Water Quality Analysis
Physical, Chemical and Biological Analysis
Physical parameters : Colour, Temperature, Transparency, Turbidity and Odour
Chemical parameters: pH, Electrical Conductivity (EC), Total Solids (TS), Total Dissolved Solids (TDS), Total Suspended Solids (TSS), Total Hardness, Calcium Hardness, Magnesium Hardness, Nitrates, Phosphates, Sulphates, Chlorides, Dissolved Oxygen (D.O), Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Fluorides, Free Carbon-di-oxide, Potassium and Sodium
Heavy metals: Lead, Copper, Nickel, Iron, Chromium, Cadmium and Zinc
Biological parameters: The biological parameters involved the qualitative analyses of planktons (zooplankton and phytoplankton).
Field measurement: The field parameters measured include pH, conductivity, dissolved oxygen, temperature and transparency
In natural water, colour is due to the presence of humic acids, fulvic acids, metallic ions, suspended matter, plankton, weeds and industrial effluents
Visual comparison: take about 20ml of the sample and 20ml of distilled water in two separate wide mouthed test tubes. The results are tabulated (as clear, greenish, greyish, brownish, blackish, etc) by comparing the colour of the sample with distilled water
Impinging solar radiation and atmospheric temperature brings about spatial and temporal changes in temperature, setting up convection currents and thermal stratification. Temperature plays a very important role in wetland dynamism affecting the various parameters such as alkalinity, salinity, dissolved oxygen, electrical conductivity etc. In an aquatic system, these parameters affect the chemical and biological reactions such as solubility of oxygen, carbon-di-oxide-carbonate-bicarbonate equilibrium, increase in metabolic rate and physiological reactions of organisms, etc. Water temperature is important in relation to fish life. The temperature of drinking water has an influence on its taste
Apparatus required: Thermometer
Procedure: Temperature measurement is made by taking a portion of the water sample (about 1litre) and immersing the thermometer into it for a sufficient period of time (till the reading stabilizes) and the reading is taken, expressed as °C
Transparency (Light Penetration)
Solar radiation is the major source of light energy in an aquatic system, governing the primary productivity. Transparency is a characteristic of water that varies with the combined effect of colour and turbidity. It measures the light penetrating through the water body and is determined using Secchi disc
Apparatus required: Secchi disc, a metallic disc of 20cm diameter with four quadrats of alternate black and white on the upper surface. The disc with centrally placed weight at the lower surface, is suspended with a graduated cord at the center
Procedure: Transparency is measured by gradually lowering the Secchi disc at respective sampling points. The depth at which it disappears in the water (X1) and reappears (X2) is noted. The transparency of the water body is computed as follows:
Transparency (Secchi Disc Transparency) = (X1 + X2 )/2
Turbidity is an expression of optical property; wherein light is scattered by suspended particles present in water (Tyndall effect) and is measured using a nephelometer. Suspended and colloidal matter such as clay, silt, finely divided organic and inorganic matter; plankton and other microscopic organisms cause turbidity in water. Turbidity affects light scattering, absorption properties and aesthetic appearance in a water body. Increase in the intensity of scattered light results in higher values of turbidity
Apparatus required: Nephelometer
Principle: Nephelometric measurement is based on comparison of the intensity of scattered light of the sample with the intensity of light scattered by a standard reference suspension (Formazin polymer) under similar conditions. Turbidity is measures in unit NTU.
Procedure: The nephelometer is calibrated using distilled water (Zero NTU) and a standard turbidity suspension of 40NTU. The thoroughly shaken sample is taken in the nephelometric tube and the value is recorded
Turbidity (NTU) = (Nephelometer readings) (Dilution factor*)
* If the turbidity of the sample is more than 40 NTU, then the sample is diluted and the dilution factor is accounted in final calculations
Turbidity can also be measured by Jackson Turbidity Meter. It is a visual method, where a flame of candle is seen till its view is obscured by the turbidity of water.
Chemical Parameters : pH
The effect of pH on the chemical and biological properties of liquids makes its determination very important. It is one of the most important parameter in water chemistry and is defined as -log [H+], and measured as intensity of acidity or alkalinity on a scale ranging from 0-14. If free H+ are more it is expressed acidic (i.e. pH<7), while more OH- ions is expressed as alkaline (i.e. pH> 7).
