Increase of microbial growth potential in municipal secondary effluent by coagulation
Graphical abstract
Introduction
Freshwater shortage is becoming increasingly serious in many big cities or arid areas all over the world because of population growth, climate change, and degradation of existing sources of water. That makes the reuse of municipal wastewater a necessity. Wastewater reclamation and reuse has become a common practice in some big cities.
Microbial growth in reclaimed water is one of the most important issues attracting much attention (Ryu et al., 2005, Jjemba et al., 2010, Thayanukul et al., 2013). Microbial growth in storage and distribution systems is a practical problem that could generate undesired odor and color (Narasimhan et al., 2005), corrode the pipeline of distribution and cooling systems (Li et al., 2011, Wang et al., 2012), and pose potential health risks to human (Jjemba et al., 2010).
Organic nutrient in reclaimed water is an important factor related to microbial growth. The organic matter, especially the fraction called biodegradable organic matter (BOM), is generally the limiting nutrient for microbial growth in reclaimed water (Funamizu et al., 1998). Assimilable organic carbon (AOC) is largely used as a significant indicator for the growth potential of heterotrophic microorganisms in water. AOC is determined by measuring the maximum growth of the test bacterial strains inoculated in water samples. Many researchers have reported the relationship between AOC concentrations and microbial regrowth or biofilm formation in drinking water and reclaimed water distribution systems (LeChevallier et al., 1996, Volk and LeChevallier, 1999, Escobar et al., 2001, Thayanukul et al., 2013). It should be noted that, AOC levels represent the microbial growth potential other than a simply direct measurement of BOM in the samples, because some organic compounds in water may inhibit the microbial growth (Chudoba, 1985, Ross et al., 1998, Ichihashi et al., 2006).
Coagulation is a widely used conventional technique in drinking water and reclaimed water treatment processes. Some researchers have studied the variation of AOC in drinking water during coagulation, but the results were inconsistent. Some results revealed that the AOC concentration decreased after coagulation and filtration (maximum 36–75%) (Charnock and Kjonno, 2000, Kasahara and Ishikawa, 2002, Lehtola et al., 2002, Liang and Ma, 2009, Lou et al., 2012), while other researchers reported that the removal efficiency of AOC was low by the conventional treatments (Volk et al., 2000, Shu et al., 2008, Tian et al., 2008).
There are significant differences in organic composition and concentration between municipal secondary effluent and drinking water. Natural organic matter (NOM) accounts for the majority of organic component in drinking water sources, but the secondary effluent of the biological wastewater treatment process contains a variety of soluble microbial products (SMP) (Barker and Stuckey, 1999, Shon et al., 2006). The difference in water quality between them may lead to different behaviors of microbial growth potential after coagulation. However, the knowledge about the impact of coagulation on AOC in secondary effluents is very limited. One study performed by Thayanukul et al. (2013) investigated the AOC level of reclaimed water produced by different full-scale treatment processes. Their results revealed that coagulation could reduce AOC in secondary effluents in most cases. In order to give a clearer understanding on the effect of coagulation on microbial growth potential of secondary effluents, more studies are in great need.
This study systematically investigated the apparent AOC behavior in municipal secondary effluents during coagulation. By fractioning the organic matters in secondary effluents into different molecular weight (MW) fractions, MW distribution variation of microbial growth potential was determined so as to elucidate the mechanisms of the changes of AOC during coagulation. Furthermore, the removal of specific organic contents by coagulation was measured to identify the key substances related to the changes of microbial growth potential.
Section snippets
Water samples
In this research, water samples were collected from the effluents of secondary sedimentation tanks of three municipal wastewater treatment plants (WWTPs) in Beijing, China. Following primary sedimentations, the secondary biological treatment processes of the WWTPs are all anaerobic–anoxic–oxic (A2O) processes. Fourteen samples were collected from July 2012 to May 2013 (WWTP1: samples A–K; WWTP2: samples L and M; WWTP3: sample N). Each sample of 10–20 L was collected into organic carbon-free
Removal of organic matters in secondary effluents
The coagulation tests of the secondary effluents were conducted with various coagulant dosages of 10–60 mg L−1 at room temperature (20–25 °C). The results indicated that the removal efficiency of organic matters by coagulation was not very high, but increased with the PACl dosage (Fig. SM-1 in Supplementary Material (SM)). At the PACl dosage of 60 mg L−1, the removal efficiencies of DOC and UV254 for all the samples collected were 10–30% and 13–30%, respectively (Table SM-1).
In order to explain the
Conclusions
This study firstly gave an insight into the impact of coagulation on microbial growth potential of secondary effluents. We found that the AOC levels increased significantly after coagulation in the samples investigated.
The microbial growth potential in the fractions with different MWs indicated that the maximum cell densities of microbial growth in whole secondary effluent sample were lower than those in the fraction with MW < 10 kDa. Furthermore, the organic component with MW > 10 kDa showed an
Acknowledgments
This study was funded by Key Program of the National Natural Science Foundation of China (No. 51138006/No. 21221004). The study was supported by the Collaborative Innovation Center for Regional Environmental Quality. And the authors would like to thank the wastewater treatment plants for their supports on water sampling.
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