Sewage sludge settled on the bottom of a beaker

Most people probably don’t think of sewage very much. So most people don’t realize that our sewage is contaminated with heavy metals, among other things. Sludge, the solids left over from wastewater treatment, makes good fertilizer, but only after removal of the heavy metals.

Wastewater comes mostly from four sources:

• Household wastewater from toilets, sinks, tubs, and the washing machine and dishwasher
• Stormwater runoff
• Industrial wastewater, including from mining and manufacturing procedures
• Leachate from landfills

The last two sources contribute most of the heavy metals. Removing them has been a cumbersome and expensive process. Although a lot of sludge becomes fertilizer, it’s cheaper just to put it in a landfill. But, of course, sludge in a landfill adds heavy metals to the leachate. It’s an endless cycle.

Scientists at Florida State University have recently announced an inexpensive way to recover heavy metals. It uses something like a microwave oven.

The problem with heavy metals in sewage sludge

This is what sludge land application looks like when it is finished: a nearly even distribution of the sludge. Now, it has to be covered with soil.

Heavy metals include arsenic, cadmium, copper, iron, lead, mercury, and zinc. We need trace amounts of copper, iron, and zinc in our diets. We have no dietary need for others.

It is possible to make any sewage sludge into fertilizer or apply it directly to the soil. If it contains heavy metals, however, plants grown in it will absorb them.

Heavy metals bioaccumulate. That is, animals that eat the plants take heavy metals into their bodies. They never excrete them. Animals that eat those animals take in those heavy metals. The metals never go away. The higher on the food chain an animal is, the more heavy metals it has in its system.

Humans are omnivores at the top of the food chain. We ingest heavy metals by eating grains and vegetables grown in soil contaminated with heavy metals. And the farm animals that supply our meat if they eat contaminated food.

True heavy metal poisoning rarely occurs in the US. Excess amounts, however, cause headaches, weakness, tiredness, joint and muscle pain, and constipation.WebMD warns against online heavy metal tests and detox treatments. At best, they waste consumers’ money. At worst, they can be dangerous to health.

Conventional removal of heavy metals from sewage sludge

Soil samples with lead

Wastewater contains organic waste, including all the nutrients that make up our food. In sewage treatment plants, bacteria decompose the waste and clean the water. They multiply as they work. At the end of the process, the plant discharges the treated water into a river or lake.

Solid matter, called sludge remains. Much of the sludge comprises the mass of all those bacteria, called biosolids. Unfortunately, heavy metals accumulate on the biosolids with a very strong chemical bond.

The conventional method of removing heavy metals from biosolids relies on strong acids. It is expensive and cumbersome. It also produces hazardous wastes that come with their own disposal and treatment problems.

Electromagnetic energy breaks the chemical bond between the bacteria and the metals. Treating biosolids with radiation reduces, but does not eliminate, the need for chemicals. Too much radiation, however, can also destroy the biosolids in the process.

How innovation can happen

Gang Chen, a professor of Civil and Environmental Engineering at Florida State University, calculated how much energy it would take to release the metals from the biosolids without destroying the nutrients in them.

While he used the microwave in his kitchen, he wondered if the same principles that cook his food could “cook” biosolids.

As it turns out, microwave radiation occupies the sweet spot on the electromagnetic spectrum. It can break the chemical bonds that bind the heavy metals to the biosolids and leave the nutrients intact.

Chen’s research team found that irradiating dewatered biosolids with 140 watts of electromagnetic power for 10 seconds effectively immobilized the heavy metals. It was still necessary to mix the treated biosolids with one of three standard acids but at greatly reduced concentrations.

Using microwave irradiation before acid extraction nearly tripled the amount of metals removed from the biosolids.

Not only that, but the process is efficient, environmentally friendly, and probably scalable to industrial use. The team’s cost analysis concluded that the method could decrease the cost of handling biosolids by as much as 62.7%.

Microwaving biosolids to remove heavy metals

Microwave treatment of biosolids can potentially allow wastewater treatment plants to consider them a revenue-producing product instead of a costly disposal problem. It can keep more material out of landfills and reduce the use of petrochemical fertilizers.

The method could reduce the cost of extracting heavy metals by more than 60%. The cost reduction would make it possible for wastewater treatment agencies to produce more fertilizer and rely less on landfills. At present, American wastewater treatment facilities recycle only about half of the biosolids.

So far, Chen’s team has only managed to remove 50% of the heavy metals from a mass of biosolids. Improvements in the design of the process and equipment put 80% removal in sight.

This research has two lessons for the average reader. First, as I always insist, wastes can and ought to be viewed as resources, not as disposal problems.

Second, as Chen points out, we need to observe what we’re doing. It can suggest solutions to all kinds of problems. Like most of the rest of us, Chen frequently uses his microwave to prepare food. And when he thought about what he was doing, it suggested the answer to a problem he was considering at work.

Sources:

Microwave-induced heavy metal removal from dewatered biosolids for cost-effective composting (abstract. Link no longer works) / Simeng Li et al., Journal of Cleaner Production 241 (December 20, 2019)
Microwave treatment is an inexpensive way to clean heavy metals from treated sewage / Bill Wellock, Physics.org. December 5, 2019
Microwaving sewage waste may make it safe to use as fertilizer on crops / Gang Chen, Physics.org. January 15, 2020
What is heavy metal poisoning? / Web MD

Photo credits:
Sewage sludge in a beaker. Public domain from Wikimedia Commons
Land application of sewage sludge. Some rights reserved by SuSanA Secretariat
Soil samples with lead. Some rights reserved by PlaxcoLab
Microwave oven. Public domain from Wikimedia Commons
Electromagnetic spectrum. Opensource Handbook of Nanoscience and Nanotechnology via Wikimedia Commons