How To Assess The True Denitrification Capacity Of An Anoxic Tank

In wastewater treatment plant operation, excessive Total Nitrogen (TN) in the effluent is often one of the more difficult problems to troubleshoot. When an effluent-quality alarm is triggered, the first reaction is often to assume that the anoxic tank does not have enough volume. Operators may then respond by adding more external carbon or increasing the sludge concentration.

However, these measures can quickly increase operating costs and may even cause secondary problems such as poor sludge settling. This raises an important question:

In engineering practice, the Hydraulic Retention Time (HRT) shown on the design drawings is often used as the primary indicator of an anoxic tank's capacity. HRT is easy to calculate, but it describes only the hydraulic conditions of the tank. It does not directly tell us how much nitrate the biological system can actually remove.

The conventional assumption is that if wastewater remains in the anoxic tank for a sufficiently long period, denitrification will be completed. In reality, this assumption does not always hold. Denitrification is a biological process, and its rate is strongly affected by factors such as water temperature, sludge activity, MLVSS concentration, influent nitrate loading, carbon availability, and dissolved oxygen.

Therefore, when evaluating the actual capacity of an anoxic tank, HRT should not be considered in isolation. A more meaningful approach is to evaluate the specific denitrification rate and use it to verify the actual treatment capacity of the system.

The formula for hydraulic retention time is simple:

HRT = Tank Volume ÷ Flow Rate

For example, if an anoxic tank has a volume of 300 m³ and the flow rate is 100 m³/h, the theoretical HRT is 3 hours.

But in biological nitrogen removal, three hours does not always mean the same thing.

Denitrification is a microbially driven biochemical process, so its reaction rate changes with operating conditions. Water temperature is one of the most important factors. During winter, lower temperatures can significantly reduce microbial activity and denitrification performance.

At the same HRT of three hours, the denitrification rate at 25°C can be more than twice that at 12°C. This means that a tank volume that provides adequate nitrogen removal during summer may have only around half of the effective nitrogen removal capacity during winter.

Temperature, however, is only one factor.

Sludge concentration and influent loading are also critical. When the activated sludge concentration in an anoxic tank increases from 2,500 mg/L to 4,000 mg/L, the available biomass for denitrification increases significantly. The resulting nitrogen removal capacity therefore does not necessarily change in simple proportion to the increase in sludge concentration.

Similarly, when influent nitrate concentration increases from 20 mg/L to 35 mg/L, the amount of nitrate that must be removed within the same hydraulic retention time increases substantially.

This is why an HRT value by itself provides only limited information about the actual biological capacity of an anoxic tank.

If HRT alone cannot fully describe the capacity of an anoxic tank, what should engineers look at instead?

One important parameter is the specific denitrification rate.

The specific denitrification rate describes how much nitrate nitrogen can be removed by a given amount of active biomass within a certain period of time. Unlike HRT, which describes only hydraulic conditions, the specific denitrification rate reflects the actual biological performance of the sludge under specific operating conditions.

The denitrification rate can vary significantly depending on factors such as water temperature, sludge activity, MLVSS concentration, carbon availability, influent nitrate loading, dissolved oxygen, and the presence of inhibitory substances.

Therefore, there is no single denitrification rate that can be applied universally to every wastewater treatment plant.

For existing plants, site-specific testing is the preferred approach. Mixed liquor can be collected from the anoxic tank and tested under controlled conditions using actual wastewater and, where appropriate, an external carbon source. The change in nitrate concentration over time can then be used to evaluate the actual denitrification performance of the sludge.

When laboratory testing is not available, operating data can also be used to estimate the actual nitrogen removal capacity of the system. The key is to evaluate nitrate removal together with the amount of active biomass in the anoxic tank rather than relying on HRT alone.

This approach provides a more realistic basis for determining whether the existing anoxic tank truly lacks capacity or whether its available biological capacity is simply not being fully utilized.

The most reliable approach is to conduct a site-specific batch test.

Mixed liquor can be collected from the outlet of the anoxic tank and tested under controlled conditions using actual influent wastewater or an appropriate carbon source. The change in nitrate concentration over time can then be used to determine the denitrification rate.

