NOVELTY - The system has several microbial fuel cells (MFCs) of each having an anode connected to a cathode by a conductive material. The anode is physically associated with primary microbial population, and the cathode is physically associated with secondary microbial population or an abiotic catalyst. A proton selective material is provided between the anode and cathode. A porous insulating material is provided between the anode and cathode, and has a pore diameter of no more than about 30 micrometer.    USE - Modular water treatment system for treating non-potable water and potable water, for electricity generation. Can also be used in treating water such as graywater, near-potable water, sludge, a wastewater, runoff water, industrial wastewater, such as the byproducts of paper making or milling, a food industry wastewater, such as a brewery wastewater, an agricultural wastewater, a residential wastewater, a municipal wastewater, an animal or agricultural wastewater, a sewage, a water from a river body of water, and an effluent of anaerobic digesters.    ADVANTAGE - The system produces treated non-potable water or the potable water and carbon dioxide from the anode, new water from the cathode and electricity as a result of the bioelectrochemical reactions in both compartments. The decrease in biomass production and the direct generation of electricity reduces the overall cost relative to maintaining anaerobic or aerobic digesters, while providing comparable or improved organic oxidation rates and efficient energy recovery. The systems reduce production of the waste gas methane, the significant greenhouse gas, by moving energy away from the microbes as electricity and forms the environment where extracellular respiration is energetically more favorable than reduction of carbon dioxide to methane. The carbon dioxide produced from the systems can be harnessed and can be used as fuel for photosynthetic ponds, as biomass for fertilizer or to produce biofuels. The systems provide versatility with respect to the wastewater processed and energy consumed e.g. the system can be optimized for sludge reduction at low peak demand and electricity production at high peak demand that is used in driving energy requiring units in the system.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method of optimizing microbial population on anode of microbial fuel cell for wastewater treatment.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the modular water treatment system.