TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred System: The system further comprises a mechanism for receiving nutrients for the microorganisms into the liquid, and a fan for producing a gas flow between the gas inlet and the exhaust outlet.Preferred Component: The mechanism for populating the media further comprises:(a) an inlet conduit for receiving make-up water;(b) a reservoir containing a concentration of the microorganisms; and(c) a feeder connected between the inlet conduit, the reservoir, and the fill conduit for mixing a dosage of the concentration of microorganisms with the make-up water to produce the fill water.The mechanism for controlling the pH comprises:(i) pH probe (50) for sensing the pH of the recirculating liquid,(ii) an inlet conduit for receiving make-up water into the sump,(iii) an overflow drain for preventing overfilling of the sump, and(iv) a control valve (42) fluid connected in series with the fill conduit for blocking the inlet conduit in response to the pH probe when the pH reaches the high limit. The fill water comprises primary effluent. The make-up water is secondary effluent.Preferred Condition: The pH low limit is at least 1.5, preferably 2.0 and the high limit is at most 4.0, preferably 3.0. The gas stream has a velocity of at least 50 ft/minute through the porous media and a static pressure drop of at most 3.0 inches of water across a gas stream travel distance of approximately 10 feet through the porous media. The porous media has a surface area of greater than 1000 but not less than 10000 times the cubic dimension of the media. The flow rate of the recirculating liquid is 1.5-2.0 gallons/minute/ft squared of plan area of the porous media.Preferred Configuration: The nozzles are spaced at least 10 ft. above a lowermost media supporting surface of the media support structure.    TECHNOLOGY FOCUS - BIOLOGY - Preferred Component: The microorganisms comprise thiobacillus bacteria.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The porous media comprises a concentration of an iron compound, particularly lava rock.    TECHNOLOGY FOCUS - POLYMERS - Preferred Material: The tower vessel is a fiberglass-reinforced plastic structure.    TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The pH is maintained by diluting the recirculating liquid with water without requiring pH-balancing chemicals in the liquid. The method further comprises receiving make-up water, filtering chlorine from the make-up water, maintaining a pH of the recirculating liquid, maintaining a flow rate of the recirculating liquid, and maintaining a gas stream velocity.