Evaluation of inoculum source and saponification pre-treatment in anaerobic digestion of dissolved air flotation waste from dairy industry

Abstract

Milk is an important nutrition for human-being. However, milk processing industry produces wastewater which has quite high lipid content. Usually, lipid content in dairy waste stream is separated by mean of dissolved air flotation (DAF) unit before the liquid waste is sent to aerobic treatment. Processing in DAF unit results in semi-solid fatty waste called DAF float. Commonly, DAF float is burnt or buried as landfill but problems remain as the resulting smell and the risk of polluting ground water is unacceptable. Having high organic content, DAF float is potential to be anaerobically digested to produce methane as additional energy source for the dairy plant. However, wastes with high lipid content tend to form insoluble scum layer which limits the biodegradability of the waste. Saponification pre-treatment is a possible solution to increase the solubility of the scum layer thus to increase the biodegradability of DAF float. Using sodium hydroxide as reactant resulted in insoluble hard soap while using potassium hydroxide resulted in softer soap. Therefore, combination of both were used to produce suspended homogenous soap. This preliminary study aimed to test the effectiveness of alkali reactants with various doses for saponification pre-treatment and to test the effect of different sources of inoculum on the anaerobic digestion of pretreated DAF float. The available substrate for anaerobic microbes was measured as soluble chemical oxygen demand (sCOD). To test the effect of different sources of inoculum, three reactors were used with cow dung digester effluent (RKS) and biodiesel waste digester effluent (R04) as inoculum sources, along with control reactor without inoculum addition (RCI). Meanwhile, to test the effect of different doses of base addition in saponification pre-treatment, three reactors were used, i.e. reactor with addition of 0.04 mole base/g sCOD (R04), reactor with addition of 0.02 mole base/g sCOD (R02), and reactor without saponification pre-treatment of the substrate (RCS). Results showed that saponification pre-treatment significantly increased the solubility of the DAF float as shown by higher initial sCOD value in reactors with saponified substrate. However, it was inferred that saponification pre-treatment inhibited the indigenous bacteria in DAF float so that additional bacteria from inoculum was needed to be able to produce biogas. This preliminary experiment indicated that effluent of active digester treating biodiesel waste was a better source of inoculum to be compared to the effluent of digester treating cow manure.