I have a small bathroom which originally had a sink, shower and toilet running through a macerator. Because it was so noisy i asked the plumber to remove the sink and shower. He did this by teeing in shower and sink pipes to the outgoing macerator waste pipe underfloor before it entered the soil pipe. Soil pipe within 2m of macerator. After it was done i complained about the noise the sink was making when flushing so he fitted a Durgo valve which made the noise stop. However, one day when flushing the toilet the trap in the shower was blowing out water. I complained and he fitted another durgo valve in the shower waste pipe (just before shower). A few days later the sink started blowing out water from its trap even though already had a Durgo. It happened after two people used the shower and toilet. Most of the time it doesn't happen, but has now happened twice. My feeling is that he should have run the shower and sink directly to the soil pipe rather than teeing into a pressurised pumped out pipe (of the macerator), he said wouldn't make much difference i would still get the same problem. But i don't see how. I need to solve this problem what can i do? Please help.