Slave Lake Pulp cuts electrical, sludge, chemical use with ADI-BVF reactor

Slave Lake Pulp, a subsidiary of West Fraser Mills Ltd., is a 240,000-tonne/y BCTMP mill in northern Alberta, Canada; it primarily processes aspen to produce market pulp for the global market.

The mill’s high-strength wastewater stream was being treated in an energy-intensive, conventional activated sludge (CAS) system. The mill frequently alternates pulp brightness grades resulting in fluctuations in the wastewater’s chemical oxygen demand (COD) concentrations from 5,000 to 19,000 mg/L. This meant that process stability and the ability to handle variations in hydraulic and organic loads were important factors when selecting an anaerobic digestion system.

The company decided to explore biomethane for power generation, for both its environmental and cost-saving benefits and embarked on a project that would anaerobically digest pulp mill effluent to generate biogas to be used as green energy.

Slave Lake chose ADI-BVF® technology for the project, and ADI Systems, Fredericton, NB, designed and installed a 120,000-m3 (4.2-million- ft3) reactor for anaerobic effluent pre-treatment. The low-rate treatment system is believed to be the first in Canada’s pulp and paper sector.

ADI says the up-graded wastewater treatment system consistently achieves a high organic removal efficiency and biogas production rate, yet the technology is relatively simple and has few moving parts. The reactor was constructed as an earthen basin with a concrete perimeter wall lined with a geotextile underlay and geomembrane liner and insulated cover.

Also included was an ADI BioGasclean biological scrubber system to reduce the hydrogen sulfide (H2S) content in the biogas generated from the BVF reactor. The BioGasclean system consists of two parallel 550-m3 (19,423-ft3) scrubbing tanks filled with packed media and Thiobacillus bacteria which biochemically convert the sulfide in the biogas to sulfate. They are among the world’s largest biological scrubbers installed in terms of hydrogen sulfide load treated.

Biogas generated in the system migrates to the reactor cover perimeter where blowers pull gas through the scrubbing system, boost its pressure and transmit the scrubbed biogas to a set of three 3-MW generator sets designed for biogas applications.

The ADI wastewater treatment system is now helping Slave Lake Pulp to improve the consistency of final effluent quality. The BVF reactor can handle swings and peaks in hydraulic and organic loads, and digests over 85% of the BOD load anaerobically with no aeration and minimal mixing.

Anaerobic pre-treatment has allowed Slave Lake Pulp to decommission two of the four existing aeration basins, simplifying the over-all treatment system and reducing operating costs. The system requires little operator attention and no granular sludge, and decreases sludge generation for the over-all wastewater treatment system. Anaerobic digestion has also reduced chemical consumption by 50-70% and significantly reduced costs for electrical and sludge disposal associated with biological wastewater treatment.

Currently, Slave Lake Pulp generates 10.4 GJ of biogas energy per tonne of COD anaerobically removed, resulting in up to 6 MW of electrical power generation through biogas scrubbing and utilization.

Following a recent visit by staff from Lake Utopia Paper in New Brunswick, that mill will install an ADI-BVF system. See Supplier Orders, page 9.

ADI Systems, www.adisystemsinc.com

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