The basement drain had backed-up and I thought I fixed the problem. I poured a drain cleaning agent and used a snake. Found out that one drain had black grease/oil water and a grainy sediment. I took out all I could and it was fine for a month. Then this foul odour started and there was more! Cleaned it again and again the same thing. Any ideas what might be taking place? Or how serious this is? All the other drains are fine in the basement (washroom and laundry). This drain runs from the backyard to the water tank and I assume then to the sewer. Any help would be appreciated. Thanks.

The basement drain had backed-up This drain runs from the backyard to the water tank and I assume then to the sewer. This is confusing, Please expain. What "water tank" are you referring to? What's the back yard got to do with a basement floor drain? Can you give more details and be more specific? Back to you, tom