Elsevier

Energy

Volume 173, 15 April 2019, Pages 1021-1029
Energy

Enhancement of methane production from anaerobic digestion of waste activated sludge with choline supplement

https://doi.org/10.1016/j.energy.2019.02.076Get rights and content

Highlights

  • Choline addition enhanced methane production by 30.0%.

  • Kinetic analysis of methane production showed an increase in hydrolysis rate.

  • Functional enzyme activities throughout anaerobic digestion were stimulated.

  • Enhanced electron transfer was revealed from EDC investigation.

Abstract

This study investigated the effects of choline supplement on methane production from anaerobic digestion of sludge. Batch experiment with varied dosages (0.1–1.3 g/L sludge) of choline in terms of methane production, sludge degradation and process stability were carried out. The results showed that the addition of choline could greatly improve methane production, and the highest cumulative methane production of 225.7 mL/g volatile solids (VS) could be achieved with the optimal choline addition of 0.3 g/L, which was 30.0% higher than that of the control. Model simulation indicated that modified Gompertz model and Cone model well fitted the actual evolution of methane production. The parameters analysis showed the promotion of hydrolysis rate and methanogenesis potential of sludge by choline addition. Furthermore, the process lag phase was reduced by 32.8%. This study provided an alternative for efficient biomass stabilization and bioenergy recovery from sludge.

Introduction

Waste activated sludge (WAS) is the by-product from biological wastewater treatment process. Over 10 million dry tons of sludge is generated annually from nearly 4100 municipal wastewater treatment plants (WWTPs) in China [1]. Increasing sludge production resulting from the rapid development in the establishment of WWTPs requires more advanced technologies for the sludge treatment and disposal. Sludge stabilization is an essential process before the final disposal to avoid secondary pollution resulting from improper treatment. Meanwhile, since WAS is well worth recycling due to its high content of organic matters which could be converted to bioenergy (methane), sludge utilization is also a significant task in face of the current energy shortage problem [2].

Anaerobic digestion (AD) is a widely-used technology of waste sludge stabilization with high feasibility and good cost performance. Various attempts were investigated for better digestibility to enhance AD process in different aspects, including the promotion of hydrolysis rate, electron transfer, and syntrophism. Different pretreatments including physical (thermal and/or ultrasonic), chemical (alkali or ozone application) and biological (enzymes, thermophilic hydrolysis) ways were applied for purpose of cell disruption and further degradation [[3], [4], [5], [6]], leading to the accelerating hydrolysis of sludge. But high consumption of energy and difficulties in industrial operation were the limiting factors in industrial application [7].

Application of accelerants for the enhancement of biogas production had also achieved a lot, especially for electron transfer and syntrophy of anaerobic species. Different conductive materials like EPS, biochar, magnetite (Fe3O4), hematite (Fe2O3), had been employed to stimulate direct interspecies electron transfer (DIET) [[8], [9], [10]]. Zhou et al. [11] studied the syntrophic cooperation between Methanosarcina and Geobacter under the co-occurrence of iron (III) oxides and humics in anaerobic paddy soil and showed an increase in methane production. Biochar was also proved to stimulate the electron transfer efficiency between methanogen and other species [12,13]. Chen et al. [12] studied the electron transfer kinetics between Geobacter metallireducens and methanosarcina barkeri with conductive carbon cloth application and found an increase in DIET. Some researches had also illustrated the relief of inhibition on methanogen from ammonia nitrogen and volatile fatty acids (VFAs) after the addition of biochar [[14], [15], [16], [17], [18]]. Reductive additions like cysteine [19] and iron [20] were also proved to have positive effects on AD process. Liu and Chen [19] applied cysteine and found that the activity of homoacetogens was improved by 34.8%. In Zhang's study [20], scrap iron was added during the high-solids AD process of WAS and found that methane yield was increased by 29.5%. Apart from these achievements, there are still problems in the application of accelerants waiting to be settled, including low recovery and potential secondary pollution of chemical accelerants like iron (III) oxides and biochar, also some accelerants might add the pressure of further disposal like landfill. Hence, a new and environmentally friendly technology with higher efficiency and better atom utilization is under urgent requirement.

Choline is a water-soluble vitamin-like essential nutrient. According to its structure, choline is considered as an important methyl donor. Due to its strong reductivity and basicity, it may provide better anaerobic environment for methanogen. Furthermore, choline could be transferred into acetylcholine, phosphatidylcholine, trimethylglycine (betaine) - essential components for cell composition and metabolism activities [21]. Especially in the cycling of methionine and serine, choline addition was proved to stimulate the reaction and improve the utilization ratio of methionine [22]. There were also researches proved an enhancement of the growth and swarming of some anaerobic microorganisms. In Jameson's research [23], choline could rapidly be utilized by Proteus mirabilis for enhanced growth rate and cell yield in broth culture, as well as the swarming-associated colony expansion, but yet no published researches were found to have investigated choline application for promoting sludge stabilization or organic matter decomposition.

The objective of this study was to investigate the feasibility of choline added into AD for waste activated sludge. Various dosages of choline supplementation were conducted to examine the effect on biogas production. The efficiency was evaluated based on biogas production, methane purity of biogas, sludge degradation, changes in dissolved organic matter (DOM) and intermediates kinetics. Enzyme activities, cyclic voltammetry curves and the electron exchange capacity were also applied for deeper investigation of choline's enhancing mechanism from both biological and electrochemical perspective.

Section snippets

Substrate and inoculum

The substrate sludge was the municipal WAS collected from the secondary clarifier of Minhang Wastewater Treatment Plant located in Shanghai, China, with a treating capacity of about 30000 m3/d using A2/O activated sludge process. About 15 tons sludge (80% moisture) was generated every day. A physical screening (mesh diameter < 1 mm) for the removal of coarse solids such as grit, wood pellets and hair was applied to the collected sludge, and which was then stored at 4 °C before further use. The

Biogas and methane production

Biogas and methane production were the main indicators for resource utilization of WAS, the results of which were showed in Fig. 1. Similar trends were found in both the accumulative biogas (Pbiogas) and methane (Pmethane) production, with an increase in the prophase and were stabilized after day 20. Pbiogas and Pmethane were enhanced by choline supplementation with dosage from 0.1 g/L to 0.3 g/L, while no improving effects were found in higher dosage (from 0.6 g/L to 1.3 g/L). Compared to the

Conclusions

This study demonstrated an enhancement of hydrolysis and methane production from WAS with proper addition of choline. The results showed that dosage of choline at 0.3 g/L sludge achieved the optimal AD performance by increasing the cumulative CH4 production as well as the VS removal rate by 30.0% and 14.5%. Kinetic analysis of methane production presented a larger maximum methane potential and production rate, shorter lag-phase time and higher hydrolysis rate after choline addition. Potential

Acknowledgement

This work is supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (No. 2017ZX07403002-03) and National Natural Science Foundation of China (No. 21876110).

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