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From biosolids, big things grow for utilities

Engineer and keen gardener Todd Williams has been spreading the word about the benefits of biosolids for the past 38 years. Now, as a principal technologist at consultancy firm Jacobs in the US, he helps businesses find the right solution for managing their sludge.

Williams’ interest in biosolids was piqued in the 1980s while he was working as the manager of a 20 million gallons/day wastewater treatment plant. He was tasked with starting a large sludge composting facility, where sewage sludge is mixed with wood chips or ground green waste to produce solids that can be used as a soil amendment.

The solids produced by the facility were put through numerous tests that showed the compost was a great growing medium for many types of plants.

“Being an avid gardener, I found the qualities of biosolids compost to be far superior to anything else you could buy from a garden centre for use on lawns, shrubs, trees and in garden beds,” Williams said ahead of his presentation at the AWA/ANZBP Biosolids National Conference next month.

“I was convinced composting was a great process to produce an excellent biosolids product that was much more environmentally friendly than the alternatives at the time.”

Today, Williams is just as passionate about the environmental benefits of using biosolids, but said the technology utilities are using to treat sewage sludge is changing.

Producing a Class A product

In 2012, Williams led a team of engineers in evaluating different biosolids management technologies for a wastewater business that had been sending all of its sludge to landfill.  

The team assessed a number of approaches, including landfilling, composting, thermal drying, anaerobic digestion and thermal hydrolysis in conjunction with anaerobic digestion.

From this, Williams decided thermal hydrolysis – which at this point had not been used by a utility in North America – was worth further investigation, and so he began visiting treatment plants in Europe.

“What got my attention from the evaluation was that the thermal hydrolysis process produced a high-quality Class A product, and the economics on a lifecycle basis were comparable to the least expensive alternative – landfilling – but with many more environmental benefits,” he said.

“[Visiting European facilities] convinced me thermal hydrolysis with anaerobic digestion is a viable biosolids treatment technology that produces a superior product compared to conventional Class B digestion products that are used for agriculture.”

Williams’ hunch about thermal hydrolysis was right. The first US operation was set up in 2014, and there are now more than 60 facilities across the globe.

He said interest in the technology continues to grow because utilities want to produce Class A biosolids, improve the dewaterability of their solids and reduce the amount of solids that must be managed.

Williams will present a study of 22 different thermal hydrolysis systems at the AWA/ANZBP Biosolids Conference. This research, which he conducted with two Jacobs colleagues, found some key benefits of the thermal hydrolysis process compared to conventional digestion.

This includes a higher percentage of solids being fed into anaerobic digesters, which means less digester capacity is needed. The study also showed improved volatile solids reduction and biogas production.

“The resultant biosolids product is Class A and very low odour, allowing more versatile end-use options than simply agricultural land application,” Williams said.

“As a result of these benefits, the economics of thermal hydrolysis systems has become competitive with, or may even be lower than, other more traditional options of biosolids management.”

Want to learn more about biosolids? Hear from Todd Williams and other industry experts at the AWA/ANZBP Biosolids Conference in February. To learn more and to register, click here.