Characteristics and Composition of Sewage and their Significance

CHARACTERISTICS AND COMPOSITION OF SEWAGE AND THEIR SIGNIFICANCE

The quality of sewage can be checked and analysed by studying and testing its physical, chemical and bacteriological characteristics.

 

Physical Characteristics of Sewage

(i) Physical characteristics include:

(i) Colour

(ii) Odour 

(iii) Temperature and 

(iv) Turbidity,

 

(i) Colour:

❖ The colour of sewage can normally be detected by the naked eye, and it indicates the freshness of sewage.

❖ If the colour is yellowish, grey, (or) light brown, it indicates fresh sewage.

❖ If the colour is black (or) dark brown, it indicates stale and septic sewage.

❖  Industrial waste water imparts colour to sewage and the color depends on the chemical process in industries.

(ii) Odour:

❖ Fresh sewage is practically odourless.

❖ After 3 to 4 hours, it becomes stale when all the oxygen present in sewage gets exhausted.

❖ After few hours, it starts omitting offensive odours, especially (H,S) hydrogen zsulphide gas, due to decomposition of sewage.

(iii) Temperature:

❖ Affects the biological activity of bacteria present in sewage. When temperature is high, the bacteria will be more active in decomposition of waste in sewage.

❖ When temperature is high, the solubility of gases in sewage reduces.

❖ The dissolved oxygen content (D.O) of sewage also gets reduced with high temperature.

❖ Temperature affects the viscosity of sewage, which inturn affects the sedimentation process in sewage treatment.

❖ The normal temperature of sewage is generally slightly higher than the temperature of water.

❖ The average temperature of sewage in India is 20°C, which is ideal for biological activities.

(iv) Turbidity:

❖ Solids in suspension cause turbidity.

❖ Sewage is normally turbid, having floating matter like pieces of paper, match sticks, greases, vegetable dehris, fruit skins, soaps, etc.

❖ The turbidity increases as sewage becomes stronger.

❖ It is an optical light-emitting property.

❖ The degree of turbidity can be measured and tested by turbidity rods (or) by turbidity meters (Jackson's turbidity meters, Nephlometric turbidity metres etc.)

 

Chemical Characteristics of Sewage

(i) Total Solids

❖ Sewage normally contains very small amount of solids (0.05 to 0.1%) in relation tabiat to the huge quantity of water (99.9%). borgiow It only contains about 0.05 to 0.1 percent (i.e., 500 to 1000 mg/L) of total solids.

Classification

(i) Suspended solids are solids which remains floating in sewage.

(ii) Dissolved solids remain dissolved in sewage.

(iii) Colloidal solids are finely divided solids remaining either in solution (or) in suspension.

(iv) Settleable solids are solids which settle out, if sewage is allowed to remain undisturbed for a period of 2 hours.

❖ The solids can also be categorised as:

(a) Organic Solids,

(b) Inorganic Solids.

❖ The organic matter is about 45% of total solids and remaining 55% is inorganic matter.

❖ Inorganic matter consists of minerals, salts, sand, gravel, debris, chlorides, sulphates, etc. The presence of inorganic solids in sewage is not harmful and requires simple treatment.

❖ Organic matter consists of carbohydrate (cellulose, cotton, fibre, starch, sugar, etc.), Fats and oils received from kitchens and Nitrogeneous compounds like protein. Organic matter in sewage requires proper treatment before disposal in water bodies.

Measurements:

a) Total Solids (S, mg/l) - A known volume of sewage is evaporated and the dry residue is weighed.

b) Suspended Solids (S, mg/l)/Non-Filterable Solids.

❖ A known volume of sewage sample is passed through a glass-fibre filter apparatus ni (10) no of 1 μm pore size. The dry residue retained on filter is weighed.

c) Filterable Solids = Total Solids (S,) – Suspended Solids (S2)

(Dissolved + Colloidal Solids).

d) Total Suspended Solids (S,) may be volatile (or) fixed.

