Izmir

Wastewater Treatment Plant of Izmir...

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Description of the Plant Operation...

Inlet Works...

The sewage coming from the pumping station is received in an inlet chamber and is distributed into 3 parallel running screen channels through motorised penstocks. Each channel equipped with fine screen that works for removing fine particles in the wastewater. The screens are cleaned automatically under level control or timer control. The raked screenings drop onto a belt conveyor which carries them into a screenings press. Dewatered screenings from the screenings press are collected in containers for removal off site for disposal. Screened effluent flows into a distribution chamber and from there it is distributed into 4 parallel running aerated grit channels through motorised penstocks. Separated grit at the bottom of the grit channels is pumped out into a grit through by submersible grit pumps mounted on to the travelling bridges. The grit pumped into the grit through flows into the grit washer / classifier units. Organic substances washed away from the grit is returned to the works inlet and the separated grit is collected in containers for disposal. Degritted effluent from the grit channels passes through 3 parallel running Parshall flumes for flow measurement and is then collected in a distribution chamber. The inlet works has been sized for 12 m³ / sec maximum wet weather flow. The subsequent biological treatment section shall be dimensioned according to a hydraulic load of 12 m³ / sec for a duration of 3 successive hours. In case the wet weather flow 12 m³ / sec, exceeds the duration of 3 hours, the flow to the biological treatment shall be limited to 9 m³ / sec for additional 3 hours. The biological plant unit is designed to treat a pollution load according to table given at the section of "Design Parameters" in compliance with the values mentioned in the "Design Basis of the Contract Volume 1/2". By using the total flow value obtained from the 3 Parshall flumes and the flow distributed into each of the 3 lines it will be possible to accept the a.m. flows automatically into the biological treatment section of the plant. Additional quantities above these values will be passed by means of automatic penstocks.

Primary Settlement...

The effluent from the distribution chamber is equally distributed into 3 parallel running lines. The flow into each line will be monitored by ultrasonic flow meters and received into 3 distribution chambers before each line and from where it will be distributed into 4 circular settlement tanks in each line. According to the season, operation mode is changed. At winter operation (nutrient removal is required) the complete wastewater flow is directed to 2 of the 4 primary sedimentation tanks. The tanks out of operation will be emptied by primary sludge pumps. In summer, these tanks are taken into operation to achieve the highest possible removal efficiency with respect to organic pollution load. Settled sludge in each settlement tank will be pumped out by a dedicated sludge pump under timer control and sent to the sludge treatment plant. Sludge suction lines will be flushed automatically by high pressure wash water after each desludging period to avoid blockages in pipework. Rodding eyes will be provided at appropriate locations in the sludge pipework to interene manually in the event of blockages occurring. There will be a electro - magnetic type flowmeters in each of the 3 sludge discharge main to monitor the quantity of sludge produced. The flowmeters will also be used to protect sludge pumps against dry running by shutting down pumps when no flow is detected after a preset time from the start of desludging. Scum developed at the surface of the settlement tanks will be swept out by the descumming devices and collected in a sump. Submersible scum pumps installed in each of the 3 scum pumps will pump the scum into the sludge holding tank. Clarified effluent will gtavitate into the biological treatment system.

Biological Treatment...

