UNIT –I

                                   IMPORTANT QUESTIONS

1-   What do you mean by collection and conveyance of sewage and sources of sewage?

Before discussing collection and conveyance of sewage we can discuss about sewage. It indicates that the liquid waste originating from the domestic uses of water. It includes sullage, discharge from toilets, urinals, wastewater generated from commercial establishments, institutions, industrial establishments and also the groundwater and stormwater that may enter into the sewers. Its decomposition produces large quantities of gases, and it contains numerous pathogenic or disease producing bacteria, along with high concentration of organic matter and suspended solids.

The objective of sewage collection and disposal is to ensure that sewage discharged from communities is properly collected, transported, treated to the required degree so as not to cause danger to human health or unacceptable damage to the natural environment and finally disposed off without causing any health or environmental problems. Thus, efficient sewerage scheme can achieve the following:

• To provide a good sanitary environmental condition of city protecting public health.

• To dispose the human excreta to a safe place by a safe location .

• To dispose of all liquid waste generated from community to a proper place to prevent a favorable condition for mosquito breeding, fly developing or bacteria growing.

• To treat the sewage, as per needs, so as not to endanger the body of water or groundwater or land to get polluted where it is finally disposed off. Thus, it protects the receiving environment from degradation or contamination.

The waste management essentially comprises of collection , transport , disposal and or incineration of wastes. A sustainable management system is founded on 3 R namely  reduce , reuse and recycle. The major focus is to reduce the quantity of waste production by efficient process control, try to reuse the by products from a process and try to recycle the left out waste products by value added transformation.

Sources of sewage-

The wastewater generated from the household activities contributes to the major part of the sewage. The wastewater generated from recreational activities, public utilities, commercial complexes, and institutions is also discharged in to sewers. The wastewater discharged from small and medium scale industries situated within the municipal limits and discharging partially treated or untreated wastewater in to the sewers also contributes for municipal wastewater.

2-   Explain variations in sewage flow and write down the various methods of flow quantity estimation?

    Practically as we know that the average sewer never flows in the sewers; it continuously varies from hour to hour of the day and season to season. Hence the design of the sewers should be done for the maximum possible flow.

(i)The seasonal maximum rate of flow may be taken as the 1.3 times of average daily rate of flow

(ii)The monthly maximum rate of flow may be taken as the 1.4 times the average rate of flow.

(iii)The daily maximum rate of flow may be taken as the 1.8 times or so the average daily rate of flow.

· The variations are due to many factors such as habits of people climatic conditions, types of industries, level of ground water, fluctuations in the activities of factories, etc. the above figures for deviation from the average for seasonal, monthly and daily flow are taken for illustration only. The composition of water in summer is more than in winter or rainy season, and this change in consumption of water directly affects the quantity of sewage. There is also change in the sewage quantity if the city has seasonal industries such as sugarcane crushing, fruit canning, brewing etc.

Thus the time as to when the peak flow occurs will depend upon the flow time in the sewers and the type of district served.

For areas of moderate sizes maximum daily of hourly sewage flows can be expressed as-

Maximum daily flow= 2 times of average daily flow

Maximum hourly flow=1.5 times of max daily flow

                                        =3 times of average daily flow

The flow of sanitary sewage alone in the absence of storms in dry season is known as dry weather flow (DWF).

Quantity= Per capita sewage contributed per day x   Population

Sanitary sewage is mostly the spent water of the community draining into the sewer system. It has been observed that a small portion of spent water is lost in evaporation, seepage in ground, leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.

Methods for Estimation of Quantity of Storm Water

1. Rational Method

2. Empirical formulae method

In both the above methods, the quantity of storm water is considered as function of intensity of rainfall and coefficient of runoff.

Rational method- the result obtained for larger areas from this method is misleading. Hence this method is generally used for smaller size areas.

