Issue

Until recently, the Cholla 30 mgd WTP provided drinking water to the central and southern portions of Glendale. The city’s system-wide running annual average total trihalomethanes (TTHMs) concentration was below the Stage 2 Disinfectants/Disinfection Byproducts Rule (D/DBP Rule) limit of 80g/uL, but this count was rising.   

SOLUTION
Malcolm Pirnie conducted pilot testing of GAC post-filter adsorbers for DBP formation reduction for Glendale in 2001. The pilot testing results led Glendale to consider GAC to reduce DBP formation.

Glendale contracted Malcolm Pirnie to pilot test GAC monomedia filter-adsorbers and design a filter retrofit to convert their dual media granular filters to GAC monomedia. This would achieve adsorption removal of total organic carbon (TOC) species in addition to particulate removal, resulting in lower capital investment than a post-filter GAC process.

The filter retrofit project included other features to improve TOC removal, including: 

• Carbon Dioxide Addition for Enhanced Coagulation 
  A permanent carbon dioxide storage and feed system was included to reduce pH before or after pre-sedimentation.
• Enhanced Sedimentation Sludge Removal 
  SuperScraper[tm] sludge removal equipment was installed at the end of the sedimentation basins to remove sludge that accumulates under the finger weirs. Improving sludge removal is anticipated to reduce the release of organics by into the process flow.
• GAC Filter-Adsorbers 
  Cholla's four granular media filters were converted to GAC monomedia filter-adsorbers. Backwash troughs were raised to increase maximum bed depth from 39 inches to 68 inches, the underdrain system was replaced with a stainless steel underdrain that attached directly to the filter floor slab, and the carbon steel surface wash system was replaced with a stainless steel system. Pilot test results did not show any benefit to air scour for cleaning of the media over water wash alone. However, filter-adsorbers were provided with piping stubbed out to the pipe gallery for potential future use of air scour to prevent the need to work in the filter cells in the future. Two additional filter-adsorbers were constructed to provide an empty bed contact time (EBCT) of 10 minutes at design flow.
• Filter-to-Waste Capability 
  Filter-to-waste capability was provided for the filters, which were originally constructed in the 1970s.
• Backwash and Filter-to-Waste Equalization and Pumping
  The WTPs existing backwash water equalization basin was modified to also handle filter-to-waste flows prior to pumping to the solids handling facilities.  
• Turnkey GAC Services Access 
  GAC transfer stations were retrofitted to the exterior of the filter piping gallery. Truck drainage and washdown water is fully contained and sent to the equalization facilities.

BENEFITS
Malcolm Pirnie has been conducting a follow-up water quality optimization study with Glendale to maximize the benefit of the new facilities and the value of GAC. Operations staff have seen improved treatment performance through the entire process through optimization of enhanced coagulation and chlorination processes, including reduced chemical usage.  The $14 million upgrades have already resulted in a reversal of running annual average TTHM concentrations in the distribution system.