There are many ways to dewater residuals from drinking water treatment and wastewater treatment. Infilco Degremont offers one of the newer approaches in its Dehydris Twist technology, based on a piston press.

The technology was designed for the liquid-solid separation in the agribusiness and food processing sectors but is now being deployed in water and wastewater applications. The manufacturer states that the process is fully automated, can operate continuously with low maintenance, reaches high levels of dryness without lime addition, is energy efficient and has a relatively compact footprint.

Infilco Degremont offers the technology in a partnership with Bucher Unipektin of Germany. Hao Pham, product manager for biosolids with Infilco Degremont, talked about the process in an interview with Treatment Plant Operator.

TPO: What is the history of this technology?

Pham: What we are offering is not a novel technology but a reapplication of an existing technology. It has been used in food processing, specifically for the juicing industry, producing products like grape juice, apple juice and apple cider. It extracts the maximum juice from the fruit. So we thought, why not use the same process in water and wastewater? Here, instead of throwing away the extracted fruit pulp, we’re discarding the filtrate and keeping the solids.

TPO: What percent solids can this technology achieve in the wastewater sector?

Pham: That depends on the sludge characteristics, which vary with the upstream process. For mixed sludge at 60 percent primary and 40 percent secondary, we expect to see percent dry solids in the mid- to upper-30s. If it’s mixed digested biosolids, we typically see 30 percent dry solids, and we have seen up to 34 or 35 percent. On pure biological sludge, which is the most difficult to dewater, we see dry solids in the mid-20s.

TPO: How does it perform on residuals from drinking water treatment plants?

Pham: It does very well. We did a pilot study at a plant in Seneca, S.C., that had been using a plate-and-frame filter press on a dissolved air flotation sludge with alum floc, which is tough to dewater. We achieved up to 50 percent dry solids average performance.

TPO: What are the advantages to incremental improvements in dryness?

Pham: Of course, water is heavy, so dryer material means lower transportation costs and landfill tipping fees. In addition, our process yields a homogeneous material that breaks apart readily and is easy to handle and land-apply.

TPO: Are there benefits for facilities that dry or incinerate biosolids?

Pham: If we introduce this process upstream of thermal processes, there is a huge payback. Evaporating water requires about 1,400 to 1,500 Btu per pound. We can remove that water mechanically at a fraction of that cost. We did a study for a large city with an incineration process handling digested biosolids at 44 dry tons per day. They were getting 22 percent solids using centrifuges, and the fuel oil needed to achieve autothermal conditions in the incinerator cost $2 million a year. We determined that we could mechanically dewater to that autothermal condition. That represents $2 million in savings per year.

It’s the same principle for drying. Less moisture means less to evaporate and less energy cost. In addition, less need for evaporation means you can reduce the size of a dryer, and so the capital cost to build a plant is lower. If you have an existing plant, you can increase the dryer’s capacity to accommodate growth.

TPO: In summary, how does this process work?

Pham: It uses a hydraulic piston in a cylindrical chamber that rotates at 6 rpm. A batch cycle takes about two hours. The first hour is filling and pressing. The chamber fills with thickened sludge at about 3 percent solids. The piston moves to press the sludge as the chamber rotates, removing the water through polypropylene filter cloth elements strung across the length of the chamber. The water is channeled out through a polyurethane core. That process repeats until enough solids accumulate in the chamber — about one hour.

The second hour involves continuous rotation and pressing. That’s where we get the additional moisture removal. The cylinder rotates until the system senses a point of diminishing returns where very little water is being removed. Then the process stops. The chamber opens but continues to rotate to break up the material inside. Meanwhile the piston pushes the material out into a container or onto a conveyor.

TPO: What is the importance of chamber rotation to the process?

Pham: While the piston presses the material and removes the water, the rotation breaks up the sludge so that the end product has consistent dryness. We remove the interstitial moisture inside the sludge. As an analogy, consider a wet towel. If you wring it once, you remove a lot of water. But if you want to remove more moisture, you release it and re-wring it. That re-wringing compares to the effect of the chamber rotation.

TPO: What operator attention does the technology require?

Pham: The process is fully automated, but operators have flexibility to choose different automatic setpoints. They can set the machine to dewater to the maximum. If they are backlogged and need to move inventory, they can set it for a lower level of dry solids and save some time. They can also program for a specific cycle time, or for a certain volume of moisture removal from a batch.

TPO: What about operating costs, such as for maintenance and energy?

Pham: It’s a slow-moving machine, so there is low wear and tear and low electricity consumption. For processing 1,500 dry pounds per hour, the electricity usage is less than 30 kilowatt-hours.

TPO: How large is this equipment?

Pham: There are four standard sizes that range from 8.5 to 10 feet wide, from the low 20s to 30 feet long, and from 8 to 10 feet tall.

TPO: What kinds of capacities can the system accommodate?

Pham: We can process as low as 250 to 300 dry pounds per hour. Our largest unit can treat up to 1,500 dry pounds per hour, and of course users can add more machines for larger volumes.