Aeration and iron removal: column versus pipe
Water from the borehole runs clear out of the tap, and half an hour later there is a rusty sediment at the bottom of the bucket you filled. The filter is in place and working — it simply has nothing to catch: the iron in this water is dissolved, and it passes straight through any granular media. Here is what aeration does about that, and how a pressure column differs from an aeration pipe.
Why a filter does not see dissolved iron
A mechanical filter and granular media catch particles. While the iron stays dissolved there are no particles to catch — only ions, and they travel through the bed along with the water. Straight from the borehole that water looks colourless and clear, so the filter is usually the first thing to be blamed.
The rusty sediment shows up later, once the water has stood in contact with air: oxygen oxidises the iron, it turns into an insoluble form and settles out as flocs. The answer, then, is not denser media. The oxidation has to happen upstream of the filter and under control — not in the storage tank and not in the boiler.
The same goes for manganese. It behaves much like iron — dissolved, invisible at the tap, and showing up later as dark streaks on the fixtures.
What aeration does
Aeration saturates the water with oxygen from the air before it reaches the filters. It does three jobs at once:
- ✓Iron oxidation. Dissolved iron turns into an insoluble form that the filter media can then catch.
- ✓Manganese oxidation. The mechanism is the same, but manganese oxidises more slowly than iron and needs more contact time.
- ✓Stripping of dissolved gases. Hydrogen sulphide, the source of the smell, and carbon dioxide leave the water with the air that is vented off.
The key word here is time. Oxidation is not instant: the water has to travel some distance between the point where air goes in and the filter. That is why aeration is a stage in its own right, not a fitting teed into the pipe with a compressor on it.
The second point is that the excess air has to go somewhere. If an air pocket reaches the filter, it will break up the media bed and throw the valve off. Every aeration system therefore has a component that separates the surplus air and vents it.
Pressure column: a contact chamber and a reserve of time
An aeration column has three parts: a contact chamber, a distribution head with tubes, and an air release valve. The compressor feeds air straight into the pipeline, the air-water mixture enters the contact chamber, it is held there for the contact time, and the water goes on to filtration with practically no loss of the original pressure. That is what sets a pressure system apart from aeration into an open tank, after which the water has to be lifted again by a separate pump.
The air release valve sits at the top of the chamber: it removes excess air and the gases released from the water — the same hydrogen sulphide and carbon dioxide.
You size a column by flow rate and by how long that particular water needs to oxidise. The designation shows both: 0844/AS19/MAC01 is an 8×44″ vessel with an AS19 compressor, 1054/AS19/MAC01 a 10×54″ vessel. Diameter sets the flow, height sets the volume of the contact chamber and with it the holding time. The range runs from 0844 to 1665, and the weights stay easy enough to handle on site — the 1044 weighs 18.9 kg, the 1054 20.3 kg.
If the water contains manganese or hydrogen sulphide, a margin on chamber height matters more than a flow-based calculation makes it look.
Aeration pipe: an aerator inside the line
The AWT aeration pipe tackles the same job differently. Oil-free compressors feed air in ahead of the aerator, governed by a control unit of their own, and one device both saturates the water with oxygen and separates the excess air. No separate contact column is needed.
The size follows the diameter of the pipe and the flow in the line the aerator is teed into: AP 17-20/D90, AP 30-40/D110, AP 50-60/D140, AP 75-100/D160, AP 90-120/D200. The device is horizontal and elongated: the D110 is 1560×430×220 mm and 22 kg, the D140 is 1640×530×250 mm and 30 kg.
| Aeration column | AWT aeration pipe | |
|---|---|---|
| What sits in the system | contact chamber, head, air release valve | aerator combining saturation and air separation |
| Separate contact vessel | the column itself | not required |
| Sized by | flow and the oxidation time required | pipe diameter and line flow |
| Geometry | vertical body, from 8×44″ to 16×65″ | horizontal body, from D90 to D200 |
| Space it takes | room height | length along the line, about 1.5 m for mid sizes |
| Air is supplied by | AS19 compressor included | oil-free compressors with a control unit |
| Pressure in the line | retained | retained |
More often than not the site decides, not the technology. A column takes up height and needs clearance above it for service, but gives you a contact volume that is easy to grasp and easy to scale up. A pipe runs along the line, fits into a low room and does not add one more vertical tank beside the filters.
What goes after aeration
Aeration on its own does not clean the water — it only turns the impurities into a form that can be caught. Next comes a reagent-free filtration unit: a fibreglass vessel with an internal polyethylene lamination, granular filter media, a supporting gravel layer, a drain and distribution system, and an automatic control valve on the outside. Oxidation carries on within the media itself; the oxidised iron, manganese and hydrogen sulphide stay in the bed and are flushed to drain during backwash — no reagents needed.
The filtration unit puts two conditions on the site:
- ✓Water for backwash. Backwash runs at a higher flow rate than the service flow, and that water has to come from somewhere.
- ✓A drain with enough capacity. All of the backwash water goes to the sewer, and the discharge point has to take it.
Industrial vessels from 24 inches up come with the larger Runxin valves — 2 and 3 inch connections, capacity from 10 to 50 m³/h. At domestic flow rates the range starts with 8×17″ vessels and valves such as the F71Q1.
What next
Sizing aeration takes five things: iron and manganese from a laboratory analysis report, whether hydrogen sulphide is present, the peak flow of the site, the working pressure in the line after the pump, and the dimensions of the room — height for a column, or free length for a pipe. Decide early where the backwash water will discharge: without that point there is nowhere to put the filter that follows the aeration.
With those in hand you can turn to the catalogue: pressure aeration columns — when the room has the height and you need a margin of contact time, AWT aeration pipes — when it is simpler to tee the aerator into an existing line. If you have no water analysis, start with one: nobody sets the air dose or the chamber volume from how the water looks and smells.
