Abstract
Biomass from various sources such as cow dung is a significant source of renewable energy (as biogas) in many regions globally, especially in India, Africa, Brazil, and China. However, biogas production from biomass such as cattle dung is a slow, inefficient biochemical process, and the specific biogas produced per kg of biomass is relatively small. The improvement of specific biogas production efficiency using various dilution ratios (and, hence, total solids [TS]) is investigated in this work. A wide range of feed dilution (FD) ratios of cow dung: water (CD: W) was tested in batch biogas digesters with total solids ranging from 1% to 12.5% and FD ratio ranging from 2:1 to 1:20. To further verify the results from the above batch experiments, semi-batch experiments representative of field-scale biodigesters were conducted. Semi-batch reactors have a steady-state process, unlike batch reactors, which have an unsteady state process. Our results suggested that specific biogas production (mL/g VS) increased continuously when the total solids decreased from 12.5% to 1% (or when dilution increased). Our experiments also indicate that the commonly used 1:1 feed dilution ratio (TS ~ 10% for cow dung) does not produce the maximum specific biogas production. The possible reason for this could be that anaerobic digestion at higher total solids is rate limited due to substrate inhibition, mass transfer limitations, and viscous mixing problems that arise at higher total solids concentration. Hence, a higher feed dilution ratio between 1:2 and 1:4 (TS between 4 and 6.7%) is recommended for a more efficient biomass utilization of cowdung. Empirical relationships were also developed for variation of specific biogas yield with the total solids content of the cow dung slurry.Graphic abstract
Highlights
Solid waste disposal is increasingly becoming a challenge due to population rise, and the importance of renewable green energy is more than ever
In the 1:1 feed dilution (FD) ratio experiment, 500 g of cow dung was taken with 450 g of distilled water and 50 g of inoculum, which was 1:1 dilution in mass since the density of water and the inoculum was taken as approximately as 1 g/maximum total biogas (mL)
FD ratio possibly reduced the inhibition caused by mass transfer, mixing limitations, and ammonia concentrations
Summary
Solid waste disposal is increasingly becoming a challenge due to population rise, and the importance of renewable green energy is more than ever. The cattle population in the world has grown steadily and is estimated to be around 1.6 billion [1]. Improper disposal of cattle dung is an environmental and health challenge that can be overcome by using cattle dung in biogas plants, which simultaneously produces valuable cooking gas and organic fertilizer. There are over 4.5 million biogas plants in India and over 27 million biogas plants in China alone [2]. The potential for biogas production from cattle manure and agricultural waste is very high. Even a slight increase of 25% in biogas production efficiency can cut down the use of fossil fuels by a significant amount
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