Mixers Improve Biological Treatment Process At Paper Mill

The pulping process at any containerboard mill involves water — lots of it — and the conversion process generates significant effluent.

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The pulping process at any containerboard mill involves water — lots of it — and the conversion process generates significant effluent. The mill operated by Smurfit-Stone Container Corp. in Stevenson, AL, is no exception. The increasingly stringent environmental regulations make the efficiency of every link in the effluent treatment process essential.


Rebuilt mixer is installed on one of 16 rafts, while raft is held ashore by cables. Once installed, raft will be re-positioned out in the ASB.
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The efficiency of the biological treatment process is enhanced by the mixers, which keep the organic solids in suspension during the biological treatment process. The agitation produced by the mixers directly complements the work of thirteen 75 hp low-speed fixed aerators and 17 high-speed floating aerators in Aerated Stabilization Basins (ASBs) at the head of the multi-stage secondary treatment chain.

A preventive maintenance program adopted recently at the mill has improved the reliability of the raft-mounted mixers. The proactive approach to maintenance, using factory authorized service from Jim House & Associates, has kept the mixers operational by correcting a problem that arose after the units were rebuilt. The program also has delivered significant savings to the mill.

The mill produces approximately 2400 tons of containerboard per day (830,000 tons per year) from recycled corrugated stock and virgin hardwood fiber subjected to the neutral sulfite pulping process. Primary screening and clarification remove the fiber and other major solids subjected to an on-site wastewater treatment plant. The overall supporting infrastructure for the effluent treatment process consists of bar screens, two primary and two secondary clarifiers; four aerated stabilization basins; an activated sludge unit and three settling lagoons for final polishing. Screw presses dewater the primary sludge, which is typically burned as boiler fuel.

The harsh effluent entering the four 22 MG ASBs demands reliability from the mixers and aerators for the biological treatment process to work at peak efficiency. Sixteen ITT Flygt Model 4660 mixers, which are mounted aboard custom-made rafts by Clark Technology Systems, provided the increased circulation needed for the aerators to keep the biological treatment operating evenly throughout the ASBs.

Anchored at various points in each of the four ASBs, their agitation combined with the action of the aerators maintain the vitality of the biological process as it breaks down the lignin, fibers and other suspended solids in the effluent. The process removes up to 80% of the organic solids during this initial round of treatment before the effluent flows through the ASU.

"The mixers are important in maintaining the aerobic process within our ASBs," said Bill Roddy, Environmental Manager. "We use them as flow developers to reintroduce the liquid material back into the aerators and heightened exposure to the biological treatment process. We can raise and lower the mixers so that the prop angle keeps the solids from collecting and infilling the bottom of the ASBs."

Concern arose, therefore, when the rebuilt mixers began failing with increased frequency soon after their return from an outside motor shop, which repairs many different types of motors for the mill, said Robert Sims, who manages a 10-member group of electrical instrumentation (E&I) technicians.


View of ASB Basin #3, with two mixer-rafts and two high-speed aerators in operation.
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The rash of recurring failures began affecting the budget's bottom line. A breakdown often required four journeymen E&I technicians up to six hours to put on the safety equipment, launch a boat to pull the mixer platform to the shoreline of the ASB, and then retrieve the failed mixer with a heavy hoist. If the rebuilt unit was beyond repair a second time, downtime ensued and the level of agitation sought in the ASB was compromised while the unit was out of service, reducing the efficiency of the biological treatment.

"The replacement for nearly a dozen of the mixers — exclusive of man-hour costs — eventually exceeded $150,000," Sims said.

The staff called in Jim House & Associates, an authorized factory representative that supplied the ITT Flygt mixers, to help assess the problem and develop appropriate recommendations. Working as a team, they attributed the unacceptable failure rate of the rebuilt mixers to improper procedures followed by the motor shop and the absence of a regularly scheduled preventive maintenance program, according to Ed Swain, the planner for Power & Recover, E&I Group, at the mill.

In order to instill greater agitation in the harsh environment of the ASBs, the team installed props with a different pitch and removed their jet rings (propeller shrouds) to prevent limbs, wood and other floating debris from jamming the mixers and burning out the motors.

Under the preventive maintenance program now enacted for the equipment, one mixer is pulled each month and immediately substituted with a previously rebuilt unit by Jim House & Associates. Additionally, the mill now keeps a spare unit on site, eliminating down time if any problems arise.

In a disciplined sequence, the factory-trained technicians inspect the mixers and any visible damage or missing pieces are fully noted. The equipment is then cleaned with a high-pressure sprayer and re-inspected for damage. Next, the cable is inspected and its integrity tested with a megohmer, which is then used to diagnose any possible electrical problems with the stator windings.

If the windings reveal an electrical short from water intrusion, the stator is replaced. A specialized press is used to remove and replace its watertight housing. The previous motor shop had often cut the stator housings and then welded them back together, often failing to achieve a watertight seam.

The company's shop technicians also inspect the integrity of the oil seal, including pressure testing the units to 8 psi, before adding a precisely measured amount of fresh lubricant. If any of these previous steps detect problems, they disassemble the unit and compile a complete list of the components and labor hours necessary to restore the mixer back to factory standards. If the cost to rebuild and retest a unit appears prohibitive, the mill replaces it with a new mixer kept in stock now as a term in the agreement.

Since enacting the proactive program, all of the mixers have now gone through one and one half maintenance cycles, resulting in greatly improved performance records.

"The more preventive maintenance you do, the less reactive work you have to do later on," Sims said. "We've experienced 100% uptime since we adopted the preventive maintenance program and gained the consistent reliability needed for the effluent treatment process."

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