Better mixer maintenance ensures cleaner effluent at Smurfit-Stone

Pulp & Paper, Sep 2003 by Grella, Patrick

ISSUE Focus:

ENVIRONMENTAL TECHNOLOGIES

To achieve consistent reliability from its wastewater treatment process, SmurfitStone uses a preventive maintenance program that reduces treatment downtime

The pulping process at any mill involves large amounts of water and generates significant process effluent.The containerboard mill operated by Smurfit-Stone Container Corp. in Stevenson, Ala., is no exception. With environmental regulations becoming increasingly strict, every link in the mill's effluent treatment process is critical to returning water to the Tennessee River at the same quality level it was prior to intake.

At the Stevenson mill, mixers are used to agitate the effluent and keep the organic solids in suspension. This action complements the work of 13 lowspeed fixed aerators and 17 high-speed floating aerators in aerated stabilization basins (ASBs) at the head of the mill's multi-stage secondary treatment chain.

A preventive maintenance program adopted at the mill has clearly improved the reliability of 16 raftmounted mixers that help stimulate the biological treatment process removing organic solids from process effluent.The proactive approach to maintaining the mixers, utilizing factory-authorized service, has corrected problems with the previously rebuilt units and delivered significant savings to the mill.

WASTEWATER TREATMENT: NOW AND THEN. The mill produces approximately 2,400 tpd of containerboard (830,000 tpy) from recycled corrugated stock and from virgin hardwood fiber subjected to the neutral sulfite pulping process. Primary screening and clarification remove the fiber and other settling solids in an onsite wastewater treatment plant. The supporting infrastructure for the biological treatment process now consists of a bar screen, two primary and two secondary clarifiers, four ASBs, an activated sludge unit (ASU), and three settling lagoons for final polishing. Screw presses dewater the primary sludge, which is burned later as boiler fuel.

The mill originated in 1974 with a 600-tpd capacity and a 4.2-mgd wastewater treatment capacity. At that time, the process consisted of one primary and one secondary clarifier designed to handle a biological oxygen demand (BOD) loading of 20,000 lb/day. The secondary clarifier effluent was designed for a maximum overflow rate of 50 mg/1 solids measured as total suspended solids.The biological sludge off the secondary clarifier was wasted to a 165,000-gal clarifier thickener before a belt press dewatered the organic waste prior to disposal.

In 1998, the Stevenson mill increased its production capacity to 2,400 tpd, leading to additional investment in the wastewater treatment facilities, A second primary and secondary clarifier increased the flow rate to 8.1 mgd.The ASBs remained the same but received additional high-speed aerators and mixers, along with a new gravity-fed 3.6-mgd ASU, an additional screw press to process the primary sludge, and upgraded transfer piping.

After the upgrades, the effluent flow off the two secondary clarifiers averaged approximately 620 mg/1, with a BOD loading of 70,000 lb/day. In 2002, this resulted in 38 million gal of secondary sludge off the thickener, 8 million gal of return activated sludge, and 46 million additional gal cleaned from the ASBs and effluent ponds.

MIXERS MAXIMIZE TREATMENT PROCESS. For the biological treatment process to work at peak efficiency at the head of the multi-stage treatment chain, the flow entering the four 22-mgd ASBs demanded higher reliability from the mixers and aerators. Sixteen ITr Flygt Model 4660 mixers, mounted aboard custom-made rafts by Clark Technology Systems Inc., were installed in 1998 to help with the circulation of wastewater throughout the ASBs.

Anchored at various points in each of the four ASBs, agitation by the mixers, 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 secondary treatment then removes another 70-90% of any residuals.

By the time the 14- to 21-day process fully cycles, the 70,000 lb originally introduced into the ASBs have now been, on average, reduced to less than 7,000 lb when the effluent exits the three polishing ponds, says Bill Roddy, environmental manager at the Stevenson mill.

"The mixers are important in maintaining the aerobic process within our ASBs," explains Roddy. "We use them as flow developers to reintroduce the liquid material back into the aerators and heighten exposure to the biological treatment process.We can raise and lower the mixers so that the propeller 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 that repairs many different types of motors for the Stevenson mill. It quickly became apparent that the shop personnel responsible for rebuilding the mixers required specialized training.