Thornton Water Treatment Plant Replacement

Summary

We provided design-build services for a new water treatment plant with a treatment capacity of 20 million gallons per day (MGD). The plant also can treat two water sources, Standley Lake and the South Platte River. Additional hydraulic capacity (up to 30 MGD) and the ability to treat a future third water source are also included in the design. Our role included everything from final design criteria development through detailed design and construction, including pilot testing. The six-month pilot included preoxidation, pretreatment, intermediate ozone and biofiltration. 

Garney Construction was our partner on the design-build team, and Carollo was the city's owner's adviser. Working with Garney, we generated value engineering ideas that ultimately saved significant funds for the city, and we contributed substantively to the maintenance of the overall project schedule.

The process design includes raw water oxidation, pumped rapid mix, flocculation, sedimentation with plate settlers, advanced oxidation with ozone and hydrogen peroxide, biologically active filtration and chlorine disinfection. Treatment goals include oxidation of iron and manganese, reduction of taste-and-odor associated with MIB and geosmin, and mitigation of bromate. 

The project also included development of a greenfield site and connection to the existing distribution system. 

The overall project schedule was allowed for design and construction to happen at the same time. The pilot period was also simultaneous with design work. The project was divided into multiple work packages for construction and permitting, which allowed for early procurement of equipment and a construction start before final design was complete. These three factors saved 15 months on the schedule. The goals of the pilot were to achieve a variance for an increased filter loading rate (resulting in a smaller footprint required for the water treatment plant), to confirm and finalize design criteria values, and to test emergency scenarios and mitigation strategies. The pilot began during basis of design and ended near 30% design completion. 

The design submittal to the Colorado Department of Health and Environment included our corrosion models for blending ratios from the multiple water sources. Additionally, this development study reviewed similar systems, including the adjacent facility being replaced. The assessment reviewed the calcium carbonate precipitation potential, aggressiveness index, chloride to sulfate mass ratio, the Ryznar index, and dissolved inorganic carbon. The results indicated that the water is not aggressive. 

To address corrosion impacts in the unit process selection, ferric sulfate was used in lieu of ferric chloride to lower the chloride to sulfate mass ratio, which is a key indicator for a water's corrosivity. A caustic soda feed system addresses pH and alkalinity adjustment. 

Achieving date-certain operational status means that water is delivered to the distribution system by a certain date. In order to complete sufficient performance testing on a new facility, the completion date must be during a time of high system demand so maximum flow rates move through the facility. The existing plant could not treat one of water sources (EGL) due to the taste- and odor-causing compounds in the source water. The new facility includes treatment processes (ozone and biofiltration) that can treat the EGL water.

Services

• Construction management
• Engineering design

Features

Treatment processes include:

• Flash mix
• Flocculation
• Sedimentation
• Ozone
• Biological filtration
• Disinfection

Ozone, in combination with hydrogen peroxide as an advanced oxidation process, oxidizes taste and odor compounds present in one raw water source. Biologically active granular media filters are downstream of the ozone process. The biological filters remove suspended solids via filtration and organic compounds (including disinfection byproduct precursors and taste- and odor-causing compounds), as well as metals such as iron and manganese via biodegradation. Biofiltration is often used downstream of ozonation, as it removes the assimilable organic carbon (AOC) formed from oxidation of organic compounds by ozone, preventing growth of microbes and biofilms in the distribution system.