Consider the following statements: 1. Ammonia nitrogen is a measure of nitrogen present as ammonium hydroxide and ammonium salts. It will progressively decrease as sewage gets treated. 2. Organic nitrogen is the total nitrogenous matter in sewages excepting that present as ammonia nitrogen, nitrites and nitrates. It becomes ammonia in anaerobic decomposition and nitrites or nitrates in aerobic decomposition.
Both 1 and 2
Understanding Nitrogen Forms in Sewage Treatment
The question asks us to evaluate the correctness of two statements concerning different forms of nitrogen found in sewage and how they change during treatment processes.
Analysis of Statement 1: Ammonia Nitrogen
Statement 1 says that ammonia nitrogen is a measure of nitrogen present as ammonium hydroxide and ammonium salts and that its level decreases as sewage gets treated.
- What is Ammonia Nitrogen? In wastewater, nitrogen exists in several forms. Ammonia nitrogen typically refers to the sum of un-ionized ammonia (\(NH_3\)) and ionized ammonium (\(NH_4^+\)) present. These forms are in equilibrium, which is affected by pH and temperature. Ammonium hydroxide is \(NH_4OH\), which dissociates into \(NH_4^+\) and \(OH^-\). Ammonium salts, like ammonium chloride (\(NH_4Cl\)), also contain the ammonium ion (\(NH_4^+\)). So, the statement's definition aligns with the presence of nitrogen in these forms.
- Does it decrease during treatment? Yes, effective sewage treatment processes aim to remove nitrogen. A key biological process is nitrification, an aerobic process where ammonia (\(NH_3\)) or ammonium (\(NH_4^+\)) is oxidized by bacteria first to nitrite (\(NO_2^-\)) and then to nitrate (\(NO_3^-\)). Subsequent denitrification (anaerobic process) converts nitrate to nitrogen gas (\(N_2\)), which escapes into the atmosphere, thus removing nitrogen from the water. Other removal methods like ammonia stripping can also reduce ammonia nitrogen levels. Therefore, the concentration of ammonia nitrogen generally decreases as sewage undergoes proper treatment.
Based on this analysis, Statement 1 is correct.
Analysis of Statement 2: Organic Nitrogen
Statement 2 defines organic nitrogen and describes its transformation during decomposition.
- What is Organic Nitrogen? Organic nitrogen is nitrogen bound within organic compounds, such as proteins, amino acids, urea, and nucleic acids, which come from human waste, food scraps, and other organic matter. Total Kjeldahl Nitrogen (TKN) is a common measure that includes both organic nitrogen and ammonia nitrogen. The statement correctly points out that organic nitrogen is the total nitrogenous matter *excluding* inorganic forms like ammonia nitrogen, nitrites (\(NO_2^-\)), and nitrates (\(NO_3^-\)).
- How does it transform? The statement describes transformations during anaerobic and aerobic decomposition.
- In anaerobic conditions (lack of oxygen), microorganisms break down organic nitrogen compounds through a process called ammonification. This process converts organic nitrogen into ammonia (\(NH_3\)). So, organic nitrogen becomes ammonia.
- In aerobic conditions (presence of oxygen), organic nitrogen is also first converted to ammonia via ammonification. Subsequently, this ammonia is oxidized to nitrites and nitrates through nitrification, as mentioned earlier. So, organic nitrogen indirectly leads to nitrites or nitrates in aerobic decomposition via the intermediate formation of ammonia.
Based on this analysis, Statement 2 is correct as it accurately describes organic nitrogen and its decomposition pathways.
Conclusion
Both Statement 1 and Statement 2 are accurate descriptions of ammonia nitrogen and organic nitrogen and their behavior during sewage treatment and decomposition processes.
| Nitrogen Form | Description | Transformation during Treatment |
|---|---|---|
| Organic Nitrogen | Nitrogen in organic compounds (proteins, urea, etc.) | Converted to ammonia (ammonification) |
| Ammonia Nitrogen (\(NH_3\), \(NH_4^+\)) | Nitrogen as ammonia or ammonium ions | Converted to nitrite/nitrate (nitrification), or removed (stripping) |
| Nitrite (\(NO_2^-\)) | Intermediate inorganic form | Converted to nitrate (nitrification) or nitrogen gas (denitrification) |
| Nitrate (\(NO_3^-\)) | Oxidized inorganic form | Converted to nitrogen gas (denitrification) |
| Nitrogen Gas (\(N_2\)) | Gaseous form | Released to atmosphere (final removal stage) |
Revision Table: Key Nitrogen Forms in Sewage
| Nitrogen Form | Chemical Representation | Origin | Role in Treatment |
|---|---|---|---|
| Organic Nitrogen | Part of complex organic molecules (e.g., proteins) | Human waste, food scraps | Decomposes to ammonia |
| Ammonia Nitrogen | \(NH_3\) (ammonia) and \(NH_4^+\) (ammonium) | Decomposition of organic nitrogen, urine | Undergoes nitrification to nitrite/nitrate; can be removed by stripping |
| Nitrite | \(NO_2^-\) | Intermediate product of nitrification | Converted to nitrate in aerobic conditions; can be reduced to \(N_2\) in anoxic/anaerobic conditions |
| Nitrate | \(NO_3^-\) | Product of nitrification | Converted to \(N_2\) gas during denitrification in anoxic/anaerobic conditions |
Additional Information: Nitrogen Cycle in Wastewater Treatment
The nitrogen cycle is crucial in understanding sewage treatment. Wastewater contains nitrogen primarily in organic and ammonia forms. Treatment processes aim to convert these forms and ultimately remove nitrogen from the water before discharge to prevent eutrophication in receiving water bodies.
- Ammonification: Organic nitrogen is broken down into ammonia (\(NH_3\)) or ammonium (\(NH_4^+\)) by heterotrophic bacteria. This occurs in both aerobic and anaerobic conditions.
- Nitrification: A two-step aerobic biological process carried out by autotrophic bacteria (Nitrosomonas and Nitrobacter). First, ammonia/ammonium is oxidized to nitrite (\(NO_2^-\)) by Nitrosomonas. Second, nitrite is oxidized to nitrate (\(NO_3^-\)) by Nitrobacter. This process requires oxygen and alkalinity.
- Denitrification: An anoxic/anaerobic biological process where nitrate (\(NO_3^-\)) is reduced to nitrogen gas (\(N_2\)), which is then released into the atmosphere. This process is carried out by facultative heterotrophic bacteria using organic matter as a carbon source. It requires the absence of dissolved oxygen but the presence of nitrate.
- Other removal methods: These include ammonia stripping (physical removal of \(NH_3\) gas at high pH), ion exchange, and plant uptake (in constructed wetlands).
Effective sewage treatment typically involves stages that promote aerobic conditions for nitrification and anoxic or anaerobic conditions for denitrification to achieve significant nitrogen removal.
Important Questions from Quality and Characteristics of Sewage
Conventionally, the biochemical oxygen demand (BOD) is measured for _______ days.
Identify the correct relation from the following:
Where BOD, COD, and TOD are Biochemical, Chemical, and Theoretical oxygen demand, respectively.
What is the correct range of \(\frac{{BO{D_u}}}{{COD}}\) for a waste water to be considered as fully biodegradable?
(where BODu = ultimate Bio-chemical oxygen demand and COD = chemical oxygen demand)
Determine ultimate BOD for sewage having 5-day BOD at 20°C as 180 ppm. Assume the de-oxygenation constant as 0.8 per day.
What does Chemical Oxygen Demand (COD) indicate?
