North Davis County Sewer District

"Expanding and Upgrading Wastewater Treatment Plant"

What's the most effective way to expand and upgrade a wastewater treatment plant in the United States?

MWH has taken on this challenge with North Davis County Sewer District. The US $75 million design-build project involves upgrading and expanding the District's wastewater treatment plant from an average daily wastewater treatment capacity of 25 mgd to 34 mgd. It will have a maximum capacity of 65 mgd, and will serve the District's needs until at least 2025.

The regional facility serves seven communities south of Ogden, Utah, as well as several industrial wastewater users including Hill Air Force Base. The plant meets government requirements for discharges into the Great Salt Lake. However, winter and spring weather can cause effluent suspended solid concentrations to approach the approved limit.

The facility also uses solar drying beds to dewater biosolids. This causes periodic seasonal solids backup and contributes to the effluent quality problems. Through a series of design workshops with the District, MWH incorporated solutions to these challenges in the plant upgrade project.

Project Funding

From the beginning, MWH helped secure funding for the project. We continue to look for ways to conduct the work in a cost-effective manner, using innovative technological tools and procurement strategies.

Our professionals assisted the District in securing US $48.5 million through a general obligation bond election. This passed with a 90 percent voter approval level. We worked with the District and their bonding company to develop a public information campaign outlining the need for improvements. This was presented at city council meetings and public meetings in the communities served by the North Davis County Sewer District.

The District also secured a US $20 million low interest loan from the State of Utah Water Quality Board. This was the largest such loan issued to date in the State of Utah.

Staging Projects

To extend the budget, MWH is maximizing the use of the existing plant. This includes converting the facility's two-stage trickling filter process to a biotower/solids contact (BT/SC) process, and adding mechanical biosolids processing. These plant improvements are designed to maximize existing facilities, while incorporating new ones within the existing process flow stream.

MWH is sequencing eight design-build task orders to be constructed over a five-year period. This approach considers the plant's operational needs, as well as the District’s cash flow and schedule for receipt of bond and loan proceeds.

Project Specifics

The eight task orders for the North Davis County Sewer District project include:

• Chlorine Scrubber – Adding 20-ton chlorine scrubbing system and building near the existing chlorine storage building to enhance public safety.
• Headwork Improvements – Expanding the headworks building to house new grit transfer augers and storage receptacles. Replacing existing bar screens with fine screens (step screens). Removing the existing screening conveyor system and replacing with a vacuum screenings transfer system. Adding aeration and grit removal improvements to existing aerated grit chambers, as well as replacing the headworks building HVAC system.
• Digester Mixing Improvements – Installing four external draft tube mixers on each of two existing 85-ft diameter primary digesters. The mixers replace existing pump mix systems.
• Solids Processing Facility – Constructing a new 25,000 square foot building to house biosolids thickening and dewatering equipment. This includes three gravity belt thickeners and four belt filter presses. The solids processing facility replaces existing solar sludge drying beds.
• Primary Clarifier Improvements – Adding a fourth 150-ft diameter primary clarifier and installing additional air diaphragm pumps and sludge grinders in the primary sludge pump station.
• Biotower/Solids Contact Process - Converting the existing two-stage trickling filter process to a single stage process. Building two 120-ft diameter by 30-ft high biotowers, 90-mgd biotower/trickling filter recirculation pump station. Adding a solids contact process with eight aeration basins; blower building; return and waste sludge pump station; and three new 160 –ft diameter final clarifiers. Adding four 20-mgd pumps to the plants' influent pump station, and constructing a central warehouse building.
• Disinfection System Expansion – Building two additional chlorine contact chambers and a new utility water pump station.
• Plant Power System Improvements – Replacing 480-volt plant electrical power distribution system with a 12.7 kV distribution system. Including new substations, transformers, and switchgear. Also adding a 2 MW diesel emergency power generation system.

Three-Dimensional CAD

MWH applied 3-D CAD technology in the design process. The 3-D process allowed the District to visualize the proposed design, and make changes directly to the model. This sped up the approval process and eliminated the need for multiple, expensive design review drawings. It also helped identify conflicts that might have caused costly changes during construction.

The system also tracks all technical information associated with equipment, instruments, valves and pipes. Whenever changes are made, all drawings are automatically updated. Equipment data sheets can also be printed directly from the database for incorporation into the project specifications. This greatly streamlined the design preparation process.

This innovative use of advanced technology has become a model for other MWH projects around the globe.

Procurement Subcontractor Prequalification

To further save money and time, MWH procured most equipment before the design was completed. This not only shortened the construction schedule. It also ensured the use of quality equipment on the project, and reduced costs to the owner.

MWH served as the general contractor for construction of the project. We developed a pool of pre-qualified subcontractors in various construction disciplines eligible to bid on the task orders over the life of the contract. This helps secure competitively priced work for the District, and streamlines administrative aspects of the project.

Disruptions Minimized

For the North Davis County Sewer District, it was important that MWH could provide a single point of responsibility for phasing and coordinating construction and startup. This helps us minimize disruptions to plant operations. MWH has worked closely with subcontractors and plant staff using a team approach to execute critical activities in a way that reduces operational interruptions.

Key facilities (such as screening; influent pumping; biological treatment and disinfection) — which require bypassing or temporary alternate facilities to maintain operations during reconstruction — are handled by the same team during design and construction phases of the project. This team effort is paying off.

The project team devised a bypass of the existing vacuum system, when replacing the chlorine feed system piping and vacuum regulators. It minimized the chlorine feed system's downtime to five minutes.

This bypass saved about 8 hours in downtime. The extra time would have been required to replace the chlorine piping, while using an alternate disinfectant system (sodium hypochlorite feed) as originally planned. As a result, plant employees were able to hold water in the contact basins for the five minutes required for the bypass — and they prevented a discharge of untreated water.

Collection System Expansion

Under another contract, MWH is serving as engineering consultant for the expansion of the District's wastewater collection system. The District owns and operates approximately 100 miles of large diameter sewer outfall lines serving its 80 square mile region.

Planned improvements for the US $25 million project include traditional design-bid-build construction of about 20 miles of 24-inch to 60-inch diameter sewer outfall lines. These lines will replace five existing outfall lines to the treatment plant and increase capacity for future plant flows.