Coagulation Chemistry: Effects on the Acid/Base Balance Via chemical equilibrium reactions, consumption of OH  in the precipitation step has a domino effect on the concentrations of H +, OH , H 2 CO 3, HCO 3 , and CO 3 2 . The net changes can be determined by solving several non-linear equations:

The exact results can be obtained numerically, but the approximate change is conversion of one HCO 3  to H 2 CO 3 for each OH  consumed, while TOTCO 3 remains constant: Coagulation Chemistry: Effects on the Acid/Base Balance

The ultimate “reservoir” undergoing most of the change is not the one where the change is initiated, like water removal from connected reservoirs: OH  HCO 3  If water is removed from “OH  reservoir”, equilibration replenishes most of it from other reservoirs; the ultimate loss is mostly from the “HCO 3  reservoir”. Coagulation Chemistry: Effects on the Acid/Base Balance

To a good approximation, the final pH can be calculated from the initial conditions and the conversion of HCO 3  to H 2 CO 3. The calculations are often presented in the context of alkalinity, which is the net capacity to bind H + : where the approximation holds at pH less than ~9.0 Coagulation Chemistry: Effects on the Acid/Base Balance

Typically, Alk init, pH init and coagulant dose are known. Approximate (HCO 3  ) init as Alk init, compute (H 2 CO 3 ) from K 1. Compute TOTCO 3,init as (HCO 3  ) init + (H 2 CO 3 ) init. Compute Alk fin from Alk init and coagulant dose. Approximate (HCO 3  ) fin as Alk fin, compute (H 2 CO 3 ) fin from TOTCO 3 and (HCO 3  ) fin. Compute pH fin from (H 2 CO 3 ) fin, (HCO 3  ) fin, and K 1. If pH fin is too low, choose acceptable value, re- compute Alk fin, and determine required lime dose. Coagulation Chemistry: Effects on the Acid/Base Balance

A water supply at pH 7.3 and containing 0.8 meq/L Alk is dosed with 40 mg/L FeCl 3. Estimate the final pH. Example: Coagulation Chemistry 1.Approximate (HCO 3  ) init as Alk init. Each mmole of HCO 3  contributes one meq of Alk, so (HCO 3  ) init  0.8 mmol/L. Then, (H 2 CO 3 ) is computed as: 2.Compute Alk fin from Alk init and FeCl 3 dose:

3.Approximate (HCO 3  ) fin as Alk fin, compute (H 2 CO 3 ) fin from TOTCO 3 and (HCO 3  ) fin. 4.Compute pH fin from (H 2 CO 3 ) fin, (HCO 3  ) fin, and K 1. The pH is quite low, and lime would probably have to be added to increase it to at least 6.0.

Conditions in typical natural waters. Lots of dissolved NOM. Low doses of Fe 3+ or Al 3+ partially neutralize the charge on the NOM. The NOM exerts a “coagulant demand.” O OHO OOCCOO OH OH O COOH COO O OH HOOC O O O HO O O OH Fe 3+ High doses of Fe 3+ or Al 3+ generate new surfaces to which the NOM can bind. Coagulation and NOM

–Requires NOM removal from many surface waters –Removal requirement depends on NOM conc’n (quantified as Total Organic Carbon, TOC) and Alkalinity –“Escape clause” available if a point of diminishing returns is reached –Enhanced coagulation is a “BAT.” If it doesn’t work, you are off the hook The Enhanced Coagulation Rule TOC (mg/L) ALK (mg/L CaCO 3 ) 0-60>60-120>120 <2N/A 2-435* > *Required percentage reduction in TOC

Flocculation

Paddle Flocculators at Everett WTP (Note the CMRs-in-Series Arrangement)

A Paddle Flocculator at Everett WTP

Fluid Shear: Particles Collide by Traveling on Different Streamlines at Different Velocities Brownian Motion: Particles Collide Due to Random Motion Differential Sedimentation: Particles Collide Due to Different Terminal Velocities The rate of reaction by all mechanisms is expected to be first order with respect to each type of particle second order overall: Flocculation Theory: Particles Flocculate by Three Mechanisms

The Rate of Collisions by Each Mechanism Can be Predicted from Theory

Different mechanisms dominate for different size ranges. The only controllable mechanism is shear, by controlling the shear rate, G.

The optimum coagulant dose and mixing rate are determined by simulating both coagulation and flocculation in “jar tests.” Coagulation and Flocculation Practice