Chapter 1 Sewage

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1. Introduction

Sewage is the used water supply of a community, the excrements, dirts from washing, laundry,and other wastefrom places of human habitation, together with such wastes produced from

various manufacturing, and industrial processes as well as ground surface rain water as may be

present. Sewage is usually regarded as extremely foul and dangerous, a conception well justifiedsince it may contain colour, suspended solids, soluble matter, odor, and harmful bacteria, but still

the fact that 99.9% of its water should not be forgotten.

1-1 Reasons for Public Sewage System

With increasing population and industrial expansions the need for safe system of collecting,

treatment, and disposal of sewage is becoming more and more evident. Among the specific

reasons for safe sewage and waste treatment are.

1-1-1 Health Protection

From the health stand point the greatest problem of pollution is its effect on water supplies, byoverloading the water purification devices so that they may pass intestinal irritating organsisms.At the same time sewage pllution of swimming and bathing waters and shell-fish areas may

result in epidemics of typhoid, and other intestinal toubles Carefully controlled sewage treatment

plants reduces the pollution load on water strs aids in the productiot of safe drinkirg wcer, proven

continmation of shell-fish beds as well as permitting bathing and other water sports.

1-1-2 Comfort

Overloading the streams and water bodies with savage discharges will produce odors anddiscoloration, nuisances from sludge deposits as well as interferring with bathing facilities while

floating refuse on the water surface may give objectionable appearance to the water, as shown in

Fig. (1-1).

1-1-3 PropertyThe discharge of sewage into water bodies affects their usefnlness by changing the character of

the water. In addition sewage continated water may cause damage to boats.

The vater carriage system lends itself readily as a metxcd which ueets the principal requirements

of safe sanitary system. In such a system,, liquid wastes are carried out of houses and factories

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into a piping system to a cou point where it is scientifically treated. Such wastes, being highly

diluted with water will freely flow in the pipes according to the general rules of hydraulics.

1-2 Sources of SewageThe water finding its way into sewerage system consists principally of water from cue or more of

the following sources:

1-2-1 House WastesThis is generally called domestic wastes, it can be divided into:

a- Bathroom wastes including human wastes, paper, fibrous material as well as soap.

b- Kitchen wastes including waste food, soap, grease, sugar, sand , and frequntly quantities of

garbage.

c- Laundry wastes including soap, dirts arid starch.

Fig. (1-1) Engineering Measures to Prevent Water-Borne Disease

1-2-2 Storm Sewagei.e. rain water including aceria1 washed out by rain from roofs and streets. Such material


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contains relatively little organic natter arid settles rapidly cating some difficulties in sewers with

flat grades arid some parts of the sewage treatment plant.

1-2-3 Industrial Wastes

Ranging from cooling water containing little suspended solids to slaughtery house and distillerywastes with high concentrations of both organic arid inorganic m.atter. Industrial wastes include,

oil, grease, sand, dirts, from garages, filling stations arid factories in addition to floor washing

and score wastes.

1-2-4 Infltration Water

This is the ground water that may find its way into the sewerage piping system through leakage

into faulty joints or relatively porous bodies of sewer pipes.

1-2-5 Garbage

This is discharged to certain extent into severs as evidenced by examination of the screens atsewage treatment plants. With the introduction of house hold graiaders which are attached to thesinks the quantity of garbage intc the sewerage syst is expected to gradually increase.

1-3 Composition of Characterstics of Sewage

The average yearly used water production is never constant but some variations in the qualityand composition of sewage samples is expected depending on the following factors:

1. .3. 1 Age of Sewage

Fresh sewage is grayish in color with se fecal matter grease, paper and vegetable floating on its

surface. Its odor is sty but not offensive.

If sewage is allowed still, in contact with air, for twenty four hours settable solid particles insuspension will readily settle but the supernatant Liquid is still turbid and en offensive odor is

given off as a result of purification of organic matter present in sewage.

Again if sewage is allowed still for several weeks, the liquid becomes clearer, while the odordisappear since the organic sediments are no longer in a state of purification.


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Fig. (1-2) Activities and pollutant Sources Potentially

1.3.2 Time of CollectionThe strength of sewage varies fr season to season, fr day, as well as fr hour to hour on the se day,depending an the habits of the people and house hold activities. Type of food consumed in

different reasons, as well as changes in industrial operations may influence the composition of

sewage from season to season, while laundry work changes the character of sewage during theweek. In one day, the strongest sewage is during the morning but may be received several hours

later at the trteatment plant, while night sewage is mch weaker than day sewage, as shown in Fig.

(1-2).


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1-3-2 Existence of Air in Contact with Sewage

Fresh sewage contains oxygen which is rapidly consumed in the activity of existing aercbic

bacteria, if this oxygen is exhausted. Aeorbic bacteria will die and sewage becomes scale and

dark in color, and araerobic bacteria will get active, decomposing organic matter into ammoniaand ocher gases. Such sewage is called septic sewage, which causes damages to sewers due to

corrosion by sulferic acid

On the other hand, if sewage is in continuous contact with the acosphere, it will absorb oxygen

from it and such septic condition will not prevail, simple decopositioa of organic acter present in

sewage will only tak.e place.

Fig. (1-3) Hourly Variation in flow and strength of municipal wastewater


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1-3-4 Treatment of Sewage

High temperatures greatly speeds up the decomposition and disintegration of sewage solids,

resulting in stale dark in colour sewage with more finally divided substances present in

suspension.

1-3-5 Mechanical Factors

Factors such as pumping will break up the course solids into finer particles.

