A practical guide to aluminum sulfate, how coagulation actually works, why it consumes alkalinity, and how it is supplied, from a US-based distributor.
Aluminum sulfate (alum, Al2(SO4)3, CAS 10043-01-3) is the most widely used coagulant in water and wastewater treatment. Dissolved in water it hydrolyses to aluminum hydroxide floc, which sweeps up the fine colloidal particles that cause turbidity and colour and carries them out in settling or filtration. Commercial grade runs 17% Al2O3. Stratus Chemical supplies powder grade in bags and supersacks across the United States.
| Property | Value |
|---|---|
| Chemical name | Aluminum sulfate |
| Synonyms | Alum, filter alum, cake alum, papermaker's alum, sulfate of alumina, aluminium sulphate |
| CAS number | 10043-01-3 |
| Molecular formula | Al2(SO4)3 |
| Molecular weight | 342.15 g/mol (anhydrous) |
| EC / EINECS | 233-135-0 |
| Appearance | White powder |
| Water solubility | Readily soluble |
| Grades supplied | Technical, powder |
| Packaging | Bags, supersacks |
| Documentation | COA & SDS provided on request |
The figures below are representative of technical grade aluminum sulfate powder. They are indicative rather than guaranteed: specifications vary by source and by lot, and the certificate of analysis for your material governs.
| Characteristic | Specification |
|---|---|
| Total Al2O3 | 17% |
| Basic Al2O3 | 0.2% min |
| pH at 25 °C | 2.9 min (1 in 20) |
| Size (powder) | 99% through 30 mesh |
| Appearance | White powder |
Alum is bought and dosed on its Al2O3 content rather than on product weight, because that figure is what determines how much aluminum reaches the water. Two products at different alumina strengths are not comparable on price per ton.
The particles that make water cloudy are mostly too small and too stable to settle. They carry a negative surface charge, repel one another, and will stay suspended indefinitely. No amount of settling time removes them.
Alum breaks that stalemate in two ways at once. The aluminum ion carries three positive charges, which neutralises the surface charge on the colloids and lets them approach each other and stick together. At the same time the aluminum hydrolyses and precipitates as aluminum hydroxide, a bulky gelatinous floc that forms throughout the water and physically sweeps particles down with it as it settles. In most plant conditions the sweep mechanism does the majority of the work.
The result is floc large enough to settle in a clarifier or be caught by a filter. Colour, much of the organic load, and phosphorus come out with it, which is why the same chemistry serves drinking water plants and wastewater phosphorus removal.
Alum hydrolysis releases acid. That has consequences an operator has to plan for, and it is the most common source of trouble when a plant changes dose or source water.
Every unit of alum dosed consumes alkalinity from the water. In a well-buffered source the water absorbs it and pH barely moves. In a soft, low-alkalinity source the buffer runs out, pH falls sharply, and two problems follow at once: coagulation stops working because the chemistry has left its effective window, and the finished water turns corrosive to the distribution system. Plants treating soft water routinely dose lime, soda ash, or caustic soda alongside alum for exactly this reason.
The effective pH window for alum coagulation is roughly 5.5 to 7.5, and it is narrower than many operators expect. Outside it, aluminum hydroxide becomes more soluble, floc formation suffers, and residual dissolved aluminum in the finished water rises. Jar testing against the actual source water, repeated when the source changes seasonally, is the only reliable way to set dose and pH together.
The primary use. Alum is the standard primary coagulant in municipal treatment, removing turbidity, colour, and natural organic matter ahead of filtration and disinfection. Removing that organic matter matters beyond clarity, because it would otherwise react with chlorine downstream to form disinfection by-products.
Aluminum precipitates phosphate as insoluble aluminum phosphate, which is how many treatment plants meet phosphorus discharge limits. Alum is also used for sludge conditioning and for general suspended solids removal in industrial effluent.
In papermaking, alum sets rosin size onto the cellulose fibre, which is what stops ink feathering and gives paper its water resistance. It is also used for retention and drainage control on the machine.
Used as a mordant in dyeing, fixing dye to fibre, and in tanning.
