BIOLOGICAL PLANT
SIZING
Ing. Alberto Scaunich

EXISTING PLANT
STATISTIC
ELABORATION
(or available data flowrate
and pollutants concentration)
- Number of values
N
- Average Value
M
- Standard Deviation
s
- Typical Values
WHICHEVER
NORMAL
DISTRIBUTION
DISTRIBUTION
M+s
Generally are available data for:
Flow
Pollutant concentration
68.3%
M+2s
75%
95.4%
M+3s
91%
99,7%
Q
c
[m3/d]
[mg/l]
C
[kg/d]

Pollutant Load
= Q*c/1000

EXISTING PLANT
(or available data flowrate
and pollutants concentration)
When are available a lot of data, it’s better to eliminate single data (only flow
or only concentration).
Hence you proceed in statistic elaboration.
At the end, when you have average values of flow and loads, calculate the
value ratio:
average load
average flow
(concentration)
which generally is different from concentration average values and is more
significant, representing the weighted average of concentrations.

NOT EXISTING PLANT
1.
MUNICIPAL WASTE WATER
You have to refer your design to the SPECIFIC CONTRIBUTION PER
CAPITA, which generally result prudential values.
2.
INDUSTRIAL WASTE WATER
You have to refer your design to the available SPECIFIC CONTRIBUTION
PER UNIT OF PRODUCTS, adopting some security factors.
POLLUTANTS BALANCE
In biological plant sizing the ratio COD/BOD and BOD/TKN (or
COD/TKN) are very important
In Denitrification you need organic load to remove Nitrogen.
assume:
3 kgBOD/kg(N-NO3)DEN
4 kgBOD/kg(N-NO3)DEN
sizing oxidation
sizing post-denitrification
(methanol requirements)
Calculate Pollutants balance for these following cases (to verify section
sizing):
M (BOD) + M(TKN)
M(BOD) + M+2s (TKN)
M+2s (BOD) + M(TKN)
NITROGEN BALANCE
TKNin+(N-NO2)in+(N-NO3)in =
= TKNSED+(N-NO3)DEN+TKNox+TKNout+(N-NO2)out+(N-NO3)out
Where:
TKNin
(N-NO2)in
(N-NO3)in
TKNSED
TKNin(N-NO3)DEN
TKNox
TKNout
(N-NO2)out
(N-NO3)out
= inlet Nitrogen (organic ed ammonia)
= inlet Nitrogen (nitrite):
generally absent
= inlet Nitrogen (nitrate):
present only in industrial wastewater
= organic Nitrogen removed in primary sedimentation: 10÷15% TKNin
= nitrogen to remove by denitrification
= TKN removed by bacterial metabolism (5% BOD removed in
biological treatment = 0,05 (BODin Den – BODout)
= outlet Nitrogen (organic ed ammonia) - assume: 1 mg/l
= outlet Nitrogen (nitrite) - negligible
= outlet Nitrogen (nitrate) - project requirement(10÷15 mg/l)
Normally you can’t have in the same time significant values of (N-NH3)out and (N-NO3)out
DENITRIFICATION DESIGN
DENITRIFICATION VELOCITY (municipal effluents)
(nD)T = (nD)20 * qT-20
Where:
(nD)T [gN-NO3/kgVSS*d] = Denitrification velocity:actual operative conditions
(temperature = T);
(nD)20 [gN-NO3/kgVSS*d] = Denitrification velocity: max value at T = 20 °C,
without any limiting factor;
q = Temperature correction coefficient (higher value, higher T dependence)
Process parameter
Max Denitrification velocity
Temperature correction
coefficient
Symbol
M.U.
(nD)20
g N-NO3/
q
