TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Method: Enrichment comprises either limiting the concentration of a microbial growth-limiting substrate in the first reactor, biological kinetic control, providing a stochiometric amount of a compound that is an oxyanion or an electron donor or enriching the methanogenic culture in the second reactor by providing a growth substrate of methanogen.The first treatment step comprises metabolizing at least a portion of the hydrocarbon and a biological denitrification step. The second treatment step comprises biological reductive dehalogenation of at least a portion of the halogenated hydrocarbon to produce a breakdown product at least a portion of which is metabolized in the second step to produce an alkane. Molecular oxygen is delivered to the third reactor with the second effluent to accomplish aerobic respiration and the production of a third effluent. Metals dissolved in the waste stream are precipitated as metal sulfidesPreferred Conditions: The first reactor is operated under conditions that exclude molecular oxygen at 20 - 40 degreesC, pH 7 - 10 and mean cell residence time of no more than 15 (preferably 10) hours to produce a first effluent and a gas comprising nitrogen. The mean cell residence time of the first reactor enriches the bacteria capable of biological reduction of an oxyanion and prevents methanogenesis. It is operated as a suspended growth reactor and is a completely mixed continuous stirred tank reactor. A vacuum is imposed on the headspace provided in the reactors to remove the gases.The second reactor is operated under anaerobic conditions at pH 6.8 -7.4, 20 - 65 degreesC and mean cell residence time of at least 30 hours to produce a second effluent and methane gas.The reactor comprises the pore space of a porous underground formation.Preferred Substances: The mixed hydrocarbons comprise a halogenated hydrocarbon such as tetrachloroethylene, trichloroethylene or 1,1,1-trichloroethane and under methanogenic conditions serve as an electron acceptor. The oxyanion is an oxide of nitrogen, at least one is a dissolved nitrate and another is dissolved nitrite. EXAMPLE - No example is given.