Ammonia and Nitrogen Removal in STP

Introduction: Why Clear Water Can Still Fail the Nitrogen Test

A sewage treatment plant can produce water that is clear, odourless and low in BOD, and still fail a sample because of nitrogen. Raw domestic sewage typically carries tens of milligrams per litre of nitrogen, mostly as ammonia and organic nitrogen from urine and food waste. A basic biological plant designed only for BOD removal does little about it. Some ammonia is taken up into new bacterial cells, but much of it passes through, or is converted to nitrate, which is no longer ammonia but still nitrogen.

Why does it matter? Ammonia is toxic to aquatic life, it consumes oxygen as it converts in a river or lake, and excess nitrogen feeds algae. Regulators have started to treat it seriously. The NGT order of April 2019 on STPs set limits that vary by category, and for the most stringent category it calls for BOD of about 10 mg/L, TSS of about 20 mg/L, COD of 50 mg/L and total nitrogen of 10 mg/L, with faecal coliform of 100 MPN per 100 mL (desirable). Many state boards write similar numbers into consent conditions for apartments, hospitals and hotels.

Removing nitrogen is a two step biological job. First, bacteria convert ammonia to nitrate in an oxygen-rich zone, a process called nitrification. Then, other bacteria convert nitrate to harmless nitrogen gas in an oxygen-free zone, a process called denitrification. A plant needs both, in the right proportions, with enough alkalinity, carbon and time.

This guide explains how the two steps work, what conditions they need and how to design for them, with a worked example for a 200 KLD plant. At Trity Enviro, we design MBBR, SBR and extended aeration plants for nitrogen limits, and the numbers below reflect common design practice. It is written for consultants, facility managers and plant operators.

The Nitrogen Problem and the Norms

Where STP Nitrogen Comes From

Nitrogen enters sewage mostly as total Kjeldahl nitrogen (TKN): organic nitrogen plus ammonia. In domestic sewage, TKN is often around 35 to 50 mg/L, higher in hostels, hospitals and hotels where water use per person is low and the sewage is stronger. Industrial effluent from food, pharma and fertiliser units can carry much more.

In the plant, organic nitrogen is converted to ammonia, a small part is taken up into biomass and removed with waste sludge, and the rest must be nitrified and denitrified, or removed by other means. Nitrate and nitrite from other sources add to the total.

What the Limits Look Like

Standards come at three levels. Schedule VI general standards set ammonia and nitrogen limits for some discharge routes. The NGT's STP norms add limits for BOD, COD, TSS, nitrogen and coliforms by category, with total nitrogen of about 10 mg/L for the stricter category and somewhat higher for others. Your SPCB consent sets the value that binds you, and it may be stricter or looser than the NGT table. Check the exact numbers in your consent before designing.

How Nitrification Works

Oxygen, Alkalinity and Sludge Age

Nitrification is carried out by slow growing bacteria that need plenty of oxygen and time. The reaction consumes about 4.57 kg of oxygen per kg of ammonia nitrogen oxidised, and it destroys about 7.14 kg of alkalinity (as CaCO₃) per kg of ammonia nitrogen. For a sewage with 33 mg/L of ammonia to be nitrified, that is about 235 mg/L of alkalinity gone, so a sewage with low alkalinity may need chemical dosing to keep pH above about 7.

The bacteria grow slowly, so the plant must keep them: sludge age (solids retention time) must be long enough that they are not washed out. Many extended aeration plants run a sludge age of 15 to 30 days, which is well above the minimum for nitrification at warm temperatures. At lower temperatures, the required sludge age rises sharply, as described in our guide on STP performance in winter.

What Slows or Stops It

• Low dissolved oxygen: Keep DO at about 2 mg/L or higher in the aerobic zone.
• Low pH or alkalinity: Nitrification slows rapidly below about pH 7 and almost stops below 6.5.
• Cold water: Rates fall with temperature, and nitrifiers are more sensitive than the bacteria that remove BOD.
• Short sludge age: Aggressive sludge wasting washes nitrifiers out.
• Toxic or inhibitory inputs: Disinfectants, some detergents, heavy metals and industrial chemicals inhibit them. Hospital and laboratory wastewater needs particular care.
• High organic loading: A heavily loaded tank favours fast growing BOD removers, which compete for oxygen and space.

How Denitrification Works

Anoxic Zones, Internal Recycle and Carbon

Denitrification happens when bacteria that normally breathe oxygen switch to breathing nitrate, in a zone with no free dissolved oxygen, called anoxic. They need a carbon source, which is the BOD in the incoming sewage. Each kg of nitrate nitrogen reduced returns about 2.86 kg of oxygen equivalent and about 3.57 kg of alkalinity, which recovers roughly half of what nitrification used. That is a bonus: good denitrification lowers aeration energy and stabilises pH.

