Magnetic Separation
Magnetite ballast — twice as dense as silica and magnetically recoverable — for ultra-low phosphorus, tertiary polishing without filters, and multiplying the capacity of clarifiers you already own.
A heavier ballast, recovered magnetically
Magnetite ballasted clarification works on the same principle as microsand ballasting, but with a ballast that is denser, finer and separable by magnetic force rather than by density.
Magnetite (Fe₃O₄) has a specific gravity of about 5.0–5.2 — roughly twice that of silica — and is dosed at 20–40 µm, around a quarter of the diameter of microsand. Finer and denser means more attachment surface per unit mass, heavier floc for the same ballast dose, and markedly less abrasion in the equipment it circulates through. It is fully oxidised, insoluble, non-toxic and will not degrade in service.
The decisive difference is recovery. Because magnetite is ferrimagnetic, it is reclaimed by a permanent-magnet drum instead of a hydrocyclone. Waste sludge passes through a high-shear mixer that liberates the ballast from the floc, then across the drum, which captures the magnetite and returns it to the reaction tank. Recovery exceeds 99 %. Crucially, because the separation is magnetic rather than density-based, it remains efficient on light, low-density biological and tertiary sludges — exactly the streams on which a hydrocyclone struggles.
That combination is what makes the process the strongest of the three for very low phosphorus limits and for tertiary polishing: total phosphorus below 0.05 mg/L, suspended solids below 2 mg/L and turbidity below 1 NTU are achievable without downstream media or membrane filtration.
Ultra-low phosphorus consents, tertiary polishing without a filtration stage, and multiplying the capacity of clarifiers already on site.
Process sequence
- Coagulation and magnetite dosingCoagulant is dosed into the influent and recovered magnetite is introduced into the first reaction tank. Simple mixing is enough — magnetite is hydrophobic and associates readily with chemical floc and biological solids.
- MaturationPolymer is added in a second reaction tank and the ballasted floc is grown to settling size at low shear. 2–5 minutes.
- ClarificationThe heavy floc settles rapidly in a high-rate clarifier, which may be a new unit or an existing tank. Surface overflow rates up to about ten times conventional and solids loading rates up to twenty times conventional are achievable.
- Sludge recycle80–90 % of the clarifier underflow returns to the reaction tanks. The resulting high-density slurry absorbs shock loads and sweeps up fine residual particulate, which is what holds effluent quality steady through influent swings.
- Shear and magnetic recoveryThe waste stream passes through an in-line high-shear mixer that separates magnetite from the floc, then across a rotating drum with stationary permanent magnets inside. More than 99 % of the magnetite is lifted out and returned; the sheared sludge continues to dewatering.
The magnets are permanent and stationary inside the drum. They attract the magnetite, not the surrounding equipment, and consume no power. There is no electromagnet, no field to maintain and nothing to demagnetise.
| Design parameter | Typical range |
|---|---|
| Ballast medium | Magnetite Fe₃O₄, 20–40 µm, SG 5.0–5.2 |
| Ballast concentration in reaction tanks | 2–6 g/L |
| Ballast recovery | > 99 % |
| Ballast make-up consumption | typically < 1 g per m³ treated |
| Reaction time — dosing plus maturation | 4–8 min total |
| Clarifier surface overflow rate | 25–50 m/h (up to ~10× conventional) |
| Solids loading rate | up to ~20× conventional |
| Clarifier underflow recycled | 80–90 % |
| Start-up for wet-weather duty | < 15 min |
| Effluent total phosphorus | < 0.05 mg/L |
| Effluent suspended solids | < 2 mg/L |
| Effluent turbidity | < 1 NTU |
| UV transmittance of effluent | > 75 % |
| Colour, metals, pathogens | substantial reduction without downstream filtration |
Ranges are typical engineering values for this process type and are given for orientation only. The design basis for a specific plant is fixed from raw water characterisation, jar or pilot testing and the agreed effluent guarantee.
Where it fits
Chosen where the limit is very tight, the footprint is fixed, or the existing tankage has to do more work.
- Tertiary polishing to ultra-low phosphorus consents, below 0.1 mg/L and down to 0.05 mg/L
- Chemically enhanced primary treatment, including ahead of an overloaded biological stage
- Wet weather, stormwater and combined sewer overflow treatment
- Capacity uprating of existing circular or rectangular clarifiers without new tankage
- Reverse osmosis and membrane pretreatment where fouling potential must be minimised
- Metals, colour and pathogen reduction ahead of disinfection
- Sites where a filtration stage would otherwise be needed but there is no room for one
- Duties where the sludge is too light for hydrocyclone-based ballast recovery
Because the ballast is recovered magnetically and the clarifier itself can be an ordinary tank, the process is frequently installed into clarifiers a plant already owns. The added equipment — dosing, two reaction tanks, a shear mixer and a recovery drum — is comparatively compact. An existing clarifier can often be uprated several-fold rather than replaced, which is usually the difference between a capital project and a plant extension.
Magnetite grains are roughly a quarter the diameter of microsand and rounder in habit, so the abrasive duty on pumps, valves and pipework is far lower than a sand-ballasted loop. Where a plant has limited maintenance capacity, that difference matters more than the ballast cost.
Common questions
Is magnetite acceptable in drinking water treatment?
Magnetite is a fully oxidised, insoluble and non-toxic iron ore, and it is used in potable applications. For drinking water duty we specify ballast media and wetted components certified to the relevant national standard for contact with drinking water — NSF/ANSI 61 in North America, or the equivalent in your jurisdiction — and the certification is stated in the supply documentation rather than assumed.
How does it compare with microsand ballasting?
Both add density to the floc. Magnetite is about twice as dense, roughly a quarter the grain size, much less abrasive, and recovered magnetically rather than by centrifugal force — which is why it holds up on light tertiary and biological sludges. Microsand is cheaper per tonne, achieves the highest raw hydraulic rates, and has a longer track record on very large drinking water plants. In practice the choice usually turns on the effluent limit and on whether an existing clarifier is being reused. The comparison table sets the two side by side.
Do we still need filters downstream?
For suspended solids and phosphorus, generally not — that is the main economic argument for the process. Whether a filtration stage can be omitted altogether depends on the full consent, including any requirement on dissolved constituents that clarification does not address. We would examine your consent in detail before recommending that a filter be deleted from a scheme.
What happens to magnetite that is not recovered?
It leaves with the sludge. Iron oxide is inert and adds no disposal constraint; at typical make-up rates the contribution to sludge mass is minor. Losses are made up automatically from a small storage silo or bag station.
How quickly can it respond to a storm event?
The unit can be brought online in under fifteen minutes and taken offline again when the event passes, or run as a tertiary polishing stage in dry weather. Because the ballast inventory is retained in the system between events, there is no long re-establishment period.
Working to a tight phosphorus limit?
Send the consent, the current clarifier dimensions and the design flow. We will tell you whether magnetite ballasting can meet it in the tankage you already have.