Nitrogen depletes oxygen in water. This stimulates plant growth and causes toxicity to aquatic life. It also presents a public health hazard and affects the resuse of wastewater for reuse purposes.
This is critical as water becomes more scarce.
Nitrification is the conversion of ammonia (NH3) to nitrate (NO3).
This is a two-step process in the presence of oxygen and two types of nitrifying bacteria, nitrosomonas and nitrobacter.
- Ammonia (NH3) + Oxygen (O2) + Alkalinity + Nitrosomonas = Nitrite (NO2)
- Nitrate (NO2) + Oxygen (O2) + Alkalinity + Nitrobacter = Nitrate (NO3)
Nitrification is reliant on removal of biochemical oxygen demand (BOD) from the influent by the dominant heterotrophic bacteria, allowing the nitrifying bacteria to proliferate.
The two-step reactions usually occur very rapidly and it is rare to find nitrite levels higher than 1 mg/l.
Nitrification is required because ammonia is toxic to fish and other aquatic life, places a high oxygen demand on the receiving waters and provides nutrient that can lead to algal blooms.
The ammonia values are approximately 60% of the total kjeldahl nitrogen (TKN) values in raw wastewater and the remaining organic nitrogen is generally converted to ammonia or removed in the settled sludge in a septic or primary stage of treatment which can remove 10% to 30% of Total Nitrogen. Also, TKN generally equals 15 to 20% of the BOD of the raw sewage.
The total conversion of ammonia to nitrate takes 4.6 parts oxygen and 7.1 parts alkalinity to convert 1 part ammonia. You should maintain the pH of the aeration tank above 6.5. Below this level biological activity will become inhibited and toxic ammonia (NH3) can bleed through the system.
When alkalinity drops below 50 mg/l pH can drop dramatically. If the ammonia multiplied by 7.1 mg/l is less than 100 mg/l of the influent alkalinity concentration, you need to add alkaline such as sodium hydroxide or lime prior to the aeration tank.
High levels of nitrite in the system indicate there is, or there is about to be, a problem with the nitrification cycle. Nitrosomonas bacteria are harder to kill than nitrobacter bacteria. If nitrobacter bacteria are killed off, nitrosomonas bacteria will continue working in the ammonia resulting in a jammed cycle with high levels of nitrite.
The important parameters for nitrification include the hydraulic retention time of the aeration stage and the provision of sufficient oxygen. The nitrification process is temperature dependent with the nitrification rate decreasing as temperatures reduce below 10 degrees C.