NaOH–urea pretreatment for biogas enhancement from algal biomass anaerobic digestion

With the gradual increase in algal waste, its disposal has become a key problem in the field of marine environmental protection. Anaerobic digestion (AD) of algal biomass is one potential method for its treatment in a beneficial manner. However, AD of algae is hindered because of strong resilient cell walls, and hence, pretreatment of algae biomass before AD is an essential process. In this study, green algae (Enteromorpha) were used as the research substrate to explore the damage and impact of NaOH–urea pretreatment on biomass. Scanning electron microscopy and thermogravimetric analysis were used to analyze the damage of the cell wall structure of Enteromorpha via NaOH–urea pretreatment. The response surface method (RSM) was used for the optimization of pretreatment conditions to obtain the best experimental conditions. The experimental independent variables were pretreatment time, pretreatment temperature, and NaOH–urea concentration. RSM results showed that the optimal conditions are pretreatment time of 50 min, NaOH–urea concentration of 5.89%, and pretreatment temperature of −16 °C. AD was carried out under these conditions. The results showed that cumulative gas production for the control group was 231.5 ml whereas the biogas yield for best optimal conditions was 521.5 and 413 ml, respectively. In addition, AD reaction kinetics was evaluated by two mathematical kinetic models. It was found that modified Gompertz model is more suitable for describing the AD behavior.