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Sand-Filter Trenches for Purification of Septic Tank Effluent: III. The Microflora¹

ABSTRACT

The microbial flora of sand filters play an important role in purification and clogging processes in sand. The organisms were studied by plate counts, microscopic counts, and biomass calculations. Fecal coliforms were estimated by MPN techniques.

In sand-filter trenches A, B, and C, operated at different temperatures and loading rates for further treatment of septic tank effluent, the number of viable bacteria in the infiltrative surface generally increased from 10⁶ to 10⁷–10⁸/g sand, and was about 10⁷ in the sandfill below. The total number of bacteria was a 100–1,000 times the viable count, and decreased with depth. This was most profound in a heavily clogged filter (A) which was kept at the highest temperature (12–15°C). Fecal coliform numbers followed the same pattern with depth.

Rods (ca. 0.3 µm³) and cocci (ca. 0.06 and 0.5 µm³) dominated the bacterial flora of the sand-filter trenches. Cocci (ca. 0.5 µm³) and small rods penetrated the whole profile in the least clogged unit, but were retained to a certain degree in unit A.

In the crust layer of a heavily clogged sand filter, 44 mg C and 13 mg N/100 g sand were bound in bacteria. The bottom sand layers contained about 0.2 mg C and 0.006 mg N/100 g sand bound in the biomass. In a slightly clogged filter these values were about 40–50 times as high. In the most clogged unit the biomass was strongly correlated with the amount of total C. This was not the case in a slightly clogged unit.

Concentrations of fecal coliforms in sand and outlet water from filters with low clogging intensities operated at ambient temperature were considerably higher than concentrations from a filter operated at a higher temperature and thus a higher clogging intensity.

Key Words: viable counts • size classification • biomass • fecal coliforms

¹ Contribution of Dep. Microbiol, Agric. Univ. Norway. Funded by Agric. Res. Coun. of Norway.

² Now Research Coordinator, Agric. Res. Coun. of Norway, Section Soil Pollut. Res., 1432 Aas-NLH, Norway.

Received for publication August 16, 1980.

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