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The Process of Flocculation

When pool water is labelled "cloudy", there are usually two possibilities. The first is that there is a microbial build-up due to a failure to maintain adequate sanitiser levels. The solution here is relatively straightforward - the water needs a shock dose of chlorine to destroy bacteria and restore clarity. The second possibility is that there is a suspension of finely-divided (colloidal) particles in the water. These may be organic (from plant, animal or human sources), inorganic (from metals such as calcium, copper or iron), or a combination of both. The solution here is to use a clarifier or, in cases of extreme cloudiness or where speed is of the essence, a flocculant.

The mechanism by which a clarifier or flocculant works is the same - the small particles that make up the suspension are brought together, forming larger aggregates. In the case of a clarifier, these aggregates are eventually large enough to be trapped by the filter. In the case of a flocculant, the process continues until the aggregates are so large and heavy that they settle out of the water onto the bottom of the pool, where they can be vacuumed to waste.

So why do cloudy suspensions form in the first place? If it was possible to bring the individual particles that make up a suspension close enough together, natural coagulation would take place and the aggregates would form on their own. Unfortunately, suspensions are stabilised by repulsion forces that keep the particles far enough apart to prevent the attraction forces from taking hold, and prevent settlement by keeping the particles in constant motion. These repulsion forces come from two sources:

  • Water is adsorbed onto the surface of many particles forming a "solvation sheath" which repels the sheathes on other particles.
  • All particles have an electrical charge upon their surfaces, the sign (positive or negative) and strength of which are dependent on the type of particle involved. For instance, the particles in aqueous (water) suspensions at a pH of 4 or more will invariably have an overall negative charge. As such, individual particles will repel each other.

There are three ways of overcoming these repulsion forces:

1. Using an appropriate physical process, bring the particles close enough so that the attraction forces overcome the repulsion ones. Unfortunately, in most "real world" situations, this is next to impossible to achieve. Moreover, the aggregates formed by this process tend to break down very easily simply by agitation.
2. Using an appropriate additive, reduce the electrical charge on the particles so that they can come closer to each other. Recalling that the particles are negatively charged at a pH of 4 or more, the chosen additive must have a positive charge for neutralisation to occur. There are a number of inorganic salts that work well, the most widely used of these being aluminium sulfate (alum), which provides the highly charged Al3+ ion. Ideally, alum should be used in water that has a pH of between 5.5 and 7.0. (1) Unfortunately, it is all too easy to overdose with an additive like alum, resulting in a positively charged suspension. In addition, the aggregates formed are again subject to breakdown if they are agitated too vigorously.
3. Using a high molecular weight, water-soluble polymer (plastic), form a "bridge" between the individual particles. The requirements for a polymer to act as a flocculant is that it must be strongly adsorbed onto the particles, and must be long enough to span the gap between successive particles. Ideally, it should enable several particles to attached themselves at once, forming a stable 3-dimensional network, as depicted below. In general, the higher the molecular weight, the better the bridging flocculant. Two cationic polymers that readily fall into this category are the poly[acrylamides] and the poly[DADMAC]s (poly[diallyldimethylmethylammonium chloride]). A third polymer, poly[aluminium chloride] (PAC), falls somewhere between the second and third categories here.

Clarifying Pools with Polysheen Plus

Apart from Water Sparkle (alum), there are three options in the BioGuard range for dealing with cloudy water due to suspended material. The first of these, Polysheen Plus, is a highly concentrated blend of non-toxic, cationic polymers that works exceptionally well in clarifying pool water. It is particularly suitable for clearing up suspensions of inorganic material,
such as finely-divided particles of calcium carbonate and clay.

Directions for Use:

1. For best results, the pool water pH should be 7.4 – 7.6 at the time of treatment.
2. The filter, skimmer and pump strainer should all be clean.
3. Operate the pump and filter continuously during treatment.
4. Add 30mL of Polysheen Plus per 10,000 litres of pool water, adding directly into the water, preferably at the deep end.
5. Backwash the filter if the pressure reaches the maximum recommended by the manufacturer.
6. Repeat the treatment with Polysheen Plus if water clarity has not been restored within 36 hours.
Note: NEVER exceed dosage rate indicated above. Increasing the dosage rate will not clear the water faster!

Maintaining Crystal Clarity with Super Clear Tabs

Super Clear Tabs are a blend of non-toxic, cationic polymers and alum in a convenient 60-gram tablet form that prevents the build-up of small particulates in pool water. Whilst they are ideally suited to maintaining pools fitted with sand filtration systems, they are also applicable to pools fitted with DE and cartridge filters. In cases where cloudiness is already present, they can be used as an alternative to Polysheen Plus.

Directions for Use with Sand Filters:

1. For best results, the pool water pH should be 7.4 – 7.6 at the time of treatment.
2. Backwash and rinse the filter according to normal maintenance procedures.
3. Apply one Super Clear Tab per 50,000 litres to the skimmer basket. Once the water is crystal clear, application is weekly or after every backwash.
4. In cases of high bather load, another Super Clear Tab may be required.
Note: Initial use of Super Clear Tabs may cause filter pressure to rise within a few hours of application. This increase is normal. Pressure differential and flow rate should be used to gauge the need for backwashing.

