Planned Excellence

New Brightwater plant in King County, Wash., combines efficient treatment with odor control technologies that make the plant a good neighbor.
Planned Excellence

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After more than a decade of planning and about five years of construction,

the 36 mgd (design) Brightwater Treatment Plant began operating in September 2011. It was the largest capital improvement in the history of King County, Wash., and the largest expansion of the regional wastewater system since the two original plants were built in the 1960s.

"It's a really significant project, and one we're really proud of," says communication specialist Annie Kolb-Nelson. The final $1.85 billion cost includes an on-site environmental education/community center and over 70 acres of public open space and restored habitat. The treatment plant includes a technologically advanced odor control system to address King County's commitment to odor-free operation.

While Brightwater can make a claim to be one of the world's largest membrane bioreactor plants, a plant under construction in Dubai, due to come online in a year or two, will be larger. Still, it was a major project for the county Wastewater Treatment Division.

Brightwater joins the West Point plant in Seattle and the South plant in Renton in serving a total of 1.5 million people in 34 cities and sewer districts in King County and parts of Snohomish and Pierce Counties in the Seattle area. Brightwater itself will serve about 189,000 people in northern King and southern Snohomish Counties.

King County and their engineering consultant team of CH2M HILL and Brown and Caldwell, selected the ZeeWeed hollow-fiber membrane bioreactor (MBR) from GE Water & Process Technologies. It combines clarification, aeration and filtration in a single process, saving capital and operating costs.

"Hollow-fiber ultrafiltration is very applicable to wastewater treatment because the microscopic particles have biological components like viruses and bacteria," explains plant manager Ron Kohler. "The membranes filter out pollutants to the bacterial level, achieving high-quality water suitable for landscape irrigation and reducing pollutant loading into Puget Sound substantially compared to conventional secondary treatment. We get very low turbidity and zero coliform."

Plant operator Angelo Archuleta, who has been working in wastewater plants for about 20 years, adds, "The dynamics of an MBR are very interesting and different."

Two-stage screening

One of the keys to membrane filtration is proper screening of wastewater to protect the membranes from damage by debris and to prevent accumulation of solids on the fibers. Improper screening can also void membrane warranties.

"Coming into the headworks, we have four 9 mm punch plate screens (WasteTech, a Division of Kusters Water) followed by aerated grit channels and five primary sedimentation tanks for chemically enhanced primary clarification," says Kohler. "Before it gets to secondary treatment, we have 2 mm traveling band screens (Ovivo)."

Two screening stages were selected to prevent having fine screens prior to primary treatment, which carried the risk of blinding the screens, although some other MBRs use single-stage screening. "We didn't want to take that risk," Kohler says. After screening, the influent goes to three standard activated sludge secondary aeration tanks with anoxic zones up front.

"With the nitrification-denitrification process available with membranes, you sometimes get low alkalinity," Kohler says. "A 1 mgd recycle pump (Fairbanks Morse) sends the flow from the ends of the tanks back to the beginning for alkalinity recovery."

The wastewater is then pumped to the membranes to remove remaining suspended solids. When the flow leaves secondary treatment, it is at about 8,000 mg/L mixed liquor suspended solids (MLSS).

"We withdraw so much clean water through the hollow tubes that MLSS in the membrane basins goes up to 11,000 or 12,000," says Kohler. "That is starting to act as a solid." To combat that, the plant uses a 5Q recycle flow rate: For every 5 million gallons pumped to the membranes, 1 million gallons of clean water comes out and 4 million gallons is needed to carry the mixed liquor back to the front of the aeration tanks.

Smaller footprint

There are 10 membrane basins, each 21 feet deep, 70 feet long and 28 feet wide. Each has 13 hollow-fiber cassettes 5 feet square and 13 feet tall. "The advantage of hollow-fiber membranes is that for the same treatment throughput, you get a significantly smaller footprint, which means much less expense," says Kohler. Because of the split-flow treatment process, which processes peak wet-weather flows through chemically enhanced primary clarification, the MBR requires about 40 percent less space than a traditional secondary clarifier installation.

The membranes remove 80 to 90 percent of viruses, which are too large to get through the pores. Bacteria are also filtered along with other suspended solids. "That physical barrier gives you better effluent, and it improves efficiency of disinfection through lower dosing," says Kohler. "So membranes do improve effluent quality."

The clean water flows from the membranes to chlorine disinfection with liquid sodium hypochlorite dosed at 1.7 ppm for 30 minutes. Programmable logic controllers (PLCs) monitor reclaimed water production to make sure it meets a limit of 1 ppm residual chlorine; it is normally about 0.5 ppm. "If it doesn't meet that quality, it is automatically diverted back to the influent pump station," says Kohler. "The control strategies assure us that out-of-spec water doesn't get to our reclaimed water customers."

Final effluent flows to an outfall with a two-tiered weir that directs the water to meet the demand of the reclaim system first. Whatever is left flows 13 miles to Puget Sound. For the time being, all effluent goes to the Sound, as the plant team wanted a year to refine operations while the 10-mile reclaimed water system is finished (2013). "We will be able to ultimately deliver up to 21 mgd to the Class A reclaimed water system," says Kohler.

