Series – The Removal of Metals from Wastewater
December 18, 2024Series – The Removal of Metals from Wastewater
December 18, 2024Removal methods of nitrogen in Wastewater Treatment
On occasion, untreated wastewater will contain nitrogen compounds that need to be processed to ensure excess quantities do not accumulate in the environment. In conjunction with phosphorus, excess nitrogen can lead to the eutrophication of waterways, which in turn, can promote the accelerated growth of its plant life, a rapid depletion of oxygen levels and the eventual death of all other organism living within.
Nitrogen can be present in wastewater in a variety of forms1:
- Organic nitrogen (Norg)
- Total ammoniacal nitrogen (TAN) – all forms of Ammonia (NH3) and Ammonium (NH4+)
- Oxidized as nitrites (NO2–) and nitrates (NO3–)
To promote their removal, biological and chemical treatment application strategies can be applied. Of the two, biological nitrogen removal applications are most common and often have a nitrification and denitrification step built in.
Nitrification is a two-part process that uses Autotrophic Bacteria (Nitrosomonas and Nitrobacter) to oxidize ammonia waste (Organic and TAN Materials) and transform it to nitrites and nitrates. The reactions used to promote the formation of both nitrite and nitrate are listed as equations 1 and 2 below:
| NH4++1.5 O2 → NO2– + H2O + 2H+ | (eq.1) |
| NO2– + 0.5 O2 → NO3– | (eq.2) |
Denitrification is a biological process that is used to remove oxidized forms of ammonia from wastewater. In an anoxic environment, any nitrates/nitrites that are naturally occurring or present via nitrification, are chemically reduced and transformed to nitrogen gas.
The chemical reaction best used to describe denitrification is listed in equation 3 below:
| 2 NO3– + 10e– + 12 H+ → N2 + 6 H2O | (eq.3) |
Denitrification processes can be applied before or after nitrification, however, it is most common to see it applied afterward.
Another treatment method that is relatively new and worthy of consideration is a process called Partial Nitrification-Anammox (Anaerobic Ammonium Oxidation) or PNA. Under slightly anaerobic conditions, a portion of the ammoniacal nitrogen1 entering a reactor will get converted to nitrite. From there, Anammox bacteria, under anoxic conditions, will convert the resultant mix of ammoniacal nitrogen and nitrite to nitrogen gas and water. As one can appreciate, this process is a bit more complex in terms of its environmental and process control requirements, however, if high levels of organic and ammoniacal nitrogen waste need to be treated using less oxygen and a smaller sludge footprint, PNA may be a good fit.
By and large, nitrification and denitrification processes are subject to specific environmental and process control conditions to ensure appropriate bacterial response. Some of these key conditions are summarized in the table below.
Table 1. Key parameters to ensure the optimal environment for the growth of the nitrifying and denitrifying bacteria1.
| Parameters | Nitrification | Denitrification |
| pH range | 6.5 – 8 | 6.5 – 7.5 |
|
Dissolved Oxygen Concentration |
2 mg O2/L |
< 1 mg/L is necessary < 0.1 mg/L is preferable |
|
Temperature |
30 oC to 35 oC |
Not efficient lower than 15 oC |
|
Respiration mode |
Aerobic |
Anaerobic |
|
Nutrition mode |
Autotrophic |
Heterotrophic |
In terms of chemical treatment, it is possible to oxidize ammonia using chlorine to form nitrogen gas. This process is called breakpoint chlorination2 and is essentially an applied form of chemical disinfection whereby chlorine or a chlorine derivative (sodium hypochlorite, calcium hypochlorite, or chlorine dioxide) is used to promote the desired effect.
Even though this type of chemical treatment can fully oxidize all forms of ammonia that are present in a wastewater stream, it does have its fair share of disadvantages. The likes of which are described in the table below.
Table 2. Breakpoint chlorination disadvantages for the removal of nitrogen from wastewater 2.
| Disadvantages | Descriptions |
| Acid Generation and Buildup | HCl is formed during the breakpoint chlorination process, reacting with the alkalinity of the wastewater causing a drop in the pH. |
| Buildup of total suspended solids (TSS) | Adding chemicals to achieve breakpoint chlorination creates an excess of TSS that may inhibit any reuse application. |
| Formation of unwanted chloro-organic compounds |
The formation of unwanted byproducts alters the shape of the breakpoint curve and might create odor problems. |
It is important to note that the disadvantages listed above are just a few of the many reasons why breakpoint chlorination is not commonly used in industry today. In short, theses processes are often complex, difficult to control and not nearly as effective/efficient as the biological system that are so widely used.
Aquasan’s technical knowledge and expertise can help you with the optimization and control of all critical parameters that affect your biological treatment processes and their part in removing nitrogen. Our goal is to be the best at providing the services you need to achieve your operational targets. Your success is how we measure ours!
Sources:
2 Wastewater Engineering – Treatment and Reuse – Metcalf & Eddy (4th Edition) )
