Sludge, the suspended and dissolved solids produced during water treatment, has long been a waste product. Both sewage and waterworks sludge are considered a non-hazardous material. Most of it has been dewatered and make into cakes – which are sent to the landfill.
Some of the potential ways to recycle waterworks sludge include odor gas purification in wastewater treatment plants, absorbent for phosphorous, pesticides and heavy metals, coagulant, substrate in constructed wetlands, and as building material. Some of these ideas are newer than others, but today we are going to focus on just two: building materials and absorbing heavy metals.
Eco-Friendly Bricks from Sludge
Published in the November 2021 edition of the Journal of Environmental Management, a study done by researchers at universities in Portugal and Brazil described how waterworks sludge could be repurposed as an additive to red ceramic bricks.
Unlike waterworks sludge, which is from the treatment of drinking water, sewage sludge can contain heavy metals and pathogens. This limits its uses to ones that don’t risk human health or releasing those contaminants to the environment.
This research effort reflects the need to reuse and repurpose as much as we can. Not only can we replace materials we use with recycled waste, but we can also minimize the waste that is sent to the landfill. In developing countries, the improper disposal of sewage treatment waste can potentially harm human health.
The study found that incorporating up to 15 wt% sewage sludge into the brick clay mixture is highly feasible. The use of sewage sludge as an addition resulted in similar compressive strengths and total linear shrinkage after drying and firing as regular bricks. They found that in the bricks heavy metals are inert and can’t leach out.
Adsorbing Heavy Metals
Heavy metals are a huge threat to human and ecology health – even in small, trace amounts. Because of the high threat, a lot of research has been done to find solutions for removing heavy metals. Adsorption using various additives is the most common technique used.
One possible additive to use is the waterwork sludge produced during water treatment. Unlike some of the other adsorbents used, waterwork sludge, also called drinking water treatment residual (DWTR), is easy to use, local, and readily available.
The exact properties of DWTR can vary based on the original quality of the water, and the treatment and chemical used. Regardless, all DWTRs have similar amorphous structures and functional groups which gives them all similar abilities to remove heavy/semimetal ions.
Adsorbing Heavy Metals: The Next Step
The next direction for studying this use of waterworks sludge seems to be modifying the waste to make it more effective. Currently there are two directions being taken with this. One involves enlarging the specific surface area to create more sorption sites. The other uses a combined application with other material to make the physical appearance more regular and stable.
There is even thought of turning DWTR into a reusable adsorbent by removing metal ions. Although, this is still very conceptual. Research would need to be done to develop an efficient and inexpensive way to remove the metal ions before reusability could be a possibility.