WATER TREATMENT

WATER TREATMENT

It is the alteration of colloids so that they will be able to approach and adhere to each other to form larger floc particles. Theses flocs are then conditioned so that they will be readily removed in subsequent processes.

1. COAGULATION

WATER TREATMENT processesThree key properties of a coagulantTrivalent cationNontoxicInsoluble in the neutral ph rangeCoagulantIs a substance (chemical) that is added to the water to accomplish coagulation.Commonly used coagulants are aluminum and ferric iron

4 basic types of coagulant aidsPh adjustersacids and alkalies are both used to adjust the ph of the water into the optimal range for coagulation.sulphuric acid is the most commonly used acid for lowering the pheither lime [ca(oh)2] or soda ash (na2co3) are used to raise the ph

4 basic types of coagulant aids (cont.)Activated silicait produces a stable solution that has a negative surface charge and thus unite with the coagulants resulting into a larger, denser floc that settles faster and enhances enmeshment.

4 basic types of coagulant aids (cont.)Clay just like activated silica but has a slight negative charge and can add weight to the flocs. Clays are also most useful for colored, low-turbidity waters but are rarely used.

4 basic types of coagulant aids (cont.)Polymerscan have a negative charge (anionic), positive charge (cationic), both (polyamphotype), or no charge (non-ionic). it is a long-chained carbon compound of high molecular weight that have many active sites. These active sites adhere to flocs and consequently producing a larger, tougher floc that settles better and this process is called interparticle bridging.

hardnessRemoving some of the hardness in water which is by definition has the allowable hardness of only in the range of 75 to 120 mg/l as Caco3.2. SOFTENINGIs used to characterize water that does not lather well, causes scum in the bath tub, and leaves hard, white, crusty deposits (scale) on coffee pots, tea kettles, and hot water heaters. The reactions of calcium and magnesium accounts for these occurrences.

HARD Water classificationHARDNESS RANGE(mg/L CaCO3)DESCRIPTION0-75SOFT75-100MODERATELY HARD100-300HARD>300VERY HARDTotal hardness (th)Th = ca2+ + mg2+

it is also often broken down to two components namely: carbonate hardness (ch), those associated with hco3- anion, and noncarbonated hardness (nch), associated with other anions.Th= ch + nch

Carbonate hardnessAmount of hardness equal to the total hardness or total alkalinity, whichever is less. Often called temporary hardness because heating the water removes it.Total hardness in excess of alkalinity. Also called permanent hardness because it is not removed when water is heated.nonCarbonate hardnessSummary of softening reactionsNeutralization of carbonic acidPrecipitation of carbonate hardness

Precipitation of noncarbonate hardness due to calcium

Precipitation of noncarbonate hardness due to magnesium3. MIXINGMixing or rapid mixing is the process whereby the chemicals are quickly and uniformly dispersed in the water.

4. Flocculation is a process wherein colloids come out of suspension in the form of floc or flake; either spontaneously or due to the addition of a clarifying agent. 5. Sedimentationis a physical water treatment process using gravity to remove suspended solids from water.6. Filtrationis the process of removing solids from a fluid by passing it through a porous medium.

7. Disinfectionis accomplished both by filtering out harmful micro-organisms and also by adding disinfectant chemicals.8. Adsorptionis the capacity of a solid particle to attract molecules to its surface. An effective method for removing dissolved organic substances that cause tastes, odours, or colours is adsorption by activated carbon.9. Membrane processeselectrodialysis and reverse osmosis are two processes used for desalination. Desalination refers to any of several processes that remove some amount of saltand othermineralsfrom salinewater.