The aeration tanks are arrange in 3 lanes each consisting of 2 lines. One line of aeration tanks consist of 2 oxidation ditch tanks operating in series (cascade - I and cascade - II). Clarified effluent will first be let into ditch type anaerobic tanks for biological phosphorus removal. Recycled activated sludge from the final sedimentation tank will be directed to the first Bio - P tank. The clarified effluent from the primary sedimentation tank will also enter this tank. Mixing in the anaerobic tanks will be achieved by submersible mixers. Phosphorus present in the effluent will be metabolised under anaerobic conditions by the microorganism in the recycled sludge. Effluent from the anaerobic tanks will flow into a distribution chamber. From this distribution chamber the wastewater will flow to 2 aeration lanes consisting of 2 race track oxidation tanks operating in series. Horizontal mixing in the oxidation ditches (process tanks) will be achieved by 6 submersible mixers installed from the fixed bridges. Air blowers will provide the necessary dissolved oxygen in the system. There will be one blower building for a total of 5 blowers of which one unit as stand-by. Membrane type high efficiency fine bubble diffusers will be used to provide dissolved oxygen from pressurised air delivered by the blowers. The necessary diffusers will be arranged 40 % in the first aeration tank whereas 60 % will be located in the second tank of the lane. This facilitates alternating compartments (oxic and anoxic) within the same tank and providing simultaneous denitrification effect. With the effective internal recycling between the 2 tanks the nitrate rich effluent of the second tank is denitrified in the first cascade where the concentration of carbon source is higher due to influent from the Bio - P tank. For the internal recycling 2 submersible propeller pumps are installed for each line of the 3 lanes. Effluent from oxidation ditches will flow over weirs and is collected in a distribution chamber in each line before let into final settlement.

A significant portion of the electrical energy used in the plant is consumed by the aeration system. In order to economise energy consumption by the aeration system, a dissolved oxygen control system will be provided to control the level of dissolved oxygen in the tanks and thereby adjusts the blowers output.

Final Settlement...

Effluent from the distribution chambers will be distributed into 4 circular final settlement tanks in each line. Settled activated sludge in final settlement tanks will be desludged into a sludge sump by motorised valves. Sludge from the sumps will gravitate into the return sludge pumping station and from there it will be returned to the first Bio - P tank. Surplus sludge generated will be pumped to the sludge mixing compartment of the sludge mixing and equalization tank by 2 (1 duty + 1 stand-by) submersible pumps. Any scum accumulating on tank surfaces will be collected by descumming arms into a scum sump in each line. There will be 2 submersible scum pumps operating under level control (1 duty + 1 stand-by) in each scum sump to pump scum into the sludge holding tank. Final effluent from the final settlement tanks will be collected and is discharged into the sea through a discharge channel.

Irrigation Pumping Station...

Final effluent from the final settlement tanks can be taken into a pumping station by a penstock when required for irrigation. Since there is no information available on the distance of area to be irrigated and the head required, a pumping station based on the following assumptions is included in the tender on a provisional basis.

Item	Value
Total flow	7 m³ / sec
Number of pumps	8 (7 duty + 1 stand-by
Pump duty	1 m³ / sec
Pump head	40 m WC

Sludge Treatment System...

The primary sludge obtained from the primary sedimentation tanks with dry matter content of approximately 2 % is collected in a sludge holding tank 9/A. The second sludge holding tank (9/B) will be used as sludge mixing and equalisation tank and consists of 2 compartments. 1 small mixing compartment serves for the mixing and homogenising of primary sludge and surplus activated sludge. The second compartment is used for storing the mixed sludge for a short time in case of a short term excess amount of the sludge. The primary sludge holding tank 9/A will be equipped with two submersible mixers which homogenise the sludge and prevent the dry substance from settling. Also in 9/A two submersible pumps are located which transfer primary sludge to the mixing compartment of 9/B in such an amount, that a constant DS-content of the mixed sludge is achieved. The mixing compartment of 9/B is equipped with diffusers. This diffusers will be supplied with compressed air by two blowers. The air is used for mixing and homogenising the primary- and excess sludge and to keep the sludge under aerobic conditions to avoid biological phosphorus release. Homogenised sludge from the sludge equalization tank will be pumped into mechanical sludge thickeners by 10 positive displacement progressive cavity type pumps.

