The extensive use of antibiotics in healthcare and agriculture has created an alarming rise in antibiotic-resistant genes and antibiotic-resistant bacteria, posing significant risks to human, animal and environmental health.
But new research facilitated by Water Research Australia and South East Water, and led by Professor Qilin Wang from the University of Technology Sydney, has analysed how anaerobic digestion influences the fate and removal of these resistant bacteria in sludge from wastewater treatment plants (WWTP).
Researchers conducted experiments using sludge from a WWTP in Melbourne to assess the impact of various anaerobic conditions on antibiotic-resistant genes and antibiotic-resistant bacteria.
Wang said antibiotic-resistant genes are genetic elements that can exist in microorganisms, but there are opportunities to reduce the health risks posed by working with WWTP processes.
“If microorganisms become resistant to antibiotics it will not be good for human health, as it will mean the use of antibiotics will not work as well, or at all,” he said.
“Sludge produced in WWTPs has already been recognised as a hotspot for antibiotic resistance genes. Also, in Australia, around 80% of sludge produced in WWTPs will be used on agricultural land.
“If the sludge contains antibiotic resistant genes, when we apply the sludge onto the land, those antibiotic-resistant genes end up in the soil. This poses another risk to human health, particularly if the soil is used for agricultural production.
“The overall aim of this project is to understand and reduce the spread of antibiotic resistance genes in the anaerobic sludge digestion process.”
Wang said the first objective of the research project was to understand the fate of antibiotic resistance genes in temperature-phased anaerobic digestion (TPAD).
“TPAD is a type of anaerobic digestion process that is not very common here in Australia. In WWTPs in Australia, the most commonly used process is normal anaerobic digestion, or mesophilic anaerobic digestion, which uses a temperature of around 37 degrees,” he said.
“In comparison, TPAD consists of two parts. The first is thermophilic, which treats at about 55 degrees. The second part involves mesophilic anaerobic digestion, at around 37 degrees.
“The reason TPAD is used is not to remove antibiotic-resistant genes, but to enhance the biogas production within the treatment process. It’s about enhancing the sludge degradation in the WWTP.”
Wang said the fate of the antibiotic-resistant resident genes in the TPAD process had not yet been evaluated.
“We took samples from one WWTP in Melbourne to try to understand this process. There are plenty of antibiotic-resistant genes. I selected 10 typical ones and found that TPAD can reduce about 90% of those antibiotic-resistant genes in sludge,” he said.
“The first part of the process – thermophilic anaerobic digestion – reduces antibiotic-resistant genes by around 60%, and the second part of the process – mesophilic anaerobic digestion – reduces antibiotic-resistant genes by a further 30%.
“To the best of my knowledge, this is the first study to evaluate the fate of antibiotic-resistant genes in the TPAD process.”
Wang said the second objective of the research was to try to develop a technology to reduce antibiotic-resistant genes in the more commonly-used anaerobic sludge digestion process – mesophilic anaerobic digestion.
“Here, we wanted to look at normal anaerobic digestion at 37 degrees. This process already exists in many WWTPs in Australia, so we tried to develop a technology to reduce antibiotic-resistant genes in this process,” he said.
“We used free ammonia technology for this aim. We applied free ammonia to pre-treat the sludge. We then added the free ammonia pre-treated sludge into the anaerobic digester.
We found that this worked to enhance the removal of antibiotic-resistant genes.
“We then tested a few antibiotic-resistant genes. Our results showed that we can further reduce antibiotic resistant genes between about 20-70% if we use free ammonia technology. We do not yet know why this works more effectively for some genes and less effectively for others.”
During mechanism studies, the research team tried to connect antibiotic-resistant genes with their host, Wang said, which are antibiotic-resistant bacteria.
“We found that applying free ammonia reduced the host of the antibiotic-resistant genes, too. This means the process killed the antibiotic-resistant bacteria. This could be one of the potential reasons for the variance in effectiveness.”
Wang said that by adopting these advanced treatment methods, WWTPs can play a pivotal role in safeguarding public health and preserving environmental integrity.
“In the future, when we are selecting technologies for WWTPs, we now know that in addition to the normal parameters of biogas production and degradation, we can also consider different approaches,” he said.
“Maybe we can consider this less common process of TPAD as a means of also helping to manage the prevalence of antibiotic-resistant genes in our sludge products.
“Another great outcome of this study is that we know that applying free ammonia is an effective approach for reducing the abundance of antibiotic-resistant genes in sludge.”
Wang said the research has showcased how well these approaches work, but future studies will be focused on optimisation to try to further enhance the removal of antibiotic-resistant genes.
“Currently, we have only shown that this technology can work, but we haven’t done a lot of optimisation yet. That’s what we plan to do next – optimise the technology to achieve better results.”