Bacteria Helps Wastewater System Turn a Profit

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Bacteria Helps Wastewater System Turn a Profit

Hamilton is one of only a handful of Canadian cities that has expanded the appetite of anaerobic digestion to create enough energy to run its wastewater treatment facilities as well as generate excess gas for profit. It's a similar situation in the U.S.

These organisms, usually bacteria, are the type that thrives only in the absence, or near-absence, of oxygen. It's their role in the treatment of sewage sludge, aided by new technology to produce energy-rich biogases, that is bringing them into the spotlight.

Drivers crossing the Burlington Skyway in Ontario can see one 24-metre tall sphere where it's all being put into action. Inside the sphere is biogas, about 65 per cent methane produced from sewage, that's being put to profitable use.

Hamilton's Woodward wastewater treatment facility's anaerobic digesters produce about 17,000 cubic metres of biogas every day. That gas is turned into electricity by combined heat and power units − enough to run the treatment plant's entire operations as well as contribute toward city coffers. Each year, the city earns about $1.7-million from selling the extra electricity generated from the biogas."We took gas that would be flared off and created revenue," says Tom Chessman, Hamilton's manager of energy initiatives.

Hamilton is one of only a handful of Canadian cities that has expanded the appetite of anaerobic digestion to create enough energy to run its wastewater treatment facilities as well as generate excess gas for profit. It's a similar situation in the U.S.

But in a landscape of accelerating energy costs and rising levels of greenhouse gases, wastewater treatment is cleaning up its act.

The process has traditionally been a big energy hog. In the U.S., 3 per cent of all energy produced is used for wastewater treatment, representing one-third of a typical municipality's total energy costs. In addition, noxious gases created by the process, such as methane, are often flared into the atmosphere.

A 2013 study released by the Biogas Association reported that roughly 55 per cent of sludge-derived biosolids from Canada's largest treatment facilities are processed into biogas. Capturing more biogas via anaerobic digestion could reduce greenhouse gas emissions by 2.8 million tonnes, similar to taking 560,000 cars off the road.

Anaerobic digestion has been used to treat wastewater for about a century, but recent advances are maximizing the usefulness of what used to be dumped into waterways or burned into the atmosphere.

"We've pulled it all together to capture and digest more sludge, produce more gas from the sludge and destroy more volatile solids," says Aidan Cumiskey, business leader for advanced digestion technology with GE Power and Water.

GE has developed Advanced Anaerobic Digestion (AAD), a multi-step process using its Monsal line of systems that can yield over 25 per cent more biogas from sludge.

Traditional anaerobic digestion takes place in one vessel, usually over 20-40 days, says Cumiskey, a chemical engineer based in the U.K.

GE's process begins by removing sludge from settlement tanks using LEAPprimary - a novel system for removing sludge in a footprint one-tenth the size of traditional primary settlement tanks.

Next come the biological hydrolysis pre-treatments, using new reactors that make the sludge more soluble and acidified, maximising biogas production in the second phase: the AAD process. By pre-treating the sludge, the overall retention time can be reduced to less than 20 days, Cumiskey says.

The anaerobic organisms (usually bacteria) that break down the sludge are naturally-occurring and flourish even in our guts."We don't have to cultivate a super little bug," Cumiskey says. But conditions during AAD are stringently controlled to keep the anaerobes happily chomping.

Once AAD is complete, the resulting biogas can be converted to thermal energy or electricity.

Water is taken out of the final biosolids using a centrifuge to create a stackable cake product which can be readily applied to land. Depending on which GE Monsal system was used, the biosolids can be turned into an organic fertilizer - an important development given that other means of disposing of sludge are increasingly limited."It looks and feels like a soil substitute and is a viable alternative or supplement to inorganic fertilisers," says Cumiskey.

Source: GEReports

 

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