EXAMPLE: DESIGN OF CSTR WITH BIOMASS RECYCLE

Oxygen supply is an important constraint for process design.

Oxygen transfer through conventional aeration devices is limited, with 1,000 g-O2/m3-d to be assumed to be a maximum rate of oxygen supply

Aeration via compressed air supplied by mechanical blowers (bubble) or turbines is costly – especially in terms of energy consumption

Purpose of this example: develop a CSTR-biomass recycle process to minimize plant size while maintaining oxygen requirement < 1,000 g-O2/m3-d.

Given

SSO = 250 g/m3 COD

SS ≤ 10 g/m3 COD (discharge permit)

SNHO = 40 g/m3 NH4-N

SNH ≤ 0.5 g/m3 NH4-N

Kinetic and stoichiometric data,

μ̂H = 4 d−1

μ̂A = 1 d−1

KS = 25 g/m3 COD

KNH = 3 g/m3 NH4-N

b = 0.05 d−1 for both heterotrophs and nitrifiers

YH = 0.64 g-heterotrophic cell COD/g-substrate COD

YA = 0.12 g-autotrophic cell COD/g-NH4-N

fD = 0.2 g-debris COD/g-biomass COD decayed

iNXB = iNXD = 0.09 g-NH4-N/g-cell COD

μH = μ̂H SS / (KS + SS)

μA = μ̂A SNH / (KNH + SNH)

assume XBA << XBH and nitrogen synthesis requirement to autotrophs can be neglected

1. Relation between Θ and SS and SNH

For Θ > 0.5 d, discharge permit for COD is satisfied.

For Θ > 11 d, discharge permit for ammonia-nitrogen is satisfied.

2. Relation between Θ and XBH

Recall that the product XBHT = function of (Θ) and decreases as Θ increases. Also products XDT and XTT both increase as Θ increases.

3. OXYGEN CONSTRAINT for design of CSTR

rO,max = RO/V = 1,000 g-O2/m3-d

Note that minimum volume of CSTR is achieved at minimum τ = τmin.

for COD oxidation only,

τmin = 1/rO,max (SSO − SS)(1 − YHOBS)

and Xmax = (XT)/τmin where XT = XT(Θ)

4. Other constraints:

3 days < Θ

XT > 1,500 g/m3 COD

5. Adding nitrification

τmin = (1/rO,max) [(SSO − SS)(1 − YHOBS) + 4.57 [SNHO − SNH) − iNXB YHobs (SSO − SS)]

6. Note that wasting rate, observed yield and nitrogen synthesis requirements all decrease as Θ increases