A Concrete Solution: Recycling Dewatered Sludge

Written by AOS Treatment Solutions on April 18, 2017

Custom sludge dewatering is having a profound impact on landfills around the world. The sheer volume of dewatered sludge from wastewater treatment plants is filling landfills more quickly than is acceptable. This is why scientists all over are now searching for a solution to the dewatered sludge issue plaguing landfills.

Sludge is an inevitable by-product of water treatment. Wastewater sludge is composed of organic polymers, biomass waste, chemicals and surfactants, and a variety of other waste and treatment components.

But more than anything else, sludge is composed of water — between 80 and 97 percent water. As a result, sludge disposal is both costly and space prohibitive.

While not inherently toxic, it certainly can be, according to research by IOP Science: “Sewage sludge presents an important problem because of its negative impact on the environment, connected with the contain of heavy metals, microorganisms and contamination retained in the process of sewage treatment and, at the same time, its considerable volume.”

Sewage sludge must be disposed of underground. Not only because of the potential for toxins, but the odor is often extremely foul. Leaving wastewater treatment sludge on the surface is not an option.

But burying it, even in landfills, is costly. And landfills that would otherwise serve as a dump site for an extended period of time are filling at a much faster rate than expected because of wastewater sludge.

This is why scientists are looking into recycling options.

Sludge as a Construction Material Solution

A logical solution is to use sludge as a construction material. Scientists have discovered that dehydrated sludge — sludge ash — has many of the properties found in concrete aggregates.

In the Middle East and Europe, scientists are using dewatered sludge in construction applications. One solution that has shown promising results is the use of dry sludge as a concrete mix component. When used in the right ratio, sludge ash is a suitable concrete aggregate.

Value of Sludge Ash as Concrete Aggregate

Certain qualities of sludge ash make it a sound concrete aggregate:

“Digested and dewatered sludge, after incineration at a high temperature, yields a hard, cellular, porous mass with low unit weight. This hardened mass of sludge ash can be crushed to smaller sized aggregates, which, when graded in suitable proportions, manifest the basic attributes required of lightweight aggregates. When used as aggregates in the production of lightweight concrete, experimental results show that the resulting concrete satisfies the physical requirements of a lightweight concrete in terms of unit weight, strength, heat‐insulating properties and fire resistance, thus indicating that sludge ash could be a potential source of suitable lightweight aggregates.”

While wet sludge reduces the compressive strength of concrete by up to 15 percent, dry sludge does not reduce the compressive strength to the same degree. As a result, up to 20 percent of concrete aggregate can be dry sludge.

Sludge Ash as Brick Material

Unlike concrete, which loses structural integrity when too much sludge ash is used as an aggregate, bricks made of sludge ash proved to be stronger than those bricks manufactured without sludge ash:

“The appropriate percentage of ash content for producing quality bricks was in the range of 20 to 40 percent by weight, with a 13 to 15 percent optimum moisture content prepared in the molded mixture and firing at 1,000°C for 6 h. With 10 percent ash content, the ash-clay bricks exhibited higher compressive strength than normal clay bricks.”

Other Recycling Options for Dewatered Sludge

Although the most promising means of recycling dewatered sludge thus far is as a concrete aggregate or as an aggregate for brick material, sludge as is also being tested as an aggregate for other building materials, such as drywall and plaster.

The greater the number of recycling uses for dewatered sludge and ash, the less landfill space will be used for disposal.