Two recurrent topics among the scientific community are the use of microalgae for biofuel production, and as a biological agent for nutrient removal in wastewater treatment plants. We have analysed the possibility of coupling these two processes, using microalgae that naturally form biofilms on wastewater treatment tanks to produce methane. A methane assay using microalgae biofilms from the primary (T1) and secondary (T2) treatment tanks from a wastewater treatment plant (WWTP) on the Canary Islands showed  that, when this substrate is added to suitable methanogenic bacteria, in this case the microbial community in marine sludge from a fish farm, it gives a methane yield of 0.104 Nm³ kg^‑1VS (biofilm from T1), and 0.076 Nm³ kg^-1VS (T2). We also checked the microalgal biomass production of each tank (40.13 g AFDW  d^-1 and 106.41 g AFDW d^-1, for the T1 and T2 respectively), and the growth of this biofilms in photobioreactors (PBRs). When grown in PBRs, the algae community from the primary tank was dominated by a unicellular Chlorophyta (specific growth rate: 0.42 d^-1), while biofilm from the secondary tank was dominated by filamentous Chlorophyta (specific growth rate: 0.25 d^-1). The biofilms growing in the WWTP in Gran Canaria are a free, naturally available source of biomass, and we have shown that this biofilm, besides being a natural agent for nutrient removal in a WWTP, has also the potential of being used as a low cost, green source of biomass for methane production when used in combination with other substrates.