Alum as Clarifier and Flocculant – Effect of Alum on Filtration Systems

Although modern polymeric alternatives (eg. poly(aluminium chloride), polyDADMACs and polyacrylamides) are widely available, alum (aluminium sulfate, Al2(SO4)3) is still extensively used as a flocculant for swimming pools and, for that matter, other bodies of water, such as dams, ponds and reservoirs.


Alum as Clarifier and Flocculant

The role of clarifiers and flocculants in swimming pool applications is to aid in the removal of suspended particles (sand, silt, clay, dead algae, etc.) that are too small to be trapped by the pool’s filter. Many of these particles develop a negative electrical charge and, as such, repel each other. One way to bring them together and form aggregates large enough for filtration or settling is to neutralise this negative charge. This can be done by adding minerals that release positively charged metal ions (cations) upon dissolving. Whilst any metal could theoretically be used, those which give cations of multiple charge (valency) are preferred. The higher the valency, the better and faster the clarification or flocculation; a trivalent cation (M3+) is approximately ten times more effective than a divalent cation (M2+).(1)

When alum is dissolved in water, it releases positively charged aluminium cations (Al3+) which neutralise the negative charge on suspended particles. Depending on parameters such as pH, Total Dissolved Solids (TDS) and the presence of humic acids from decomposing plant material, the aggregates which result may be just large enough to be trapped within the pool’s filter (the process of “clarification” or “coagulation”), or they may be so large as to overcome the buoyancy of the water and sink to the floor of the pool (the process of “flocculation”), where they can subsequently be vacuumed to waste. Just like many clarifiers and flocculants, alum is more efficient in higher pH water; We recommends raising the pH to above 7.8 when using alum as a flocculant. And, just like all clarifiers and flocculants, adding too much alum will actually impede the coagulation/flocculation process

Alum tends to behave more as a clarifier than a flocculant. This is primarily due to the presence of acidic chlorines in the blends – trichloroisocyanuric acid (trichlor) and sodium dichloroisocyanurate (dichlor).

One proposal put forward by opponents of alum-based technologies is that alum accumulates in the filter and “blinds” it; that is, it fills the pores between the filter media, pushing up the filter pressure and preventing ingress of any further filterable material from the pool. This ultimately leads to cloudy water and, more importantly, a damaged filtration system. Whilst failure to maintain filter pressures within manufacturers’ stated limits is potentially a costly exercise, the chief culprit isn’t the alum (as it remains dissolved in the water) but, rather, the particulate material it coagulates in the pool.

So what are the effects on the various pool filtration systems? Let us consider each in turn.


Sand Filters

Sand filters are the most prevalent, accounting for around 70% of the market. The performance of a sand filter can be markedly enhanced by the addition of a filter aid. Alum, too, improves the efficiency of a sand filter, particularly a “rapid rate” unit. Given that the pH and Total Alkalinity (TA) are maintained within recommended ranges, the alum will form a gelatinous substance that adheres to the sand.(2) As is the case with Quick Clear, particles are then trapped within the filter by electrostatic attraction.

Given that the pore size of sand filters are greater than that for either DE or cartridge filters, alum tends to have the most positive effect on the filter efficiency, whilst having the least impact on filter pressures. Continuous use of alum-based products will eventually increase the pressure in a sand filter, but this can usually be redressed by a slight increase in the frequency of backwashing.

Diatomaceous Earth (DE) Filters

DE and cartridge filters contribute almost equally to the remaining 30% of filtration systems fitted to Australian pools. DE filters have a smaller pore size than does sand, and are consequently more effective in removing particulate matter from the water. Predictably, this improvement in filterability is matched with a commensurate increase in the frequency of backwashing. That being said, DE filters can still benefit greatly from the addition of filter aids or the use of alum-based products, particularly in the removal of colloidal-sized contaminants (particles having a maximum dimension of less than 1 micrometre). As for sand, there have been no reports of excessive filter pressure increases in DE filters using the blended sanitisers or oxidisers listed above.

Cartridge Filters

Although a far smaller percentage of the pool filter market, cartridges are the filtration system of choice in spas. As the latter are regularly emptied, the use of alum is normally not required for flocculation purposes. Of the pools fitted with cartridge filters and running one or more of the blended products. In the majority of these, the pools were quite cloudy to start with, meaning that the filter was removing far more waste material than was normally the case. The simply rule here is that pool (and spa) owners should periodically check their filter pressure, irrespective of whether they are using alum based products or not.

Modular Filters

Introduced in the 1990’s and growing in popularity in the US and Canada, these expensive devices can filter out extremely small particles without the need for filter aids. As might be expected, this feature has a downside to it in that the filter demands frequent backwashing and is unsuitable for pools in which high concentrations of undissolved particulates regularly accumulate. Of all four systems considered here, the modular filter is the one most prone to excessive increases in pressure when using alum-based products.

Aluminium and Alzheimer’s Disease

One final comment. Alum was originally chosen as a clarifier and flocculant for drinking water because of the minimal toxicity of aluminium (as Al3+). During the 1990’s, there was significant publicity given to investigations that suggested a causal link between aluminium ingestion and the degenerative condition Alzheimer’s Disease. However, much of the data that this conclusion was based upon has since been refuted and it is now no longer believed that aluminium, even at high concentrations in the body, contributes to the onset of Alzheimer’s. At the very low levels to be found in swimming pools (usually less than 30 ppm after treatment with alum), there are no health concerns whatsoever.



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