Sand‐Filter Trenches for Purification of Septic Tank Effluent: I. The Clogging Mechanism and Soil Physical Environment

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Abstract
Investigations were conducted for 17 months in three pilot plant sand filters to elucidate the relationship between clogging and soil physical and chemical environment in sand filters. Two trenches (A and C) were loaded with septic tank effluent at a rate of 4–6 cm/day, and one (B) at 12–18 cm/day. The temperature in A was 12–15°C, and in B and C was ambient (4–16°C).The septic tank effluent concentrated in one end of the filters. Here the sand surface clogged and ponding occurred. Ponding rates of 0.33, 0.27, and 0.16 m2/month and crust resistances of 35, 5.5, and 4 days were found in units A, B, and C, respectively. The C/N ratio in the crust zone was 5.9 ± 0.2, and in the rest of the sandfill was 5.8 ± 0.3. Bacterial cells occupied 0.74% of the pore volume in the clogged layer of A. Production of extracellular slimy material, which probably filled a larger proportion of the pore volume than the cells, was probably temperature dependent.Increased clogging intensity induced reduced soil water content below the crust zone. Because the filters were aerated from below, the increased clogging led to improved aeration that was shown by gas and redox measurements. While low Eh values (2 contents (4 was measured. In zones of sand containing 2, up to 93% N2 was found. N2O contents above the detection limit (200 ppm) were never measured.

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