Aeration is the stage where water meets air under pressure: oxygen dissolves into the water, while the gases dissolved in it travel the other way and are vented to atmosphere. Aeration does not take iron out of the water — it converts iron into an insoluble form so that the filter downstream can retain the precipitate.
Principle
An oil-free compressor feeds air straight into the pressurised pipework, and the air-water mixture that forms enters a contact volume: in a column that is a vertical vessel, in an aeration pipe it is a section of enlarged diameter cut into the line. There the water is held, the excess air collects at the top and leaves through the air release valve. The line stays at the pressure the borehole pump has already created and needs no second pumping stage.
Two opposite processes run inside. Oxygen dissolves into the water and begins to oxidise ferrous iron, turning it into an insoluble form. At the same time the gases dissolved in the water — hydrogen sulphide and carbon dioxide — pass into the air bubbles and leave with the vent. Saturation and stripping happen in the same vessel, which is why this one stage deals with the smell and with the preparation for the filter at once.
The governing quantity in sizing is not the air supply but the residence time. Oxidation by oxygen is not instantaneous, and its rate depends strongly on pH: the lower the pH, the longer the reaction takes. Residence time is the volume of the contact chamber divided by the flow, so it has to be worked out at the peak draw rather than the daily average: in the morning hour the flow is higher and the water has less time.
Separating the air is not an accessory detail but the second half of the job. Air not separated here travels on down the line: an air pocket collects under the filter valve, the working depth of the bed is reduced, turbidity begins to break through, and at the tap the water turns milky and the pipes knock. The catalogue describes the aeration pipe as combining saturation and separation of excess air in a single device, so that no separate contact column is needed with it.
The two versions in the catalogue solve the same task in different ways. A column is a narrow vertical vessel that stands next to the filter in an ordinary plant room; it is selected by flow and by borehole yield so that the contact chamber holds enough volume for oxidation at the peak draw. A pipe is cut into existing pressurised pipework and is selected to match its diameter — from D90 to D200 in the catalogue; it takes no floor space, but at 15 to 55 kg it needs supports of its own.
Principle
Water looks worse after aeration, and that is the sign the stage is working. While the iron is dissolved the water is clear; as soon as it has been oxidised, turbidity and a brown tint appear — the precipitate has formed but has not yet been removed. The expectation that installing aeration will make the water clearer is wrong in principle: it becomes clear after the filter, and before the filter it is bound to become cloudier.
Aeration changes more than the oxygen content. Dissolved carbon dioxide leaves with the air, and carbonic acid goes with it, so the pH shifts upwards. For the scheme that is sometimes more important than the oxygen itself: manganese only starts to come off on catalytic media at a raised pH. The shift cannot be relied on without an analysis — how large it is depends on the alkalinity of the raw water.
All eight columns in the catalogue carry the same AS19 compressor, although the vessels range from 8×44″ to 16×65″. That shows directly what is being scaled: not the air supply but the volume of the contact chamber. Little oxygen is needed by mass, and what is in short supply at this stage is not air but the time the water spends inside the vessel.
Practice
The smell of hydrogen sulphide is removed physically rather than masked: the gas leaves the water and goes to atmosphere with the vented air. This is the request aeration is asked about most often.
No reagents appear on site. The oxidant here is the air in the room, so the stage adds no solution tanks, no dosing pump and no one to handle chemicals.
The pressure in the line is kept. Unlike aeration into an open tank, a pressurised scheme needs neither an intermediate tank nor a second pump set — and creates no further point at which the water being treated meets the outside environment again.
The stage works for its neighbour: with it, catalytic media receives water carrying dissolved oxygen and reaches its design duty. Without it, and with iron high, filter backwashes become more frequent with no improvement in the result — the same conclusion the filtration page draws from the other side of the boundary.
The units are built from standard parts — a vessel or an in-line section, a compressor, a control unit, an air release valve. We do not manufacture them: our part of the work is the analysis, the residence time calculated at peak flow, the choice between pipe and column, tying the stage into the filter scheme, supply, commissioning and service.
