Nogales International Wastewater Treatment Plant Rehabilitation Project, Arizona, USA

Scheduled for completion in October 2009, the upgrade of the Nogales International Wastewater Treatment Plant represents the culmination of years of planning involving a variety of agencies at local, state, federal and international levels.

Located in Santa Cruz County, on Arizona’s international boundary with the Mexican state of Sonora, the plant treats wastewater from both sides of the border. Though this makes it a unique facility, its administration has inevitably been complicated by the legal, political and environmental issues that have arisen as a result of this distinctive status – leading to the current rehabilitation project itself being delayed for a number of years.

The work includes significant upgrades to the secondary treatment to meet regulatory standards for TSS and total nitrogen discharge, together with improvements to disinfection and sludge treatment. In addition, the International Outfall Interceptor (IOI), which transports wastewater from both sides of the border to the plant, will receive much-needed repair.

In December 2005, the City of Nogales, Arizona received a $59.5m grant agreement from the North American Development Bank to meet the project costs. The plant upgrade itself is expected to cost $50m.

BACKGROUND

The Nogales International Wastewater Treatment Plant was first conceived in 1943 to provide a regional approach to the issue of wastewater treatment and disposal. Serving the cities of Nogales, Arizona and Nogales, Sonora, together with the surrounding areas of Santa Cruz County, the original works was designed to meet the needs of 20,000 people. As this population has grown over the years, the plant has been subject to a series of upgrades and enhancements.

Between 1990 and 2000, the population on the Mexican side grew from around 108,000 to more than 206,500. With an eye on the increase which was taking shape, in the mid-1990s, the US Environmental Protection Agency sponsored a bi-national discourse on wastewater management. This led to a variety of options being considered including treating Mexican sewage in Mexico, at the existing international facility or operating treatment works on either side of the border.

As a result, plans emerged for a major renovation of the Nogales facility and in 1995, international agreement was reached. The US Border Environment Cooperation Commission certified the concept and the Environmental Protection Agency authorised a $60m grant for the project.

Unfortunately, forging agreement between the wider stakeholders was to prove more troublesome and considerably more time consuming. A number of issues, including operational costs, responsibility for over-spending and planned capacity allocation proved difficult to resolve.

One of the main drivers on the need to upgrade was the continuing environmental and public health hazard posed by the plant’s illegally high discharges into the Santa Cruz River and the Nogales Wash. In 2000, the Sierra Club filed a legal action alleging ongoing and continuous violations of the Clean Water Act, which ultimately resulted in a court-ratified agreement that the plant would comply with federal standards by 2004 – a deadline which came and went.

Facing the possibility of losing federal funding, the Arizona Department of Environmental Quality took the driving seat and policy and technical committees were formed to progress the work. Ultimately, with consensus reached over the major sticking points and the technical specifications of the project, everything was in place to enable work to start on the long-awaited programme of rehabilitation.

THE PLANT

Described as a "low tech, low cost plant", the facility treats water to a secondary level, removing floating debris and settled solids to exclude around 75% of the influent organics. Ammonia and hydrocarbons are not specifically removed and the plant has problems meeting its TSS and nitrogen discharge limits.

Currently it provides primary screening and grit removal at the head-works, secondary treatment comprising both complete-mix and partial-mix lagoons with subsequent filtration. The final effluent is disinfected before discharge using chlorine and UV; the sludge is periodically removed. The rehabilitation project calls for the head-works and primary treatment facility to have a new medium screen installed in parallel with the existing coarse one, along with a new washer/compactor conveyor system. Two new aerated grit chambers will also be constructed, each 520m³ in volume and a biofilter odour control system put in place.

The main upgrade to secondary treatment is the intended provision of a new Modified Ludzack-Ettinger (MLE) system, with a target effluent total nitrogen of 10mg/l as a five-month rolling geometric mean and a 90% confident limit of 8mg/l. The two process basins will between them provide a total anoxic volume of 3,800m³ and 24,600m³ in total aerated volume and an internal recycle of up to 3.3 times the design in-flow.

In addition, four 28.5m diameter final clarifiers are also planned, the disinfection facilities are to be upgraded and improvements are to be made to the arrangements for solids including sludge thickening and dewatering.

KEY PLAYERS

The City of Nogales, Arizona and the International Boundary and Water Commission (IBWC) are co-owners and operators of the plant. Project funding is being provided by a grant from the North American Development Bank (NADB), through the Border Environment Infrastructure Fund, financed by the US Environmental Protection Agency (EPA).

The technology evaluation was done by Camp Dresser and McKee. Atkins an Faithful & Gould have been appointed to provide the design review and project construction management. The working group consisted of the Arizona Department of Environmental Quality, the EPA, the Border Environment Cooperation Commission, the University of Arizona, along with the NADB, the IBWC and the City of Nogales itself.

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