Capacity Boost

Dec. 11, 2008

Kruger Inc., a Veolia Water Solutions & Technologies Co.
401 Harrison Oaks Blvd., Suite 100 • Cary, NC 27513
tel: 919.677.8310 •

The city of Somersworth, N.H., has a historical background dating back to the early 1900s, when it became the first community to start using chlorine to disinfect its drinking water. The city worked with Wright-Pierce Engineers to preserve the historic value of the buildings at the Somersworth Water Treatment Plant (WTP)while improving both total organic carbon (TOC) removal rates and the plant’s disinfection regime. New technology had to be integrated into an 1895 plant environment, while maintaining existing operations during construction and upgrade of the plant from a 3-million-gal-per-day (mgd) flow to a 6-mgd flow.

Plant Overview

In 1971, a 1-mgd filter system was created. It was used until 1991. The Somersworth plant then experienced a second upgrade that included a single train upflow clarifier. The 1991 plant design capacity was 3 mgd to help with the city’s growth demands, but the existing finish water high-lift pumps limited pumping capacity at 1.4 mgd.

In 2001, the plant went through another upgrade to increase the finished water pump capacity to 3 mgd and integrate a plant-wide SCADA system. To screen new clarification processes, pilot testing was conducted in Fall 2004. The decision was made to use Kruger’s ACTIFLO microsand ballasted clarification process based on the results, process footprint, installation base and utilization of existing buildings.

A phased construction plan began in 2006 and included clarification, filtration, disinfection and sludge handling upgrades—all while maintaining 3 mgd of existing facility operations.

Process Solution

To answer the challenge, the city of Somersworth and project engineers from Wright-Pierce looked at different clarification and filtration options. Following onsite pilot testing work and detailed process evaluations, the project team determined that installing Kruger’s ACTIFLO clarification process in conjunction with converting the existing upflow clarifier tanks into dual media filters was the best solution for the project.

The ACTIFLO process consists of three process tanks with mixers followed by a settling tank. At the Somersworth WTP, alum is properly dispersed and thoroughly mixed through in the first process tank. The water and flocculated materials then enter the second process tank, where the recycled microsand is continuously injected. With the addition of polymer in the third process tank, heavy ballasted floc form and the high-rate settling begins as the flow exits the third tank and enters the settling tank. Settled material is pumped from the bottom of the settling tank to a hydrocyclone for separation and microsand recovery.

The use of microsand greatly enhances the settling rate of the flocculated material, allowing for a small process footprint and for the process to produce consistent effluent quality under varying influent water quality and flow rates.


The effectiveness of the ACTIFLO process followed by dual media filtration has allowed the Somersworth WTP to produce a finish water quality well below its current Stage II disinfection byproduct limits. Since its start of operations in the summer of 2007, the ACTIFLO clarification process has greatly improved the plant’s TOC removal capabilities year-round, especially during the difficult-to-treat summer months.

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