Anaerobic biohydrogen production from dairy wastewater treatment in sequencing batch reactor (AnSBR): Effect of organic loading rate☆
Introduction
Nowadays, global energy requirements are mostly dependent on fossil fuels, which eventually lead to foreseeable depletion due to limited fossil energy resources [1], [2]. In recent times a great deal of attention is being paid to the usage of hydrogen (H2) as alternative and eco-friendly fuel throughout the world. Presently H2 is produced mainly from fossil fuels, biomass, and water. About 90% of H2 is produced by the reactions of natural gas or light oil fractions with steam at high temperatures. These methods mainly consume fossil fuels as energy source, and are considered to be energy intensive, and not always environmental friendly. Present utilization of H2 is equivalent to 3% of the energy consumption, and is expected to grow significantly in the years to come [2], [3]. Biological production of H2 is one of the alternative methods where processes can be operated at ambient temperatures and pressures, and are less energy intensive and more environmental friendly. Broadly, biological H2 production processes can be classified as biophotolysis of water using algae and cyanobacteria, photodecomposition of organic compounds by photosynthetic bacteria, and fermentative H2 production from organic compounds [2], [3], [4]. So far H2 production by photosynthetic microorganisms was extensively studied [2], [5], while H2 evolution by fermentation was treated with little attention [3]. The fermentative evolution is more advantageous than photochemical evolution for mass production of H2 by microorganisms, where various wastewaters can be used as substrates. Of late, H2 production through anaerobic fermentation using wastewater as substrate has been attracting considerable attention [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].
Exploitation of wastewater as substrate for H2 production with concurrent wastewater treatment is an attractive and effective way of tapping clean energy from renewable resources in a sustainable approach. This provides dual environmental benefits in the direction of wastewater treatment along with sustainable bioenergy (H2) generation. However, the microbial conversion of substrate by anaerobic fermentation is a complex series of biochemical reactions manifested by diverse group of selective bacteria [14]. Dairy wastewater contains complex organics, such as polysaccharides, proteins and lipids, which on hydrolysis form sugars, amino acids, and fatty acids [15]. In subsequent acidogenic reaction, these intermediate products are converted to volatile fatty acids (VFA), which are further degraded by acetogens, forming acetate, CO2, and H2. Lastly, both acetate and H2/CO2, are converted by methanogens to CH4 [16]. To harness H2 as end product from anaerobic process instead of CH4, inhibition of specific biochemical reaction (methanogenic) and enhancement for specific biochemical reaction (acidogenic) are important prerequisites. Also optimized operating conditions can result in good H2 yield. In this direction, we have made an attempt to harvest H2 from dairy wastewater treatment through anaerobic fermentation in suspended growth bioreactor using anaerobic mixed consortia, by restricting the methanogenic activity and manipulating operating conditions of the reactor.
Section snippets
H2 producing mixed consortia
Anaerobic mixed microflora acquired from an operating laboratory scale upflow anaerobic sludge blanket (UASB) reactor treating chemical wastewater for the past three years was used as parent inoculum. Prior to inoculation, dewatered sludge acquired from UASB reactor was subjected to cyclic pretreatment sequences (four times) changing between heat-shock (100 °C, 2 h) and acid [pH 3 adjusted with ortho-phosphoric acid (88%), 24 h] treatment to restrain the growth of methanogenic bacteria (MB), at
Biohydrogen production
After inoculating with selectively enriched mixed consortia, the bioreactor was initially operated with dairy wastewater as feed at OLR of 2.4 Kg COD/m3-day by adjusting the influent pH to 6.0 for a period of 52 days. Subsequent to stable operation, the reactor was shifted to higher OLR of 3.5 Kg COD/m3-day, and operated for a period 36 days, until the reactor attained stable performance. The reactor was then operated at OLR of 4.7 Kg COD/m3-day for a period of 28 days. The experimental data depicted
Conclusions
The study demonstrated the feasibility of H2 generation from dairy wastewater treatment by anaerobic fermentation in suspended growth bioreactor using anaerobic mixed inoculum. However, the process of H2 generation was found to be dependent on the OLR applied. The pretreatment steps adopted for enumerating the H2 production from anaerobic inoculum were found to be effective. The selected reactor operating conditions (acidophilic pH 6) were found to be optimum for effective H2 yield. Integration
Acknowledgment
The authors gratefully acknowledge the financial support of Department of Biotechnology (DBT) [BT/PR/4405/BCE/08/312/2003], Government of India in carrying out this research work
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Communication No.: IICT 061105.