There will be a electromagnetic type sludge flowmeters in each sludge pumping line to monitor sludge flow into the thickeners. Flowmeters will also be used for pump protection against no-flow conditions. Pumps will be shut down in such cases and an alarm will be given for operators. Polyelectrolyte solution required for sludge conditioning before thickening will be prepared by 2 package type polyelectrolyte preparation units. There will be 10 (9 duty + 1 stand-by) polyelectrolyte dosage pumps. A flocculation tank with a mechanical mixer will be provided in each sludge line. Conditioned sludge will be thickened to approximately 6 % DM content in 10 belt type mechanical thickeners and the thickened sludge will flow by gravity into the belt presses for dewatering. The mechanical thickeners will be installed at an elevated position in the press building to facilitate gravity flow into belt presses. Filtrate from mechanical thickeners will be collected by gravity in a filtrate sump. Thickened sludge from the thickeners will be fed into 10 belt presses by gravity. Belt presses will produce sludge cake at 26 - 30 % DM content. Filtrate from the belt presses will flow through channels into the filtrate sump. Sludge cake produced in belt presses will be transported outside the building by 2 belt conveyors into the cake storage area. Filtrate collected in the filtrate sump will be returned to the works inlet by 3 (2 duty + 1 stand-by) submersible pumps.

Process Calculations...

Design Parameters...

Design wastewater flowrates are given below :

Wastewater flow	Flowrate (m³ / sec)	Flowrate (m³ / hour)	Flowrate (m³ / day)
Average dry weather flow	7	25,200	605,000
Maximum dry weather flow	9	32,400	-
Maximum wet weather flow	12	43,200	680,400

During wet weather conditions (3 hours) the concentration of pollutants will be reduced due to the dilution effect of the rain water. It was assumed that the concentration during the first 3 hours of the rain event (flow = 9 m³ / sec) will remain constant. After this 3 hours, the flow was assumed to be 12 m³ / sec. The sewage composition is given below.

Parameter	Concentration (mg / L)	Load (kg / h)
BOD₅	233	10,066
COD	350	15,120
SS	292	12,614
Total - N	35	1,512
Total - P	3.5	151

The load for the static design calculation for the plant under this "wet weather conditions" have been calculated as shown in the following example for the parameter "Tot-N" :

[ (60)(25,200)(18) + (60)(32,400)(3) + (35)(43,200)(3) ] / 1,000 = 37,584 kg / day

Using this estimations, the sewage composition for dry and wet weather conditions are given as follows.

Parameter	Dry weather condition		Wet weather condition
Parameter	Concentration (mg / L)	Load (kg /day)	Concentration (mg / L)	Load (kg /day)
BOD₅	400	242,000	368	250,560
COD	600	363,000	552	375,840
SS	500	302,500	460	313,200
Total-N	60	36,300	55	37,584
Total-P	6	3,630	5.5	3,758
T_winter	15^oC	-	-	-
T_summer	22^oC	-	-	-

Discharge Criteria...

Parameter	Summer concentration (mg / L)	Winter concentration (mg / L)
BOD₅	20	20
COD	100	100
SS	30	30
NH₄ - N	-	10
Total - N	-	12
PO₄ - P	-	1

Fine Screens...

Item	Value	Unit / Specification / Explanation
Maximum flow	12	m³ / sec
Velocity at maximum flow	1.2	m / sec
Bar opening	20	mm
Minimum velocity	0.5	m / sec
Number of units	3	2 duty + 1 stand-by
Type	-	Mechanical raked bar screen
Maximum flow in each unit	6	m³ / sec
Channel width	2,400	mm
Maximum water depth	2,080	mm
Minimum water depth	1,500	mm
Estimated quantity	0.035	L / m³

Grit Chamber...

Item	Value	Unit / Specification / Explanation
Maximum flow	12	m³ / sec
Horizontal velocity at max. flow	0.175	m / sec
Retention time at max. flow	2	min
Number of units	4	Automatically cleaned
Maximum flow in each unit	3	m³ / sec
Flow area	17.2	m² / sec
Channel width	5,200	mm
Depth	4,800	mm
Channel length	21,000	mm
Air supply requirement	0.30	m³ / min . m
Air supply to each basin	6.3	m³ / min
Total air supply	25.2	m³ / min
Estimated amount	30	m³ / day

Primary Settling Tank...

