Semi-passive remediation systems have the potential to treat low-volume, sulphaterich, mining impacted waters in a cost-effective and sustainable way. This paper describes the "proof of concept" evaluation of a hybrid linear flow channel reactor, capable of sustaining efficient biological sulphate reduction and partial oxidation of the sulphide product to elemental sulphur. Key elements include the presence of a sulphate-reducing microbial community, immobilised onto carbon fibres and the rapid development of a floating biofilm at the air-liquid interface. The biofilm consists of heterotrophic species and autotrophic sulphide oxidisers. It impedes oxygen mass transfer into the bulk volume and creates a suitable pH-redox microenvironment for partial sulphide oxidation. Demonstration of the concept was successful, with near 20 complete reduction of the sulphate in the feed (1 g/l), effective management of the sulphide generated (95-100% removal) and recovery of a portion of the sulphur by harvesting the elemental-sulphur-rich biofilm. The biofilm re-formed within 24 hours of harvesting, with no decrease in volumetric sulphate reduction rate during this period. Colonisation of the carbon microfibers by sulphate reducing bacteria ensured biomass retention, suggesting the reactor could remain effective at high volumetric flow rates..

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Semi-passive remediation systems have the potential to treat low-volume, sulphaterich, mining impacted waters in a cost-effective and sustainable way. This paper describes the "proof of concept" evaluation of a hybrid linear flow channel reactor, capable of sustaining efficient biological sulphate reduction and partial oxidation of the sulphide product to elemental sulphur. Key elements include the presence of a sulphate-reducing microbial community, immobilised onto carbon fibres and the rapid development of a floating biofilm at the air-liquid interface. The biofilm consists of heterotrophic species and autotrophic sulphide oxidisers. It impedes oxygen mass transfer into the bulk volume and creates a suitable pH-redox microenvironment for partial sulphide oxidation. Demonstration of the concept was successful, with near 20 complete reduction of the sulphate in the feed (1 g/l), effective management of the sulphide generated (95-100% removal) and recovery of a portion of the sulphur by harvesting the elemental-sulphur-rich biofilm. The biofilm re-formed within 24 hours of harvesting, with no decrease in volumetric sulphate reduction rate during this period. Colonisation of the carbon microfibers by sulphate reducing bacteria ensured biomass retention, suggesting the reactor could remain effective at high volumetric flow rates. 2

The coupled application of biological sulphate reduction (BSR) and partial sulphide oxidation to treat sulphate-rich wastewater is an effective strategy to mitigate pollution and recover elemental sulphur for re-purposing. The recent development of the hybrid linear flow channel reactor (LFCR) achieves simultaneous BSR and partial sulphide oxidation with biosulphur recovery via a floating sulphur biofilm (FSB). Here, we explore the microbial community zoning and dynamics facilitating the process. Three continuous LFCRs were used to evaluate the effect of reactor zones, hydraulic residence time (HRT), carbon source, namely lactate and acetate, as well as reactor geometry and scale on process performance and microbial community dynamics. Community composition of sessile and planktonic microbial consortia were resolved at a 5- and 2-day HRT through 16S rRNA amplicon sequencing. Preferential attachment and prevalence of specific phylotypes within the sessile and planktonic communities revealed clear adaptation of key microorganisms to different microenvironments. Key microbial taxa affiliated with sulphate reduction and sulphide oxidation as well as those implicated in fermentation and syntrophic metabolism, fluctuated in response to changes in HRT and process performance. Through understanding the relationship between microbial community dynamics and process performance this research will inform better process design and optimisation of the hybrid LFCR.