Operational Control of Nitrification and Denitrification
The two part process of biological nitrogen removal through nitrification/denitrification requires that ammonia if first converted into nitrite and then nitrate, and then that the nitrate be converted into nitrogen gas, which is released to the atmosphere.
The nitrification portion of biological nitrogen removal can be accomplished using either fixed film or suspended growth reactors. In either case, a sufficient quantity of oxygen and ample time is required for the microorganisms to carryout the process. No matter which type of reactor is used, the process must be operated in such a way that ammonia is oxidized completely to nitrate. If the process results in the formation of only nitrite but not nitrate, disinfection problems will result where chlorine disinfection is employed. This is because nitrite exerts a high demand (2.5:1) as nitrite is oxidized to nitrate by chlorine.
Please note that a new approach to nitrogen removal is being employed that oxidizes ammonia to nitrite and then denitrifies the nitrite immediately without the creation of nitrate. This is accomplished biologically, all within the same aeration basin. The advantage to this new process is that significantly less oxygen is required. Control of the process can be difficult and requires on-line sensors and computer control of the aeration system.
Nitrification in fixed film processes can be accomplished using a variety of treatment units. Fixed film processes have an advantage over suspended growth processes when it comes to nitrification, because the organisms responsible for nitrification prefer environments where they can attach to fixed surfaces. This allows nitrifiers to grow in stable colonies. Constructed wetlands, RBCs, fluidized beds, recirculating sand filters and trickling filters have all been used successfully for nitrification. Of these treatment units, trickling filters are most commonly employed and the following discussion is applicable mainly to the trickling filter process. However, many of the important points apply to all fixed film processes that are intended to nitrify.
Trickling filters (and all fixed film processes) provide BOD removal by converting soluble BOD into material that can be removed through solids separation (gravity settling or filtration). If nitrification is also a requirement, most of the BOD must be removed first. This is because the microorganisms responsible for nitrification cannot compete with the large numbers of Heterotrophic bacteria that form when a large BOD source is available for food. For this reason, dual stage trickling filters are often employed to provide BOD removal in the first stage and then nitrification in the second stage. BOD removal and nitrification can occur in the same trickling filter, with the BOD conversion occurring in the upper portion and nitrification in the lower portion of the filter media, but efficiency suffers.
Very low organic loadings (< 25 lbs. BOD/day/1000 cuft of media) are necessary to allow the growth of nitrifying bacteria in the same filter with large numbers of Heterotrophs. Recirculation through the trickling filter is critical to maintaining efficient nitrification. Because the nitrifiers are sensitive to changes in their environment, the filter media must be kept wet at all times and sufficient dissolved oxygen must be maintained. Recirculation provides for both requirements. Recirculation rates of 50 – 200 % (and higher) have been used for nitrification. Occasionally, forced air ventilation will be used to improve the nitrification rate of trickling filters. Activated sludge processes can be operated in a variety of modes. Not all modes of operation are conducive to nitrification. Conventional activated sludge lends itself well to nitrification because the plug flow through the basin allows for the assimilation of BOD by the Heterotrophs prior to the start of nitrification. Adequate detention times and dissolved oxygen levels must be maintained. It is also of great importance to maintain a large enough population of nitrifiers in the system (higher SRT). This requires an increased solids inventory over that which is required for BOD removal alone. Extended aeration activated sludge has even greater advantage for nitrification due to long detention times and high sludge ages. Contact stabilization activated sludge generally does not provide for nitrification because of the high F:M that these systems operate at and the short detention times in the contact zone. Step feed activated sludge can be used for partial nitrification, however, because influent is introduced near the end of the aeration basin, ammonia can pass through without being fully oxidized to nitrate.