Infiltration of Wastewater in a Newly Started Pilot Sand‐Filter System: III. Transformation of Nitrogen

Abstract

AbstractThe objective of the study was to monitor the chemical dynamics associated with N transformations from start‐up until a steady state was reached in a pilot sand‐filter system consisting of a three‐chambered septic tank, a sand filter with a one person capacity, and eight columns. The system was loaded with 67 L m−2 d−1 of an artificial wastewater. The untreated wastewater contained 114 mg Kjeldahl N L−1 and 0.12 mg NH+4‐N L−1. Mineralization reached a steady state in the septic tank after 2 wk of operation; thereafter, 65 ± 6% of the effluent N was NH+4‐N. In the column effluent the concentrations of NH+4‐N increased during the first 6 wk of operation, whereafter a stabilization at 60 to 80 mg NH+4‐N L−1 occurred. The mean proportion of effluent N in mineralized form for the whole period was 88 ± 6%. Three weeks after start‐up, all effluent N from the sand filter was in mineralized form and remained high for the rest of the study period. The effluent concentrations of NH+4‐N increased rapidly during the first 2 wk after which it decreased to less than 3 mg L−1. The concentration of NO−3‐N was initially low. It then increased until about Day 30 when it stabilized at 64 to 71 mg NO−3‐N L−1. Elevated concentrations of NO−2‐N were observed between Days 8 and 45. Nitrifying activity reached a steady state after 6 wk of operation. During this state a considerable amount of nitrification took place in the upper 1.5 cm and all wastewater N had been nitrified after 15 cm of percolation. By the end of the operating period the total amount of influent N in the septic tank had been reduced by 17%. Corresponding reductions for the columns and sand filter were 18 and 33%, respectively.