Denver's Metro Wastewater Reclamation District is nearing peak construction on its $212-million PAR 1085 South Secondary Improvements project at the district's Robert W. Hite Treatment Facility north of the city. The construction cost is nearly $135 million.

The project will continue the process of upgrading the aging 1960s-era treatment facility by adding capacity, improving operating capability and upgrading treatment methods to meet more stringent effluent requirements for nitrogen and ammonia removal, as required by the district's discharge permit. To meet the new permit requirements, the upgrade must be completed by January 2015.

The district is currently undergoing one of the largest conversions from pure-oxygen treatment to biological nutrient processes of any wastewater district in the U.S., according to Metro Wastewater's public information officer. At completion, the south aeration complex will be able to reach 214 million gallons a day of peak capacity. The Robert W. Hite Treatment Facility is the largest wastewater treatment plant between the Mississippi River and the West Coast.

The Metro District serves nearly 1.7 million people in a 715-sq-mile service area that includes Denver and 45 water and sanitation districts in several of its suburbs. The district currently treats about 140 mgd of wastewater, discharging it into the nearby South Platte River, where for nine months of the year it constitutes nearly 85% of the river's flow northeast of the plant.

The facility's collection process includes taking in 140 mgd from sewer lines through 232 miles of inceptor sewers, treating it in about 14 hours and removing nearly 95% of pollutants to make the river discharge clean enough for agriculture, industrial use, fish habitat and recreation.

The district also produces about 80 dry tons of biosolids a day—treated solids that result from cleaning wastewater—most of which it provides for use as "cake" on agricultural land in eastern Colorado. The Environmental Protection Agency awarded the district a first-place national award for its biosolids management in 2004. The remaining 5% of biosolids not applied to land are mixed with wood chips and turned into compost.

Complete Upgrade

Work on the South Secondary Improvements project includes adding six activated sludge aeration basins, plus associated centrate and return activated sludge re-aeration basins, five new pump stations and other upgrades to existing systems.

Denver's Carollo Engineers is the design engineer and owner's representative on the project, responsible for construction management, inspection services and engineering services during construction. Carollo substantially completed design in mid-2010, and construction began early last year, according to John Luna, the firm's construction manager.

He says that the 16-month design effort was split among the firm's three offices—Denver, Phoenix and Orange County—using 4D modeling to help track "the more than 12,000 highly integrated activities in the schedule," which also includes hundreds of maintenance operation protocols, or MOPs. It is the largest virtually modeled job Carollo has done so far.

All of the construction pieces must be precisely coordinated to avoid any need for unscheduled shutdowns or operational surprises, "because there's no such thing as a 'good' surprise in wastewater treatment plant operations," Luna says.

The improved nitrification/de-nitrification treatment process being installed on South Secondary was piloted on the recently completed $67-million improvements to the facility's north side, also designed by Carollo and built by the Denver office of Garney Construction. That project finished more than a year early.

The process treats ammonia using centrate and return-activated sludge re-aeration basins, or CaRRBs. The activated sludge process uses centrate with a high ammonia concentration as a source that the re-aeration basins convert from ammonia to nitrate. The CaRRB process takes far less space to provide comparable treatment capacity. Its use on both the north and south sides of the facility are among the first full-facility applications of the process in the U.S., Frank says.

The joint venture contracting team of Western Summit/McCarthy (WSMC) started construction last year on the facility's south side, which includes building demolition, substantial concrete work and under-slab piping, extensive dewatering of groundwater, relocation of utilities and installation of primary effluent bypass pumping.

The project is being done in two parts. Completing construction on the new south aeration complex and upgrading secondary clarifiers and the high-purity oxygen system, or HIPOS, on the existing western half will mark the first big project milestone. That must be complete and operational by February 2014 for WSMC to earn a $1.8-million bonus, or about $10,000 per day.