I have an upstairs bathroom and one in the basement. I just had an injector pump installed in the basement as we have a septic system. Twice now I was washing cloths and after the third load the pump went on to drain at the same time and there was an explosion of water where the washing machine hose meets the pvc drain line. The second time was a week later and someone used the toilet when the washing machine was draining. The septic tank was just cleaned out. We had the plummer out twice, he left telling us the it was fixed. Lst time he said the pump plug was not in all the way. The second time he changed the water pipes that fill the washing machine. It still explodes.
Please help.

I don't have an answer for your question. But, I care that your question gets answered. By my posting a reply now, your question will get moved more towards the top of the list of questions needing answers. Hopefully, someone will come along soon with the expertise to answer your question.

We have some people who are very knowledgeable about plumbing. I hope that one of them comes along.