Thickening of waste activated sludge (WAS) before anaerobic digestion can reduce costs substantially. Typically, WAS is more difficult to thicken than primary sludge. Traditional methods include gravity belt and rotary drum thickeners and dissolved air flotation (DAF).

Now, Centrisys, a manufacturer of dewatering centrifuges and other products for the wastewater treatment industry, offers a hybrid technology designed to thicken without polymer and with low electricity consumption, and therefore at an attractive cost.

The company’s THK hybrid thickening system also offers a compact footprint, an automated process that requires minimal operator attention and a closed-system design that simplifies the control of odors. Michael Kopper, president of Centrisys, talked about the technology in an interview with Treatment Plant Operator.

TPO: What was the rationale for bringing this technology to the market?

Kopper: We looked at the market and noted issues in thickening processes, including high consumption of polymer, odor concerns and hydrogen sulfide discharges causing corrosion and requiring extensive air-handling systems. We set out to develop a technology to address those concerns with a more hygienic, more controllable and more reliable process.

TPO: Why is thickening of WAS so beneficial?

Kopper: Primary sludge thickens relatively easily to 3 to 4 percent solids. WAS coming from final clarifiers is typically at 0.8 to 1.1 percent solids. If you feed WAS to anaerobic digesters at 1 percent solids, there is a high energy cost to maintain those digesters at the optimum 97 degrees F. In addition, the volume of the digesters will be extremely high. If we increase the solids content from 1 percent to 4 percent, we reduce the overall volume by 75 percent.

TPO: Why is your technology called a hybrid thickening system?

Kopper: We combine centrifugal forces for settling the solids with air injection that helps to float the solids out of the machine. We basically use the centrifuge concept and the DAF concept in a single device.

TPO:  In basic terms, how does the thickening process work?

Kopper: The WAS is pumped from the final clarifiers into the centrifuge at a prescribed flow rate. The solids particles settle within the centrifuge and are conveyed below a divider disc to a small solids discharge chamber. Into this chamber we introduce a small amount of air — about 1 to 20 cfm. Air bubbles then attach to the solids particles, reducing their specific gravity to below the specific gravity of the water, causing them to float. Through that and a system of piping, hydraulic pressure of the liquid in the machine forces the solids out of the chamber. The water separated from the solids is sent to the headworks or, preferably, back to the aeration basins.

TPO: How does the design of the system control odor?

Kopper: The system is completely enclosed, and therefore the odor cannot escape. So, rather than having to vent an entire room for odor control, we only have to vent the device itself into an air-handling system and ultimately to an odor-control system.

TPO: How much operator attention does the technology require?

Kopper: The system is fully automated. We would expect an operator to walk by once in the morning, take a sample and make sure the machine is running properly. The daily labor should amount to about half an hour.

TPO: How does the footprint of this technology compare with alternatives?

Kopper: We have about one-thirtieth the footprint of a DAF system of similar throughput, and about 50 percent of the footprint of a gravity belt or rotary drum thickener.

TPO: How much energy input does this system require?

Kopper: The system uses electricity at an average rate of 0.11 kW/gpm. That means if we run 1,000 gpm for one hour — thus 60,000 gallons processed — the machine would consume about 110 kWh.

TPO: How much can facilities expect to save with this technology by eliminating polymer for WAS thickening?

Kopper: Generally speaking with a gravity belt or rotary drum thickener, the polymer dose is 6 to 10 pounds per dry ton of solids. If we assume the avoidance of 8 pounds of polymer per dry ton, and a price of $2.50 per pound, a 120 gpm THK system processing 2,360 dry tons per year would yield annual polymer savings of $52,600.

TPO: How are these systems sized, and for what size facilities are they designed?

Kopper: We offer units in 200 gpm, 300 gpm and 600 gpm capacities. Generally, the technology is suitable for facilities with average wastewater flows of at least 5 mgd. The larger the facility, the better the payback will be. A point of interest for larger facilities is that by adding a small amount of polymer — on the order of 1 to 3 pounds per dry ton of solids — we can double the flow rate through the machine. That means we don’t need mechanical standby capacity — we add capacity by adding that small dose of polymer.

TPO: What has been your experience with this technology under field conditions?

Kopper: We have six units in operation. We have done an enormous amount of testing around the world, in China, in Europe and in the United States in cities such as San Francisco, New York, Miami, San Antonio and Austin [Texas]. The results to date are very favorable. A unit in our home city of Kenosha, Wisconsin, has been running for three years, and so far there are no issues with the machine. We feed an average of about 150 gpm, achieving on average 94 percent solids capture at 4.5 to 5 percent discharge solids. Power consumption is 16.5 kWh for 9,000 gallons of WAS processed. The economic payback is about two years.