Storm water quantity can be estimated by rational method as below: Storm water quantity, Q = C.I.A / 360

Where, Q = Quantity of storm water, m3 /sec

C = Coefficient of runoff

I = intensity of rainfall, mm/hour, and

A = Drainage area in hectares

OR

Q = 0.278 C.I.A Where,

Q is m3 /sec;

I is mm/hour,

And A is area in square kilometer

Empirical Formulae –

Empirical formulae are used for determination of runoff from very large area. Various empirical relationships are developed based on the past observations on specific site conditions suiting a particular region. These empirical formulae can be used for prediction of storm water runoff for that particular catchment.

A] Burkli – Zeiglar formula

B] dicken formul(Q=CM3/4)

C] ryve formula(Q=CM2/3)

3-   What are the design steps of circular sewer?

   Sewers of any shape are hydraulically designed as open channels, except in the case of inverted siphons and discharge lines of pumping stations.  Following formulae can be used for design of sewers.

Hydraulic formulas are used in the design of circular sewer are-

• Chezy’s formula-

C= chezy constant

S=hydraulic gradient

R= hydraulic radius

for circular sewer where D = dia

The value of C may be determined by various formulae-

1-kutter formula

-

2-bazin formula-

• Manning ‘s formula-

• Crimp and burge’s formula-

Shield’s expression for self cleansing velocity-

The velocity that would not permit the solids to settle down and even scour the deposited particles of a given size is called as self-cleansing velocity. This minimum velocity should at least develop once in a day so as not to allow any deposition in the sewers. Otherwise, if such deposition takes place, it will obstruct free flow causing further deposition and finally leading to the complete blocking of the sewers. This minimum velocity or self-cleansing velocity can be worked out as below:

K= constant, for clean inorganic solids = 0.04 and for organic solids = 0.06

Hence, for removing the impurities present in sewage i.e., sand up to 1 mm diameter with specific gravity 2.65 and organic particles up to 5 mm diameter with specific gravity of 1.2, it is necessary that a minimum velocity of about 0.45 m/sec and an average velocity of about 0.9 m/sec should be developed in sewers.

Hydraulic characteristics of circular sewer-

1-   Depth of partial flow-

d= depth of partial flow

D= dia of full section

2-   Proportionate depth-

3-   Proportionate area-

4-   Proportionate perimeter-

5-   Proportionate velocity-

6-   Proportionate discharge-

4-   Explain pumping of sewage & location of pumping station?

 Sewage is required to be pumped under the following circumstances:

(1) When some area of a town or city is low lying it may not be possible to drain the low lying area by gravity to discharge into a sub main or main located at a higher level, unless the entire sewerage system in the remaining higher area is laid at a correspondingly lower level. In such circumstances it is more economical to collect the sewage of the low lying area into a sump well by gravity and then pump it into the gravity main sewer of the high level area.

(2) When the land is flat the laying of sewers at their designed gradients may involved deeper and deeper excavations in the forward direction of flow .

(3) When an outfall sewer is lower than the entrance to treatment works or body of water or any other point of discharge, then sewage is required to be lifted by pumps either for its treatment or for its disposal into the body of water or to any other point of discharge.

(4) When a sewer is required to be taken across a high ridge then a tunnel is required to be driven for this purpose. However, instead of driving a tunnel, pumping sewage into sewers laid across the slope of the ridge at reasonable depths may be more economical.

(5) When it is required to take out sewage from cellars or basements of buildings the floor of which is at a level lower than the invert level of sewer into which sewage is to be discharged, then the sewage is required to be pumped.

(6)For disposing of sewage of the basements of large commercial buildings sewage may have to be pumped, as the street sewer may be higher than the level of basement floor.

Location of pumping station-

The following points should be considered while locating the site for a pumping station-

1-   The topographical condition of the city should be thoroughly studied to locate the best site of pumping station.

2-   If the quantity of sewage is very large , the site should be near to the disposal point or at a place where the sewage can be directly disposed off during emergencies.

3-   The site should be such that during flexed it should not flooded with river water or seepage from the ground.

Provision should be made to pump all the sewage which will received during worst condition of rains.

5-   What are the various types of characteristics of sewage & explain in brief?

