Withstanding the Test of Time

Rarely does a design engineer have the chance to return to a facility ten years later and evaluate a project's progress. It is even rarer to return and find the original operator still on the job. Such an occurrence is a great opportunity to see what worked well in operation and what lessons can be applied to future projects.

03/01/2001


Rarely does a design engineer have the chance to return to a facility ten years later and evaluate a project's progress. It is even rarer to return and find the original operator still on the job. Such an occurrence is a great opportunity to see what worked well in operation and what lessons can be applied to future projects.

In 1988, an engineering team from HNTB Corporation, headquartered in Kansas City, Mo., designed an industrial wastewater pretreatment facility for an automotive-parts factory being created by Aisin U.S.A. Manufacturing, Inc., Seymour, Ind. The design for the wastewater facility was based primarily on the operational experiences at a similar facility located in Japan.

The new manufacturing facility and wastewater-treatment plant were constructed simultaneously and placed into operation in October 1989. In January of 2001, HNTB took advantage of an opportunity to visit the wastewater treatment plant to compare the design criteria with the actual conditions based on a decade of operation.

In the beginning

The treatment plant was originally designed to accommodate any future manufacturing facility expansions, offering a total design capacity of 200,000 gallons per day (gpd). Constructed with random-rib precast concrete panels, the wastewater treatment plant is housed in a single-story, 7,000-square-foot building.

The two major wastewater streams-oily and heavy metal-are segregated within the manufacturing facility and conveyed separately to the plant. At the same time, waste oil, heavy-metal waste, anodizing waste and chromic waste are separately piped to the treatment facility.

The top figure on the opposite page illustrates the oily wastewater treatment processes and highlights some of the changes to the original design. The oily wastewater treatment facility consists of an American Petroleum Institute (API) oil/water separator along with a 45,000-gallon holding tank and a dissolved-air flotation (DAF) unit.

The bottom figure displays the processes and some of the changes for the heavy-metal system. This treatment system also includes a 45,000-gallon holding tank and an inclined-plate settler. Chromic waste is reduced through a separate batch-treatment process and combined with the heavy-metal wastewater.

Sludge from each wastewater treatment process is first combined and mixed in a sludge holding tank and then dewatered using a 38-cubic-foot plate and frame filter press.

Ten years down the road

Overall, the year 2001 finds the plant operating well within the pretreatment effluent limitations set by the municipal wastewater treatment plant. The Table on page 30 summarizes the effluent parameters for the year 2000, showing how well the manufacturing plant is meeting the requirements.

The manufacturing facility operates an average of 20 to 24 hours per day, six days a week, while the wastewater treatment plant operates approximately nine hours a day, six days a week. The average flow in the year 2000 was 87,000 gpd-roughly half the plant's design capacity. The ratio of the volume of oily wastewater to heavy metal wastewater is approximately 60 percent to 40 percent, and between 60 and 70 percent of the wastewater flows are generated by two zinc-plating lines, one acid-anodizing line and two paint-dip lines in the factory.

The pretreatment facility itself has come a long way from its design and construction to the present day, offering a chance to look at how some of the systems have stood up over time:

  • Building structure and systems. The owner made a deliberate decision during design to use precast-concrete panels for the treatment facility. While concrete is more expensive, the walls are flatter and easier to wash than those of more typical metal buildings, which are more difficult to maintain because beams and joists corrode and the material is harder to clean. The owner's upfront investment has paid off in long-term durability. For the first time since their construction in 1989, the outside walls are being sealed as part of the building's routine maintenance. The building's interior is still sound and corrosion-free.

While the building's all-concrete construction has resisted damage, the ventilation wall louvers did not, due to a damper motor that was not designed for a corrosive atmosphere. However, the fume-venting system, consisting of plastic ductwork, has worked well in the facility, keeping corrosive fumes to a minimum.

Additionally, as an alternative to placing pipes above ground, a 4-foot-high crawl space was originally designed underneath the entire ground-floor slab. This helped avoid a cluttered maze of pipes, protected the pipes from breakage and proved very useful for maintenance. The operator, who appreciated the inherent safety and accessibility provided by the crawl space, would continue to request an additional 2 to 3 feet of height clearance in future designs.

  • Chemical-feed systems. A 7,000-gallon fiberglass-reinforced plastic tank was originally provided for the sodium-hydroxide feed system. The caustic-feed metering pumps were installed on top of the 6-foot-high spill-containment wall, and although the metering pumps were designed for the 6-foot suction lift, pumping chemicals from the tank's lower levels proved difficult. This, coupled with a chronic tank-leakage problem, led to the replacement of the original tank with a smaller tank mounted on an elevated stand in 1998. This has allowed the metering pumps to easily pump the entire contents of the tank.

The original metering pumps for feeding sodium hydroxide to the inclined-plate settler and DAF rapid-mix tanks were furnished with both automatic speed-based on flow-and stroke control-based on pH. Simpler metering pumps with pulse control eventually replaced the originals. Since most of the pH adjustment occurs in the holding tanks, little, if any, sodium hydroxide is added to the rapid mix tanks.

