Retention and removal of the fish pathogenic bacterium Yersinia ruckeri in biological sand filters.

AIMS: To investigate the retention and removal of the fish pathogenic bacterium Yersinia ruckeri in biological sand filters and effects on the microbial community composition. METHODS AND RESULTS: Sand filter columns were loaded (70 mm day(-1)) with fish farm wastewater and a suspension (10(8) CFU ml(-1)) of Y. ruckeri. Bacterial numbers and protozoan numbers were determined by plate counts and epifluorescence microscopy, respectively, and microbial biomass and community composition were assessed by phospholipid fatty acids (PLFA) analysis. Concentrations of Y. ruckeri in the filter effluent decreased from 10(8) to 10(3)-10(5) CFU ml(-1) during the experiment. Numbers of Y. ruckeri in the sand decreased from 10(6) CFU g(-1) dry weight (DW) sand to 10(4) CFU g(-1) DW sand. In contrast, microbial biomass determined with plate counts and total PLFA increased during the whole experiment. Principal component analysis (PCA) revealed a change in microbial community composition with time, with the most pronounced change in surface layers and towards the end of the experiment. Protozoan numbers increased from ca 0-600 cells g(-1) DW sand, indicating the establishment of a moderate population of bacterial grazers. CONCLUSIONS: The removal of Y. ruckeri improved during the experiment. Introduction of Y. ruckeri to the sand filter columns stimulated growth of other micro-organisms, which in turn caused a shift in the microbial community composition in the sand. SIGNIFICANCE AND IMPACT OF THE STUDY: This study increases the understanding of the dynamics of sand filters subjected to a high loading of a pathogenic bacterium and can therefore be used in future work were the overall aim is to provide a more reliable and efficient removal of pathogenic bacteria in biological sand filter systems.

Intermittent filtration of wastewater--removal of fecal coliforms and fecal streptococci.

Removal of fecal coliforms and fecal streptococci was monitored over a period of 13 months in 14 buried pilot scale filters, treating septic tank effluent. The effects of grain size, hydraulic dosing rate and distribution method were investigated. Two different natural sands (sorted sand and unsorted sand) and three different types of light weight aggregates (LWA 0-4 mm, LWA 2-4 mm and crushed LWA 0-3 mm) were used. Intermittent dosing rates from 20 to 80 mm/day in 12 doses per day were applied to the filters by uniform pressure distribution or point application by gravity dosing. Removal of fecal coliforms was more than three orders of magnitude higher in the media with the finest grain sizes (unsorted sand) as compared to the coarsest media (LWA 0-4 mm and LWA 2-4 mm) operated under same conditions. Fecal streptococci were determined only in effluent from filters with LWA 0-4 mm and LWA 2-4 mm. Higher removal of fecal coliforms was observed in pressure dosed filters compared to gravity dosed filters. A lower removal was observed by increasing the hydraulic dosing rate. Minimum retention time was found to be a key parameter for predicting removal of bacteria in unsaturated, aerobic filters. At minimum retention times lower than about 50 h, there was a correlation of 0.96 between retention time and removal of fecal coliforms. Retention times longer than 50 h gave almost complete removal of fecal coliforms.

Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters.

The transport potential of Cryptosporidiim parvum (C. parvum) through intermittent. unsaturated, sand filters used for water and wastewater treatment was investigated using a duplicated. 2(3) factorial design experiment performed in bench-scale, sand columns. Sixteen columns (dia = 15 cm, L = 61 cm) were dosed eight times daily for up to 61 days with 65,000 C. parvum oocysts per liter at 15 degrees C. The effects of water quality, media grain size, and hydraulic loading rates were examined. Effluent samples were tested for pH, turbidity, and oocyst content. C. parvum effluent concentrations were determined by staining oocysts on polycarbonate filters and enumerating using epifluorescent microscopy. At completion, the columns were dismantled and sand samples were taken at discrete depths within the columns. These samples were washed in a surfactant solution and the oocysts were enumerated using immunomagnetic separation techniques. The fine-grained sand columns (d50 = 0.31 mm) effectively removed oocysts under the variety of conditions examined with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (d = 1.40 mm) yielded larger numbers of oocysts which were commonly observed in the effluent regardless of operating conditions. Factorial design analysis indicated that grain size was the variable which most affected the oocyst effluent concentrations in these intermittent filters. Loading rate had a significant effect when coarse-grained media was used and lesser effect with fine-grained media while the effect of feed composition was inconclusive. No correlations between turbidity, pH, and effluent oocyst concentrations were found. Pore-sizc calculations indicated that adequate space for oocyst transport existed in the filters. It was therefore concluded that processes other than physical straining mechanisms are mainly responsible for the removal of C. pavum oocysts from aqueous fluids in intermittent sand filters used under the conditions Studied in this research.