Anaerobic digestion performance of concentrated municipal sewage by forward osmosis membrane: Focus on the impact of salt and ammonia nitrogen
Graphical abstract
Introduction
Forward osmosis (FO), which is a natural phenomenon based on water transport towards a solution with higher osmotic potential (Cath et al., 2006), is regarded as a breakthrough technology worldwide due to its advantages compared to traditional pressure-driven membrane processes (Ortega-Bravo et al., 2016). Based on the above advantages, FO membrane technology has been applied in a number of aspects (Choi et al., 2009, Efraty, 2016) like water treatment (McHugh et al., 2017), in which FO can be used as a highly effective water pre-treatment process (Soler-Cabezas et al., 2018) for wastewater, juice, and urine concentration (Ansari et al., 2017, Bell et al., 2017).
With the development of society and the boom of the population, issues caused by resource shortage are constructing a threat to the sustainable development (Honda et al., 2015, Iskander et al., 2017, Lee et al., 2014, Liu et al., 2017). To alleviate the problem, in the latest years the recovery of nutrient and energy from sewage has aroused increasing interest (Soler-Cabezas et al., 2018). The recovery of water, energy and nutrient from sewage is of great importance because sewage can become a source of value and economic gain if its treatment is re-oriented to highlight the recovery. In our previous study, it was proved that real sewage could be concentrated by FO membrane (Gao et al., 2018), which indicated that dealing with real sewage by FO membrane was feasible, and the concentrated sewage, with high concentration of organics, ammonia nitrogen (NH4+-N) and phosphorous, could also be further treated by other methods.
Commonly, using aerobic bioreactor, wherein the costs of aeration and sludge handling remained as the major disadvantages (Chen et al., 2014), is applied for treatment of low-strength sewage with no energy recovery (Castelló et al., 2018). But concentrated sewage, which is significantly high in organics, is an alternative for the feed of traditional anaerobic digestion (AD), in consideration of energy conservation, nutrient and energy recovery (Chen et al., 2014). However, there is one thing to be noticed that the AD process of typical high-strength sewage was quite different from that of FO concentrated sewage, which is not only high in organics, but also in NH4+-N and salinity caused by salt reverse osmosis (using NaCl as the draw solution) determined by FO membrane process characteristic. In AD process, COD can be converted into methane by anaerobe to achieve energy recovery. However, speaking of FO membrane combined with anaerobic treatment (like anaerobic FO membrane bioreactor), simulated sewage was adopted in most of previous research (Yap et al., 2012). In simulated sewage, the main source of COD came from glucose, which could be easily degraded. Due to the complex composition of organics, the gas production potential of FO concentrated real sewage could not be predicted and identified easily. There might be difference between it and simulated sewage biogas production potential. Among these characteristics, high salinity and NH4+-N concentration might have influence on AD process. In previous research, some study pointed out that the individual presence of high NH4+-N (Poirier et al., 2016, Sung and Liu, 2003) and high salinity (Li et al., 2017, Wu et al., 2017b) did have negative influence on the methane production of AD process. Nevertheless, until now, to the best of our knowledge, few reports dealing with the potential and feasibility of FO concentrated sewage in AD was reported. The influence of synergistic effect of many aspects (mainly organics, nutrient, salinity and NH4+-N) contained in FO concentrated sewage on biogas production is unknown. And if inhibiting effect does exist, is there any solutions to relieve it? The AD performance of FO concentrated sewage, which is in great request of investigation, is not sufficiently analyzed yet. Further study on the impact of high salinity and NH4+-N concentration in FO concentrated sewage and its remission is also demanded.
Based on the above, to investigate the AD performance of FO concentrated sewage, as well as the influence of salinity and NH4+-N on biogas production is worth studying. In this research, a laboratory-scale anaerobic bioreactor (continuous stirred tank reactor, CSTR) was built for the removal of chemical oxygen demand (COD) and the production of biogas, to evaluate the AD performance of FO concentrated real sewage. Inhibitory batch tests were also carried out for the impact of both single and synergistic presence of NaCl and NH4+-N on methane production. Meanwhile, sludge acclimatization was applied as one approach to relieve the impact.
Section snippets
AD reactor
The AD reactor was a laboratory-scale CSTR with a working volume of 2 L (Fig. 1(a)). Anaerobic sludge, with initial mixed liquor suspended solids concentration of 3.55 g/L, sampled from anaerobic fermenters of a local municipal wastewater treatment plant (Beijing) with no pre-treatment was used to inoculate the reactor. The reactor was maintained at 36–38 °C using water bath heating, in which the circulating water passed through and was recycled in the interlayer of the reactor. The pH of the
COD removal and first-order kinetic model
Fig. 2 illustrates the decrease in COD concentration with the extension of reaction time of FO concentrated sewage and simulated concentrated sewage (water index shown in Table 1), respectively, indicating the degradation of organic compounds during the AD process. The initial COD concentration of the simulated concentrated sewage was about 3000 mg/L. It was observed that the COD concentration was greatly affected by the reaction time. Shorter reaction time (12 h) led to lower COD removal
Conclusions
An anaerobic bioreactor was built for FO concentrated sewage treatment. The results showed FO concentrated sewage could be purified to a certain degree with 80% COD removal. Energy recovery could be achieved. The results of the inhibitory batch test showed (i) when NH4+-N concentration was lower (<200 mg/L), the biogas production was promoted, with the concentration went high, the inhibition appeared; (ii) single existence of NaCl had negative influence on methane production; (iii) the
Acknowledgments
The research was financially supported by the International Program of MOST of China (No. 2016YFE0118500).
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