Sand-Filter Trenches for Purification of Septic Tank Effluent: II. The Fate of Nitrogen
1981
Kristiansen, Rolv
In some areas septic systems are suspected to cause an increased content of ground-water N. For that reason, turnover of N in sandfilter trenches for septic tank effluent was investigated from July 1976 until November 1977 in three pilot-plant sand filters operated at 12–15°C (unit A) and ambient temperature (units B and C), and at loading rates of 4–6 cm/day (A and C) and 12–18 cm/day (B). Results from chemical analysis of N in sand and water were compared to redox and other soil properties. Insignificant amounts of N were found to be removed from the effluent passing through the filters. Nitrification in the cold season was significantly affected by the sand temperature in equally loaded units. Nitrification occurred between 7.5 and 27 cm below the crust in unit A, which also had the highest clogging intensity. In the other units nitrate was predominately found near the bottom air vent. This was due to low clogging intensities leading to low contents of air-filled pore spaces. Nitrogen bound in the microbial biomass, adsorbed to the cation exchange complex, and bound in dead organic material could amount to about three months of loading of N from a single-family home to a standard 20-m sand filter. Because of aerobic conditions and lack of suitable energy sources in zones containing nitrate, insignificant denitrification occurred in the sand-filter trenches. Intermittent loading or recycling of nitrified effluent are suggested as methods of improving denitrification in septic systems.
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