Quality of Water

Quality of WaterWater Quality and StandardsWater delivered to consumer should be clear, colorless, odourless and safe to drink.It should not contain conc of chemicals that are physiologically harmful, aesthetically objectionable or economically damaging.WHO provides guidelines for minimum level of quality for public supply of water.The quality standards provide a description of various inorganic and organic chemicals that may be found in drinking water.

Water Quality and StandardsPurpose of these standards is to ensure a safe and palatable drinking water and operators of treatment systems have obligation to meet these requirements.If level of any parameter exceeds its permissible limits, it indicates adverse water quality and must be reported.

Analysis of Water QualityThe parameters of analysis depends on the specific use the water is to be put to.Analysis is essential for designing water treatment system as it is operated on basis of impurities present.After treatment, analysis is done again to ascertain whether water is purified and meets the standards.Quality is variable with respect to time and space so periodic tests are necessary.Following parameters are routinely analyzed:PhysicalChemicalMicrobiologicalPhysical Water Quality Parameters Characteristics that respond to the senses of sight, touch, taste or smell.

Suspended Solids Turbidity Color Taste Odor TemperatureSuspended solids Solids may consist of inorganic or organic particles or of immiscible liquids. Inorganic- clay, silt and soil constituents. Organic materials- plant fibers and biological solids (algal cells, bacteria, etc)

Significance Aesthetically displeasing provides adsorption sites for chemical and biological agents. organic solids may be degraded, resulting in objectionable by-products.

Testing Passing the water sample through Filter paper and then evaporating the paper to dryness and measuring the residue. Expressed as mg/l.Turbidity measure of extent to which light is either absorbed or scattered. due to suspended solids.

Significance colloidal particles may provide adsorption site for chemicals and biological organisms. disinfection is difficult as solids may shield organisms. may interfere with light penetration and photosynthetic reactions in streams and lakes.Testing Measured photometrically by determining the percentage of light of given intensity that is either absorbed or scattered. Earlier determined in Jackson Candle Turbidity Unit (JTU). Now replaced by Nephelometric Turbidity Unit (NTU). For filtered water, it should be less than 1.0 NTU. Color Pure water is colorless. water is nature is often colored by foreign substances.

Significance not aesthetically acceptable. highly colored water is unsuitable for laundering, dyeing, papermaking, beverage manufacturing, dairy production and other food processing, and tectile and plastic production.

Testing Color of water is compared with standard color solution or color discs. Color produced by 1 mg of platinum in litre of distilled water has been fixed as unit of color (CU). For aesthetic purposes, it should be below 5 CU.Taste and Odor Due to presence of dead or living micro-organisms, dissolved gases such as H2S, CH4, CO2 or O2 with organic matter, mineral substances.

Significance aesthetically displeasing.

Testing Intensity of Odor is measured in terms of Threshold Odor Number (TON), and is equal to:

Where, A= vol of odorous water in mlB= vol of odor-free water required to produce 200ml of mixture where smell is barely detectable.

Temperature Governs to large extent, the biological species present and their rates of activity. Has effect on most chemical reactions that occur in natural water systems. Pronounced effect on solubility of gases in water.

Significance For public supply, most desirable temp is between 4.4 C - 10 C. Above 26 C is undesirable and above 35 C is unfit as it may be prone to bacterial contamination.

Chemical Water Quality Parameters Water is universal solvent and chemical parameters are related to solvent capabilities of water.

Total Dissolved Solids pH Alkalinity Hardness Metals NitratesTotal Dissolved Solids material remaining in water after filtration for suspended solids analysis is considered to be dissolved.

Significance dissolved minerals and organic constituents may produce aesthetically displeasing colors, tastes and odours. toxic chemicals may be present.

Testing by evaporating to dryness, a sample which has been filtered to remove suspended solids. The remaining residue is weighed and represents TDS. Expressed as mg/l.pH ValueDepending upon nature of dissolved salts and minerals, water may be acidic or alkaline.Acidity is determined on pH scale.pH is defined as negative log of hydrogen ion [H+]in water.Expressed on scale of 0-14.pH7 Alkaline

TestingDetermined using specially prepared electrodes; instrument is pH meter.Alkalinity Quantity of ions in water that will react to neutralize hydrogen ions. Measure of ability of water to neutralize acids. primarily due to presence of bicarbonates & carbonates

Significance reactions can occur between alkalinity and certain cations in water. resultant precipitate can foul pipes and other water-systems appurtenances. imparts bitter taste to water.

