Editor's note: The following is the last in a series of articles summarizing the findings of a recent Clemson University study on aerated lagoons by its Department of Environmental Engineering and Science. The performance of aerated lagoon systems, as well as the diagnosis and remedies of their operational problems, are the focus of the series. Part 1 was on "Fluent BOD5-A misleading parameter for the performance of aerated lagoons treating municipal wastewaters." Part II focused on "Aerated lagoon effluents" and the "Control of algae." Today's installment will discuss "Nitrites and their impact on efflent chlorination." Two more technical papers are in the works and will be published by the author at a future date. They include "Aerated lagoon for secondary treatment" and "Nitrification in aerated lagoons and with intermittent sand filters." Complete text of the study findings may be found at the Clemson University website.
Chlorination of secondary treatment effluents is sometimes impaired by an immediate chlorine demand exerted by nitrites. Such a demand is erratic and can be so large as to prevent maintaining a chlorine residual regardless of the dosage. In most instances, the condition appears to be transitory and soon disappears. However, in some instances, especially in the case of aerated lagoons with long retention times, the condition lasts long enough to result in fecal coliform violations of the effluent discharge permit. Remedial measures to remedy the problem depend primarily on the understanding of the conditions that favor nitrite production.
During warm weather, some nitrification will occur in most aerated lagoon systems treating domestic wastewaters, especially if there is sufficient retention time in the aerobic regions of the system.
Nitrification (oxidation of ammonia to nitrate) is a two-step process:
Unlike in nitrification, the second step appears to be the slowest step (Dawson and Murphy 1972), particularly if the carbon is limiting growth (Freedman 1999) or if the carbon source is complex (McCarty et al. 1969). Furthermore, it has been found that the second step, nitrite reduction, is inhibited by the presence of nitrates (Kornaros et al. 1996), and is more sensitive to oxygen (Koraros and Lyberatos 1998).
Since the second step is the slowest step, nitrite accumulates in larger concentrations. Effluent concentrations as high as 15 mg/L (as nitrite nitrogen) or more have been found aerated lagoons (verbal communications).
When chlorine is added to effluents with nitrites but with little ammonium nitrogen (NH4+-N), the chlorine reacts as free chlorine and is removed quickly by a chemical reaction with the nitrites, thereby increasing significantly the amount of chlorine that must be used to meet the required limit of coliform concentration. Each mg/L of nitrite nitrogen reacts with 5 mg/L of chlorine. Thus, a nitrite concentration of only 10 mg/L will exert a chlorine demand of about 50 mg/L.
Adding ammonia may help counteract the effect
It has been shown, however, that when chlorine is added to effluents with nitrites and with a high concentration of ammonium ion (>20 mg/L), the chlorine reacts preferentially with the ammonium ion forming chloramines (Phoenix 1995). This suggests that, if insufficient ammonium is already present in the effluent, the chlorine demand exerted by effluent nitrites can be minimized by adding ammonia along with chlorine in a well-mixed chlorine contact basin. Chloramines, while effective as disinfectants, will not react with nitrites.
When the condition of excessive nitrites in the effluent of an aerated lagoon persists, the best remedy is to attempt to nitrify the nitrites to nitrates by ensuring a completely aerobic environment (all aerators on all the time) along with the presences of sufficient alkalinity to support rapid nitrification. When cooler weather intervenes, nitrification diminishes and eventually the inventory of nitrites will disappear.
For more information, contact the author at email@example.com or 864-656-5575 or access the website at www.clemson.edu/ees/Rich/technotes.
Edited by Joyce Jungclaus, Editor, Public Works Online