WASTEWATER TREATMENT CONNECTED WITH THE ELECTRICITY GENERATION

1. WASTEWATER TREATMENT CONNECTED WITH THE ELECTRICITY GENERATION Iliev M1., Tasheva L1 ., Groudeva V1.Groudev S2. 1-Department of Microbiology, Faculty of Biology, University of Sofia, Sofia 1421, Bulgaria 2-University of Mining and Geology, Sofia 1700, Bulgaria Objective of the research The main objective of the research was to combine the treatment of acid drainage waters by means of a permeable multibarrier with the electricity generation by means of microbial fuel cell. Materials and methods The permeable multibarrier was a pond dug into the ground and its bottom and walls were isolated by impermeable plastic sheets. The multibarrier had a volume of about 6.5 m3 (4.0 m long, 1.0 m wide, and 1.6 m deep) and was filled by a mixture of solid biodegradable organic substrates (cow manure, plant compost, straw) and crushed limestone and was inhabited by a microbial community consisting mainly of sulphate - reducing bacteria and other metabolically interdependent microorganisms. The water flow rate through the multibarrier varied in the range of about 2 – 5 m3/24 h, reflecting total water residence time of about 210 to 84 hours. The efficient removal of the heavy metals from the waters being treated was due to different processes such or chemical neutralization followed by hydrolysis, microbial dissimilatory sulphate reduction and biosorption mainly by the plant biomass. The microbial activity in the multibarrier markedly depended on the temperature and during the cold winter days was negligible. However, at air temperatures close to the freezing point, the temperatures inside the multibarrier, within the deeply located layers, usually were in the range of about 4 – 6 oC. Under such conditions the microbial sulphate reduction still proceeded, although at much lower rates. In any case, the water clean up efficiency of the multibarrier was much higher during the warmer months of the years. The microbial fuel cell was a Plexiglas rectangular container 80 cm high, 10 cm wide, with perforated slab graphite – Mn4+ anode and graphite – Fe3+ cathode located in the bottom and in the top sections of the container, respectively. The two sections were separated by a permeable barrier of 5 cm thickness consisting of a 2.5 cm layer of glass wool and a 2.5 layer of glass beads. The feed stream, i. e. the portions of effluents from the multibarrier, was supplied to the bottom anodic section and exited at the top continuously. Air was injected during the treatment to the cathode section. There was a clear gradient of the dissolved oxygen between the anaerobic anodic section and the aerobic cathode section. Biofilm consisting of a consortium of different electro-chemically active microorganisms (mainly of the genera Geobacter, Shewanella, Rhodoferax, Desulfovibrio, Desulfobacter, Desulfococcus and Clostridium) was formed on the anode electrode in the course of time. Apart from the multibarrier effluents, several mixed microbial populations present in anaerobically digested sludge taken from different wastewater treatment plants were also involved in the formation of this biofilm. During the treatment of the organics – bearing wastewater in the MFC the maximum Chemical oxygen demand (COD) removal rate was 102 mg/l.h, and the maximum power density was 284 mW/l. Results The above processes were connected with the gradual formation of biofilm on the anode electrode of the cell. The biofilm consisted of a consortium of different anaerobic microorganisms, including some acidophilling bacteria able to transfer electrons from their respiratory chains directly to the anode, as well as some bacteria able to perform this transfer by means of self-produced mediators. The continuous operation of the microbial fuel cell was connected with continuous generation of electricity at a steadily increasing power density (the maximum value achieved by this system was 284 mW/l). At the same time, the quantity of the produced activated sludge was about 3-4 times lower per liter of wastewater treated by a classical system of this type but without microbial fuel cell. Table 1 Treatment of acid mine drainage by means of the permeable multibarrier used in this study Table 2 Microflora of the multibarrier effluents and of the waters treated in the microbial fuel cell Table 3 Sulphate – reducing bacteria in the sediments and in the organic matter in the permeable multibarrier Table 4 Content of heavy metals in the dead plant biomass in the permeable multibarrier Conclusions The combination of permeable multibarrier with a microbial fuel cell facilitated the removal of organics from the wastewaters, decreased the amount of produced sludge and the consumption of energy from outside by generating electricity during treatment. Table 5 Data about the optimum conditions for the electricity generation during this study