Influent control structure
Liquid treatment process
Wastewater and stormwater are pumped into the treatment plant.
What went wrong
It took workers nearly 40 minutes to stop the flood of wastewater destroying the West Point treatment plant, by manually shutting off raw-sewage pumps bringing in more flow. That stopped the flood, and tripped an emergency bypass gate, sending about 180 million gallons of untreated wastewater, including about 18 million gallons of raw sewage, into Puget Sound. However, that was far too late to save the plant from the flood.
Trash screening
Liquid treatment process
Bar screens separate trash and large solids from the flow. Collected trash is sent to a landfill.
Raw-sewage pumps
Liquid treatment process
Raw sewage and stormwater are pumped into the plant for primary and secondary treatment.
Grit removal
Liquid treatment process
By injecting air into the flow, sand and grit settle to the bottom of the tank while human and food wastes become suspended.
Primary sedimentation tank
Primary treatment
The influent flow is slowed to allow human and food organic waste to settle on the bottom of the tank. A skimmer removes the oil and grease that float to the surface.
What went wrong
Damaged float switches in these tanks jammed and failed. No alarms were sent to the plant’s electronic control system from those devices to warn the crew of water rising in the two giant primary tanks -- though other alarms sounded. Nor did the float switches shut down the raw-sewage pumps as they should, so wastewater continued surging into the plant. Both primary sedimentation tanks began overflowing at 2:25 a.m. An estimated 15 million gallons of raw sewage and stormwater poured into equipment galleries, destroying millions of dollars worth of electrical panels, equipment and motors.
Intermediate pump station
Primary treatment
Controls flow into the secondary treatment phase or into the bypass line to the disinfection tank.
Oxygen aeration tank
Secondary treatment
In this four-stage tank, added oxygen activates bacteria to reproduce, grow and consume the dissolved organic waste suspended in the water.
Clarifier tank
Secondary treatment
Gravity causes engorged bacteria and any remaining organic waste to settle at the bottom of the tank, where it is collected and sent into the solids treatment process.
Disinfection tank
Liquid treatment process
Hypochlorite, a strong bleach, kills any remaining bacteria in the water.
Dechlorination tank
Liquid treatment process
Bleach is neutralized to protect the marine environment.
Effluent pump station
Liquid treatment process
Final effluent is pumped into Puget Sound. The effluent is at least 85 percent free of the pollutants coming into the plant that the process was designed to remove.
What went wrong
An electrical fault stopped pumps 1 and 2, needed to pump wastewater out of the facility, at 2:12 a.m. All four effluent-pump discharge valves closed. Two minutes later, pumps 3 and 4 also failed and shut down because of high vibration. Crews tried for an additional 46 minutes to restart pumps.
Blend tank
Solid treatment process
Sludge from primary and secondary treatments is blended together.
Gravity belt thickener
Solid treatment process
Water is released as the sludge is moved through a gravity belt. A thicker sludge cake is then discharged.
Anaerobic digester
Solid treatment process
Anaerobic bacteria in digesters break down the organic sludge into nutrients and methane.
What went wrong
In the hours before daybreak, the incoming sludge pumps continued to add material into the anaerobic digester, even after the digester mixing pump and the export-transfer pump failed. Without mixing the anaerobic digester, trapped gases cause the sludge to expand and rise. The expansion and continued addition of sludge increased until it finally overflowed digester tanks.
Centrifuge dewatering
Solid treatment process
The material is spun in a centrifuge to remove remaining water.
Exported biosolid
Solid treatment process
A final, nutrient-rich biosolid product is trucked from the plant for use as fertilizer.