Primary clarifiers have been basic equipment in wastewater treatment for decades, and they remain effective.

Now, Hydro International offers an alternative form of primary treatment that uses a screening process instead of gravity settling to get the job done. The company says its Hydro MicroScreen system can do the same work as primary clarifiers in a significantly smaller footprint with lower power input and at lower cost of installation.

The technology uses a rotating belt screen to separate out solids, which in smaller plants can include grit and fine screenings as well as the material that comprises primary sludge. The company says the system removes 50 to 60 percent of TSS, along with 20 to 40 percent of BOD and 10 percent of phosphorus by removing their particulate forms.

The process allows engineers and operators to choose from a range of screen mesh sizes and thereby customize the removal rate and optimize the efficiency of downstream processes including nitrification. It is suited for a variety of industrial uses. Matt Bokenkroger, industrial sales manager, talked about the technology in an interview with Treatment Plant Operator.

TPO: What problems in the marketplace are addressed by this technology?

Bokenkroger: First, land values were becoming astronomical so that plants couldn’t obtain land affordably, or the land they acquired was so degraded that it cost significant money to upgrade it. And second, plants were looking to manage solids so as to feed material directly to energy recovery systems. Those include anaerobic digesters, as well as gasification and pyrolysis processes, which are starting to make waves.

TPO: So your technology is basically replacing the primary clarifier?

Bokenkroger: That’s correct. For small plants, under about 4 mgd peak flow, we can combine fine screening, grit removal and advanced primary clarification in one component.

TPO: In simple terms, how does the screening process work?

Bokenkroger: The equipment is typically installed in a stainless steel vessel. The influent level is controlled by the rotational speed of the screen. Heavy solids require a higher screen speed. Lighter solids loading would require a lower screen speed. In either case, the water level stays the same. A level transducer monitors the inlet water cavity, and an algorithm directs the screen to speed up or slow down. In that way we maintain a uniform, thick mat of solids on the screen.

TPO: What does this screen look like?

Bokenkroger: It looks identical to a window screen, except we can vary the mesh size. If the screen has a 100-micron mesh, then it has 100-micron squares. The screen doesn’t stretch or expand. It has greater than 300 pounds per square foot loading capacity, which will never be reached. The material we remove doesn’t get trapped inside the screen. It rides on top of it. Because we have a fairly steep but not too steep angle of incline, close to 90 percent of the material falls off by gravity. The rest falls off with low-volume, high-pressure wash water and a secondary scraper.

TPO: What happens to the material that is removed?

Bokenkroger: Under the screen we have a built-in auger. The auger conveys the material to a discharge point. Along with the auger we can add a dewatering press, if the customer wants it. If there is no press, we expect material at 3 to 5 percent solids, similar to primary clarifier sludge. With a press we expect 20 to 25 percent solids. That material will pass a paint filter test and has been approved to go to landfill. However, being a renewable energy proponent, I would prefer to see the liquid slurry go to digestion or the compressed material to gasification.

TPO: How would the footprint of this technology compare to primary clarifiers in different-size treatment plants?

Bokenkroger: For large-scale plants, the technology would have as low as 10 percent of the footprint of primary clarifiers and use 20 percent of the power. For a 2 mgd or 3 mgd facility, it’s closer to 50 percent of the footprint and 50 percent of the power. We did a theoretical project with an engineering firm for a plant in Oklahoma that had six primary clarifiers. Our MicroScreen would have fit within one-third the space of one of the primary clarifiers.

TPO: How can this technology affect downstream processes?

Bokenkroger: There are so many variables, but to cite one example, in a biological nutrient removal process, it can reduce sludge wasting and make it easier to maintain the desired food-to-microorganism ratio. Many facilities have found that they can dial down their aeration requirement.

TPO: What has been done to ensure that this system is reliable?

Bokenkroger: The system was originally designed for use in tanneries, where they can have from 2 to 12 pH, from 0.2 to 3 percent solids loading, and average H2S content at 100 to 125 ppm. For municipal plants, our system is several times more robust than what they need. We also designed the components to be maintenance-friendly. For example, on rotational components, we wired the bolts to reduce maintenance. We’ve also hard-plumbed the grease ports to the top of the unit.

TPO: What safety features does this system include?

Bokenkroger: Our screen is 100 percent designed to meet OSHA standards for rotational equipment. The access ports where operators can see the screen are guarded with wire mesh. If they lift a lid to get inside, the rotation stops instantly. They can then perform whatever maintenance or service is needed, and the moment the lid is closed, operation automatically starts again.

TPO: How can the equipment be customized to meet a given plant’s requirements?

Bokenkroger: We offer screen porosities from 70 microns to 5,000 microns. A 70-micron screen has 13 percent open area, while a 1,000-micron screen has 50 percent open area. Most municipal plants just want primary clarification, and we can do that reliably with a 300- to 400-micron screen, unless they also want grit removal, in which case we need a 158-micron screen. Industrial users would often choose a smaller mesh. They want to remove as much material as possible to reduce BOD and TSS surcharges from municipal treatment plants.