In natural waters pH is governed by the equilibrium between carbon dioxide/bicarbonate/carbonate ions and ranges between 4.5 and 8.5 although mostly basic. It tends to increase during day largely due to the photosynthetic activity (consumption of carbon-di-oxide) and decreases during night due to respiratory activity. Waste water and polluted natural waters have pH values lower or higher than 7 based on the nature of the pollutant. The parameter need to checked in field to avoid any changes.
Electrical Conductivity (EC)
Conductivity (specific conductance) is the numerical expression of the water's ability to conduct an electric current. It is measured in micro Siemens per cm and depends on the total concentration, mobility, valence and the temperature of the solution of ions.
Electrolytes in a solution disassociate into positive (cations) and negative (anions) ions and impart conductivity. Most dissolved inorganic substances are in the ionised form in water and contribute to conductance. The conductance of the samples gives rapid and practical estimate of the variation in dissolved mineral content of the water supply. Conductance is defined as the reciprocal of the resistance involved and expressed as mho or Siemen (s).
G = 1/R
G – Conductance (mho or Siemens) and R - Resistance
Apparatus required: Conductivity meter
Procedure: The electrode of the conductivity meter is dipped into the sample, and the readings are noted for stable value shown as mS/cm. Electrometric method and instrument looks similar to pH meter.
Solids in Water
Total Solids (TS)
Total solids is the term applied to the material residue left in the vessel after evaporation of the sample and its subsequent drying in an oven at a temperature of 103-105 degree C.
Total solids include Total Suspended Solids (TSS) and Total Dissolved Solids (TDS)
Principle: A known volume (50 ml) of well-mixed sample is evaporated in a pre-weighed dish and dried to constant weight in an oven at 103-105C. The increase in weight over that of the empty dish gives the total solids
Total Dissolved Solids (TDS)
Dissolved solids are solids that are in dissolved state in solution. Waters with high dissolved solids generally are of inferior palatability and may induce an unfavourable physiological reaction in the transient consumer
Principle: The difference in the weight of total solids and the total suspended solids expressed in the same units gives the total dissolved solids.
Total Suspended Solids (TSS)
Suspended solids are the portions of solids that are retained on a filter of standard specified size (generally 2.0 µ) under specific conditions. Water with high-suspended solids is unsatisfactory for bathing, industrial and other purposes
Principle: A well – mixed sample is filtered through a weighed standard glass fibre filter and the residue that is retained on the filter is dried to a constant weight at 103-105 C. The increase in the weight of the filter determines the total suspended solids. This is called Gravimetric Method, however these days Electronic Methods are also used for TDS. You might have seen your home RO service provider always bring a small electronic device to check and adjust TDS of water.
Where; W1 and W2 are initial and final weight respectively.
Hardness of Water
The presence of calcium (fifth most abundant) in water results from passage through or over deposits of limestone, dolomite, gypsum and such other calcium bearing rocks. Calcium contributes to the total hardness of water and is an important micro-nutrient in aquatic environment and is especially needed in large quantities by molluscs and vertebrates. Small concentration of calcium carbonate prevents corrosion of metal pipes by laying down a protective coating. But increased amount of calcium precipitates on heating to form harmful scales in boilers, pipes and utensils
Magnesium is a relatively abundant element in the earth's crust, ranking eighth in abundance among the elements. It is found in all natural waters and its source lies in rocks, generally present in lower concentration than calcium. It is also an important element contributing to hardness and a necessary constituent of chlorophyll. Its concentration greater than 125 mg/L can influence cathartic and diuretic actions.
Principle: Magnesium hardness can be calculated from the determined total hardness and calcium hardness. It is measured by Colorimetric Method (EDTA Titration)
Nitrates are the most oxidized forms of nitrogen and the end product of the aerobic decomposition of organic nitrogenous matter. The significant sources of nitrates are chemical fertilizers from cultivated lands, drainage from livestock feeds, as well as domestic and industrial sources. Natural waters in their unpolluted state contain only minute quantities of nitrates.
The stimulation of plant growth by nitrates may result in eutrophication, especially due to algae. The subsequent death and decay of plants produces secondary pollution. Nitrates are most important for biological oxidation of nitrogenous organic matter. Certain nitrogen fixing bacteria and algae have the capacity to fix molecular nitrogen in nitrates. The main source of polluting nitrates is domestic sewage. Nitrates may find their way into ground water through leaching from soil and at times by contamination. They can be measured by the phenoldisulphonic method.