Testing by titrating water with an acid and determining the hydrogen equivalent. expressed as mg/l of CaCO3. Hardness presence of multivalent metallic cations in solution. classified as carbonate and non carbonate hardness, depending upon anion it associates with.

Significance scaling in boilers. more soap consumption as lathering does not occur, causing economic loss. pH changes in water distribution system may result in deposits of precipates.Testing by chemical titration to determine quantity of calcium and magnesium ions. titration with EDTA using eriochrome black T as an indicator.Metals All metals are soluble to some extent. by dissolution from natural deposits and discharges from domestic, industrial & agricultural wastewaters.

Non-toxic metals not harmful in relatively small amounts. sodium, iron, manganese, aluminum, copper and zinc are commonly found in water. excessive conc can cause bitter taste and health hazard to cardiac & kidney patients. Toxic metals harmful to humans and other organisms in small quantities. due to mining, industrial or agricultural sources. arsenic, barium, cadmium, chromium, lead, mercury and silver. can get accumulated and concentrated in food chain, posing greatest danger to organisms at top of food chain.

Testing measurement is usually made by atomic absorption spectrophotometry.

Nitrates Nitrogen presence indicates presence of organic matter. May be present as: nitrites, nirates, free ammonia, albuminoid nitrogen. presence of nitrites is extremely dangerous. nitrites are converted to nitrates by full oxidation of organic matter. nitrates should not exceed 45 mg/l; causes methamoglobamine.

Biological Water Quality Parameters Microbiological composition is most important of drinking water because of possible presence of disease causing organisms or pathogens. organisms include bacteria, protozoa, viruses, cysts and parasitic worms. specific disease causing organisms are not easily identified; as a result screening tests have been developed to indicate degree of contamination. diseases often spread by micro-organisms in fecal waste. widely used method involves testing for indicator organisms that are present if fecal contamination is there.Methods used to test coliform bacteria are: Presence/ Absence Test (P/A) Membrane Filter Test (MF) Heterotrophic (standard) plate count (HPC) Multiple Tube Fermentation Test

The common method of expressing bacterial density in a water sample is its Most Probable Number (MPN).

Chemistry in Water TreatmentSolutions

Percent Solution: conc of solute is expressed as a percentage of the total weight of solution.Molar Solution: The number of moles (gram-solute/gram-molecular weight of solute) of the solute dissolved in one litre of solution.Normal Solution: The number of gram-equivalents (gram-solute/gram-equivalent weight of solute) of solute present in one litre solution. gram-equivalent = gram-molecular weight / Z Z = equivalence; no. of H atoms that an element or radical can hold in combination or replace in a reaction.Chemical Feeding

Majority of processes require chemical feeding.Correct dosing is important in maintaining efficiency of process.

Following calculations are required to perform:Dosage and Dosage RateMaking solutions of desired strengthFeed rate of chemicalsCalibration of chemicalsDosageDefined as mass of chemical added to a given volume of water.Allows us to calculate the mass (m) of chemical required to treat a given volume (V) of water at a dosage (C).

Dosage rate refers to amount of chemical applied per day to treat a given quantity of water. It is expressed in kg/d.Can be determined by multiplying dosage and water flow rate.

Dilution Formula

Expression for mass balance of the chemical.Useful in performing following calculations:Diluting a solutionMaking a solution using liquid chemicalDetermining the vol of solution to be fed in order to apply a desired dosage of the chemical.

Dilution formula:

Density Considerations

When density of liquid chemical is different from water, strength of 1% is not equal to 10000 mg/L.For correct conversion, it is needed to multiply by density of liquid chemical, r. converting concentration:

where Cm/m is strength of solution, SG is specific gravity, r is the density of solution, rw is density if water.

