In many biological reactors bicarbonate is the major species determining pH buffering capacity, or alkalinity. In anaerobic digesters bicarbonate levels should be within 10 to 50 mM for stable operation. Bicarbonate alkalinity in wastewater treatment processes in routinely measured off‐line titrimetrically. Recently we have described the principle of a novel on‐line method of measuring bicarbonate alkalinity. In the prototype device described here, a continuous stream (15 cm3 min−1) of the substrate to be monitored was saturated with gaseous CO2, acidified by the addition of excess acid, and the rate of carbon dioxide evolution, proportional to the concentration of bicarbonate/carbonate in the liquid flow, continuously measured by a sensitive gas meter. The instrument was robust and its response was satisfactory for wastewater treatment process control applications, with linearity in the range 5 to 50 mM HCO3−, a response time in the order of 30 min, and accuracy of the order of 7% in the concentration range 5 to 50 mM sodium bicarbonate. The device was not affected by interference from volatile fatty acids, does not make use of pH probes which in many wastes are subject to fouling, and may form the basis of a digester control strategy. © 1994 John Wiley & Sons, Inc.