There are three types of sewage characteristics-

1-   Physical characteristics

2-   Chemical characteristics

3-   Biological chracteristics

 Physical characteristics of sewage-

Following are the detailed physical characteristics of Sewage:

Temperature:

• The normal temperature of sewage is generally higher than water supply due to domestic and industrial activities. Depending on geographical location, the mean annual temperature of sewage is in the range of 10 to 21°C. Temperature of sewage is an important parameter because of its effect on chemical reaction rates and aquatic life. The avg temperature of sewage in india is 20°C.
• Increase temperature can cause a change in fish species that are present in water bodies.
• Similarly, oxygen is less soluble in warm water, while some species of aquatic life population increases with temperature causing more demand of oxygen and result in depletion of dissolved oxygen in summer.
• Similarly, sudden change of temperature cause mortality of species.

Colour:

• Fresh sewage is light brown grey colour.
• At a temperature of above 20 °C, sewage will change from fresh to old in 2 - 6 hours.
• The old sewage is converted to dark grey and black color due to anaerobic activities, known as stale or septic color.
• Some industrial sewage also add color to domestic wastewater.
• The grey, dark grey and black color is due to formation of sulfide produced under anaerobic conditions reacts with the metals present in wastewater.

Odor:

• Fresh domestic sewage is odourless but after 3 to 4 hours it becomes stale.
• Stale sewage has a pronounced odour of Hydrogen Sulphide (H2S).
• The odor at low concentration has no effect, but high concentration causes poor appetite for food, lower water consumption, impaired respiration, vomiting etc.

Solids:

• Solids comprise matter suspended or dissolved in water and wastewater.
• Solids are divided into several different fractions and their concentration provide useful information for characterization of wastewater and control of treatment processes.

Total solids:

• Total solids (TS) are the sum of total suspended solids and total dissolved solids (TDS). Each of these groups can further be divided into volatile and fixed fractions.
• Total solids (TS) is the material left in the evaporation dish after it has dried at 103-105 °C.
• Total solids can be expressed in mg/L.

Total suspended solids:

• Total suspended solids (TSS) are referred to as non-filterable residue.
• It is determined by filtering a well mixed sample through 0.45μm to 2 μm pore sized membrane. The residue retained on the filter is dried in an oven at a temperature of 103-105 °C for at least 1 hour.
• TSS is expressed in the unit mg/L.

Turbidity

• Measure of light‐transmitting properties of water, used to indicate quality of waste discharges and natural waters with respect to colloidal and residential suspended matter
• Measurement based on comparison of intensity of light scattered by a sample to the light scattered by reference suspension under same conditions. Formazin suspensions are used as primary reference standard
• Results of turbidity reported as nephelometric turbidity units (NTU)

Chemical characteristics of sewage-

Sewage contains both organic and inorganic chemicals in addition to various gases like H2S, CO2, CH4, and NH3 etc that are formed due to the decomposition of sewage. The chemical characteristics of wastewater of special concern are pH, DO (dissolved oxygen), oxygen demand, nutrients, and toxic substances.

PH

PH is used to describe the acid or base properties of water solutions. The pH of sewage is initially high and drops when the sewage becomes septic but becomes increases again with the treatment processes.it may also be mentioned that sewage is generally alkaline in nature.(ph<7)

Dissolved oxygen (DO)

Wastewater that has DO is called aerobic or fresh. The solubility of oxygen in fresh water ranges from 14.6 mg/L at 0oC to about 07 mg/L at 35oC at 1.0 atm pressure. It is ensure that DO is atleast 4ppm. The DO is generally determined by winkler’s method.

Oxygen Demand

It is the amount of oxygen used by bacteria and other wastewater organisms as they feed upon the organic solids in the wastewater.

The COD may be defined as the oxygen required to oxidize the organic matter.

BOD

BOD is defined as the amount of oxygen required by the bacteria while stabilizing decomposable organic matter under aerobic condition. It is written as by BOD or BOD520. “It is the amount of oxygen required by aerobic bacteria to decompose/stabilized the organic matter at a standard temperature of 20oC for a period of 05 days”. For domestic sewage 05 days BOD represents approx. 2/3 times of demand for complete decomposition.

