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ABSTRACTA high microbiological quality of drinking water must be ensured to protect public health. The filtration techniques that are used in treating drinking water play an important role; however, a biofilm can form on granular‐media filters and the accumulated bacteria can slough off and enter the filtered water.The aim of this research was to examine (a) the potential for biofilm formation and detachment from filter sand, and (b) the effect of different backwash regimes on biofilm removal. During the operation of the filter, bacteria became attached to the sand media, particularly in the top 30 mm of the filter bed. A water‐only backwash at 20% and 40% bed expansion demonstrated poor removal of biofilm throughout the depth of the bed. Collapse‐pulsing is a more efficient method and results in a reduction in the number of bacteria in the filtered water.

AbstractRecent legislation has led to stringent ammoniacal‐nitrogen consents and the need for first‐time sewage treatment in coastal areas where land is limited. This has led to the need to improve 'small footprint'sewage‐treatment processes (such as biological aerated filters) which can be used for carbonaceous treatment, nitrification, or for combined treatment. The removal of ammoniacal nitrogen in filters containing different sizes of Lytag medium and operated for combined carbonaceous treatment and nitrification, was compared at different hydraulic and volumetric loading rates. The results suggest that filters containing the smallest media size (2–4 mm), gave optimum nitrification at ammoniacal‐nitrogen loading rates up to 0.6 kg/m3. d. At higher loading rates there was a rapid decrease in nitrification for this size of medium but, with 2.8–5.6 mm medium, nitrification continued at loading rates up to 1 kg amm. N/m3 d. The filters containing larger media sizes (4–8 mm and 5.6–11.2 mm) exhibited low levels of nitrification above a loading rate of 0.2–0.4 kg amm. N/m3. d.

ABSTRACT At present there is no generally accepted test available for characterizing the friability of granular filter media in the context of its ability to withstand handling and placing, as distinct from abrasion resistance during backwashing – which is the subject of other, existing, test methods. Most existing procedures tend to be grinding tests and have little meaning. The approach selected in this work was to adapt the ring shear test, in which a sample (large enough to be analysed by sieving) is subjected to shear under a known compression load and the change in the undersize fraction is assessed. A representative selection of dry media was submitted to the test and the results proved to be reproducible and capable of distinguishing (clearly) between friable and non‐friable resistant types. The procedure is likely to become the basis of a standard test.