• Waste water is the dilute mixture of various waste from residential areas, commercial, industrial and other public place.
• Before treatment and disposal it is essential to know it’s composition, quality and characteristics.
• Contains organic, inorganic matter and living organism.
• Physical constituents/impurities: grit, ash, sand, dirt etc.. They affect physical characteristics of waste water.
• Chemical constituents/impurities: All kind of dissolved, suspended and colloidal impurities that affects the chemical characteristics of the waste water.
• Biological constituents/impurities: Presence of all living microorganism (mostly micro organism) determines the biological characteristics of the waste water.

1.2 Characteristics of wastewater:

1. Physical characteristics and its examination

Four major parameter indicates the physical characteristics of waste water

• Turbidity:

Turbidity is the degree of clarity of waste water. Naturally, presence of different types of impurities causes sewage to be turbid. Higher turbidity implies that waste water is rigorously polluted. The degree of turbidity is measured by turbidity rod or turbidimeters. Turbidity rod is used for measuring turbidity in silica scale, it consist of an aluminum rod of about 203 mm long and graduated in ppm of silica scale (1 ppm=1mg/lt in silica scale=turbidity obtained from the solution of 1mg of silica in 1 liter of water). A screw containing a standard platinum needle (1mm diameter and 25mm long ) is inserted in the lower end. Graduated aluminum rod is lowered in water at the depth where platinum needle ceases to be seen and the value can be read on the graduated scale near the water surface.

This method gives the rough value only.

The other instruments used for the measurement of turbidity are

1. Jackson turbidity meter
2. Baylis turbidity meter
3. Helge turbidity meter
4. Nephelometer

Nephelometer is the digital instrument, that measures turbidity after calibration with standard turbidity solution. It measures turbidity in NTU.

• Color: Color in sewage appears due to presence of all different kind of impurities. Fresh sewage is brownish in color or slightly grey and with the passage of time, sewage becomes stale and darkens in color. It can be easily detected by naked eyes.

It is measured by platinum cobalt method. Various different concentration of standard solutions are made by dissolving various amount of platinum cobalt in distilled water and the color sample is compared with these solutions using tintometer. Tintometer consist of an eyepiece with two holes inside \, placing standard water and the sample water in these two different eyepieces, comparison is made. The standard solution is changed until the color in two slides matches.

• Odor: Fresh sewage has slightly soapy, earthy, relatively unpleasant and can be sometime completely odorless. As it becomes stale, it starts smelling due to the action of microbial activity which results hydrogen sulphide and other decomposition product.

Ordor is examined by threshold ordor number. In this test the water sample of known volume is mixed with taste and odor free water is mixed which makes the sample diluted to just make it ordor free.

Threshold order number = (A+B)/A

Where, A= volume of waste water sample

B= Volume of taste/ Odor free water added.

• Temperature: Normally the temperature of domestic and municipal sewage is slightly higher than that of the water supply, this happens with the presence of all kind of impurities and chemical reactions taking within. Observation of temperature of sewage is useful in indicating the solubility of oxygen which affects oxygen transfer capacity of aeration equipment’s and rate of biological activity. The ideal temperature for the biological activities is 20^oC. Temperature variation also occurs due to variation in season of the year.

2. Chemical Characteristics:

Chemical characteristics of sewage indicate the sewage pollution extent and the type of treatment required. The basic chemical characteristics with test for determining are:

• Total solid, suspended and dissolved solid and settle able solid

Present in small amount (0.1%), solid can  be in four different forms.

These solids can be both organic and inorganic matter. Organic matter consist of carbohydrate, fat, nitrogenous compounds etc and inorganic matter consist of minerals and salts.

Total solid is the sum of all kind of solid.

The estimation of suspended solids, both organic and inorganic, gives a general picture of the load on sedimentation and grit removal system during sewage treatment. Dissolved inorganic fraction is to be considered when sewage is used for land irrigation or any other reuse is planned.

Suspended solid: solids whose specific gravity are lighter and remains in floating stage.

Dissolved solid: those solids that are in dissolved stage in waste water.

Settleable solids: Those portions of solids (organic or inorganic) matter which settles out if sewage is allowed to remain undisturbed for a period of 2 hours.

Suspended solid content in a treated waste water must not exceed 50mg/lt before disposal.

• pH values:

pH of sewage represents the –ve logarithm of concentration of hydrogen ion. It  is a valuable parameter in the operation of biological units. The pH of the fresh sewage is slightly more than the water supplied to the community. However, decomposition of organic matter may lower the pH, while the presence of industrial wastewater may produce extreme fluctuations. Generally the pH of raw sewage is in the range 5.5 to 8.0.

Dissolved oxygen DO

DO content represents the amount of oxygen in dissolved state in sewage. It is an important parameter that indicates the purity of water and hence waste water. In case of sewage, if it is too polluted, a situation may arise when it has no DO content, in such cases it becomes important to increase the DO content of sewage by aeration. Presence of DO content facilitates aerobic decomposition of waste water. After treatment, it is important that sewage contain at least 4 ppm DO content before disposing to the receiving water value. 4 ppm DO content is required to maintain the aquatic life of the water bodies. It is determined by wrinklers method in lab.

Organic Matter

Heterogenous mixture of various organic compounds, Main component carbohydrate, protein and fats. Organic matter are determined by two method

Direct method: This involves determination of total organic carbon.

Indirect method: BOD₅, COD and ultimate BOD

 BOD and COD

 Organic compounds present in sewage are of particular interest for environmental engineering. A large variety of microorganisms (that may be present in the sewage or in the receiving water body) interact with the organic material by using it as an energy or material source. The utilization of the organic material by microorganisms is called metabolism. To describe the metabolism of microorganisms and oxidation of organic material, it is necessary to characterize quantitatively concentration of organic matter in different forms. In view of the enormous variety of organic compounds in sewage it is totally unpractical to determine these individually. Thus a parameter must be used that characterizes a property that all these have in common. In practice two properties of almost all organic compounds can be used: (1) organic compound can be oxidized; and (2) organic compounds contain organic carbon. In environmental engineering there are two standard tests based on the oxidation of organic material:

The Biochemical Oxygen Demand (BOD) and  Chemical Oxygen Demand (COD) tests.

 In both tests, the organic material concentration is measured during the test. The essential differences between the COD and the BOD tests are in the oxidant utilized and the operational conditions imposed during the test such as biochemical oxidation and chemical oxidation. The other method for measuring organic material is the development of the Total Organic Carbon (TOC) test as an alternative to quantify the concentration of the organic material.

