Comparative evaluation of anaerobic digestion for sewage sludge and various organic wastes with simple modeling

Comparative anaerobic digestion of sewage sludge at different temperatures with and without heat pre-treatment

Anaerobic digestion of sewage sludge is generally conducted under mesophilic (around 35 °C) or thermophilic (around 55 °C) conditions, whereas it is conducted at lower temperatures in some wastewater treatment plants without heating. In this study, we compared the anaerobic digestion of sewage sludge at 15, 25, 30, 35, 45, and 55 °C following hyperthermophilic pre-treatment at 80 °C for 24 h. Laboratory-scale reactors were operated continuously for more than 1000 days, and batch experiments were performed to evaluate the reaction kinetics. Biogas production rates at 15 °C with and without pre-treatment divided by that at 35 °C without pre-treatment were 0.73 and 0.78, respectively. The dewaterability of the digested sludge was evaluated by the capillary suction time (CST). The CST was approximately 50 s at 15 °C with and without pre-treatment and was slower than the CST at 35 °C. Compared to the shear rate, viscosity was higher at lower temperatures; however, it decreased with pre-treatment, which reduced the energy required for mixing in the reactors. Pre-treatment eliminated Escherichia coli from the sludge; however, E. coli (approximately 10⁵ colony forming unit/g-total solids) was detected after digestion at temperatures ≤30 °C. Pre-treatment was also useful to replace a part of heat treatment required for digested sludge before it was used as fertilizer. Gene sequencing analyses indicated the effects of pre-treatment and digestion temperature on the microbial community in the digested sludge. Co-generation of biogas is useful to obtain both electricity and heat; however, heat from co-generation is sometimes limited. To maximize electricity recovery, the use of low temperature digesters has the potential to reduce fuel costs. The results indicate that anaerobic digestion at low temperatures with or without heat pre-treatment can be an efficient and cost-effective method of treating sewage sludge.

Co-Digestion of Extended Aeration Sewage Sludge with Whey, Grease and Septage: Experimental and Modeling Determination

The potential of co-digestion mixing thickened secondary sludge (TS) from extended aeration wastewater treatment plant and locally available substrates (whey, grease and septage) has been studied in this work, using three steps. The first step was a batch test to determine the biological methane potential (BMP) of different mixtures of the three co-substrates with TS. The second step was carried out with lab-scale reactors (20 L), simulating anaerobic continuous stirred tank reactors, fed by three mixtures of co-substrates that were determined according to the previous step results. Modeling was applied in the third step, using ADM1 as a mechanistic model to help understand the co-digestion process. According to the BMP step, septage used as a co-substrate has a negative effect on performance, and the addition of 10–30% grease or whey would lead to a gain of around 60–70% in the production of methane. The results from the reactor tests did not validate the positive effects observed with the BMP assay but confirmed good biodegradation efficiency (> 85%). The main purpose of co-digestion in this scenario is to recover energy from waste and effluents that would require even more energy for their treatment. The protein and lipid percentages of particulate biodegradable COD are important variables for digester stability and methane production, as predicted by modeling. The results of simulations with the ADM1 model, adapted to co-digestion, confirmed that this model is a powerful tool to optimize the process of biogas production.

Energy generation from anaerobic co-digestion of food waste, cow dung and piggery dung

The study investigated bioenergy generation from anaerobic co-digestion of food wastes (FW), cow dung (CD) and piggery dung (PD). The physicochemical parameters of the substrates were determined before and after digestion following standard procedures after mechanical pretreatment. Throughout the study, pH remained slightly alkaline while temperature varied between 26 and 32 °C. The highest cumulative biogas yield of 0.0488 L was recorded from the digestion of FW + CD + PD on the ninth day. After analyses, the highest methane content of 64.6 was obtained from the digestion of FW + PD while the lowest (54.0%) was from the digestion of FW only. Overall, cumulative biogas production for the four digestion regimes followed the order: FW + CD + PD, FW + PD, FW + CD and FW only respectively. Accumulation of VFAs was recorded at a slow rate during the digestions.

Utilization of high solid waste activated sludge from small facilities by anaerobic digestion and application as fertilizer

Anaerobic co-digestion of sewage sludge with organic wastes has recently gained attention in small facilities. For small facilities, high solids sludge is suitable for transportation to a centralized co-digester, and direct utilization of the digested sludge as liquid fertilizer is recommended. Effects of high solid and hyperthermophilic pretreatment (80 °C, 24 hr) on anaerobic digestion at low temperatures and utilization as fertilizer are investigated by anaerobic/aerobic digestion and paddy soil incubation experiments. The volatile solids (VS)/total solids (TS) ratio decreases to 0.57(-), and the VS removal rate is approximately 0.7 (-) after long-term aerobic digestion. This is possibly the limitation of biodegradation, even with pretreatment, within engineering time. Substrate TS of 16% (not diluted), 10% and 5% are compared. The effect of substrate TS on biogas production performance (0.2-0.3 NL/gVS-added) is not statistically observed. Laboratory-scale paddy soil incubation experiments are performed fed with anaerobically digested pretreated or not pretreated dewatered sludge as liquid fertilizer. Pretreatment promotes nitrogen mineralization before use as fertilizer, which is helpful to prevent an outflow of surplus ammonia to the environment. The effect of soil type on microbial communities is more significant than that of anaerobically digested sludge conditions.

