With standard notations, the total subsurface soil area required for the soak pits or dispersion trenches is given by
- AQ = 100 / √t
- BQ = 130 / √tCorrect
- CQ = 200 / √t
- DQ = 260 / √t
Solution
Concept
A percolation test gives the maximum rate at which septic tank effluent can be applied to soil. This rate fixes the area of a soak pit or dispersion trench.
Formula
Q = 130 / √t (Q in L/m²/day, t in minutes for 25 mm fall of water level); Area = Daily effluent flow / Q
Step-by-step solution
1. Do a percolation test and note t = time (minutes) for water level to fall 25 mm 2. Allowable rate of effluent application: Q = 130 / √t (litres per m² of leaching area per day) 3. Required subsurface area = total daily effluent flow / Q 4. So the standard relation is Q = 130 / √t
Explanation
The standard formula from the percolation test is Q = 130/√t. Here Q is the maximum effluent loading rate in L/m²/day and t is the time in minutes for a 25 mm drop in water level. The total soil area for the soak pit or trench is the daily sewage flow divided by Q.
Why the other options are wrong
Constants 100, 200 and 260 are not used in the standard percolation test relation; the correct constant is 130.
Shortcut
Remember '130 by root t' for soak pit and dispersion trench design.
Common mistake
Thinking Q is the area itself. Q is the application rate, and area = flow / Q.
Correct answer: B) Q = 130 / √t