Mikey
Aspiring Old Fart, EE, computer & networking geek
I was talking to a septic system designer the other day in connection with a questionable septic drainfield on a home I bought. I mentioned that I was planning to replace all the old 3-gallon toilets with modern low-flow or dual flush units (specifically, Drake or Aquia).
He suggested I think about that some more, arguing that there are two failure modes for septic systems: hydraulic failure, where the drainfield cannot absorb/perc the incoming fluid effluent, and biologic failure, which can be accelerated by an effluent with a high solids content. Low-flow toilets result in a more solids-rich effluent, and could thus aggravate a borderline situation.
I'm just starting to do the research, but it seems to be a reasonable argument, somewhat dependent on the ratio of flushes to showers and other "grey water" waste. Our 2-person household currently averages about 100gpd, using 1.6gpf Drakes. Estimating 10 flushes per day, that means about 16% black-water, 84% grey or better. The ratio would change to 26/74 if we had 3-gallon toilets installed. I have no idea what an ideal ratio might be; at what point do you start worrying about biologic vs hydraulic failure?
He suggested I think about that some more, arguing that there are two failure modes for septic systems: hydraulic failure, where the drainfield cannot absorb/perc the incoming fluid effluent, and biologic failure, which can be accelerated by an effluent with a high solids content. Low-flow toilets result in a more solids-rich effluent, and could thus aggravate a borderline situation.
I'm just starting to do the research, but it seems to be a reasonable argument, somewhat dependent on the ratio of flushes to showers and other "grey water" waste. Our 2-person household currently averages about 100gpd, using 1.6gpf Drakes. Estimating 10 flushes per day, that means about 16% black-water, 84% grey or better. The ratio would change to 26/74 if we had 3-gallon toilets installed. I have no idea what an ideal ratio might be; at what point do you start worrying about biologic vs hydraulic failure?