There is a terrible raw sewage smell coming from the drain waste vent (DWV) on top of my roof. The sewage smell can be smelled from 4 houses away. Because the sewage smell was also coming from manhole covers, the city has been out twice to flush the main sewer line and also plugged the holes in two of the manhole covers. Ever since they did that, the smell from my roof vent is even worse. I now smell raw sewage in my front yard AND backyard, and the smell is worse at night around 8pm (we figure it's when people are home from work/school and more waste is being produced). While we think the smell is coming from the roof vents of several houses (all on my side of the street), the smell is worse at my house. When we were on the roof we smelled one of the DWV's and it was terrible. We used a hose to run water down it and it was not blocked (you could see the water coming through the main clean out). When you open my main clean out in front of my house you can actually see the sewer gas coming out! I am 4 houses up from where the city's sewer line cross connects with another sewer pipe. Is my house being hit the hardest (by the gas/smell) because the main city sewer is backing up where those two city pipes connect? One plumber suggested I put a p-trap before my main line clean out, at a cost of $1,500! But, wouldn't it be difficult to roto rooter (if needed) my main line if I had a p-trap on it? The city said I should install a backwater prevention device. Our homes are about a foot above the manhole cover and we do not have basements, so do I really need a backwater prevention device? The research I did on BPD's does not say anything about them keeping sewer gas smells from escaping through roof vents, so would that even help? My other option is some type of DWV filter to put over the DWV. I have read that you can purchase filters that use carbon or charcol. I contacted several plumbing stores and went to Home Depot and no one heard filters for your DWV, but I know they exist, I've seen them on the internet! I live in California and am hooked up to the city sewer, and I guess filters are mainly used for septic tanks? I'm at my wits end, I have headaches and my eyes burn from the smell. Any advice you can give me would be greatly appreciated. Thank you.