Suspended Growth Processes (Nitrification)
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.
When an adequate population of nitrifiers is present, and suitable conditions of dissolved oxygen, alkalinity and temperature are maintained, nitrification systems are relatively easy to operate. The control of nitrification in a fixed film reactor depends mainly upon:
(1) the recirculation rate, and
(2) the applied loading.
The control of nitrification in suspended growth reactors depends upon:
(1) the SRT or MCRT, and
(2) the dissolved oxygen levels.
Temperature affects nitrifiers as it affects all biological activity. Colder temperatures slow the process.
For activated sludge, the detention time in the aeration basin must be at least 4 hours and preferably >8 hours. The typical MLVSS concentration required to maintain an active population of nitrifiers is > 1500 mg/L. Dissolved oxygen levels of 2 – 4 mg/L are typical for conventional activated sludge nitrification processes, while extended aeration plants typically need only 1 – 1.5 mg/L of D.O.
Alkalinity is consumed during the nitrification process at a rate of 7.2 parts of alkalinity for each part of ammonia oxidized. Because of this, alkalinity determinations (along with D.O. readings) offer one of the best day-to-day operational controls for nitrification. A drop in the pH of wastewater may or may not accompany nitrification, depending upon the alkalinity available and pH at the start of the process. Any wastewater containing less than 50 mg/L of alkalinity is likely to experience a significant drop in pH during nitrification.
If the pH drops below 6.5, nitrification will effectively cease. For this reason, it is sometimes necessary to add alkalinity in order to maintain nitrification. If you suspect that low alkalinity is inhibiting nitrification, investigate carefully before taking action. Remember that 24-hour composite samples can often mask fluctuating alkalinity and pH drops. If the pH drops low enough to inhibit nitrification, alkalinity will have to be added to the influent. Lime, soda ash and sodium hydroxide are the most commonly used chemicals that are added to increase alkalinity in nitrifying systems.
The optimum wastewater temperature for nitrification ranges from 15º – 30º C (60º – 95º F). Nitrification is inhibited at low temperatures, and as much as five times the detention time may be necessary to accomplish complete nitrification in the winter as opposed to the summer. Because there is no way to control the wastewater temperatures, operators must adjust other process parameters to compensate for the lower growth rate of the nitrifiers during low temperatures. Typically, increasing the MLVSS concentration, increasing the MCRT, and increasing the pH slightly are the methods used to accomplish this. Under warm weather conditions, nitrification will proceed at a lower pH, lower MCRTs and with lower MLVSS concentrations.
The growth of nitrifying organisms is affected by the nitrogenous load applied to the system. In fact, the population of nitrifiers will be limited by the concentration of ammonia in the influent. Organic nitrogen, phosphorous and trace elements are essential for the growth of the microorganisms in any nitrifying system. The generally recommended BOD to nitrogen to phosphorous ratio is 100:5:1 for BOD reduction alone. In nitrifying systems, ratios containing significantly more nitrogen can be treated. If any of these constituents is not available in sufficient quantities, treatment will suffer.
In some circumstances, it is only necessary to nitrify ammonia into nitrate. For example, for a treatment plant that discharges into a large river, ammonia toxicity may be a problem but nutrient loading may not. Under this situation, the facility’s discharge permit may limit the ammonia form of nitrogen in the discharge, but not limit nitrogen in other forms, such as nitrate. Simply converting the incoming ammonia into nitrate is sufficient to meet the permit limit for ammonia in this case. In many other situations, it is necessary to actually remove the nitrogen by finishing the nitrification/denitrification cycle.