By Heather Jennings, PE
Today we are going to focus on nitrifiers, those wastewater treatment autotrophs that get energy from oxidizing ammonia. (Autotrophs are microorganisms that produce complex organic compounds using inorganic carbon from simple substances as a food source.) Oxidizing ammonia is a fancy way of saying ripping off hydrogens to stick oxygens onto nitrogen—the essence of nitrification.
In wastewater treatment we rely on bacteria to perform nitrification. While Nitrobacter and Nitrosomonas are the most commonly recognized, they are only part of a suite of autotrophic nitrifying bacteria that do the job. Mind you, there are also heterotrophic nitrifiers that are part of the floc-forming bacteria that help in the oxidation of ammonium, but we’re going to focus on the autotrophs this time around.
In municipal wastewater systems, the primary sources of nitrogen found in BOD come from commercial and industrial sources and from human waste. In other industries such as food processing, sources of organic nitrogen can be fats, oils, and grease, or food preparation treatments. The most common forms of nitrogen in wastewater are organic nitrogen, ammonia, and/or ammonium.
Diving a little deeper into organic nitrogen, you’ll find that it is a combination of proteins, amino acids, and urea that are broken down to provide ammonia or ammonium. Regardless of the nitrogen source, once organic nitrogen is broken down to ammonium or ammonia as a nitrogen source, nitrification can start. Realize, though, that there will always be a portion, typically 5%–10%, that is not economically feasible to treat as it can take more energy or time to treat than the wastewater system has capacity. This portion is released in the effluent and into the environment.
Nitrifiers have many requirements to work. First and foremost, they need oxygen: in chemical terms, 1.5 parts of oxygen to nitrify 1 part of ammonium to nitrite and 0.5 parts of oxygen to nitrify 1 part of ammonia to nitrate. For best results, it is recommended that a system maintain at least 2 mg/L of dissolved oxygen to provide sufficient oxygen to accomplish nitrification. This dissolved oxygen requirement is the real driver in sizing aeration systems and the main contributor to the high-energy requirements of aeration equipment.
Nitrifiers not only need oxygen, they need residence time as well. Nitrifiers typically take between 22 and 48 hours to multiply and, if they had their druthers, more than 5 days of retention time. Retention times can be even longer if the temperature of the influent is below 59°F. Nitrifiers also have a sweet spot for nitrifier growth, temperature-wise, which is 77°F. Nitrification can also be significantly reduced above 86°F, due to reduced dissolved oxygen retention in the water column.
Last, but not least, nitrification needs carbonate alkalinity, or “alkalinity,” as a carbon source for the autotrophic nitrifiers. The lack of alkalinity can be a significant limiting factor. Nitrification requires 7.1 lb. of alkalinity for every 1.0 lb. of ammonia-N oxidized. Approximately half of the alkalinity can be recovered during the denitrification process, but if the other half of it doesn’t come with the influent you will need to add sodium bicarbonate, soda ash, or lime to augment the alkalinity. Alkalinity also supports wastewater systems by buffering them from the subsequent increase in acidity associated with nitrification.
So, in summary, you need nitrifying bacteria, oxygen, suitable temperatures, and enough alkalinity and time to nitrify. Which on paper seems so easy, but in reality can be a challenge to achieve and maintain.