9.0 COAGULATION Virtually all surface water sources contain turbidity. ost of the suspended matter in water are present as finally divided or colloidal particles and these do not settle due to gravitational forces alone. For example in a depth of 1 ft; bacteria would take about 1 year to settle clay particles would take about 8 hours sand about 10 secs Colloidal size particles may be formed from soluble compounds (ex. soap. starch, bentonite) which do not grow into crystals of size large enough to settle or be filtered out in plain sedimentation. To assist in the agglomeration of particles, coagulants are used to increase effective size, which increases settling velocities. 9.1 Zeta Potential Since the properties that prevent the natural agglomeration vary for suspended solids, it is important to review the significant effects. majority of particles in surface waters are negatively charged these like charges cause the particles to repulse, or in other words remain in suspension such colloidal suspensions are called stable coagulation is an attempt to destabilize the suspensions and allowing the particles to "join" When two colloids come in close proximity, there are two forces on them as shown in Figure 1: i) Zeta Potential which causes the particles to repel ii) van der Waals force which promotes attraction The net force is repulsive at great distances and attractive only after passing through a maximum net repulsive force. Once the force becomes attractive, contact between the particles takes place. To overcome the repulsive forces: Figure 1: Net Force Using Zeta Potential molecular momentum where the particles overcome the energy barrier and collide reduce the zeta potential so that van der Waals' force can take 359-Stu_99F.wpd F-1
SCULZE-HARDY Rule: The precipitation of a colloid is effected by the ion of the added electrolyte which has an charge opposite to that of the colloid particles. Thus coagulants must have an opposite charge to that of the particles to do a good job. 9.2 Stages of Coagulation rapid mixing slow mixing to allow contact undisturbed settling Coagulating Power of Several Electrolytes Electrolyte Relative Power of Coagulation Positive Colloids Negative Colloids NaCl 1 1 Na 2 SO 4 30 1 Na 3 PO 4 1000 1 BaCl 2 1 30 gso 4 30 30 AlCl 3 1 1000 Al 2 (SO 4 ) 3 (a) 30 >1000 FeCl 3 1 1000 Fe 2 (SO 4 ) 3 (b) 30 >1000 (a) popular in water treatment; (b) popular in raw sewage Categories of Synthetic Organic Coagulants and Flocculants General Chemical Characteristics Inorganic Alum Acidic; forms gelatinous floc Ferric chloride Acidic; gelatinous floc, residual iron Ferrous salts Acidic; gelatinous floc, residual iron Aluminate Acidic; gelatinous floc Lime Alkaline, fine floc Silica Neutral; promotes grainy floc clay Ion exchange properties assist flocculation Organic Cationic polyelect. Usually works best on biological solids usually effective below ph 9 overdose may cause dispersion Non-ionic polymer Anionic polyelect Usually best at neutral ph for floccing high solid slurries, strengthens inorganic floc Usually best for raw wastewaters solids or fibrous industrial waste; usually effective over ph 6; often used to condition solids before treatment with cationic polyelectrolytes; strengthens floc 359-Stu_99F.wpd F-2
typical reactions Aluminum sulfate Al 2 (SO 4 ) 3 *18H 2 O + 3Ca(HCO 3 ) 2 ------> 2Al(OH) 3 + 3CaSO 4 + 18H 2 O + 6CO 2 Ferric chloride 2FeCl 3 + 3Ca(HCO 3 ) 2 -----> 2Fe(OH) 3 + 3CaCl 2 + 6CO 2 Ferric sulfate Fe 2 (SO 4 ) 3 + 3Ca(HCO 3 ) 2 ------> 2Fe(OH) 3 + 3CaSO 4 + 6CO 2 Ferrous sulfate and lime FeSO 4 *7H 2 O + Ca(OH) 2 -----> Fe(OH) 2 + CaSO 4 + 7H 2 O followed by and in the presence of oxygen 4Fe(OH) 2 + O 2 + 2H 2 O -----> 4Fe(OH) 3 Chlorinated copperas 3FeSO 4 *7H 2 O + 1.5Cl 2 ----> Fe 2 (SO 4 ) 3 + FeCl 3 + 21H 2 O followed by Fe 2 (SO 4 ) 3 + 3Ca(HCO 3 ) 2 ----> 2Fe(OH) 3 + 3CaSO 4 + 6O 2 and 2FeCl 3 + 3Ca(HCO 3 ) 2 ----> 2Fe(OH) 3 + 3CaCl 2 + 6CO 2 Optimum ph ranges for each coagulant is as follows: alum 4.0 to 7.0 ferrous sulfate 8.5 and above chlorinated copperas 3.5 to 6.5 and above 8.5 ferric chloride 3.5 to 6.5 and above 8.5 ferric sulfate 3.5 to 7.0 and above 9.0 9.3 Chemistry of Coagulation check alkalinity of water sufficient alkalinity must be present to maintain proper ph control as the hydrolysis of the chemical produces H + ions. 359-Stu_99F.wpd F-3
