The total daily design sanitary sewage flow above which a treatment unit must be used with a filter bed system is

• A. 1,000 L
• B. 3,000 L
• C. 5,000 L
• D. 10,000 L

Answer: (C)

The needed concept is that larger daily wastewater volumes require pre-treatment before a filter bed to protect the filtration system and ensure consistent performance. Five thousand litres per day is the threshold because, beyond this level, the load of solids and organic matter entering the filter bed becomes high enough that a primary treatment unit (like a septic tank or other treatment device) is needed to remove solids and reduce biological oxygen demand. This pre-treatment lowers the risk of clogging the soil pores, maintains proper infiltration rates, and helps the system meet effluent quality goals. Below this flow, the filter bed can often operate with basic pre-treatment, but once you exceed five thousand litres daily, the code typically requires a treatment unit ahead of the filter bed.