January 25, 2011 | General

Building Organics Diversion Around Treatment Plant Digester

completed digestersBioCycle January 2011, Vol. 52, No. 1, p. 31
The City of Saint-Hyacinthe, Quebec installed an anaerobic digester to process wastewater treatment plant solids. Planning is underway for an expansion to receive SSO and cheese whey.
Nora Goldstein

FACED with annual costs of $1.3 million to manage its biosolids, the City of Saint-Hyacinthe, Quebec decided to invest in anaerobic digestion technology and a biosolids drying unit. For years, the city had been transporting 13,500 metric tons/year of biosolids at 25 percent solids over 60 miles to the Town of Saint-Rosaire, where it was landfilled. Average cost for disposal was ± $100/metric ton. Installing anaerobic digesters and a thermal dryer had the potential to reduce the volume of solids generated significantly.
Another incentive to install anaerobic digesters was gaining the operating experience to enable expansion into digestion of source separated municipal organics from city households and businesses. In February 2010, a total of $650 million in financial assistance was made available from the province of Quebec and the federal government for projects to divert organics from landfill for biogas production and composting. Quebec had adopted a Policy on the Management of Residual Materials, which gradually bans the burial of organic matter by 2020 while contributing to attainment of the province’s goal of reducing greenhouse gas emissions. The Government of Canada’s contribution comes from a $1 billion, five-year, nationwide Green Infrastructure Fund, part of Canada’s Economic Action Plan announced in the January 2009 budget. This Fund supports sustainable energy generation and transmission, along with municipal wastewater and solid waste management infrastructure.
Saint-Hyacinthe wasn’t able to take advantage of that funding or installation of the biosolids digesters, as the monies were not yet available. However, Phase II of the city’s project involves installing two additional anaerobic digesters to process municipal organics. “The city hopes to move ahead in 2011 to add additional capacity,” explains Pierre Mathieu, Head of the Division of Water Treatment in the town of Saint-Hyacinthe. That project would be a first of its kind in the province to benefit from the Quebec’s Residuals Management program.

Of the options studied by the city to reduce its biosolids management costs, solids digestion and biogas production provided the best financial and environmental compromise, explains Mathieu. “The process easily became a part of the existing installations and could be operated by the on-site personnel,” he says. “And after analyzing different ways to reuse biogas, it appeared that thermal drying of the digested sludge offered the best financial profitability and made it possible to easily use the dried solids in agriculture. Via anaerobic digestion, the volume of the city’s sludge was reduced by 50 percent. With thermal drying, this decreases to 85 percent.”
The city purchased three LIPP KomBio Digesters from Bio-Methatech Inc., a Montreal-based company that represents the German technology. LIPP has numerous installations in Europe, including many farm digester projects. When multiple substrates are being digested, LIPP installs a hydrolizer ahead of the methanizer to thoroughly mix the feedstocks.
Construction of the digesters began in September 2009; start-up was four months later in January 2010. Each unit has capacity of 1,600 cubic meters; capital costs for the system were $4.5 million. “Payback is about five years,” says Michael Brown, Bio-Methatech’s Director of Business Development. A Fenton biosolids dryer was installed for final treatment of the material.
The digesters operate at mesophilic temperatures of 30°C to 37°C and are equipped with side-mounted mixers that create a rotating current in the substrate, enabling materials that float, such as fats and oils, to be recirculated. The drainage system is in a central cavity of the digester floor where the rotating current permits sediment to accumulate in this area; sediment is periodically extracted without interrupting the process. Retention time in the digesters is about 24 days.
digester construction
Perhaps the most unique aspect of the LIPP digester technology is the construction process. At the manufacturing facility, a thin layer of stainless steel is glued to a thick band of galvanized steel to produce Verinox, the digester “walls.” The stainless steel prevents corrosion of the inside wall of the digester; the galvanized steel provides the strength of the wall. As it is produced, Verinox is wound onto “spools,” which are transported to the digester construction site.
The cement base of the digester is built, with the necessary coils and drainage installed. The spool of Verinox is fed into LIPP’s patented Double Fold System, which “double-folds” the joints of the combined stainless and galvanized steel, creating a watertight seal. The unique feature of the LIPP folded system is the spiral shape, which gives the tank a high level of stability and makes the entire construction resistant to horizontal pressure. The fold has a smooth interior surface with no grooves or projections. The tank is built several feet off the ground so that it rotates (and creates the spiral) as the Verinox feeds into the Double-Fold machine. (The installation equipment remains stationary.) After several layers of Verinox have been installed, the roof is placed on the top layer, and then gradually rises as construction continues.
Once the tank is completed, the bottom is cut to make it level, and then it is lowered and secured onto the round cement bed. The bottom is sealed with high-strength industrial caulk. “The construction process is very rapid, independent of the digester size,” explains Brown. “It takes less than two weeks for the erection of the digester and less than two weeks for the insulation and cladding to be installed. The actual dimensions of the tank are determined by the client’s types and volume of the substrates to be digested.”
biosolids dryer
The heating coils are wrapped around the exterior as the spiral tank is being erected. To protect the heating system, the glycol-filled coils are located between the inner and outer walls of the digester. In the case of the Saint-Hyacinthe installation, excess heat from the biosolids dryer is used to heat the glycol in the coils. A low-pressure reservoir with 600 m3 capacity, located at the top of each digester and protected by a solid external structure, stores the biogas at ambient pressure until it is used.
“We initially explored using the biogas from the digesters to produce electricity, as that is where our most significant expenses are at the plant,” says Mathieu. “But this method of biogas utilization is not possible because our utility, Hydro-Quebec, doesn’t allow any hook-ups to its network. That’s why thermal drying of the biosolids quickly became an interesting option. The methanizer can function using the recovered heat of the dryer, which makes all the biogas available to dry the sludge after digestion.”
Phase II of the Saint-Hyacinthe project is to add additional anaerobic digestion capacity to process 15,000 tons/year of source separated organics. This waste source will be combined with a large volume of cheese whey from an adjacent cheese producer. If the same technology is utilized, a hydrolizer will be part of the system, as multiple substrates will be digested. The hydrolizer is constructed using the same materials and double-fold system. Retention time in that phase is about five days.
The plan for the additional biogas, estimated to be 5 million cubic meters/year, is to upgrade and inject it into the local area gas network. The city will use a portion for its buildings and vehicles, with a majority of the remaining gas available to the provincial utility company’s residential and commercial consumers. “GazMétro, the provincial natural gas provider, has opened up its grid to support the production of biogas in Quebec,” says Brown. “About 4 million cubic meters will be available to them.”

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