Miramar Water Treatment Plant Upgrade and Expansion Project, San Diego, USA

Although little more than half-way through its twelve year course of construction, the work to upgrade and improve Miramar has already received recognition as an "outstanding civil engineering project" by the San Diego section of the American Society of Civil Engineers.

The project forms a major component of the San Diego City Water Department's capital improvements programme – initiated in July 1998 – to increase capacity and reliability to meet the current and future water needs of its fast-growing population. The work will increase the plant's daily treatment capacity by 285,000m³ to a final 815,000m³/day and provide a state-of-the-art facility with a 75 year design lifespan.

Implemented in six component phases, the scheme includes a new rapid mix facility, de-aeration basins, disinfection and chemical facilities, new and refurbished administration buildings, flocculation and sedimentation basins, wash-water recovery system and water filters. In addition, new pipelines have been laid and other preparatory improvements have been undertaken outside the WTP site itself.

The project cost will be slightly over US$200m, with the plant's upgrade and expansion accounting for $171m of this budget.

BACKGROUND

A combination of growing demand, ageing infrastructure and regulatory requirements acted as drivers on the project.

The City of San Diego's current population stands at more than 1.3 million and it is projected to rise by a further 600,000 by 2030. The Miramar plant is the sole source of drinking water to the half a million inhabitants of the northern section of the city – itself the fastest growing region in San Diego County. With peak demand in this service area already having exceeded the plant's existing 530,000m³/day capacity, ongoing local water shortage has been predicted by 2008.

In common with the city's two other WTPs, Miramar currently uses conventional water treatment processes – coagulation, flocculation/sedimentation, filtration and disinfection – to provide a multi-barrier approach to comply with federal and state drinking water standards. Ferric chloride and an organic polymer are added in the coagulation step; chlorine forms the primary disinfectant, ammonia being subsequently introduced to react with the chlorine to form chloramines providing a residual disinfectant throughout the distribution system. The water is also treated with potassium permanganate to improve taste and odour, and sodium hydroxide to adjust the pH, before being supplied.

However, having first been brought into service in 1962, much of the facility and its associated equipment is now approaching the end of its useful life-span and is required to be either rehabilitated or replaced.

These factors, coupled with recent changes in drinking water regulations and the California Department of Health Services compliance order – issued in 1997 – make the expansion and upgrade of Miramar essential to the city's future ability to supply treated water.

PROJECT IMPLEMENTATION

The project has six constituent elements – a two-phase programme of early start improvements which spanned 1998–2004, followed by four further contracts which take the work through to its anticipated completion towards the end of 2010.

Phase I of the early start improvements involved the construction of distribution pipelines, the valve vault and tunnel, together with modifications of the raw water pump station. In phase II, the distribution pipeline was connected to existing clear-wells and mass grading work was undertaken to prepare the plant for subsequent construction. In addition, a new Diego County Water Authority feed pipeline and temporary chemical facilities were also provided as part of this phase.

Beginning in the summer of 2003, contract A forms a major construction component of the overall project. It comprises the provision of new filters, rapid mix facilities, de-aeration and ozone facilities together with chemical and main administration buildings. Completion of this part of the scheme is expected by autumn of 2007.

Additional flocculation/sedimentation basins are to be constructed as part of contract B, which also includes the demolition of the existing filters and flocculation/sedimentation basins. The existing operations building will undergo a programme of rehabilitation as part of this phase of the work, which – when completed in the autumn of 2009 – will ultimately expand the plant's capacity by 285,000m³/day.

The two remaining contracts – C and D – involve the provision of the ozone equipment and ozone generation facilities and a final clean up/landscaping, respectively.

San Diego's capital improvement programme has the stated objective of providing a safe and reliable water treatment and distribution system, offering state-of-the-art facilities at the lowest possible cost to its consumers. A number of the new design features are intended to make this possible.

With around 90% of the raw water arriving at the city's WTPs drawn from the Colorado River or northern California and conveyed along a series of pipelines, pump stations and conduits, large quantities of air can become entrapped in the micro-bubbles which form. The new de-aeration basins – constructed as part of contract A – have been included to aid the removal of this air, which will greatly enhance downstream water filtration performance.

The upgraded plant will include ozone disinfection. In order to arrive at a final design which was robust, cost effective and efficient, computational fluid dynamics modelling was used to investigate the flow characteristics of the proposed contactor – largely examining the straightening vanes which were intended to reduce dead zones.

Computer simulations performed with and without these vanes revealed that they had no effect on residence time – enabling them to be omitted from the final design and saving around $300,000 in construction cost.

In addition, the new waste equalisation basin will improve the backwash and the 'filter-to-waste' cycle – typically around 5–10% of the total flow – and up-to-the-minute technology monitors water levels and regulates pressure within the distribution system.

KEY PLAYERS

The City of San Diego Water Department is the plant owner/project sponsor. Archer Western worked on the early start improvements and contract A was been awarded to Western Summit. COM provided computational fluid dynamics modelling for the ozone contactor design.

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