This type of test provides a more realistic understanding of the biological activity of the sludge under actual plant conditions.

When laboratory testing is not available, a simplified back-calculation approach can also be used.

The basic concept is to calculate the amount of nitrate nitrogen removed per unit time and then divide it by the total MLVSS mass available in the anoxic tank.

In simplified form:

Specific Denitrification Rate = Nitrogen Removal Load ÷ Total MLVSS Mass

The nitrogen removal load can be estimated from the difference between the influent nitrate concentration and the target effluent nitrate concentration multiplied by the corresponding flow rate.

One important point should not be overlooked: the calculation should be based on MLVSS rather than MLSS whenever the specific denitrification rate is expressed on a volatile-solids basis.

MLSS contains both organic and inorganic solids. The inorganic fraction, or ash, does not directly participate in biological denitrification. MLVSS therefore provides a better representation of the active biological solids involved in the process.

In general, maintaining an MLVSS concentration of approximately 2,000–4,000 mg/L in the anoxic tank is considered appropriate, although the optimum value depends on the overall process configuration and operating conditions.

If calculations indicate that the existing anoxic tank cannot provide sufficient denitrification capacity, expanding the tank is not necessarily the first or only solution.

Several operational strategies can be considered before undertaking expensive civil engineering modifications.

1. Increase Effective Biomass (MLVSS)

Increasing the amount of active biomass is one of the most direct ways to increase denitrification capacity.

However, simply increasing sludge concentration is not always beneficial.

The additional biomass must be supported by the solids-handling capacity of the secondary clarifier. If the clarifier is already operating close to its solids flux limit, excessive MLSS may result in poor settling, sludge blanket rise, or sludge washout.

The mixing capability of the anoxic tank must also be considered. A higher solids concentration increases the demand for effective mixing. If the mixing system cannot maintain the sludge in suspension, increasing MLVSS may provide little practical benefit.

Therefore, the target MLVSS should be determined by considering both the biological requirements and the hydraulic and solids-handling capacity of the overall system.

2. Optimize External Carbon Addition

External carbon addition can be an effective way to improve denitrification when the readily biodegradable carbon available in the wastewater is insufficient.

However, more carbon does not necessarily mean better nitrogen removal.

Excessive carbon dosing increases operating costs and may result in residual organic matter in the treated effluent, creating a risk of secondary pollution.

Bench-scale testing is therefore recommended to determine how a particular carbon source affects the specific denitrification rate of the plant's sludge. Based on the test results, the theoretical carbon requirement can then be estimated.

Where appropriate, multi-point carbon dosing or feedback control can also be considered to improve dosing accuracy and respond to variations in influent nitrogen loading.

3. Reduce DO at the Outlet of the Aerobic Zone

This is an often-overlooked operational measure.

When dissolved oxygen (DO) at the end of the aerobic zone is excessively high, a significant amount of oxygen can be carried into the anoxic zone through internal nitrate recycle.

This can interfere with the anoxic environment because denitrifying microorganisms will preferentially use readily available dissolved oxygen before using nitrate as an electron acceptor.

For example, reducing the DO concentration at the aerobic-zone outlet from approximately 2.5 mg/L to 1.5 mg/L can, under suitable operating conditions, improve the effective utilization of the anoxic zone without requiring additional equipment or civil construction.

This is why DO should be considered not only as an aeration-control parameter but also as an important factor in biological nitrogen removal.

4. Optimize the Aeration Pattern

For plants with suitable process configurations, intermittent aeration at the end of the aerobic zone may provide another way to improve nitrogen removal.

During aeration periods, the zone performs its aerobic treatment function. During non-aeration periods, however, the same area can effectively function as a post-anoxic zone.

During these non-aeration periods, dissolved oxygen can be consumed by the microorganisms, creating more favorable conditions for denitrification. This can improve nitrate removal before the recycled mixed liquor returns to the anoxic zone.

In this sense, intermittent aeration can be viewed as a "trading time for space" strategy: instead of physically increasing the anoxic tank volume, operators use available tank volume and process time more efficiently to create additional denitrification capacity.