ISA Seattle Section Monthly Meeting

Our February meeting featured a tour of the Metro-King County sewage treatment plant on West Point, in Seattle. This is a huge facility that has recently completed a major up-grade to meet Federal requirements for secondary treatment of sewage. It was fascinating to learn some of what’s there and how it got there.

The facility is designed to handle a maximum flow rate of 440 million gallons/day. The storms between last Christmas and New Year’s Day tested the plant at maximum capacity for several days. The plant worked as designed.

In some ways, the Seattle area presents a difficult situation for sewage treatment. The storm drains generally go straight into the sanitary sewers, so the sewage treatment plants must treat storm run-off along with sewage. West Point is designed to deal with that situation. Under summer conditions, the plant handles about 80 million gallons/day. When the flow gets above 300 million gallons/day it becomes necessary to bypass secondary treatment but primary treatment continues, and the sewage is so diluted by rainwater that the output still meets all the applicable standards. It wasn’t clear just what happens if the total flow exceeds 440 million gallons/day. I have the impression that the pipelines that feed the plant can’t handle more than that, so sewage is probably spilled somewhere in the system, but it wasn’t apparent where.

The plant is fed by a pipe that is 9 feet in diameter. The flow through the plant is driven by gravity, with some help from large pumps. There is a pump rated for more than 1000 horsepower at the entry to the plant, which raises the liquid level 12 feet so it can flow through primary treatment. A similar pump raises it again so it will flow through secondary treatment. At the outlet of the plant there is a third pump which moves the treated water out into Puget Sound. At low tide the outlet pump isn’t needed, but at high tide it works pretty hard.

The original West Point Plant was built starting in 1965. There was a separate water quality agency, Metro, which was tasked to clean up the local sewage situation. At that time, a lot of raw sewage was dumped into the nearest body of water. As a consequence, Lake Washington and other local lakes were not fit to swim in and people had decided that something should be done.

The original plan included the construction of treatment plants at West Point and Renton and large, pumped, pipelines to collect sewage throughout the area and get it to the treatment plants. The construction program took several years, and the result has been very successful. However, in recent years the laws have changed and they now require a much cleaner quality of water that leaves the plants after treatment. To achieve the required level of water quality, Metro had to add ‘secondary’ treatment to the primary treatment that was originally implemented.

The program to add secondary treatment at West Point started in 1987 and was just recently completed. The program was budgeted at $578 million and came in slightly under that amount. There were some interesting complications along the way.

Archaeological work on a 3000 year old Indian village (or garbage dump, depending on who you ask) delayed construction for about 4 months.

Secondary treatment involves a lot or aerating and settling to clean up the water. This takes room, and there are only 30 acres at West Point, which isn’t enough room for a conventional secondary treatment plant. The solution to that is an oxygen plant, which draws 95% pure oxygen from air. The oxygen is bubbled through the water to accomplish the aeration in a much smaller space than would be required otherwise.

There is a very high, very strong, reinforced-concrete retaining wall that keeps Discovery Park from sliding into the treatment plant. This winter has tested the wall pretty thoroughly and demonstrated its value.

The plant can draw electric power from either of two substations. In theory, it should not lose power from both substations more often than once every 100 years: in fact, it has already happened 2 times during the 30 year life of the plant.

The facility is controlled by 11 medium sized PLCs providing setpoint information to 135 PID controllers and directly controlling roughly 17,000 discrete Inputs/Outputs. I’d call that a large control system.

If you look at West Point from Puget Sound, you probably won’t see the treatment plant at all. There is a large berm that screens all but its tallest buildings from view on the water side. There is also about $5 million of landscaping and shrubbery to help it blend in.

Our thanks to Phil Daniels of Metro-King County for a great presentation and tour of the West Point Treatment Plant.