Waste activated sludge treatment based on temperature staged and biologically phased anaerobic digestion system

A perspective on the combination of alkali pre-treatment with bioaugmentation to improve biogas production from lignocellulose biomass

Anaerobic digestion (AD) is a bioprocess technology that integrates into circular economy systems, which produce renewable energy and biofertilizer whilst reducing greenhouse gas emissions. However, improvements in biogas production efficiency are needed in dealing with lignocellulosic biomass. The state-of-the-art of AD technology is discussed, with emphasis on feedstock digestibility and operational difficulty. Solutions to these challenges including for pre-treatment and bioaugmentation are reviewed. This article proposes an innovative integrated system combining alkali pre-treatment, temperature-phased AD and bioaugmentation techniques. The integrated system as modelled has a targeted potential to achieve a biodegradability index of 90% while increasing methane production by 47% compared to conventional AD. The methane productivity may also be improved by a target reduction in retention time from 30 to 20 days. This, if realized has the potential to lower energy production cost and the levelized cost of abatement to facilitate an increased resource of sustainable commercially viable biomethane.

Performance and microbial community evaluation of full-scale two-phase anaerobic digestion of waste activated sludge

"Temperature Staging and Biological Phasing" (TSBP) is an improved two-phase anaerobic digestion (AD) technology. This technology hydrolyzes waste activated sludge (WAS) at 45 °C and converts methane at mesophilic temperature (35-38 °C), with hydraulic retention times of 3-5 d and 14-17 d, respectively. In this study, the performance and microbial community dynamics of full-scale TSBP-based sludge anaerobic digestion system were studied, and the technology was evaluated by energy balance and ecological benefit analysis. The stable operation for 390 d showed that the cumulative biogas yield was about 349,041 m³, the maximum biogas yield rate was 563.68 L/kg VS, and the VS degradation rate of organic matters in the sludge was 47.19%. Proteobacteria and Firmicutes were found to be the dominant bacteria in both thermophilic and mesophilic reactors. Methanobacterium and Methanosarcina were the two most abundant methanogenic genera in the AD samples. The aceticlastic methanogenesis was likely the predominant production pathway of methane in AD processes based on metagenomics. The TSBP system operated stably, and the recovered energy could achieve energy self-sufficiency, which provided technical reference for the anaerobic treatment of sludge.

Impacts of Temperature and Solids Retention Time, and Possible Mechanisms of Biological Hydrolysis Pretreatment on Anaerobic Digestion

Anaerobic digestion (AD) has benefits in sludge management, energy recovery, and pathogen reduction. In order to better understand the mechanisms of biological hydrolysis (BH) pretreatment on AD, biochemical methane potential (BMP) and continuous stirred-tank reactor (CSTR) tests were utilized to compare untreated municipal combined sludge with pilot-scale BH pretreated sludge. During the BH process, there was 15%, 30%, and 33% (w/w) volatile solids (VS) reduction after BH at 42 °C (BH42) for 24, 48, and 72 h, respectively; under BH61 (42 °C for 36 h and 61 °C for 6 h), and there was 10% and 30% (w/w) overall VS reduction after 36-h and 42-h hydrolysis, respectively. BMP results showed that BH42-pretreated sludge had 22.6% enhancement of methane yield compared to untreated sludge, and BH61 pretreated sludge had 29.4% enhancement of methane yield. Both temperature and solids’ retention time (SRT) contributed to the enhanced AD performance within 36 h, while temperature played more important roles after 36-h BH pretreatment. CSTR tests confirmed the acceleration of anaerobic digestion by BH pretreatment, and higher enhancement was observed when SRT of anaerobic digestion was shorter than 16 days. Through a literature review of BH-related studies, the possible mechanisms were highlighted for further optimization on the scale-up systems in order to reduce carbon footprint and operating expenditure for wastewater treatment plants.

