Conceptual Approach / Calculations

We made the calculations below to check the caloric value of the dry matter in feces. This is important because the caloric value of dry feces must be sufficient to dry the same amount of other feces when using the dry feces as fuel for a fire. Please follow critically the below calculation, set up for the burning of the collected feces of 100 persons.

100 people produce an average of 0,2 kg/day x 100 = 20 kg/day.
Suppose the feces contain 70% water, total amount of 20 kg x 0.7 = 14 kg
Calories as needed for evaporating the water:
Warming up: 14 kg x (100* - 25*) = 1050 kcal (4375 kJ)
Vaporizing: 14 kg x 500 kcal/kg = 7000 kcal (29.170 kJ)
App. total caloric value as needed: 8000 kcal (33.333 kJ)

Net heat generated at a supposed efficiency of 50% of the combustion of the feces with a supposed caloric value of 4000 kcal/kg:
6 kg x 50% x 4000 kcal/kg = 12.000 kcal
Surplus in heat generated: 12.000 kcal - 8000 kcal = 4000 kcal (16.666 kJ). This surplus could be sufficient to keep going the drying and burning of the collected feces, even in a simple furnace.

• A diagram of such a furnace will be added shortly
• A calculation should be made at what price (per week) a toilet unit could be leased to a household, while this weekly amount includes the service of emptying and removing the excreta of this household. This weekly amount, multiplied by the number of households serviced should be economically attractive for a local entrepreneur to run such a sanitary facility

Conceptual Approach / Calculations