Problem Solving with PowerFloc

PowerFloc replaces LiquiFloc as BioGuard's front line flocculant. A proprietary polymeric blend, PowerFloc has been shown to work up to four times faster than its predecessor, is less pH-dependent and is ideally suited to pools with high levels of suspended organic matter, such as plant material and dirt. The recommended dosage rate is 60mL per 10,000 litres of pool water. For extremely cloudy pools (can’t see drain in deepest part of pool), you should use 120mL of PowerFloc per 10,000 litres.

Directions for Use:

1. Check the filter and backwash or clean if necessary. Caution: Under no circumstances must the filter pressure be allowed to exceed the value recommended by the filter manufacturer.
2. Adjust the pH to 7.5 – 8.0 for best results.
3. Remove the skimmer basket and any products (eg. Smart Sticks) from the skimmer.
4. With the pump on and the filter valve in "Recirculate" or "Whirlpool" position, slowly pour required amount of PowerFloc into the skimmer.
5. For filters without these settings, adjust to the "Filter" position, dilute PowerFloc into a large plastic bucket, then pour evenly across the surface of the pool water. NEVER exceed 60mL of PowerFloc in 4 litres of water.
6. Replace the skimmer basket and allow the pump to circulate water for 2 hours to distribute Power Floc evenly throughout the pool.
7. Shut the pump off and allow PowerFloc to settle particulate matter for 12 – 24 hours. DO NOT disturb the pool water during this time.
8. Slowly vacuum any settled material directly to waste. As settling may continue over several days, repeat vacuuming where necessary. Caution: NEVER vacuum debris through the filter.
9. Backwash and chemically clean the filter with Filter Brite, then reset to normal filter cycle.
10. Adjust the pH to 7.2 – 7.6 and resume normal pool operation.
Note: PowerFloc will not kill algae, but will aid in the removal of dead algae after treatment with any of the BioGuard range of algaecides. Use the chosen algaecide as directed on the label. After algae treatment is complete, follow Steps 1 through 8 as listed above for cloudy water.

Difficulties with Flocculation

As intimated earlier in this Technical Information Bulletin, there can be difficulties experienced when using flocculants. The first concerns overdosing, particularly with a compound like alum. How do you determine what is the "right" dose of clarifier or flocculant? The fact is that this is an extremely difficult task which is constantly encountered, not just in the pool and spa industry, but in a range of industrial processes, from drinking water purification to paper pulp manufacture. Generally, optimum doses increase with increasing molecular weight of the chosen polymer flocculant, but the trade-off is that the rate of settling is very much slower. Fortunately, there is a procedure which greatly reduces the chance of overdosing whilst ensuring a reasonable rate of settlement.

  • ALWAYS follow the dosage rates recommended above and on the product labels. These rates have been determined by the T & D Department at Bio-Lab in the US after extensive testing on a wide variety of pools with varying degrees of cloudiness from a range of sources.
  • Wait 24 - 48 hours with the pump OFF, then vacuum up any sediment from the bottom of the pool. Take care when vacuuming not to stir up the sediment too much, or you will need to let it settle again. If the pool is still cloudy, you should nevertheless perform this vacuuming step, as it will reduce the "load" on any flocculant that either remains in the water, or is subsequently added to the system.
  • If cloudiness persists, repeat the dose as specified on the bottle.

Also as mentioned earlier, flocculants (and clarifiers, for that matter) are pH-dependent to one extent or another. Whilst this dependence is most pronounced for alum, it is recommended that you ensure the pH falls within the label-specified range for a product like PowerFloc or Polysheen Plus in order to optimise the rate of particle aggregation and, hence, water clarification.

Importantly, no matter what the choice of flocculant, NEVER agitate the sediment more than necessary when vacuuming the bottom of the pool or you run the risk of breaking the aggregates apart. Whilst the effect is again most pronounced when using alum, it has been found that aggregates formed in the presence of excess polymer flocculant degrade faster than those that form at the optimum concentration.(1)

NOTE: It has been found that PowerFloc works very well in flocculating organic matter, but is somewhat slower handling inorganic matter. If you know that the suspension in a pool is mainly due to inorganics like clay-based material (often washed into pools after heavy rainfall) or finely-divided insoluble metal deposits, use Polysheen Plus as your flocculant.

For best results:
1. Adjust the pH to 7.4 – 7.6.
2. Add 30mL of Polysheen Plus per 10,000 litres of pool water, adding directly into the water, preferably at the deep end.
3. Allow the pump to circulate water for 2 hours to distribute Polysheen Plus evenly throughout the pool.
4. Shut the pump off and allow Polysheen Plus to settle particulate matter for at least 24 hours. DO NOT disturb the pool water during this time.
5. Slowly vacuum any settled material directly to waste. Settling may continue over several days, so repeat vacuuming where necessary. Caution: NEVER vacuum debris through the filter.
6. Backwash and chemically clean the filter with Filter Brite, then reset to normal filter cycle.
7. Adjust the pH to 7.2 – 7.6 and resume normal pool operation.

Reference
1. N. Moss and B. Dymond, "Flocculation: Theory and Application", reprint from Mine and Quarry Journal, May 1978.

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.