For a better brick, just add poop
The world produces about 1.5 trillion bricks per year, mostly from China (67%) and India (13%), and the number is rising. The ovens used to bake clay into bricks use 375,000,000 tons of coal, so it would be better for the environment to find an alternate means. Most bricks use clay as the main substance, but the annual removal of clay from the ground is enormous. Imagine 12,000 holes that are each as deep as 100-story buildings and have the area of soccer fields. That's what is being taken from the Earth to make bricks every year. A research team from Brazil has found a way to take solid human waste and combine it with clay to reduce that burden. Its reuse of the waste material serves another beneficial purpose; the material would otherwise be burned (using more fossil fuels) and buried (taking up landfill space, 100,000 tons in Brazil alone every year).
Clay is made basically from 6 components, 7 if you count water:
- 50-60% silica (silicon dioxide, SiO2)
- 30% alumina (aluminum oxide, Al2O3)
- 8% iron oxide (Fe2O3)
- 5% magnesia (MgO)
- 1% lime (calcium oxide, CaO)
- 1% organic matter
Alumina makes them moldable, silica binds to alumina for strength and to prevent cracking, shrinking, and warping. Lime and iron oxide help in the binding process, and the iron adds color. Magnesia adds a yellow color and reduces shrinkage. Organic matter is inevitable and adds unwanted porosity.
Dr. Tuani Zat and colleagues from the Department of Structures and Civil Construction, Federal University of Santa Maria, Brazil have come up with a method to improve the composition by adding human waste. Not raw feces, but sludge, which is the end result of processing in a wastewater treatment plant. Sewage is filtered then shredded in a comminutor before being sent to a settling tank to separate liquid from the biosolid known as sludge. (In the diagram below, you see that in the lower left. Additional steps will aerate the liquid layer, in the top half of the diagram, to remove dissolved gases and break down organic debris.)
Raw sludge is further treated and then disposed by dumping into the ocean, fertilizing farms, or burying in landfills after pathogens are killed with the addition of quicklime. But Zat decided that this dried human waste might be a valuable addition to brick clay because in addition to the organic material, it contains silicon, aluminum, and calcium. Baking the mixture with clay would effectively kill any bacteria and parasites and destroy viruses.
Ceramics around the world use sludge mixed with clay, too, with the amount of sludge varying from 2.5% to 20% as a wet material. The amount of organic material in sludge is what affects how well it will bind to the clay particles, and sludge from different populations of people do not have the same composition. So, Zat and her team tested various mixtures to make bricks. Another Brazilian team measured the range of particle sizes of clay and human sludge. The graph below shows that clay has many more particles less than 0.08 mm in diameter, and that is good because those can surround the larger sludge particles to hold them together when heated.
That other Brazilian team tested mixtures of sludge and clay to assess how economical it would be for brick making. They also compared different baking temperatures and found that a fairly low temperature could be used. Regular bricks cost about $0.16 each to make, but when mixed with sludge, and accounting for energy savings of materials and lower heat, they were able to make comparable quality bricks for $0.025 per brick. So, Zat's team was on the right track.
A combination of 15% sludge and clay were used by Zat's team, and researchers elsewhere found similar results to make useful bricks. Some dried the sludge first, and then filtered (sieved) it and clay to a certain particle size maximum before adding water and then molding & baking.
Here is a cool YouTube video showing how bricks are made from start to finish.
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