grit pit
n.— «Grit pits are pits about 10-feet-by-10-feet wide that run under main sewer lines and catch debris filtered through the main line. The purpose of the plastic-walled pit is to collect debris that would normally accumulate inside the main sewage lines, causing backups and overflows.» —“Grit pits to help with sewer debris” by Luke E. Saladin Kentucky Post (Cincinatti, Ohio) Apr. 13, 2007. (source: Double-Tongued Dictionary)