Principle: Nitrates react with phenoldisulphonic acid and produce a nitrate derivative, which in alkaline solution develops yellow colour due to rearrangement of its structure. The colour produced is directly proportional to the concentration of nitrates present in the sample.Instrument used is Spectrophotometer.
The high concentration of nitrate in water is indicative of pollution.
Phosphates occur in natural or wastewaters as orthophosphates, condensed phosphates and naturally found phosphates. Their presence in water is due to detergents, used boiler waters, fertilizers and biological processes. They occur in solution in particles or as detritus. They are essential for the growth of organisms and a nutrient that limits the primary productivity of the water body. Inorganic phosphorus plays a dynamic role in aquatic ecosystems; when present in low concentration is one of the most important nutrients, but in excess along with nitrates and potassium, causes algal blooms.
Principle: In acidic conditions orthophosphate reacts with ammonium molybdate forming Molybdophosphoric acid, reduced further to molybdenum blue by stannous chloride. The intensity of the blue colour is directly proportional to the concentration of phosphate. The absorbance is noted at 690nm using spectrophotometer. It is calculated by the stannous chloride method.
High phosphorus content causes increased algal growth till nitrogen becomes limiting, although blue green algae continues to dominate because of its ability to utilize molecular nitrogen. Besides sedimentation, high uptake by phytoplankton is one of the reasons for fast depletion of phosphorus in water.
Sulphates are found appreciably in all natural waters, particularly those with high salt content. Besides industrial pollution and domestic sewage, biological oxidation of reduced sulphur species also add to sulphate content. Soluble in water, it imparts hardness with other cations. Sulphate causes scaling in industrial water supplies, and odour and corrosion problems due to its reduction to hydrogen sulphide.
Principle: Sulphate ions are precipitated in acetic acid medium with barium chloride to form barium sulphate crystals of uniform size. The scattering of light by the precipitated suspension (barium sulphate) is measured by a Nephelometer and the concentration is recorded.It is measured by turbidometric method.
The presence of chlorides in natural waters can mainly be attributed to dissolution of salt deposits in the form of ions (Cl-). Otherwise, high concentrations may indicate pollution by sewage, industrial wastes, intrusion of seawater or other saline water. It is the major form of inorganic anions in water for aquatic life. High chloride content has a deleterious effect on metallic pipes and structures, as well as agricultural plants.
Principle: In alkaline or neutral solution, potassium chromate indicates the endpoint of the silver nitrate titration of chlorides. Silver chloride is quantitatively precipitated before the red silver chromate is formed. They are calculated by Argentometric method
Dissolved Oxygen (DO)
Oxygen dissolved in water is a very important parameter in water analysis as it serves as an indicator of the physical, chemical and biological activities of the water body. The two main sources of dissolved oxygen are diffusion of oxygen from the air and photosynthetic activity. Diffusion of oxygen from the air into water depends on the solubility of oxygen, and is influenced by many other factors like water movement, temperature, salinity, etc. Photosynthesis, a biological phenomenon carried out by the autotrophs, depends on the plankton population, light condition, gases, etc. Oxygen is considered to be the major limiting factor in water bodies with organic materials. It is one of the most important parameter and is also required for testing BOD so discussed in details.
Either electronically by Membrane electrode method or by titration through Winkler’s Iodometric method.
Principle: Oxygen present in the sample oxidizes the dispersed divalent manganous hydroxide to the higher valency to precipitate as a brown hydrated oxide after addition of potassium iodide and sodium hydroxide. Upon acidification, manganese reverts to its divalent state and liberates iodine from potassium iodide, equivalent to the original dissolved oxygen content of the sample. The liberated iodine is titrated against N/80 sodium thiosulphate using fresh iodine as an indicator
Reagents: Manganese sulphate, Alkaline iodide-azide reagent, Conc. sulphuric acid, Starch indicator, Stock sodium thiosulphate
Procedure: Take BOD Bottles add 2ml of manganous sulphate and 2ml of potassium iodide This is mixed well and the precipitate allowed to settle down. At this stage 2ml of conc. sulphuric acid is added, and mixed well until all the precipitate dissolves. 203ml of the sample is measured into the conical flask and titrated against 0.025N sodium thiosulphate using starch as an indicator. The end point is the change of colour from blue to colourless
V = mL thiosulphate solution used
M = molarity of thiosulphate titrant
203ml because (200) (300)/ (300-4=296) = 203ml (202.7ml)
Biological Oxygen Demand (BOD)
Biological Oxygen Demand (BOD) is the amount of oxygen required by microorganisms for stabilizing biologically decomposable organic matter (carbonaceous) in water under aerobic conditions. The test is used to determine the pollution load of wastewater, the degree of pollution and the efficiency of wastewater treatment methods. 5-Day BOD test being a bioassay procedure (involving measurement of oxygen consumed by bacteria for degrading the organic matter under aerobic conditions) requires the addition of nutrients and maintaining the standard conditions of pH and temperature and absence of microbial growth inhibiting substances.
