Effects of alkaline pretreatment on the anaerobic digestion process of vegetables wastes

Methane is produced by methanogenic bacteria from acetic acid, hydrogen and carbon dioxide and from other substrates of which formic acid and methanol are the most important (Bouallagui et al., 2005) during the anaerobic digestion of organic biomass. The objectives of this study consisted in setting up laboratory–scale biochemical methane potential
(BMP) assay digesters at mesophilic conditions to assess the effectiveness of alkaline (NaOH) pretreatment in improving the anaerobic digestion process for biogas production from mixed vegetable wastes.

Further Authors:
A. Mudhoo - National Research Chair in Solid Waste Management, Mauritius
V.D. Ramdeen - National Research Chair in Solid Waste Management, Mauritius

The parameters monitored were cumulative biogas generation, pH, temperature, Total COD, Soluble COD, total solids (TS), volatile solids (VS) and volatile fatty acids (VFA) with time. Mixed vegetable wastes (120.6±6.00g) were subject to anaerobic digestion after alkaline pretreatment with NaOH(aq). A total solid (dry matter) to liquid ratio was 10.09±0.14% (w/w) and inoculum to wet feed ratio 30.14±1.67% (w/w). Digesters (run in duplicates) were incubated at 30±0.1ºC. Waste activated sludge was used as inoculum in this study and had been taken from the wastewater treatment anaerobic reactor in a textile factory. Controls (pH 7.01±0.67) showed a decrease in VFA to 378±25 meq/L, a decrease in COD from 159,420±8,306 mg/L to 143,478±8,306 mg/L and a 38.9±19.7% decrease in TS. The other two experiments were A1 7% NaOH(aq) (w/w) and A2 12% NaOH(aq) (w/w). For A1 at an average pH of 6.28±1.14, a 34.2±10.2% decrease in COD from 160,870±3,682 mg/L, 18.8±7.56% decrease in SCOD, 85.9±6.8% decrease in VFA from 1,128±416 meq/L, 50.5±5.8% decrease in TS and 52.9±14.6% decrease in VS were recorded. For A2 at an average pH of 6.25±1.12, a 19.1±9.52% decrease in COD from 166,667±15,868 mg/L, 12.6±8.04% decrease in SCOD, 88.9±36.6% decrease in VFA from 1,125±412 meq/L, 45.2±25.6% decrease in TS and 50.0±16.1% decrease in VS were deduced. The average cumulative volumes of biogas produced after 35 days of anaerobic digestion were 285±7.0 mL for the control, 234±5.0 mL for A2 and 650±11 mL for A1. In this study, it has been found that chemical treatment of vegetable wastes with alkali (NaOH(aq)) enhanced biogas yield over a 35–day retention period of anaerobic digestion. A limitation which has been observed to the anaerobic digestion of the fruit and vegetable wastes studied here was a high volatile fatty acids production, which may have stressed and inhibited an optimum activity of the methanogenic bacteria (Ghasimi et al., 2009; Schievano et al., 2010). On the whole, it may be deduced that alkaline pretreatment with NaOH has had a positive effect in improving the rate and yield of biogas production (López Torres and Espinosa Lloréns, 2008; Lin et al., 2009).

Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2012 (Juni 2012)
Seiten: 8
Preis inkl. MwSt.: € 8,00
Autor: Prof.Dr. Romeela Mohee

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