Filter Improvement Project 259: NTMWD Water Treatment Plant in Wylie, TX
Clean drinking water supply is a growing concern throughout much of the US. Most of the water treatment infrastructure is coming to the end of its design shelf life and new construction costs can be prohibitive. Space for new facilities is at a premium but capacity must be increased.
The NTMWD was established as a regional waters system north of Dallas Texas in 1954 and it currently provides drinking water to over 1.6 million people in over 55 cities, towns, special utility districts and water supply corporations. The area is growing and population is expected exceed 3.7 million in the next 50 years. Some of the older plants in the NTMWD system are running inefficiently, leaking, and underperforming. Concurrent with conservation efforts and new construction the district wanted to improve the current facilities to better serve their member cities and customers.
Engineering Evaluation and Solution:
After non-destruction testing (visual inspection, surface penetrating radar, corrosion testing, and impact echo location) and destructive testing (coring for compressive strength and petrography) the decision was made to improve the filtration systems of the existing concrete structures instead of replacing the entire treatment facility.
Incomplete and unreadable existing drawings coupled with 35 year old geotechnical studies made the redesign challenging and an on-going process. Scheduling, shutting down parts of waters supply, and construction windows of opportunity also made it difficult to develop an accurate scope. Many of the design standards, product standards, and practices from original construction are not in use currently. There were risk factors, safety factors, and differential settlement issues that all had to addressed.
Many of the problems in the existing facilities could be traced to water leaks at joints, excessive cracking and subsequent leaks, as well as deterioration of inferior polymer waterstop products. The Filter Improvement project would have numerous areas where new concrete structures were to be joined to the existing structure. The Carollo Engineers were careful not to repeat the mistakes of the past and established the best practice methods to use the waterstops to seal the systems correctly.
Carollo has had many successful projects with J P Specialties all over the world and both companies gave much thought as to the best way to address the joining of new concrete structures to the existing facility. Traditionally this task was accomplished by the use of a standard embedded waterstop within a saw-cut and epoxy-grout filled channel. This labor-intensive and costly method has often produced questionable results due to the potential cold joint formed between the epoxy and existing concrete surface from shrinkage. Furthermore, by saw-cutting the existing concrete, the contractor may be inadvertently destroying the reinforcing steel within the concrete.
The best choice was to use a retrofit waterstop system that would provide a liquid-tight seal, ease of installation, and flexibility so the product could be installed in slab to wall applications, wall to wall applications. or wall to slab applications. The JP450T Retrofit waterstop system seals the interface both chemically (epoxy gel) and mechanically (SS batten bars and SS anchoring devices). The back of the waterstop that is bound to the existing concrete has a combed profile to increase the surface area and tightly bond the waterstop to the concrete.
One other key component of the choice of the JP450T was the waterstop polymer. The flexible PVC polymer that was developed over 50 years ago is still used extensively in many commercial applications. However, the filler material used to change the PVC from its normal rigid state to a flexible elastomeric product is a phthalates — Phthalates are a known carcinogen and are listed in the California Proposition 65 as a product known to cause cancer. Because of this, no PVC based waterstop has NSF certification for use with drinking water.
JP450T is manufactured from unadulterated, fully cross-linked thermoplastic vulcanizate. The compound has a rubber phase (EPDM), and a plastic phase (polypropylene), which are inter-linked to provide a rubber-like finished product that can be heat welded like a thermoplastic. This new generation of polymers provide superior performance in every category.
The advantages of the TPV polymer can be seen in the following table:
In some of the NTMWD plants a key component of the water treatment program is ozonation - the injection of ozone into water to kill various bacteria, viruses and yeast. The ozone process is quicker than the older chlorine process and does much less harm to the environment but the ozone is aggressive to many materials in used in construction. Specifically PVC will deteriorate in the presence of ozone and concrete joints will leak. PVC cannot be used in ozone facilities.
Installation of Waterstop:
PCL worked with J P Specialties to insure that the waterstop would be installed in a liquid tight fashion. Some of the important steps taken include:
Changes of direction for the JP450T included 90° surface mounted corners, 90° inside wall corners, 90° outside wall corners, and some specialty custom fabrications. Electronic prints were exchanged and J P Specialties provided the contractor with all fittings that had changes of direction. These joints can be done in the field but at our facility the miter cuts, the specialty irons, the controlled environment, the experienced highly trained crews, and QC testing certified that all fabs were shipped to the site pinhole free. The goal is to have the field welding just be straight edge butt-welding.
J P Specialties sent a certified plastic welding instructor to the job-site to spend a half day training PLC workers the correct welding techniques in a safe and effective manner. The training program included preparation, welding instructions, safety concerns, QC testing techniques, and hands-on welding. All attendees made at least 2 pinhole free butt-welds before they were designated as certified plastic welders.
Since some of the as built structures were not exactly what was on the 35 year old plans it was key that PCL and J P Specialties worked closely when new issues cropped up. J P Specialties had on site representation when phone and email correspondence were not adequate.
Current Status of NTMWD Filter Rebuilds:
There have been several major filter rebuild projects that have successfully used the JP450T and the owner, the engineers, the contractors, and the waterstop vendors are all pleased with the installation and performance of the JP450T.
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