I have a query, Not sure of the best way to describe it, but here goes, we had a modular building installed on a plot of land, the waste systems,ie toilet, sink etc were connected by the builders to the drain, this was about three months ago, the drains blocked back to the toilet, I fetched up the manhole covers to find total blockage, I rodded and cleared right back to the edge of the property, once cleared I found that the drain run inside the manhole nearest the road was at different levels(highest going out) which I found strange, on calling back the builders they tell me that this is some kind of grease/silt trap, I have never seen this in a sewage drain before, can anyone advise if this is correct, could this be a rainwater drain run??, your advice is welcome. Thanks...