It is calculated based on total oxygen demand and oxygen transfer efficiency (OTE). Total oxygen demand can be calculated as follow using the equation (4) here for the condition described in the case example.
=1,112.8 kg O2/day
Specific oxygen demand per wastewater volume is calculated at 0.309g/L by dividing the oxygen demand (O2) by feed flow rate, Q (3,600m3/day).
As shown in equation (2) here, α-, β-, and θ-factors, average saturation DO in aeration tank (DOS), fouling factor, f, and specific SOTE are required to estimate OTE. First of all, α-factor is calculated using the equation in Fig. 1 here (Krampe, 2002).
= 11.0 mg/L
=0.078 (or 7.8%)
Total biological air flow rate can be calculated using the already calculated oxygen demand, OTE, and the oxygen content of air, i.e. 0.275 kg O2/m3 at the condition blower performances are rated for (20 oC, 36% relative humidity, sea level).
=52,000 m3 air/day or 0.60 m3/s
The moisture content of air is typically not a significant source of error in the calculation considering the other errors in oxygen demand, OTE, etc. But, it might be worth while to take humidity effect into account at high ambient temperature (>30oC) with high relative humidity. The spread sheet here provides a method to calculate accurate oxygen content in the air.
The head pressure required is calculated as
= 49.0 kPa
Scouring air flow is given as 0.9 Nm3/s the case study. The specific aeration demand based on membrane surface area (SADm) is calculated at 0.36 m3/m2/hr. The total head pressure required can be calculated by adding nozzle submergence and head loss in the pipe and the nozzle (HS+HL,S), i.e. 3.5 m H2O or 34.3 kPa.
© Seong Hoon Yoon