Optimizing the treatment of municipal sludge

Process uses pre-treatment to improve the production of biogas from organic biosolids

June 06, 2017 — Montréal, Quebec

Full scale installations: in the foreground, we can see wastewater awaiting to be treated; in the background, two biomethanizers and hydrolyser (smallest of the three cylindrical installations).
Photo credits: Gilbert Samson

A Quebec municipality recently leveraged NRC expertise to successfully implement a new, highly efficient method of treating their biosolids. The method harnesses latent energy to substantially reduce both the volume of biosolids and disposal costs.

La Régie de l’assainissement des eaux du bassin La Prairie (RAEBL) is a group of five suburban municipalities located on Montréal’s south shore. Treating and disposing of excess sludge after wastewater treatment is an expensive proposition for RAEBL, as it is for municipalities across Canada. As of 2020, municipalities will face an additional challenge: organic waste will be banned from landfills under provincial law. To comply with the law, RAEBL plans to significantly reduce the amount of excess biosolids it generates, and partnered with the NRC to optimize its new waste-treatment system.

To treat the approximately 65,000 cubic metres of waste that flows into its wastewater system each day, RAEBL has long followed the same general process as most other municipalities across North America: sewage is directed into aerated large tanks, where part of the solids are degraded, and liquids are drained off and treated separately. The remaining solids, known as sewage sludge, are then dewatered and trucked away for use as soil amendment (where allowed) or to be buried in landfill sites.

Recycling a by-product of digestion

When organic material – such as sewage sludge – is processed in an anaerobic digester, it releases biogas, including methane. In recent years, more and more cities have begun to harvest biogas and use it as renewable fuel. RAEBL planned to recycle the biogas generated during anaerobic digestion and use it to power an additional step in the treatment process – one that would significantly reduce the total volume of leftover biosolids. To make this plan a reality, the municipality turned to the NRC for help.

For three decades, the NRC’s Montreal-based bioengineering team has researched various aspects of anaerobic digestion – the decomposition of organic material in sealed (i.e. no oxygen added) environments. In a range of academic and industrial studies, the team has explored and documented the use of anaerobic digestion for a variety of purposes, including the optimal production of biogas.

Innovative approach improves efficiency

RAEBL’s plan involves pretreating sewage sludge immediately prior to anaerobic digestion. Before entering the anaerobic digester, sludge is stored in a hydrolysis tank and heated. This process breaks down the sludge and increases the amount of biogas later produced during anaerobic digestion. The biogas provides the fuel needed to warm the hydrolysis tank and the digester and dry the digestate. The NRC team built a scaled-down model of the proposed hydrolysis-digester process to analyze the efficacy of RAEBL's plan.

The combination of 24 hours hydrolysis and 19 days of anaerobic digestion proved to be optimal for increased methane production and organic solids reduction. An innovative control instruction, which the NRC added to the process, removed an inhibition pattern that would have hindered the operation of the digester. The experiment de-risked the start-up of the full-scale plant, validated a reduction in solids produced, and produced a cleaner liquid effluent, easing disposal.

“When operating at optimum efficiency, the combined treatment system reduces the organic solids by over 55%,” says Jean-Claude Frigon, NRC Project Manager. “Drying of the digestate will further reduce this volume and save RAEBL a significant amount of money over the life of the facility.”

With testing complete, La Prairie’s hydrolysis system began operating in April 2017. Ultimately, it will process approximately 110,000 wet tons of secondary sludge per year and enable the municipality to comply with provincial regulations. By validating the efficacy of incorporating hydrolysis to anaerobic digestion for wastewater treatment, the project also benefits municipalities across Canada.

The NRC has the unique combination of technical and scientific expertise that municipalities need to optimize wastewater treatment by optimizing the biogas produced during anaerobic digestion. Learn more about the NRC’s Bioenergy Systems for Viable Stationary Applications program.

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