Principle: The method consists of filling the samples in airtight bottles of specified size and incubating them at specified temperature 20 degree C for 5 days or else 27 degree for 3 days. The difference in the dissolved oxygen measured initially and after incubation gives the BOD of the sample.
Reagents: all are same as that for DO but need to prepare dilution water in BOD testing.
Preparation of dilution water: To 1000ml of water, 1ml each of phosphate buffer, magnesium sulphate, calcium chloride and ferric chloride solution is added, before bringing it to 20 C and aerating it thoroughly.
Procedure: The sample having a pH of 7 is determined for first day DO. Various dilutions (at least 3) are prepared to obtain about 50% depletion of D.O. using sample and dilution water. The samples are incubated at 20 C for 5 days and the 5th day D.O is noted using the oximeter. A reagent blank is also prepared in a similar manner.
- D initial - 1st day D.O of diluted sample
- Dfinal - 5th day D.O of diluted sample
- p – Dilution
- p = Vol of ww/ vol of DO bottle
Types of BOD
- Aerobic decomposition : CO2, Orthophosphate, Sulfate, Nitrate
- Anaerobic Decomposition : H2S, NH3, CH4 Swamp Gas
BOD in two parts
CBOD – Carbonaceous (OC to CO2); NBOD – Nitrogenous (Ammonia to Nitrate)(ON to Amm to Nitrite to Nitrate) – does not exert till 5-8 days
Ultimate BOD (UBOD)= 4.57 X TKN
5 Day BOD
Total amount of oxygen consumed by microorganism in initial 5 days;No light – keep algae away which can add O2 by photosynthesis and air tight;DO should be above zero at the end of experiment. Seeding to provide sufficient microorganism. In this case it is necessary to subtract OD caused by seed from mixed sample OD (one seeded dilution water other with sample)
D1 = DO of diluted sample immediately after preparation, mg/L,
D2= DO of diluted sample after 5 d incubation at 20°C, mg/L,
P= decimal volumetric fraction of sample used,
B1= DO of seed control before incubation, mg/L,
B2= DO of seed control after incubation mg/L, and
f = ratio of seed in diluted sample to seed in seed control = (% seed in diluted sample)/(% seed in seed control).
If seed material is added directly to sample or to seed control bottles: f = (volume of seed in diluted sample)/(volume of seed in seed control)
Chemical Oxygen Demand (COD)
Chemical oxygen demand (COD) is the measure of oxygen equivalent to the organic content of the sample that is susceptible to oxidation by a strong chemical oxidant. The intrinsic limitation of the test lies in its ability to differentiate between the biologically oxidisable and inert material. It is measured by the open reflux method.
Principle: The organic matter in the sample gets oxidized completely by strong oxidizing agents such as potassium dichromate in the presence of conc. sulphuric acid to produce carbon-di-oxide and water. The excess potassium dichromate remaining after the reaction is titrated with Ferrous Ammonium Sulphate (FAS) using ferroin indicator to determine the COD. The dichromate consumed gives the oxygen required for the oxidation of the organic matter.
Apparatus required: Reflux apparatus, Nessler’s tube, Erlenmeyer flasks, hot plate and lab glassware.
Procedure: 15ml of conc. sulphuric acid with 0.3g of mercuric sulphate and a pinch of silver sulphate along with 5ml of 0.025M potassium dichromate is taken into a Nessler's tube. 10ml of sample (thoroughly shaken) is pipetted out into this mixture and kept for about 90 minutes on the hot plate for digestion. 40ml of distilled water is added to the cooled mixture (to make up to 50ml) and titrated against 0.25M FAS using ferroin indicator, till the colour turns from blue green to wine red indicating the end point. A reagent blank is also carried out using 10ml of distilled water.
