Activated Sludge

I designed an activated sludge aeration basin. Qmax=3Qavg.

if i don't want to use an equalization tank so the basin volume (V) should be design according Qmax. and for calculating HRT, we have HRT=V/Qavg. HRT &SRT calculated from this method is much more from standards (also SRT may negative).

I Think SRT must be under 30  for activated sludge. Can anyone help?

Qavg= 5000 m3/day

Qmax= 14400 m3/day

F/M=0.3 1/day

MLSS=3000 ppm

BODin=240 BODout=20 ppm

-Calculate V: F/M=(Qmax.BODin)/(V.MLVSS)  

v= 5485 m3

-Calculate HRT= V/Qavg=1.09 day

Calculate SRT using Y=0.4 and kd=0.04 1/day: MLSS=SRT×Y×(BODin-BODout)/(HRT×(1+kd×SRT))

SRT=-75 day !!!!!