Solutions · Wastewater

Gas analysis for wastewater treatment plants

Q: Which gases are measured at a wastewater treatment plant?

At the aeration tank, operators measure nitrous oxide (N₂O), methane (CH₄) and CO₂ as emissions. In the digester gas between digester and CHP unit, the focus is on methane and hydrogen sulphide (H₂S), often with CO₂ and oxygen. In the CHP exhaust, formaldehyde matters. In sludge halls, methane monitoring serves explosion protection. In exhaust air, the key gases are H₂S and ammonia (NH₃). When process water is stripped, ammonia is measured.

Q: What is the difference between digester gas and sewage gas?

There is none: both terms mean the gas from anaerobic sludge digestion in the digester. It consists mainly of methane and carbon dioxide and contains traces such as hydrogen sulphide. It is measured mainly before the CHP unit uses it.

Q: How is nitrous oxide measured at a wastewater treatment plant?

A floating hood such as the Gas Dome 300 captures the off-gas from 1.00 m² of tank surface. The gas analyser measures the concentration. A flow measurement records the volume flow. Together, they give the emission rate E = c × Q / A in mg/(m²·h).

Q: Why is H₂S measured in digester gas?

Hydrogen sulphide is toxic and corrosive. In the CHP unit it attacks the engine and exhaust aftertreatment. The H₂S value after desulphurisation shows whether it is still working. This lets you replace an activated carbon filter, for example, only when it is actually needed.

Q: How is methane monitored in a sludge hall?

An NDIR device monitors methane continuously in a range up to 10 vol.%. This range covers the lower explosive limit of 4.4 vol.%. Unlike pellistors, NDIR is not poisoned by H₂S or siloxanes. It also needs no oxygen for its signal.

Q: How is gas measured when the sampling point is in an Ex zone?

Extractively: a sampling line carries the sample gas to the device. The device stands outside the zone. On the BioBasic, ATEX-certified flame arresters protect the plant. With the GA 210, the sampling line reaches up to 40 m.

Q: Do wastewater treatment plants have to account for nitrous oxide and methane?

The new Urban Wastewater Treatment Directive (EU) 2024/3019 requires treatment plants to account for their methane and nitrous oxide. EU member states transpose it by July 2027. Measured values replace default emission factors.

Q: How do the readings get into the control system?

The devices output readings via 4–20 mA analogue outputs and relays. Depending on the device, a fieldbus is added, such as Modbus TCP, Profibus DP, EtherNet/IP or Profinet. No cloud is required.

Q: Which solution fits my treatment plant?

That depends on the measuring point, the gases and the number of sampling points. Tory, our digital advisor, asks for the details needed. She then takes you to a quote request. Or you talk to our team directly.

From the aeration tank to the CHP unit: this page shows which gases you should measure at each point of your wastewater treatment plant. It also shows which process measurement technology from Fresenius Instruments does the job.

Explore measuring points Request advice

7measuring points from aeration to exhaust air
N₂O · CH₄ · H₂Sthe gases that matter most
2027Urban Wastewater Directive: account for N₂O and methane

Interactive plant map

Which gas you should measure where on your plant

Pick a measuring point or filter by substance. Below, you can see which measurement technology works there.

Measuring point 1

Aeration tank

N₂ONitrous oxide
CH₄Methane
CO₂Carbon dioxide

The Off-Gas System comes as Smart with the Smart Control 300 or as Expert with the GA 320. Both versions work with the Gas Dome 300 floating hood.

Off-Gas System More on this point

1.00 m² collection area per hood
The gas analyser calculates the emission rate E = c × Q / A in mg/(m²·h)
N₂O detection limit 10 ppmV (Smart) or 0.5 ppmV (Expert)
No blower, no zero or span gas

Measuring point 2

Digester gas

CH₄Methane
H₂SHydrogen sulphide
CO₂Carbon dioxide
O₂Oxygen

The BioBasic measures methane and H₂S, and optionally CO₂, O₂ and H₂. It measures at one or several points between digester, gas holder and CHP unit.

BioBasic More on this point

Measuring cycle per measuring point 22 min 20 s: interval from 15 min, plus pre-purge, 20 s measurement and post-purge
H₂S via a bypass three times a day per measuring point
Sampling line 40 m as standard
1 sample gas inlet, expandable to up to 5

Measuring point 3

CHP exhaust

CH₂OFormaldehyde

The GA 315 Hot Gas measures formaldehyde hot and wet, directly in the exhaust stream. It uses NDIR and needs no sample gas cooler. So there is no washout loss.