TSS= Volatile Solids + Fixed Solids

e) Volatile Solids - Non-filtered residue (of step b) is burnt and ignited at about 550°C in an electric muffle furnace for about 15 to 20 minutes.

❖  Loss of weight due to ignition will represent the volatile solids (S, mg/L).

f) Fixed Solids S₅ = S₂ – S₄ i.e., Suspended Solids - Volatile Solids.

g) Settlable Solids (S₆):

❖ Conical glass vessel called Imhoff cone is used.

❖ The capacity of the cone is 1 litre and it is graduated upto about 50 ml.

❖ Sewage is allowed to stand in this imhoff cone for a period of 2 hours and the quantity of solids settled in the bottom of the cone can be directly read out.

 

(ii) pH Value:

❖ pH value indicates negative log of hydrogen ion concentration

pH = -log H⁺ (or) H⁺ = (10)^-pH

pH <7-acidic range

pH > 7 - alkaline range.

❖ The fresh sewage is generally alkaline in nature (pH more than 

❖ But as time passes, sewage turns acidic and its pH tends to fall due to production of acids by bacterial action.

❖ The pH value can be measured quickly and automatically with the help of potentiometer, which measures the electrical potential exerted by the hydrogen $2015 ions.

❖ The efficiency of sewage treatment depends on pH.

❖ If pH of sewage is low, lime is added to create alkaline condition.

 

(iii) Chloride Contents:

❖ Chlorides are generally found in domestic sewage, and are derived from the kitchen wastes, urinary discharges, feces etc.

❖ Large amount of chlorides may enter from industries like ice cream plants, meat salting industries.

❖ Chloride in sewage may also be due to infiltration of sea water (NaCl).

❖ The normal chloride content of domestic sewage is 120 mg/l.

❖ The chloride content can be measured by titrating the waste water with standard silver nitrate AgNO, solution using potassium chromate as indicator.

 

(iv) Nitrogen Contents:

❖ The presence of nitrogen in sewage indicates the presence of organic matter, and it may occur in any of the following forms.

a) Free ammonia called ammonia nitrogen.

b) Albuminoid nitrogen, called organic nitrogen.

c) Nitrites and

d) Nitrates.

NH₃ ↑ +H₂0 NH₄⁺+OH^-

If further oxidation occurs as

NH₄⁺+ 20₂ → NO₃^-+2H⁺+H₂0

❖ The sewage treatment is done using microorganisms which decompose the organic waste into stable compounds.

❖ The presence of nitrogen in various forms is indicative of the stages of decomposition and level of treatment.

❖ The free ammonia indicates the age of waste water and the very first stage of nogobert decomposition of organic matter.

❖ Albuminoid nitrogen indicates quantity of nitrogen in sewage before the decomposition of organic matter is started.

❖ The nitrites indicate the presence of partly decomposed (not fully oxidised) organic matter viz., treatment is in progress.

❖ The nitrates indicate the presence of fully oxidised organic matter viz., treatment is complete.

 

a) Free Ammonia

❖ Indicates age of waste water.

❖ Its presence indicates stale or old sewage.

❖ Indicates very first stage of decomposition of organic matter in sewage.

❖ Measured by distillation process.

 

b) Albuminoid Nitrogen

❖ Indicates nitrogen content in waste water before decomposition of organic matter has started.

❖ Measure of undecomposed Nitrogen.

❖ Measured by treating sample with alkaline solution of potassium permanganate.

 

c) Nitrites Nitrogen

❖  Presence of nitrites indicates presence of partly decomposed organic matter.

❖  Indicates treatment is still incomplete and sewage is stale. 

❖ Measured by Colourimetric Method [Colour Matching Method].

[Adding Sulphonilic Acid and Naphthamine]

 

d) Nitrate Nitrogen

❖  Presence of nitrate indicates presence of fully oxidised organic matter.

❖  Stable form of nitrogenous matter.

❖  Indicates oxidised/treated wastewater.

❖  Measured by colourimeteric method (colour matching methods) and compared with standard colours. (Adding Phenol di-sulphonic acid and potassium hydroxide).

