A septic tank filter is designed to filter particles of what size in millimeters and requires a minimum flow area of how many square centimeters?

• A. 0.5 mm; 300 cm²
• B. 1.6 mm; 550 cm²
• C. 2.0 mm; 700 cm²
• D. 1.0 mm; 450 cm²

Answer: (B)

A septic tank effluent filter is meant to stop solid particles from moving on to the drain field while still allowing liquid to pass smoothly. The pore size sets what gets trapped: a size around 1.6 mm (about 1/16 inch) filters out finer solids that could clog laterals but isn’t so fine that it rapidly clogs itself. Along with that, the filter must have enough flow area so that the effluent can pass through without building up pressure or creating a bottleneck; a minimum flow area of about 550 cm² provides sufficient room for typical flow rates, helping prevent backpressure and early clogging. Smaller pore sizes or smaller flow areas would increase clogging risk and restrict flow, while the 1.6 mm with 550 cm² combination matches standard practice, making it the best choice.