HomeMy WebLinkAboutIR 9377To the Mayor and Members of the City Council
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No. 9377
April 19, 2011
SUBJECT: FILTER REPAIRS AT ROLLING HILLS WATER TREATMENT PLANT
The purpose of this informal report is to provide information regarding certain urgent repairs
required at the Rolling Hills Water Treatment Plant.
The Rolling Hills Water Treatment Plant (RHWTP) is the largest water plant operated in Fort
Worth, with a peak capacity of 200 million gallons per day. The treatment process consists of raw
water ozonation, coagulation, flocculation, sedimentation, filtration, and residual disinfection. The
filtration component of the plant treatment process is one of the most critical. At the RHWTP,
there are twenty filters, each rated at 10 million gallons per day, consisting of filter underdrain
blocks topped with a porous plastic cap followed by 48" thick granular anthracite filter media.
The RHWTP filters were retrofitted in 2003 and 2004 to increase the plant capacity to 200 million
gallons per day and to operate biologically for removal of organic matter produced during the
ozonation process. The filter retrofits included the installation of new underdrain blocks, which
are topped with a porous plastic cap that supports the anthracite filter media and evenly
distributes both the air and water across the media during backwashing. In the past, the RHWTP
has experienced problems with the cap pores becoming clogged by organic matter, resulting in
high uplift forces across the block during backwash events damaging the filter underdrain system.
On the morning of March 15th, during a routine backwash of Filter #3, a portion of the underdrain
blocks failed, lifting upward from the filter box floor slab.
Recommendation
In the subsequent weeks, City staff removed the anthracite media from Filter #3 and had the
underdrain system inspected by both the underdrain manufacturer and the design engineer. The
recommendation from both the manufacturer and the design engineer was to completely replace
the existing underdrain block in Filter #3. The replacement underdrain block would be installed
with 12" of gravel media over the blocks instead of the porous cap that is subject to clogging.
Furthermore, in order to measure and document uplift forces through the block during backwash
events, a pressure transmitter should be installed at each filter, recording and logging the
headloss through the filter during backwash events.
Authorization
During summer months, full treatment capacity at the RHWTP is critical in meeting water
demands. In order to make repairs and regain capacity prior to the peak summer water
demands, emergency procurement of a contractor to perform the repairs is required.
ISSUED BY THE CITY MANAGER FORT WORTH, TEXAS
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To the Mayor and Members of the City Council
No. 9377
April 19, 2011
On April 12, 2011, the City Manager's Office and the Department of Law authorized the work
required to make the repairs to the Rolling Hills filters under Section 252.022 of the Local
Government Code, which exempts from normal bidding requirements expenditures that are
necessary to preserve and protect public health or safety of the municipality's residents; and
procurements necessary because of unforeseen damage to public machinery, equipment, or
property. The authorization covers the following:
1. Filter #3 Repairs: Salvage or reuse of the existing underdrain system in Filter #3 is not
recommended. Therefore the existing underdrain blocks and grout must be removed and
replaced with new underdrain blocks and gravel media. A bid package is being prepared
and will be bid through an expedited bidding process utilizing pre-qualified contractors.
During repairs, two adjacent filters will be taken out of service in order to store the
anthracite media from Filter #3. The estimated cost to repair Filter #3 is $200,000.
2. Other Filter Modifications: Because of biological growth in the porous underdrain caps,
the engineer recommends removal of the caps and replacement with 12" of gravel media
on four additional filters prior to the summer peak demands. This will allow plant staff to
conduct full scale pilot testing to determine operation requirements without losing plant
capacity. Should this testing be successful, the remaining filters will be modified in the
future. The initial modification to four filters will be incorporated into the Filter #3 repair
contract. The estimated cost to retrofit four filters is $120,000.
3. Purchase of Pressure Transmitters: In order to monitor the headloss through each filter
and minimize filter underdrain damage due to excessive uplift forces resulting from clogged
underdrain caps, it is necessary to procure and install pressure transmitters to monitor,
record, and transmit the filter headloss data to the plant SCADA system. The estimated
purchase cost of the pressure transmitters is $35,000. Pressure transmitters will be
installed by city staff.
A confirming M&C will be circulated as soon as possible after the filter repair bids have been
received and the contract executed. Should you have any questions, please contact Frank
Crumb, Water Director, at 817-392-8246.
T&h-Higgins
Interim City Manager
ISSUED BY THE CITY MANAGER
FORT WORTH, TEXAS