Evaluation of the feasibility of beneficiating sewage sludge from municipal wastewater treatment works

Mudzanani, Khuthadzo Ednah
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The study aims to enhance biogas production using the Anaerobic Digestion (AD) from the municipal wastewater sludge and evaluate the feasibility of the value-added products from the by-products of the biogas production. The main objectives are to first understand the physicochemical & microbiological characteristics of the digester feed, optimize the digestion process, evaluate the enhancement by co-digestion with different industrial bio-wastes, and conduct a model-based simulation to predict the performance of the AD process and synthesis of (Sludge-based Activated Carbon) SAC. Most research efforts have focused primarily on the use of co-substrate to boost biogas production and few studies have explored anaerobic co-digestion of specific organic wastes as co-substrates to optimize biogas production; however, none have conducted a comparative study of various co-substrates or bio-wastes. The knowledge gap that was identified is the lack of understanding of the interactive mechanism between the primary substrate and co-substrates in the digestion system. By use of the simulation model, this study provides insights into the physicochemical analysis of the parameters that catalyze and those that can inhibit anaerobic digestion. Literature reports frequently on the synthesis of the adsorption materials from the bio-wastes and a few explore synthesis from the digestate such as SAC. Further research reported the feasibility to synthesize SAC from waste sludge at a low cost of the modification process, highly efficient, and with low installation, operations, and maintenance costs. The physicochemical and microbiological characterization was performed on the wastewater treatment works (WWTW) sewage sludge, from the City of Tshwane (CoT) and Joburg Water (JW) using the standard procedure for examination of water and wastewater. The study examined the anaerobic co-digestion of sewage sludge and six organic waste materials as possible cosubstrates for enhancement using the bio-methane potential technique, the semi-continuous, and the pilot plant trials. The simulation was carried out using MATLAB Simulink, and additional equations and simulations were added to the standard model to develop tools to investigate the feasibility of co-digesting sewage sludge with biowaste. The synthesis technique to modify waste sludge into SAC involved physical and chemical activation to increase surface area and enhance the porous structure. Prior to the batch studies; the textual, physical, and composition of the material were evaluated using SEM, TGA, FTIR, and EDX. The performance test was carried out by the removal of Cr(VI) & Cd(II) heavy metals, from wastewater. The results presented show that WWTW sewage sludge contains a high potential methane production, 28.6 & 27.8 g CH4/kg feed in COT2 PS and JW1 NW sewage sludge, respectively. However, a well-balanced high COD with C/N ratio and VS concentration in the ranges of 10-100 g/kg feed, C/N of 17–36, and VS 80–98% TS is recommended. The microbiology data showed that the high-throughput sequencing analysis showed clear differences in microbial communities available in the sewage sludge along with the AD inhibiting microbes. The microbial analysis indicated that Proteobacteria is the most abundant phylum, followed by the Bacteroidetes, Firmicutes, and Actinobacteria. The archaea community of hydrogenotrophic methanogen genus enrichment includes methanogens such as Methanobacterium, thermoautotrophicum, Methanosarcina barkeri, and Methanobacterium wolfei. Thus both sewage sludge from COT and JW WWTP can be a suitable substrate for efficient anaerobic digestion. Co-substrates such as molasses, food waste, animal manure, and fresh produce waste performed well at co-digestion maximum ratios. Soft drink waste and sewage sludge co-digestions had a maximum co-digestions ratio of 25%. Furthermore, except for manure, co-digestion of all other solid co-substrates resulted in additional VS and COD residuals in digested sludge. In contrast, the majority of liquid co-substrates tested here demonstrated a significant synergistic effect, increasing the removals of TS, VS, and COD during anaerobic digestion. Thus, anaerobic co-digestion might be considered a cost-effective solution that could contribute to the energy self-efficiency of WWTWs and sustainable waste management. A significant reduction in the biogas yield; from 10.23 to 2.02 NL/day was also observed in the semi-continuous setup when the OLR reached the level of 2.90 gVS/L.day. The simulation results report that the optimal mixture and parameter dependence were two aspects to be investigated. The best combination of two separate input variables identified was for SIN, SIC, and q, based on the trial and error observation this propagates during a simulation. One noticeable peak each was discovered in both SIC and SIN , stages 1 and 2. Even though the flow rate shows a poor fitment to the particle swarm optimization, the increase of gas production is noticed until the maximum of 161 m3/day is noticed at stage 20. This corresponds to the operating conditions that were mentioned previously. The SEM results showed that SAC material has a rougher surface with spheres of various dimensions indicating a well-developed porous structure compared to what was reported previously in the literature. TGA results showed a remarkable weight loss of ~30.64% and ~64% observed for raw sludge and SAC material, respectively. The activation led to an increase of oxygen-containing functional groups (C-OH, O-H, and C=O) which might increase the adsorption capacity of the material. The elements present in the sample based on EDX analyses were mainly P, Na, K, Fe, and Ca. All these characteristics contributed to a high adsorption capacity of metal ions such as Cr(VI), Cd(II), dyes, and other impurities on SAC. The material performance was carried out by adsorption of Cr(VI) and removal adsorption efficiency was reported. Batch adsorption studies reported that the highest Cr(VI) removal efficiency was achieved with the SAC by 99.99 %, whereas 60,50 & 40,71 % removal efficiency was achieved from CGAC & raw sludge, respectively. The maximum or equilibrium removal (99.99 %) of Cr (VI) and Cd (II) was achieved by 0.8 and 1.4 g SAC dosage, respectively. This behavior is due to the fact that at increased sorbent dosage, there are more available active adsorption sites for the adsorbate to adhere to, hence, a high percentage removal. The adsorption capacity at equilibrium (qe) was found to increase from 8.91 to 312.50 mg/g with an increase in the initial dye concentrations from 50 to 500 mg/L.
A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering to the Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2022