Dear Behnam,
Welcome to SuSanA. 

I am glad to know that you are using the SFD tool and found the right place to seek answer to your questions. It will be helpful if you could tell a bit more about the town - population size, administrative divisions, the information you are seeking to collect from household surveys, etc. for anybody to be able to help.

The manual does not suggest any sampling method for household surveys. It will, however, depend on the level of SFD you are planning to prepare and the resources available with you. In some of the comprehensive SFD reports from India, information from 3-5 households from each administrative unit (ward) was collected and used.  In Alleppey we surveyed nearly 2000 households as the survey was carried out as part of a project (see here for details; let me know if you don't have access). 

You could however, refer to sampling strategy for observations;  the following sentence captures the gist very well in my opinion.

Observations should aim to look at services, facilities and procedures adopted through all stages of the sanitation service chain. The observations should reflect the range of practices.

SFD manual, p.113

Please feel free to keep asking questions and updating us about the SFD and your PhD as you make progress.

Regards
paresh

Hi Professor
I am a PhD student in environmental sciences and engineering from Isfahan University of Technology and I want to prepare a septage management evaluation report (SFD). In this regard, I need your guidance to choose the right number of families to answer the relevant questionnaires.
Thank you for your kindness