Optimisation of hydraulic performance to maximise faecal coliform removal in maturation ponds

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 sub-optimal. The research used intensive field assessments (bacteriological, general quality and hydraulic) to identify the parameters that affect the bacteriological quality of the effluent from 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 L/W ratio from 9:1 to 79:1. Hydraulic tracer studies in the South pond with Rhodamine WT showed that the dispersion number ‘d’ was reduced from 0.37 (dispersed flow) to 0.074 by this intervention under similar flow conditions (4.5 l/s). Hydraulic retention time was thus increased by 5 h, delay in jet flow short-circuiting was increased from 2.5 to 17.5 h thus increasing the exposure times for all elements. As a result of the intervention FC removal increased substantially. Maximum channel-lagoon efficiency of 99.84% was obtained at 4.5 l/s and 19 °C, when exposure to sunlight was 17 h in summer. 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.