Biochemical Oxygen Demand (BOD):

The BOD of the sewage is the amount of oxygen required for the biochemical decomposition of biodegradable organic matter under aerobic conditions. The oxygen consumed in the process is related to the amount of decomposable organic matter. The general range of BOD observed for raw sewage is 100 to 400 mg/L. Treated wastewater BOD must not exceed 30mg/lit before disposal to receiving body.

Per capita load

It represents average contribution of each individual expressed in terms of pollutant mass per unit time. Commonly used is (gm/person.day). Eg BOD gm/person.day for domestic sewage is 40.

Load=population*per capita load

Note: g/m³=mg/L

BOD calculation in lab

Only in the condition when the waste water has sufficient oxygen content required for complete decomposition of organic matter, it is possible to find the BOD. Highly polluted water often lacks DO content to meet the demand. In this case, waste water sample are diluted with clean water having sufficient DO content.

BOD₅=DO consumed by the diluted sample*(Dilution factor)

1% diluted sample means 1ml of sewage is diluted to make 100ml of test solution

Chemical Oxygen Demand (COD):

The COD gives the measure of the oxygen required for chemical oxidation. It does not differentiate between biological oxidable and non oxidable material. However, the ratio of the COD to BOD does not change significantly for particular waste and hence this test could be used conveniently for interpreting performance efficiencies of the treatment units. In general, the COD of raw sewage at various places is reported to be in the range 200 to 700 mg/L. In COD test, the oxidation of organic matter is essentially complete within two hours, whereas, biochemical oxidation of organic matter takes several weeks. In case of wastewaters with a large range of organic compounds, an extra difficulty in using BOD as a quantitative parameter is that the rate of oxidation of organic compounds depends on the nature and size of its molecules. Smaller molecules are readily available for use by bacteria, but large molecules and colloidal and suspended matters can only be metabolized after preparatory steps of hydrolysis. It is therefore not possible to establish a general relationship between the experimental five-day BOD and the ultimate BOD of a sample, i.e., the oxygen consumption after several weeks. For sewage (with k=0.23 d-1 at 20oC) the BOD5 is 0.68 times of ultimate BOD, and ultimate BOD is 87% of the COD. Hence, the COD /BOD ratio for the sewage is around 1.7.

• Nitrogen

 The presence of nitrogen in sewage indicates the presence of organic matter, the principal nitrogen compounds in domestic sewage are proteins, amines, amino acids, and urea.

The various form of  nitrogen in sewage are

1. Free ammonia
2. Albuminoidal nitrogen called organic nitrogen
3. Nitrites
4. Nitrates

 Ammonia nitrogen in sewage results from the bacterial decomposition of these organic constituents and indicates the very first state of decomposition. Nitrogen being an essential component of biological protoplasm, its concentration is important for proper functioning of biological treatment systems and disposal on land. Generally, the domestic sewage contains sufficient nitrogen, to take care of the needs of the biological treatment. For industrial wastewater if sufficient nitrogen is not present it is required to be added externally.

Albuninoid nitrogen indicates the quantity of nitrogen present in sewage before the decomposition of the organic matter is started.

Nitrites indicate the presence of partly decomposed organic matter.

Nitrate indicates the presence of fully oxidized organic matter.

The test on nitrogen confirms the decomposition stage of organic matter. For example if nitrite is present then it indicates incomplete decomposition however if nitrate is present it shows that sewage is well oxidized. It is however important to limit the concentration of nitrate before disposal as it may cause nitrate poisoning.

In infant it nitrate rich water causes blue baby disease.

Nitrogen compound leads to a phenomenon of eutrophication.

In the process of conversion of ammonia to nitrite and nitrate, oxygen are consumed this reduces DO content.

Free ammonia is measured by boiling of sewage and measuring the gas.

Albuminoid nitrogen is measure by adding potassium permanganate to the boiled sewage.

If unboilded sample is used to add potassium permanganate, it gives the sum of ammonia nitrogen and organic nitrogen which is known as kjedahl nitrogen.

Nitrite is measured by color matching method by adding sulphonilic acid and napthamine.

Nitrates is measured by adding phenol di sulphonic acid and potassium hudroxide. Generally nitrogen content in the untreated sewage is observed to be in the range of 20 to 50 mg/L measured as TKN or 45 ppm nitrate.

• Phosphorus

Phosphorus is contributing to domestic sewage from food residues containing phosphorus and their breakdown products. The use of increased quantities of synthetic detergents adds substantially to the phosphorus content of sewage. Phosphorus is also an essential nutrient for the biological processes. The concentration of phosphorus in domestic sewage is generally adequate to support aerobic biological wastewater treatment. However, it will be matter of concerned when the treated effluent is to be reused. It is essential for microorganism growth but in excess amount it leads to eutrophication. The concentration of PO4 in raw sewage is generally observed in the range of 5 to 10 mg/L.

• Chlorides:

Municipal water contains large quantity of chlorides, derived form the kitchen wastes, human feaces and urinary discharges etc. The normal chloride content is 120 ppm however in water supplies it is accepted to 250ppm.

 Concentration of chlorides in sewage is greater than the normal chloride content of water supply. The chloride concentration in excess than the water supplied can be used as an index of the strength of the sewage. The daily contribution of chloride averages to about 8 gm per person. Based on an average sewage flow of 150 LPCD, this would result in the chloride content of sewage being 50 mg/L higher than that of the water supplied. Any abnormal increase should indicate discharge of chloride bearing wastes or saline groundwater infiltration, the latter adding to the sulphates as well, which may lead to excessive generation of hydrogen sulphide.

• Oil and grease:

These are fraction of organic matter that are soluble in hexane. It constitutes about 10 % of total organic matter.

3. Biological characteristics:

Bacterial and Micro-organisms present abundantly in sewage (5-15 billion per liter of sewage). All kind of microorganisms such as algae, fungi, protozoa etc are present. Bacteria are the most diverse group of microorganisms which  are the minute single cell microorganisms possessing no defined nucleous and having no definite shapes. Most of the bacteria area harmless called nonpathogenic bacteria, however some of them are harmful and causes serious diseases and called as pathogenic bacteria.

Most of the bacterial require oxygen for their survival, they consume DO and decompose the refuse and organic matter present in water or sewage. They are called aerobic bacteria. Some of them do not require oxygen and are called anaerobic bacteria, they also decompose organic matter for their consumption where are there is one different set of bacteria that can survive with and without oxygen.

