Elsevier

Bioresource Technology

Volume 249, February 2018, Pages 298-306
Bioresource Technology

Enhancement of sludge reduction by ultrasonic pretreatment and packing carriers in the anaerobic side-stream reactor: Performance, sludge characteristics and microbial community structure

https://doi.org/10.1016/j.biortech.2017.10.043Get rights and content

Highlights

  • Improving sludge reduction of anaerobic side-stream reactor (ASSR) was studied.

  • Combining ultrasonication and packing carriers enhanced sludge reduction by 426%.

  • Packing carriers played more important roles for enhancement than ultrasonication.

  • Packing carriers in ASSR improved sludge settleability and dewaterability.

  • Packing carriers enriched slow-growing, fermentative and floc-forming bacteria.

Abstract

Effects of ultrasonic pretreatment and packing carriers on sludge reduction, settleability, dewaterability and microbial community structure in the anaerobic side-stream reactor (ASSR) were investigated with three anaerobic reactors operated in parallel. Ultrasonication from 3.65% in the ASSR to 5.08%, and packing carriers further enhanced the efficiency to 19.2%. Ultrasonic pretreatment of sludge decreased oxidation–reduction potential in ASSR and enhanced the release of intracellular substances. The deterioration of sludge settleability and dewaterability in the ASSR after ultrasonic pretreatment was improved by packing carriers. Illumina-MiSeq sequencing showed that microbial richness and diversity increased after ultrasonic pretreatment and packing carriers in the ASSR. Packing carriers favored the growth of slow grower (Dechloromonas), fermentative bacteria (Draconibacteriaceae, Fusibacter, Acidaminobacter) and floc-forming bacteria (Zoogloea), while hydrolytic and predatory bacteria (Saprospiraceae_unculture) and slow grower (Thauera) was enriched in the ASSR.

Introduction

Because of the stringent environmental regulations, sludge treatment and disposal has become a technical and economical challenge for wastewater treatment plants (WWTPs), in which the activated sludge (AS) process is extensively applied to remove organic matters and nutrients from the wastewater by the growth of biomass (Semblante et al., 2016, Troiani et al., 2011, Zhou et al., 2015a). In order to minimize the costs associated with downstream sludge treatment and disposal, various technologies have been proposed to decrease sludge production within the AS process. They include inserting anaerobic side-stream reactor (ASSR) (Cheng et al., 2017, Semblante et al., 2016, Troiani et al., 2011, Zhou et al., 2015a) or micro-aerobic tank (Niu et al., 2016b) for sludge hydrolysis and uncoupling, using advanced oxidation and ultrasonication to disintegrate cells (Niu et al., 2016a, Yang et al., 2015, Zhou et al., 2015c), and adding uncouplers to minimize biomass growth (Li et al., 2016). As an efficient and low-cost way to reduce sludge production, the ASSR and its derivation, oxic-settling-anaerobic (OSA) process, have been intensively investigated in lab and pilot-scale systems (Cheng et al., 2017, Semblante et al., 2016, Zhou et al., 2015a), and successfully applied in full-scale applications (Troiani et al., 2011, Velho et al., 2016).

Compared to the ASSR, the AS process coupled with physicochemical treatment in the side-stream reactor (SSR) usually achieved higher sludge reduction. Mohammadi et al. (2011) constructed a pilot-scale sequencing batch reactor with ultrasonic treatment, and achieved 78% reduction of sludge yield by applying energy of 35,000 kJ/kg volatile suspended solids (VSS). Qiang et al. (2015) operated a pilot-scale anaerobic/anoxic/oxic process coupled with sludge ozonation, and obtained a maximum sludge reduction efficiency of 85% at dosage of 100 mg O3/g suspended solids (SS). Yang et al. (2015) combined an ozone/ultrasound SSR with anaerobic/anoxic/micro-aerobic/oxic process, and decreased sludge production by 55.08% at 154 mg O3/g SS and 867 kJ/L. These methods have proved reliability and high cost, and their application as a pretreatment for ASSR at low dosage or energy consumption was probably more applicable than their separate usage for sludge reduction. The physicochemical pretreatment can destruct sludge flocs and disrupt microorganism cells (Niu et al., 2016a, Yang et al., 2015), and thus is expected to accelerate the hydrolysis process in the ASSR to convert particulate matters into small molecular compounds.