- Measures pollution potential of organic matter
organic matter + oxidant ⇒ CO2 + H2O
- Decomposable organic matter results in consumption of DO in the receiving streams
- Does not differentiate between biologically degradable & nondegradable organic matter
The COD test (1) (Digestion)
- Sample is refluxed with known amount of excess dichromate in presence of acid
- AgSO4 catalyst is used for oxidation of low molecular weight fatty acids
- Remaining dichromate is titrated with FAS to determine that used for oxidising the organic matter
The COD test (2) (Titration)
- Ferroin indicator gives a sharp change to brown colour on complete reduction of dichromate
- FAS is a secondary standard, must be standardised frequently
- Result expressed as mg/L COD
A = FAS used for blank, mL
B = FAS used for sample, mL
M = Molarity of FAS
- BOD value is always lower than COD value
- For domestic and some industrial wastewater COD is about 2.5 times BOD
Fluorides have dual significance in water supplies. High concentration causes dental fluorosis and lower concentration (<0.8 mg/L) causes dental caries. A fluoride concentration of approximately 1mg/L in drinking water is recommended. They are frequently found in certain industrial processes resulting in fluoride rich wastewaters. Significant sources of fluoride are found in coke, glass and ceramic, electronics, pesticide and fertiliser manufacturing, steel and aluminium processing and electroplating industries. It is calculated by SPADNS method.
Principle: The colorimetric method of estimating fluoride is based on the reaction of fluorides (HF) with zirconium SPADNS solution and the 'lake' (colour of SPADNS reagent), which is greatly influenced by the acidity of the reaction mixture. Fluoride reacts with the dye ‘lake’, dissociating (bleaching) the dye into a colourless complex anion (ZrF6 2-). As the amount of fluoride increases, the colour produced becomes progressively higher or of different hue.
Apparatus: Spectrophotometer and lab glassware.
Potassium and Sodium
Potassium ranks seventh among the elements in order of abundance, behaves similar to sodium and remains low. Though found in small quantities (<20mg/L) it plays a vital role in the metabolism.
Principle: Trace amount of potassium can be determined by direct reading of flame photometer at a specific wavelength of 766.5nm by spraying the sample into the flame. The desired spectral lines are then isolated by the use of interference filters or suitable slit arrangements. The intensity of light is measured by the phototube.
Working principle of Flame photometer: The emission of characteristic radiations by alkali and alkaline earth metals and the correlation of the emission intensity with the concentration of the element form the basis of flame photometry. The principle of the flame photometer depends on the "Emission Spectroscopy" in which the electrons of the metals after absorbing energy get excited from ground state to higher energy level and return back to the ground state with emission of light. The sample under test is introduced into flame in solution by means of atomizer. The radiation from the flame enters a dispersing device and isolates it (radiation) from the flame to the desired region of the spectrum. The phototube measures the intensity of isolated radiation, which is proportional to the concentration of the element present in the sample.
Sodium is one of the most abundant elements and is a common constituent of natural waters. The sodium concentration of water is of concern primarily when considering their solubility for agricultural uses or boiler feed water. The concentration ranges from very low in the surface waters and relatively high in deep ground waters and highest in the marine waters. It is calculated by flame photometric method.
Heavy metals are elements (properties of metals satisfied) of high atomic numbers. They have high utilities in industrial applications from papers to automobiles, by their very characteristic properties. They are found in the deep bowels of the earth as ores (complexes of mixtures). The metals are segregated from these ores, leaving behind the tailings that find their way into the environment as toxic pollutants. They get into the water bodies directly from point sources as sewage, and non-point sources as runoff and more insidiously as atmospheric deposition that are transported from long distances. Heavy metals affect every level of the food web, from producers in the trophic levels to the highest order carnivore by residing in the system and magnifying at every trophic status.
Instrument : Atomic absorption spectrophotometer (AAS)
Working principle: Atomic absorption spectrometer's working principle is based on the sample being aspirated into the flame and atomized when the AAS's light beam is directed through the flame into the monochromator, and onto the detector that measures the amount of light absorbed by the atomized element in the flame. Since metals have their own characteristic absorption wavelength, a source lamp composed of that element is used, making the method relatively free from spectral or radiational interferences. The amount of energy of the characteristic wavelength absorbed in the flame is proportional to the concentration of the element in the sample.