Shift in microbial community structure of anaerobic side-stream reactor in response to changes to anaerobic solid retention time and sludge interchange ratio
Introduction
The more stringent regulatory limits imposed to guarantee high environmental standards caused an inevitable increasing of the production of excess sludge. The EU, the USA, and China each produce 6–11 million tons of sludge as dry solids per year; Australia produces 0.3 million tons each year (Semblante et al., 2016). The handling, treatment and disposal of sewage sludge are challenging waste management problems common to many countries that could account for 25–65% of the total plant operating costs (Chon et al., 2011a, Chon et al., 2011b). In this view, the development of technologies able to reduce the sludge production within the conventional activated sludge (CAS) process inspired an overall rising interest, representing nowadays one of the first priorities in the waste management hierarchy. Among all, the insertion of an anaerobic side-stream reactor (ASSR) in the return sludge line of a CAS system could significantly enhance the sludge reduction. The rate of produced sludge passed through the ASSR is usually expressed as a percentage per day of the total mass present in the CAS, and it is known as sludge Interchange Rate (IR).
Over the last two decades, many studies demonstrated that the CAS-ASSR system can reduce the observed sludge yield (Yobs) by up to 40% (Chudoba et al., 1992), 55% (Chen et al., 2003, Saby et al., 2003), 60% (Novak et al., 2007), 66% (Ferrentino et al., 2016b) as compared to a CAS process. Despite the good sludge reduction performance obtained without any negative effects on effluent quality and sludge settleability, the process is still little applied to full scale because the mechanisms leading to sludge reduction has not yet been fully understood (Ferrentino et al., 2016a). The lack of exact information causes uncertainty regarding the appropriate operating and design parameters, and thus a wide variation in sludge reduction efficiency.
A link theory that basically connects all the mechanisms proposed so far, linking the sludge decay, the cell lysis, the extracellular polymeric substances (EPS) destruction and the presence of slow growing microorganisms has been proposed in our previous study (Ferrentino et al., 2016b). In particular, the important role of slow growing bacteria, such as sulphate reducing bacteria (SRB) and denitrifying phosphate accumulating organisms (DPAOs) has been pointed out.
In the literature, several studies had investigated the microbial community in CAS-ASSR systems. The development of a specific microbial community in the ASSR seems to be of prime importance for sludge reduction process (Chon et al., 2011a). Chudoba et al. (1992) was probably the first to find that 60% of the microbial populations in the CAS-ASSR could be classified as phosphorous accumulating organisms (PAOs), contrary to 10% in the reference CAS system. The Authors showed that the periodic passageway of facultative aerobic activated sludge microorganisms through the anaerobic zone in the CAS-ASSR system created conditions of uncoupled growth. In the ASSR, under anaerobic conditions and in the absence of substrate, PAOs use adenosine triphosphate (ATP) and polyphosphates as a source of energy, then, in the water line, under aerobic conditions and in the presence of exogenous substrate, they rebuild their energy reserves at the expense of growth, resulting in a consecutive reduction of activated sludge production.
Wang and Zhao (2011) reported that most bacteria in their ASSR were phylogenetically related to PAOs, denitrifying bacteria and anaerobes. Chon et al. (2011a) showed about 75% similarity for microbial composition between a sequencing batch reactor (SBR) - ASSR system and an anaerobic digester. Recently, Zhou et al. (2015) showed that the insertion of an ASSR could enhance the selection of anaerobic bacteria such as fermentative, hydrogenogenic and acidogenic bacteria that are able to improve the biomass decay and hydrolysis of particulate organic matters. The Authors confirmed also the shift of the main microbial populations from fast growers to slow growers.
However, there is still insufficient literature on the correlation between the sludge production and the microbial community in the ASSR as function of design and operating parameters of the CAS-ASSR system.
For this aim a laboratory scale SBR – ASSR system was established for sludge reduction. The SBR-ASSR system was operated for 9 months under different design parameters: the anaerobic retention time in the ASSR (SRTASSR) and the sludge Interchange Ratio (IR). The estimation of the sludge reduction and the comparison of the microbial community population have been carried out.
Section snippets
System operation
The laboratory scale system consisted of an anoxic -aerobic SBR for nutrient removal from real urban wastewater and a continuously mixed ASSR where the produced sludge was treated. A denitrifying side-stream reactor (DSSR) was introduced in the treatment scheme both to increase the solid concentration in the sludge to cycled back to the ASSR and to complete the nitrate removal in order to ensure a tightly anaerobic environment in the ASSR (oxidation − reduction potential (ORP) = −400 mV). Fig. 1
Observed sludge yield
In each experimental period, the cumulative generated sludge was plotted against the cumulative consumed sCOD (Fig. 2). The slope of linear regression curves for each period was determined for Yobs. The effluent solid and the solids wastage for sampling had been considered in the determination of the Yobs. The Yobs of the SBR-ASSR system in Periods I, II, III were 0.2092 g TSS/g COD (R2 = 0.9935), 0.1432 g TSS/g COD (R2 = 0.9926), 0.117 g TSS/g COD (R2 = 0.9955), respectively. The Yobs of the CAS
Discussion
The sludge production in the SBR-ASSR decreased from Period I to Period III, reaching the high sludge reduction efficiency working at the highest IR (100%) and a SRTASSR of 2.5 d. At those conditions, Ferrentino et al. (2016b) showed that the release of soluble microbial products (SMPs), the hydrolysis of particulate matter and cell lysis in the ASSR improved. Further, increasing the produced sludge passed thought the ASSR, also the removal efficiency of sCOD, total nitrogen, and orthophosphate
Conclusions
Under 10 d SRTASSR and 27% IR, 5 d SRTASSR and 50% IR, 2.5 d SRTASSR and 100% IR, the SBR-ASSR system reduced sludge production by 41, 62 and 66%, respectively. The resulting reduction was correlated to the microbial community. In each experimental period, microbial analysis revealed a large diversity of bacterial populations. Increasing IR and decreasing SRTASSR, results revealed a shift of the microbial community towards fermenting bacteria, able to hydrolyze organic matter and release EPS,
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