Item	Value	Unit / Specification / Explanation
Number of tanks	12	Circular
Average flow	7	m³ / sec
Maximum flow	12	m³ / sec
Diameter	40.9	m
Side wall depth (water)	3.4	m
Bottom slope (to center)	5	^o
Area of each tank	1,305	m²
Operating volume	5,200	m³
Weir length for each tank	120	m
Scraper type	-	Side driven - half bridge
Peripheral speed	3	cm / sec
Average flow to each tank	2,100 - 4,200	m³ / h
Average retention time	2.48 - 1.24	h
Average surface loading rate	1.61 - 3.22	m³ / m² . h
Average weir loading rate	17.5 - 35.0	m³ / m . h
BOD₅ removal efficiency	35 - 25	%
SS removal efficiency	72 - 65	%
Total - N removal efficiency	10 - 10	%
Total - P removal efficiency	8 - 8	%
1^st value for summer and 2^nd value for winter

Primary Sludge Production and Effluent Characteristics...

Item	Value	Unit / Specification / Explanation
Influent SS	302,500 - 302,500	kg / day
Dry matter removed SS	217,800 - 196,625	kg / day
Dry matter concentration	2.0 - 2.0	%
Primary sludge flowrate	10,890 - 9,831	m³ / day
Number of withdrawal pumps	12 - 6	pcs
Capacity of one pump	120 - 120	m³ / h
Total capacity	1,440 - 720	m³ / h
Daily operation time	7.5 - 13.7	h
Effluent BOD₅	260 - 300	mg / L
Effluent COD	390 - 450	mg / L
Effluent SS	140 - 175	mg / L
Effluent total - N	54 - 54	mg / L
Effluent total - P	5.5 - 5.5	mg / L
1^st value for summer and 2^nd value for winter (for DWF)

Biological Treatment...

The design of the biological treatment stage is determined by the winter operation mode of the plant. Therefore the following design calculation is based on the respective data. Combined nitrification - denitrification in an oxidation ditch process with separate biological phosphate removal 15 ^oC in winter conditions.

Average wastewater flow = 7 m³ / sec = 605,000 m³ / day
Maximum wastewater flow = 680,400 m³ / day

Effluent of Primary Sedimentation...

Parameter	Dry weather condition		Wet weather condition
Parameter	Concentration (mg / L)	Load (kg /day)	Concentration (mg / L)	Load (kg /day)
BOD₅	300	181,500	280	190,640
COD	450	272,250	420	285,960
SS	175	105,875	167	113,400
Total-N	54	32,670	50	33,826
Total-P	5.5	3,328	5.1	3,445
T	15^oC	-	15^oC	-

Basic Design Parameters...

pH = 7.2
MLSS = 3,800 mg / L
MLVSS / MLSS = 0.785
DO = 2.0 mg / L
µ_max = 0.52 day^-1
K_OX = 0.30 mg / L
Y_N = 0.20 kg MLVSS / kg NO₃ - N
Y_{C - max} = 0.80 kg MLVSS / kg BOD_{5 - removed}
K_N = 0.50 mg / L
k_dN = 0.05 day^-1
k_d = 0.05 day^-1
S_{NO - DWF} = 54 mg / L
S_{NO - WWF} = 50 mg / L
S = 20 mg / L

Aerobic Sludge Age...

The corrected maximum specific growth rate is calculated using the following expression :

µ_{max - corrected} = ( µ_max ) ( e ^{0.098 ( T - 15 )} ) [ DO / ( K_OX + DO ) ] [ 1 - 0.833 ( 7.2 - pH ) ]
µ_{max - corrected} = 0.452 day^-1

The maximum substrate utilisation rate is calculated as follows :

k_N = µ_{max - corrected} / Y_N
k_N = 2.26 day^-1

The minimum residence time for autotrophic microorganisms is calculated as follows :

1 / theta_CM = ( Y_N ) ( k_N ) - k_dN
theta_CM = 2.49 day

Allowing the safety factor for nitrification of 2.0 during dry weather conditions, the aerobic sludge age is calculated as :

theta_C = 4.97 day

As further calculations show (the results of the calculation show the same aeration tank volume), the safety factor theoretically decreases to 1.87, if pollution values during wet weather conditions are used for calculation. Using this reduced safety factor, the aerobic sludge age is calculated as :

theta_C = 4.66 day

Nitrification...