Presence of bacteria facilitates sewage decomposition however the concern is of pathogenic bacteria. In case of sewage, test of microorganism is not performed because of the high concentration of bacteria but at the time  of epidemiological investigation certain test may be required.

Indicator organism

All bacteria present in nature are not harmful to us, only disease causing bacteria poses direct threat to human health, these are called pathogens. Large number of bacteria resides in the colon of human and animal, naturally, these different bacteria make their way out to the atmosphere through human and animal excreta. The main objective of bacteriological test is to ascertain contamination of excrete, since excreta contains different types of bacteria, the most common E coli is taken as an indicator organism. E coli is relatively in large number  about millions per 100ml of crude waste water, whereas pathogens are few and their survival rate is quite low. The benefit of adopting E coli as indicator organism is that the presence of E coli indicates the contamination of water with excreta and hence alerts about the possible presence of pathogens.

Introduction to biological analysis: membrane filter test

Test conducted for examine microorganism with use of membrane filter. The filter consist of large number of microscopic pores that are capable of retaining bacteria.

The procedure for the test is

1. Waste water sample is filtered through the membrane filter and the membrane is then placed on a sterilized pad absorbed with nutrient (M-Endo’s medium)  this inhibits the growth of bacteria other than coliform group.
2. It is then placed in an incubator at 37^oC for a period of 20 hours
3. If the coliform bacteria are present the colonies of coliform group bacteria can be seen through microscope.

1.3 Aerobic and anaerobic decomposition:

1. Aerobic decomposition:

• Natural biological degradation and purification process in which bacteria grows in presence of air or dissolved oxygen.
• Example: Aeration tank, trickling filter etc.

Advantage:

• Aerobic bacteria are very efficient in breaking down waste product.
• Aerobic pathway releases substantial amount of energy.

2. Anaerobic decomposition:

• Biological process in which decomposition of organic matter occurs without oxygen.
• Example: Septic tank, digestion tank etc.

Advantage:

• Waste water pollutant are transformed into methane.
• Biomass growth is less.

1.4 Cycle of decomposition:

1. Nitrogen cycle:

2. Carbon cycle:

3. Sulphur cycle:

1.5 Expression of BOD:

BOD represents amount of oxygen required for biological decomposition of organic matter. However oxidation of organic matter occurs in two stage, first stage is a rapid process which completes in 7 to 10 days where most of the carbonaceous matters get oxidized, this stage is dependent on temperature and characteristics of sewage. On the other hand second stage BOD curve represents rate of oxidation of nitrogenous matter and the process is called nitrification.

-ve sign indicates that with passage of time the value of  decreases

Where K is decomposition constant and its unit is per day.

Integrating on both sides

Where C is constant of integration, the value of which is calculated from the boundary condition.

When t=0, L_t = L_o

Therefore, C=Log_e L_o Substituting value  of C in equation 1

Using, 0.434K= K_D

Where K_D is the decomposition constant or BOD rate constant on base 10 at the given temperature.

Rewriting equation 2

For BOD exerted,

We know that,

Relative stability

Ratio of available oxygen to the required oxygen for decomposition of organic matter present in waste water. The available oxygen can be in the form of DO, nitrite or nitrate.

Indicates the character of sewage

S = 100 (1 – 0.794T₂₀) or

S=100 (1-0.630 T₃₇)

Where T₂₀ = no. of days, required to decolorize a standard volume of methylene blue solution by a sewage when incubated at 20⁰C and 37 degrees respectively

Population Equivalent

It is a term used to indicate strength of industrial sewage equivalent to population as of BOD.

References:

• Modi, P.N. Environmental Engineering, Volume II : Waste Water Treatment, Disposal and Air Pollution Engineering. Delhi: Standard Book House.
• Garg. S.K. Environmental Engineering (Vol. II): Waste Water Engineering. Delhi: Khanna Publishers.

The post Quality of Waste Water: Constituent and Characteristics of waste water appeared first on OnlineEngineeringNotes.

• Quality of waste water or sewage must be known to design of sewer.
• Underestimate of waste water quantity may cause overflow on the street and unsanitary condition will be created.
• Overestimate of waste water quantity may cause uneconomical section and self-cleansing velocity may not be calculate.
• Waste water are classified as:

a. Sanitary sewage (Dry weather flow [DWF])

b. Strom sewage (Wet weather flow [WWF])

1.2 Sanitary sewage (DWF):

• Flow through sewer that is available during non-rainfall period.
• Minimum amount of flow throughout the year.
• Include waste water through residence and industries.

Source of sanitary sewage:

1. Public water supply:

• Include waste supplied from industries, commercial places and hospital.
• All supplied water are not converted into waste water.

2. Private waste supply:

• Water drawn from well, tube well and canal is part of private water supply.
• Water supplied to private institute, household etc.

3. Ground water sewage:

• Water from leaky joint is the source of sanitary sewage.
• When head of ground water is more than sewage head it occurs.

4. Unauthorized connection:

• Occurs due to unauthorized connection made by people in the sewer arrangement.
• Strom water is also considered in this section.

Factors affecting DWF:

• Population growth
• Rate of water supply
• Ground water infiltration
• Unauthorized connection
• Types of area served

Steps to determine DWF:

• Fix the design period.
• Forecast population for the design period.
• To obtain average sanitary sewage.

DWF = (70 to 90%) * (Population * Rate of water supply)

• To obtain peak sanitary sewage.

Q_max = Pf * DWF

Where,

Pf = Peak factor

1.3 Strom water or WWF:

• Include run off available from roof, yards, pavement and open space during rainfall.

Factors affecting WWF:

1. Catchment area:

• Catchment area is determined with help of map.
• Coefficient of surface runoff is calculated based upon the surface character.

2. Coefficient of runoff:

• Representation of fraction of total rainfall available in the form of storm water.
• Runoff coefficient increases with decrease in perviousness.

Rational method:

• Quantity of storm water from and area can be determined by rational method.
• This method gives reasonable estimate up to a maximum area of 400 ha.

Limitation and Assumption:

• It is limited to small catchment area.
• Precipitation is uniform over the entire catchment.
• Runoff is dominant by overland flow.
• Storm duration is equal to time of concentration.
• Basin storage is neglected.

1.4 Concept of time of concentration:

• Time of concentration is the time required for rain water to flow over ground surface from extreme point of catchment area to point under concentration.

