Abstract
Anaerobic digestion is considered unsuitable for the bioremediation of tannery effluent due to process inhibition, mainly due to high concentrations of sulfur species, and the accumulation of H2S and/or NH3. This study using the standardized biochemical methane potential protocol showed that efficient processing is possible with slaughterhouse wastewater, provided sufficient functional biomass is present at the start of the process and the SO42− concentration is below inhibition threshold. Methanogenic activity (K = 13.4–17.5 and µm = 0.15–0.27) and CH4 yields were high when reactors were operated ISR ≥ 3 and/or lower SO42− ≤ 710 mg/L while high SO42− ≥ 1960 mg/L and ISR < 3.0 caused almost complete inhibition regardless of corresponding ISR and SO42−. The theoretical optimum operating conditions (922 mg/L SO42−, ISR = 3.72) are expected to generate 361 mL biogas/gVS, 235 mL CH4/gVS with reduction efficiencies of 27.5% VS, 27.4% TS, 75.1% TOC, 75.6% SO42−, and 41.1% COD. This implies that tannery sludge will be reduced by about 27% (dry mass) and SO42− by 76%, with a fraction of it recovered as S0. The models displayed a perfect fit to the cumulative CH4 yields with high precision in the order Logistic > Cone > modified Gompertz > first order.
Highlights
The leather tanning and products industries play prominent roles in the world’s economy, for developing countries
A Merck Spectroquant Pharo® Spectrophotometer (Darmstadt, Germany) together with Merck cell tests or kits were used to determine the concentration of substances (Table 1): chemical oxygen demand (COD), 5-day biological oxygen demand (BOD5), total organic carbon (TOC), total volatile organic acids (VOAt) as acetic acid equivalents (AAE), total sulfate (SO42−), total (S2−)as HS−, total nitrogen (TN), nitrate (NO3−), nitrite (NO2−), total phosphate (TP) as phosphorous (PO42− − P), total ammonia nitrogen (TAN) (NH3 − N), and total alkalinity (Alk) as calcium carbonate (CaCO3), following the prescribed procedures
These results suggest that, if organic solids, notably fats, were responsible for the high total solids (TS), TVS, and COD concentrations in batch 4, these were mainly recalcitrant in nature
Summary
The leather tanning and products industries play prominent roles in the world’s economy, for developing countries. South Africa is a net exporter of hides/skins, and is a renowned producer of exotic ostrich leather. A tonne of raw hide/skin yields approximately 200 kg of leather, 500 kg wet sludge, and 50–15,000 m3 of liquid effluent containing residual processing chemicals [1]. TWW contains high loads of chromium (Cr), sodium (Na), chlorides (Cl), nitrogen (TN), sulfate (SO42−), sulfide (S2−/H2S/HS−), and suspended solids (SS) [2]. The process generates in-plant solid wastes such as untanned raw trimmings, fleshings, tanned waste blue splits, trimmings, and shavings. Tanneries are regarded as one of the most polluting industries in developing countries that dominate the industry and supply >60% of the world’s skins/hides [1]
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