COD

By definition the COD is the amount of oxygen required to stabilized the organic matter chemically, i.e. the COD is used as a measure of the oxygen equivalent of the organic matter contents of a sample that is susceptible to oxidation by a strong chemical oxidant.

Biological characteristics of sewage-

The three biological organisms present in wastewater are bacteria, viruses, and parasites.

Bacteria

Sewage consists of vast quantities of bacteria, most of which are harmless to man. However, pathogenic (disease-causing) organisms such as typhoid, dysentery, and other intestinal disorders may be present in wastewater. The bacteria in raw swage may be expected to in the range from 500, 000 to 5,000,000 per mL. These bacteria are responsible for the decomposition of complex compounds to stable compounds with the help of some extracellular and intracellular enzymes. Depending upon the mode of action of bacteria may be divided into the following three categories;

• Aerobic Bacteria
• Anaerobic Bacteria
• Facultative Bacteria

Algae:

• Some algae found in sewage includes Chlorella phormidum, Ulothrix etc
• Algae are used in trickling filter in sewage treatment plant

Fungi:

• Fungi like Fusarium and Sporotricum are found in sewage which play important role in trickling filter.

Virus:

• Some viruses causing human disease such as Poliovirus, Rotavirus, Hepatitis A and E etc are found in sewage which get access through stool of patients.

6-   Define self purification of natural streams & classify rivers on the basis of MoEF ,govt of india?

 The self purification of natural water systems is a complex process that often involves physical, chemical, and biological processes working simultaneously. The amount of Dissolved Oxygen (DO) in water is one of the most commonly used indicators of a river health. As DO drops below 4 or 5 mg/L the forms of life that can survive begin to be reduced. A minimum of about 2.0 mg/L of dissolved oxygen is required to maintain higher life forms. A number of factors affect the amount of DO available in a river. Oxygen demanding wastes remove DO; plants add DO during day but remove it at night; respiration of organisms removes oxygen. In summer, rising temperature reduces solubility of oxygen, while lower flows reduce the rate at which oxygen enters the water from atmosphere.

This phenomenon which occurs in all natural streams or rivers is known as self-purification of natural streams or rivers. It is thus seen that natural streams or rivers, polluted by sewage, are purified in natural course by the phenomenon of self-purification. The rate of self-purification depends on various factors such as rate of re-aeration, type of organic matter present in sewage, temperature, velocity of flow, presence of available oxygen in receiving waters, sedimentation, etc.

River classification as per MoEF &CC, govt of india -

The rivers of India can be classified into four groups viz., the Himalayan rivers, the Deccan rivers, the coastal rivers, and rivers of the inland drainage basin.

The main Himalayan River System includes the Indus and the Ganga-Brahmaputra-Meghna system. The Indus originates near Mansarovar in Tibet, flows through India and Pakistan, and finally falls into the Arabian Sea near Karachi. Its important tributaries flowing through Indian territory are Sutlej (originating in Tibet), Beas, Ravi, Chenab and Jhelum. The Ganga-Brahmaputra-Meghna river system creates principal subbasins of the Bhagirathi and the Alaknanda, which join at Dev Prayag to form the Ganga. It then traverses through Uttarakhand, Uttar Pradesh, Bihar, and West Bengal. Below the Rajmahal hills, Bhagirathi, which used to be the main course in the past, takes off, while Padma continues eastwards and enters Bangladesh. The Yamuna, Ramganga, Ghaghra, Gandak, Kosi, Mahananda and Sone are the important tributaries of Ganga. Rivers Chambal and Betwa are the important sub-tributaries, which join Yamuna before it merges with Ganga. The Padma and the Brahmaputra join in Bangladesh, and continue to flow as River Padma or Ganga. The Brahmaputra rises in Tibet, where it is known as Tsangpo and runs a long distance till it crosses over into India in Arunachal Pradesh under the name of Dihang. Near Passighat, Debang and Lohit join river Brahmaputra and together run all along Assam in a narrow valley. It crosses Bangladesh as a downstream of Dhubri. In the Deccan region,most of the major river systems flow in the eastern direction and fall into the Bay of Bengal. The major Eastflowing rivers are Godavari, Krishna, Cauvery and Mahanadi. Narmada and Tapti are the major West-flowing rivers. River Godavari in the Southern peninsula forms the second largest river basin, covering ten per cent of the total area of the country, while the Mahanadi has the third largest basin. River basin of the Narmada in the uplands of the Deccan, flowing into the Arabian Sea, and of Cauvery in the south, falling into the Bay of Bengal is about the same size, though with different character and shape. There are numerous coastal rivers, which are comparatively smaller. While only a handful of such rivers drain into the sea along the East Coast, there are as many as 600 such rivers on the West Coast.

