Water purification at a modern urban wastewater treatment plant involves removing undesirable chemicals, suspended solids and gases from contaminated water.
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Biosolids – primarily dead bacteria – from sewage plants are usually dumped into landfills. However, they are rich in nutrients and can potentially be used as fertilizers. But farmers can’t just replace the normal fertilizers they use on agricultural soil with these biosolids. The reason is that they are often contaminated with toxic heavy metals like arsenic, lead, mercury and cadmium from industry. But dumping them in the landfills is wasting precious resources. So, what is the solution?
I’m an environmental engineer and an expert in wastewater treatment. My colleagues and I have figured out how to treat these biosolids and remove heavy metals so that they can be safely used as a fertilizer.
How treatment plants clean wastewater
Wastewater contains organic waste such as proteins, carbohydrates, fats, oils and urea, which are derived from food and human waste we flush down in kitchen sinks and toilets. Inside treatment plants, bacteria decompose these organic materials, cleaning the water which is then discharged to rivers, lakes or oceans.
The bacteria don’t do the work for nothing. They benefit from this process by multiplying as they dine on human waste. Once water is removed from the waste, what remains is a solid lump of bacteria called biosolids.
This is complicated by the fact that wastewater treatment plants accept not only residential wastewater but also industrial wastewater, including the liquid that seeps out of solid waste in landfills – called leachate – which is contaminated with toxic metals including arsenic, lead, mercury and cadmium. During the wastewater treatment process, heavy metals are attracted to the bacteria and accumulate on their surfaces.
If farmers apply the biosolids at this stage, these metals will separate from the biosolids and contaminate the crop for human consumption. But removing heavy metals isn’t easy because the chemical bonds between heavy metals and biosolids are very strong.
Microwaving waste releases heavy metals