The removal rate of ammonia nitrogen can calculated as :

1 / theta_C = ( Y_N ) ( U_N ) - k_dN
theta_{C - DWF} = 4.97 day
theta_{C - WWF} = 4.66 day
U_{N - DWF} = 1.26 day^-1
U_{N - WWF} = 1.32 day^-1

The concentration of residual ammonia nitrogen in the effluent is calculated as :

U_N = [ ( k_N ) ( S_N ) ] / ( K_N + S_N )
S_{N - DWF} = 0.62 mg / L
S_{N - WWF} = 0.71 mg / L

The ammonia nitrogen oxidation time is calculated as :

theta_N = { ( S_NO - S_N ) / [ ( U_N ) ( MLVSS_N ) ] } 24
MLVSS_N = 2,983 x 5.67 % = 169.1 mg / L
theta_{N - DWF} = 6.03 h
theta_{N - WWF} = 5.25 h

Surplus Sludge Production...

The effective yield coefficient for biomass growth is calculated as :

Y_C = Y_{C - max} / [ 1 + ( k_d ) ( theta_CT ) ]
theta_{C - T - DWF} = 9.7 day (see the total sludge age)
theta_{C - T - WWF} = 9.1 day (see the total sludge age)
Y_{C - DWF} = 0.54 kg MLVSS / kg BOD_{5 - removed}
Y_{C - WWF} = 0.55 kg MLVSS / kg BOD_{5 - removed}

The net biomass growth rate during aerobic phase is calculated as :

M_B = ( Y_C ) ( Q ) ( S₀ - S ) ( 10^-3 )
Q_DWF = 605,000 m³ / day
Q_WWF = 680,400 m³ / day
S_{0 - DWF} = 300 mg / L
S_{0 - WWF} = 280 mg / L
M_{B - DWF} = 91,162 kg MLVSS / day
M_{B - WWF} = 97,268 kg MLVSS / day

Assuming the portion of MLVSS to be 78.5 % :

SAS_DWF = 116,100 kg / day
SAS_WWF = 123,900 kg / day

SAS volume assuming 1 % concentration :

QSAS_DWF = 11,600 m³ / day
QSAS_WWF = 12,400 m³ / day

Note : The higher SAS volume during wet weather flow is not decisive for the design of the sludge line because of the buffering capacity of the process volume (aeration tanks and anaerobic tanks).

Nitrogen Assimilation...

Assuming 40 % cell protein and protein : N ratio = 6.25, the amount of nitrogen which is taken up by microorganisms for growth will be :

N_biomass = ( 0.40 ) ( 1 / 6.25 ) ( M _B )
N_{biomass - DWF} = 5,834 kg / day
N_{biomass - WWF} = 6,225 kg / day

Denitrification...

The net amount of nitrate nitrogen that will be denitrified :

Total - N_{into aeration - DWF} = 32,670 kg / day
Total - N_{into aeration - WWF} = 33,826 kg / day

Total - N_effluent = ( Q _d m³ / day ) ( 0.012 kg / m³ )

Total - N_{effluent - DWF} = 7,260 kg / day
Total - N_{effluent - WWF} = 8,165 kg / day

Amount of nitrogen to be denitrified = ( Total - N ) - ( Total - N_effluent ) - ( N _biomass )

N_{DN - DWF} = 19,576 kg / day
N_{DN - WWF} = 19,436 kg / day
or
N_{DN - DWF} = 32.4 mg / L
N_{DN - WWF} = 28.6 mg / L

Total Sludge Age...

As the following calculation shows, 51.1 % of the total volume of the ditch is required for nitrification. The total sludge age will be :

theta_CT = theta_C / 0.511
theta_{CT - DWF} = 9.7 day
theta_{CT - WWF} = 9.1 day

Aerobic and Anaerobic Residence Time...