Mathematically,

Time of concentration (t_c)

t_c = t_e + t_f

where,

t_e = Time of entry

t_f = Time of flow

Cases:

a. If time of runoff equals to time concentration.

• Maximum runoff occurs at point of concentration.

b. If time of concentration greater than time of runoff.

• Less runoff at point of concentration.

c. If time of runoff is greater than time of concentration.

• Rainfall intensity is less.

1.5 Concept of time area graph:

• Graph obtained by plotting duration storm in X – axis and area in Y – axis.
• To obtained proper result time-area graph is adopted.

Consider a drainage area with respective areas A₁,A₂, A₃ and A₄. At the commencement of the storm, rain water will be arriving at P only from the area A₁ which is in the immediate vicinity of P. As the time passes, water from the respective areas A₂, A₃ and A₄ will arrive and thus at the moment of the time of concentration water from the total area (A₁ + A₂ + A₃ + A₄) will be arriving at P.

Based on time of storm three cases are possible:

1. Duration of rainfall t>t_c

When t=t_c, peak discharge occurs till Tc and then it recedes.

2. Duration of rainfall t<t_c

3. Duration of rainfall t = t_c

References:

• Modi, P.N. Environmental Engineering, Volume II : Waste Water Treatment, Disposal and Air Pollution Engineering. Delhi: Standard Book House.
• Garg. S.K. Environmental Engineering (Vol. II): Waste Water Engineering. Delhi: Khanna Publishers.

The post Quantity of Waste Water: Description of sanitary & storm water and concept of time of concentration and time area graph appeared first on OnlineEngineeringNotes.

Important considerations while designing sewer lines:

• Meeting self-cleaning velocity:

Sewage that flows in sewer has some solid particles on it (0.1%), in order to avoid clogging of these solid particles it is necessary that sewer pipe be so designed and laid at such gradient that it causes self-cleaning velocity at different possible discharges.

• Maintaining continuous gradient in the downward direction

Sewer conduits carry sewage as open channel and the source of energy is gravity. It is important that sewer be laid in downward direction up to the outfall point.

Hydraulic formula for sewer design

1. Manning’s formula

Most common formula used for calculation of velocity in open channel

Where, N= manning’s roughness coefficient

R=hydraulic mean depth=wetted area/wetted perimeter

S=bed slope of the sewer

For Cement concrete, (hume pipe) N=0.013

2. Chezys formula

 Velocity calculated from chezy formula

Where,

R=hydraulic mean depth

S=bed slope of the sewer

For Cement concrete, ( hume pipe)  N=0.013

C= Chezys roughness coefficient

C depends on the nature of surface, flow characteristics, shape and size of sewer, slope so it is complex. C is calculated from Kutters formula or Bazins formula.

3. Kutters formula for C calculation

Where,

R=hydraulic mean depth

S=bed slope of the sewer

N= rugosity coefficient, the value of which depends on the nature of inside surface of the sewer. For cement concrete pipe N may be taken as 0.013

4. Bazin’s formua for C calculation

Where,

R=hydraulic mean depth

K= Bazins constant, the value of K can be taken as 0.833 for cement concrete surface.

5. Hazens William formula

Velocity from hazens William formula is calculated as

Self-cleansing velocity, minimum velocity and maximum velocity

The flow velocities in the sewer should be such that neither it creates clogging of suspended materials nor it scours the pipe material due to excessive abrasion. This conditions set two limiting velocity.

Minimum velocity:

The problem of silting of solid particles in sewer can be prevented by setting velocity that can automatically create self-cleaning effect. It is important that this velocity is maintained in the sewer line at least once a day.

Maximum velocity or non-scouring velocity

Shield’s expression for self-cleaning velocity

Self-cleaning velocity can be determined as follows:

Consider a layer of sediment of unit length, unit width and of thickness t is deposited at the invert of a sewer of gradient Ө. Let γ_sub be the submerged unit weight of the sediment.

The weight of the sediment, W= γ_sub *(1*1)*t

But, γ_{sub =} γ_w * (S_s – 1) / (1+e)

Where

γ_w = unit weight of water

S_s = Specific gravity of the sediment

e= void ratio of the sediment

 Relation of void ratio e and porosity of sediment n

Therefore, 

                  (1 – n) = 1 / (1+e)

Therefore, γ_{sub =} γ_w * (S_s – 1) * (1- n)

W= γ_w * (S_s – 1) * (1- n) *t

In order to scour the deposited sediment and to cause it to slide down the inclined plane, it is necessary that the drag force Ʈ exerted by the flowing water on the surface of the channel equals the frictional resistance R

Ʈ=R

But R=WsinӨ=WtanӨ, for small Ө

Therefore, Ʈ= WsinӨ

Ʈ= γ_w * (S_s – 1) * (1- n) *t sinӨ

Drag force exerted by flowing water on the sediment is given by

Ʈ= γ_w * r * s

Where, r=hydraulic mean depth of the channel

                        s=bed slope of the channel

γ_w * (S_s – 1) * (1- n) *t sinӨ = γ_w * r * s

Considering, (1-n) sinӨ=k, k being a characteristics of sediment

s = k/r * (S_s – 1) t

Since t is volume per unit area, it can be considered as d for a unit grain of sediment

Therefore self-cleaning slope is given as
s ∝ k/r * (S_s – 1) d

This self-cleaning slope is replaced in chezy formula for getting self-cleaning velocity

V = C* (RS)^1/2

C can be replaced in the form of friction factor, by comparing chezys formula with Darcy weisbach formula

Self-cleaning velocity in the form of friction factor is

The usual value of friction factor for sewer is 0.03

Similarly, self-cleaning velocity using manning’s formula is

The usual value of  for cement concrete pipe is 0.013

From the above expression, for inorganic particles of 1mm diameter a minimum velocity of 0.45m/sec is to be maintained likewise for removal of organic particle of 5mm diameter it is necessary to have velocity of 0.45m/sec for self-cleaning action. If it is required to cleanse inorganic particle greater than 1mm diameter and organic particle greater than 5mm diameter then the minimum velocity will be consequently higher.

The importance of maintaining minimum velocity is

1. It helps in controlling the size of sewer
2. It prevents silting of organic particles and inorganic particles. By flushing organic particles the problem of anaerobic decomposition in the sewer is controlled, which otherwise would create a problem of corrosion due to release of corroding gases such as hydrogen sulphide and methane. Likewise by flushing inorganic particles, problem of clogging and its effect is controlled.

Note: The larger the diameter of the sewer pipe, the gentler the slope is.

