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The renovation of domestic effluent through a forest soil Bryck, John Michael Guy

Abstract

An investigation of the trend over time of the concentration of selected soil leachate constituents contained in domestic effluent applied to a ferro-humic podzol forest soil was undertaken. From July 15, 1976 to November 12, 1976, 2 cm of domestic effluent, selectively supplemented with commercial fertilizer, was sprayed twice weekly to a 900 m² plot situated on a mountain slope. Ten porous plate tension lysimeters installed at various depths and locations in the forest soil continuously extracted the resulting soil leachate. The soil leachate samples collected twice per week over the duration of the study, as well as the applied effluent samples, were analysed for chloride, nitrate and total Kjeldahl nitrogen and total and orthophosphorus. Based on. the concentrations of nitrate nitrogen, total Kjeldahl nitrogen, and total phosphate in the applied effluent as well as the average volume of effluent applied over the total irrigation plot, k8A kg/ha (^3.2 lb/acre) nitrate nitrogen, 93-7 kg/ha (83-6 lb/acre) total Kjeldahl nitrogen, and ^3.5 kg/ha (38.6 lb/acre) of total phosphate were applied to the plot during the project period. The chloride ion gradually increased in concentration over the duration of the study until it eventually equalled the value of the chloride in the domestic effluent. This gradual increase was a possible result of the retention of the initial amounts of applied chloride on the anion exchange sites in the soil, which when satisfied, allowed subsequent amounts of applied chloride to move through the soil with the applied effluent. No ortho or total phosphorus was detected in the soil leachate samples collected over the duration of the study. While recognizing the possibility of microbiological immobilization of a portion of the applied phosphorus, the most likely retention mechanism was the adsorption of the phosphorus by the amorphous iron and aluminum compounds in the soil. Nitrate and total Kjeldahl nitrogen were detected in low concentrations in the soil leachate samples only in the time interval following the application of domestic effluent containing greatly increased effluent concentrations of nitrate and total Kjeldahl nitrogen. The increased concentrations resulted from the addition of commercial fertilizers to the treatment lagoon. Such trends suggest microbiological immobilization was playing a role in the removal of the nitrogen compounds. The retention of ammonia, the major component of total Kjeldahl nitrogen, on the cation exchange sites and the retention of nitrate nitrogen on the anion exchange sites would also be occurring. In order to investigate the magnitude of the reaction between phosphorus and the iron and aluminum compounds in the podzol soil, batch scale laboratory tests were undertaken. The time-phosphorus adsorption results indicate that the phosphorus reacts very quickly with the amorphous iron and aluminum compounds in the soil on the plot. The Langmuir and Freundlich isotherms were calculated as was the phosphorus adsorption maximum based on the Langmuir isotherm. The calculated phosphorus adsorption would probably give a conservative estimate of the phosphorus removal potential since the isotherm is based only on the initial and not the long term phosphorus adsorption reactions. Despite the fact that the soil water content was continually above the soil water content at the field capacity, no phosphorus and very little nitrogen was detected in the soil leachate samples. Such results suggest that a phosphorus or nitrogen balance may be a useful irrigation operating criterion rather than the commonly accepted water balance criterion. In such a situation the phosphorus or nitrogen requirements over the growing season of the crop would be evaluated and met by the application of domestic effluent.

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