This Technology Offers Yet Another Way to Dewater Biosolids

A newly introduced rotary press offers efficiency and flexibility in dewatering a wide range of wastewater treatment solids streams.

This Technology Offers Yet Another Way to Dewater Biosolids

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There are various ways to dewater wastewater solids: Belt presses, filter presses, centrifuges and rotary presses all have their applications.

A new entry to the field is a rotary press from Evoqua Water Technologies. The unit uses pressure and friction to cost-effectively dewater wastewater sludges and a variety of industrial solids streams in agriculture, petrochemicals, food processing, and pulp and paper.

The press operates continuously at low speed. With few moving parts and a simple design, it runs with minimal maintenance and low energy demand. The press comes in a wide range of capacities. It is designed to be deployed in new facilities and to integrate efficiently with existing solids processing systems.

Systems are packaged at the factory and optimized to accommodate each facility’s sludges. The skid-mounted units include PLC controls, polymer feed and mixing system, feed pump, discharge valves and piping. Mike Jager, senior product manager for dewatering, talked about the offering in an interview with Treatment Plant Operator.

What was the reason for bringing this product to market?

Jager: We wanted to deliver a product that is cheaper to operate and runs automatically to minimize operational costs and produce a drier cake.

What would you say are the most significant advantages this rotary press has?

Jager: There are few moving parts and few parts in general per unit. The machine is backed by Evoqua’s long-standing ability to test sludge and help customers optimize dewatering devices for their applications. We have test-run more than 12,000 applications in our lab.

Please briefly describe how the process works.

Jager: Sludge conditioned with polymer is fed continuously into the space between two filter screens. The solids are held by the filter screens while additional moisture is continuously pressed out as friction and pressure build. The material advances to the discharge end of the press, where pneumatically controlled restrictor gate arms slow the material, allowing a cake plug to form. The slow rotation of the press continues until enough cake has built up to push the plug past the restrictor gate and exit the press.

What is the capacity range of these machines?

Jager: We have models with one to four channels. The hydraulic flow ranges from 5 to 260 gpm. The dry solids per hour ranges from 225 to 3,900 pounds, comparable to a 2.5-meter belt press. With a four-channel unit a facility can run one channel or all of them, depending on solids volume that day. Often we sell a two-channel unit, and the facility will run one channel and keep the other for the heaviest days, or for redundancy.

What levels of dryness can this press achieve?

Jager: That depends on the material. With waste activated sludge, we would be in the range of 12%-16% solids; with lime sludge we can get to 45%; with a mix of primary and secondary sludge, the range is 20%-32%. We test each individual sludge, but we’ve found that our cake is about 25% drier than with a belt filter press for each type of material. The nice thing is that the dryness is extremely adjustable.

How can the solids content be adjusted?

Jager: We can adjust the dryness by adjusting the back pressure on the restrictor gates. That is controlled by a PLC. Each channel is pressure-controlled separately.

What does the unit include on delivery?

Jager: It is delivered complete on a compact skid. It includes a positive-displacement rotary-lobe feed pump, all piping, the polymer injection system and an inline mechanical mixer, the washing system, and controls.

What design features help make this press reliable?

Jager: We use a small drive motor with a large reducer gear. Our two-channel 48-inch unit, for example, has a 5 hp motor and runs at 1 to 2 rpm. The components that come in contact with the solids slurry are stainless steel. The seals are internal and also do not contact the sludge. The housings are powder-coated. Spare parts costs are almost nonexistent — about 0.5% of total costs over 10 years.

How does the press washing system operate?

Jager: There are internal and external channels to continuously wash the screens. The wash runs only as needed; usually washing for about five minutes at the end of the day will suffice, so the amount of wash water is very limited. We can also set a timer to wash every couple of hours, but normally that is not necessary.

How would you describe this equipment’s ease of operation?

Jager: We offer less maintenance and allow operators to spend less time standing around the machine, so they can work on other important tasks. The press can run intermittently. If they run out of sludge, it will turn off and go into a clean-in-place mode. Then after about five minutes it will turn off and be ready to start back up. We are capable of sending fault messages to a plant control system or to smartphones or laptops.

What is involved in programming the press at initial startup?

Jager: It’s completely set up at the factory based on our testing of the customer’s material. We provide operator training, usually an eight-hour session involving the operators and the polymer supplier. We connect the material feed, water, electricity and the source of polymer, and then it’s basically plug and play.

Can an installation include ancillary handling equipment?

Jager: Yes. We can supply belt or screw conveyors that pick up the cake and move it hundreds of yards if need be to an appropriate container, or to discharge into the back of a dump truck. It depends on the facility, how high-tech they need to be, and where they need the material to go.

What odor controls are available with this equipment?

Jager: The system is completely contained. The feed material comes in on a pipe from the bottom of the digester, so there is no odor until after the unit. By that time it’s pretty dry, and so the odor is reduced. We can also pull odorous air off at that point and join it to the plant’s odor-control system.

What other innovations are included with the technology?

Jager: We have see-through inspection piping so users can ensure that the floc is developing properly. There are inspection windows that open, so they don’t have to turn the machine off and open housings to make sure it’s running well. We also have an element in the top that stirs the drying sludge and releases any interstitial water. That way we don’t form a hard plug toward the top that’s hiding water inside it.

What is the primary market opportunity for this product line?

Jager: It is available for new construction and for plant expansions, but the primary target is for replacement of aging infrastructure.   


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