In practical cases, the gentlest slope that can be adopted is (1/diameter)

In flat terrain, where it is difficult to maintain minimum velocity  on the beginning part of sewer network, it may be impossible to maintain self-cleaning velocity, in such case, flushing tanks are provided that helps to flush out the clogged sediments.

Maximum velocities:

It is observed that if the velocity of flow in sewer is very high then the smooth interior surface starts getting scoured due to continuous abrasion caused due to the high speed particles colliding on it. It thus becomes necessary to limit non scouring velocity.

For different types of sewer material non scouring limiting velocity (maximum velocity) is provided in the table below

This problem frequently occurs in hilly terrain with steep slope. This problem is addressed by providing drop manholes at regular interval.

Shapes of sewer

The most common shape of sewer is circular. However there are also other shapes of sewer that can be constructed and used.

1. Non-circular shaped sewer section

a. Rectangular shaped sewer section

b. Horshoe shaped sewer section

2. Circular shaped sewer section

Sewer material

Sewer carries sewage that contains all kind of solid particle in it such as ash, sand, organic impurities, acids, alkaline etc. Sewer material should be durable enough to provide service for its intended design period and it must be resistant against the detrimental effects of solid particles flowing in sewage. In addition to it sewer material also has to withstand forces acting on it. Sewer material should in general should have following attributes:

1. Withstand internal pressure of sewage
2. Withstand pressure of external load
3. Bear temperature stress
4. Bear flexural stresses
5. Resistant to bear detrimental effects of solid particles flowing in it.
6. Must have minimum weight so that it can be easily handled,  transported and laid in position
7. Must not easily corrode
8. Must not easily scour
9. Able to create watertight joints
10. Impervious so that water does not seepage in to it.

Types of sewer material

1. Salt glazed stoneware or vertified clay

Made from clay and shales of special qualities, mixed with water and placed in pipe and pressed with pressure of 0.85N/mm2. Pipe are burned in clinker at 150 ^o C at beginning, 6000c to 7500c and then finally 1200⁰ C. Small quantity of salt is added into it which vaporize at high temperature and form glaze.

Merits                         

• Resistance to corrosion
• Cheap, durable, easily laid and joined
• Withstand hydraulic pressure

Demerits

• Heavy , bulky difficult in handling and transportation
• Weak

2. Plain or reinforced cement concrete pipe

PCC are manufactured in small size up to 450mm diameter and for larger diameter pipe RCC pipe are constructed with circumferential reinforcement to carry internal or external stresses.

The RCC pipes are further categorized into NP2-light duty RCC, non pressure pipes used for drainage  and irrigation, NP3-medium duty non pressure pipes, for carrying medium traffic and NP4 popes , heavy duty non pressure pipes.

Merits

• Strong enough to withstand internal and external pressure
• Easily manufactured

Demerits

• Crown corrosion
• Heavy and brittle

3. Cast Iron Pipe

Cast iron pipe manufactured by sand molding method and the centrifugal process are structurally stronger and capable of withstanding tensile, compressive and bending stresses.

These pipes are costlier and hence used in special situation such as when sewage is to be pumped, when sewer is to be laid under the building and at insufficient depth below ground.

Advantages:

• High resistant to corrosion
• Withstand high internal pressure and external loads
• Strong and durable

Disadvantages

• Heavy and brittle
• Costly

4. Plastic sewer:

These are new types of pipes. Double wall corrugated pipes are recently being used as sewer. The major benefits of using these pipes are its light weight and easy transportation. Use of these pipe are still at its early phase.

Advantages

• Resistant to corrosion
• Economical
• Light weight, longer length so easy in handling and transportation
• Cheap and easily available

Disadvantages

• Affected by acidity and alkalinity
• Reduction in strength with increase in temp
• Easily cutoff

5. Asbestos cement pipe

These popes are manufactured from a mixture of asbestos fiber, silica and cement and converted under pressure to a dense homogenous material possessing considerable strength called asbestos cement.

Advantages

• Light in weight so easy to transport
• Easily cut and easily joined
• Resistant to corrosion

Disadvantages

• They  are costly
• Unable to withstand pressure
• Brittle

6. Steel sewer

Sewer made up of steel material possesses high external and internal pressure bearing capacity. The major advantage of steel sewer is its light weight yet high flexibility and it can also absorb vibration and shock loads. It can be made corrosion resistant by galvanization or by bituminous coating. It can be easily coated. Despite having major benefits it is costly and used only on special cases.

Design of sewer network

• Calculation of maximum sanitary discharge Qs max for given sector from Upper Manhole(UMH) to lower Manhole(LMH) as per indexing of manholes & other related structures.
• Selection of Manning’s roughness coefficient (n).
• Calculation of ground slope of the section between UMH & LMH.
• Selection of internal diameter (D) of sewer pipe of circular crosses section & cross section in the case of rectangular reinforced conduit.
• Selection of slope (i) of sewer pipe in the given sector.
• Calculation of full discharge (Q_full) from nomograph based on Manning’s formula.
• Derivation of design depth of flow (h/D) from partial flow diagram (Q/Q_full).
• Derivation of actual velocity (V_actual) from partial (V/V_full).
• Calculation of invert drops of manhole.
• Calculation of invert drops of sewer line.
• Calculation of invert elevation of upper manhole.
• Calculation of invert elevation of lower manhole.
• Calculation of depth of excavation down to invert level of UHM for the given sector.
• Calculation of depth of excavation down to invert level of LMH.

Sewer Design criteria

1. Select the system of sewerage
2. Determine the quantity of sewage
3. Select the shape of sewer
4. Select the sewer size range
5. Sewer gradient (a site dependent factor, generally minimum gradient must meet self cleaning velocity and maximum gradient must meet non scouring velocity)
6. Design depth, should not exceed 2/3 of diameter at full flow condition

Basic hydraulic formula

Discharge=Area*velocity

Velocity=manning’s or chezys formula

Hydraulic elements of sewer

Circular section running just full

Circular section running just full 

A= π D² / 4

P = πD

Hydraulic mean depth (R) =A/P=D/4

Circular section running half full

A= π D² / 8

P = πD/2

Hydraulic mean depth R = D/4

Circular section running partially full

Consider a sewer section of diameter D, d is the flow depth of sewage in sewer. let Ө be the central angle subtended  in degrees at this flow depth.

