- 27 July 2006 -

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Previous page: How does the clarifier work?

Case study: improving effluent quality in Minnesota, USA

A Minnesota , USA-based wastewater treatment facility found mobile clarification to be a viable solution when it was experiencing problems with effluent quality.

The 0.75-million-gallon-per-day (MGD) aerated lagoon treatment plant was permitted to discharge wastewater with a maximum average monthly total phosphorus level of 1.0 milligram per litre (mg/l) - but the effluent from the facility's newly installed disc filters was exceeding the US Environmental Protection Agency's National Pollutant Discharge Elimination System (NPDES) permit limit. The plant's operator knew that the extremely cold temperatures would cause problems with total suspended solids (TSS). The disc filters were installed to remove TSS and the associated phosphorus.

Soon after the disc filters were placed online, however, the plant's operators determined that the phosphorus concentration was not adequately reduced. The engineers sent a feed water sample to Siemens Water Technologies 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 - so that 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 were able to remove them.

Siemens suggested high-rate mobile clarification to remove the suspended solids and thus reduce phosphate levels. A trailer was delivered to the facility, and modified to withstand temperatures as low as -12°C and winds of 60 miles per hour. Heaters were placed both inside and underneath the trailer, and a structure was built over the trailer to keep out the high winds.

Four days after the mobile clarifier began treating the water, the total phosphorus level of the treated effluent was 0.75 mg/l, comfortably below the permitted 1.0 mg/l maximum. The facility was able to meet the conditions of its permit the following month.

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.

Case study: avoiding plant shutdown in a Midwest, USA, refinery

Timing is everything when it comes to a scheduled plant outage. But sometimes even the best plans are derailed when a piece of water treatment equipment fails. This is what happened at a US refinery when its conventional clarification system stopped working, some time before the plant's actual scheduled shutdown. The refinery had planned for one clarifier to be taken off-line for the outage, and for three temporary mobile clarification trailers to be installed and operational well before then. However, before this could happen, the plant operators found that the conventional clarifier had failed and they needed the trailers sooner than planned in order to avoid costly production downtime.

The operators therefore requested a mobile clarifier on an emergency basis to treat the refinery's water while the facility repaired the conventional clarifier. The clarifiers were operating in three days, and were making water that met the refinery's specifications in five days. Combined, the three clarifiers delivered approximately 2,900 gpm of treated water.

The mobile clarification trailers remained on site for six weeks, allowing the plant operators to repair their failed clarifier without having to shut down the plant before the normally-scheduled outage.

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