(Kg VSS*d)
/
Value
Reference
80÷100
Ekama – Beccari
1,06÷1.08
Ekama - Beccari
DENITRIFICATION VELOCITY
INTERNAL CARBON
DENITRIFICATION
VOCE
Organic fraction
Denitrification velocity
PRE-DEN
Average
vel.
POSTDEN
Average
vel.
Unità di misura
Scaunich
vecchio
Scaunich
attuale
Forte
influenza
T
Esercizio
attuale
Debole
influenza
T
SSV/SST
0,7
0,7
0,7
0,7
0,7
1,12
1,065
1,200
1,080
1,030
Temperature correction coefficient
Denitrification velocity
PRE-DEN
Inizial
velocity
a °C
20
gN-NO3/kgSSTxd
70,0
56,0
504,0
70,7
50,4
a °C
18
gN-NO3/kgSSTxd
55,8
49,4
350,0
60,6
47,5
a °C
16
gN-NO3/kgSSTxd
44,5
43,5
243,1
52,0
44,8
a °C
14
gN-NO3/kgSSTxd
35,5
38,4
168,8
44,6
42,2
a °C
12
gN-NO3/kgSSTxd
28,3
33,8
117,2
38,2
39,8
a °C
10
gN-NO3/kgSSTxd
22,5
29,8
81,4
32,7
37,5
a °C
20
gN-NO3/kgSSVxd
100,0
80,0
720,0
101,0
72,0
a °C
18
gN-NO3/kgSSVxd
79,7
70,5
500,0
86,6
67,9
a °C
16
gN-NO3/kgSSVxd
63,6
62,2
347,2
74,2
64,0
a °C
14
gN-NO3/kgSSVxd
50,7
54,8
241,1
63,6
60,3
a °C
12
gN-NO3/kgSSVxd
40,4
48,3
167,4
54,6
56,8
a °C
10
gN-NO3/kgSSVxd
32,2
42,6
116,3
46,8
53,6
DENITRIFICATION VOLUME CALCULATION
V=
(N-NO3)DEN
------------------(nD)T * X
Where:
V [m3] = Minimum design Denitrification volume
T [°C] = Minimum design Temperature
(N-NO3)DEN [kg N-NO3/d] = nitrogen to remove by denitrification
X
[kgSSV/m3]: = Volatile Suspended
(Denitrification – Nitrification)
Solids
concentration
in
biological
basins
Note: It’s opportune to assure a minimum residential time of 3÷4 h at the maximum flow, to
give to mixed liquor enough time to reduce its O2 content (DO concentration of 0,5
mg/l reduce denitrification efficiency to 10%)
MIXED LIQUOR TO RECYCLE CALCULATION
QML =
1000 * (N-NO3)DEN
------------------------- - QR
24 * N-NO3 out
Where:
QML [m3/h] = flowrate of recirculated Mixed Liquor
QR [m3/h] = return sludge flowrate
(N-NO3)DEN [kg N-NO3/d] = nitrogen to remove by denitrification
N-NO3 out [g/m3] = concentration of nitrogen in outlet stream (design value)
1000 = conversion factor (kg  g)
24 =
conversion factor (d  h)
MIXING - DENITRIFICATION
Above
8÷10 W/m3 energy density is required (normal submersible mixers)
Mixer rotation velocity must be chosen as low as possible (< 700 rpm)
OXIDATION DESIGN
PRELIMINARY SIZING
V=
BODin
--------------X * F/M
Where:
BODin [kgBOD/d] = Inlet BOD, coming from Denitrification
X [kgSST/m3]
= Total Suspended Solids concentration in biological basins
(Denitrification – Nitrification): Values: 4÷6
SSV/SST
= Organic fraction: typical
F/M [kgBOD/kgSST*d] = Ratio Food/Mass:
- extended aeration
- nitrification (according T)
- carbon removal only (h =85-90%)
= 0,7
Typical values
0,075
0,15
0,25
range
(0,06÷0,09)
(0,12÷0,18)
(0,2÷0,35)
OXIDATION DESIGN
NITRIFICATION VERIFING
Where:
(nn)T = Nitrification velocity: actual operative conditions (temperature = T [gTKN/kgSSV/d];
(nn)20
= Nitrification velocity: max value at T = 20 °C, without any limiting factor;
[gTKN/kgSSV/d];
q = Temperature correction coefficient;
