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COMPUTER AIDED DESIGN OF

WASTE WATER TREATMENT PLANT

WITH ACTIVATED SLUDGE PROCESS

K. SUNDARA KUMAR

Associate Professor, Department of Civil Engineering, K L University, Guntur, Andhra Pradesh, India

Abstract

There are two fundamental reasons for treatment of wastewater viz., prevention of pollution and thereby

protecting the environment, and protecting the public health by safe guarding water supplies and

preventing the spread of water borne diseases. Proper design, construction together with good operation

and maintenance are essential for waste water treatment plants (WWTP), in order to produce effluents

which are satisfying the safe disposal standards prescribed by the regulatory authorities. In this work a

computer program in C++ has been developed for comprehensive design of wastewater treatment plant

which incorporates activated sludge process as biological treatment method. All the units of WWTP are

included in the design and the program is developed in a very user friendly manner by referring various

standard procedures and manuals. The validity of the software has been verified by test running and

comparison with an existing plant data. This program not only helps in sizing the treatment units but also

helps in understanding the plants capacity as well as in deciding the future expansion works needed for

increased hydraulic and organic loadings.

Keywords: Wastewater treatment, Activated Sludge Process, Aeration tank, Computer Aided Design

1. INTRODUCTIONWastewater is essentially the water supply of the community after it has been fouled by a variety of uses. Thewater supplied to a community receives a range of chemical substances and microbial flora during its use suchthat the wastewater acquires a polluting potential and becomes a health and environmental hazard.

Communicable diseases of the intestinal tract such as cholera, typhoid, dysenteries and water borne diseaseslike infectious hepatitis etc., can be spread from uncontrolled disposal of waste water, and therefore preventionof communicable diseases and protecting public health attracts the primary objective of sanitary waste waterdisposal [1].

Given the characteristics of raw wastewater and the requirements of disposal or reuse, the wastewater usuallyrequires some type of preparation or treatment before it is rendered fit for disposal or reuse. Generally, in manysituations involving domestic wastewater, the treatment consists of removal of suspended solids and 5-day,20C BOD, which are the two usual parameters of prime interest. The purpose of waste water treatment plant isto separate inorganic particulates and to stabilize the decomposable organic matter present in waste water so asto produce an effluent and sludge which can be disposed of in the environment without causing health hazardsor nuisance. The complete treatment of wastewater is brought by a sequential combination of various physicalunit operations, and chemical and biological unit processes.

Environmental Engineers are entrusted with designing wastewater treatment plants that are efficient and at thesame time cost effective. Very often the designer has to compare various operations in order to achieve theabove said objective, which requires colossal effort if done manually. Computer aided design is not only helpfulin sizing the treatment units but also useful in checking the designs of existing plant with relevant input data.

OBJECTIVES OF THE STUDY

The objectives of the present study are the following:

To develop a comprehensive design program for the design of waste water treatment plant UsingC++

K. Sundara Kumar / International Journal of Engineering Science and Technology (IJEST)

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To incorporate all field and other practical constraints in the design program To make the software be more user-friendly, which can provide guidance to the user while

entering input data (range of values) where ever necessary

To validate the developed program by checking the design of an existing treatment plant2. CASE STUDY FOR VALIDATION:Nesapakkam Sewage Treatment Plant

The Chennai city(Tamilnadu) sewerage system has been divided into five zones. Each zone has been provided

with individual collection areas, pumping stations, force mains etc. Nesapakkam Sewage Treatment Plant is

located on the western part of city and receives the sewage collected from zone 4, covering the areas like

Saidapet, Westmambalam, Ashoknagar, M.G.R Nagar, K.K.Nagar, Thirunagar etc,. The plant is designed to

cater for an average flow of 23 MLD (million liters per day) with a peak factor of 2.67. Fig 2.1 gives the layout

of the plant. The biological treatment process used in the plant is Activated sludge process. The plant is having

one inlet chamber through which the waste water enters the plant & one screen chamber by which debris may be

removed. The influent will be fed into two primary clarifiers where suspended particles are removed by settling.

