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CEU- 401 Environmental Engineering - II Contents: 1. Activated Sludge Process - Design 2. Trickling filters – Details & Design 3. Oxidation Pond - Design
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Page 1: ASP - ppt - EE-TT

CEU- 401Environmental Engineering - II

Contents:

1. Activated Sludge Process - Design

2. Trickling filters – Details & Design

3. Oxidation Pond - Design

Page 2: ASP - ppt - EE-TT

Modification in the ASP

1.Tapered aeration process

� Higher air-supply at the inlet

� 45%, 30% & 25% @ each 1/3 lengths

2. Step aeration process

� Sewage introduced along the length

� Require uniform O2 throughout

Q

Qr

Page 3: ASP - ppt - EE-TT

3. Contact Stabilization process

� shorter aeration in the recycle line

� 0.5 to 1.5 hours

Modification in the ASP

Aeration tank

SST

Stab. TankQw

Qr

Page 4: ASP - ppt - EE-TT

4. Complete Mix process

� Hourly variation high

� Biological instability problems occur

� Uniform supply & uniform with drawl

Modification in the ASP

Qr

Q

SST

Page 5: ASP - ppt - EE-TT

Modification in the ASP

5. Extended Aeration Process

� PST is avoided

� Aeration ~ 12 to 24 hours

� BOD removal ~ 95 to 98%

� High MLSS concentration

� O2 required so high

� Sludge in endogenous respiration

Page 6: ASP - ppt - EE-TT

Size & Volume of Aeration Tank

tc

w R

V X

Q Xθ = ( )w R y o E e tQ X Q Y Y k X Vα= − −

( )te t y o E

c

V Xk X V Q Y Yα

θ+ = −

1( ) ( )e t y o E

c

k X V Q Y Yαθ

+ = −

( )

(1 )y o E c

te c

Q Y YV X

k

α θθ

−=

+Gives, V for any assumed

values of θc & Xt

o

t

Q YF

M V X=

Page 7: ASP - ppt - EE-TT

O2 Requirements of the Aeration Tank

O2 Reqd. for,

� Oxidation of influent organic matter

� Endogenous respiration of microbes

02

( )( / ) 1.42E

w R

Q Y YO gm d Q X

f

−= −

5 0.68u

BODf

BOD= ≈

O2 Reqd. for nitrification = 4.56 kg O2/kg NH3-N

Page 8: ASP - ppt - EE-TT

O2 Requirements of the Aeration Tank

Aerators provided � to transfer O2

Under, T = 20°C ; P = 760 mm Hg

O2 transfer capacity (N) under field condition,

20( ) (1.024)

9.17

Ts S LN D D

Nα−−=

Ds – DO saturation value for sewage

DL – Operation DO level in aeration tank

α- correction factor for O2 transfer for sewage (0.8- 0.85)

Ns – O2 @ std. condition � 1.2 - 2.4 kg O2/kWh

Page 9: ASP - ppt - EE-TT

Design of ASP

1.Compute daily sewage flow, YO and YE based

on desired BOD removal

2. Find the F/M ratio and MLSS conc. from

the std. table

3. From F/M, calculate V of the tank

4. Check the HRT (t = V/Q) & verify the value

Page 10: ASP - ppt - EE-TT

Design of ASP

5. Check the SRT (θc) to be maintained

6. Check the volumetric loading = Q YO/V

7. Check return sludge ratio (QR/Q) – 0.25 to

0.50

8. Calculate tank dimensions,

d � 3-4.5 m; W � 5 to 10 m &

L � 30 to 100 m

Page 11: ASP - ppt - EE-TT

Design of ASP

9. Calculate the rate of air supply

� 100 m3 of O2/kg of BOD removed

10. Calculate no. of diffusers reqd.

� each length, 1.2 m3 O2/min/m2

11. Design the SST by assuming the surface

loading rate; SST = 20 m3/d/m2

12. Finally design the sludge drying bed loading

Page 12: ASP - ppt - EE-TT

Trickling Filters (TFs)

