Biodigester and Septic Tank Installation in Nairobi

Biodigester or conventional septic

A biodigester uses bacteria to break down solids, so it needs emptying far less often and takes less space. A conventional septic tank is cheaper up front, needs an exhauster more regularly and needs a working soakaway. Which is right depends on your plot size, your soil and how often you want to see an exhauster truck.

Where a plot has heavy black cotton soil, common across much of Nairobi, percolation is poor and a conventional soakaway struggles. That is the situation a biodigester genuinely solves, and it is worth the extra cost.

Sizing

Systems are sized by occupancy, not by house size. A four-bedroom house with two occupants and a four-bedroom house with nine occupants need different systems. We size on the realistic number of users, plus any staff quarters, and we allow for the house being fuller than it is today.

Siting

Three constraints decide where it goes: distance from any borehole or well, distance from the boundary and the house, and the fall available from the lowest fitting. Getting fall wrong is the single most expensive mistake, because it cannot be fixed without re-laying the run.

Enzymes and maintenance

A biodigester needs its bacterial culture topping up, an enzyme pack roughly annually for a household. It also needs you to keep bleach, paraffin and heavy disinfectant out of the system, because those kill the culture the tank depends on. We hand over written instructions on that, because a biodigester that has been bleached for a year behaves exactly like a failed septic tank.