NOVELTY - The wastewater treatment system comprises a tank having opposing first and second side walls, an inlet wall (70) defining an inlet for receiving wastewater, an outlet wall (72) defining an outlet for discharging treated wastewater, a floor and an upper surface. The first and second side walls, the inlet wall and the outlet wall connect the floor and the upper surface. The upper surface defines at least one opening. The tank comprises a first baffle (80) having an upper edge extending above the inlet and a lower edge extending below the inlet and above the floor.    USE - The wastewater treatment system is useful as a residential septic system, a municipal wastewater treatment facility, an industrial wastewater treatment facility, a commercial wastewater treatment facility and an agricultural wastewater facility.    ADVANTAGE - The affordable treatment system effectively treats the wastewater with high-fidelity and reduced biochemical oxygen demand and chemical oxygen demand. The bio-electrochemical sensor of the treatment system enables real-time and rapid detection of the toxic chemicals in the wastewater.    DETAILED DESCRIPTION - The wastewater treatment system comprises a tank having opposing first and second side walls, an inlet wall (70) defining an inlet for receiving wastewater, an outlet wall (72) defining an outlet for discharging treated wastewater, a floor and an upper surface. The first and second side walls, the inlet wall and the outlet wall connect the floor and the upper surface. The upper surface defines at least one opening. The tank comprises: a first baffle (80) having an upper edge extending above the inlet and a lower edge extending below the inlet and above the floor; a second baffle (82) having an anchored end anchored to the floor of the tank and a free end projecting into the tank; and a fixed-film aeration apparatus coupled to the first and second baffles. The fixed-film aeration apparatus is raised from the floor at a position above a level of accumulation of bio-solids in the tank. The first baffle extends between the first and second side walls along an entire width of the tank. The first baffle and the inlet wall together define a first anaerobic zone in the wastewater treatment system. The free end is positioned at a position higher than the lower edge of the first baffle. The second baffle is coupled to the first and second side walls, and extends between the first and second side walls along the entire width of the tank. The second baffle and the outlet wall together define a second anaerobic zone in the wastewater treatment system. The fixed-film aeration apparatus comprises: at least one fixed-film media support; a base for supporting the fixed-film media support; an air line comprising an internal aeration connector and an external oxygen source connector for coupling to an external oxygen source; and an air dispersal mechanism coupled to the internal aeration connector. The air dispersal mechanism is arranged for delivering oxygen to the fixed-film media support. The fixed-film aeration apparatus defines an aerobic zone in the wastewater treatment system. The first and second baffles direct a flow of the wastewater along a treatment path. The treatment path extends from the inlet through the first anaerobic zone, the aerobic zone defined by the fixed-film aeration apparatus, the second anaerobic zone and to the outlet. The upper edge of the first baffle is anchored to the upper surface of the tank at a position closer to the inlet wall than the outlet wall. The base comprises a tray for supporting the air dispersal mechanism. The tray comprises raised side portions, which engage with a bottom surface of the fixed-film media support to restrict bio-solids from clogging the air dispersal mechanism. The air dispersal mechanism comprises at least one air stone. The wastewater treatment system further comprises: at least one bio-electrochemical sensor for detecting the presence of wastewater constituents in the wastewater; an oxygen pump positioned externally to the tank and reversibly coupled to the oxygen source connector, for providing oxygen to the air line; and an effluent filter (50). The bio-electrochemical sensor is positioned in the first or second anaerobic chamber and outside the tank. The tank is in fluid communication with a leech field. The bio-electrochemical sensor is further positioned in the leech field. The detected wastewater constituents comprise toxic chemicals. The effluent filter comprises: a filter inlet for receiving wastewater to be filtered; a first chamber in fluid communication with the filter inlet; and a filter outlet in fluid communication with the first chamber. The first chamber comprises: a porous material for filtering solid material from the wastewater; and the bio-electrochemical sensor. The filter outlet is arranged for discharging filtered wastewater from the filter. The porous material comprises a conductive material for supporting the growth of exoelectrogenic bacteria. The first chamber further comprises: electrode pairs coupled to the porous material, where each electrode pair comprises an anode and a cathode; a terminal electron acceptor electrically coupled to the cathode for receiving the electrons from the cathode and for generating an electrical output; and a resistor electrically coupled to the terminal electron acceptor, where the electrical output is measured across the resistor using a data acquisition system. The exoelectrogenic bacteria are in proximity to the anode and release electrons to the anode. The released electrons are flowed from the anode to the cathode. The electrical output of the resistor is indicative of a metabolic activity of the exoelectrogenic bacteria. The effluent filter comprises a second chamber in fluid communication with the first chamber and the filter outlet, where the second chamber houses an adsorptive material for clarifying the filtered wastewater. The bio-electrochemical sensor is configured for: detecting nutrient over-loading in a leech field associated with the wastewater treatment system; detecting changed in hydraulic loading in the leech field; and detecting the presence of toxic chemicals in the wastewater treatment system. INDEPENDENT CLAIMS are included for:    (1) a fixed-film aeration apparatus;    (2) a method for controlling a wastewater treatment system;    (3) a bio-electrochemical sensor; and    (4) an effluent filter.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a wastewater treatment system.    Effluent filter (50)    Inlet wall (70)    Outlet wall (72)    First baffle (80)    Second baffle. (82)