GA 315 Hot Gas More on this point

Measuring range 0–200 mg/m³ (0–160 ppm)
Detection limit 2 mg/m³ (1.6 ppm)
Measurement uncertainty < 5 % of reading
Hot gas measurement up to 300 °C, without gas cooler

Measuring point 4

Sludge hall

CH₄Methane

Here, the GA 210 in its LEL version does the job. This version is optimised for 0–10 vol.% methane. The device stands outside the Ex zone. The sampling line brings the sample gas to it.

GA 210 More on this point

Measuring range 0–10 vol.% CH₄
NDIR dual beam with reference detector, no cross-sensitivity to H₂S and CO₂
Measurement every second, analyser T₉₀ approx. 3 s
Sampling line up to 40 m, internal pump approx. 4 l/min

Measuring point 5

Exhaust air

H₂SHydrogen sulphide
NH₃Ammonia

The GA 210 suits one component, the GA 220 several. Depending on the gas, they use an NDIR or an electrochemical sensor. The right version depends on what you measure: room exhaust air or gas from covered tanks.

GA 220 More on this point

GA 210: NDIR, electrochemical or zirconium oxide
Up to 8 sample gas inlets per device
Continuous (1 s) or quasi-continuous (20 s) measurement
Profibus DP, Modbus TCP, EtherNet/IP, Profinet, 4–20 mA

Measuring point 6

NH₃ stripping

NH₃Ammonia

The GA 315 Hot Gas measures highly water-soluble substances directly in the hot, humid gas stream. It uses NDIR and needs no gas cooling. We design the ammonia version to suit your process.

GA 315 Hot Gas More on this point

Ammonia version on request
Fully heated sample path – no washout loss
The humidity in the gas is compensated
Heated sampling probe and sample line are part of the system

Measuring point 7

Spot check

CH₄Methane
CO₂Carbon dioxide
H₂SHydrogen sulphide
O₂Oxygen

The GAP 100 measures methane, CO₂, H₂S and oxygen in the field.

GAP 100 More on this point

CH₄ 0–100 vol.%, CO₂ 0–65 vol.%
H₂S 0–2,000 ppm, O₂ 0–25 vol.%
NDIR dual beam for CH₄ and CO₂, electrochemical for H₂S and O₂
Full specification after 5 minutes

Measuring points and solutions

Seven measuring points – and the right measurement technology for each

Gas Dome 300 on the aeration tank of a wastewater treatment plant

Measuring point 01

Aeration tank: nitrous oxide and methane as an emission rate

N₂ONitrous oxide
CH₄Methane
CO₂Carbon dioxide

Nitrous oxide (N₂O) forms in the aeration tank during biological nitrogen removal. Methane (CH₄) enters the tank with the influent and from anaerobic zones. Aeration strips it out. The off-gas is measured above the water surface. A floating hood covers a known area for this.

Solution: The Off-Gas System comes as Smart with the Smart Control 300 or as Expert with the GA 320. Both versions work with the Gas Dome 300 floating hood.

1.00 m² collection area per hood
The gas analyser calculates the emission rate E = c × Q / A in mg/(m²·h)
N₂O detection limit 10 ppmV (Smart) or 0.5 ppmV (Expert)
No blower, no zero or span gas
One or two measuring points per system

Digesters and gas holder of a wastewater treatment plant at dusk

Measuring point 02

Digester and gas holder: digester gas ahead of the CHP unit

CH₄Methane
H₂SHydrogen sulphide
CO₂Carbon dioxide
O₂Oxygen

Digester gas is also called sewage gas. It forms in the digester during anaerobic sludge digestion and consists mainly of methane and carbon dioxide. Two values matter for the CHP unit. The methane content determines the energy content. Hydrogen sulphide (H₂S) attacks the engine and gas lines. The H₂S value also shows whether desulphurisation is working.

Solution: The BioBasic measures methane and H₂S, and optionally CO₂, O₂ and H₂. It measures at one or several points between digester, gas holder and CHP unit.

Measuring cycle per measuring point 22 min 20 s: interval from 15 min, plus pre-purge, 20 s measurement and post-purge
H₂S via a bypass three times a day per measuring point
Sampling line 40 m as standard
1 sample gas inlet, expandable to up to 5
NDIR dual-pressure reference method for methane and CO₂ with automatic zero-point correction
ATEX-certified flame arresters; the device stands outside the Ex zone
4–20 mA, Modbus TCP, Profinet; wall housing IP 54

BioBasic

Measurement technology for biogas plants →

Measuring point 03

CHP exhaust: measuring formaldehyde

CH₂OFormaldehyde

The gas engine of the CHP unit forms formaldehyde (CH₂O) during combustion. Emission limits apply to the exhaust, in Germany under the 44th BImSchV. Formaldehyde is highly water-soluble. If you cool the sample gas, a large share is lost with the condensate. At a dew point of around 4 °C, the loss is typically 30–60 % or more.