For analysis we are considering following notation

a)Proportionate depth

b) Proportional area

The area subtended by angle Ө

c) Proportional wetted perimeter

d) Proportional hydraulic mean depth

e) Proportional velocity of flow

f) Proportional discharge

It is seen from the above expression that for different value of Ө, proportionate hydraulic elements can be calculated

It is important to note that the velocity and discharge in the partial flow sewer exceeds to that of full flowing sewer. For the case when N and n is considered to be same, maximum velocity occurs when depth is about 0.81 times the full depth. On other hand when N and n is considered different velocity is maximum in upper 20% depth only.

An important information can be derived from this result, sewers flowing between  50% to 80% depth need not be placed on steeper grades to be as self-cleansing as sewers running full.

Velocity and discharge are function of tractive force intensity which depends on the friction coefficient and the flow velocity. Ratios of vs/V, qs/Q and ss/S , where subscript s is denoting flow depth needed for maintaining self-cleansing velocity as that obtained in full flow condition can be computed with the help of equation on the assumption that equality of tractive force intensity implies equality of cleaning.

Eg: A 300mm diameter sewer is to flow at a 0.3 depth on a grade ensuring a degree of self-cleaning equivalent to that obtained at full depth at a velocity of 0.9m/sec. Find the required grade and associated velocity and rate of discharge at this depth. Assume Manning’s roughness coefficient n as 0.013. The variation of n with depth may be neglected.

Use of tables and nomograms for hydraulic computation for the design of sewers

While designing a sewer network, it becomes cumbersome to make calculation for computing the sizes and gradient of sewers required for developing requisite velocities at different discharge. To reduce this cumbersome task, readymade charts and tables, based on their original formulas are generally prepared, kept in design office and used.

For N=0.013, the unknown values say (Diameter and slope) can be find out for given two values (discharge and velocity).

300mm diameter sewer is to flow at a 0.3 depth on a grade ensuring a degree of self cleaning equivalent to that obtained at full depth at a velocity of 0.9m/sec. Find the required grade and associated velocity and rate of discharge at this depth. Assume Manning’s roughness coefficient n as 0.013. The variation of n with depth may be neglected.

Use

V=1/n R^2/3 S^1/2

R=A/P

d/D=0.3

Sewer is to be laid at such slope which will result in velocity of 0.9m/s as in full flow condition.

V=1/NR^2/3S^1/2

V=.9

N=0.013

R=D/4

S=.0043

r/R=.684

Vs=.846m/s

Q=AV=0.064 cumecs

q=av=0.015 cumecs

Figure Nomogram based on manning’s formula for sewer running full(N=0.013)

For different value of N , the parameter obtained must be multiplied by factor (0.013/N).

Stages of construction of sewer

1. Setting out of centerline of the sewer:

• Position of manholes are located on the ground from longitudinal section
• Sewer is laid from tail end or outfall end proceeding upward
• Sewer laid between two manholes at a time
• Center line is marked by

a. Offset method

• Central line of the sewer is marked on the ground .
• Offset line is marked parallel to the central line at suitable distance, about half the trench width plus 0.6m.
•  line can be drawn by fixing the pegs at 15 m intervals.

b. Sight rail method

• Two vertical posts are driven into a ground at a known distance
• From center line peg and one horizontal rail (sight rail ) is fixed between these posts at some convenient  height above ground.

2. Alignment and gradient

• Boning rods and sight  rails are used to correct alignment and gradient

3. Excavation, timbering and dewatering of trenches

• Width of trench depends upon the external diameter of pipe.
• The depth of trench depends upon gradient
• Sewer line are below the GWT the ground water enters the trenches dewatering of trenches is done

Following precaution should be taken

• Timbering of trench should be done if the depth of trench is greater than 1.5m.
• In water logging areas continuous pumping to be done.

4. Laying and jointing of sewer

• Sewer pipes are lowered carefully by hanging on ropes
• Should be laid to the correct alignment and gradient
• Joining of pipes required to achieve required length
• Before jointing straightness is tested
• Types of joints depends upon sewer material
• Should be water tight

5. Testing of Sewer

a. Water Test:

• The sewers are tested after giving sufficient time for the joints to set for no leakage.
• sewer pipe sections are tested between the manholes to manhole under a test pressure of about1.5 m water head.
• the downstream end of the sewer is plugged and water is filled in the manhole at upper end.
•  depth of water in manhole is maintained at about 1.5m.
• sewer line is inspected and the joints which leak are repaired.

b. Test for Straightness of alignment

• This can be tested by placing a mirror at one end of the sewer line and a lamp at the other end.
• If the pipe line is straight, full circle of light will be observed.

c. Air test:

• If no sufficient water  and if the size of sewer is larger
• Subjecting pipe to an air pressure of 100mm of water by hand pump
• If pressure is maintained at 75mm of water , joint is assumed to airtight

d. Backfilling the trench:

• After the sewer line has been laid and tested, the trenches are backfilled.
• The earth should be laid equally on either side with layer of 15 cm thickness.
• Each layer should be properly watered and rammed.

Sewer appurtenances

These are structures constructed at suitable interval of sewer line that helps to connect sewer lines, allows bend in case of change in direction, joins branch sewer to main sewer, gives access for inspection for maintenance, facilitates hydraulic function, make the design economic and technically feasible.

 Manhole

A manhole is a structure along a sewer line connecting sewers and providing an opening for access helping to inspect and maintain sewer performance. Manhole may be rectangular or conical section.

Location of manhole

Manhole is provided along the center line of sewer. It helps to connect laterals to the branch sewer and branch to the main sewer line. It is provided whenever there is change in direction of sewer line, change in diameter of sewer, change in gradient of sewer line.

Spacing of manhole

Spacing of manhole depends on the change in direction of sewer line, diameter of sewer pipes and change in gradient. However in general sense the spacing of manhole is close if the diameter of the pipe is small and it increases with diameter of pipe. If there is no obstruction, sewers of diameter less than 0.3m can be spaced in 45m whereas in the case of bigger size sewer, the spacing can be made up to 100m.

Components of manhole

• Top cover and frame

Every manhole is provided with top cover with frame as it is needed for covering access hole of the manhole. Depth of the frame is 20-25cm and width is 10cm whereas the opening should be at least 50cm.

• Access shaft

The upper portion of the manhole is called access shaft  which provides an access to the working chamber.  Its size is narrower than the bottom part. In rectangular manhole its size is 0.6m*.75m and for  circular it is 0.6m diameter

• Working chamber

It is the lower portion of manhole which provides working space and have a minimum of 0.9m *1.2, size for rectangular and 1.2m diameter for circular shape.

