Nitrification in a Sewage Treatment Plant

Nitrification is a crucial biological process in wastewater treatment that converts ammonia nitrogen, a significant pollutant, into less harmful forms. This process predominantly occurs in the Aerobic Chamber of a wastewater treatment plant, such as an aeration tank in an activated sludge system.

How Nitrification Happens in an Aerobic Chamber

Nitrification is a two-step biological oxidation process performed by specific groups of autotrophic bacteria (meaning they get their energy from inorganic compounds and use CO2 as their carbon source). These bacteria are called nitrifiers.

The process unfolds as follows:

Step 1: Ammonia Oxidation (Nitration)

Bacteria Involved: Primarily Ammonia-Oxidizing Bacteria (AOB), mainly from the genus Nitrosomonas (e.g., Nitrosomonas europaea).

Reaction: These bacteria oxidize ammonia (NH3) or ammonium ions (NH4+) to nitrite (NO2−).

Equation: NH4++1.5O2→NO2−+2H++H2O+Energy

Key Requirement: This step requires oxygen (aerobic conditions). It also produces hydrogen ions (H+), which consume alkalinity and can lower the pH.

Step 2: Nitrite Oxidation (Nitration)

Bacteria Involved: Primarily Nitrite-Oxidizing Bacteria (NOB), mainly from the genus Nitrobacter (e.g., Nitrobacter winogradskyi).

Reaction: These bacteria oxidize the newly formed nitrite (NO2−) to nitrate (NO3−).

Equation: NO2−+0.5O2→NO3−+Energy

Key Requirement: This step also requires oxygen (aerobic conditions).

Overall Nitrification Reaction: Combining both steps, the overall reaction is: NH4++2O2→NO3−+2H++H2O+Energy

This shows that for every 1 mg of ammonia-nitrogen (NH4+−N) oxidized, approximately 4.57 mg of oxygen is theoretically consumed, and 7.14 mg of alkalinity (as CaCO3) is consumed. These are critical considerations for aeration system design and alkalinity management in the aerobic chamber.

Reasons that Affect Nitrification in an Aerobic Chamber

Nitrification is a sensitive process, and its efficiency can be significantly impacted by various environmental and operational factors to ensure optimal nitrogen removal:

1. Dissolved Oxygen (DO) Concentration:

Requirement: Nitrifying bacteria are obligate aerobes, meaning they absolutely need oxygen to survive and perform their metabolic functions.

Effect: A DO concentration of at least 1.5 - 2.0 mg/L is generally recommended for effective nitrification. Below this, the nitrification rate will decrease significantly, and below 0.5 mg/L, it can be completely inhibited. Insufficient aeration is a common cause of poor nitrification.

2. Temperature:

Effect: Nitrifying bacteria are mesophilic, meaning they thrive in moderate temperatures.
- Optimal Range: Generally, 20°C to 30°C (or 25°C to 35°C, depending on the specific strain).
- Lower Temperatures: Below 15°C, nitrification rates decrease significantly. Below 10°C, they become very slow, and below 5°C, they are practically inhibited. This is a significant challenge in colder.
- Lower Temperatures: Below 15°C, nitrification rates decrease significantly. Below 10°C, they become very slow, and below 5°C, they are practically inhibited. This is a significant challenge in colder.

3. pH and Alkalinity:

Optimal pH Range: Nitrifying bacteria are highly sensitive to pH, with an optimal range typically between 7.0 and 8.5.

Alkalinity Consumption: As shown in the equations, nitrification consumes alkalinity (HCO3−). If the wastewater has insufficient buffering capacity (low alkalinity), the pH can drop significantly during nitrification, inhibiting the nitrifiers.

Consequence: A pH below 6.5 or above 9.0 can severely inhibit.

Action: In some cases, alkalinity supplementation (e.g., adding sodium bicarbonate or lime) may be necessary to maintain in the optimal pH.

4. Sludge Age (Mean Cell Residence Time - MCRT / SRT)

Effect: Nitrifying bacteria have a much slower growth rate than heterotrophic bacteria (which remove BOD/COD).

Requirement: To ensure that nitrifiers are retained in the system long enough to grow and reproduce, the sludge age (SRT) of the activated sludge system must be sufficiently long. Typical SRTs for nitrification range from 5 to 20 days, depending on temperature and other factors. If the SRT is too short, nitrifiers will be washed out faster than they can grow, leading to poor nitrification.

5. Presence of Inhibitory / Toxic Substances:

Effect: Nitrifying bacteria are more sensitive to toxic compounds than heterotrophic bacteria.

Examples: Heavy metals (e.g., copper, chromium, mercury), certain organic compounds (e.g., phenolics, cyanide), high concentrations of free ammonia (NH3) or free nitrous acid (HNO2), and some industrial chemicals can inhibit or kill nitrifiers.

Action: Careful monitoring of industrial discharge and potentially pre-treatment of toxic wastes are essential.

6. BOD/COD Load (C:N Ratio):

Competition: High concentrations of readily biodegradable organic matter (BOD/COD) can lead to competition for oxygen between fast-growing heterotrophic bacteria (which remove BOD/COD) and slow-growing nitrifying bacteria. If the C:N ratio is too high, heterotrophs will consume most of the available oxygen and often outcompete nitrifiers for space and nutrients.

Effect: While some BOD/COD is necessary for overall biomass growth, an excessively high organic load can suppress nitrification.

7. Ammonia Concentration:

Substrate Requirement: There must be sufficient ammonia (NH4+) in the influent for nitrification to occur.

Inhibition at High Concentrations: Paradoxically, very high concentrations of free ammonia (NH3) (which forms from NH4+ at higher pH and temperature) can be inhibitory, especially to Nitrobacter (the nitrite oxidizers). This can lead to nitrite accumulation in the effluent.

8. Mixing:

Effect: Adequate mixing in the aerobic chamber is necessary to ensure that the nitrifying bacteria are constantly in contact with the ammonia and oxygen. Poor Mixing can lead to localized anoxic zones or substrate limitations.

Comparison: Extended Aeration vs. SBR for Nitrification & Denitrification

Feature

Nitrification

Denitrification

Carbon Utilization for Denitrification

Operational Flexibility

Space Requirements

Complexity

Sludge Settling

Extended Aeration (Standard, without major modifications)

Highly effective due to long SRT and continuous aeration.

Limited or incidental in a basic design. Primarily occurs unreliably in a clarifier or as a minor SND. Requires significant modification (e.g., separate anoxic tank with recycle) for effective denitrification.

Poor, as most carbon is consumed aerobically before anoxic conditions might be achieved. Often requires external carbon.

Less flexible. Requires flow equalization if the influent is highly variable. Fixed tanks for each process.

Generally requires more space due to separate tanks (aeration, clarifier, and potentially anoxic).

Simpler control for basic BOD/TSS removal. More complex with added BNR zones.

Relies on a separate clarifier. Susceptible to clarifier bulking or upset.

Sequencing Batch Reactor (SBR)

Highly effective during the aerobic "React" phase due to long SRT and controlled aeration.

Highly effective and easily integrated into the cycle. Alternating aerobic/anoxic phases are inherent. Efficient use of influent carbon for denitrification.

Excellent, as raw influent carbon can be utilized in a dedicated anoxic phase (e.g., Mix-Fill).

Highly flexible. Single tank for all processes. Cycle times can be adjusted for varying loads or effluent requirements (e.g., longer anoxic for more N removal).

Typically requires less footprint as all processes occur in one.

Easy with controls and programming for managing different phases within the cycle.

Excellent, as quiescent settling occurs in the same tank. Strong hydraulic selection for good-settling sludge.