Tannery effluent characterisation & culture enrichment for enhancement of biological treatment

Abstract

Pollution of water resources has become a critical problem around the world. The tanning process, used in leather manufacturing, is water and chemical intensive and has developed a reputation as critically water polluting. The hides and skins are salted to preserve them. Sodium hydroxide and sodium sulphide are used in the initial phase of the tanning process to remove the hair. Therefore, tannery effluent is characterised by a high organic load, high salinity and high concentrations of sulphur species. Consequently, comprehensive wastewater treatment is required before the effluent can be discharged. Tannery effluent has a high organic load, which makes it theoretically attractive for anaerobic digestion (AD) to achieve energy recovery as part of the treatment process. The high sulphur species load, in particular sulphide (HS-), inhibits the methanogenic microbial species responsible for biogas generation. Therefore, in its raw form, the effluent is not a suitable substrate for AD. Pre-treatment, particularly to remove the HS- components, would require organisms that are capable of withstanding the high salinity associated with the effluent. However, there is relatively little research on saline tolerant bacterial consortia capable of removing sulphur compounds. The primary aims of this study were to collect and enrich microbial communities from saline, anaerobic environments and use them to test the hypothesis that tannery effluent can be pre-treated in a novel reactor system to manage the sulphur species, making the effluent more amenable to AD. Samples were collected from five marine and estuarine environments and enriched in batch reactors of increasing volumes, on lactate-supplemented artificial seawater. Their ability to reduce sulphate (SO42-) was evaluated and promising candidates were selected for further tests. Tannery effluent was obtained from a local tannery that processes ostrich skins and another that processes bovine and ovine hides. These were characterised in detail and used as substrates to evaluate biological sulphate reduction (BSR) and sulphide oxidation (SO), initially in batch reactors, and then in a hybrid linear flow channel reactor (HLFCR) under continuous operation at a 4-day hydraulic residence time (HRT). The HLCFR was designed to support simultaneous reduction of SO42- in the bulk volume and partial oxidation of HS- to elemental sulphur (S0) within a floating sulphur biofilm (FSB). A molecular biology approach was used to characterise and quantify the sulphate reducing communities in the environmental samples, tannery effluent, and reactor contents, thereby elucidating the relationship between microbial community structure and function and process performance. This was based on metagenomic sequencing and quantification of the dissimilatory sulphite reductase (dsr) gene. The ostrich tannery effluent was characterised by low sulphide concentrations (< 5 mg/ℓ) and relatively low sulphate (average 650 mg/ℓ), so it did not require pre-treatment prior to AD. The bovine/ovine tannery effluent (BTE) exhibited a high salinity (electrical conductivity (EC) 32 mS/cm) and variable sulphide and sulphate concentrations, reaching almost 900 mg/ℓ and 4000 mg/ℓ, respectively. Preliminary AD tests using raw BTE confirmed the need for pre-treatment. The performance of a HLFCR that was inoculated with enriched microbial consortia was compared with the performance of a non-inoculated reactor. The results demonstrated that both the endogenous and exogenous microbial communities were capable of BSR and SO, and were able to maintain a FSB. The non-inoculated reactor was able to reach 99% reduction of SO42- over 21 days of batch operation, while the inoculated reactor attained 80% sulphate reduction (SR) efficiency during batch operation. Together, when operating both the reactors continuously in series for > 100 days, the system reached a maximum SR of 96.6% in the bulk liquid at a 4-day HRT and was able to achieve an average sulphate reduction rate (SRR) of 170 mg/ℓ.day (maximum of 444 mg/ℓ.day), with a maximum of 81% reduction of influent SO42-. The SRR improved under continuous operation due to the near first order relationship of BSR to SO42- concentration. Both reactors were consistently capable of near-complete HS- removal during stable operation. The effluent from the non-inoculated HLFCR had an average HS- concentration of 8.4 mg/ℓ when operated alone, while the average HS- concentration in the effluent from the system in series was 9.7 mg/ℓ when the reactors were operated in series. The SRB communities in the enrichment consortia were dominated by the Desulfovibrio genus, in particular, the uncultured Desulfovibrio. sp. MCM B_508 (34.2-60.5% relative abundance (RA)), with contributions from a variety of other species. The BTE batches contained an endogenous SRB community dominated by an unidentified Desulfovibrio sp. (enrichment culture HCB4; 18-59% RA across the batches). The community structure was relatively diverse and varied across the five batches of bovine/ovine tannery effluent. During continuous operation, the SRB community within the HLFCRs treating BTE changed significantly. The inocula were dominated by Desulfovibrio species, but continuous operation selected for Desulfobacterium autotrophicum and Desulfomicrobium orale. The metabolic versatility of D. autotrophicum is likely responsible for its swift emergence within the system. This research provided a technical proof of concept for the process using the enriched SRB/SOB community to reduce SO42- under saline conditions. Preliminary AD studies on the BTE treated in the HLFCRs showed substantial improvement in biogas generation relative to AD of raw and partially treated BTE. It was concluded that pre-treatment of BTE using a HLFCR inoculated with well acclimated microbial consortia, followed by AD is a feasible option for the simultaneous remediation and valorisation of tannery effluent.