A Quick, Low-Cost Treatment Method for Secondary Effluent Nitrate Removal through Denitrification

Abstract

The objective of this work was to develop an experimental treatment method for nitrate removal from secondary effluents through denitrification. Three plant substrates (pine bark, almond shells, and walnut shells) were used as organic carbon sources for denitrification and as a medium for the denitrifying bacteria. Experiments were carried out using eight cylindrical open-air batch reactors with a diameter of 16 cm and a height of 21.5 cm. Each pair of batch reactors was filled with one of the plant substrates, and additionally, one pair was filled with gravel and used as a control. Additional experiments were conducted using a rectangular, open-air, continuous-flow reactor filled with almond shells, which had a length of 70.5 cm, a width of 28 cm, and a height of 30 cm. Nitrate removal was evaluated as a function of hydraulic retention time (1, 2, and 3 days for batch reactors and 16 and 24 h for the continuous reactor), water temperature (13 and 20°C for batch reactors and 15, 20, and 25°C for the continuous reactor), influent nitrate concentration (only in the case of the batch reactors, 18.5 and 35 mg NO₃^--N/L) and type of reactor. The efficiency of the nitrate removal was seen to be dependent on the hydraulic retention time, temperature, and influent concentration. Under all the conditions tested, walnut shell batch reactors were more efficient (removal rates from 9.6 to 18.4 g NO₃^--N/m3 · day) than the batch reactors filled with the other substrates (removal rates from 4.6 to 8.5 g NO₃^--N/m3 · day for bark pine and 4.7 to 7.3 g NO₃^--N/m3 · day for almond shells). The control gravel reactors did not remove nitrate. From the data obtained it is concluded that walnut shells are more effective than almond shells and pine bark, because they produce a larger quantity of organic matter, which is also more easily biodegradable. The continuous-flow reactor filled with almond shells was more effective for nitrate removal than the batch reactors filled with the same substrate (11.3 to 17.6 g NO₃^--N/m3 · day). Water circulation in the continuous-flow reactor apparently favors the rate of organic matter release and dispersion, which in turn, improves denitrification. The main concern of this treatment method is the increase in effluent COD concentration produced by organic matter release; nevertheless, the effluent COD was, in general, below the limit set by the European Directive 91/271 in all experiments.