Some of the worst sewer lines are in steakhouses, as I often have sewer line maintenance and drain complications at the steakhouses downtown where I work.
Most of those sites empty their grease down the sink and in the dishwasher and all of that grease and sludge goes somewhere.
Occasionally parts of the grease and sludge remain in the pipe and as more grease and sludge comes through, the contents build up over time. Occasionally the contents can completely block the sewer pipe causing a backup. I went to a chicken and fish steakhouse down the corner from the commercial plumbing business, and I was there for supper when the manager called the store to request services. I knew something was up in the steakhouse because there was quite a commotion and stir going on, but everyone was talking in whispers and the manager was walking around with a weird look on his face. My boss called me on the cellphone to see where I was and I told him I was finishing up with supper and she told me about the commercial plumbing job. I told my boss that I was actually at the steakhouse and I figured something was going on with the plumbing comment because there was a pressing sign on the bathroom that said “closed”. I finished my chicken and fries and then both of us asked the store manager what was going on and how I could help. The sewer line was totally clogged up with grease and fat and all of the grease and fat needed to be removed in order for the sewer drain to run freely once again.