Anaerobic treatment of tannery effluents for resource recovery in a circular bioeconomy

Abstract

The current economy of tanneries in developing countries is mainly linear, with certain elements of circularity as it promotes the recycling of skin/hides and indirect water reuse. In view of the global efforts to achieve sustainable development, the concept of a circular bioeconomy through the application of anaerobic digestion (AD) has gained significant traction in solving the tannery industry’s economic and waste management challenges. However, its application is susceptible to inhibition by recoverable toxicants particularly sulfur species, metals and ammonia (NH3). This study investigated the AD raw slaughterhouse–ostrich (SOTE) and bovine–ovine (BOTE) tannery effluent for resource recovery. It further investigated the use of a novel hybrid linear flow channel reactor (HLFCR) for the pre-treatment of TWW to remove inhibitory sulfur species and recover it as S0, while making TWW more amenable for methane (CH4) recovery in anaerobic sequential batch reactors (AnSBR). Standardised biochemical methane potential (BMP) tests were conducted on raw SOTE and BOTE to determine the effects of inoculum to substrate ratio (ISR), sulfate concentration [SO42−], and/or substrate to substrate ratio (SSR) on the anaerobic biodegradability (Bo), CH4 yields and process kinetics. The effect of these factors and process optimisation was assessed using response surface methodology (RSM). The optimal conditions were then applied in AnSBR with/out pre-treatment with HLFCR. The BMP tests on SOTE demonstrated the inhibition effect of high [SO42−] and/or lower ISR on CH4 yields while tests on BOTE showed that beamhouse effluent (BHE) was more amenable for AD than tanyard effluent (TYE). Bioreactors with higher TYE compositions (v/v) and operating at very high/low ISR (3<ISR≤2) suffered severe methanogenesis inhibition. However, it was established that all reactors were active regardless of inhibition. It was also apparent that at least a fraction of the HS− formed from sulfidogenesis was oxidised into S0 and a white-yellowish layer formed at the interface of the bulk liquid and head space for both BOTE and SOTE. As expected, higher CH4 yields (93–130 mLCH4/gVS) were achieved at [SO42−] within the range (646±417 mg/L) measured in SOTE batches. This meant there was no need for the pre-treatment of SOTE to remove sulfur species. The optimum theoretical operating conditions for maximal CH4 yield and biodegradability (Bo) for SOTE were [SO42−] = 922 mg/L and ISR = 3.7, while for BOTE they were determined as ISR = 2.5 and 100% BHE. The AD of SOTE with [SO42−] ≈ 680 mg/L in AnSBRs at ISR = 4 while mixing at 50–300 rpm using a pitched four-blade marine impeller achieved 31–208 mLCH4/gVS. The optimal mixing conditions at 200 rpm (continuous) achieved 208 mLCH4/gVS and 49% total solids reduction.

The feasibility of using a novel integrated biological system (IBS) that comprised of a novel HLFCR and an AnSBR connected in series was investigated in treating 100%BHE. The IBS’s optimum operating configuration and conditions were single-stage (8 days HRT) or two-stage (4 days HRT) HLFCR and 50 rpm continuous mixing in an AnSBR. These operating conditions achieved the removal of 80–91% total COD, 78–98% TOC, 89–91% SO42−, 92–93% S2−, 50–73 PO42−, and 48–60% total nitrogen while recovering 241±4 mLCH4/gCODadded after 20–24 days. Single-stage HLFCR (8 days HRT, pH=7.0–7.8, ambient temperature and –366<ORP<–322) recovered about 16–25% of the inlet sulfur as S0. The composition range of the harvested floating sulfur biofilm (FSB) was 10.8–14% C, 1.5–1.9% N, 1.9–4.4% H, and 26–39.6% S. Mixing continuously at 50 rpm significantly improved (F test, p>0.05) the cumulative CH4 yield by 82–98%, 33– 65% and 63–71% compared to raw BHE, unmixed and intermittently mixed AnSBRs treating HLFCR pre-treated BHE, respectively. The AD of settled solids from HLFCR in an unmixed reactor operating at ISR = 4 produced 88±2 mLCH4/gVSadded. The final treated effluent of SOTE, BHE and that of IBS met many of the irrigation standards for most developing countries leading in leather production except for Na, Cl, and NH3. In the South African context, the treated SOTE and BHE met the standards for irrigating up to 500 m3/day for BMP tests and 50 m3/day for BHE in the IBS. Similarly, metal concentrations in the SOTE and BHE digestate were within the recommended limits for class A1a/b sludge suitable for agricultural use in South Africa (SA) and other developing countries, except for Cr. Chrome (III) is insoluble in water and hence it attaches and settles with solids. Nonetheless, the treated BHE from IBS can be further treated for the recovery of VOA (2284– 2465 mg/L), and NH4 (232–444 mg/L) that were still present in high concentrations due to hydrolysis and/or diluted to meet irrigating up to 2000 m3/day. The full-scale application of the IBS at a local medium-sized tannery treating 2258 m3/day of BHE would produce a floating sulfur biofilm with 33% S0, 3420 m3 of CH4, and 31 tonnes of biofertiliser. The sale of recovered resources and potential savings from 72% reduction in electricity demand and 62% in sludge disposal had a potential revenue of US$5559. The integrated system has a payback period of about 5 years and an internal rate of return of about 18%.