Local authorities
To remember
  • Reduce sludge amounts
  • Produces renewable energy, biogas
  • The essential tool on wastewater treatment plants for achieving energy neutrality

100,000
The sewage sludge from a city of 100,000 inhabitants can produce around 5 GWh of biomethane per year, which can power 20 buses or waste collection vehicles, or supply 900 homes with biofuels for one year

1.7
million m³
of anaerobic digestion plants have been built worldwide

> 80
biogas production references worldwide

Anaerobic digestion: reducing sewage sludge and producing recoverable biogas

It is an essential step in making wastewater treatment plants energy neutral. Drawing on our over 45 years of expertise in anaerobic digestion, we can help you reduce sludge quantities and maximise biogas production.

We offer a range of technological processes to meet your needs and adapt to the characteristics of the sludge and the technical and environmental constraints of your sites.

We are constantly innovating to improve the technical and economic performance of our processes, and to maximise the energy potential of sewage sludge.

Our processes

It is a conventional mesophilic anaerobic digestion process that operates at 37°C. 

This compact solution enables methanisation with around 40% less digestion volume and is ideal for sites with limited space. It is a conventional thermophilic anaerobic digestion solution that operates at 55°C.

This process is particularly suitable for low-methanogenic sludge, such as biological sludge resulting from prolonged aeration. Compared to the conventional mesophilic process, it increases biogas production and reduces outgoing sludge quantities. It also enables the production of a sanitised digestate. It is an innovative anaerobic digestion solution with thermal hydrolysis at temperatures above 150°C.

Digelis BH is a compact solution that enables anaerobic digestion in two structures in series with approximately 40% less digestion volume. Compared to the conventional mesophilic process, it increases biogas production. There is also a variant that operates with a first stage at 55°C, which is fed in batches to produce a sanitised digestate. It is an innovative anaerobic digestion solution with biological hydrolysis at 37°C or 55°C.

We have also developed the advanced Dehydris Ultra dewatering workshop with hydrothermal carbonisation. When positioned downstream of anaerobic digestion, it increases biogas production by 20-30% and produces biocoal for use in agriculture or as a biofuel.

Discover also our wastewater treatment offer

Our construction solution

Conventional sewage treatment and anaerobic digestion plants are made of concrete. However, we have expanded our range to include the innovative, low-carbon Digelis Simplex: a metal-design digester developed in partnership with the German company LIPP. To date, we have installed more than 35 Digelis Simplex units at wastewater treatment plants.

Digelis Simplex – Stainless steel digester for sewage sludge and organic waste | SUEZ

Credit: SUEZ group

What happens to the outgoing digestate?

Digestate can be used in agriculture as an alternative to chemical fertilisers, depending on the local regulation. Thanks to anaerobic digestion, which stabilises the organic matter, spreading operations cause less odour nuisance. In the case of sludge digestate, it can be used in agriculture either directly or after composting. However, if they are incompatible with agricultural recovery or there are insufficient outlets, they can be directed to thermal recovery channels, such as pyrolysis, sludge incineration furnaces, EfW and cement plants.

Recovery of nitrogen and phosphorus from dewatered digestate

Liquids produced after the dewatering step of anaerobic digestion contain high levels of nitrogen and phosphorus. We use recovery systems to make treatment simpler and produce a bio-based fertiliser that can replace chemical or mineral fertilisers.
Discover also our wastewater treatment offer
Diagram anaerobic digestion process and utilisation of biogas and biomethane

Biogas: a renewable and sustainable energy source produced through anaerobic digestion

Biogas can be recovered as heat and/or electricity, or as biomethane. We can support you in implementing the most suitable biogas recovery solution for your needs, whether for self-consumption or for use outside the site.
Methanisation principle

Heat and/or electricity production

We can upgrade on-site pre-treated biogas for use in a gas boiler to produce heat, or in a cogeneration unit to produce heat and electricity simultaneously. This is an established approach for improving the energy balance of remote sites, particularly those not connected to the gas grid.

Biomethane production

Since 2014, we have supported you in recovering biogas produced and pre-treated at your plants and converting it into biomethane. This green gas has a neutral CO₂ balance, is compatible with injection into city gas networks, and can be used as a biofuel (BioCNG) for developing green mobility, improving air quality, and reducing emissions.