Aluminum sulfate lowers soil pH quickly, which growers use for acid-loving plants such as blueberries, azaleas, and hydrangeas. It acts faster than elemental sulfur because it does not depend on microbial oxidation, though sulfur is generally preferred where time allows.
| Coagulant | Where it wins | Trade-off |
|---|---|---|
| Aluminum sulfate (alum) | Lowest cost per unit of coagulation, well understood, widely available | Consumes alkalinity, narrow pH window, performs poorly in cold water |
| Polyaluminium chloride (PAC) | Works over a wider pH range, consumes far less alkalinity, better in cold water | Substantially more expensive per unit; often justified on soft or cold sources |
| Ferric chloride / ferric sulfate | Effective over a wider pH range, strong on colour and phosphorus | Stains and is highly corrosive to equipment; residual iron colours the water if overdosed |
| Polymer coagulant aids | Build stronger, faster-settling floc when used with a primary coagulant | Not a primary coagulant on their own; overdosing blinds filters |
How to choose: start with the raw water. On a well-buffered source at moderate temperature, alum is difficult to beat economically and remains the default. On soft, low-alkalinity water, the cost of the supplementary alkali needed to hold pH can erase alum's price advantage, and PAC often wins on total treated cost. Cold water is the other decider: alum's performance falls off noticeably near freezing while PAC holds up better, which matters for northern plants in winter. Ferric is the usual answer where colour or phosphorus removal is the governing duty and the plant can live with the corrosion and staining.
Products used to treat drinking water are normally expected to carry NSF/ANSI 60 certification. Certification is specific to the individual product and manufacturer rather than to aluminum sulfate as a chemical, so request the certification documentation for the exact grade before specifying it for a potable system. Contact us for the documentation covering the grade you are considering.
Store aluminum sulfate dry in closed original packaging, off damp floors. This matters more than it does for most dry chemicals: the material is hygroscopic, and moisture pickup causes both assay variation and hardening of the product in the bag. A hardened supersack is a genuine operational problem, not a cosmetic one.
Alum solutions are acidic and corrosive to mild steel, galvanised surfaces, concrete, and aluminum itself. Use plastic, lined, or suitable stainless equipment for dissolving tanks, day tanks, pumps, and lines, and protect concrete containment.
The dust and solution are irritating to skin, eyes, and the respiratory tract. Use the protection specified in the Safety Data Sheet, add product to water rather than water to product when making up solution, and avoid dry sweeping of spills.
Aluminum sulfate, commonly called alum, is used primarily as a coagulant in drinking water and wastewater treatment, where it removes turbidity, colour, natural organic matter, and phosphorus. It is also used as a sizing agent in papermaking, as a mordant in textile dyeing and in leather tanning, and as a fast-acting soil acidifier for acid-loving plants. Stratus Chemical supplies technical grade powder in bags and supersacks across the United States.
The CAS number for aluminum sulfate is 10043-01-3. Its molecular formula is Al2(SO4)3 and its molecular weight is 342.15 g/mol on an anhydrous basis. Its EC number is 233-135-0. It is also known as alum, filter alum, cake alum, papermaker's alum, and sulfate of alumina.
The particles that cause turbidity carry a negative surface charge, repel one another, and will not settle on their own. Alum works on both fronts at once. The trivalent aluminum ion neutralises that surface charge so particles can collide and stick together, and the aluminum simultaneously hydrolyses into bulky aluminum hydroxide floc that forms throughout the water and physically sweeps particles down as it settles. In most plant conditions the sweep mechanism does most of the work.
Because the hydrolysis reaction that forms aluminum hydroxide floc releases acid, and that acid is neutralised by the alkalinity present in the water. In a well-buffered source the pH barely moves. In a soft, low-alkalinity source the buffer is exhausted, pH drops sharply, coagulation stops working properly, and the finished water becomes corrosive to the distribution system. Plants treating soft water commonly dose lime, soda ash, or caustic soda alongside alum to hold pH.
Roughly 5.5 to 7.5, and the window is narrower than many operators expect. Outside it, aluminum hydroxide becomes more soluble, floc formation deteriorates, and residual dissolved aluminum in the finished water rises. Because the optimum shifts with source water chemistry and temperature, jar testing against the actual raw water is the only reliable way to set dose and pH, and it should be repeated when the source changes seasonally.
Alum is cheaper per unit of coagulation and remains the default on well-buffered water at moderate temperature. Polyaluminium chloride works across a wider pH range, consumes far less alkalinity, and performs better in cold water, but costs substantially more per unit. On soft, low-alkalinity sources the cost of the supplementary alkali alum requires can erase its price advantage, and PAC often wins on total treated cost. Cold northern plants frequently make the same switch for winter operation.
Store it dry in closed original packaging, off damp floors. Aluminum sulfate is hygroscopic, and exposure to moisture or humidity causes both assay variation and hardening of the product in the bag, which is an operational problem rather than a cosmetic one. Solutions are acidic and corrosive to mild steel, galvanised surfaces, concrete, and aluminum, so use plastic, lined, or suitable stainless equipment for dissolving and dosing.
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