Brightwater Project, Membrane Bioreactor (MBR), King County, WA, USA

Construction of the new Brightwater WWTP, which began with preliminary demolition and clearance work on the 114 acre site in November 2005, continues on schedule for the plant to enter service in 2010/11. When completed, the facility will have an initial peak flow capacity of 495,000m³/day, rising to a daily 645,000m³ by 2040.

King County selected Zenon to supply the membrane technology for what will become the largest membrane bioreactor (MBR) in the world and appointed Hoffman Construction as general contractor/construction manager.

In addition to the new plant itself, the project also requires the construction of a major conveyance pipeline system, together with a new marine outfall which will discharge the treated effluent directly into Puget Sound.

The cost of the MBR will approach $30m, with a further $50m allocated for state-of-the-art odour control measures; the total construction cost is estimated at $280m, with an additional $88m ear-marked for landscaping and mitigation.

BACKGROUND

The population of the Puget Sound region has more than doubled in the last 40 years and projections of the continued growth indicated that by 2010 the King County wastewater treatment system would have insufficient capacity.

In November 1999, the Regional Wastewater Services Plan was approved and the process of identifying and evaluating potential sites, marine outfall zones and construction methods for a new plant began the following year. This ultimately led in December 2003 to the selection of a site for the planned facility – known as the Route 9 site – near the City of Woodinville, with an associated conveyance pipeline along the border of King and Snohomish counties and an outfall off Point Wells.

With adequate wastewater treatment widely seen as one of the key factors in regional sustainable economic development, in 2004, the Puget Sound Regional Council placed Brightwater on its top ten list of priority projects.

After a comprehensive series of environmental impact assessments and an extensive programme of public consultation, this site selection was formally reaffirmed in July 2005 and preliminary work began four months later.

MEMBRANE BIOREACTOR (MBR) PLANT

Preliminary treatment of the influent will be conventional, using mechanically cleaned screens and grit removal, with both processes being covered and vented to minimise odour.

The primary treatment approach, which will treat most of the inflow into the plant, will also be of traditional design and consist of covered rectangular clarifier units, vented through an odour control system.

The secondary treatment process, however, will employ MBR technology as an alternative to the more conventional activated sludge method. This approach comprises 2mm fine screens to exclude any remaining debris and inorganic material, bioreactor aeration basins and, finally, the membrane tanks where the last stage ultra-filtration takes place.

The facility has a design potential which allows for up to 220 membrane cassettes, providing a final average daily capacity of 205,000m³ by the year 2040.

The ZeeWeed MBR technology selected consists of a suspended growth biological reactor integrated with an ultra-filtration membrane system and provides substantially better final effluent quality than conventional secondary treatment. The membranes are immersed within the aeration tank and bathed in the mixed liquor. A pump then draws water through the ultra-filtration membranes, while periodic intermittent airflow introduced at the bottom of the module cleans the surface of the membrane fibres.

This approach effectively overcomes the poor sludge settling often characteristic of conventional activated sludge processes and allows for significantly higher mixed liquor solids concentrations - typically in the range of 8,000mg/L to 10,000mg/L. The elevated biomass concentrations supported result in the highly effective removal of both soluble and particulate biodegradable material. The combination of high quality effluent and the system's operational reliability were major drivers in the decision to adopt this technology.

At times of particularly high flows, provision will be made to split the influent after the preliminary stage, with the excess undergoing an enhanced primary treatment before being recombined with the effluent from the MBR for disinfection with sodium hypochlorite and discharge to Puget Sound.

ODOUR CONTROL

Stringent criteria have been established to ensure effective odour control, with King County having committed to a goal of no odours detectable beyond the site boundary. To achieve this, all process units will be covered and the air collected will be routed through the odour control system. This comprises multistage chemical scrubbers, with a final carbon adsorption stage, which will also be used to treat any digester gases discharged through pressure release vents.

MBR PLANT OUTPUT

Of the four classes of reclaimed water identified by Washington state, Class A – which may be used for non-potable industrial processes and irrigation – is the most stringent. The plant will initially offer a 20,000m³/d Class A water reclamation facility, with space reserved on-site for future expansion to a final daily production of 200,000m³.

King County has been successfully recycling biosolids for more than a quarter of a century, using it for applications in agriculture, forestry, landscaping and gardening. Biosolids from the new plant will undergo thickening, anaerobic digestion and dewatering processes on-site before being removed for beneficial off-site use in these established outlets.

STORM WATER MANAGEMENT

As befits a major 21st Century construction site, environmental concerns have been given great consideration – the innovative approach to storm water management being a case in point. Rainwater running across the bare soil of construction sites can quickly pick up a variety of particles and pollutants, which inevitably then end up in local waterways, eroding the soil and increasing turbidity in the receiving waters. In an attempt to avoid this, silt fences, sediment traps and check dams are often used.

Chemical treatments are also employed where building work sits alongside particularly sensitive watercourses, or when construction coincides with high seasonal rainfall; a situation which can create unique issues for storm water management. Such chemical treatment – though effective – is expensive. To avoid this costly route and maximise both environmental and economic gains, the Brightwater site makes use of forest filtration – allowing the site’s large undisturbed areas of trees and vegetation to treat the water runoff.

Storm water run-off from the southern end of the site where most of the building work is taking place is pumped to an installed sprayer system, which irrigates the forestry to the north of the site minimising the use of chemicals while still meeting the appropriate standards. Ground filtration itself is not a new idea, but it is not usually used on construction sites and seldom at this scale.

KEY PLAYERS

King County's Wastewater Treatment Division is the client, with Hoffman Construction acting as the general contractor and construction manager. The membrane system for secondary treatment will be supplied by Zenon.