For wastewater treatment plants, producing more biogas creates an opportunity to recover more value from existing waste streams. But before that gas can be used for energy generation, contaminants such as hydrogen sulphide (H₂S) need to be managed.
At the St. Cloud Nutrient, Energy and Water (NEW) Recovery Facility in Minnesota, biological desulphurisation forms part of a wider strategy to turn wastewater and organic residuals into renewable energy.
The facility serves more than 122,000 customers across six cities and has progressed from conventional wastewater treatment towards resource recovery. Today, biogas production, gas cleaning, combined heat and power (CHP) and solar generation all contribute to an energy system that helped St. Cloud achieve net-zero energy in 2021.
More biogas through co-digestion
Anaerobic digestion has long been part of wastewater treatment at St. Cloud. Municipal solids are processed in two primary mesophilic digesters, where they are converted into biogas and nutrient-rich biosolids.
When processing municipal solids alone, the digesters produced approximately 150,000–175,000 ft³ of biogas per day.
In 2014, the facility began testing the co-digestion of high-strength liquid waste from food sources alongside municipal solids. Today, approximately 26,000 gallons per day of high-strength liquids can be co-digested at the facility.
The result has been a substantial increase in biogas production, to approximately 300,000–400,000 ft³ per day.
That additional gas gives the facility a larger renewable energy resource. It also makes reliable biogas treatment increasingly important.
Why H₂S removal matters
Raw biogas can contain hydrogen sulphide, moisture and siloxanes that need to be removed before the gas is used in downstream equipment.
At St. Cloud, the biogas is cleaned before being supplied to the facility’s CHP system. Hydrogen sulphide removal is particularly important because H₂S can contribute to corrosion and damage in gas engines and other downstream equipment.
For wastewater treatment plants considering how to remove H₂S from biogas, the treatment technology therefore has to be considered as part of the complete energy system.
In 2020, St. Cloud selected and installed Biogasclean’s DS-BTR biological desulphurisation technology. The system was chosen for reliable H₂S removal and its ability to self-adapt to changing operating conditions.
Biological desulphurisation uses naturally occurring sulphur-oxidising bacteria to remove H₂S from the gas. Biogas and air enter the reactor, while recirculated scrubbing liquid passes through structured packing material that provides the conditions required for the biological process.
As the biogas passes through the packing, hydrogen sulphide is biologically oxidised and removed from the gas.
For St. Cloud, the operational experience has been positive:
“Our experience has been nothing but positive with biological desulfurization. It’s safer and easier to use than alternatives as it self-adapts and does not require hazardous chemicals. We highly recommend the biological desulfurization solution from Biogasclean and we know that we would not have achieved net zero without it.”
St Cloud Plant Manager
From treated biogas to electricity and heat
Gas treatment is not the end goal at St. Cloud. Its purpose is to make the biogas suitable for productive use.
The facility operates two 633 kW engine-driven CHP units with a combined capacity of 1.26 MW. The CHP units convert biogas into renewable electricity while also recovering thermal energy.
Each unit can provide up to 2.2 MBtu/h of heat in the form of hot water. That heat is fed into the facility’s central heating system, where it supports process heating as well as building demand.
For approximately six to eight months of the year, the CHP units can provide the facility’s full heat requirement. Natural gas purchases for the existing boilers have been reduced by 42%.
The CHP system operates alongside solar generation. St. Cloud has four solar arrays totalling 545 kW, creating a combination of renewable energy sources that reduces the facility’s dependence on grid electricity.
Once the facility achieved net-zero energy, grid power effectively became a backup to the renewable electricity generated onsite.
Biological desulphurisation as part of resource recovery
St. Cloud’s experience shows why H₂S removal should not be viewed as an isolated gas-cleaning process.
The facility first increased the amount of biogas available through co-digestion. That gas then needs to be treated before it can be used reliably for energy generation. CHP converts the treated gas into electricity and useful heat.
Each stage supports the next.
The wider resource recovery model also creates value beyond energy generation. High-strength liquid waste is delivered to the facility by nearby commercial businesses, creating tipping-fee revenue while providing additional feedstock for anaerobic digestion.
St. Cloud continues to explore other forms of resource recovery, including renewable natural gas, nutrient recovery, green hydrogen and green oxygen.
The facility’s progress reflects a broader change in how wastewater infrastructure can be used: not only to treat waste, but to recover energy and other useful resources from it.
Making more of the biogas already being produced
For wastewater treatment plants with anaerobic digestion, increasing biogas production is only valuable if that gas can be used effectively.
St. Cloud provides a practical example of how biological desulphurisation can support that objective. Co-digestion produces more biogas. H₂S removal helps prepare that gas for downstream use. CHP then converts it into electricity and useful heat.
The result is a connected system in which gas treatment supports a much larger objective: recovering more useful energy from the resources already entering the wastewater treatment plant.
