“We run very lean,” says Fehr. “We needed something that wasn’t going to be a big drain on our resources.” High-rate digestion systems can be very time-consuming, she says. “We felt the operator input, for us, would be prohibitive.” Another advantage of the BVF tech-nology is that it doesn’t require a tank. The costs of building a tank in Alberta at that time were prohibitive, she recalls. “So, from a cost perspective, BVF was a leader.” Also, Fehr says ADI Systems offered performance guarantees that satisfied West Fraser. As part of the tendering process, sev-eral anaerobic pilots were commissioned. Alberta Innovates – Technology Futures performed a lab-scale low-rate anaerobic pilot followed by an aerobic pilot stage to simulate the final effluent quality of an anaerobic pre-treatment stage followed by conventional aerobic treatment. SIMPLE YET EFFECTIVE TECHNOLOGY In a technical paper co-authored by Fehr, McCarthy, Daniel Bertoldo of ADI Systems and Megan DiJulio of Slave Lake Pulp, Slave Lake Pulp Biomethanation with Power Generation Project , the authors describe the anaerobic process as it applies to Slave Lake: “In general, anaerobic digestion involves a series of steps in which micro-organisms break down organic matter in the absence of oxygen… resulting in the production of biogas, a mixture pri-marily consisting of methane and carbon dioxide (with hydrogen sulfide and trace amounts of other gases). The high heating value of methane allows for the biogas energy to be recovered and offset fossil fuel consumption.” Financing for this project was assisted by the Climate Change and Emissions Management Corporation (CCEMC), which provided $5 million and the EcoTrust Fund, which contributed $10 million. A further $25 million was con-tributed by West Fraser. In December 2015, Fehr reported that the project was under budget, but one year behind schedule because of some difficulties associated with building tanks for the scrubber system. Now, $40 million and three-and-a-half years after the tenders began, the final piece of the system is in place. The biogas scrubbers were commissioned in the final days of 2015. They are among the world’s largest biological scrubbers, in terms of hydrogen sulfide loading. In their technical paper, Fehr and McCarthy note that the BVF reactor is effective at consistently achieving high organic removal efficiency, yet is relatively simple. “The anaerobic reactor itself is an earthen basin with a concrete perimeter wall lined with a geotextile underlay and geomembrane liner….. piping evenly dis-tributes incoming wastewater throughout the front-end of the reactor, where the majority of the biologically degradable organics are digested.” Biogas generated in the system migrates to the reactor cover perimeter, where blowers pull gas through the biogas scrubbing system, boost the gas pressure and transmit the scrubbed biogas to the generators. The process characteristics of the BVF reactor enhance process stability, and have an inherent ability to handle variations in hydraulic and organic loads. “This factor is important for Slave Lake Pulp, as the mill frequently alternates pulp brightness grades, resulting in swings in the waste-water’s COD concentrations from 5,000 to 19,000 mg/l,” say the authors. The biological scrubber system, called BioGascleaners, consists of two parallel 600-m3 scrubbing tanks filled with packed media and Thiobacillus bac-teria which biochemically convert the hydrogen sulfide in the biogas to sulfate and sulphuric acid. Cleaned biogas is transferred to the GE Jenbacher “gensets” – gas-fired recip-rocating engine generators designed for biogas applications. The three 3-MW engines at Slave Lake Pulp are dual-fuel capable; they can also run on natural gas. “There’s been a learning curve for us regarding anaerobic digestion,” Fehr admits. Biological systems generally take a long time to commission, and the digester took months, she recalls. CONSTRUCTION HAD TO WORK AROUND WINTER WEATHER Detailed design of the anaerobic diges-tion and biogas scrubbing systems began in February 2013. It was decided to perform the construction in two phases due to the large size of the reactor and the harsh winter conditions typical of northern Alberta. The first construction phase extended from June to December of 2013. The objectives were to have the basin, liner and internal items installed, and to fill the reactor with water. An area 95 metres by 185 metres had to be stripped and excavated. The reactor liner and internal piping were installed by Geomembrane Technologies Inc. In November, the reactor was filled with effluent from Slave Lake Pulp’s 12 Canadian BIOMASS CBM_Uzelac_Quarter_SeptOct_MLD.indd 1 OCTOBER 2016 2016-09-16 8:44 AM
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