Grey water pipe work - how to flush?

11 Sep 2007
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United Kingdom
I'm noticing that the grey water downpipe in the ground that is fed from my kitchen sink and washing machine is running a bit high. In other words when water is pouring into it the water level in the downpipe is very close to the top (but not overflowing). This is though something that I've never really paid attention to before so perhaps it's normal?

The downpipe goes about a foot into the ground and when there's no water flowing into it the standing water level is about 6 inches (which I assume is normal to prevent smells backing up). This of course connects to the main drain which, further 'downstream', runs into my septic tank system (the pipe work for this is all clear, septic tank levels are fine, etc).

I've had my arm down the downpipe and lifted out a bit of grey sludge but on feeling around the pipe seems to be shaped with only small gaps at the sides for water to exit. I'm wondering if there's some sludge built up in parts that I can't of course reach, but how to clean out the system? I have a pressure washer but of course that isn't going to go around the bend at the bottom of the pipe and I also don't want to risk breaking anything with pressure that's too high.
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Thanks, didn't know that. Is there a chance that a pressure washer could damage the inside of a plastic pipe? My washer only goes up to 120 Bars.
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