Design-Build Accelerates WWTP Expansion Project

The design-build construction delivery approach is being used with increasing frequency on municipal wastewater projects, as its benefits receive broader recognition. Design-build differs from the conventional design-bid-build scenario in that some or all of the design development is assigned to the prime contractor, which may be either an engineering or a construction firm.

The design-build construction delivery approach is being used with increasing frequency on municipal wastewater projects, as its benefits receive broader recognition. Design-build differs from the conventional design-bid-build scenario in that some or all of the design development is assigned to the prime contractor, which may be either an engineering or a construction firm.

On public projects, the owners design staff typically develops the preliminary specifications and project scope, which form the basis of the bid. Pre-qualified bidders then compete for the role of design-build prime contractor responsible for everything from design calculations to procurement to construction.

The biggest attraction of design-build for public clients is often the one-point responsibility it offers. In the traditional construction process, engineers and construction contractors can always point the finger at each other when something goes wrong. But a firm that both designs and builds a facility or system can be held solely accountable for meeting the citys expectations. Other important advantages include an earlier knowledge of real project costs, smoother implementation of design changes during construction, and significant time savings.

It was the potential time savings that prompted the City of Memphis to take the design-build route when faced with an urgent need to expand its 20-year-old Maxson Wastewater Treatment Plant. In Memphis, industrial waste producers usually pay the city to treat their wastewater at the plant instead of building their own treatment facilities.

The system has worked efficiently for many years. But in 1995 two local food processing facilities simultaneously announced plans to expand. The city recognized that its existing, 80 mgd wastewater treatment facility would not be able to handle the increased solids loads that these expansions presented. It had to increase its handling capacity, and do so quickly.

Familiarity Breeds Confidence

Memphis administrators had used the design-build approach before with success, and they had a list of qualified bidders that they felt confident could handle the Maxson expansion. The citys engineering staff developed a preliminary design including one 180 foot diameter primary clarifier; two 135-foot diameter activated biofiltration trickling filter towers with three 350-horsepower recycle pumps; a belt filter press; two final clarifiers; a new hydraulic piston pump to improve the function of the belt conveyor; and other pumps, electrical equipment, piping, controls, conveyance systems and appurtenances necessary to complete the system.

Familiarity Breeds Confidence

From the 10 prequalified bidders who competed for the job, Black & Veatch Engineers was selected as the low bidder and was charged with design, permitting, procurement, construction, testing and startup for the project. A lump sum contract was issued for $12 million with a mandate to complete the expansion within two years.

Familiarity Breeds Confidence

By the time the bid was awarded, a great deal of the procurement work had already been performed. During the intensive pre-bid cost assessment process Black & Veatch had spoken or met with dozens of equipment and material suppliers to determine the cost and technical appropriateness of their products, and incorporated the information into the bid. This not only resulted in a very competitive bid, but also provided a head start on the project.

Value Engineering

The bidding process also presented an opportunity to offer value engineering ideas to the city. The engineers were very familiar with the Maxson plant, having worked on 12 prior modifications and upgrades since 1972. Based on this experience and the citys drawings, 13 design alternatives were developed which were ultimately adopted.

Value Engineering

One of these was the replacement of a costly hydraulic piston pump which had been specified to enhance sludge dewatering by improving the performance of the belt conveyor. A simpler solution was offered consisting of a conveyor extension, new lime/sludge blender, and second conveyor modification, for a net savings of about $150,000.

Value Engineering

A number of pipe material changes were also recommended. The city was shown where different types of pipe could provide comparable life and performance for less money. The final design called for the use of steel pipe on the biotower influent pipe, to connect with steel pipe that was already in place. Ductile iron pipe was used for the primary sludge pump discharge and dewatering tunnel, while ADS pipe was selected for the filtrate drain line.

Value Engineering

The location of the final basin influent pipe was riddled with obstacles that precluded good bedding and soil compaction. Prestressed concrete cylinder pipe was accordingly selected for this application, due to its stiffness and ease of installation. The pipe was readily suited to the custom design required in the treatment plant piping system, with obstructions and potentially difficult bedding and backfill situations. The pipe was encased in a thick Portland cement mortar coating, making it highly corrosion resistant as well.

Value Engineering

In addition, Hobas fiberglass pipe, corrugated high density polyethylene pipe, and regular HDPE pipe were also used on the project.

Fast Track Approach

A big source of time savings on a design-build project is the ability to begin construction before the design is complete. That was true on the Maxson project, where the subcontractor, Max Foote Construction of Louisiana, was installing equipment just one month after the bid was awarded, long before all the final design details had been worked out. Just as important, from a time-savings point of view, was the elimination of delays from faulty communication that can result when a number of different parties are involved. Since the team had all been an integral part of the design process, it allowed for knowledgeable, on-the-spot decisions when construction questions arose.

Combining Clarifier, Thickener

Work with an independent consultant, Orris Albertson, also enabled the design-build team to explore an idea for improving the design after construction had begun. Instead of installing the originally specified primary clarifier, the notion arose of combining the clarifier with a thickener, to thicken the sludge from all four primary clarifiers.

Combining Clarifier, Thickener

This had never been done before, and no one could guarantee that it would work. But changes that had already been made in the piping layout to accommodate other design modifications supported this option. Moreover, if the idea didnt work, it would be relatively painless to return to the original plan. The city approved the change, and because the parties had worked so closely on developing the concept, the cost increase was easily negotiated.

Combining Clarifier, Thickener

Throughout the project the parties put a premium on frequent, detailed communication. City representatives met at regular intervals with Black & Veatch to discuss and approve design concepts. During construction, the citys resident engineer was constantly on hand to consult with the design-build team and its three major subcontractors. This constructive interaction kept problems at bay and made it easy to incorporate minor changes as the project progressed.

Combining Clarifier, Thickener

The Maxson Wastewater Treatment Plant expansion was completed four months ahead of schedule and 20 percent under the initial budget. The new facility is operating even better than expected: the four presses are running at 5,500 pounds per hour, and the combination clarifier/thickener has raised primary sludge solids concentrations from 4 to 7 percent. All of the Citys wastewater customers are being served, and the facility is well within its permit discharge limits.

Combining Clarifier, Thickener

Projects like this prove that design-build can work in public construction. What owners must bear in mind is that the success of such a project will depend almost entirely on the expertise of the prime contractor. They should ensure, therefore, not only that the contractor is properly insured, but that it is staffed with experienced people who bring a familiarity with wastewater treatment processes to the job. By doing so they will greatly enhance their chances of a successful outcome.

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