Interview Series #4

Combating antibiotic resistance: The important role of antibiotic removal in waste treatment

Nazanin Moradi is a researcher with a passion for environmental health engineering and wastewater treatment. She earned her BSc degree in Applied Chemistry from Arak University, Iran, in 2008 and has been actively involved in this research field ever since. For over 12 years, Nazanin worked at the Water and Wastewater Consulting Engineers Company in Isfahan, Iran, where she gained valuable expertise in water and wastewater management. During this time, she completed her MSc in Environmental Health Engineering at Isfahan University of Medical Sciences. In 2020, she started her PhD at the IHE Delft Institute for Water Education.

Q: Why is antibiotic removal an issue?

The problem is that some organisms and bacteria are resistant to antibiotics, either by nature or by acquiring resistance from another organism. If antibiotics are discharged into the environment, for example through digestate, the more exposure of this bacteria to the antibiotics offers a convenient condition for proliferation of antibiotic resistance bacteria (ARBs) and horizontal transfer of antibiotic resistance genes (ARGs) among different microorganisms. It happens via vertical transfer, which is defined by transmission of DNA from parents to offspring, and by horizontal gene transfer (HGT) – moving genetic material between unicellular or multicellular organisms plays an important role in organism evolution.

HGT is the main mechanism for proliferation of ARBs since antibiotic resistance in one species of bacteria can be transferred to another species through various mechanisms. This rapid spread of antibiotic resistance genes has become a medically challenging issue. In the future people may die from a simple infection because the antibiotic doesn’t work to treat the disease anymore.

Even now, based on the data obtained from across Europe, ARBs contribute to 33,000 deaths each year.

In our case digestate could enter a river, or in agricultural field and can be taken up by plants. Animals or humans then eat the plant; either way it goes through the food chain and ends up in the human body. That’s why we need to treat the digestate before it’s discharged to the environment, to be sure the antibiotic is not there anymore.

Q: Why is your lab important to the NOMAD project?

The NOMAD project’s goals are nutrient recovery and antibiotic removal from organic waste, among other aspects. One part of this waste stream is water. This grey water can be used in an agricultural field for irrigation and also has the potential to be used as a source of potable water after more treatment. In both cases we need to remove antibiotics from the water because otherwise it will end up in the human body. One of our objectives in NOMAD is to figure out how to remove these compounds from the outflow. Before scaling up the antibiotic removal test, a series of lab and bench experiments were required for better understanding about the optimum treatment and technology to remove antibiotics from the digestate.

Q: What is digestate, and what is the material you work with?

Digestate is the converted organic waste that had gone through anaerobic digestion in a biogas plant. The very first step is to pasteurise the digestate to remove pathogens and microorganism for cleaner production After pasteurisation we do ‘solid-liquid separation’ before any other step.

The separation starts with the centrifuge step, where coarse and solid material precipitates at the end of the process and the decanted liquid, which still contains small particles, goes through further steps, mostly micro and ultra filtration to remove the fine material.

The penetrate of ultra filtration at this stage can go through the advanced oxidation process (AOPs), either ozone-based AOPs or UV-based AOPs, for antibiotic removal.

Alternatively, the ultra-filtration penetrate goes through the SED process for nutrient recovery, and the outflow stream of SED would be applied in AOPs before discharge.

Q: What is ozone doing to this liquid?

Ozone works in two different mechanisms. The ‘direct mechanism,’ via molecular ozone and the indirect mechanism via OH radicals.

In the direct mechanism, molecular ozone at a neutral to acidic pH attacks the target compounds in the organic matter and breaks them into smaller molecules. More specifically, it attacks the electron-rich site of organic matter, i.e.double bond and phenolic ring, which degrades the target compounds.

In the indirect mechanism ozone in the liquid with the contribution of a catalyst or UV light produces OH (known as hydroxyl) radicals. These will have a higher oxidation capacity than ozone and it attacks non-selectively to all organic matter. In other words, it can degrade a wider range of organic matter, including the antibiotics.

In NOMAD project, we have a hybrid system called UVOX technology, where  both molecular ozone and hydroxyl radicals with the contribution of UV light attack the target compounds and the antibiotic removal can take place faster and more efficient than the removal by using lab-scale ozonation.

We are even thinking about further steps to possibly make drinking water from this. The outflow of AOPs could go for reverse osmosis for further treatment to produce even potable water without concern about the presence of antibiotics in the RO concentrate or penetrate.