• Bottom or invert or benching

The bottom is made up of concrete bed with slope towards center.

• Steps or ladder

Steel steps are provided for access to the bottom part of the manhole.

Manholes are of various types, one classification is done in the basis of its depth

1. Shallow manhole: Manholes with depth ranging from 0.75m-0.9m . Generally occurs on the branch line.
2. Normal or medium manhole: Manhole with depth ranging from 0.9m -1.5m.
3. Deep Manhole: When the depth of the manhole exceeds 1.5m then it is called deep manhole.

Drop Manhole

These are the type of manhole constructed when there is change in elevation of incoming sewer and the outgoing sewer. The drop should be made by means of an outside connection. Sometimes when a lateral sewer needs to be placed deep enough to meet the deeper main sewer line, use of a drop manhole allows the lateral to be maintained at a shallow slope thereby reducing the amount of excavation. The sewage drops into the lower sewer through the vertical pipe at the manhole.

Street inlet

These are openings constructed at the road junctions and intersections for intercepting storm water and conveying it to the sewer line. The top part of the inlet is provided with MS grating cover that is durable enough to bear heavy traffic and pedestrian step.

Street inlets are further categorized as

• Curb inlet

It is also called as vertical inlet as it provides vertical opening for storm water to be accumulated in it.

• Gutter inlet

It is also called as horizontal inlet as it has horizontal opening on its top for the entrance of storm water

• Catch Basin or pits

It is a special type of inlet which allows grit, sand and debris etc. flowing in the storm water settle out. The outlet is provided with a trap to prevent escape of odor from the sewer.

Flushing Tanks

These are the devices or tanks in connection with sewer line and placed at the location where it is difficult to maintain self-cleaning velocity of a sewer line. The water stored in these tanks are flushed which helps in cleansing of the sediment that silts due to insufficient self-cleaning velocity. Normally in flat terrain where it is difficult to maintain the velocity due to gentle slope these tanks are used. There are two types of flushing tanks a. Hand operated flushing tank and b. Automatic flushing tank.

Inverted siphon

Inverted siphon is a depressed sewer laid at a gradient deeper than its HGL line. The need of this type of sewer is in situation when it is necessary to cross through road, railway tracks, canals etc lying on the level lower than sewer. In these sewer flow is in pressure greater than atmosphere. The arrangement consists of inlet and outlet chamber connected by three different sizes of pipes parallel to each other. The use of three pipe helps in meeting self cleaning velocity of 1m/sec in case of minimum flow. It is important to meet this self cleaning velocity at the minimum flow as cleaning of inverted sewer is extremely difficult. By providing lateral weirs when the flow is minimum only pipe 1 which is of smallest size will function and as the flow increases pipe 2 and pipe 3 subsequently comes into operation.

Ventilation in shaft

The decomposition of sewage release harmful and odorous gas. These gases are poisonous causing a threat to maintenance person and they are also corrosive which can reduce the life of the sewers. It thus becomes necessary to expel these gases through suitable opening. These opening are called ventilation which are provided at every 80m-300m. In open area ventilation is connected to the manhole cover but in crowded area air tight ventilating shaft with concrete base is used.

Sewer outlet

Outlets are the last structure of sewer from where treated waste water or storm water is discharged. If sewer is discharging to large water bodies of water it is usually extended beyond the banks into fairly deep water where dispersion and diffusion will aid in the mixing sewage with the surrounding water. In some cases outlets are anchored to prevent movement of the sewer line.

References:

• Modi, P.N. Environmental Engineering, Volume II : Waste Water Treatment, Disposal and Air Pollution Engineering. Delhi: Standard Book House.
• Garg. S.K. Environmental Engineering (Vol. II): Waste Water Engineering. Delhi: Khanna Publishers.

The post Design and Construction of Sewer: Hydraulic formula for sewer design and Types of sewer material appeared first on OnlineEngineeringNotes.

Sanitary Engineering: It is the branch of public health engineering which deal with the prevention & maintenance of the health of individual & the community. Its studies methodical collection, conveyance, treatment & disposal of waste water to land or water bodies meeting the criteria of pollution load as prescribed by the national guidelines.

Major goals of sanitary engineering:

1. Protect the public health: Waste water is nuisance, carries disease spreading pathogens, harmful toxic compounds, and heavy metal. Proper collection, treatment and disposal of such waste are the major objective of sanitary engineering.
2. Protecting the environment: Untreated waste water when discharged directly to receiving water or land body leads to the deterioration of surface and groundwater quality and pollutes the soil.
3. Reuse of treated waste water and solid: Treated waste water that is free of harmful impurities can be used for agricultural, industrial and ground water recharge. This helps in addressing growing concern water scarcity and water stress. Similarly, the treated solid particles when treated and stabilized can be used as manure or filling material.

Basic terminology

• Black water: Water from flush toilet (faeces and urine with flushwater)
• Gray Water: Washing water from kitchen, bathroom, laundry (without excreta)
• Rubbish: Dry and combustible waste from office eg: Building broken material, broken furniture, paper etc.
• Refuse: Refuse includes all rejected solid, semi solid and liquid waste.
• Garbage: It indicates dry refuse & it includes decayed fruit, grass, leaves, paper pieces, sweeping, vegetables etc.
• Sullage: It indicates the waste water from the bathroom, kitchen (human excreta not mixed)
• Night soil: Waste water mixed with human excreta.
• Infiltration: It is the water which inters the sewers from ground water through Leaks from loose joints or cracks.
• Sewer: Sewers are underground pipes or conduits which carry sewage. 
• Trunk or main sewer: Sewer receiving sewage from branch or sub mains and serves as an outlet for a large area.   
• Branch sewer: Sewer receiving sewage from a no of lateral sewers and delivering to main sewer.
• Lateral sewer: Sewer receiving sewage from house though house sewer and delivering it to a branch or sub main sewer.
• Sewage/waste water: The Liquid waste from a community is called sewage. Sewage is classified into domestic and non-domestic sewage. The non domestic sewage is classified into industrial, commercial, institutional and any other sewage that is not domestic.                                                                                            
• Sewerage: The entire system used for collection, treatment and disposal of Liquid waste. This includes pipes, manholes, and all structures used for the above mentioned purposes.                                                           
• Manholes: A supporting structure for connecting sewers.
• Outfall: The last point of sewer network.