Standards of Quality for Water UseFor Drinking Purpose

Water Related DiseasesDiseases Related to WaterWater-borne DiseasesWater-washed DiseasesWater-based DiseasesWater-related DiseasesWater-borne Diseases Diseases caused by ingestion of water contaminated by human or animal excrement, which contain pathogenic microorganisms.Include cholera, typhoid, amoebic and bacillary dysentery and other diarrheal diseases

37Diarrheal DiseasesGiardiasis (Protozoan)Cryptosporidiosis (Bacteria)Campylobacteriosis (Bacteria)Shigellosis (Bacteria)Viral Gastroenteritis (Virus)Cyclosporiasis (Parasite)38Stomach flu virusesCryptosporidiosis-Milkawakee outbreak, the largest recorded waterborne outbreak in US history, affecting 25% of citys population 1993Campylobacter is the most common diarrheal illness in the US.In addition, water-borne disease can be caused by the pollution of water with chemicals that have an adverse effect on health

ArsenicFlourideNitrates from fertilizersCarcinogenic pesticides (DDT)Lead (from pipes)Heavy Metals

39Water-washed DiseasesDiseases caused by poor personal hygiene and skin and eye contact with contaminated water.These include scabies, trachoma, typhus, and other flea, lice and tick-borne diseases.

Water-based DiseasesDiseases caused by parasites found in intermediate organisms living in contaminated water.Includes Schistosomiasis and Dracunculiasis

Water-related DiseasesWater-related diseases are caused by insect vectors, especially mosquitoes, that breed or feed near contaminated water. They are not typically associated with lack of access to clean drinking water or sanitation services.Include dengue, filariasis, malaria, onchocerciasis, trypanosomiasis and yellow fever.

ProblemsWhat, in your own words, do you understand when it is stated that MPN of a certain water sample is 25/100 mL.MPN test was performed on a wastewater sample, and the results given in the table below obtained.

Describe in detail how you would determine the MPN of the sample based on the above results.

A laboratory runs a solids test. The weight of the crucible = 48.6212g. A 100 ml sample is placed in the crucible and the water is evaporated. The weight of the crucible and dry solids = 48.6432g. The crucible is placed in a 600oC furnace for 24 hr and cooled in a desicator. The weight of the cooled crucible and residue, or unburned solids = 48.6300g. Find the total, volatile and fixed solids.

A water sample contains 60mg/l of calcium, 60 mg/l of magnesium, and 25mg/l of sodium. What is the total hardness in (a) meq/l and (b) mg/l as CaCO3?

Nitrite standards for gaseous NO2 measurement are prepared by dissolving 2.03 g NaNO2 in 1000 mL of distilled water. This primary standard is then diluted 100 times to get the secondary standard. Prove that 1 mL of this secondary standard is equivalent to 10 mL of NO2 (at 298oK, 1 atm.), given that 0.72 moles of NaNO2 produces same color as 1 mole of NO2.What is the theoretical oxygen demand in mg/l for a 1.67 x 10-3 molar solution of glucose, C6H12O6 to decompose completely ?What is the theoretical oxygen demand in liters of air for a 50 mg/l solution of acetone, CH3COCH3, to decompose completely ?Assuming a deoxygenation constant of 0.25/d. Calculate the expected BOD5 if the BOD3 is 148 mg/l.Propionitrile has a general formula of CH3CH2CN. It is completely oxidized by dichromate during the COD test. Write a balanced equation for reaction of C-2H3C-2H2C+2N-3 with Cr2O7--. The nitrogen end-product of the reaction is NH3. What is the COD of 50mg/l of Propionitrile.Discuss the sources and impacts of suspended solids, tastes and odours in water supplies. How are they measured ?What is an indicator organism ? Discuss the characteristics of the ideal pathogen indicator and indicate which organisms most nearly exhibit these characteristics.Name toxic metals that may be dissolved in water, identify their principal sorces and discuss their impacts.A 200 ml sample of water has an initial pH of 10. Thirty milliliters of 0.02 N H2SO4. Is required to titrate the sample to pH 4.5. What is the total alkalinity of the water in milligrams per litre as CaCO3?

Tests for common ions are run on a sample of water and the results are shown below. If a 10 percent error in the balance is acceptable, should the analysis be considered complete? Ca+2 = 55mg/l, Mg2+ = 18mg/l, Na+ = 98mg/l, HCO3- = 250mg/l, SO42- = 60mg/l, Cl- = 89mg/l