Chemical Treatment Processes Coagulation-flocculation-sedimentation Chemical Treatment Processes Process Purpose Coagulation To remove turbidity or suspended solids The process is usually employed for treatment of river or lake water for domestic purposes It may also be used in treating industrial wastewater Precipitation A chemical reaction is involved in this process through which the pollutant reacts with an induced chemical to form a precipitate that settles down Examples: removal of hardness by lime and soda ash and removal of iron and Mn by oxidation Ion exchange Removal of undesirable ions (cations or anions) by replacing them with other ions Activated carbon Removal of trace organic compounds by adsorption Disinfection Inactivation of microorganisms using chlorine, ozone, etc Coagulation Objective: To remove suspended solids (turbidity) Suspended solids are negatively charged and repel each other in solution When a coagulant is added the particles are neutralized and agglomerate Setup in Treatment Plants Coagulant Coagulation Flocculation Sedimentation Suspended solids ½ minute ½ hr rapid mixing slow mixing Flocculation 1-10 hr Sedimentation Coagulants: Al2(SO4)3.18H2O Alum Al FeSO4 FeSO Turbidity removal Optimum removal of colloidal particles from water and wastewater depends, among other factors on, •type of coagulant •dose •pH Turbidity removal FeCl3 FeCl Dose pH Schematic of the Jar Test Apparatus Reactions Coagulants need alkalinity to form a precipitate of the metal hydroxide [Al(OH)3 or Fe(OH)3] that precipitates with the suspended solids Thus, if turbid water lacks natural alkalinity, then lime Ca(OH)2 or soda ash Na2CO3 are added Without the alkalinity, flocs will be poorly formed Al2(SO4)3.14.3H2O+3Ca(HCO3)2= 2Al(OH)3+3CaSO4+14.3H2O+6CO2 According to the above reaction, mole of alum (m wt 600 g/mole) requires moles of natural alkalinity (m wt 100 g/mole as CaCO 3) Thus, mg/l alum requires 0.5 mg/l alkalinity as CaCO Mixing and Sedimentation Tanks The Great Lake Upper Mississippi River Board GLUMRB recommended the following values Q for the design of rapid mixing, flocculation and sedimentation tanks in water treatment plants Example: coagulation-flocculationd or h sedimentation, or softening units As Q Ax W Rapid mixing: Detention time= V/Q ≤ 30 sec Flocculation: L Detention time > 30 Velocity through tank= Q/Ax=0.5-1.5 ft/min Paddle velocity= 0.5-3 ft/sec Sedimentation: Detention time ≥ hrs Velocity through tank ≤ 0.5 ft/min Over-flow rate= vo= Q/As=500-800 gpd/ft2 Weir loading= Q/Lweir ≤ 20,000 gpd/ft Example Q=3000 m3/d A clarifier 27 m long, m wide, and 3.8 m deep settles 3000 m3/d The effluent weir length is 50 m Calculate the detention time, flow-through velocity, over-flow rate, and weir loading Q 3.8 Solution V 27 × × 3.8 m 24 hr th = = × = 4.1 hr Q d 3000 m / d Q 3000 m / d d v= = × = 0.11 m / Ax 24 × 60 × m Q 3000 m / d vo = = = 22.2 m / d / m 2 As 27 × m 3000 m / d Q weir loading = = = 60 m / d / m Lweir 50 m 5m 27m