The overflow liquid is then sent into a battery of aeration tanks where aeration is done by mechanical aerators.

The aeration supplies oxygen required for biological decomposition of organic matter present in the liquid. The

liquid effluent from aeration tank will be sent to two secondary clarifiers where the sludge is separated from the

liquid. The supernatant from the secondary treatment will have the desired quality suitable for disposal. The

sludge is sent for drying in sludge drying beds.

Fig 2.1 Layout of the waste water treatment plant.

3. METHODOLOGYSewage treatment plants are designed to convert a raw sewage into an acceptable final effluent and to dispose of

the solids removed in the process. Activated sludge process is the most commonly used biological treatment

method for treating municipal waste waters of large cities. The entire treatment process depends on physical as

well as biological principles and no chemical additions are provided to protect the ecosystems that receive the

treated effluents. In the present study a comprehensive C++ program has been developed for the design of the

following units as they are commonly used in the field of waste water treatment.


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1. Screen Chamber2. Grit Chamber3. Primary Clarifier4. Activated Sludge Process(Aeration Tank)5. Secondary Clarifier6. Sludge Drying Bed

The various principles and rational, scientific as well as empirical formulae used in the design of the abovetreatment units are derived from standard references, hand books and manuals[2],[3]. To improve the suitabilityof the software for various field conditions and limitations, various constraints and compatibility range valuesare incorporated. For example in the design of a sedimentation tank the diameter of the tank must be compatiblewith the scrapping mechanism system supplied by the manufacturers. Like this in each and every unit all thepossible field constraints, climatic and other conditions related to the Indian topography are included to makethe software versatile.

PROGRAM DEVELOPMENT

The advantage of computer aided design of wastewater treatment plant is an easy way to repeat the designcalculations with different sets of input data and optimal size of the system may be obtained. Using C++language, a computer program has been developed for the comprehensive design of wastewater treatment plant.

The flow chart of the program is shown in Fig. 3.1.The entire program has been written comprising of functions,with object oriented programming (OOPs) concept. Object oriented programming allows one to write programsin a much more rational manner than procedural oriented programming. One can update the program by addingdesired units. C++ inherits the qualities of Object Oriented Programming, which are data encapsulation, classinheritance and polymorphism and speed of traditional C[10]. C++ is the most commonly used and powerfulOOP paradigm to date. The main features of the software are as follows:

The software is completely user friendly. Menu is displayed to select a particular unit for design. Design procedures followed are according to standard practice and field oriented. Permissible ranges of the parameters are provided to guide the user for entering the input data. A warning message is displayed when value of any parameter entered yields a design value, which

exceeds or falls short of the expected range usually practiced and, also an option to modify thatparticular parameter until a satisfactory design is obtained.

The program is written in a user friendly environment, and supports necessary information for designof the units.

The software will not allow entering any data which is incompatible and prevents from obtainingerroneous results.

Some of the formulae used in the design procedures are given below :

Head loss in screen chamber, HL =(W/B)4/3 HV sin

= shape factor for bars, W = maximum width of bars, B = clear spacing of bars, HV = velocity head, = angleof inclination of rack.

Settling velocity of particle in Grit chamber, VS = {gd(G-1)/3Cd}1/2

d = diameter of particle, g = acc due to gravity, G = sp. gravity of particle, Cd is the coefficient of drag.

Hydraulic retention of aeration tank, = V/Q

Mean cell residence time c =VX/(Qw.Xs)

V= volume of aeration tank, Q = flow rate, X = Mixed Liquor Suspended Solids(MLSS), Qw = waste activatedsludge, Xs = MLSS in waste activated sludge.

Oxygen required =Q{(So-S)/f}-1.42Qw.Xs

So = influent BOD, S = Effluent BOD, f= ratio of BOD5 to Ultimate BOD.