 

* Blue Baby Disease (Methaemoglobinemia) - affects Infants

❖  If the nitrate content is above 45 ppm in water, it may cause nitrate poisoning in infants.

❖  This happens because there exists lower acidity in the intestines of the infants, which permits the growth of nitrate-reducing bacteria, which converts nitrates to nitrites.

❖  The nitrites prove very harmful because they have greater affinity for haemoglobin than oxygen and thus completely reduce oxygen level in blood causing suffocation, turning the body of infants to blue colour.

(v) Presence of Fats, Oils and Greases:

❖  Greases, fats and oils are derived in sewage from the dischages of animals, vegetable matter from garages, hotels, restaurants and industries etc.

❖  They interfere with sewage treatment and they form scum on the top of sedimentation tanks and clog the voids of the filtering media.

❖  They are not decomposible by bacterial action and therefore should be removed from sewage.

❖  Measured by evaporating sewage sample, residual solids left after evaporation are mixed with ether (hexane) and again evaporated, leaving behind the fats and greases as a residue, which can be weighed.

 

(vi) Sulphides, Sulphates and Hydrogen Sulphide Gas:

❖  Formed due to the decomposition of various sulphur containing substances in sewage.

❖  This decomposition also leads to evolution of hydrogen sulphide (H,S) gas, causing bad smells odours and corrodes the sewer pipes.

❖  Formation of sulphides hinders the process of sludge digestion.

(vii) Dissolved Oxygen (D.O):

❖  D.O. is the oxygen present in dissolved state in waste water, which prevents noxious odours.

❖  D.O test performed on sewage before treatment helps in indicating the condition of sewage and decides selection of treatment methods.

❖  Fresh sewage contains some dissolved oxygen, which is soon depeleted by aerobic decomposition.

❖  Presence of D.O. in treated sewage indicates oxidation during treatment stages.

❖  Treated sewage effluent should have atleast 4 ppm D.O. in it, otherwise it will affect the aquatic life when sewage is discharged into water bodies.

❖  The dissolved oxygen in fresh sewage depend upon temperature.

❖  If the temperature of the sewage is more, the D.O content will be less.

❖  The D.O content of sewage is determined by Winkler's method which is an oxidation-reduction process, wherein iodine liberated is equivalent to the D.O.

 

(viii) Oxygen Demand:

❖  Oxygen required for oxidation of both inorganic as well as organic matter.

BOD - Biochemical Oxygen Demand.

COD - Chemical Oxygen Demand.

TOD - Total Oxygen Demand.

ThOC - Theoretical Oxygen Demand.

TOC - Total Organic Carbon.

 

 (ix) Chemical Oxygen Demand (COD):

❖  Measure of Oxygen (O₂) required to oxidise organic matter in sewage into CO₂, H₂O and oxidised species.

❖  Chemicals are used to oxidise both biologically active and inactive organic matter in sewage.

❖  COD test - Oxidation using potassium dichromate.

❖  A known quantity of waste water is mixed with known quantity of potassium dichromate and the mixture is heated. The organic matter is oxidised by K₂Cr₂O₇ (in the presence of H₂SO4). The resulting solution of K₂Cr₂O₇ is titrated with Ferrous Ammonium Sulphate (FAS) and the oxygen used in oxidising the waste water is determined. This is called Chemical Oxygen Demand (COD).

 

(x) Biological Oxygen Demand (BOD):

❖  Measure of oxygen required to oxidise biologically active organic matter in sewage by microorganisms.

* Biologically Active → The organic matter which can be oxidised by microorganisms is called biologically active (under aerobic conditions at standard temperature).

* Significance of BOD:

BOD of waste water decides the following: noir

(1) Quantity of O₂ required for biological stabilization of organic matter in sewage.

(2) Size of treatment facilities.

(3) Measure of efficiency of treatment.

(4) Dilution required for disposal of waste water.

Aerobic bacteria utilises the organic matter and oxygen in sewage and starts multiplying, the bacterial count increases which causes faster decomposition of organic matter in sewage. The oxygen consumed by the bacteria for decomposition of organic matter in sewage is BOD.