For this to work, nitrate made in the aerobic tank must be brought back to the anoxic zone. A pump sends internal recycle of mixed liquor from the end of the aeration tank to the anoxic tank at a flow commonly two to four times the inflow. A rule of thumb is that a pre-anoxic zone needs about 4 to 5 kg of readily degradable BOD for each kg of nitrate nitrogen removed. When the sewage is weak, or when BOD has been consumed before the anoxic zone, an external carbon source such as molasses, methanol or acetate may be dosed.

Process Layouts: Extended Aeration, SBR and MBBR

• Extended aeration with a pre-anoxic tank: The common modification of the standard design. Sewage and return sludge enter an anoxic tank, which is mixed but not aerated, followed by the aeration tank and clarifier, with an internal recycle from the end of the aeration tank to the anoxic tank.
• SBR: Nitrogen removal is built into the cycle. Aerated periods nitrify, and unaerated mixed periods denitrify, in the same tank. The cycle timing and blowers can be programmed for it. See our sequencing batch reactor page.
• MBBR: Biofilm on media retains nitrifiers independent of sludge age, and an anoxic MBBR zone with media can denitrify. It is a good retrofit route for existing plants. See our MBBR sewage treatment plant and MBBR media pages, and our comparison of MBBR vs SBR vs MBR.

A Worked Design Example

Consider a 200 KLD STP with raw sewage TKN of 40 mg/L, BOD of 250 mg/L and alkalinity of 250 mg/L as CaCO₃. The target is effluent total nitrogen of 10 mg/L or less.

1. Nitrogen to be nitrified: TKN 40 mg/L, less about 5 mg/L taken up in sludge and about 2 mg/L left as ammonia and organic nitrogen, leaves about 33 mg/L. Daily load: 200 m³ × 33 g/m³ = 6.6 kg N/day.
2. Oxygen for nitrification: 4.57 × 6.6 = about 30 kg O₂/day. This is in addition to the BOD oxygen demand.
3. Alkalinity used: 7.14 × 6.6 = about 47 kg as CaCO₃ per day, or about 235 mg/L. Raw alkalinity of 250 mg/L would leave too little, so recovery from denitrification and, if needed, dosing is important.
4. Nitrate to be removed: To end with about 7 mg/L of nitrate, which with ammonia and organic nitrogen keeps total nitrogen near 10 mg/L, denitrify about 26 mg/L, or 5.2 kg NO₃-N per day.
5. Oxygen credit and alkalinity recovery: 2.86 × 5.2 = about 15 kg O₂/day returned, roughly half of the nitrification demand, and 3.57 × 5.2 = about 19 kg alkalinity recovered.
6. Internal recycle: The fraction of nitrate to be removed is 26 ÷ 33, about 79 percent. With a return sludge flow equal to the inflow, an internal recycle of about three times the inflow, roughly 600 m³/day, achieves this in a simple pre-anoxic design.
7. Carbon check: 5.2 kg N × about 4.5 = about 23 kg of BOD needed in the anoxic zone. Raw BOD of 250 mg/L gives 50 kg/day, so carbon is sufficient if the anoxic zone gets enough of the incoming load.

These are design estimates. Actual plants vary with temperature, loading and hydraulics, and the final sizing should follow a proper design calculation. Our guide to air blower selection shows how the extra oxygen demand feeds into blower sizing, and air diffuser selection covers the layout, while how to calculate STP capacity covers the hydraulic side.

Cost, Upgrade Options and Choosing a Partner

If your existing STP fails ammonia or nitrogen limits, the usual routes are, from least to most disruptive:

• Operating changes: Longer sludge age, better DO control, alkalinity dosing, equalization to smooth peaks
• Adding MBBR media to the aeration tank for a protected nitrifying biofilm
• Converting part of the tank to an anoxic zone with an internal recycle pump and mixer
• Adding a new anoxic tank, or a second reactor
• Tertiary polishing, such as ion exchange, breakpoint chlorination or biological filters, for tight limits on ammonia, as described on our tertiary treatment plant page

Cost depends on tank volumes, mixers, pumps, blowers and civil work, and a media retrofit is usually the least costly. Get a short monitoring programme first, with temperature, TKN, ammonia, nitrate, alkalinity, BOD and MLSS, so that the design fits the actual plant.

Questions to Ask Before You Order

• What data did you use for TKN, ammonia, alkalinity and temperature?
• What outlet ammonia and total nitrogen do you guarantee, and at what temperature?
• How do you size the anoxic zone, internal recycle and aeration?
• How much carbon source or alkalinity chemical will be needed?
• What is the effect on power consumption and operating cost?
• Can the work be phased, and how long will the plant be out of service?
• What operation and maintenance support do you provide?

Trity Enviro's sewage treatment plant designs and upgrades can be built for nitrogen limits, and an annual maintenance contract keeps probes, pumps and blowers working.

Industry Comparison / Operational Parameter Table

Why Choose Trity Enviro

Trity Environ Solutions is an ISO 9001:2015 certified and QCI-approved sewage treatment plant manufacturer and supplier in India, delivering robust, high-efficiency systems engineered for CPCB and SPCB regulatory compliance.

Every installation is supported by pan-India Annual Maintenance Contract and Operation and Maintenance services.