Most greywater filters, like the ones you describe, are designed to remove the particles in greywater and to prevent clogging of a irrigation system that sends greywater through small outlets (like a drip irrigation system). These physical filters do not remove potentially harmful substances dissolved in the water from the soaps. If you design you system with larger outlets you won’t need a physical filter, and a redwood tree definitely doesn’t need to be irrigated with drip irrigation!
There really is no practical/economical way to remove the potentially harmful substances that can be found in soaps in an affordable residential greywater system- but luckily it’s very easy to avoid these harmful substances- just use “plant friendly” soaps and detergents. In general, shampoos, conditioners, and other common products used in the shower will not harm plants. The harmful ingredients are boron (found in some laundry detergents and cleaners) and high concentrations of salts (found in powdered detergents and powdered cleaners). Chlorine bleach is another one to avoid. In a shower system you should divert the water away from your plants (back to the sewer/septic) when using powdered cleaners, hair dye, bleach, or other harsh chemicals (for example, drain cleaner). Other than that, the water with normal products will be a fine quality to irrigate your redwood tree.
You should create mulch basins around the tree (near the drip line) to absorb the greywater and the mulch will catch the gunk in the water and allow it to decompose in the basin. My book, The Water Wise Home, has a lot of info on design and construction of different types of systems, if you need more details.
Good luck with your system!
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