No abstract

In situ studies on waste stabilization ponds (in Portugal) showed that faecal coliform numbers were lowest at positions in the water column where pH, temperature, dissolved oxygen and algae were high. Numbers were not always lowest at the surface where light intensities were highest or in ponds where light penetration had increased through algal grazing by Daphnia. Laboratory studies showed that pH values approaching 9.0 or above increased faecal coliform die-off particularly under nutrient-poor conditions. Elevated temperatures enhanced the pH effect but the level of dissolved oxygen made little difference. Streptococcus, Salmonella and Campylobacter isolates all behaved similarly to the faecal coliforms. Daphnia grazing of the algal population in maturation ponds may reduce the microbial quality of the final effluent. These findings are discussed in relation to pond design.

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.

No abstract

Waste stabilization ponds are widely used in many European countries, such as France and Germany, for the treatment of wastewater from mainly small communities. There would seem to be no reason why they should not also be a suitable treatment process for populations of under 2000 in the UK. The relative advantages and disadvantages of ponds are compared to other small community treatment systems, such as rotary biological contactors, biological filters and reed‐bed systems, particularly in relation to effluent standards with respect to algae, nutrient removal and relative costs.