Abstract
Maturation ponds are potentially the most economical and easily maintained unit process for the removal of faecal indicator bacteria in sewage treatment, especially when climate and sunlight conditions are favourable. The present study was conducted with the aim of improving faecal coliform (FC) and faecal streptococcus (FS) removal efficiencies in tertiary maturation stages of a sewage treatment plant in Southern England, where climatic conditions are less favourable The research used intensive field assessments (bacteriological, general quality and hydraulic) to identify the parameters that affect the bacteriological quality of the effluent from the maturation ponds, and optimising both the engineering design and system operation. The tertiaiy maturation ponds comprise three parallel maturation ponds (North, Central and South) of similar geometry and dimensions. An engineering intervention was carried out to convert the South pond to three channels to increase the LAV ratio from 9:1 to 79:1 in order to improve hydraulic performance and FC and FS removal. Hydraulic tracer studies with Rhodamine WT at the plant showed that the dispersion number 'd' was reduced from 0.37 (dispersed flow) to 0.074 (piston flow) by this intervention under identical flow conditions. Hydraulic retention time was thus increased by 5 hours. As a result of the intervention FC removal increased substantially. When operating at 8.6 l/s and 14°C, and with approximately 10-hours sunlight exposure the removal efficiency was 92%. Maximum channel-lagoon efficiency of 99. 84%, was obtained at 4.5 l/s and 19°C, when exposure to sunlight was 17 hours. The most important variables for FC removal were shown to be retention time followed by sunlight exposure. Under similar operating conditions (4.5 l/sec), delay in jet flow short-circuiting was increased by 86%, from 2.5 hour to 17.5 hours thus increasing the exposure times for all elements. As a consequence the final effluent met the WHO guideline of less than 1,000 FC/100ml for the first time. It is concluded that the channel configuration produces a higher hydraulic efficiency than conventional maturation ponds. It is therefore recommended as a viable engineering solution which permits a low-cost upgrading of plant performance, requiring no additional land, and with minimal maintenance costs.