Biodigester Sizing

300 People

• About 7 cubic meters for 300 people boarding school in the tropics - [1]

24 People

• For toilet water - we have 4 gallon max per person per day - at 0.5 gpf for liquids.
• Shower water goes directly to septic fertigation
• Biodigester waste is held for 30 days. 24 people *4 gal = about 100 gpd. The actual solids, if they settle, they stay as long as they like - there is only under a liter of feces per day [2].
• It seems that septic tank calculations allow for no retention time, just one day's hold. This makes sense if solids settle on a daily basis, so that discharge water is not too loaded. [3]
• Design: send shower and wash sink water directly to septic fertigation. Toilet water goes into digester.
• If we produce 5 gallons per person per day or about 100 gallons per day - a 2000 gallon (8 tote) gets us 20 days of detention time. But if solids sink and we are baffled, effective retention time is much greater and we are good to go.
• Conclusion: bath and shower water to septic fertigation, and kitchen sink + toilets to biodigester.