Four years later, the Annecy WWTP provides the equivalentof 1,182 housholds with biogas per year

Interview with William Perrier, Director of the operation of sanitation facilities at the Syndicat Mixte du Lac d’Annecy (SILA) on the results of the injection into the GRDF network (French gas distribution network) of biomethane produced by the Annecy WWTP.

On 10 January 2017, carrying out pioneering work, the SILA commissioned its biogas recovery unit from the anaerobic digestion of sludge at its main wastewater depollution plant (SILOE), so taking advantage of an energy source hardly exploited so far. Thanks to this process, sludge and fats are methanised, i.e. digested by bacteria to produce biomethane. The objective is to obtain a significant reduction in the quantity of sludge to be treated in the order of 35% to 40%, while producing renewable energy. In 2019, SILA produced in biomethane the gas equivalent used by 1,182 households (average GRDF client consumption of 12Mwh/year).

William Perrier, after its commissioning in 2017, the sludge anaerobic digestion project has evolved. Can you tell us how?

WP: Taking advantage of new regulations enacted at national level, we started by carrying out, in collaboration with BG, a study to determine whether it was better to produce green electricity by cogeneration or inject green gas directly into the GRDF network. The study showed for the SILA project that it was much more environmentally and economically attractive to inject biomethane than to produce electricity, because the energy efficiency is much better.

What was the goal of this project?

WP: The SILA elected officials wanted to recover 100% of the biomethane produced by the by-products of the territory’s sanitation to be part of the energy transition policy of France and Greater ANNECY, which aims for a target of 32% renewable energy by 2030 and 60% by 2050. To optimise the injection of biomethane, the heating of the digesters is mainly provided by heat pumps and sludge/sludge heat exchangers. The implementation of anaerobic digestion of sludge also made it possible to reduce the plant’s carbon footprint.

In 2020, with two years of hindsight, would you say that the results are convincing?

WP: They are more than convincing. The average injection rate is already 150 normal cubic metres per hour (Nm3/h), the installation is very reliable and we will eventually be able to produce 180. In 2019, 95.3% of the biogas was YEARtransformed into biomethane, 2% was used for process heating and 2.7% of biogas remained, which we had to burn during maintenance periods.

This represents high performance, almost optimal for very complex industrial equipment. Best of all, the plant produces more energy than it consumes, with a positive balance of 2,460 Mwh in 2019.

To put it concretely, in 2019, the installation generated the production of 14,184 MWh, which was sold to ENGIE; that corresponds to the gas equivalent used by 1,182 households over a year, or to the annual consumption of 73 buses that would run on natural gas. And, from an economic point of view, the resale of biomethane generated 1.5 million euros in revenue per year for the sanitation budget.

What was the mission of BG within the framework of the project?

WP: BG participated in this project as general project manager and was a true partner from the inception of the project to its completion. It carried out all the sizing and safety studies and supervised all the work.It was a permanent collaboration, with a partner attentive to the objectives of the owner. BG has allowed us to find optimisation solutions, especially with regard to the installation of sludge/sludge heat exchangers and heat pumps.

SILOE was the first wastewater treatment plant in France to operate a biomethane production unit under government control, meaning that the injection was not entrusted to a private company, which remains a source of pride for elected officials and SILA agents.

PICTURE: Guerrin
(Article taken from BG Magazine 2021, updated version on the site)