Variations in raw water turbidity and total organic carbon (TOC) loading can make drinking water treatment challenging, often forcing plant operators to vary coagulant doses to maintain finished water quality. Enhanced coagulants and coagulant/polymer blends have an extra coagulant charge that can provide a reserve of coagulant energy to overcome those variations or provide operators with time to adjust treatment.
Coagulation is often the first line of defense against raw water excursions. Plants that do not change coagulation rapidly enough can spiral out of control. If coagulant demand increases greatly and a plant does not respond, an underdose situation arises that produces cloudy, unfilterable water. On the other extreme is the plant that overdoses with coagulant. Overdosing degrades finished water by creating an overcharged, dispersed floc that does not settle or filter well.
Coagulation depends, in part, on how much positive charge is present to neutralize the negatively charged particles and colloids in raw water. In conventional coagulants, the aluminum in alum has a charge of +3, and the iron in iron salts has a charge of either +2 or +3. Enhanced coagulants like polyaluminum hydroxychloride (PACl) have a +6 or +7 charge, which gives them twice the strength of traditional coagulants.
Organic polymers, which contain numerous +1 charges along their chains, can be added at the plant intake. This helps the primary coagulant by removing some of the increased charge demand and can help the settling process, which prevents the raw water excursion from affecting filtration. Plants that continuously feed supplemental chemicals may simply increase them during a time of change.
While this can be effective, it can lead to problems. If the addition rates are too high, the excess positive charge can disperse the floc so it does not settle. This produces floc carryover or haziness that can blind filters and cause filter breakthrough.
Plants using conventional coagulants can handle change by switching to an enhanced coagulant, such as PACl, a PACl/polymer or alum/polymer blend. PACls used for high-turbidity or rapidly changing waters typically are high-basicity grades, because they have a high positive charge.
The choice of an enhanced coagulant should be based on testing, internal treatment goals and a variety of operating factors. For instance, a plant using alum can easily switch to an alum/polymer blend without having to clean its storage and feed systems. A change to a PACl in this case would require cleaning of the storage tank and the entire feed system.
Many plants use enhanced coagulants or blends seasonally as their primary coagulant to improve cold weather performance or handle variant water. Some of these plants have switched to continuous, year-round use of a PACl or blended product to ease operations by, for example, reducing sludge or decreasing or eliminating the need for alkali used in pH or alkalinity adjustment.
Pilot Tests
One water treatment plant regularly copes with water released by a dam in midwinter. The water, which is usually held back for five months, is discharged each year to adjust upstream water level before spring rains arrive.
Pilot Tests
To improve its ability to deal with this extreme shift in raw water, the plant compared two coagulants (alum and a high basicity PACl/polymer blend) in parallel trains during the mid-winter event (See chart on page 23). As the test progressed, raw water turbidity rose from an average of less than 5 NTU before the release to a high of 70 NTU. Similarly, TOC increased from 2 mg/L to more than 8 mg/L just after the dam opening.
Pilot Tests
In treatment train A, the alum dose was increased from an average 35 ppm before the raw water event to about 200 ppm during the difficult treatment period, while the PACl/polymer dose in treatment train B went from 10 ppm to about 50 ppm.
Pilot Tests
Settled water turbidity was 60 percent better with the PACl/polymer blend during the event than with alum, while filtered water turbidity was more than 85 percent better with the blend. The plant also applies cationic polymer, anionic polymer and lime. The enhanced coagulant adapted to the change more readily than alum, so significantly less of these supplemental chemicals were needed.
Pilot Tests
Another plant draws water from a small river that is significantly affected by relatively minor weather changes. Given the highly variable raw water source, plant operators use jar tests to confirm coagulant dose and effectiveness. In order to find a coagulant that could deal with a wide range of conditions, the plant evaluated ferric sulfate and a high basicity PACl one at a time during two large raw water turbidity changes. The plant does not use a cationic polymer to supplement the primary coagulant during high demand periods, so the coagulant must handle the entire turbidity change.
Pilot Tests
The two episodes saw comparable changes in raw water turbidity (See chart on this page). Dosage for ferric sulfate went from 80 ppm to 130 ppm, while PACl went from 37 ppm to 45 ppm. PACl gave somewhat lower settled water turbidity and about 60 percent lower filtered water turbidity compared to ferric sulfate.
Pilot Tests
The PACl had a faster response time than ferric sulfate since a smaller dose was needed for acceptable coagulation. Caustic demand was nearly 90 percent less with PACl than with ferric sulfate, which improved operations and reduced cost. PACl also decreased chemical sludge almost by half during the high turbidity episode when the chemical demand was high. The plant reported that treatment solids generated by PACl dewatered more effectively than those from ferric sulfate treatment.
Basic Strategiesasic Strategies
For Dealing With Change
Plants often use operating history and operator experience to anticipate raw water problems. If a reservoir’s level rises six inches after a heavy rain, for instance, historical data can indicate how long it will take for a problem to appear at the raw water intakes and how intense the change could be. Operators temper this information with their own knowledge. For example, and experienced operator might know that a surge in TOC after a rise in reservoir level is likely to be worse after the first rain following a dry spell or that a more drastic coagulant change is needed at low temperatures when coagulation can be slower.
Basic Strategiesasic Strategies
For Dealing With Change
Jar testing also is an important tool, since it simulates the raw water-chemical environment during mixing and settling. It allows for a visual determination of the size and speed of floc formation. Zeta potential and streaming current detectors, which measure the charge on particles and colloids in water, can help plant operators draw conclusions about coagulant demand in the raw water. Some plants also use pilot filters, which can give an early warning of coagulant demand shifts and indicate if the turbidity applied to the filters is adequately conditioned.
Basic Strategiesasic Strategies
For Dealing With Change
When these tools are used regularly prior to upset conditions, they define a baseline for effective performance. Once a plant has a good understanding of acceptable coagulation, these tools can guide coagulant changes in the face of a difficult raw water.
Conclusion
Coagulation must be adapted as turbidity, TOC and other factors change. Systems that use conventional coagulants alone or in combination with supplemental coagulants generally have a more difficult job because they are not as highly charged. Those using supplemental coagulants may face the challenging task of adjusting two chemicals simultaneously.
Conclusion
Plants that use enhanced coagulants, such as high basicity PACls or blended coagulants, have a coagulation advantage because of the high charge built into the coagulant system. Also, enhanced coagulants may only need modest change to meet most raw water variances.