The sulfuric-acid feed system initially incorporated a 7,000-gallon mild steel acid storage tank with a spill-containment wall and a feed-pump set-up similar to the sodium hydroxide feed system. The acid-metering pumps also had difficulty pumping chemicals from lower levels of the tank. However, the acid tank was decommissioned in place and may be used for the future storage of waste coolants. Sulfuric acid is now occasionally fed by a simple air-operated diaphragm metering pump from a 55-gallon drum to the wastewater holding tanks and chrome reduction tank. The demand for sulfuric acid for both chrome reduction and pH adjustment has proven to be far less than originally anticipated. In general, most of the pH adjustment occurs in the holding tanks.

The original sodium metabisulfite (chrome reduction), ferric chloride (heavy-metal wastewater) and alum (oily wastewater) feed-system design has worked well at the treatment facility.

  • Polymer feed systems. Two separate polymer feed units were originally installed as part of the design. One unit feeds liquid polymer to the inclined-plate settler's flocculation tank and the other feeds the polymer to the DAF unit's flocculation tank. While this set-up works, the operator would have preferred the option of feeding polymer upstream of the flocculation to achieve better mixing and flocculation. The operator occasionally hand-feeds cationic polymer immediately downstream of the API effluent weir to enhance the removal of emulsified oils in the DAF unit.

 

  • Wastewater holding tanks. The volume of each 45,000-gallon wastewater holding tank is usually larger than the total daily volume of either the oily or heavy-metal wastewater. As a result, pH neutralization can reliably occur in the holding tanks rather than in the rapid mix tanks. Ferric chloride can now be added directly to the holding tank-simplifying that part of the chemical-feed operation. The operator found that adding ferric chloride to the rapid-mix tanks didn't allow enough time to adjust the pH properly.

 

  • Inclined-plate settler. The inclined-plate settler has a rapid-mix tank followed by a flocculation tank, and a design flow capacity of 130 gallons per minute (gpm). During the course of operation the settler has operated at a level of between 100 and 130 gpm and the sludge recirculation function has not been used.

Ferric chloride, sodium hydroxide and sulfuric acid are now fed to the heavy-metal wastewater holding tank rather than the rapid-mix tank, while polymer is fed to the flocculation tank. The unit operates best with a pH set point of 9.0; once a week, the plate settler is acid-washed at 2.2 pH.

  • DAF unit. The DAF unit, which has a rapid-mix tank followed by a flocculation tank, has a design raw flow of 130 gpm with 130-gpm recycle flow. The unit has operated with a raw-feed flow of 140 gpm and a recirculation flow of 160 gpm.

The rapid-mix tank mixer is no longer operated because it appeared to entrap air, causing sludge to float in the flocculation tank. Sodium hydroxide and sulfuric acid are now fed only to the oily wastewater holding tank rather than to the rapid-mix tank, and alum is added to the rapid-mix tank to drop the pH level to 9.0. The unit operates best with a pH set-point of 10.2 in the oily wastewater holding tank. The DAF unit is acid-washed with 2.2 pH once every three to four weeks.

  • Chrome-reduction tank. It has been found that there is no need to add sulfuric acid to the chrome reduction batch-treatment tank because the pH of the chromic waste stream is already low enough at 4.0.

 

  • Electrical systems. The motor-control center, installed in the laboratory and office, has been sufficiently protected from corrosion due to chemical fumes. However, the wastewater feed pump variable-frequency drive (VFD) units were installed next to the pumps. Because the original enclosures for the VFD were ventilated, the units had corrosion problems and were eventually replaced with units in nonventilated National Electrical Manufacturer's Association (NEMA) 4 enclosures.

Lessons learned

After reviewing conditions at the plant over 10 years of operation, plans were made to add a supervisory control and data-acquisition system to remotely read the process water meters, upgrade the existing graphic control panel and add automatic dial-up capability in the event of high alarm conditions.

The manufacturing facility is currently implementing International Standards Organization (ISO) 14000 and may eventually reduce its wastewater flows by 20 percent through recycling at the plating lines.

Revisiting the wastewater treatment plant 10 years after it was built provides a valuable look at the evolution of design through a decade of operation. By taking the opportunity to compare expected results with actual results, the lessons learned can be applied to future work to make sure similar facilities stand the test of time in terms of function, durability, maintenance, cost and efficiency.

Year 2000 Comparison


Effluent Limitation2000 Data


Daily average Flow


None


87,000 gpd


pH:


Daily maximum


10


9.39


Daily minimum


5


8.86


Biochemical oxygen demand


None


78 mg/l


Chemical oxygen demand


None


199 mg/l


Suspended solids


None


18 mg/l


Oil (maximum grab)


100 mg/l


31 mg/l


Zinc (daily average)


1.48 mg


0.45 mg


Total chrome (daily average)


1.71 mg


0.14 mg


Overall, the plant is operating well within the pretreatment effluent limitations set by the municipal wastewater treatment plant.





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