Practically it is not feasible to determine the ultimate BOD.

Hence BOD at 20°C during 5 days is taken which is 68% of the total BOD.

BOD_5d =0.68 BOD

BOD₅ = 5 day BOD

BOD _u = Ultimate BOD.

 

* BOD₅/BOD Test: (Dilution Method)

❖ Known volume of sewage sample is diluted with known volume of aerated pure water.

❖ Diluted sample is incubated for 5 days at 20°C.

❖ The D.O of the diluted sample before and after incubation period is measured.

❖ The difference between the initial D.O value and final D.O value indicates the oxygen consumed by microorganisms in 5 days (causing aerobic decomposition).

BOD/BOD, = D.O consumed by diluted sample x

[Volume of diluted sample /Volume of undiluted sample]

[DF = 1% = 1 ml. of sewage diluted to make 100 ml, hence multiplied by 100]. Normally, 300 ml BOD test bottles are used

If 4 ml of sewage sample is taken - Incubation at 20°C-5 days.

The BOD rate at any time depends on temperature and also on the amount and nature of organic matter present in sewage at that time.

At a certain temperature, the rate of deoxygenation is assumed to be directly proportional to the amount of the organic matter present in sewage at that time i.e.,

L₁ = Oxygen equivalent of carbonaceous oxidisable organic matter present after t days

(mg/l)

Now L is the organic matter present at the start of BOD reaction and L, is the organic matter left after t days, which means that during t days, the quantity of organic matter oxidised = L-L_i

If Y, represent the total amount of organic matter oxidised in t days, then we have,

Taking L out of bracket on R.H.S.

Substituting equation (3),

Yt is the oxygen absorbed in t days.

Ultimate BOD (Y_u )i.e., When t= ∞ days.

Y_u =L = Ultimate BOD = Organic matter in sewage.

The ultimate BOD (Yu) is equal to the oxygen equivalent organic matter present in sewage (L).

BOD rate constant (K_D)

The value of K_D determines the speed of the BOD reaction.

❖BOD rate constant is temperature dependent.

❖ K_D is higher at higher temperature and rate of oxidation will be faster.

The oxygen demand during the first 20 days is due to oxidation of organic matter in sewage and is called Carbonaceous BOD or first stage demand (CBOD). The latter oxygen demand occurs due to biological oxidation of ammonia and is called as second stage BOD or nitrogenous BOD (NBOD).

OAB - 1^st stage (or) Carbonaceous Stage (CBOD).

AC - 2^nd stage (or) Nitrogenous Demand (NBOD).

OAC - Combined Demand (or) Combined BOD Curve.

 

* Why COD values are higher than BOD values?

❖ In COD test, the oxygen required by chemicals for oxidation of both biologically active and biologically inactive organic matter is measured.

❖ But in BOD, the oxygen required by microorganisms for oxidation of biologically active organic matter only is measured.

❖ Microorganisms cannot act on biologically inactive organic matter and hence COD > BOD.

 

❖ BOD to COD ratio

BOD_u BOD = Measure of Biodegradable organics.

COD = Measure of both Biodegradable + Non-Biodegradable organics.

Since COD > BOD => BOD_u/ COD ratio always < 1

If ratio = 0.92 to 1.0 = Waste water is fully biodegradable.

BOD₅ /COD = 0.63 to 0.68 →→ Indicates that Wastewater can be biologically treated → Indicates minimum quantity of non-biodegradable organic matter.

... BOD₅= 0.68 BOD_u

 

(xi) Total Organic Carbon (TOC) and Theoretical Oxygen Demand (Th.OD)

❖ Carbon is the primary constituent of organic matter. TOC therefore gives a measure of organic matter in sewage.

❖ Organic matter is expressed as carbon content.

❖ Chemical formula (C_n H_n O_n N_n) and concentration of compounds present in wastewater enables us to calculate theoretically the carbon content in wastewater per litre of solution.

(C_n H_n O_nN_n ) → Carbon, Hydrogen, Oxygen, Nitrogen

For example: