A comparative study on denitrifying sludge granulation with different electron donors: Sulfide, thiosulfate and organics
Graphical abstract
Introduction
Biological nitrogen removal (BNR) process has been widely applied to remove nutrients (e.g. nitrogen) from wastewater in view of its effectiveness, environmental friendliness and energy conservation (Tchobanoglous et al., 2003). In BNR process, chemolithotrophic denitrification plays an essential role in the reduction of nitrate to nitrogen gas. With respect to denitrification, both heterotrophic denitrification (HD) and autotrophic denitrification (AD) have been reported extensively with each advantage. For example, higher nitrogen removal rate was demonstrated in HD reactor while lower sludge (bio-wastes) production was observed in AD reactor (Qian et al., 2015a, Oh et al., 2000). However, the floatation of the flocculent sludge and its washout from the conventional both HD and AD reactors are common problems during denitrification reactors' operation, which seriously deteriorates the nitrogen removal performance (Eiroa et al., 2005). Therefore, effective biomass retention in the denitrification reactor should be improved to maintain the stability of the sludge. And sludge granulation is one of the most promising technologies for the biomass retention (Val del Río et al., 2015).
Sludge granulation is an advanced environmental biotechnology in wastewater treatment based on the principle of biomass self-immobilization (Liu et al., 2003). Granular sludge (GS) appeared in the form of aggregates, is the collective of numerous cells. Various functional microorganisms stick to each other in the aggregates, which have the tightly compact structure and where diversified microbial communities are spatially distributed (Adav et al., 2008a, Adav et al., 2008b). The characteristics of GS include abundant microbial biodiversity, high retainable biomass concentration, large specific density, low sludge yield, excellent sludge settling ability, and robust ability to withstand high pollutant loading rates (Liu et al., 2009). Therefore, granulation technology attracts tremendous academic interests in both industrial and domestic wastewater treatments. Despite the fact that denitrifying granular sludge (DGS) has been successfully cultivated in the lab-scale study (Val del Río et al., 2015), most of the these works focused on the effects of ratio of electron donors (e.g. organics and sulfide) to nitrogen (Yi et al., 2016), nitrogen loading rate (Li et al., 2013), nature of carbon sources (Lew et al., 2012), feeding strategy (Franco et al., 2006), hydraulic shear force (Xue et al., 2016), calcium/magnesium (Chen et al., 2015, Liu and Sun, 2011) on the DGS cultivation and stability. The comparison between the sludge granulations in HD and AD is missing in the literature.
As for AD, sulfide is the most commonly used electron donor and sulfide-based AD has been applied for simultaneous bio-desulfurization and denitrification (Guo et al., 2016). Alternative to sulfide, thiosulfate is a potential electron donor for AD, characterized as high bioavailability (Cardoso et al., 2006). Recently, we proposed a sulfur cycle-based bioprocess for co-treatment of wet flue gas desulfurization (FGD) wastes with freshwater sewage. In this process, organics in the sewage are removed through biological sulfate/sulfite (via alkaline absorption of SO2 from the fossil power plant) reduction. After the biological sulfate/sulfite reduction, three major compounds, i.e. sulfide, thiosulfate and organics residuals co-existed in the effluent. These compounds act as three types of electron donors in the subsequent denitrification reactor for BNR (Qian et al., 2015b) (the schematics of the co-treatment process of wet FGD wastes with freshwater sewage could be referred to Fig. S1). However, to the best of our knowledge, no detailed comparative studies on the denitrifying sludge granulation with these three electron donors, i.e. sulfide, thiosulfate and organics have been carried out so far.
In this study, three sequential batch reactors (SBR) were set up and operated in parallel with sulfide, thiosulfate and organics (i.e. acetate) as the electron donor for denitrification. The specific focuses are placed on: 1) the rate of sludge granulation fed with sulfide-, thiosulfate- and organics, respectively; 2) the nitrogen removal performance in the three DGS-SBR reactors and denitrification activities of the three DGS in the batch reactors 3) the physiochemical and biological characteristics of the three DGS.
Section snippets
Experimental set-ups and DGS-SBR reactors' operation
Three lab-scale identical DGS-SBR reactors with effective volume of 2.2 L (height: 50 cm; diameter: 7.5 cm) were fabricated and set up, as shown in Fig. S2. The raw sludge was taken from a heterotrophic denitrification reactor of the modified A2O process in a municipal sewage treatment plant in Xi'an, China. About 1 L of raw sludge was seeded in each reactor, resulted in the initial biomass concentration of 1600 mg MLVSS/L (MLVSS: mixed liquor volatile suspended solids). 120 mg N/L sodium
Nitrogen removal performance in SBR reactors with different electron donors
In the first week after SBR reactors' start-up, nitrate removal efficiencies ([Influent NO3− concentration – Effluent NO3− concentration]/Influent NO3− concentration × 100%) in both sulfide- and thiosulfate-SBR reactors were less than 50% (Fig. 1a and b). Also NO2− accumulations in the effluent of both reactors were found. The low nitrogen removal capacities in these two reactors should be attributed to non-acclimation of the biomass to the new condition and biomass washout from the reactors,
Conclusions
DGS was cultivated with different electron donors in three parallel SBR reactors in this study. The denitrifying sludge granulation fed with sulfide, thiosulfate and organics were compared. The main findings are summarized as follows,
- (1)
The DGS could improve the nitrogen removal capacity (in terms of kg N/kg MLVSS/d) and denitrification activities (including both denitratation and denitritation) to a large extent compared with flocculent sludge.
- (2)
Longer time was required for sulfide-DGS cultivation
Acknowledgements
This work was financially supported by Natural Science Foundation of China (No. 51608444), the Fundamental Research Funds for Central Universities (No. 3102017zy002) and Natural Science Foundation of Shenzhen (No. JCYJ20170306153655840). Dr. Jin Qian gratefully acknowledges the financial supports from the Open Fund of State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (No. SKLGP2017K026), and the Open Fund of State Key Laboratory of Hydraulics and Mountain River
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