Abstract

During advanced biological wastewater treatment, a huge amount of sludge is produced as a by-product of the treatment process. Hence, reuse and recovery of resources and energy from the sludge is a big technological challenge. The processing of sludge produced by Wastewater Treatment Plants (WWTPs) is massive, which takes up a big part of the overall operational costs. In this regard, anaerobic digestion (AD) of sewage sludge continues to be an attractive option to produce biogas that could contribute to the wastewater management cost reduction and foster the sustainability of those WWTPs. At the same time, AD reduces sludge amounts and that again contributes to the reduction of the sludge disposal costs. However, sludge volume minimization remains, a challenge thus improvement of dewatering efficiency is an inevitable part of WWTP operation. As a result, AD parameters could have significant impact on sludge properties. One of the most important operational parameters influencing the AD process is temperature. Consequently, the thermophilic and the mesophilic modes of sludge AD are compared for their pros and cons by many researchers. However, most comparisons are more focused on biogas yield, process speed and stability. Regarding the biogas yield, thermophilic sludge AD is preferred over the mesophilic one because of its faster biochemical reaction rate. Equally important but not studied sufficiently until now was the influence of temperature on the digestate quality, which is expressed mainly by the sludge dewateringability, and the reject water quality (chemical oxygen demand, ammonia nitrogen, and pH). In the field of comparison of thermophilic and mesophilic digestion process, few and often inconclusive research, unfortunately, has been published so far. Hence, recommendations for optimized technologies have not yet been done. The review presented provides a comparison of existing sludge AD technologies and the gaps that need to be filled so as to optimize the connection between the two systems. In addition, many other relevant AD process parameters, including sludge rheology, which need to be addressed, are also reviewed and presented.

Highlights

  • Wastewater Treatment Plants (WWTPs) are huge liquid waste management facilities for the existing and emerging cities worldwide

  • Afterwards, this review suggest the ways for optimization of sludge anaerobic digestion (AD) among biogas yield, dewater ability, and reject water quality, as it is an important issue in the forthcoming research

  • Though various aspects of sludge AD have been studied under different conditions, an integrative and holistic approach must be applied with emphasis on operational temperature of digestion

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Summary

Introduction

WWTPs are huge liquid waste management facilities for the existing and emerging cities worldwide. The reuse of sludge for land application and recovery of energy is getting more attention In this regard, the anaerobic digestion (AD) of sewage sludge is favored for its advantages over other sludge management alternatives. The anaerobic digestion (AD) of sewage sludge is favored for its advantages over other sludge management alternatives These include recovery of clean energy (biogas), a reduction of 30% to 50% of sludge amounts, destruction of pathogens, removal of bad odor, and some others [3]. Reviewing articles and comparing mesophilic and thermophilic sludge AD is important to identify such related gaps, in order to achieve process optimization. Afterwards, this review suggest the ways for optimization of sludge AD among biogas yield, dewater ability, and reject water quality, as it is an important issue in the forthcoming research

Massive Sludge Sources
Sludge Composition and Discharge Limits
Comparison of Mesophilic and Thermophilicanaerobic Digestion of Sludge
Sludge Dewaterability and Temperature of Digestion
Sludge Pretreatment and Its Impact on Dewaterability
Sludge Management and Rheology
Findings
Conclusions
Full Text
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