Performance and Microbial Community Dynamics in Anaerobic Digestion of Waste Activated Sludge: Impact of Immigration

Waste activated sludge (WAS) is a byproduct of municipal wastewater treatment. WAS contains a large proportion of inactive microbes, so when it is used as a substrate for anaerobic digestion (AD), their presence can interfere with monitoring of active microbial populations. To investigate how influent cells affect the active and inactive microbial communities during digestion of WAS, we operated model mesophilic bioreactors with conventional conditions. Under six different hydraulic retention times (HRTs; 25, 23, 20, 17, 14, and 11.5 d), the chemical oxygen demand (COD) removal and CH4 production of the AD were within a typical range for mesophilic sludge digesters. In the main bacteria were proteobacteria, bacteroidetes, and firmicutes in both the WAS and the bioreactors, while in main archaeal methanogen group was Methanosarcinales in the WAS and methanomicrobiales in the bioreactors. Of the 106 genera identified, the estimated net growth rates were negative in 72 and positive in 34. The genera with negative growth included many aerobic taxa. The genera with positive growth rates included methanogens and syntrophs. In some taxa, the net growth rate could be positive or negative, depending on HRT, so their abundance was also affected by HRT. This study gives insights into the microbial dynamics of a conventional sludge anaerobic digester by distinguishing potentially active (growing) and inactive (non-growing, dormant) microbes and by correlating population dynamics with process parameters.

A multi-criteria sustainability assessment framework: development and application in comparing two food waste management options using a UK region as a case study

Preventing food wastage is a key element of sustainable resource management. But as food waste is still generated at high volumes, priority is placed on its proper management as a resource, maximising sustainability benefits. This study, by integrating a multi-criteria decision analysis with a sustainability assessment approach, develops a screening and decision support framework for comparing the sustainability performance of food waste management options. A structured process for selecting criteria based on the consideration of environmental, economic and social aspects related to region-specific food waste system planning, policy and management has been developed. Two food waste management options, namely the use of food waste disposal units, which grind food waste at the household's kitchen sink and discharge it to the sewer, and the anaerobic co-digestion of separately collected food waste with sewage sludge, were selected for comparison due to their potential to create synergies between local authorities, waste and water companies, with local circumstances determining which of the two options to adopt. A simplified process used for assessing and comparing the two food waste management options in the Anglian region in the UK, indicated that there are benefits in using the framework as a screening tool for identifying which option may be the most sustainable. To support decision-making, a detailed analysis that incorporates stakeholders' perspectives is required. An additional use of the framework can be in providing recommendations for optimising food waste management options in a specific region, maximising their sustainability performance.

Anaerobic co-digestion: A critical review of mathematical modelling for performance optimization

Anaerobic co-digestion (AcoD) is a pragmatic approach to simultaneously manage organic wastes and produce renewable energy. This review demonstrates the need for improving AcoD modelling capacities to simulate the complex physicochemical and biochemical processes. Compared to mono-digestion, AcoD is more susceptible to process instability, as it operates at a higher organic loading and significant variation in substrate composition. Data corroborated here reveal that it is essential to model the transient variation in pH and inhibitory intermediates (e.g. ammonia and organic acids) for AcoD optimization. Mechanistic models (based on the ADM1 framework) have become the norm for AcoD modelling. However, key features in current AcoD models, especially relationships between system performance and co-substrates' properties, organic loading, and inhibition mechanisms, remain underdeveloped. It is also necessary to predict biogas quantity and composition as well as biosolids quality by considering the conversion and distribution of sulfur, phosphorus, and nitrogen during AcoD.

Effects of the low-temperature thermo-alkaline method on the rheological properties of sludge

Municipal sewage sludge (hereafter referred to as sludge) in increasing amounts is a serious threat to the environment and human health. Sludge is difficult to dispose because of its complex properties, such as high water content, viscosity, and hazardous compound concentration. The rheological properties of sludge also significantly influence treatment processes, including stirring, mixing, pumping, and conveying. Improving the rheological properties and reducing the apparent viscosity of sludge are conducive to economic and safe sludge treatment. In this study, the low-temperature thermo-alkaline (LTTA) method was used to modify sludge. Compared with the original sludge with an apparent viscosity at 100 s(-1) (η100) of 979.3 mPa s, the sludge modified under 90 °C-Ca(OH)2-1 h and 90 °C-NaOH-1 h conditions exhibited lower η100 values of 208.7 and 110.8 mPa s respectively. The original sludge exhibited a pseudoplastic behavior. After modification, the pseudoplastic behavior was weakened, and the sludge gradually tended to behave as Newton fluids. The hysteresis loop observed during the shear rate cycle was mainly caused by the viscoelasticity of the sludge. The hysteresis loop area (Hla) reflected to a certain extent the energy required to break the elastic solid structure of the sludge. The larger the Hla, the more energy was needed. However, this result should be evaluated comprehensively by considering other sludge parameters, such as yield stress and apparent viscosity. Hla may also reflect the damage degree of the sludge structure after shearing action. The irreversible destruction of the structure during shearing may also increase Hla.

Mesophilic anaerobic co-digestion of cattle manure and corn stover with biological and chemical pretreatment

Biological and chemical pretreatment methods using liquid fraction of digestate (LFD), ammonia solution (AS), and NaOH were compared in the process of mesophilic anaerobic co-digestion of cattle manure and corn stover. The results showed that LFD pretreatment could achieve the same effect as the chemical pretreatment (AS, NaOH) at the performance of anaerobic digestion (AD). Compared with the untreated corn stover, the cumulative biomethane production (CBP) and the volatile solid (VS) removal rate of three pretreatment methods were increased by 25.40-30.12% and 14.48-16.84%, respectively, in the co-digestion of cattle manure and corn stover. T80 was 20-37.14% shorter than that of the control test (35 ± 1 days). LFD pretreatment not only achieved the same effect as chemical pretreatment, but also reduced T80 and improved buffer capacity of anaerobic digestion system. Therefore, this study provides meaningful insight for exploring efficient pretreatment strategy to stabilize and enhance AD performance for practical application.