Focusing on the ASSR itself, the enhancement of sludge reduction efficiency by manipulating operational parameters has been intensively explored, including oxidation–reduction potential (ORP) level (Saby et al., 2003, Wang et al., 2015), hydraulic retention time (HRT) (Semblante et al., 2016, Ye et al., 2008) and side-stream ratio (Cheng et al., 2017). These investigations were very important for understanding mechanism and creating appropriate micro-ecological environment for sludge reduction in the ASSR. It is worthwhile to note that the HRT of the ASSR for effective sludge reduction, ranged from 6 to 7 h in the OSA process (Ye et al., 2008) to 20 d in the ASSR (Semblante et al., 2016), is comparable to the HRT of main stream and too long for WWTPs even after optimization (Cheng et al., 2017). Therefore, further shortening HRT of the ASSR by accelerating sludge lysis and particulate biomass hydrolysis is a spot in the field of sludge reduction. For instance, Wang et al. (2015) decreased ORP level by using gravity thickening to increase sludge concentration in the ASSR, and resulted in the enhancement of sludge reduction and the improvement of settleability. In the microbiological perspective, there are three groups of bacteria related to sludge reduction in the ASSR, including the anaerobic fermentative bacteria responsible for particles hydrolysis and fermentation, the predatory bacteria functioned for biomass lysis, and the slow grower with low sludge yield (Cheng et al., 2017, Li et al., 2014, Zhou et al., 2015a). Many previous studies have proved that packing carriers in the conventional AS process could enrich biomass with long generation time and form longer microbial food chains, and thus decrease waste activated sludge production (Li et al., 2014). Packing carriers in the ASSR probably achieves the enrichment of biomass functioned for sludge reduction, and accelerates sludge reduction with more abundant microbial communities and longer microbial food chains.

In this study, effects of ultrasonic pretreatment and packing carriers on sludge reduction in the ASSR were investigated with three anaerobic reactors operated in parallel. The migration and transformation of dissolved organic matters (DOM) and extracellular polymeric substances (EPS) components were also analyzed to expound mechanisms of sludge decay and secondary substrate release. The settleability, dewaterability and morphological characteristics of sludge in the three ASSRs were compared. Illumina-MiSeq sequencing was applied to investigate the microbial community structure and population composition in ASSR under different pretreatment.

Section snippets

Experimental setup and operating conditions

Three circular types of anaerobic reactors with effective volume of 2 L were operated airtight in parallel for sludge reduction (Fig. 1). The seed mixed liquor for the three reactors with mixed liquor suspended solids (MLSS) of 4.5 ± 0.3 g/L was taken from the ASSR coupled membrane bioreactor (MBR) (Cheng et al., 2017) in Dongqu WWTP, Shanghai, China. AS from oxic tank of the ASSR-MBR was fed into three reactors at 1 cycle/day, and each cycle comprised of 0.2 L of decanting and 0.2 L of filling. The

Release of organic and nitrogenous matters

Fig. 2 summarizes variations of COD, NH4-N, NO3-N, TN, MLSS and MLVSS concentrations in the supernatant of sludge fed into and discharged from the three ASSRs. The average soluble COD (SCOD) in the raw sludge (RS) fed into the conventional ASSR was 33.7 ± 6.6 mg/L, and then increased to 56.6 ± 18.2 mg/L in the supernatant of ASSR. After ultrasonic pretreatment, the average SCOD in the ultrasonic treated sludge (UTS) was increased to 176.1 ± 53.1 mg/L, and rose to 209.6 ± 83.5 and 195.4 ± 80.6 mg/L in U-PASSR

Conclusion

U-PASSR, U-ASSR and ASSR achieved SRE of 19.2%, 5.08% and 3.65% in MLVSS, respectively. Ultrasonic pretreatment of sludge decreased ORP in the U-ASSR and enhanced the release of intracellular substances, while packing carriers in the U-PASSR played more important roles for enhancing sludge reduction. The deterioration of sludge settleability and dewaterability in the ASSR after ultrasonic pretreatment was improved by packing carriers. Microbial richness and diversity increased after ultrasonic

Acknowledgements

This work was supported by Shanghai Rising-Star Program (16QA1401900) and National Natural Science Foundation of China (51408352).

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