The overall aerobic phase residence time can be calculated :

theta_A = [ ( theta_CT ) ( Y_C ) ( S₀ - S ) ( V_aerobic ) ( 24 ) ] / MLVSS
theta_{A - DWF} = 6.03 h
theta_{A - WWF} = 5.36 h

This value is equal to the value of 6.03 h (DWF) and greater than 5.25 h (WWF) previously calculated. That means, that the required aerobic residence time is attained. The respective residence time available for denitrification based on the assumption of 48.9 % of ditch volume is calculated from :

theta_DN = ( theta_A ) ( 48.9 / 51.1 )
theta_{DN - DWF} = 5.77 h
theta_{DN - WWF} = 5.13 h

Calculating the denitrification rate using the below equation and biological parameteres for denitrification at 15 ^oC :

U_DN = ( U_DNS ) ( 1.09^{T - 20} ) ( 1 - DO_anoxic )
U_DNS = 0.08 kg N0₃ - N / kg MLVSS . day (specific denitrification rate for raw sewage)
T = 15 ^oC
DO_anoxic = 0.1 mg / L (average DO concentration in the anoxic zone)
U_{DN - DWF / WWF} = 0.047 kg N0₃ - N / kg MLVSS . day

The required residence time for denitrification :

theta_DN = [ ( N_DN ) ( 24 ) ] / [ ( U_DN ) ( MLVSS ) ]
theta_{DN - DWF} = 5.55 h
theta_{DN - WWF} = 4.90 h

This value is close enough to the theta_DN = 5.77 h (DWF) and 5.13 h (WWF) calculated from the overall sludge age previously. Then, total residence time :

theta_total = theta_N + theta_DN
theta_{total - DWF} = 6.03 + 5.77 = 11.8 h
theta_{total - WWF} = 5.36 + 5.13 = 10.5 h

Calculation of the BOD₅ Removal Rate...

The actual BOD₅ removal rate is calculated as :

1 / theta_CT = ( Y_{C - max} ) ( U_C ) - k_dC
U_{C - DWF} = 0.19 kg BOD_{5 - removed} / kg MLVSS . day
U_{C - WWF} = 0.20 kg BOD_{5 - removed} / kg MLVSS . day

The overall BOD₅ removal efficiency of the biological treatment is :

E = ( S₀ - S ) / S₀
E_DWF = ( 300 - 20 ) / 300 = 93.3 %
E_WWF = ( 280 - 20 ) / 280 = 92.9 %

The F : M ratio becomes :

F : M = U_C / E
F : M_DWF = 0.20 kg BOD₅ / kg MLVSS . day
F : M_WWF = 0.21 kg BOD₅ / kg MLVSS . day

Aeration Tank Dimensions...

Tank volume :

V_{aeration tank} = ( theta_total / 24 ) ( Q )
V_{aeration tank - DWF} = ( 11.8 / 24 ) ( 605,000 ) = 297,500 m³
V_{aeration tank - WWF} = ( 10.5 / 24 ) ( 680,400 ) = 297,500 m³

Number of lines = 3
Number of lanes in each line = 2
Number of cascades per lane = 2
Total number of ditches = 12
Volume of each tank = 24,790 m³
Average water depth of each tank = 6.0 m
Surface area of each tank = 4,132 m²
Overall length of each tank = 154 m
Width of each tank = 28 m

Oxygen Requirement...

The total amount of theoretical oxygen requirement is calculated using the below formulas :

ODC = { [ ( Q ) ( S₀ - S ) ( 10^-3 ) ] / f } - ( 1.42 ) ( M_B )
ODN = ( 4.6 ) ( Q ) ( N₀ - N_biomass ) ( 10^-3 ) - ( 2.86 ) ( N_DN )
OD = ODC + ODN
f = 0.67 (conversion factor for converting BOD₅ to BOD_L
ODC_DWF = 123,386 kg / day
ODC_WWF = 125,915 kg / day
ODN_DWF = 67,467 kg / day
ODN_WWF = 71,328 kg / day
OD_DWF = 190,853 kg / day
OD_WWF = 197,243 kg / day