We have developed a range of Methanis solutions, including the extensive purification of pre-treated biogas to produce biomethane, which can then be processed to meet end-use requirements.

A solution adapted to territories with a city gas network into which the produced biomethane is injected.

A solution for sites that are too far from city gas networks to produce a biofuel for urban transport or light vehicles on site.

BioCO₂: a complementary resource with clear value for recovery and use

Biogas consists of approximately 65% methane and 35% carbon dioxide (CO₂). When biogas is cleaned to produce biomethane, the methane is separated from the CO₂. BioCO₂ is largely unrecovered and represents a potential resource that can be captured and utilised locally.

Carbon dioxide (CO₂), a well-known greenhouse gas, is also an industrial product with multiple applications.


The CO₂ currently used on the market is mainly fossil CO₂ from steam reforming of fossil gas to produce hydrogen for ammonia production. BioCO₂, which is produced by anaerobic digestion, is a renewable alternative to fossil CO₂ and enables users to reduce their carbon footprint.


We can support you in your projects for the material recovery of bioCO₂ produced at your facilities, helping you to contribute to the circular economy in your area.

 

For ease of use and transportation, it is pre-treated and liquefied.


It can also be used to produce energy, for example, methanation combines it with hydrogen to produce synthetic methane.

 

They trust us

The "Biofactory" of Pau-Lescar: a resource factory

We designed, built and operate the 'Biofactory' in Pau-Lescar, a pioneering facility that produces 10 green resources and energies. It is the first wastewater treatment plant to be equipped with our Dehydris Ultra hydrothermal carbonisation process, which reduces sludge quantities by 4x, increases biogas production, and creates a new resource: biocoal, which can be used in combustion or applied to land as a soil amendment . It is also the first wastewater treatment plant to recover 100% of the CO₂ from sludge methanisation. This new facility will prevent the emission of 5,000 tonnes of CO₂ per year and contribute to the Agglomeration Community's strategy of achieving carbon neutrality by 2040.
Learn more about Pau Lescar biofactory

Our other collaborations

To recover the effluents of Alsatian sauerkraut farms
In Panama, we produce biogas to make the principles of the circular economy a reality

Frequently asked questions

Anaerobic digestion is a popular process that can be controversial.


While the anaerobic digestion of sludge at wastewater treatment plants has not been opposed in the past 45 years, the widespread development of anaerobic digestion in agriculture and industry today generates concerns and opposition from certain stakeholders, such as local residents.


The following issues are of particular concern: odour nuisances, methane leaks and the spreading of digestate. Regarding odour nuisances, these can be linked to the supply of external inputs, i.e. delivery and unloading operations carried out in open environments (e.g. unloading bulk into tanks outdoors).


However, it is possible to design factories with closed, ventilated and deodorised enclosures.

 

As industrial professionals in anaerobic digestion, we pay particular attention to odour nuisances when designing our facilities.

 

Methane leaks are a legitimate cause for concern due to the associated risks and greenhouse gas emissions (methane has a warming potential 30 times greater than carbon dioxide). As mentioned earlier, our anaerobic digestion process is well-established and under control, and industrial safety is one of our key priorities. A leak check is carried out on our industrial facilities by a third-party organisation before commissioning, and regular checks are carried out during the operational phase.


Like any fertilising material, the spreading of digestate is highly regulated (quantities added to the soil, soil type, crop type, distance from watercourses, etc.) and strict controls are carried out. Digestate, whether composted or not, is not spread on the soil if it does not comply with the spreading regulations or BSI PAS110. Land application of digestate is subject to regulatory controls, including compliance with environmental permitting or end-of-waste requirements, nutrient management planning, and consideration of soil, groundwater and human health protection criteria.

 

At SUEZ, industrial safety is one of our key priorities. Gas risks, such as explosions and fires, are an integral part of industrial risks, and we have put procedures in place at both the design and operational levels. These include the choice of materials, leak tests, measuring instruments, accessibility for maintenance, staff training, intervention procedures and ATEX zoning.