Solution: The GA 315 Hot Gas measures formaldehyde hot and wet, directly in the exhaust stream. It uses NDIR and needs no sample gas cooler. So there is no washout loss.

Measuring range 0–200 mg/m³ (0–160 ppm)
Detection limit 2 mg/m³ (1.6 ppm)
Measurement uncertainty < 5 % of reading
Hot gas measurement up to 300 °C, without gas cooler

GA 315 Hot Gas

Hot-wet measurement without washout loss → Measuring formaldehyde (CH₂O) →

Sludge dewatering with centrifuges in the sludge hall of a treatment plant

Measuring point 04

Sludge hall and dewatering: methane below the LEL

CH₄Methane

Sewage sludge continuously releases methane. The lower explosive limit (LEL) of methane is 4.4 vol.%. Continuous methane monitoring is therefore part of explosion protection in sludge halls. NDIR measures methane selectively in the infrared. H₂S and siloxanes do not poison the NDIR measurement.

Solution: Here, the GA 210 in its LEL version does the job. This version is optimised for 0–10 vol.% methane. The device stands outside the Ex zone. The sampling line brings the sample gas to it.

Measuring range 0–10 vol.% CH₄
NDIR dual beam with reference detector, no cross-sensitivity to H₂S and CO₂
Measurement every second, analyser T₉₀ approx. 3 s
Sampling line up to 40 m, internal pump approx. 4 l/min
4–20 mA for the PLC

GA 210

Case study: methane monitoring in a sludge hall →

Biofilter and exhaust air ducts at a wastewater treatment plant

Measuring point 05

Exhaust air ahead of biofilter and scrubber: H₂S and NH₃

H₂SHydrogen sulphide
NH₃Ammonia

Air is extracted from the screen house, covered tanks and sludge treatment. This exhaust air goes to biofilters or scrubbers. Hydrogen sulphide and ammonia are the main causes of odour. Their concentration in the raw gas shows how heavily the exhaust air treatment is loaded.

Solution: The GA 210 suits one component, the GA 220 several. Depending on the gas, they use an NDIR or an electrochemical sensor. The right version depends on what you measure: room exhaust air or gas from covered tanks.

GA 210: NDIR, electrochemical or zirconium oxide
Up to 8 sample gas inlets per device
Continuous (1 s) or quasi-continuous (20 s) measurement
Profibus DP, Modbus TCP, EtherNet/IP, Profinet, 4–20 mA

Measuring hydrogen sulphide (H₂S) →

Stripping column for removing ammonia from process water

Measuring point 06

Process water: ammonia during stripping

NH₃Ammonia

The process water from sludge dewatering is rich in ammonium. When it is treated by stripping, the ammonia ends up in a hot, steam-saturated gas. A sample gas cooler would wash it out with the condensate. That is why the device measures hot and wet.

Solution: The GA 315 Hot Gas measures highly water-soluble substances directly in the hot, humid gas stream. It uses NDIR and needs no gas cooling. We design the ammonia version to suit your process.

Ammonia version on request
Fully heated sample path – no washout loss
The humidity in the gas is compensated
Heated sampling probe and sample line are part of the system

GA 315 Hot Gas

Measuring ammonia (NH₃) →

Measuring point 07

Spot checks: testing digester gas on the move

CH₄Methane
CO₂Carbon dioxide
H₂SHydrogen sulphide
O₂Oxygen

Not every point needs a fixed installation. A portable device measures the same gases. It is enough for spot checks on gas lines and vessels. You can also use it to cross-check permanent measuring points.

Solution: The GAP 100 measures methane, CO₂, H₂S and oxygen in the field.

CH₄ 0–100 vol.%, CO₂ 0–65 vol.%
H₂S 0–2,000 ppm, O₂ 0–25 vol.%
NDIR dual beam for CH₄ and CO₂, electrochemical for H₂S and O₂
Full specification after 5 minutes

GAP 100

Aerial view of a wastewater treatment plant with aeration tanks and digesters at dawn

Urban Wastewater Treatment Directive

Nitrous oxide and methane enter the inventory

By July 2027 the EU member states transpose the new Urban Wastewater Treatment Directive (EU) 2024/3019. It requires treatment plants to account for their methane and nitrous oxide. Default emission factors give you a calculated value. A hood on the tank gives you a measured value. According to the IPCC, nitrous oxide has around 273 times the climate impact of CO₂. For biogenic methane, the factor is around 27.