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Recirculation rate, Qr =QX/(Xs-X)

Fig. 3.1 Flow chart of the program developed for design of WWTP

4. RESULTS AND DISCUSSIONThe computer program for the design of wastewater treatment plant was test-run and the results were compared

with those of the existing plant. The program is interactive. The program has been fed with relevant input data

for each un i t of Nesapakkam sewage treatment plant and executed. Data are entered as and when necessary as

per the guidance obtained from the program. As the aeration tank is the heart of Activated sludge process, the

output of the program for aeration tank was given below for illustration.


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The program is very interactive and on the execution of the program a menu will be displayed for selection ofthe unit for which the design is required. Fig.4.1 shows the menu display. Fig. 4.2 shows the output of end ofdesign of Aeration Tank.

Fig. 4.1 Menu displayed for the design of WWTP

Fig. 4.2 End part of output of design of Aeration Tank

Output of the program for design of Aeration tank

DESIGN OF AREATION TANK

NOTE: TYPICAL VALUES OF THE DESIGN PARAMETERS AND RANGES ARE GIVEN IN BRACKETS

ENTER FLOW IN MLD: 23

ENTER PEAK FACTOR: 2.67


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ENTER INFLUENT BOD (mg/l):320

ENTER INFLUENT SUSPENDED SOLIDS (mg/l): 450

ENTER EFFLUENT BOD IN mg/1: 20

ENTER SUSPENDED SLOLIDS IN THE EFFLUENT IN mg/1: 30

ENTER RATIO OF MLVSS TO MLSS (0.8): 0.8

ENTER RETURN SLUDGE CONCENTRATION mg/l( 10000): 10000

ENTER MEAN CELL RESIDENCE TIME IN DAYS(RANGE:5-8): 5

ENTER OXYGEN TRANSFER EFICIENCY OF AERATION SYSTEM IN DECIMAL(TYPICAL 8%): 0.08

CALCULATION OF SOLUBLE BOD (5DAY)IN THE EFFLUENT:

BOD(5DAY) IS EQUAL TO 68% OF BOD(ULTIMATE).

ASSUMING 65% SUSPENDED SOLIDS IN THE EFFLUENT ARE BIODEGRADABLE,

BOD OF EFFLUENT SUSPENDED SOLIDS IN mg/l=18.8292

INFLUENT SOLUBLE BOD ESCSAPING THE TREATEMENT (i.e.,EFFLUENT SOLUBLE BOD(5DAY) INmg/l)=1.1708

BOD IN THE INFLUENT(mg/l)=224

EFFICIENCY BASED ON SOLUBLE BOD(%)=99.4773

OVERALL PLANT'S EFFICIENCY (%)=91.0714

CALCULATION OF VOLUME OF AREATION TANK:

ENTER MLSS IN mg/l(RANGE FOR CONVENTIONAL COMPLETE MIX ACTIVATED SLUDGESYSTEM:3000-4000): 3200

MLVSS IN mg/l=2560

VOLUME OF AERATION TANK IN CUBIC METER= 4619.96

ESTIMATION OF WASTE ACTIWATED SLUDGE PER DAY: "Yobs"(OBSERVED YIELD):0.461538

CALCULATION OF INCREASE IN MASS MLVSS (OR) NET WASTE OF ACTIVATED SLUDGEPRODUCED EACH DAY (Px):Px IN kg/DAY=2365.42

INCREASE IN TOTAL MASS OF MLSS=2956.77

AMOUNT OF SLUDGE TO BE WASTED DAILY IN kg/DAY: 2266.77

RETURN SLUDGE RATE IN CUBIC METERS/DA Y=369.597

ENTER %AGE OF VSS IN EFFLUENT SOLIDS IN DECIMAL (80%):0.8

RETURN SLUDGE VSS CONCENTRATION 8000

RECIRCULATION RATIO (range 0.25-0.8 FOR COMPLETE MIX FLOW) =0.470588

HYDRAULIC EATENTION TIME (RANGE: 4-5 HOURS):4.82082

HYDRAULIC DETENTION TIME IS WITH IN THE RANGE (4-5 HOURS)HENCE O.K

F/M RATIO & VOLUMETRIC LOADING RATE:


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F/M RATIO IN kgBOD/kgMLSS-DAY (RANGE: 0.3 - 0.5):0.43561

VOLUMETRIC BOD LOADING RATEIN kgBOD/CUBlC METER-DAY= 1.11516

CALCULATION OF OXYGEN REQUIREMENTS:

MASS OF BOD(5DAY)UTILISEDIN Kg=7536.87

OXYGEN REQUIRED IN Kg/D A Y=4177.98

CALCUATION OF VOLUME OF AIR REQUIRED:

AIR CONTAINS 23.2%OXYGEN BY WEIGHT; SPECIFIC Wt OF OXYGEN: 1.201kg/ CUBIC METER

THEORITICAL AIR REQUIRED IN CUBIC METER/DAY= 14994.6

ACTUAL AIR REQUIRED IN CUBIC METER/DAY= 187433

DESIGN AIR REQUIREMENT USING A SAFTY FACTOR OF 2 IN CUBIC METERA/MIN=260.323

AIR REQUIRED PER UNIT VOLUME OF AERATION TANK CUBICMETER/CUBIC METER=8.14924

VOLUME OF AIR REQUIRED PER kg BOD REMOVED IN CUBIC METER/KgBOD=36.5717

OXYGEN TRANSFER CAPACITY OF THE AERATION EQUIPEMENT UNDER FIELD CONDITIONS:

OXYGEN TRANSFER CAPACITY OF THE AERATION EQUIPEMENT UNDER STD CONDITIONS: 1.8

SATURATION VALUE OF DISSOLVED OXYGEN CONCENTRATION FOR TAP WATER AT

STANDARD CONDITION(2ODEGREES CENTIGRADE)i.e, Cs=9.17mg/L

SATURATION VALUE OF DISSOLVED OXYGEN CONCENTRATION FOR WASTE WATER AT

OPERATING TEMPERATURE Csw = 90-98% OF Cs;

ENTER OPERATING TEMPERATURE OF WASTE WATER IN DEGREES (TYIPICAL 30):30

ENTER SATURATION VALUE OF DISSOLVED OXYGEN CONCENTRATION FOR TAP WATER AT

OPERATING FIELD CONDITION (FOR TEMP 30 DEG IT IS 7.63): 7.63

CORRECTION FACTOR FOR OXYGEN TRANSFER FOR WASTE WATER (0.8-0.85)

ENTER OPERATING DISSOLVED OXYGEN LEVEL IN AREATION TANK(0.8-1.0): 1.0

OXYGEN TRANSFER CAPACITY OF THE AERATION EQUIPEMENT UNDER FIELDCONDITIONS INkg O2/KILO WATT-HOUR= 1.52223

AERATOR POWER REQUIRED IN KILO WATTS=114.36

AERATOR POWER REQUIRED IN HORSE PORWER=152.48

The results of the test-run of the program are summarized in the Table 4.1. The specifications of the existingplant were also given for comparison and validation. It was evident from the below table that the designdetails obtained from the output of the program are in close proximity with those of the existing plant, andhence the usefulness and the authenticity of the software are proved and verified. To check the validation ofthe software further the performance characteristics and efficiency of the individual units were studied.Removal efficiencies of the individual units in the waste water treatment plant were analysed from the dataobtained from the Nesapakkam sewage Treatment plant and are given in Table 4.2. It was found that theefficiency of the treatment plant is also matching the expected values hence the softwares validity is proved,and it can be readily used in the field to design WWTP with activated sludge process for large cities.