Limits
Practice
The cases are typical examples, not site reports.
Scheme
Aeration is never a stage on its own. What it produces is a precipitate suspended in the stream, and the filter behind it has to retain that. The boundary is worth stating plainly: aeration does not remove iron, it prepares the water so that the next filter can. Installing it alone means making the water worse at the tap.
A coarse strainer goes ahead of the aeration. Sand and scale from the borehole wear the pipework and find their way into the filter valve, and that is not noticed at once.
The distance between the aeration and the filter is part of the calculation. Oxidation does not stop at the outlet of the vessel: the water goes on oxidising in the pipework, so a filter set right beside the column and a filter at the end of a long run work under different conditions.
The air vent is designed as a separate item. With sulphide-bearing water the vented air smells, so the discharge point is taken outdoors rather than left in the plant room; a little water leaves with the air, which is why a drain connection is provided under the valve.
Scheme
Operation
Energy
The only consumer at this stage is the compressor; there are no pumps and no heating of its own, and the pressure it works on is the pressure the borehole pump has already created. Specific consumption is the compressor running time referred to the volume treated, and with a flow-triggered start it is the lower the steadier the draw. The rating depends on the compressor version and is taken from its own data sheet: it cannot be derived from the vessel size — in the catalogue all eight columns carry the same compressor.
Consumables
What goes to drain
Aeration is the one stage in the scheme whose waste is a gas: what is removed goes to atmosphere with the vented air. It produces no sludge and no backwash water at all — the precipitate formed by oxidation stays in the stream and is retained on the filter, which is where the backwash water arises. There is a single practical consequence: with sulphide-bearing water the vented air smells, and the discharge point is taken outdoors. A small amount of water leaves the valve with the air, so a drain connection is provided beneath it rather than an open branch.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
No. It changes the form of the iron, not the amount: dissolved ferrous iron is oxidised and turns into a precipitate, but the precipitate stays in the same water. It is removed at the next stage, by a filter with granular media. Aeration with nothing behind it therefore does not solve the problem at all and leaves the water cloudy; as a pair the two stages close it completely.
Because the stage is working. While the iron is dissolved the water is clear and there is nothing in it to retain; once it has been oxidised there are particles, and with them turbidity and a brown tint. This is an intermediate state and it is supposed to end at the filter. If the cloudiness reaches the tap, the question is not about the aeration but about what follows it: the filter area, the filtration rate or the backwash routine.
They do the same job: give the water oxygen, hold it, and separate the excess air. A column is a narrow vertical vessel that stands beside the filter and is selected by flow and borehole yield. A pipe is cut into the pressurised line and selected to match its diameter, combining contact and air separation in a single device, so no separate column is needed with it. The choice is settled by the space in the plant room, the diameter of the line and the residence time required, not by price.
Hydrogen sulphide leaves the water by two routes at once: part of it is stripped into the air and vented, part is oxidised to finely divided sulphur which the filter retains. How completely depends on the concentration in the raw water, on the pH and on the residence time, so the figure comes from an analysis rather than from a general rule. One practical condition goes with it: the vented air smells, and if its discharge point is inside the plant room the smell will come back into the house with the equipment in perfect order.
Because the excess air is not being separated. There are usually three reasons: the air release valve is stuck or was never fitted, the compressor runs regardless of the draw and fills the vessel with air in advance, or the contact volume is too small for the flow actually taken. The consequences are not confined to the tap: the same air collects under the filter valve, reduces the working depth of the bed and leads to turbidity breaking through.
There are various ways of getting oxygen into water, and some of them need no compressor, but they all involve either contact with the atmosphere in an open tank or a loss of pressure. The catalogue holds pressurised versions precisely because they keep the pressure the borehole pump has created: no intermediate tank, no second pump set and no space for either. The air is supplied by an oil-free compressor — oil must not reach the water.
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