I work on the project where it is impossible to design non-structural measures because soil can´t infiltrate almost any water. All of the rainfall is collected in the cistern and discharges to the public sewer through filter with 85 % TSS efficiency. To meet the intent of this credit is it necessary to size the volume of the cistern to store all water from average annual rainfall? LEED reference guide requires capturing, treating (p. 101) and accommodating (p.104) 90% of the annual rainwater volume. The credit form requires only treating 90% of the annual rainwater volume.

If it is not necessary to store this amount of water, does following solution meet this credit requirement? I must for example capture 900 cu ft of water that represents 90% annual rainwater volume. Water flow to the cistern with the volume of 300 cu ft. Water is discharged to the public sewer in 8 hours and ready to capture another 300 cu ft of water. The volume of 900 cu ft is discharged in 24 h, the requirement of 80 % TSS efficiency is fulfilled. Is this cistern design sufficient to meet the credit intent?

Thanks a lot!