Filter Underdrain Blocks Improve Backwash Efficiency

July 21, 2015
Philippines potable water system improvement project optimizes operational performance

Founded in 1997, the Manila Water Co. (MWC) serves the potable water needs of more than 5 million residents of the Philippines capital and cities to the east. The company serves as the private partner in a public-private partnership with the Philippines government in operating the Metropolitan Waterworks and Sewerage System for Metro Manila’s East Zone.

MWC’s Balara water facility consists of two separate treatment plants in Quezon City: Balara 1, with a design capacity of 470 million liters per day (mld), or 124 million gal per day (mgd); and Balara 2, with a design capacity of 1,130 mld (298 mgd).

Faced with anticipated regional population growth and ever-tightening water quality regulations, MWC underwent an exploratory study and then a pilot project to determine expansionary requirements that would enable the company to optimize operational performance and meet water quality and flow demand requirements at the Balara facility.

As a result of the successful pilot project, MWC embarked on a formal program in 2009 to upgrade the Balara facility. Refurbishment of the Balara 2 plant is expected to be complete by 2011. Refurbishment of the Balara 1 plant will follow.

Study Reveals Need

In 2006, MWC undertook a study of the capacity and process optimization at the Balara 1 and 2 plants. The study highlighted the need to improve chemical dosing and the operations of flocculation, de-sludging and filtration. Many of the shortcomings identified in the study were the result of equipment that needed upgrading—an ongoing process that has been in progress since the mid-1990s.

The Balara 1 and 2 plants were designed to operate at 470 mld (124 mgd) and 1,130 mld (298 mgd), respectively. Simple hydraulic analysis of both plants confirmed they were able to handle 600 mld (158 mgd) and 1,200 mld (317 mgd), respectively, without structural modification. At these elevated flows, however, acceptable water quality levels would not be achieved without operational enhancements and refurbishments to treatment equipment.

A number of treatment enhancements were identified to enable the Balara 1 and 2 plants to operate at the higher flow rates. One enhancement included the removal of existing filter laterals and surface wash pipe work within the single filter; installation of filter underdrains with media retention plates; installation of air scour blowers; and replacement of filter media to a single filter.

The 2006 study noted that the condition of the existing filters was, at best, poor. Filters were not being operated optimally or backwashed effectively, and run times were short. There was evidence of both media depletion and media expansion. Media depletion, particularly for anthracite, is to be expected; however, media expansion can only be caused by sludge and mud accumulation resulting from ineffective backwashing. Complete refurbishment of the filter underdrain and media replacement was required.

Key: Replacement of Underdrains

In 2008, MWC executed a pilot project to prove the viability of the proposed refurbishment and upgrades for the Balara water treatment facility recommended in the 2006 study. The pilot project involved the refurbishment of the chemical dosing system, a single clarifier and a single rapid gravity filter at the Balara 2 plant. The six-month pilot was a success, highlighting the ability of the fully proposed refurbishments to optimize operational performance, meet water quality and flow demand requirements.

As a result, MWC selected the TETRA LP Block dual parallel lateral underdrain , featuring the “S-Plate” media retention plate and Grout Grip design , from Severn Trent Services.

The TETRA LP Block features a proven design to provide superior distribution for both water and air, sequentially or concurrently. The blocks cover the floor of the gravity filters, supporting the filter media. Air is distributed evenly across the entire filter bottom area to scour the media and to provide an air lift which, with the water, removes the released solids from the media. This dual backwash action provides intense and extremely effective backwashing of the filter media. It was recommended that air scouring be introduced into the backwashing process. An effective backwashing process was especially critical at the Balara facility, where filtration rates were very high and bed depth was limited.

The LP Block underdrain features the unique and patented Grout Grip design, which increases resistance to uplift from the filter floor. Additionally, the wide, low-profile design of the TETRA LP Block requires the installation of fewer blocks to cover a filter floor. Therefore, fewer joints are created and less grout is needed.

TETRA LP Blocks will be installed in 10 filters at the Balara 1 plant and 19 filters at the Balara 2 plant. The S-Plate media retention plate eliminates the need for gravel support media and will enable an increase in media depth, thus improving the solids removal capacity of the filters.

The investment in the TETRA LP Block dual parallel lateral underdrain technology will yield multiple benefits for MWC and its customers.

About the Author

Bill Stimeling