Effects of Two Earthworm Species on Movement of Septic Tank Effluent through Soil Columns
Development of a clogging layer at the soil-gravel interface often causes failure of septic tank filter fields to transmit effluent. A column study was undertaken to determine whether earthworms (Lumbricus terrestris L. and L. rubellus Hoff.), through their burrowing activity, could reverse the negative effects of clogging layer formation. Twelve soil columns, 0.20 m in diameter by 0.38 m long, were constructed using gravel and subsoil of a Peridge soil (fine-silty, mixed, mesic Typic Paleudalf). The columns were dosed for at least 263 d with 2.3 cm d−1 of septic tank effluent until clogging layers had formed and stabilized. Steady-state effluent flux densities through the columns ranged from 1.9 to 2.9 µm s−1, so that about 2 h were required for drainage of the daily effluent dose. The columns were divided into four groups based on their hydraulic performances, and the columns in each group were treated with 25 L. rubellus, 25 L. terrestris, or no worms. Within 80 d the flux densities through the columns containing earthworms increased to between 4.8 and 12.4 µm s−1, or 40 to 135% of the initial maxima before clogging layer formation. Effluent flux densities through control columns remained constant. Earthworm survival rates were not significantly different (P > 0.05) between species, with 81% of L. rubellus and 95% of l. terrestris surviving. In two columns, L. rubellus cocoons hatched. During the 96-d study, the earthworms lost less than 4% of their initial weights on a dry weight basis. The L. rubellus and L. terrestris produced channels with median diameters of 2.3 and 8.4 mm, respectively.
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