River Authority Increases Filtration Capacity

Trinity River Authority retrofits with cloth media filters

The Trinity River Authority (TRA) of Texas established its Central Regional Wastewater System (CRWS) in 1957 and began operations in December 1959. The original treatment plant served Irving, Grand Prairie, Farmers Branch and a portion of western Dallas. The plant has since expanded and now serves approximately 1.2 million people in the Dallas/Fort Worth area.

CRWS processes include a bar screen, grit removal, primary clarifiers, aeration tanks, secondary clarifiers, traveling bridge sand filters, chlorine disinfection and dechlorination. Thirty traveling bridge sand filters were sized to handle the plant design flow rates for tertiary filtration.

The Challenge

After many years of operation, several of the traveling bridge sand filters required full rehabilitation due to mechanical and performance degradation. CRWS also faced the additional challenge of soon needing another 100 million gal per day (mgd) (378,000 cu meters per day) of filter capacity. This prompted TRA to explore other filter technologies.
        
The Solution

TRA chose to retrofit the plant’s existing traveling bridge sand filters with AquaDiamond cloth media filters in order to remedy its concerns.

"We had sand filters that were in need of rehabilitation. After some research we found that the AquaDiamond units had a direct fit to our filters, with minimal structural changes, and provided higher flow capacities," said Mike Young, CRWS operations manager.

Each cloth media filter has more than twice the treatment capacity of one of the existing sand filters. CRWS initially retrofitted two of its existing traveling bridge sand filters with two filters. Each cloth media filter is designed to handle an average flow of 12 mgd (45,400 cu meters per day) and maximum flow of 24 mgd (90,800 cu meters per day). Today the plant operates six AquaDiamond filters.

Performance Testing

Shortly after installation, process performance tests were conducted on one filter for seven consecutive days. The filter was tested at the average design flow of 12 mgd (45,400 cu meters per day) for most of the testing. During the first three days, the peak flow and/or peak influent total suspended solids (TSS) concentrations were simulated for a period of two hours. Effluent TSS and backwash requirements were met during normal and simulated peak flow and solids conditions. Influent solids were as high as 58.90 mg/L during the test.

Measuring Results

The cloth media filters addressed CRWS’s treatment performance concerns. The targeted requirements for the filters are listed in the following table:

Figure 1. Target Performance Requirements

        Parameter                   

Influent

Effluent

        TSS mg/L

15

5

30

15

  •         Backwash rate limit,
  •         percent of throughput based
  •         on influent filter TSS

15 mg/L

3%

24 mg/L

6%

The operations staff was impressed with the reduction of backwash water created by the units.

"We now have six units handling more than 80 mgd on a daily basis with less return flows back to the front of the plant," Young said. "During peak flows, we have put more than 150 mgd through these filters with TSS results still in the 1 to 2 mg/L range. The biggest advantage of the units is the ability to handle 12 to 25 mgd with less backwash, resulting in a capacity gain at the front of the plant."

Robert Batek, P.E., is a senior application engineer, and Eric Roundy, P.E., is a project application engineer at Aqua-Aerobic Systems Inc. Batek can be reached at rbatek@aqua-aerobic.com. Roundy can be reached at eroundy@aqua-aerobic.com.

Company Information

One of filters retrofitted into an existing traveling bridge filter basin.
  • http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/diamond.jpg
    One of filters retrofitted into an existing traveling bridge filter basin.
  • http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/cloth.jpg
    Cloth media used at the wastewater system.