In 2008, 31% of Québec’s municipal wastewater sludge ended up in landfills. By 2020, Quebec wants to divert all residual materials from landfills in order to recycle, produce energy or dispose of the waste, in that order.

To reach this goal, Québec recently adopted a series of policies and programmes as part of the government’s sustainable development strategy. The Green Infrastructure Fund of Québec earmarked Cnd$650 million for municipalities to divert organic materials away from landfills. This new approach promotes the emergence of a green economy based on the environment and quality of life for its citizens through a zero-waste policy.

The City of Saint-Hyacinthe received some of this ‘green money’ to expand its existing wastewater treatment plant (WWTP) and produce pipeline quality biomethane. Saint-Hyacinthe currently has three digesters (1 600 m³ ea.) with a capacity of 36 tonnes of solids per day (13 000 dried tonnes/year).

73 000 tonnes per year of sewage sludge are pumped towards the digestion tanks from the primary and secondary sedimentation tanks which are heated from 10°C to 37°C. With the new funding, the city now can start Phase II of the upgrade: construction of two hydrolysers (1 200 m³ each) and three more digesters (1 800 m³ each) with Montreal based renewable energy company Bio-Methatech.

Currently, the city uses the WWTP’s biogas to dry the sludge that comes out of the digesters. This reduces the overall waste by 85% and results in a clean, light, granular, Class A biosolid that is safe and beneficial for agricultural, horticultural, municipal and domestic use.

“Before the treatment facility was in place, the city hauled 13 500 tonnes of treated sewage every year to a landfill where tipping fees averaged an expensive $90 a tonne. Plus, the 100 km round trip burned a lot of fuel,” said Pierre Mathieu, Head of the Division of Waste Water Treatment in the town of Saint-Hyacinthe.

In Phase II, instead of just using the digesters, the WWTP will first route the biosolids through the hydrolyzers to break down the cell structure and organic material. Operators will also add cheese whey and other organic waste to boost the production of biogas from the anaerobic digestion process.

To further boost biogas production and make the flow of gas more consistent, the WWTP chose a sidewall entry mixer to properly mix each digester and maintain a good distribution of temperature, pH levels and micro-organisms. A single SUMA Giantmix FR4 mixer will generate a circular flow in the tank to promote maximum digestion and a flow pattern which allows heavier solids to settle to the bottom of the digester. At the bottom, the digester tank has a special drain where heavy solids are removed without shutting down biogas production.

The mixer also has two unique features which allow a visual assessment of the agitators’ condition and will cut fibrous components that tend to adhere to propeller blade edges. For a visual assessment, operators can use the external oil inspection glass to detect seal breaks, oil contamination and shaft bending in the equipment. A cutting blade located behind the propeller is especially effective in cutting away troublesome fibrous materials.

“When you add organics to wastewater you boost your biogas production but create new, unknown variables,” said Christie Allen, international business development manager at SUMA GmbH. “Plant type, length and size of fibre will change the viscosity based on which organics are added each season. It’s important to monitor the equipment and monitor the dry matter content of the substrate. As maintenance personnel learn the impact of these materials on the digester, they can simply adjust the agitator speed of the mixer to optimize biogas production.”

In the next year, after Phase II, surplus biogas will be upgraded to biomethane and injected into the natural gas distribution grid operated by Gaz Métro. Gaz Métro anticipates selling 4-5 million m³ of biomethane each year (460-570 m³/hour) for the city’s buildings, natural gas vehicles and the gas network.

Currently, the Saint-Hyacinth digesters produce 150 m³/hour of biogas which is equivalent to 3 MBtu/hour of energy and 350 kWh electricity. Adding organic waste with the additional capacity, Bio-Methatech expects the WWTP to produce 1 200 m³/hour of biogas, 26 MBtu/hour of energy and 3 000 kWh electricity 8.5 times more energy using co-substrates then with only the municipal sludge.

Source: waterworld.com