1.3.6 Quality and Quantity of infiltration and rainwater

Infiltration water and rainwater that ay find its way into the sewer pipes, will greatly affect theamount and concentration of both suspended and soluble solids in the sewage.

1.4 Solid Matters in Sewage

Solid substances which are left behind as a residue upon evaporation of a sewage sample may be

divided into two large groups:

1-4-1 Suspended Solids

Susspended solids including all floating material whether on the surface or in the body of

sewage. These suspended solids in turn can be divided into two portions, the first of which will

settle readily upon standing quisceucly, while the other portion does not settle in a reason-able

short time. The quantity of each of these portions depe4 upon:

a- Character of Sewage:

If sewage is scale disintegration and decomposition are greater than in fresh sewage resulting ina more finely divided substances that are too difficult to remove by settling.

b- Presence of Trade Wastes:


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Such wastes may disperse or coagulate finely divided matter originally present in sewage thus

affecting its rate of settling. In general soluble alkaline material disperses acid matter cosgulate.

c- Pumping:

Which breaks up mechanically the coarser solids.

1.4.2 Dissolved Solids

Including all readily soluble substances such as salts from water supply, soluble material in

urine, meat and vegetable extact as well as acids and alkalities from industries.

The quantity of dissolved solids varies from two thirds to quarters of the total solids while the

suspended solids iron te balance, depending on the composition of sewage. In treaent works agrea porDicu of suspended solids is roved while a large portion of the soluble matter passesthrough the treatment works unchanged, the balance being oxidized stabilized.

1.4.3 Mineral and Organic Solids in Sewage

Again the total solid contents of sewage can be divided into other two large groups namely,

organic and inorganic or mineral solids. In general about half of the solid matter in sewage isinorganic and the other half is organic. At the same time two thirds of the inorganic solid content

is in solution and half of the organic solid content is in solution while the balance of each group

is in suspension depending on the character of sewage. The following cable shows the physicalconditions of principal constituents of a sewage sample, the numbers are percentages of total

solid contents:

Totalsolids-100 Suspended solids 30

Settleable solids -15Organic -10

Mineral -5

Non-Settleable-15Orgainic -10

Mineral -5

Dissolved solids-70Orgainc -30

Mineral -40


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Fig. (1-4) Sewer Corrosion to (H2S) Oxidation

1-5 Organic Matter in Sewage

The character of organic contents is of utmost important since its decomposition is the majorsource of nuisances. The principal constiuenta of organic matter are nitrogen, oxygen carbon and

hydrogen beside some minor elements such as sulfur and phosphorus.ln general, there are two

kinds of decomposition namely:


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1.5.1 Purification

i.e. anaerobic decomposition is completed by anaerobic bacteria which is highly active in the

absence of oxygen. The end products of such decomposition is methane, amonia, hydrogen

sulphide, and carbon dioxide. Such anaerobic decomposition takes place in septic tanks, imhofftanks of sludge digestion tanks in the sewage treatment works. In such tanks the dissolved

oxygen is often quickly depleted since sewage is not in contact with fresh air.

1.5.2 Oxidation

i.e. aerobic decomposition which is completed through the activity of aerobic bacteria, the end

products of such decomposition is carbon dioxide, nitrates, sulphates and other in offensivematter. Such aerobic decomposition takes place in both filters and activated sludge tanks in

sewage treatment works since sewage is brought continuously in contact with fresh supplies ofair.

1.6 Nitrogen Cycle

Since nitrogen composes the main part of organic contents. Study of the cycle of such element in

nature will assist in interpreting the behaviour of decomposing sewage. In fact the conditionson which the compounds of such elements are present in a sewage sp1e can be considered as a

yardstick to show the degree of de composition the sewage sample has undergone.

In its simplest form, as shown in Fig. (114) , (1/5), the cycle of nitrogen goes in the followingsteps:

a- Nitrogenecus organic matter decomposes to amouia through the activity of anarobc bacteria.

b- Aerobic bacteria, through its activity will oxide the amonia into nitrites and finally to nitrates

serve as a food for plants

c- Nitrogen taken up by plants is converted in the plant, to plant proteins, to be eaten by animals

or die. By death the plant protein. becomes organic matter again.

d- If plant is eaten by animals, the animal protein after death becomes organic matter again. Acthe same time the riicrogeneous wastes of animal life are organic matter.


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Fig. (1-5) Cycle of Nitrogen, Carbon and Sulphur in Anaerobic Decomposition

1.7 Micro-Organisms in Sewage

Beside the mineral and organic contents, sewage contains a great variety of microscopic

organisms as well as bacteria in great numbers, essential agents in stabilizing organic matter yet;

some of them are disease producing organisms. The conditions of organic matter as found insewage depends upon the activity of differentkinds of bacteria present in sewage.


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Anaerobicbacceria responsible for pucrficatiou i.e. reducing organic matter into smaller for

while aerobic bacteria is responsible for the oxidation and stabilization of organic matter

previously reduced by anaerobic bacteria to simpler farms.

1.8 Biochemical Oxygen Demand

This is the amount of oxygen absorbed by bacteria in the process of decomposing and stabilizingthe organic matter present in a sewage sample, in order to standardize such tests, it is made

under specific conditions which are 5 days incubation period at 20 C. The magnitude of organic

matter in the tested sample versus time at different temperatures ar shown in. Fig. (1/6), (1/7).

Fig. (1-6) Progress of B.O.D at 9 , 20 , & 30 for Average Domestic Sewage

Fig. (1-7) The B.O.D curve (a) Normal Curve for Oxidation of Organic Matter, (b) The Influence

Nitrification

Chapter 1 Sewage

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