The question of whether 80% is good is a really good one. I’m not an expert on wastewater but I have worked in stormwater treatment. In general it easier to remove a higher percentage of a pollutant if the original concentration is higher. So the average percent removal often isn’t a good indicator of how effective that method of removal will be with different starting concentrations or of what the final concentration to be.

Copied from my comment above:

> I do this for a living. $100k is very cheap. A large driver of the costs are environmental regulations. You can’t just call up a contractor to show up and jam wood in a stream. The stream needs to be assessed to understand what can be done. It needs to be surveyed so you have a base to work from. It needs to be modeled to ensure that whatever you do doesn’t increase flooding. It needs to be designed to ensure that what they put there is going to stay put. Then it has to go through a rigorous permitting process. All that work needs to be done by experts who are expensive. That’s me - though I now work for a public agency myself.

> Once it’s finally time to construct the project, again you can’t just start ripping up a stream. You need to have biologists go through an capture all the wildlife. You need to carefully pump or divert the water so as not to harm the wildlife outside of the project. You need to protect the stream from any sediment or spills that could pollute the stream. All this is on top of working with natural materials that are oddly shaped requiring field adjustments as you construct.