Temperature measurements were taken at three pilot-scale facultative ponds located at Esholt wastewater treatment works in West Yorkshire, UK over two one-year periods. The measurements were taken at different depths using a technology called Thermochron iButton. The iButton readings were compared with temperature measurements taken by a YSI sonde probe and maximum/minimum thermometers: they were found to be within 1 .C. In the temperature range 3.18 .C the mean weekly and monthly air temperatures were found to be good predictors for the mean weekly and monthly pond water-column temperatures. The level of stratification in the ponds under UK climatic conditions was assessed; the data revealed that the ponds remained thermally stratified for significant periods during the year despite their shallow depth.

Three pilot-scale facultative ponds were constructed at Esholt wastewater treatment works in Bradford, West Yorkshire, UK. The ponds were operated in parallel to test the effect of surface BOD loading on performance and the maintenance of facultative conditions. The performance criteria adopted were BOD, SS and ammonia removal. The criterion for facultative conditions was the presence of an algal population maintaining aerobic conditions at the pond surface. Filtered BOD removal was found to be more than 90%, non-seasonal, and related to areal BOD loading. SS removal was also found to be non-seasonal, at around 95% but not related to areal BOD load. Ammonia removal was seasonal (32-48% October to March; 60-81% March to July) and related to areal BOD load. At the test loadings (60, 110 and 169 kg/ha.d) an algal population was not maintained in winter. The initial results indicate that a much lower loading is required to maintain facultative conditions than to optimise BOD and SS removal.

Waste stabilisation pond systems in the UK are used to treat effluents from small rural communities where there are large fluctuations in both BOD load and inflow; the facultative ponds in these systems have a wide range of hydraulic retention times: between 11-86 days. Low hydraulic retention times in UK ponds are sometimes accompanied by a high BOD loading, although some have a low BOD loading due to high inflows of dilute wastewater. It is not certain whether the performance is affected by the short hydraulic retention time or high BOD loading. A pilot-scale experiment tested the effect of hydraulic retention time (20-60 days) on primary facultative pond performance whilst keeping the BOD loading constant at 80 kg/ha d. It was found that no significant loss of performance was experienced at the test range for BOD and ammonia removal; some loss in SS removal was noted at 20 days' retention time. The effect of BOD loading on the maintenance of algal populations during winter (November(February) was tested at loadings of 50 and 80 kg/ha d. Although there was a significant difference in the concentrations of chlorophyll a and dissolved oxygen between the two loadings, there was no effect on performance.

Three pilot-scale facultative ponds, located at Esholt wastewater treatment works in Bradford, West Yorkshire, UK were operated in parallel for 53 months, each with different BOD loadings and hydraulic retention times. Measurements of the sludge accumulation were made at 6-monthly intervals commencing at 3 months after start-up. In addition grab sludge samples were collected and analysed for total solids, volatile solids and faecal coliforms. After the first year of operation the sludge accumulation rate was 0.74 m3 yr(-1) for all three ponds (p=816), independent of BOD loading and HRT; the per capita accumulation rates were within the range 0.08-0.16 m3 person(-1) yr(-1). Evidence of degradation was found in the sludge from the percentage of volatile solids in the dry solids, but insignificant removal of faecal coliforms.

Three pilot-scale primary-facultative waste stabilisation ponds were constructed at Esholt sewage-treatment works, Bradford. Each pond received screened sewage at different BOD loading rates over a two-year period. Concentrations of BOD, SS, amm. N and chlorophyll-a were measured weekly in summer and bi-weekly in wintec and sludge accumulation was measured after 3, 9, 15 and 20 months. BOD and SS removals were consistently high, although they were lower in summer than in winter due to high levels of algae. The removal of amm. N was generally much better in summer than in winterthe removal mechanism being most likely related to algal activities. Sludge accumulation rates were veiy low.