The performance of tertiary lagoons in the United Kingdom: I

Two tertiary lagoons at Holmwood Sewage Treatment Works in the United Kingdom were sampled over a 1 year period. Each lagoon is approximately 39×28 m, receives domestic wastewater only, and serves approximately 2000 people. The aim of the work is to gain a greater understanding of the performance of the lagoon with respect to a number of chemical, biological and physical parameters. Samples were analysed for E. coli, enterococci, total coliforms, BOD, COD, suspended solids, ammoniacal nitrogen, nitrite, nitrate, total oxidised nitrogen and soluble phosphate. Occasional samples were also analysed for Cryptosporidium, Giardia and enteroviruses. Cusum charts were used to analyse the cumulative variations that were seen for the parameters being investigated, and had the advantage of emphasising their distinct seasonal trends. Although good percentage removals were seen, there was a lack of consistency in performance for all parameters over the year. No ammonia or phosphorus removal was found, although limited nitrification does appear to occur. None of the bacterial criteria for the WHO guidelines on wastewater reuse or the EC directive on bathing water were reached.