Biosolids are treated in fixed-cover modified silo anaerobic digesters to create Class B biosolids used as an agriculture and forestry soil amendment. Some material is available to the public through a vendor that treats biosolids to create Class A compost.

Membrane maintenance

Discharge permit compliance is assured with the highly automated MBR process. The membranes have three operating modes: Filtering, relaxed and backflushing. Membrane cassettes go into a relaxed period for 30 minutes about every eight hours, during which time no water flows through them and they are allowed to rest. They are backflushed every few days to clean out the pores and wash off the fibers.

A PLC runs that process. "The computers are looking at things like the membrane pressure and amount of energy required to get negative pressure in the hollow fibers," says Kohler. "They have very high-level algorithms that control when the fibers need to be relaxed and when they are backflushed."

Archuleta says it is highly automated, so operators only need to monitor the system. "We have trending to help us, and everything is working really well on its own."

Operators need to understand the membrane operating cycle, Kohler observes: "If you're planning on a 15 mgd effluent flow rate hour after hour, you do not get that with membranes. When they go through a relaxed period or backflush, there's about 20 minutes when your effluent drops until the other basins pick up the flow. It's an important element you have to plan for when you're delivering reclaimed water."

The Brightwater plant usually has one basin either in the relaxed mode or backflushing at any given time, which means nine basins are in full operation.

Besides backflushing, periodic chemical cleaning is needed to remove biofouling and dissolve lime and calcium buildup on the membranes. For those processes, the basin is filled with clean effluent and receives a light dose of hypochlorite for four hours for biofouling removal, or citric acid for lime and calcium removal.

The Brightwater team does not yet know how often citric acid will be needed — the area has very soft water. Another MBR facility in the area has operated for about three years without needing chemical cleaning, although it has been done anyway to comply with warranty requirements.

Energy-efficient design

Besides high treatment efficiency, the plant is designed for excellent energy performance. High-efficiency turbo blowers with air bearings supply air for the aeration system. "We originally specified a very sophisticated single-stage blower," says Kohler. "During design, APG-Neuros provided one of their turbo blowers for testing at another of our plants. We documented 3 to 5 percent less energy use to deliver the same amount of air at the same pressure compared to other high-efficiency blowers."

The local electric utility, Snohomish County Public Utility District, installed monitoring equipment on the blowers and documented the savings, resulting in a $265,000 rebate. "We're going to see that reduction in electrical use for as long as those blowers are in operation, and that's a great value for our ratepayers," says Kohler.

An added benefit is that they are much quieter: the blower room is at about 85 decibels, compared to 130 decibels in most blower rooms. The rooms are also cooler because the blowers recapture their own heat instead of expelling it into the room. "I've worked with different types of blowers and these are a lot quieter and more efficient," says Archuleta. "It's refreshing to see technology at work."

The air-handling system was also optimized to reduce air requirements and that is expected to lower power consumption by about 50 percent. "The membranes use a lot of air injected right under the cassettes to keep them moving so they don't stick together," says Kohler. "We worked with GE to modify that system to reduce airflow significantly."

Good neighbor

While creating a cost-effective treatment system, the plant invested heavily — to the tune of $65 million — in being a good neighbor. During design, a promise was made to the public: You will not smell Brightwater. "That was the biggest concern people had," says Kolb-Nelson. "We committed to no detectable odors past the fence line."

The plant neighborhood is low-density housing with some light industry and is likely to develop in the future. "The county conditional use permit places specific performance criteria on our odor control system," says Kohler. "All process areas are covered or inside buildings under negative pressure. All air that comes in contact with wastewater is scrubbed in three odor control phases before it is exhausted."

The plant has ten trains of Siemens odor control equipment in operation at all times, and three more serve as backups. Each train treats about 350,000 cfm. The process starts with a biological scrubber, followed by chemical scrubbing and finally treatment in carbon absorption beds containing 90,000 pounds of activated carbon.

The biological scrubber is a relatively new technique that uses organisms that feed on hydrogen sulfide. "It's a low-cost, passive system that is guaranteed to remove 60 to 80 percent of H2S," says Kohler. "That results in a direct reduction of sodium hydroxide and sodium hypochlorite in the chemical scrubbers by one to three tankers a week."

Brightwater can document that it is meeting its promise to neighbors. The conditional use permit limits H2S to 0.08 parts per billion (ppb) at the property boundary. That's below the detection limit, but mathematical calculations show it is equal to 20 ppb at the stack when dispersion is taken into account. Monitors show that the actual stack level is typically less than 3 ppb.

"There are no odors even inside the plant," adds Archuleta. "That's one thing that really stood out to me as an operator. It's just really clean — it's amazing."

Meeting expectations

Kohler accepts the modern reality that people expect sustainable operation and effective wastewater treatment, but they don't want hassles, especially with greenfield construction projects like Brightwater. "Neighbors are going to insist you cover the entire facility and scrub all of the air so you don't become a nuisance," Kohler says.

The education center is also helpful, adds Archuleta. "We're open to the public and it's a great educational experience to teach children and the public about our environment, wastewater treatment and reuse."

The biggest compliment Kohler has heard came at the grand opening, to which all neighbors received a personal invitation. "Two of our closest neighbors asked me when we were going to start operating. I laughed and told them we'd been operating for a month." Then he heard something all wastewater operators want to hear: "They told me they trusted us."


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