Organic solid waste conversion into hydrogen
The exhaustion of fossil fuels is imminent, for this reason the interest in the production of renewable fuels has been increasing over the years.
The dependence on fossil fuels as energy source contributes to global climate change, environmental pollution and health problems (Levin et al., 2004). Hydrogen (H2) is a clean energy source, accepted as a potential substitute of fossil fuels, producing only water instead of greenhouse gases when burning. The generation of hydrogen may be achieved by a range of technologies, including biological processes using algae and bacteria. For algae H2 production the use of photo-bioreactors have been reported, while for bacterial H2 production a dark fermentation has been documented as the usual process (Levin et al., 2004). Suitable substrates for dark fermentative hydrogen production include carbohydraterich media, wastewater and wastes (Das y Vezuriglu, 2001). Thus, biological hydrogen production from renewable organic waste through dark fermentation represents an important area of bio-energy production.
On the other hand, in Mexico City’s central of supplies are generated around 750 ton/day of organic solid wastes (OSW), which can be used as substrate. The microbial conversion of the organic waste into hydrogen is of increasing interest due to the simultaneous elimination of waste and the energy production. Previous studies developed in an anaerobic digester (AD) used for treating sorted OSW of the Central of supplies showed that under the conditions of the OSW, low pH and easily degraded organic matter, the H2 production was favored instead of methane (CH4) production.
The main goal of this study was to evaluate the feasibility of two pretreatment methods (heat and acid pretreatments) to increase the hydrogen producer population. The H2 production and substrate removal efficiency using different incubation temperatures was also tested.
An anaerobic sludge was used as inoculum. Heat (80 ºC, 30 min,) and acid pretreatments of the mesophilic sludge, obtained from the AD, were used to inhibit the activity of the H2 consumers and to harvest high yield of H2 producing anaerobes. Batch tests were carried out to evaluate the influence of the pretreatment and the incubation temperature (35 and 55 ºC) on H2 production. Systems without pretreatment were used as control. Glucose was used as a model substrate. Metabolic intermediates of the H2 production process were determined throughout the experiments.
The heat pretreated microbial community (HPMC) incubated at 35 ºC showed complete removal of 10 g/L of glucose as substrate in 32 h, the molar yield of H2 reached under these conditions was 2.8 mol H2/molglucose. The acid pretreated microbial community (APMC) incubated at 35 ºC achieved a hydrogen yield of 2.2 mol H2/molglucose. Lower hydrogen yields of 0.8 and 0.7 mol H2/molglucose were determined for the HPMC and the APMC systems respectively when incubated at 55 ºC. CH4 was detected only in systems without pretreatment. Volatile fatty acids (FVAs), acetic and butyric acids, and alcohols such as ethanol and methanol, were determined as metabolic intermediates. The acid pretreatment favored alcoholic and butyric fermentation, whereas heat pretreatment induced acetic acid production. On the other hand, a deviation of the metabolic pathway to an alcoholic fermentation was caused by the incubation of the system at a thermophilic temperature (55ºC)
Heat and acid pretreatments were effective for eliminating methanogenic microorganisms and enriching H2 producing inoculums from an anaerobic sludge. Nevertheless, heat pretreatment H2 yields were higher than those obtained with acid pretreatment, both under a mesophilic incubation temperature. The experimental results showed that the pretreatment of the microbial community and the culture conditions (incubation temperature, substrate concentration and pH) play a key role to enhance the H2 production.
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