Gabriela Buamscha. Master Recycler (Class 27).
On April 2022, a team of Master Recyclers toured the City of Portland’s Waste Water Treatment Plant (WWTP) located on Columbia Blvd. near the Columbia River.
Wastewater is sewage from our homes and businesses, and it can be combined with water from stormwater runoff. The city’s network of sewer and stormwater pipes ends at the WWTP. Once Inside the plant, wastewater goes through a series of steps to; (i) obtain clean and disinfected water, and (ii) recover resources such as biogas fuel and biosolid fertilizer.
The recovery of resources in wastewater mimics natural processes such as precipitation by gravity and anaerobic respiration. Anaerobic means without oxygen, while anaerobic respiration is a metabolic process that a group of microorganisms use to obtain energy and release methane gas or biogas. Biogas can generate electricity and heat for our daily use. An anaerobic digestor is an enclosed chamber with the conditions for anaerobic microorganisms to thrive and produce biogas and biosolids.
Synthesis of the water treatment and resource recovery steps at the City of Portland’s WWTP
STEP 1. Screening Facility: Wastewater flows through large metal screens that remove litter, rocks, and sticks that would otherwise plug and damage the plants pumps, pipes, and other hardware. Fact: The city’s residents can help the WWTP by avoiding flushing diapers and wipes, and disposing of cooking grease in kitchen sinks. Place those items in the trash bin instead. Photo: A series of pumps in the Mechanical Room.
STEP 2. Primary Clarifier Tanks: Water is slowed down and treated chemically so large and heavier solids sink (precipitate) to the bottom of the tank. The sunken solids are called sludge. The sludge is scrapped from the tank floor and sent to an Anaerobic Digestor. Photo: Calm waters in a primary clarifier.
STEP 3. Secondary Clarifier Tanks: These are open basins where aeration pumps (large blowers) inject air into the water. The oxygen promotes the growth of microorganisms that feed on smaller solids that are still suspended in water. Photo: Clarifier tank with aeration pump at work.
STEP 4. Final Clarifier: Water is slowed down again so microorganisms sink to the bottom, the sludge is removed with mechanical scrapers, and sent to the Anaerobic Digestor.
STEP 5. Disinfection: Water is treated with chemical disinfectants to kill harmful bacteria and viruses. Water must meet set chemical and biological guidelines before it is discharged into the Columbia River.
STEP 6. Anaerobic Digestor: You may wonder why all the sludge from the Clarifiers ends at the Digestor? Because anaerobic microorganisms “eat” the sludge and in the process release biogas. Fact: The biogas recovered at the WWTP can power 120 trucks running non-stop 24 hours per day and 365 days per year. Biosolids are another product from the anaerobic digestor. Biosolids are high in nutrients and organic matter, thus used as plant fertilizers and to remediate degraded soils. Currently, WWTP biosolids are sent to Eastern Oregon where they’re applied to Dry Wheat (non-irrigated) and cattle farm soils. Photo: Anaerobic digestor.
Acknowledgements: Special thanks to Josh Newman for leading the tour and Concha Paloma for her Interpreting services. Thanks also to Joe Donnegan and Tiago Donnegan for their thorough reviews of the English-version manuscript.