Colorado City Tackles Turbidity Treatment

DAF system proves it can handle high-turbidity application

Dissolved Air Flotation (DAF) has been characterized as a clarification process that is limited to low-turbidity algae-laden waters. Pilot testing and actual plant operations at the 63rd Street Water Treatment Plant in Boulder, Colo., has proven that this is not the case.

Background

The City of Boulder has two water treatment plants. The Betasso plant is the primary treatment plant and obtains raw water that is a result of high mountain snow melt that contains low turbidity, low hardness and low alkalinity. The raw water is gravity-fed into the plant. The Betasso treatment plant averages 14 mgd, but does not have the capacity to supply the total demand of the system. The other treatment plant—the 63rd Street facility—is a support plant that normally operates at 4 mgd during the winter and 8 mgd during summer months. This facility uses more energy than the Betasso plant because, depending on the source, raw source water must be pumped into the plant, and then treated water must be pumped into the distribution system.

In 2002, Boulder had experienced drought conditions that severely restricted its source water supply and necessitated that it reduce its water production at the main Betasso plant in order to minimize the quantity of raw source water used in producing water for its consumers. In addition, to maximize the city’s water rights, it became evident that there was a need to expand the 63rd Street plant. With the high cost of new construction, the city decided to look for a high-rate process that could be retrofitted into the existing clarification basins. The plant had a 12-ft-deep space that was 149-ft-by-132-ft containing a rectangular upflow solids contact unit and a rectangular equalization basin where chlorine was added prior to the filtration process. The Leopold Clari-DAF system’s small footprint and shallow depth offered the right clarification process to meet the facility’s needs for this application.

The raw water source for the 63rd Street plant is from the western slope basin that forms Carter Lake and then flows through an open canal to the Boulder Reservoir. The water in Boulder Reservoir is higher in minerals than the Betasso water source, which comes from a shallow body of water that is used as a recreational lake for a large portion of the year. In the summer months, when the canal has peak flows, the plant uses this source because it is closer in water quality to the Betasso water source and allows easy blending of the two effluents in the distribution system. The turbidity of the Boulder Reservoir is typically between 6 and 18 ntu, with occasional spikes as high as 100 ntu caused by high winds, heavy rains and spring thaws.

Pilot study

For purposes of simulating high-turbidity conditions during the pilot studies, the city dredged the bottom of the reservoir so that it could inject sediment directly into the pilot unit’s raw water piping. The concern with the ability of the DAF to handle high turbidity was based on the fact that heavier solids are associated with higher turbidities and, thus, it would be difficult to keep them suspended and float them to the surface. By changing chemistry, it was found that higher turbidities could be handled efficiently and effectively.

Conversion

It was decided that the 63rd Street plant upflow solids contact system and equalization basin rectangular tanks should be retrofitted with the Leopold Clari-DAF System during the low-demand season from November 2004 to February 2005. The plant was able to replace its filter pretreatment with an entirely new process scheme with only a five-month shutdown. Both the guts of the upflow clarifier, with the bottom of its tank filled in with concrete, and the reaction basin walls were removed so that a 149-ft-by-132-ft rectangular space with 12-ft sidewalls remained. With the higher designed loading rate of the Clari-DAF System, the capacity of the plant was doubled while using less than half of the rectangular space. Two 29-ft-by-132-ft Clari-DAF systems—each capable of processing 8 mgd—were installed. Each system was designed with hydraulic flash mix, 20-minute flocculation times, a 4-gpm-per-sq-ft loading rate and a 10% recycle rate.

The plant has used a number of clarification treatment chemistries to obtain the optimum clarified water quality. With conventional upflow clarifier technology, plant personnel used ferric sulfate, Sumaclear 820B and caustic. On start-up of the Clari-DAF system in March 2005, it switched to alum, Sumaclear 820B and caustic. It initiated the feed of cationic polymer in April 2005. Since March 2006, plant personnel use Sumaclear 803B as the primary coagulant with the cationic polymer and adding alum and caustic as needed. The cost of chemical treatment has declined and continues to drop since the advent of the Clari-DAF system clarification process. Total chemical costs are listed below in Table 1.

Table 1

   Clarification

Year

$/Million Gal

Upflow Clarifier

2004

153

Clari-DAF Startup

2005

151

Clari-DAF System

2006

109

This represents a 29% reduction in chemical cost for clarification.

The filter runs during upflow solids contact clarification averaged 40 hrs and have increased to 60 hrs with dissolved air flotation clarification. The plant has achieved 70 hrs between backwashes, but with fluctuations of demand, plant personnel prefer to use all of the filters and backwash at 60 hrs rather than allow some of them to sit out of service for extended periods of time. These increased runtimes are due to the Clari-DAF system providing better quality of water to the filters. Table 2 compares the turbidities of the two clarification technologies.

Table 2

 

Pre-DAF

Post-DAF

Average Daily

May, 2004

May, 2006

Raw Water

8.1 to 38.5

2.6 to 22.1

Clarified Effluent

0.72 to 3.13

0.23 to 0.47

Filter Effluent

0.04 to 0.09

0.02 to 0.08

Plant personnel have experienced turbidity spikes during the day from 20 ntu to 100 ntu and only saw a turbidity increase in the Clari-DAF effluent from 0.4 ntu to 0.7 ntu. In addition, daily pH swings from 7.5 to 8.5 have not had any affect on the Clari-DAF effluent turbidity quality.

Comparing the sludge solids during the same time frames listed above indicates upflow solids contact clarification yielding sludge solids concentrations of 0.70% and the Clari- DAF system yielding sludge solids concentrations of 4.0%. This increase in sludge solids has resulted in a significantly lower volume of solids that is pumped to the lagoon and a significantly shorter period of time for the solids to dewater to a point where they can be removed.

With the success of the new Clari-DAF dissolved air flotation system technology, plant personnel are already contemplating adding an additional 8 mgd capacity to the remaining unused rectangular tank space. This would still allow the luxury of using the additional unused space for equalization and chlorine contact tanks, if desired.

Company Information

227 S. Division Street
Zelienople
PA
724.452.6300
The 63rd Street water treatment facility in Boulder, Colo.
  • http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/Leopold%20CS%20CO%201.jpg
    The 63rd Street water treatment facility in Boulder, Colo.
  • http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/Leopold%20CS%20CO%202.jpg
    Data from the pilot unit’s SCADA system when raw water turbidities were as high as 275 ntu and DAF effluent was below 2.0 ntu.
  • http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/Leopold%20CS%20CO%203.jpg
    The old clarifier.
  • http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/Leopold%20CS%20CO%204.jpg
    The new Clari-DAF system.