To the Off-Gas System

Aerated tank of a wastewater treatment plant in backlight

Case study

Stockholm Vatten measures instead of estimating

Stockholm Vatten serves 1.1 million people. Seven measuring systems with the GA 220 and the GA 320 run at the Henriksdal and Bromma treatment plants. They continuously transmit methane, nitrous oxide and CO₂ values to the SCADA system. The systems compensate for cross-sensitivities.

1.1 mpeople served
7measuring systems
CH₄ · N₂O · CO₂continuously to SCADA

Read the case study

Overview

Measuring point, gases and solution at a glance (extract)

Measuring point	Gases	Solution
Aeration tank	N₂O · CH₄ · CO₂	Off-Gas System, Gas Dome 300, Smart Control 300, GA 320
Digester gas	CH₄ · H₂S · CO₂ · O₂	BioBasic
CHP exhaust	CH₂O	GA 315 Hot Gas
Sludge hall	CH₄	GA 210
Exhaust air	H₂S · NH₃	GA 210, GA 220
NH₃ stripping	NH₃	GA 315 Hot Gas
Spot check	CH₄ · CO₂ · H₂S · O₂	GAP 100

As of 16.10.2026. The datasheets and brochures of each product are authoritative.

Products

Process measurement technology for the treatment plant

Complete system

From sampling to your control system

Measured extractively

The device draws in the sample gas and analyses it in a cabinet or technical room. There, the devices are protected. Out in the plant, you only need the sampling point.
Sample conditioning included

Condensate trap, filter and, where needed, a sample gas cooler are part of the system. This keeps humidity from distorting the measurement.
Values straight to your control system

Readings reach your control system via 4–20 mA, relays or fieldbus such as Modbus TCP and Profinet. Your data stays with you.
Worldwide service

Our service team handles maintenance and calibration. For maintenance, you can take the devices out of the cabinet and send them in.

To our service

Your benefits

What our gas analysis for wastewater treatment gives you

Safety for plant and team

Our process measurement technology keeps H₂S and methane on your plant in view. You act before it gets critical.
Precise at every measuring point

Nitrous oxide, methane, CO₂ and H₂S: we choose the right measuring principle for every measuring point.
Protection against corrosion

Measuring H₂S in the digester gas protects the CHP unit, pipes and catalyst from corrosion.
Lower operating costs

Durable devices with automatic zero-point correction save test gas and maintenance effort.
On-site and workshop service

Our service team maintains and calibrates your devices on site or in our workshop – internationally with service partners. You can also send devices in.

Advice

Tory – your advisor for the treatment plant

Which measuring point, which gases, how many sampling points? Tory asks for the details in a few steps. She then takes you to the right solution or straight to a meeting with our team.

The right solution for each measuring point
Off-Gas System, BioBasic or GA series
Direct path to a quote request

FAQ

Frequently asked questions on gas analysis at treatment plants

Which gases are measured at a wastewater treatment plant?

At the aeration tank, operators measure nitrous oxide (N₂O), methane (CH₄) and CO₂ as emissions. In the digester gas between digester and CHP unit, the focus is on methane and hydrogen sulphide (H₂S), often with CO₂ and oxygen. In the CHP exhaust, formaldehyde matters. In sludge halls, methane monitoring serves explosion protection. In exhaust air, the key gases are H₂S and ammonia (NH₃). When process water is stripped, ammonia is measured.

What is the difference between digester gas and sewage gas?

There is none: both terms mean the gas from anaerobic sludge digestion in the digester. It consists mainly of methane and carbon dioxide and contains traces such as hydrogen sulphide. It is measured mainly before the CHP unit uses it.

How is nitrous oxide measured at a wastewater treatment plant?

A floating hood such as the Gas Dome 300 captures the off-gas from 1.00 m² of tank surface. The gas analyser measures the concentration. A flow measurement records the volume flow. Together, they give the emission rate E = c × Q / A in mg/(m²·h).

Why is H₂S measured in digester gas?

Hydrogen sulphide is toxic and corrosive. In the CHP unit it attacks the engine and exhaust aftertreatment. The H₂S value after desulphurisation shows whether it is still working. This lets you replace an activated carbon filter, for example, only when it is actually needed.

How is methane monitored in a sludge hall?

An NDIR device monitors methane continuously in a range up to 10 vol.%. This range covers the lower explosive limit of 4.4 vol.%. Unlike pellistors, NDIR is not poisoned by H₂S or siloxanes. It also needs no oxygen for its signal.

How is gas measured when the sampling point is in an Ex zone?

Extractively: a sampling line carries the sample gas to the device. The device stands outside the zone. On the BioBasic, ATEX-certified flame arresters protect the plant. With the GA 210, the sampling line reaches up to 40 m.

Do wastewater treatment plants have to account for nitrous oxide and methane?