For each type of onsite system the user needs to enter a value for the variable F3 “proportion of this type of system from which faecal sludge is emptied”.  The true value is a function of the percentage ever emptied and the proportion of the contents removed during emptying.  However, estimating both these may be challenging.   

The SFD Process defines emptying as “the manual or motorised emptying of faecal sludge from onsite sanitation systems”. The emptying event or service should remove a substantial amount of the faecal sludge from the container which would then be transported away and should certainly comprise more than just unblocking septic tanks or flushing out pit latrines. However, the SFD Process does not define an acceptable emptying frequency; this is very context specific and dependent on many factors. 

A pragmatic approach is to estimate the proportion of households with a specific type of system who ever have them emptied.  In some cases there will be data from household surveys and situational analyses. There may also be data on the capacity of the emptying service providers (i.e. numbers and volume of trucks) and their service history; these could provide a useful cross-check.  In many locations however these data will not be available.  Calculating F3 will then depend on expert opinion and should be discussed by stakeholders as part of the SFD process.  It is important to state clearly in the SFD Report the method used and any assumptions made.