to counteract, additional alkalinity is added if the form of soda ash, lime, sodium bicarbonate and liquid caustic. This allows further reaction of the flocculant adequacy of the natural alkalinity may be estimated from the overall equations Coagulant Aids polyelectrolytes high molecular weight, long chain organic polymers with a large number of ionizable sites these sites attract opposite charge colloids provides a nucleus, which allows the flocculated particles to grow in size and have a faster settling velocity mostly cationic charges are used in water treatment 9.4 Jar Test See Lab anual and Lab that was conducted 9.5 Aluminum Levels edical Profession is unsure about alums health affects; suggest that levels be reduced in water. Table 1: Alum Levels in Ontario Communities (Globe and ail, Friday April 7, 1995) Community aximum ( g/l) inimum ( g/l) Average a ( g/l) Ajax 220 55 136 Bracebridge b 44 30 34 Gravenhurst 690 120 317 Hamilton 120 49 81 Kingston 250 53 144 GTA (Harris) 240 81 134 GTA (Clark) 110 65 83 Ohsweken c 4,350 66 1088 Ottawa 220 57 115 Port Dover 389 78 189 Rainy River 1530 170 623 Sudbury * 19 7 11 Windsor 230 60 117 a - OE Guideline at 100 g/l; looking at 50 g/l b - do not use alum c - high levels due to mechanical problems 359-Stu_99F.wpd F-4
9.6 Softening reduction in water hardness (softening) is a process commonly practised in water treatment may be done by water utility if the water is extremely hard best if done by consumer as in Guelph as the consumer governs degree of softening consumers use ion-exchange units 9.6.1 Chemical - Precipitation (Lime Soda Ash) dependent on ph, temperature and ionic species precipitates carbonate hardness via CaCO 3 and Ca(HCO 3 ) 2 + Ca(OH) 2 ph=9.4 2CaCO 3 + H 2 O g(hco 3 ) 2 + 2Ca(OH) 2 ph=10.6 g(oh) 2 + 2CaCO 3 + H 2 O converts non-carbonate g 2+ hardness into g(oh) 2 which precipitates out g +2 + Ca(OH) 2 ----------> g(oh) 2 + Ca +2 non-carbonate calcium (Ca +2 ) hardness is removed by adding soda ash (Na 2 CO 3 ) to form CaCO 3 (pg 573 of text) recarbonate with CO 2 to lower ph to 8.5 some residual hardness remains due to solubility of calcium carbonate and magnesium hydroxide in practice, lime-soda process will reduce water hardness to 50 to 80 mg/l as CaCO 3 9.6.2 Ion-exchange have resin that removes hardness ions must be regenerated costs plenty all water is not treated 9.7 Aeration used to remove undesirable gases dissolved in water (degasification) or add oxygen to oxidize undesirable substances common for groundwaters as surface waters have sufficient contact time for gas transfer groundwaters contain appreciable amounts of gases such as CO 2 and (H 2 S) aeration of groundwater containing high amounts of these gases above surface water concentrations allows equilibrium to be reached 359-Stu_99F.wpd F-5
reduces iron and manganese, forming new ionic complexes that are not soluble 4Fe 2+ + O 2 + 10H 2 O ----> 4Fe(OH) 3 3+ + 8H + 2n 2+ + O 2 + 2H 2 O -----> 2n(OH) 2 4+ + 4H + degasification is governed by the principles of gas transfer methods are designed to drive the water-gas mixture toward equilibrium as quickly as possible common methods include # fountains # piping suspended over a catch basin that sprays water into the atmosphere # smaller the nozzle greater the surface to volume ratio # cascade tower # series of waterfalls that drop into small pools # tray towers # others as new technology becomes available 359-Stu_99F.wpd F-6