Abstract: A method and apparatus are provided for measuring the concentration of contaminants in an aqueous water system, wherein the contaminants are measured with the bioreactor containing a biofilm or microbial community which acts on the water to be analyzed to provide a measure of the amount of the dissolved organic carbon which is biodegradable. A bed for facilitating regulation of water flow through the bioreactor is provided, and an autosampling mechanism enables the measurement of total organic carbon, inorganic carbon and oxygen, between sample inflow and sample outflow at specified time intervals.