Nitrifiers consist of two types of bacteria. Nitrosomonas, which oxidizes Ammonia to Nitrite, and Nitrobacter, which oxidizes Nitrite to Nitrate. Each one of these naturally occurring bacteria can only perform its single function and nothing more. So, for complete Nitrification to occur, both bacteria need to be present at the same time, to complete the Nitrification Cycle. This process does not totally remove Nitrogen from the water, but rather converts Nitrogen’s form. Nitrosomonas and Nitrobacter require high oxygen content and perform best in warmer water, with a neutral pH and BOD below 200.
It can be difficult to grow Nitrifiers for use in Industrial and Commercial products because of the specific conditions they are dependent upon for growth. Bacillus bacteria, called heterotrophs, are primarily responsible for removing organic wastes, which lowers the BOD to a level where Nitrifiers can grow. Microbial growth is often be expressed in terms of their “doubling”, called “generation time”. Generation times of heterotrophic bacteria like Bacillus are normally 10-20 times less than the generation time for Nitrifiers. This is important to know, because as mentioned above, it will be difficult to nitrify your water if you have high BOD. It is not until BOD falls below 20 mg/L that Nitrifiers will start to grow well. When there is low BOD in the water, the Bacillus bacteria do not have enough carbon (food source) to grow well, so Nitrifiers will thrive and use most of the available oxygen. In contrast BOD is high, Bacillus bacteria have a lot of carbon, so grow rapidly, and thus consume most of the available oxygen. The process of Nitrification can increase oxygen consumption by as much as 50%. Each mg/L of Ammonia requires about 4.5 mg/L of Oxygen demand.
The first step of Nitrification is Nitrosomonas oxidizing Ammonia to Nitrite, which releases more energy than in the second step when Nitrobacter oxidizes Nitrite to Nitrate. In addition, Nitrosomonas are more capable of withstanding changes in pH and temperature, than Nitrobacter. These limitations of Nitrobacter bacteria can lead to partial Nitrification, where only the Ammonia gets oxidized to Nitrite, and the Nitrite buildup faster than the Nitrobacter can oxidize it to Nitrate. Both Ammonia and Nitrite are toxic to fish and other aquatic life, so it is crucial that both get oxidized all the way down to Nitrate, to avoid toxicity in the water. Once in the form of Nitrate, the Nitrogen is no longer toxic. In fact, Nitrate is the form of Nitrogen that plants take up and utilize in their growth. Although it is not toxic, Nitrate buildup creates its own problems. Usually in the form of algae blooms, because the major nutrient that algae require is Nitrate. However, Nitrate can also enter ponds from fertilizer run off during times of rain. When fertilizers are used in soils that surround ponds, the Nitrate found in the fertilizers can be washed directly into the pond by rain. Regardless of how the excess Nitrate found its way into the pond, it needs to be managed. This brings us to the second process called Denitrification.
Denitrification is the process of converting Nitrate into gaseous Nitrogen that can be released into the atmosphere. Bacillus bacteria are excellent at this task. When there is a lot of dissolved oxygen present, Bacillus will use it as first choice. However, when there is no dissolved oxygen available, Bacillus will utilize the oxygen molecule from Nitrate. As they remove the oxygen molecule from the Nitrate, the remaining Nitrogen molecule is released as a gas, which bubbles up out of the pond and is released into the atmosphere.