� Conventional TFs

� High-rate TFs (improved form)

o Also called as percolating or sprinkling filters

o Components:

� Coarser filtering media

� Spray nozzle or rotary distributor

�Under drain system

�Microbial layer (attached layer �slime layer)

Page 13: ASP - ppt - EE-TT

Trickling Filters (TFs)

o Innermost layer in anaerobic

o Deficit for OM & O2

o Sloughing � creates turbidity

o Sloughing is a function of

�Organic loading – rate of metabolism

�Hydraulic loading – creates shearing velocity

o TFs constructed above GL

o Rotation of arms – 2 to 0.5 RPM

o Diameter – 30 to 60 m

Page 14: ASP - ppt - EE-TT

Trickling Filters (TFs)

� DT – 1 to 3 min

� Filter media – 2 to 3 m

� Honey combed well

� Under drains

� Flow velocity – 0.9 m/sec

� High-rate TF � Recirculation of sewage through the filters

Page 15: ASP - ppt - EE-TT

Trickling Filter – Merits & Demerits

o High quality effluent

o Loading high

o 75% BOD removal & 85% SS removal

o Good at warm weather & Self cleansing also

o Head-loss high

o High construction cost

o Cant treat raw sewage

o More operational troubles : (1) Odour, (2) Ponding

troubles and (3) Fly nuisance

Page 16: ASP - ppt - EE-TT

Design of Trickling Filter

o Design of tank diameter and depth

o Design of rotary distributor

o Design of under-drainage system

Design based on,

� Hydraulic-loading rate

� 22 to 44 ML /ha/day

� 110 to 330 ML/ha/day

� Organic –loading rate

� 900-2200 kg BOD5/ha-m/day

� 6000-18000 kg BOD5/ha-m/day

Page 17: ASP - ppt - EE-TT

Performance of Conventional TFs

o In conventional TF – highly nitrified effluent with

stabilized sludge

o BOD reduction � 80 to 90%

o Efficiency,

o u � organic loading rate in kg/ha-m/day

o Higher loading provides lesser efficiency in the

filters

100(%)

1 0.0044 uη =

+

Page 18: ASP - ppt - EE-TT

Recirculation of Treated Sewage

o Essential in high-rate filters

o Recirculation � portion of treated or partially

treated sewage

o Single-stage or two-stage recirculation process

o Recirculation given (1) continuous dosing, (2)

equalizing, (3) longer contact and (4) influent

remains fresh all the time

o Higher influent flow � wash off the filter

o Loss of nitrates

Page 19: ASP - ppt - EE-TT

Efficiency of High-rate TFs

o Depends on � (1) volume of recirculated flow

and (2) organic loading

o Recirculation ration, = R/I

R – Vol. of sewage recirculated ;

I – Vol. of raw sewage

o Recirculation factor,

o Efficiency (%),

2

1

[1 0 .1 ]

R

IFR

I

+=

+

100

1 0.0044Y

V F

η =+

Y – Total organic loading in kg/dayV – Filter volume in ha-m

1

1 1

100

0.00441

1Y

V F

η

η

=+

2nd stage Efficiency

Page 20: ASP - ppt - EE-TT

Types of High-rate TFs

1. Biofilters

� Shallow filters with 1.2 to 1.5 m deep

� Recirculation of portion of filter eff. to the PST

2. Accelo-Filters

� 1.8 to 2.4 m deep

� Direct recirculation of unsettled filter eff. to the

distributor feed

3. Aero-Filters

� recirculation only during low sewage flow

condition (depth – 1.8 m)

� Recirculation of SST eff. to the distributor feed

Page 21: ASP - ppt - EE-TT

Steps Involved in the Design of TFs

1.Calculate total BOD to be treated per day

2.Assume organic loading rate & find volume

3.Assume, depth and find the surface area

4.Calculate, the diameter of the TF

5.Check for hydraulic loading

6.Design the rotary distributors � design for

peak flow (2.25 times average flow)

