Optimization of factors affecting biogas production from dairy wastewater using Response Surface Methodology and its kinetic and microbial analysis

• Anaerobic bacterial degradation of organic materials yields flammable biogas. • Biological treatment is the most preferred method for milk waste. • The conditions required to maximize biogas production are determined using the predictive profiler model (Modde5) in our study. Anaerobic digestion is one of the most widely used treatment methods for producing methane as an alternative energy source. This study aimed to produce biogas from milk waste via an anaerobic digestion process, which involves high nutrient concentrations and microorganisms. Two batch digesters were designed to be capable of producing biogas under the experimental protocol, and the highest daily gas yield was monitored for 64 days. In this context, we conducted an experimental study using the design of experiments (DOE) method for the optimization of biogas production and determined the ideal conditions for biogas generation, which depend on the temperature, humidity, pH, and retention time. The design used in response surface methodology (RSM) studies is a central composite design (MODDE 5.0 software). The various factors affecting anaerobic digestion were optimized with the following parameters: temperature T = 39 °C, pH = 7.45, moisture content 90 % and retention time 59 h. The percentages of nutrients before digestion were 70.6 % organic matter, 40.94 %, 2.85 mg/L and 5.56 % total organic carbon, phosphorus and nitrogen, respectively. The total volume of biogas formed was 17.206 L. The microbiological analysis focused on six kinds of bacteria: mesophilic anaerobic flora, mesophilic aerobic flora, coliforms, Staphylococcus, Pseudomonas and fungal flora. The results of this study indicate that biotechnological processes are adequate for treating milk waste to produce important products such as good-quality methane, which is very rich in methane (49.6 %).