Historical development of waste water and solid waste management

The history of sanitation records back to the 26^th century BC, in Iraq. The well documented sanitation practices are after the outbreak of cholera in 1848, 1849, 1852, 1853, and 1854. With understanding of water borne disease by poor sanitation, the sector started receiving attention. In England, Chadwick, known as father of sanitation, promoted house and cesspools connection to the sewers. Likewise, in 1868 experiments on the intermittent sand filter was carried out in England followed by experiment in aeration in 1882. Contact bed filtration was tried in Massachisetts in 1889. Digestion of sludge in lagoons was developed in Germany in 1891. First grit chamber was developed in 1904 in USA, disinfection by chlorination was developed in USA in 1906. The first municipal trickling filter was installed in USA in 1908.

Nepal stance on sanitary

“Nepal achieved its one of the most important goals of being declared as an Open Defecation Free (ODF) country in September 2019. The journey toward becoming an ODF free country commenced after the introduction of community led total sanitation (CLTS) and school led total sanitation (SLTS) in 2005. International year of sanitation 2008 was a breakthrough in shaping the robust national campaign of sanitation and hygiene. Then in 2011 Government of Nepal provided Sanitation and Hygiene Master Plan, a nationwide guideline, with a target to declare ODF Nepal by 2017. The declaration of ODF in 2019 is the result of combined cooperation and collaborative action of Nepal’s Government and different donor organizations.

The first sewers line in Nepal was constructed during RANA regime. In the heavily dense urban setting, Nepal has adopted water carriage system, however in rural area the practice is still conservancy system with septic tanks as treatment unit.

System of sanitation

The following are the two methods which are employed for the collection & disposal of refuse of a locality.

1) Conservancy system

This is a primitive system where refuse are collected manually & then suitably disposed off to the safe point of disposal. Conveyance is generally done by carts or trucks. The night soil is collected in pans, carried by labor in carts, truck etc & then buried into the ground & is thus converted into manure. The storm water & sullage are collected & conveyed separately by closed or open channel. They are discharged in natural rivers or stream.

Advantage:

• A primitive method with no sophisticated technology needed. It cartered the need of the time when the idea of water carriage system was not conceived.
• The system does not pollute water.

Disadvantage

• Insanitary condition: The decomposition of sewage starts about 4 to 5 hrs after its production and hence causes insanitary condition. There is direct contact between waste collection team and waste, hence the risk of disease spreading is high.
• Labor Problem: It entirely depends upon labor. if the labor goes on strike due to any reason, the public health is put into great danger.
• Land for disposal: The sewage, especially night soil require considerable land for its disposal.
• Night soil carts: It is highly undesirable to allow the night soil cart to pass through main road of the city.
• Open drains: If the storm water & sullage is carried in open drain. it create unsanitary condition.
• Pollution of water: There is a chances of liquid waste as it may soak in ground then the underground supply of water will be polluted.
• Risk of epidemic: If the sewage is not properly disposed then there is a chance of outbreak of epidemic.

2) Water Carriage system (sewerage system)

In this system, water is used as a medium to convey the sewage from the point of generation to treatment to the final point of disposal. Sewer lines are the conveyance structure.

Advantages

• The system facilitates compact unit as waste is not collected and carries in buckets and carts, as is required to be done in the conservancy system.
• The system is hygienic in nature as the sewage is transported in closed pipe i.e sewer line preventing contacts with waste water.
• Based on the availability of land, suitable type of treatment unit can be adopted.

Disadvantages:

• Large quantity of water is polluted.
• Unnecessary conveyance of waste water if the treatment system is central.
• Cost intensive and requires high skilled technology in designing and operation

Classification of water carriage system/sewerage systems

1)Separate system

In this system, storm water and sewage are conveyed separately in two different sets of conduits.

Advantages

1. The size of pipes is much smaller than the combined system sewers. This gives the advantage of good hydraulics in the pipe (the pipe is Designed to have a minimum velocity to prevent sedimentation of sand)
2. Separation of wastewater from storm water minimize the total quantity of sewage which has the following advantages :

1. Smaller pumping stations are needed.
2. Smaller and more efficient treatment plants are needed.
3. Overflow of combined sewers in the storm events produces pollution to environment which is not the case in separate sewer. Only unavoidable Storm water inters the system which protects the system from the accumulation of sand in the sewers in the non-paved areas.

Disadvantages:

1. Separate set of plumbing work required for collection of storm water and sewage, starting from the household unit.
2. If the sewer length is very high in compare to the quantity, laying two sets of pipe can increase the total project cost.

2)Combined system

In this system, single pipe is used to carry both storm water and sewage.

Advantage:

1. Unlike separate system, there is no need of two set of plumbing work.
2. Suitable for regions where wet seasons is short.
3. Only option in the areas where it is difficult to construct two pipelines. crowded streets.
4. Cost of the overall system is less than separate system.
5. Sewage is diluted.
6. Self cleaning velocity is maintained most of the time. Especially in the cases where sewage quantity is low.

Disadvantages:

1. Since the system carries both storm and sanitary waste, the pipe size is generally large and hence it is difficult to meet the requirement of hydraulic characteristics (such as self cleaning velocity) when only sewage is considered.
2. In dry season, sedimentation of organic matter may lead to anaerobic digestion expelling hydrogen sulphide and other gases. These gases corrode the pipe material and as thus the life of pipe is decreased.
3. Unnecessary pollution of rain water which could have been discharge directly to the river.

3)Partially combined system

In this system a portion of storm water is allowed to enter in the sanitary sewage carrying sewer and remaining storm sewage into another sewer. Storm water from roof, pavement and yards are allowed to enter sewer and remaining sewage is drained off from other drain.

Advantage:

• This is the most preferred system as it offers benefits of both systems.
• There is no need of separate plumbing work in each household or receiving units.
• The storm water collected from the road and public spaces can be directly discharged to the river through open drain.
• The storm water entering to the sewer helps in flushing deposited sediments that can possibly occur at the beginning part of the sewer line due to low sewage quantity.

Disadvantage:

• Self cleaning velocity may not be achieved in dry season.
• The loads on the treatment plant may increase.
• Only suitable for area with proper rainfall.

References:

• Modi, P.N. Environmental Engineering, Volume II : Waste Water Treatment, Disposal and Air Pollution Engineering. Delhi: Standard Book House.
• Garg. S.K. Environmental Engineering (Vol. II): Waste Water Engineering. Delhi: Khanna Publishers.

The post Introduction to Sanitary Engineering: Historical development of waste water and solid waste management appeared first on OnlineEngineeringNotes.