Page 22: ASP - ppt - EE-TT

Steps Involved in the Design of TFs

7. Assume velocity of flow @ peak flow - 2 m/sec

8. Then find the surface Area

9. Design the arms � rotary spray type

(a) Length of arm = (D/2) – width of central column

(b) Assume no. of arms

(c) Calculate area of arm by assuming, v = 1.2 m/sec

(d) find, area of arm A = (Q/V)

Page 23: ASP - ppt - EE-TT

Steps Involved in the Design of TFs

10. Design the orifice opening in the arm (no. of

orifices in the arm)

(a) Discharge in each orifice,

(b) Total no of orifices =

11. Design of under-drainage system

Area of channel =

Page 24: ASP - ppt - EE-TT

Steps Involved in the Design of TFs

12. Assume, width of under-drain �0.25 m

� Assume, rectangular channel, then

calculate depth

13. Find, slope of the channel (S)

14. Finally, design the laterals

Page 25: ASP - ppt - EE-TT

Oxidation Ponds

o Open flow earthen basin

o Longer detention period (few days to several days)

o Stabilization ponds � By aerobic bacteria

o O2 demand met by � algae and microbes

o Algal-photosynthesis or algal-symbiosis

o Algae � produce O2 during photosynthesis

o End products � CO2, NH3 and phosphates

o Very small depth � below 0.5 m

Page 26: ASP - ppt - EE-TT

Oxidation Ponds

o Practically pure aerobic ponds are difficult to

operate

o Facultative ponds are used � 1 to 1.5 m

o Oxidation pond � partially treated sewage is

introduced as influent

o Sewage Lagoon � received raw sewage

o Oxidation pond treatment

� reduced BOD/OM

� algae discharged along effluent

Page 27: ASP - ppt - EE-TT

Oxidation Ponds

o Oxidation pond

effluent not

disposed in the

U/S end of the

dams/rivers

o Oxidation Pond

effluent used for

land irrigation

Page 28: ASP - ppt - EE-TT

Oxidation Pond - Design Criteria

o Surface area worked out assuming suitable

organic loading rate

o Hot tropical countries � 300 to 150 kg/ha/day

o Colder countries � 90 to 60 kg/ha/day

o L = 2 W & depth ~ 1 to 1.5 m

o Detention time – 20 to 30 days

o Free board � 1 m

o BOD5 loading rate depends on latitude

Latitud(o N)

BOD5loading

8 325

12 300

16 275

20 250

36 150

Page 29: ASP - ppt - EE-TT

Oxidation Pond - Design Criteria

o Detention time (days)

o

o L � BOD of the effluent entering the pond

o Y � BOD removed

o BOD removal - 90% & Coliform removal - 99%

o Sludge accumulated in ponds – 2 to 5 cm/year

o Sludge removal required once in 6 years

o Min liquid depth to be maintained – 0.3 m

Page 30: ASP - ppt - EE-TT

Advantages of Oxidation Pond

o Suitable for hot dry countries (> 200 days)

o Suitable where large land available at low price

o Very cheap in installation

o Maintenance cost very less

o No skilled supervision required

o Flexible in operation

o Nuisance due to mosquito breading

o Odour problem – Far from city

Page 31: ASP - ppt - EE-TT

Design Problems1. Design a conventional ASP to treat domestic sewage

with diffused air aeration system, given the following data:Population – 35,000; Ave. sewage flow – 180 lpcd; BOD of sewage – 220 mg/L; BOD removed in PST –30%; Overall BOD reduction desired – 85%

2. Design suitable dimensions of a circular trickling filter units for treating 5 ML of sewage per day. The BOD of sewage is 150 mg/L. Also design suitable dimensions for its rotary distribution system, as well as under-drainage system.


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