1. Sizing Septic Tank Volume:
Vol = (Average Flow Rate per hour)(1 hr)
Size of Tile Field:
Determined by “perc” test

2. Clarifier (Sedimentation Tank) Design:
Primary: Hydraulic detention time = 2 hrs
Overflow rate = 40 m3/m2-day
Secondary: Overflow rate = 15 m3/m2-day
Depth = 1.3 m

3. Biological Treatment Principles:

4. The quantity of new biomass produced is a function of the growth rate of the microbes.
Under aerobic conditions “cell yield” varies from about 0.1 to 0.4 g cells per g of BOD5 used.
Under anaerobic conditions “cell yield” is much lower – 0.05 g cells per g BOD5

5. Activated Sludge Design Parameters:
F/M = 0.3 kg BOD5/kg MLSS-day
Volumetric loading: 0.8 kg BOD5/m3-day
Hydraulic residence time = 5 hrs
Air supplied = 60 m3/kg BOD5

6. Trickling Filter Design Parameters:
Hydraulic loading: 10 m3/m2-day
Depth: 2 m

7. Anaerobic Digester Design Parameters:
Solids loading: 2 kg/m3-day
Solids retention time = 20 hrs
Note that anaerobic digestion is considered to be a mesophilic process where optimum temperature is 350C ( 90oF)

8. Gas (methane) production rate in anaerobic digesters:
0.4 m3 per kg biomass digested
(biomass digested = waste biomass from aerobic process, i.e., activated sludge, trickling filter)
This methane can be used to heat the digester.

9. Design for RBC’s:
0.6 lbs of BOD5 per 1000 ft2 of media per day
Rotational speed = 1.6 rpm
Basin depth enough to cover 40% of media surface
Media should be less than 12 ft dia.
Energy to drive shaft = 2.5 kw/shaft

10. Constructed Wetland design parameter is hydraulic retention time
Constructed Wetland design parameter is hydraulic retention time. Typical number for this parameter is 10 days.
Hydraulic retention time = Vol./Flow rate
For 1000 gal/day flow and a depth of wetland = 2 ft this would give a surface area of 688 ft2.
Typical L/W = 2:1

11. Soil Type Perc rate (mpi)
Sand < 10
Sand loam
Porous silt loam
Silty clay loam
Clay, compact
Silt loams > 45
Silty clay loams