Note : For the calculation of the hourly oxygen transfer capacity peak factors must be involved. The peak factors are primarily set for dry weather conditions to calculate the maximum hourly oxygen demand. For rainy weather conditions, the peak factors are recalculated in such a way, that the maximum hourly oxygen demand is not higher than during dry weather conditions. Allowing the following peak factors for dry weather conditions :

f_C = 1.25 and f_N = 1.80
( ODC ) ( f_C ) = ( 123,386 / 24 ) ( 1.25 ) = 6,426 kg /h
( ODN ) ( f_N ) = ( 67,467 / 24 ) ( 1.80 ) = 5,060 kg /h

Assuming the nitrogen peak load and the carbon peak load will not occur at the same time, the value for f_C = 1.25 is decisive and used for further calculation of the necessary oxygen transfer capacity. The factor f_N is set to 1.00 :

( ODC ) ( f_C ) = ( 123,386 / 24 ) ( 1.25 ) = 6,426 kg /h
( ODN ) ( f_N ) = ( 67,467 / 24 ) ( 1.00 ) = 2,811 kg /h
OD = 9,237 kg / h

Recalculation of peak factor, f_C, for wet weather conditions :

OD_maximum = 9,237 kg /h
( ODC ) ( f_C ) = OD_maximum - [ ODN / ( 24 )( f_N ) ]
( ODC ) ( f_C ) = 9,237 - [ 71,328 / ( 24 )( 1.00 ) ] = 6,265 kg / h
f_C = 1.19

Calculation of the oxygen transfer capacity (clean water) :

OC = ( OD / alpha ) [ DO_{S - corrected} / ( DO_{S - corrected} - DO ) ]
DO_S = 10.14 mg / L (oxygen saturation concentration)
ID = 5.70 m (installation depth of the diffusers)
DO_{S - corrected} = ( DO_S ) [ 1 + ( ID / 20.7 ) ]
DO_{S - corrected} = 12.93 mg / L (oxygen saturation concentration at half installation depth)
DO = 2.00 mg / L (actual oxygen concentration)
alpha = 0.70
OC = 15,610 kg / h

Estimation of Airflow...

At 5.70 m submergence and using membrane diffusers with 0.30 m diameter, the specific oxygen transfer per m³_N air and m installation depth is assumed to :

Specific oxygen transfer rate = 17.7 g O₂ / m³_N . m_ID
SOTE = ( 17.7 ) ( 5.70 ) / 290 = 34.8 %

Calculating the total airflow for 15,610 kg O₂ / h :

Q_air = OC / [ ( d_air ) ( SOTE ) ]
d_air = 0.29 kg / m³
Q_air = 154,676 m³_N / h

Number of blower stations = 3
Number of blowers per station = 4 + 1
Capacity of each blower = 13,000 m³_N / h
Maximum head = 700 mbar
Nominal capacity of each diffuser = 7.22 m³_N / h
Total number of diffusers installed = 21,600
Number of diffusers in each lane = 3,600
Number of diffusers in cascade - 1 = 1,440
Number of diffusers in cascade - 2 = 2,160
Oxygen transfer capacity in cascade - 1 = 1,053 kg / h
Oxygen transfer capacity in cascade - 2 = 1,580 kg / h

Return Sludge...

Number of return sludge pumps = 9 + 3
Capacity of each pump = 3,200 m³ / h
Capacity at normal operation (6 pumps) = 19,200 m³ / h
Installed capacity (9 pumps) = 28,800 m³ / h

Return sludge ratio, operation with 6 pumps, at average DWF = 76 %
Return sludge concentration at normal operation, at average DWF = 8,800 mg / L

Internal Recirculation...

For denitrification a combined pre- / simultaneous operation mode has been foreseen.

