Lecture# 3 Water treatment The wastewater treatment mainly involves three stages: primary treatment, secondary treatment and tertiary treatment .or can be divided to: (chemical, physical and biological) Primary Treatment Coagulation, Flocculation , Skimming,Flotation,Sedimentation, Screening,Settling Secondary Treatment: Trickling filter. activated sludge process and oxidation ponds called lagoons are the important procedures involved in the secondary treatment Tertiary( Advanced Treatment): Reverse Osmosis,Ion Exchange ,Electrodialysis,Membrane,Electrocoagulation,etc.

Types of water Waters are classified as "hard" or "soft“ , acid or alkaline to describe their predominant characteristics reflected by the compounds they contain. Hardness of water is caused by calcium and magnesium salts in solution. Total hardness, as this term suggests, is a measure of calcium, magnesium,. Alkalinity of water is due to the presence of alkaline compounds. Carbonate hardness, at one time identified as temporary hardness, represents carbonates and bi-carbonates of calcium and magnesium

Owing to the high solubility of sodium salts in water, they do not ordinarily form deposits or cause scale and are usually not objectionable except at extra ordinarily high concentrations. The forms of soda present most frequently in water are sodium chloride, NaCl, Sodium carbonate, Na2CO3, sodium bi-carbonate, NaHCO3, sodium hydroxide, NaOH, sodium sulphate, Na2SO4, and sodium nitrate, NaNO3. Sodium hydroxide (caustic soda) is always present in boiler water when the feed water contains sodium carbonate or bi-carbonate due to hydrolysis under high temperatures and pressures.

Caustic soda is known to attack steel, resulting in the characteristic cracking known as caustic embrittlement or crystallization of boiler steel. All waters contain some organic matters. Most of the organic matter present in water is caused by the discharge of sanitary sewage and industrial wastes in the supplies Most of the organic matter present in water is caused by the discharge of sanitary sewage and industrial wastes in the supplies.

It includes : Coagulation, Flocculation , Sedimentation (Water treatment) Some Physical, Chemical and Biological Wastewater Treatment Methods (Clarification): It is a process applied to surface waters for the removal of suspended solids, finer solids which appear as turbidity and color, and other colloidal materials It includes : Coagulation, Flocculation , Sedimentation Co-agulation requires the addition and rapid mixing of a chemical co-agulant with the water to be treated. To settle the heavier solids by gravity - removing the "clarified" effluent .

Flocculation follows co-agulation. It brings many small flocs together, using gentle agitation, which form larger particles with faster settling speeds. The agitation must be controlled carefully to prevent disintegration of the fragile flocs. This sludge (of the previous treatments steps) is dilute and must be thickened or dewatered for disposal.

Screening: process of removing the large matters from sewage - through screens Flotation and Skimming: process of removal of floating matters such as oils, grease, etc Filtration: Suspended solids - porous medium that is made up of paper or granular material - removal of water from sludges or slurries

Filtration In the context of external water treatment, a filter is a bed of granular material which physically removes suspended matter from the water passing through it. The only change in water quality resulting from filteration is the reduction of suspended solids to 1 mg\L or less. Most filters operate for a limited period of time. When the pressure drop increases to a given level, the filter is clogged and requires cleaning by backwashing. All filters require pressure (head) to cause water to flow through the unit. The term "pressure filter" was used to force the water through the filter under pressure .

Carbon Filtration: Granular activated carbon is occasionally used as a filter medium but usually carbon beds are preceded by sand or coal . Carbon used to remove dissolved organic matter, however, has a finite capacity that is determined by its properties and the nature of the organic matter. When granular carbon is exhausted, it is either replaced with new or thermally regenerated carbon.

Factors that affecting on the rate of settling of solid bodies in water: 1. Specific gravity, 2.Shape and size of the particles, 3.Viscosity of the water. Very fine divided particles of matter may remain in suspension for weeks and are removed by subsidence only after they have coalesced. The primary consideration in the design of settling basin for the removal of the settleable solids should be the period of detention which will be available at the maximum rate of flow. The detention period which is incorporated in the design will depend on the coarseness of the suspended matter, the degree of purification required. From 4 to 24 hours of settling may be employed.

Ca(OH)2 → Ca2+ (aq) + 2OH¯ (causing pH change) (2) II. Chemical Treatment Chlorination: oxidizing chemical Cl2 + H2O ↔ H+ + Cl- + HOCl (1) Hypochlorous acid (HOCl) is the prime disinfecting agent Ozonation : More effective against cysts and viruses than chlorine Advantages - no taste or odour problems Disinfective power of ozone is limited by its relatively low solubility in water. Neutralization: Addition of acid or base to adjust pH levels back to neutrality.  Ca(OH)2 → Ca2+ (aq) + 2OH¯ (causing pH change) (2)

Chemical co-agulation Most waters can be clarified by the addition of alum or other co-agulating materials followed by settling for a short period, usually 2 to 4 hours . Alum is the form generally used has a chemical formal of Al2(SO4)3.18H2O. Although alum is fairly soluble, it is preferable to heat the water used for dissolving the chemical and to agitate it to accelerate the rate of solution. Iron co-agulant: The iron compounds used in water treatment include several of the soluble ferrous and ferric salts . Ferrous sulphate and Ferric chlorides Miscellaneous co-agulants: Many organic and inorganic materials have been used to co-agulate solids and improve settling rates. Clay, bentonite, activated carbon, causticized starches and other compounds have been found to be helpful in the treatment of some supplies.

Adsorption: Treatment by activated carbon is mostly due to adsorption. When a chemical species is adhered to the surface of a solid - Physical adsorption. When partial chemical bonds are formed between adsorbed species or when the absorbate got into the channels of the solids, Chemical Adsorption. Porous material with high surface area. .

Biological treatment methods: Method use microorganisms, mostly bacteria, in the biochemical decomposition of wastewaters to stable end products - carbon dioxide, water and other end products.  Biological treatment - aerobic and anaerobic methods - based on availability of dissolved oxygen.