NOVELTY - Increasing the specific growth rate of anaerobic, autotrophic microbes in a wastewater treatment system comprises: optimizing dissolved carbon dioxide (CO2) further comprising: establishing an optimal control value for the soluble CO2 concentration for wastewater; measuring the concentration of soluble CO2 for the wastewater; comparing the measured concentration of soluble CO2 in the wastewater against the optimal control value; and adjusting the measured concentration of soluble CO2 to match the optimal soluble CO2 control value.    USE - The methods are useful for increasing the specific growth rate of anaerobic, autotrophic microbes in a wastewater treatment system; for treating wastewater for biochemical oxygen demand (BOD) and nitrogen removal; for treating wastewater for BOD, nitrogen and phosphorus removal; and for treating sludge (all claimed).    ADVANTAGE - The methods are environment friendly, efficiently optimize CO2 concentration to increase the specific growth rate of Anammox bacteria and methanogens in wastewater and sludge treatment, and effectively treat wastewater and sludge.    DETAILED DESCRIPTION - Increasing the specific growth rate of anaerobic, autotrophic microbes in a wastewater treatment system comprises: optimizing dissolved carbon dioxide (CO2) further comprising: establishing an optimal control value for the soluble CO2 concentration for wastewater; measuring the concentration of soluble CO2 for the wastewater; comparing the measured concentration of soluble CO2 in the wastewater against the optimal control value; and adjusting the measured concentration of soluble CO2 to match the optimal soluble CO2 control value; where optimizing the soluble CO2 increases the specific growth rate of the bacteria. INDEPENDENT CLAIMS are:    (1) a method of treating wastewater for biochemical oxygen demand (BOD) and nitrogen removal comprising: flowing wastewater effluent through a first aeration basin; optimizing CO2 content of the effluent; flowing at least a portion of the optimized CO2 effluent into a second aeration basin; flowing the optimized CO2 effluent from steps (b) and (c) into an anaerobic reactor; flowing the optimized CO2 effluent from the anaerobic reactor into a secondary clarifier; and recycling at least a portion of wastewater in the secondary clarifier by channeling the wastewater into the first aeration basin;    (2) a method of treating wastewater for BOD, nitrogen and phosphorus removal comprising: flowing a volume of wastewater through a primary clarifier; optimizing CO2 content of primary effluent exiting the primary clarifier; flowing the primary effluent having optimized CO2 into an anaerobic basin for phosphorus release; flowing effluent from step (c) to an aerobic basin to remove BOD; adding nitrite to the effluent from step (d); flowing the effluent from step (e) into a second anaerobic basin; flowing the effluent from step (f) into a secondary clarifier; and recycling at least a portion of wastewater in the secondary clarifier to provide return activated sludge to the first anaerobic basin;    (3) a wastewater treatment system comprising: an aerobic treatment zone; an anaerobic treatment zone where the anaerobic treatment zone is positioned downstream from the aerobic treatment zone; and a CO2 stripping zone where the CO2 stripping zone is positioned between the aerobic and anaerobic treatment zones;    (4) an anaerobic digestion system comprising: at least one anaerobic digester having a biogas headspace; and CO2 stripping equipment positioned adjacent the biogas headspace of the at least one anaerobic digester; where biogas contained in the biogas headspace is cycled through the CO2 stripping equipment and re-enters the biogas headspace with lower CO2 concentration; and    (5) a method of treating sludge comprising: flowing sludge through at least one anaerobic digester having a biogas headspace; cycling headspace biogas from the biogas headspace through CO2 stripping equipment to lower CO2 levels in the headspace biogas; and flowing the lower CO2 biogas back into the biogas headspace of the at least one anaerobic digester; where the lower CO2 headspace biogas encourages the growth of autotrophic anaerobic microbes.