KTKN, KO = semisaturation constants, relating to TKN and DO [mg/l];
TKN, O.D.= TKN and Oxygen concentrations in biological basins [mg/l]
OXIDATION DESIGN
NITRIFICATION VERIFING
OXIDATION DESIGN
CALCULATION OF NITRIFICANT
BACTERIA FRACTION
Where:
yN
y
S0
Se
TKN0
TKNe
= nitrificant bacteria cellular yield coefficient [kgSSV/kg/TKN]
= heterotrophic bacteria cellular yield coefficient [gSSV/gBOD]
= inlet organic matter [mg/l]
= outlet organic matter [mg/l]
= inlet TKN [mg/l]
= outlet TKN [mg/l]
y/yN = 4,72 (Bonomo, 2008)
OXIDATION DESIGN
NITRIFICATION VOLUME CALCULATION
Where:
x
= Total Suspended Solids concentration in biological basins [kgSST/m3]
XN
= Total nitrificant bacteria in nitrification basins [kgSST]
OXIDATION DESIGN
RETURN SLUDGE FLOWRATE
Where:
xr = Total Suspended Solids concentration in return sludge [kgSST/m3]
OXIDATION DESIGN
RETURN SLUDGE FLOWRATE – IMHOFF CONE
(Q + Qr)Va = Qr Vr
Va (l/l)
Qr
Va
-------------- =
Q
--------------Vr - Va
If Vr = 1 l/l
Qr
-------------- =
Q
Va
--------------1 - Va
Vr (l/l)
OXIDATION DESIGN
RETURN SLUDGE FLOWRATE SVI (sludge volume index)
Imhoff cone – 30 min [ml/l] or [cc/l]
SVI =
Imhoff
--------------x
Where:
x = Total Suspended Solids concentration in biological basins [g/l]
Qr
-------------- =
Q
x
--------------1000/SVI - x
OXIDATION DESIGN
EXCESS SLUDGE FLOWRATE CALCULATION
OXIDATION DESIGN
ACTUAL OXYGEN REQUIREMENTS (AOR) & STANDARD OXYGEN
REQUIREMENTS (SOR)
Where:
a = Carbon removal coefficient = 0,5 kgO2/kgBOD
b = Endogenous respiration coefficient = 0,08 kgO2/kgSST/d
N da nitrificare = N to remove in nitrification [kgN-NH4/d]
2,86 KgO2/KgNDEN = Oxygen recovery
OXIDATION DESIGN
ACTUAL OXYGEN REQUIREMENTS (AOR) & STANDARD OXYGEN
REQUIREMENTS (SOR)
Where:
a
= rapporto tra il coefficiente di trasferimento relativo al liquido reale a 20°C e
quello relativo alle condizioni standard, fissato pari a 0,70;
b
= rapporto tra la concentrazione di ossigeno a saturazione nel liquido reale in
condizioni di esercizio e quella in acqua pulita in condizioni di esercizio;
Cs,T
= concentrazione di ossigeno a saturazione in acqua pulita alla temperatura di
esercizio T;
Cw,T
= concentrazione di ossigeno nel liquido reale alle condizioni di esercizio, fissata
pari a 2 mg/l;
Cs,*
= concentrazione di saturazione in acqua pulita in condizioni standard (20 °C);
T
= Temperatura nelle condizioni di esercizio
OXIDATION DESIGN
AIR DEMAND
Where:
24 = days
hours;
0,28 = Kg O2 / mc air in standard conditions (20°C – 0 m a.s.l.);
h = transfer efficiency O2 = 5% / m depth.
SEDIMENTATION DESIGN
Hydraulic head
(mc/mqxh)
Ci=Q/A
0,20 – 0,30
Solid load
(kg SST/mqxd)
Cs = G/A
< 5 a Q24


<9 a Qmax

Height (m)

Bridge
≥3m
Suction bridge
- Q (mc/h), flowrate
- A (mq), area
- G (kgSST/d), solid
flowrate = 2,5 Qr X
- X (kgSST/mc), activated
sludge concentration
- Qr (mc/h), return sludge
flowrate = 1 – 1,5 Q24
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