Genomic dynamics of full-scale temperature-phased anaerobic digestion treating waste activated sludge: Focusing on temperature differentiation

A robust microbial community is essential for the overall stability and performance of the anaerobic digestion process. In this study, two digesters of a full-scale temperature-phased anaerobic digestion plant treating waste activated sludge were sampled for one year. The acidogenesis reactor (AR) was run at 45 ± 2 °C for six months in Period I and was run at 38 ± 2 °C for six months in Period II. While the methanogenesis reactor (MR) was run at 36 ± 3 °C throughout the year. 16S rRNA amplicon sequencing and GeoChip 5.0 results showed that samples were clearly differentiated by reactors and periods. The elevated temperature in AR during Period I improved the effects of phase separation between the AR and MR. In AR, Fervidobacterium, assigned to Class Thermotogae, had a higher relative abundance of 8.9% in Period I. The abundance of genes involved with carbon degradation was significantly higher in Period I than Period II. In MR, the relative abundance of Methanosarcina increased from 19.8% in Period I to 30.6% in Period II. In addition, the influent characteristics, reactor performance, and operating parameters were determined as the key variables shaping the microbial community, contributing to a total of 76.3% and 69.5% of the variance of the AR and MR, respectively. Combined, this study enriches our understanding of genomic dynamics in full scale temperature-phased anaerobic digestion process.

Phase separation and microbial distribution in the hyperthermophilic-mesophilic-type temperature-phased anaerobic digestion (TPAD) of waste activated sludge (WAS)

In order to investigate the phase separation and microbial distribution in the TPAD, the conventional thermophilic-mesophilic type (TM-TPAD) and the hyperthermophilic-mesophilic type (HM-TPAD) were operated with a single-stage mesophilic anaerobic digestion (MAD) as control. HM-TPAD accomplished the volatile solids destruction 14.5% higher than MAD. Calculating conversion efficiencies distinguished the separation of acidogenic and methanogenic phases in HM-TPAD, which was not found in TM-TPAD. The differences on microbial distributions also reflected the phase separation in HM-TPAD. The protein degraders, Coprothermobacter had higher abundance in the first stage than the second stage of HM-TPAD but it had similar abundance between the two stages of TM-TPAD. Also, the archaeal communities from the two stages of HM-TPAD shared the least similarity but those from the two stages of TM-TPAD were closely similar.

Optimization of process parameters for production of volatile fatty acid, biohydrogen and methane from anaerobic digestion

The anaerobic digestion process has been primarily utilized for methane containing biogas production over the past few years. However, the digestion process could also be optimized for producing volatile fatty acids (VFAs) and biohydrogen. This is the first review article that combines the optimization approaches for all three possible products from the anaerobic digestion. In this review study, the types and configurations of the bioreactor are discussed for each type of product. This is followed by a review on optimization of common process parameters (e.g. temperature, pH, retention time and organic loading rate) separately for the production of VFA, biohydrogen and methane. This review also includes additional parameters, treatment methods or special additives that wield a significant and positive effect on production rate and these products' yield.

A comparative study on the alternating mesophilic and thermophilic two-stage anaerobic digestion of food waste

An alternating mesophilic and thermophilic two stage anaerobic digestion (AD) process was conducted. The temperature of the acidogenic (A) and methanogenic (M) reactors was controlled as follows: System 1 (S1) mesophilic A-mesophilic M; (S2) mesophilic A-thermophilic M; and (S3) thermophilic A-mesophilic M. Initially, the AD reactor was acclimatized and inoculated with digester sludge. Food waste was added with the soluble chemical oxygen demand (SCOD) concentrations of 41.4-47.0 g/L and volatile fatty acids of 2.0-3.2 g/L. Based on the results, the highest total chemical oxygen demand removal (86.6%) was recorded in S2 while S3 exhibited the highest SCOD removal (96.6%). Comparing S1 with S2, total solids removal increased by 0.5%; S3 on the other hand decreased by 0.1 % as compared to S1. However, volatile solids (VS) removal in S1, S2, and S3 was 78.5%, 81.7%, and 79.2%, respectively. S2 also exhibited the highest CH4 content, yield, and production rate of 70.7%, 0.44 L CH4/g VSadded, and 1.23 L CH4/(L·day), respectively. Bacterial community structure revealed that the richness, diversity, evenness, and dominance of S2 were high except for the archaeal community. The terminal restriction fragments dendrogram also revealed that the microbial community of the acidogenic and methanogenic reactors in S2 was distinct. Therefore, S2 was the best among the systems for the operation of two-stage AD of food waste in terms of CH4 production, nutrient removal, and microbial community structure.