted with financial support from national technology special fund (2013ZX07201007).
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