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A disposable biosensor for ammonium ions in sewage effluent was developed and based on a screen‐printed carbon electrode coated with the enzyme glutamate dehydrogenase, 2‐oxoglutarate, and NADH. This rapid and selective amperometric biosensor is capable of detecting NH 4 + in sewage effluent at concentrations in the range 1–10 ppm. The biosensor was developed as a disposable, reagentless device incorporated into a fully automated, hand‐held instrument that can be used away from the laboratory. This low cost, commercially available system permits the measurement of NH 4 + in effluent samples in only 5 min and 20 s without any pretreatment. The results obtained correlated well with the standard laboratory method, used by the Environment Agency. [ABSTRACT FROM AUTHOR]