Production of Methane From Microalgae Biofilms Growing in Wastewater Treatment Plants in the Canary Islands

Abstract

Two recurrent topics among the scientific community are the use of microalgae in wastewater treatment plants as a biological agent for nutrient removal, and, more recently, the use of microalgae for biofuel production. In this study we have analysed the possibility of coupling these two processes, using microalgae that naturally form biofilms on wastewater treatment tanks to produce methane. The proposal is to develop a low cost, environmental friendly methodology, with the economical and environmental advantages of enhancing the removal of nutrients from wastewater, and producing sustainable biofuel. A methane assay using microalgae biofilms from the primary and secondary treatment tanks from a wastewater treatment plant (WWTP) on the Canary Islands (EDAR-del Sureste, Gran Canaria, Spain) showed that, when this substrate is added to a suitable methanogenic bacteria, in this case marine sludge from a fish farm, it gives a methane yield of 0.104 Nm³ kg^-1 VS. We also checked the in situ biomass yield of the biofilm (3.16 g AFDW m^-2 d^-1 and 7.71 g AFDW m^-2 d^-1, for the primary tank and secondary tank respectively), and the growth of this biofilms in photobioreactors (PBR). When grown in PBR, the algae composition of biofilm from the primary tank becomes dominate by a unicellular chlorophyta and produces 0.24 kg AFDW m^{- 3} d^-1 of biomass; while biofilm from the secondary tank becomes dominated by the filamentous chlorophyta Stigeoclonium, and has a biomass yield of 0.48 kg AFDW m^-3 d^-1. The biofilms growing the WWTP of the EDAR del Sureste, in Gran Canaria, are a free naturally available source of biomass, and we have shown in this study that this biofilm, besides being used as a natural agent for nutrient removal in a WWTP, it has also the potentialof being used as a low cost, green source of biomass for methane production.