The development of a predictive model for the removal of helminth eggs during rapid sand filtration

Dunn, Andrew John (1991) The development of a predictive model for the removal of helminth eggs during rapid sand filtration. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The objective of the research was to establish whether rapid sand filtration, as a tertiary treatment process, could achieve the WHO health guidelines for the reuse of wastewaters in agriculture. These guidelines reflect the belief that helminth eggs represent the greatest threat to public health. In order to establish the efficiency of filtration, detection procedures for helminth eggs in sewage were evaluated. The most efficient was used to establish environmental effects on percentage recovery. It was discovered that increasing the time and temperature under which settling occurred increased the efficiency of recovery. Under best conditions, recovery efficiencies were between 50 and 60%. To investigate filtration, duplicate experiments were carried out on two identical models 125mm diameter rapid sand filters with 77cm depth of sand. 250 Ascaris suum eggs/l of influent were added for 22 hours. The removal efficiency in filters with effective sand sizes of 0.3, 0.4, 0.5, 0.6 and 0.7mm were investigated at a flow rate of 4m³/m²/h. The flow rate was varied during the 0.5mm effective sand size runs to include 2m³/m²/h and 6m³/m²/h. Experiments with anthracite/sand and sand/garnet mixed medias were also conducted. Only the sand/garnet mixed media showed total helminth egg removal. The efficiency of the other experiments varied from 99.2% to 99.8% removal, with greater efficiencies for the smaller effective sand sizes and lower flow rates. This was equivalent to between 0.5 and 2 eggs/l of effluent. The WHO guideline value is less than one egg/l of effluent and it was shown that an effective sand size of 0.4mm or under is required to achieve this under the criteria used. It was established that interception and sedimentation are the determining removal mechanisms for helminth eggs. Decreased effective sand size and flow rates improved the efficiency of these processes. From the results a predictive model was developed which determines the numbers of helminth eggs in the effluent using the initial parameters. It was concluded that rapid sand filtration can obtain the WHO guidelines for reuse provided certain operational criteria are met, and maintained between backwashing.