Number of recirculation pumps = 12
Capacity of each pump = 2,450 m³ / h
Total capacity = 29,400 m³ / h
Total recurcilation including return sludge (during normal operation) = 48,600 m³ / h (at average DWF)
Assumed effluent concentration of N0₃ - N, N0₃ - N = 10.0 mg / L
Total amount of N0₃ - N denitrified, N_DN = 19,576 kg / day
Portion of pre - denitrification = [ ( 10 ) (48,600 ) ( 24) ] / [ (19,576) (1,000) ] = 59.6 %

Bio - Phosphorus Removal...

Phosphorus removal will be achieved by application of separate anaerobic tanks.

Type of the tank = ditch
Number of lines = 3
Number of lanes in each line = 2 (serial)
Number of tanks = 6
Water depth = 6.0 m
Overall length = 90 m
Tank width = 15.5 m
Volume of each tank = 8,200 m³
Total reactor volume = 49,200 m³

Return sludge of sewage flow = 76 %
Sewage flow = 605,000 m³ / day
Return sludge flow = 459,800 m³ / day
Total flow = 1,064,800 m³ / day
Hydraulic retention time = 1.1 h

Final Settling Tanks...

The Following design parameters are used in sizing of final settling tanks :

Average DWF = 7 m³ / sec
Maximum WWF = 12 m³ / sec
Return sludge flow at normal operation = 5.3 m³ / sec
Return sludge ratio = 76 %
MLSS in aeration tank = 3,800 mg / L
MLSS concentration of underflow = 8,800 mg / L

Tank specifications :

Number of lines = 3
Number of settling tank per line = 4
Number of tanks = 12
Diameter of each tank = 60.0 m
Area of one tank = 2,828 m²
Total area = 33,930 m²
Side wall depth = 2.80 m
Bottom slope = 1 : 15
Operating volume of each tank = 9,800 m³
Total volume = 117,600 m³
Hydraulic retention time in each tank = 2.7 h

Hydraulic Loading...

Average surface loading = 0.74 m³ / m² . h
Maximum surface loading = 1.27 m³ / m² . h

Solid Loading...

SL = [ ( 1 + R_RS ) ( Q ) ( MLSS ) ] / [ ( A ) ( 24 ) ( 1,000 ) ] = 5.0 kg / m² . h

Sludge Treatment System...

Two tanks are foreseen for sludge storage. Settled sludge from primary settling tanks is pumped to one of these tanks. The surplus sludge from final sedimentation tanks will be pumped continuously into a separate compartment of the other storage tank, where it is mixed with primary sludge. The mixed sludge flows into the aerated storage compartment, before it is pumped to mechanical pre - thickeners followed by belt filter presses. The sludge is contitioned with polyelectrolyte before thickening and dewatering. Winter operation mode is desicive for dimensioning of the sludge treatment system. Sludge storage :

Dry matter primary sludge = 196,625 kg / day
Dry matter surplus activated sludge = 116,100 kg / day
Total dry matter = 312,725 kg / day
Amount of primary sludge = 9,830 m³ / day
Amount of surplus activated sludge = 11,610 m³ / day
Total amount of sludge = 21,440 m³ / day
Dry matter of mixed sludge = 1.46 %
Number of sludge holding tanks = 2
Tank diameter = 27.0 m
Tank depth = 5.0 m
Tank volume = 2 x 2,860 m³
Maximum retention time for primary sludge tank = 7.0 h
Operational retention time for mixed sludge = 1.0 h
Maximum retention time for mixed sludge = 3.2 h
Polyelectrolyte consumption = 5 kg / ton dry matter
Total polyelectrolyte consumption = 1,563 kg / day
Polyelectrolyte dosage concentration = 0.1 %
Mechanical thickener feed capacity = 99 m³ / h
Dry matter of thickened sludge = 6.0 %
Amount of thickened sludge = 5,210 m³ / day
Filtrate flowrate = 16,230 m³ / day
Number of belt presses = 9 + 1
Belt press feed capacity = 24 m³ / h
Thickened sludge volume = 5,210 m³ / day
Amount of sludge cake at 30 % dry matter = 1,042 m³ / day
Filtrate volume = 4,168 m³ / day
Total filtrate to be returned to plant inlet = 20, 398 m³ / day