The Effect of COD/N Ratios and pH Control to Biogas Production from Vinasse

Aims: Vinasse that is bottom product of distillation unit from alcohol industry contains Chemical Organic Compound (COD) in high concentration. The purpose of this study was to investigate the effect of COD/N ratios of substrate and pH control to biogas production from vinasse.

Study Design: This study used anaerobic digestion-laboratory scale at room temperature in batch system. Urea was added as nitrogen source to adjust COD/N ratios of 400/7, 500/7, 600/7, and 700/7. Initial pH for all variables was adjusted 7.0 by using NaOH solution.

Place and Duration of Study: Waste-treatment Laboratory, Department of Chemical Engineering, University of Diponegoro, Indonesia, between August 2012 and January 2013.

Methodology: Vinasse used was obtained from the alcohol industry that produced alcohol from molasses. Polyethylene bottles which had volume 5 liters were used as digester. Vinasse of 1 liter was put into digester. Urea was added to make variation of COD/N ratio. Initial pH of all variables was adjusted 7.0 by using 10 N NaOH solution. Rumen fluid (10%v/v vinasse) was added as methanogenic bacteria inoculum. Biogas formed was measured by using water displacement method every once in two days to know biogas production daily. pH of substrates in the digesters was measured by using pH meter every once in two days to know pH profile daily. COD of substrates was measured by using COD Analyzer Hanna Reactor with specification High Range of Reagent. Solution of NaOH 2 N was used to maintain pH of substrate in range neutral condition (7.0±0.2) during the fermentation process.

Results: Biogas formed at COD/N ratio of control variable (1436/7); 400/7; 500/7; 600/7; 700/7 were 3.673; 4.909; 6.079; 6.096; 5.631 mL/g COD respectively. pH profiles for all variables were decreasing from beginning until ending of fermentation. With controlled pH, pH of substrates was maintained at neutral condition, so methanogenic bacteria could grow well in the digesters. Consequently biogas formed at controlled pH was larger than that at uncontrolled pH. The values of COD removal for COD/N ratio of control variable; 400/7; 500/7; 600/7; 700/7 were 1.27±0.43; 1.59±0.43; 2.85±0.39; 3.21±0.49; 2.22±0.39 % respectively at uncontrolled pH, whereas at controlled pH the values of these were 11.98±0.56; 12.82±0.56; 12.03±0.94; 13.05±0.35; 12.61±0.56 % respectively.

Conclusions: COD/N ratios of 400/7, 500/7, 600/7, 700/7 produced more total biogas than control variable. Variable of COD/N of 600/7 generated the most total biogas which was 6.096 mL/g COD. Biogas production at pH control was greater than that at pH non-control. At non-controlled pH, COD/N ratio of 600/7 had COD removal 3.21±0.49%. Whereas at controlled pH, COD/N ratio of 600/7 had COD removal 13.05±0.35%.