7-   What is oxygen sag curve and its analysis by the streeter phelps equation with neat sketch?

  The oxygen sag or oxygen deficit in the stream at any point of time during self purification process is the difference between the saturation DO content and actual DO content at that time.

Oxygen deficit, D = Saturation DO – Actual DO

The saturation DO value for fresh water depends upon the temperature and total dissolved salts present in it; and its value varies from 14.62 mg/L at 0oC to 7.63 mg/L at 30oC, and lower DO at higher temperatures.

In order to maintain clean condition in river stream the oxygen deficit must be nil and this can be find out by knowing the rate of de-oxygenation and re-oxygenation.

The De oxygenation the polluted DO content goes on reducing due to decomposition  of volatile organic matter.the rate of de oxygenation depends on the amount of organic matter and temperature of the reaction . Hence at a given temperature the curve showing  depletion of DO with time and it is similar to first stage BOD curve as shown in fig below.

Re oxygenation curve-in order to counter balance the consumption of DO due to de oxygenation , atmosphere supplies oxygen to the water and the process is called re oxygenation. The rate at which the oxygen is supplied by the atm to the polluted water depends upon-

1-   Depth of receiving water

2-   The conditions of the body of water

3-   The oxygen deficit

4-   Temperature of water

The analysis of oxygen sag curve can be easily done by superimposing the rates of deoxygenation and reoxygenation as suggested by the Streeter – Phelps analysis. The rate of change in the DO deficit is the sum of the two reactions as explained below:

DDt/ dt = f ( deoxygenation and reoxygenation)

OR dDt / dt = K’Lt – R’Dt

Where,

Dt = DO deficit at any time t,

Lt = amount of first stage BOD remaining at any time t

K’ = BOD reaction rate constant or deoxygenation constant (to the base e)

R’ = Reoxygenation constant (to the base e)

t = time (in days)

DDt/ dt = rate of change of DO deficit

8-   What is national  river cleaning plan ?

 The river cleaning programme in the country initiated with the launching of the Ganga Action Plan (GAP) in 1985. The Ganga Action Plan was expanded to cover other rivers under National River Conservation Plan (NRCP) in the year 1995. The pollution abatement works are implemented on a cost sharing basis between the Centre and State Governments. The works include; collection, transportation and treatment of municipal sewage, River Front Development (RFD), Low Cost Sanitation (LCS), Electric Crematoria, Improved Wood Based Crematoria etc. Prevention and control of industrial pollution is being addressed by the Central and State Pollution Control Boards/Pollution Control Committee.

More than 38000 million litres of waste water enters the major river then various types of projects are running under NCRP such as AMRUT(atal mission for rejuvenation and urban transformation NAMAMI GANGE project

The objective of NRCP is to improve the water quality of the rivers, which are the major water sources in the country, through the implementation of pollution abatement works.

