The Sediment in Our Waterways
From Nuisance to Resource
From the phone in your pocket to the shirt on your back, there’s a good chance it traveled by boat on its way there. According to the United Nations Conference on Trade and Development, about 80 percent of all products come by ship. To ensure all these ships deliver their cargo, harbors need to be accessible, but sediment may block the way.
The weather, in combination with human and industrial activities, can generate billions of tons of sediment -- sand, gravel, silts, and sludges -- in our waters each year, which can impede ships. What you may not know is that there is an incredible amount of activity that goes on to deal with the challenge of sediment, with new strategies to turn sediment problems into opportunities.
"The key challenge is to change how people see sediment. Rather than viewing it as waste, we are trying to convince them that it is a useful product that can be reused, if treated properly, as materials in new civil engineering projects, such as construction."
Globally, around 23 billions tons of sediments deposit into our rivers and waterways each year. Approximately 800 million tons of this total will require some form of assisted dewatering.
"A build up of sediment can render waterways like ports basically unnavigable by ships and boats, and this is a large, costly problem," says Daniel Andrés of UTE Synchrolift, the consortium tasked with the construction of the new Shiplifting and Transfer System in Barcelona.
Pollution is another issue, Andrés says, as industrial waste can accumulate in sediment, causing it to be classified as contaminated sediment. To keep waterways open and clean, there is a continuous ongoing need to dredge, treat, recover and deposit sediments -- or quarantine, if sediments are contaminated by toxins. Separation of the larger particles, for example rocks, gravel and sand, is pretty easy as these drain naturally. But dewatering the fine sediments, those less than 63 µm where the contaminants or toxins typically concentrate, is not that simple. These particles often need to be processed differently to separate the water from the sediment -- a process known as dewatering.
Water Water Everywhere
The traditional way to dewater is mechanical, using sieves, presses, and other components in a process somewhat reminiscent of making French press coffee. The ideal outcome is a dry cake-like block of sediment and clear particle-free water. While effective, the mechanical approach can be inefficient with certain types of sediment and therefore cost and energy-intensive, requiring a hefty investment in equipment and maintenance. Moreover, sometimes this sediment contains heavy metals or other contaminants that have to be treated and disposed of as contaminated waste -- a cost and time-intensive matter. During dewatering, there is often a need to bind these trace contaminants and heavy metals within the sediment mass in order to enable the safe return of water to the environment, or reduce downstream water treatment cost and complexity. In addition to the mechanical process, the use of chemical conditioning is often necessary to enhance the dewatering performance.
Chemistry to the Rescue
Many sediment particles found near cities are coated in negative charges. As a matter of basic physics, negative charges try to stay far away from other negative charges. This repulsive force keeps small sediment particles from joining together to form larger particles.
That’s a problem for dewatering. Step one of dewatering is to clump the particles all together, separating the solids from the liquid. One strategy to overcome the repulsive forces between the negative charges and snowball the particles is to add a chemical that has positive charges into the mix, which acts as a buffer. These positive charges can sit between the negative charges and allow small sediment clusters to form, however, these remain fairly tiny and just barely hold together.
To prevent the clusters from breaking apart and to generate even larger sediment clumps, another type of chemical, called a flocculant, can be added to the mixture. These flocculants, on a molecular level, look like pieces of rope with a series of hook-up points designed to attract sediment clumps. When the flocculant is added to the sediment, it lassos the sediment clumps and ties them together in big bundles that are ready for additional dewatering steps.
Convinced that there may be a better way to address these dewatering challenges, Clariant developed INVOQUE®.
Clariant's INVOQUE dewatering system uses a unique mineral-based technology that can lead to improved sediment characteristics, which make it easier to dewater and reuse. The same electrochemical principles are applied as described above, but more efficiently, and with some neat additional features. For example, the minerals used are better at capturing harmful trace contaminants, leading to cleaner water.
Re-using the Sediment
Today there is a growing need to not only remove the sediment from the water, but also to reuse it in our circular economy.
"The key challenge is to change how people see sediment," says Andrés. "Rather than viewing it as waste, we are trying to convince them that it is a useful product that can be reused, if treated properly, as materials in new civil engineering projects, such as construction."
Andrés says his company chose to work with Clariant since INVOQUE “can actually improve the mechanical properties of the sediment when it is separated from the water, so that uncontaminated sediment can be re-used in civil engineering projects and the water can be released into the sea again.”
For example, sediment removed during the Syncrolift project in Barcelona has been used to form the foundation of their new ship lifting system.
There are environmental benefits as well. Since there is less sediment waste, INVOQUE’s dewatering system requires fewer trucks to transport waste, which means lower emissions. Also, by re-using dewatered sediment, new material does need to be extracted from quarries and transported to construction sites.
Galceran Llorens, Safety, Health and Environment Officer on the project, had clear reasons for why INVOQUE was chosen for the project: "Clariant's INVOQUE program appealed to us as a superior technology." He adds that the approach allowed them to quarantine dangerous materials, such as heavy metals, and meet strict water quality standards. When all is said and done, the clean water can often be returned to the environment.
Next time you visit a port or harbor, remember that the sediment below the water is so much more than sludge. Appropriate sediment management means cleaner water and a new source of environmentally friendly building materials.