Findings by a team of University of B.C. wastewater engineering researchers at the Okanagan campus have revealed a low-cost solution to many of the problems plaguing anaerobic digesters used by municipalities.
Anaerobic digesters are seen as a popular solution to wastewater treatment as they offer the potential for producing green energy and associated products.
Anaerobic digestion uses a series of biological micro-organism processes to break down wastewater material to produce biogas, which can then be combusted to generate electricity and heat, or can be further processed into renewable natural gas or transportation fuels. Dewatered material can also be used for compost.
But municipalities — especially those who have tourism-based economies — have not always embraced the technology because of the known odour and associated problems that come with the process, said PhD student Tim Abbott, part of the UBC research team that worked on finding low-cost solutions.
Smaller towns, where there is a concentrated population may also shy away from the technology because it fails the sniff-test.
Abbott said the team found a low-cost solution was the introduction of ferric chloride, essentially a metallic salt, introduced into the digester process.
“Not only were we able to reduce the production of the sulfuric gases by 93 per cent, to the point they became nearly imperceptible, but we unexpectedly discovered that pathogenic fecal coliforms in the digested sludge was reduced by 83 per cent,” said Abbott.
The odour comes from either ammonia gases and more commonly the byproduct hydrogen sulphide gas as the waste material is broken down in the anaerobic digester by bacteria.
“It is extremely odorous,” he said, but it also poses a safety risk.
“Even a small leak can be hazardous to workers.”
Abbott said there is also the corrosive nature of the gases produced on the equipment and pipes. And, pathogens in the spent material have to be considered, he added.
“There are many things you can add to the digester, but they would then have adverse effects on the process and it was not feasible,” he explained.
The solution also had to be easy for existing operators of digesters to implement and low-cost to make it affordable.
The study team looked at introducing metallic salts which were known to control odours in sewage systems but had not been introduced into a digester environment. Three types of metallic salt were tested but ultimately ferric chloride was chosen as the best option as it was the most stable.
During the course of the team’s two studies, it looked at not just the best kind of metallic salt, but different combinations, what was the best dosage, when it should be effectively introduced into the system and what was the associated impact.
Abbott said the team found the best place to introduce the metallic salt was prior to the digester process.
“We basically published two papers on the topic,” said Abbott, as the team of Dr. Cigdem Eskicioglu, professor of engineering, and post-doctoral research fellow Deniz Akgul, probed the effectiveness of ferric chloride and its impact.
Digester gas and performance were not affected by adding the metallic salts and the same amounts of biogas were produced.
However, the end product material, with the reduction in pathogens, became more adaptable to use as fertilizer.
In addition, dewatering the material from the digester became easier.
Abbott said the trials were run in the university’s laboratory but used real wastewater material that would normally flow into a treatment plant.
While the research was not field-trialed at a working municipal digester, Abbott said estimates are that a medium size digester operation would have annual costs of approximately $10,000 for ferric chloride.
The metallic salt ferric chloride is known to wastewater system operators and any operator of a digester or treatment plant could easily order it from a local supplier, Abbott said.
The two research papers were published in Science of the Total
Environment on the topic.
Abbott said he is hopeful the process will show municipalities that anaerobic digesters are a viable option especially in the face of global warming and the need to harvest and use more green energy.