The increasing global demand for sustainable waste management and renewable energy has spurred interest in anaerobic digestion (AD) as a viable solution. Anaerobic digestion is a biological process that breaks down organic materials in the absence of oxygen, producing biogas and digestate as valuable by-products. Among the various applications of AD, anaerobic co-digestion (AcoD) has emerged as a promising method to enhance biogas production by combining multiple organic waste streams. Specifically, the co-digestion of sewage sludge with other organic wastes in a high solid state has gained attention for its ability to maximize resource recovery, reduce greenhouse gas emissions, and address the challenges of handling complex waste compositions.
High solid state anaerobic co-digestion (HS-AcoD) operates at a reduced moisture level, offering advantages such as reduced reactor volume, higher organic loading rates, and lower water consumption compared to traditional wet digestion methods. By leveraging the synergistic effects of co-digestion, this process improves biogas yield, stabilizes the digestion process, and ensures better management of sewage sludge—a major by-product of wastewater treatment plants. Additionally, integrating diverse organic wastes such as food waste, agricultural residues, or industrial by-products with sewage sludge enhances substrate diversity, promoting microbial activity and nutrient balance within the digester.
This introduction explores the significance of HS-AcoD as a sustainable waste treatment technology, its potential environmental benefits, and the challenges that must be addressed for its broader adoption. It sets the stage for an in-depth discussion on optimizing process parameters, understanding microbial dynamics, and evaluating the technical and economic feasibility of implementing HS-AcoD on a large scale.
Hydrolysis, acidogenesis, acetogenesis, and finally methanogenesis are the four basic processes of anaerobic digestion (Wang et al., 2018). Here, the rate-limiting stage is the hydrolysis in which complicated organic molecules are converted into smaller and simpler molecules by the extracellular enzymes of microorganisms (Deepanraj et al., 2017).
Materials and Methods
Seed sludge
The seed sludge used in the experiments was the dried sludge obtained from the drying beds of El Berka wastewater treatment plant in Cairo, Egypt.
Substrates
To simulate FW generated in Egypt, municipal solid waste (MSW) was used in this study. Food waste had average TS of 13% and consisted of rice, cabbage, potatoes, carrots, cucumber, apple, milk, honey, and bread. Bones were collected and excluded.
Inoculum source
Digested sludge was collected from the inlet and outlet of mechanical dewatering system in the existing El Gabal El Asfar wastewater treatment plant.
Mixing ratio
On the basis of the previous studies in this field, the mixing ratio used in this research between the food waste and sludge was chosen according to (Marcelo et al., 2017; Prabhu & Mutnuri, 2016). To determine the optimal ratio of mixing FW with SS for anaerobic co-digestion, a biomethane potential batch was achieved.
Pretreatment conditions
Thermal pretreatment was chosen as a pretreatment method. The homogenized food waste (FW) and sludge were pretreated separately by an electric oven to the following temperatures (100, 120, 140, 160, and 180 °C).
Biological methane potential tests
The biological methane potential (BMP) tests, which is considered the most suitable method for a relatively easy evaluation of the anaerobic digestibility, was used as a tool for evaluating the biogas production and biodegradability of the mixtures of FW and SS at different pretreatment temperatures under mesophilic conditions.
Results
The best conditions and degree of improvement under co-digestion with thermal pretreatment vary noticeably. Accordingly, the temperature and duration of the optimum thermal pretreatment and the mixing ratios depend on the difficulty of the hydrolysis of the substrate and the nature of the biomass.
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All References are included in the research.
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