Mobile Clarification Trailer Helps Treatment Facility Stay in Compliance

In the winter of 2004, the Rosemount, MN, wastewater treatment facility was experiencing problems with effluent quality.

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In the winter of 2004, the Rosemount, MN, wastewater treatment facility was experiencing problems with effluent quality. The 0.75 mgd aerated lagoon treatment plant, operated by Metropolitan Council Environmental Services (MCES), was permitted to discharge wastewater with a mazimum average monthly total phosphorus level of 1.0 mg/1; however, the effluent from the facility's newly installed disc filters was exceeding the NPDES permit limit.

MCES, responsible for operating eight treatment plants in the Minneapolis/St. Paul area, knew that the extremely cold winter temperatures would cause a problem with total suspended solids (TSS). For that reason, disc filters were installed in January 2004 to remove TSS and the associated phosphorus.

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USFilter provided a hight-rate mobile clarification system for removing the suspended solids and thus reducing phosphat levels discharge from the Rosemount facility.
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Soon after the disc filters were operational, MCES determined that the phosphorus concentration was not adequately reduced. MCES engineers sent a feed water sample to USFilter’s Rockford, IL, facility for testing, which verified that the soluble and insoluble phosphate was attaching itself to the suspended particles in the water. These particles were not being efficiently removed from the waste effluent.

The temperature of the aerated lagoon can drop below 3°C in the winter, which inhibits suspended solid flocculation. The small particles were passing through the 10-micron disc filters. In warmer weather, as the water temperature rises, the solids become larger and the filters are able to remove them.

USFilter suggested high-rate mobile clarification for removing the suspended solids and thus reducing phosphate levels. The clarification systems are self-contained, trailer-mounted units that treat water with high turbidity, high total organic carbon (TOC), algae, or very cold water. The clarifiers use microsand as a “seed” for floc formation. The microsand provides increased surface area for floc attachment, and acts as a ballast or weight. The resulting sand-ballasted flocs have higher settling velocities than do flocs produced in conventional clarifiers. Turbidity of the settled water is typically less than 1.0 NTU (suspended solids below 5 milligrams/liter).

The clarifier works as follows: raw water enters the coagulation tank, where chemical coagulant (alum or ferric, etc.) is added to destabilize suspended solids and colloidal matter in the influent stream. Mixing is provided in this step to thoroughly incorporate the coagulant into the raw water. The coagulated water passes into the injection tank, where polymer and microsand are added to initiate floc formation. In this tank, the polymer acts as a bonding agent between the microsand grains and the coagulated matter, forming flocculated particles many times the size of the individual coagulated matter.

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At a nominal 100-micron diameter, the microsand is much larger and denser than the lighter coagulated particles, and the larger sand grains, covered with polymer chains, act as floc scavengers to collect and capture floc particles in the flow stream. The larger sand grains increase the occurrence of floc collision, resulting in higher water clarity.

Next, the water passes into the maturation tank, where gentler mixing provides ideal conditions for the formation of polymer bridges between the microsand and the destabilized suspended solids. The fully formed ballasted flocs leave the maturation tank and enter the settling tank, where the weighted flocs settle to the bottom of the clarifier. The clarified water flows upward through the lamella tubes in the clarifier zone, and exits the system via a series of collection troughs or weirs for subsequent filtration and disinfection.

The microsand with the floc attached settles to the bottom of the clarifier, where it is collected and recycled by a centrifugal pump to the injection hydrocyclone. The hydrocyclone separates the sand from the sludge, and the sand is re-injected into the process.

On February 16th, USFilter delivered a mobile clarification trailer to the Rosemount facility.

“We liked the fact that USFilter could get a trailer to the site quickly and had the technical support to get it up and running for us,” said Larry Rogacki, process engineering manager at MCES. “USFilter employees were there every day during start-up. After the clarifier was operating, they were there Monday through Friday, and we took over on the weekends. We knew we could always reach them by pager if we needed to get in touch with them.”

USFilter modified the trailer to withstand temperatures as low as -12°C and winds of 60 miles per hour. The modifications included wrapping the chemical tank in hay bales to insulate it against the extreme temperatures. Heaters were placed both inside and underneath the trailer, and a structure was built over the trailer to hold a special cover to keep out the high winds.

The clarification system began treating water on February 21st, and four days later the total phosphorus level of the treated effluent was 0.75 mg/l, comfortably below the permitted 1.0 mg/l maximum.

“We met the conditions of our permit in March,” said Rogacki. “If we had gone the capital equipment route, it would have taken too long to solve the problem, putting us at risk for significant noncompliance.”

The trailer operated continuously at the site for three months until the weather warmed up and the suspended solids removal was no longer an issue. MCES is currently modifying the disc filters so they can perform effectively during the coldest months.

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