A pilot-scale irrigation scheme was set up in South-West Portugal to investigate the causes of emitter clogging which has been observed when waste stabilisation pond effluents are used for drip irrigation of crops. During field trials over a period of two growing seasons, the operating characteristics of five emitter designs were investigated. Susceptibility of the designs to clogging was compared and the nature of clogging particles was analysed. Those emitters that operated most successfully utilised a long water-path labyrinth to reduce flow to the required level. The operating characteristics of the most effective emitter design are presented in detail. Clogging was shown to result from the deposition and entrapment of sand particles within the emitter labyrinth. Organic material, including pond algae, played a secondary role in the clogging process. Emitter design, rather than water quality, was shown to be the most important factor determining the degree of clogging. It was concluded that pond microalgae alone do not constitute a major hazard to the operation of drip irrigation equipment and that waste stabilisation pond effluents may be safely used for drip irrigation if safeguards are adopted to prevent the contamination of laterals by sand and soil particles. A number of recommendations are made to prevent emitter clogging in future reuse schemes.