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Table 4.1 Comparison of design details obtained from program's output and existing plant's design values

Sl.No Name of the Unit Design details obtained from

Software

Design details of the

Existing plant

1 SCREEN CHAMBERWIDTHDEPTH

1.98m1.25m

2.04m1.37m

2 DETRITUS TANKSIDE OF SQUARE TANKSEDE WATER DEPTH

8.98m1.28m

10m1.0m

3 PRIMARY CLARIFIERDIAMETERSIDE WATER DEPTH

21.4m2.65m

21.6m2.4m

4 AERATION TANKVOLUMEAERATOR POWER REQUIREDIN H.P

4619.16m3

152.484560m3

150

5 SECONDARY CLARIFIERDIAMETERSIDE WATER DEPTH

23.13m3.78m

24.4m3.1m

Table 4.2 Removal efficiency of the individual units in the waste water treatment plant

SI. No. Unit Operation/Process

% Removal Efficiency for

BOD TSS

1 Primary Clarifier 30.59 50.61

2 Aeration Tank 73.03 NA

3 Secondary Clarifier 67.67 97.79

4 Activated Sludge Plant (Aeration TankSecondary Clarifier)

91.28 86.76

BOD: Biochemical Oxygen Demand TSS: Total Suspended Solids NA: Not applicable

5. CONCLUSIONWastewater treatment plays an important role in water pollution control. Proper design, operation andmaintenance only can give good removal efficiency of pollutants. Manual design of large scale waste water

treatment plants is cumbersome and time consuming. Computer program can do all iterations with accuracy

and with in no time. A computer program in C++ has been written for interactive computer aided design of

wastewater treatment plant design. A wastewater treatment plant located in Nesapakkam was considered for

case study. The existing plant data was used for verifying the softwares authenticity. It was observed that the

design values obtained from the program are matching the design values of the existing plant and hence

concluded that the program works well. The performance of the existing plant is also evaluated by

using the relevant data obtained from the plant and it was found sat isfactory . The program can be

used for the design of any wastewater treatment plant which is having Activated Sludge Process as biological


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process with relevant input data. The program can also be used to check the design details of an existing plant

to know the expansion works needed for increased hydraulic and organic loadings occurring in future.

6. ACKNOWLEDGEMENTThe author wishes to acknowledge the Chief Engineer, Chennai Metropolitan Water Supply and SewerageBoard, for giving permission to collect all necessary data from Nesapakkam Sewage Treatment plant. Note:

The author can be contacted for complete code of the program in C++.

7. REFERENCES[1] Arceivala, Soli J. Waste water treatment for Pollution Control, Tata Me Graw - Hill, New Delhi,1986.[2] American Society of Civil Engineers and The Water Pollution Control Federation. Sewage Treatment Plant Design, New York, 1959 .[3] Central Public Health and Environmental Engineering Organisation, Ministry of Urban Development. Manual on Sewerage and Sewage

Treatment, New Delhi, 1993.[4] Garg, S. K. Sewage disposal and Air pollution Engineering, Khanna Publishers, Delhi, 1996.[5] Junna, J. and Rintala, J. (1990) Evaluation of purification efficiency of activated sludge treatment plants for pulp and paper industry

waste waters in Finland, Water Science and Technology, 22. 199-206.[6] Metcalf and Eddy, inc. Waste water Engineering Treatment Disposal Reuse, Me Graw - Hill, New York, 1991.[7] Peavy, Howard S., Rowe, Donald R. and George Tchobanoglous. Environmental Engineering, Me Graw - Hill, New York, 1985.[8] Sastry, C.A., Hashim, M. A. and Agamuthu, P. Waste Treatment Plants, Narosa Publishing House, New Delhi, 1995.[9] U.S.Environmental Protection Agency(1974) Process design manual for upgrading existing waste water treatment plants, U.S.

Environmental Protection Agency-Technology transfer, 13-17.[10] Robert Lafore, Object Oriented Programming using Turbo C++, Galgothia Publications, NewDelhi.


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