Now let us take an example of ganga-

The Ganga rises in the Garhwal Himalaya from the Gangotri Glacier, some 4100 meters above the sea level under the name of Bhagirathi. The river flows through the Himalayas till another two streams, the Mandakani and the Alakhnanda join it at Devprayag. It is below this confluence that the river is known as the Ganga proper. The Ganga Basin which is the largest river basin of the country houses about 40% population of India. The river after traversing a distance of 2525 kms. From its source, meets the Bay of Bengal at Ganga Sagar in West Bengal. During the course of its journey from the hills to the sea, municipal sewage from large urban centres, trade effluents from industries and polluting waste from several other non-point sources are discharged into the river resulting in its pollution.

Objective of this ganga plan-At the time of launching, the main objective of GAP was to improve the water quality of Ganga to acceptable standards by preventing the pollution load reaching the river. However, as decided in a meeting of the Monitoring Committee in June, 1987 under the Chairmanship of Prof. M. G. K. Menon, then Member, Planning Commission, the objective of GAP was recast as restoring the river water quality to the 'Bathing Class' standard which is as follows:

BOD--------------------------------------------------3mg/l MAX

Only  having large number of sewerage project and STP will not clean the rivers. The govt is taking up major interventions adversely affecting the rivers without so much as even pretence of environment or social impact assessments .

9-   What are the quality requirements of disposal of sewage as per central pollution control board of india?

As per CPCB various quality requirements are listed below-

10-                       Write down the short note on the following-

• Turbidity
• Methods of sampling
• BOD
• COD
• Time of concentration

1-   Turbidity-

Turbid waste water will appear cloudy, murky, or otherwise colored, affecting the physical look of the water. Suspended solids and dissolved colored material reduce water clarity by creating an opaque, hazy or muddy appearance. Turbidity measurements are often used as an indicator of waste water quality based on clarity and estimated total suspended solids in waste water.

Turbidity in wastewater is caused by suspended matter, such as clay, silt, finely divided organic and inorganic matter, soluble colored organic compounds, and plankton and other microscopic organisms. Turbid water has muddy or cloudy appearance and it is aesthetically unattractive. The Measurement based on comparison of intensity of light scattered by a sample to the light scattered by reference suspension under same conditions. Formazin suspensions are used as primary reference standard

• Results of turbidity reported as nephelometric turbidity units (NTU)

2-   Method of sampling- Wastewater sampling is generally performed by one of two methods, grab sampling or composite sampling.  Grab sampling is just what it sounds like; all of the test material is collected at one time.  As such, a grab sample reflects performance only at the point in time that the sample was collected, and then only if the sample was properly collected.  Composite sampling consists of a collection of numerous individual discrete samples taken at regular intervals over a period of time, usually 24 hours.  The material being sampled is collected in a common container over the sampling period.  The analysis of this material, collected over a period of time, will therefore represent the average performance of a wastewater treatment plant during the collection period. By definition, samples of any media are either grab samples or composite samples. Grab samples are collected at one location and at one point in time. By contrast, composite samples consist of multiple grab samples taken over an area or time period.

3-   BOD-(bio chemical oxygen demand)- BOD is defined as the amount of oxygen required by the bacteria while stabilizing decomposable organic matter under aerobic condition. It is written as by BOD or BOD520. “It is the amount of oxygen required by aerobic bacteria to decompose/stabilized the organic matter at a standard temperature of 20oC for a period of 05 days”. For domestic sewage 05 days BOD represents approx. 2/3 times of demand for complete decomposition.

4-   COD ( chemical oxygen demand)- By definition the COD is the amount of oxygen required to stabilized the organic matter chemically, i.e. the COD is used as a measure of the oxygen equivalent of the organic matter contents of a sample that is susceptible to oxidation by a strong chemical oxidant.

5-   Time of concentration- The period after which the entire area will start contributing to the runoff is called as the time of concentration.

 The rainfall with duration lesser than the time of concentration will not produce maximum discharge.

 The runoff may not be maximum even when the duration of the rain is more than the time of concentration. This is because in such cases the intensity of rain reduces with the increase in its duration.

 The runoff will be maximum when the duration of rainfall is equal to the time of concentration and is called as critical rainfall duration. The time of concentration is equal to sum of inlet time and time of travel.

Time of concentration = Inlet time + time of travel