My guess is that they rely on the whole length of the outflow being at the same level - it fills as a whole and drains equally through each tee. Or, the system fills from the end upwards/backwards. Which in theory means the last tree always gets more water than the first
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You're not wrong - the T-joint will not split the water 50-50 under normal conditions. The water will flow where it can. If you want to ensure flow through all outlets, the outlet ( ie each branch/exit) pipes should be significantly smaller than the main pipe.
This way, your main pipe will fill with water faster than the smaller outlets release it, and all outlets will receive water.
What if I connect the T this way ?
The 90 degree joint will to the rest of the line and the other end of the T will go to the basin
The round frame is the basin? I'm not sure I understand your setup, but that's OK.
The basic idea is, water flow occurs due to pressure difference. To force the water to multiple exits, you need pressure. The easiest way to achieve this is to use a large-diameter pipe as the main feed, then place small-diameter outlet pipes along it.
If you have an open exit like in your picture, the pressure will be lost.
Thanks for your answers The round box is the basin in this example. But that's not exactly my setup. In my setup, instead of an open ended exit directly there, I would take it through a pipe to the basin where it would pour open ended... So not that different I guess.
I get what you mean about the pressure. My only doubt is that since it's greywater, it might have slight dirt / hair etc coming from the shower and washing machine that may clog small pipes. I know I can add a filter but I'm trying to keep it simple
Where is the greywater treatment system in this? Do you want to water your plants with laundry detergent?
Yeah, there's no problem as long as you're using an appropriate detergent.