Clean Water Services, Tigard

Purple Flowers

Oregon highways coming down from Crater Lake were bordered for miles by these tiny purple flowers. If you are in central Oregon in late June you will be in luck!

Oregon was a treasure trove of anaerobic digesters. Our third digester on the trip was a plant in Tigard, OR outside Portland operated by Clean Water Services. Our roommate from Brown University, David Perlmutter, joined us for this tour. This was a municipality service (not a company) which seemed to have a stronger connection with other nearby water treatment systems. Clean Water Services has the slogan “Our commitment is clear.” That’s pretty good, right? After we were let through security at the front gate, we had a two hour tour and discussion with Mike Mengelkoch, the plant manager (business card says “Operations Analyst”). He was very patient with us and our questions. Thanks Mike!

Fermentation tanks to produce volatile fatty acids

The Tigard facility services 200,000 people and the waste is collected via sewer mains. The plant has been anaerobic since 1994. Before that they used aerobic digestion with incinerators to destroy the biosolids effluent. They only used about 50% of the capacity of the plant because of over estimation of waste inflow. The most novel aspect of the plant is that they use the effluent nowadays to produce a value added product without the stigma associated with using human waste as a fertilizer. The company Ostara has come in to turn the phosphorus in the waste into a slow release fertilizer used on fields and golf courses. The chemical is called struvite and wikipedia says the Tigard facility is the first waste water treatment facility to collect struvite. Interestingly, they also sell the fertilizer for dumping in the backwaters of salmon spawning areas. Before humans broke the cycle, fish would take up the phosphorus in these creeks and then return the phosphorus when they died or eaten by bears, etc. But now that fishermen take the salmon home and throw the scraps in the trash hundreds of miles from the creeks, the phosphorus levels have dropped in the creeks and decreased the salmon stocks in those creeks. After using the Ostara fertilizer, Mike says, the stocks have increased by a factor of ten. Wow! Let’s protect the cycle!

This process is particularly appropriate for Oregon. Most places are deficient in phosphates, but central Oregon, were the biosolids are eventually spread as fertilizer, has more than enough phosphate and so it is necessary to remove some of the phosphorus before spreading the biosolids.

Phosphate Fertilizer

The phosphorus recovery process works as follows. Phosphorus accumulating micro organisms take up the phosphorus in a pre anaerobic digestion process. Using some proprietary technology, the phosphorus is then collected from the organisms and turned into M&M-shaped while pellets. Doing this changes the anaerobic digestion process slightly. In order to feed the organisms the settled sludge is left to set for a few days to develop volatile fatty acids. These smell awful and require an odor control system (two tall while towers) but they feed the phosphorus accumulating micro organisms.

Odor Control

The solids at ~ 7% TS are sent to the three anaerobic digesters continuously. The digesters are run in parallel where the solids are sent to the first digester for 30 minutes, then to the second, then the third, then back to the first. In this way there is effectively new solids in each digester throughout the day. The digesters are also wasted continuously as more solids are added by displacement. I asked if there was ever a problem with pathogens from the new sludge leaving with the effluent. Mike said there wasn’t ever a problem as the new sludge made up such a small part of the effluent. But with the retention time on the digester at 15-22 days that is about 5% of new sludge in the effluent which doesn’t seem like so little to me.


The digesters are stirred by propellers and heated by cogen generators powered by biogas which produce 25% of the electricity used on at the waste treatment plant. If the generators fail, there are boilers that can take over in a pinch. When we visited, the yearly quota had just been met so the generators were being taken apart for repairs. I guess that is the problem with quotas. Once they are met, there is not much incentive to increase output.

There was no scrubber to remove carbon dioxide or hydrogen sulfide from the biogas and instead, had a H2S removal system that I had not heard of before. Ferric Chloride (Fe3+) is mixed into the solids holding tank which reduces H2S before it has a chance to bubble out of the sludge in the digester. Mike said that they first added Ferric Chloride, they added it directly to the digester and this caused the pipes to be eroded as the Ferric Chloride itself is very corrosive. Adding it beforehand turned out to work better.


On top of the digesters there is one blow off valve if the biogas is too pressurized (about 20 inches of water). There is also a system that lets methane into the digester if the headspace develops a vacuum. They don’t want to vent air into the headspace as that will destroy the anaerobic environment in the digester. Currently about a third of the gas is flared because there is not enough generator capacity to use all of the biogas generated.

Solids being hauled away

Mike said that they were required to keep the percentage of volatile solids destroyed above 38% but they are usually around 60-65%. This causes the effluent total solids to be around 3%. To remove the water before hauling the biosolids, a polymer is first added to the effluent (the three of us Brown alumni couldn’t figure out how the mechanism worked) followed by a centrifuge step to separate the liquid and solids. The final total solids are 24% and dark black

Digester Temperature: 98˚F

Retention Time: 15-22 days (≥15 days by DEQ requirement)

pH: about 7.4 (depends on how quickly the sample is tested. Dissolved CO2 changes the pH when it comes out of solution.)

16580 SW 85th Avenue

Tigard, OR 97224

About Kyle David

Thinking about distribution in developing countries
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