Reagent-free filtration is the stage where dissolved iron, manganese and hydrogen sulphide are converted into an insoluble form and retained within a bed of granular media. No oxidant is dosed from outside: the oxygen already dissolved in the water does that work, and the filter media acts as the catalyst.
Principle
The unit is a fibreglass vessel with an inner polyethylene lining, filled with granular filter media; beneath it lie a supporting gravel layer and a drainage and distribution system, and a control valve sits on top. In service the water flows downwards, passes through the bed and leaves through the lower drain. There are no moving parts inside the vessel: the only mechanism in the unit is the valve that switches the flows.
The stage has two jobs at once, and that is what sets it apart from an ordinary mechanical filter. While iron is in its dissolved ferrous form there are no particles in the water at all — there are ions, and they pass through any granular bed without resistance. The substance therefore has to be oxidised first and retained second. Catalytic media does both: oxidation takes place on its surface, and the insoluble precipitate that forms stays within the bed.
The oxidant is the oxygen dissolved in the water itself, and the rate of oxidation depends on how much of it there is and on the pH. That sets a direct limit of applicability: where iron is high or oxygen is short — which is exactly what deep borehole water looks like — the dissolved oxygen is not enough, and aeration is installed ahead of the filter. In such a scheme it is not an improvement but a condition of the filter working at all.
The accumulated precipitate is removed by backwashing: the valve sends the flow upwards, the bed expands, the grains are set in motion and what was retained is carried to drain. The backwash flow rate is set by the cross-sectional area of the vessel, not by the capacity of the unit, and is markedly higher than the service flow. This is a requirement on the source rather than on the equipment, and it is the one most often left out of the calculation.
The control valve determines who starts the backwash and when. An automatic valve counts by timer, or by actual throughput if it has a water meter; a manual one is switched by the operator. The valve type has no direct bearing on filtrate quality — it bears on whether the backwash happens on time. A manual version needs no power supply and costs less, but it needs a routine and someone who keeps to it.
Principle
Clear borehole water that turns brown in a bucket within half an hour is a sign not of a poor filter but of oxidation happening downstream of it. While the iron is dissolved there is nothing to retain: filter media works on particles, and the particles do not yet exist. The precipitate forms in the bucket, in the tank or in the pipework — wherever the water has time to take up oxygen. This is why an ordinary mechanical filter does not help here at all, however fine its rating is made.
Iron and manganese need different conditions even though they are removed at the same stage. Iron is oxidised by oxygen at a pH of about 7 already, manganese needs noticeably more, and in practice its removal starts working from a pH of around 8. Hence a common outcome that looks like a fault: the iron has gone, the manganese has stayed. The media is not to blame, an uncorrected pH is, and the cure is alkali dosing ahead of the filter, not a larger unit.
The capacity of the unit is set by the cross-sectional area of the vessel, not by the figure on the valve. The valve caps the flow from above, but the governing quantity is the filtration velocity, that is, flow divided by area. Exceed it and the water passes the bed faster than it can give up its precipitate, and turbidity breaks through into the filtrate. For that reason the «valve capacity» in the data is an upper limit rather than a working point, and a size cannot be chosen on it alone.
Practice
The stage closes the most common set of borehole water deviations in one go: iron, manganese, hydrogen sulphide, turbidity and the colour that goes with iron. A separate device for each parameter is not needed — at moderate concentrations a single bed takes all of them.
No reagents are dosed and none are stored on site. What remains among the running costs is backwash water, the electricity for the valve and periodic replacement of the media; there is no salt handling here of the kind ion exchange requires.
The unit does not change the salinity of the water. Downstream it stays as hard and as mineralised as it arrived: filtration solves the problem of turbidity and oxidisable impurities, but not the problem of salt composition.
The stage protects everything installed after it. Ion exchange resin, membrane elements and a UV chamber fail from iron and suspended solids sooner than they wear out, and the cost of restoring them bears no comparison to the cost of filter media.
The unit is assembled from standard parts — vessel, media, Runxin valve — and supplied in a version matched to a particular flow and connection. We do not manufacture it: our part of the work is the analysis, the calculation of filtration velocity and bed volume, the choice of vessel and valve, supply, commissioning and subsequent service.
Limits
Practice
The cases are typical examples, not site reports.
Scheme
Reagent-free filtration is rarely a stage on its own. A coarse strainer goes ahead of it — it takes out the sand and scale that damage the control valve — and the rest of the pretreatment is set by the analysis: aeration where oxygen is short or iron is high, pH correction where manganese is present.
Downstream of the filter come the stages it protects: softening, reverse osmosis, UV. The order here cannot be rearranged — ion exchange resin and membrane elements take the iron and suspended solids on themselves and fail early, and a UV lamp loses its effect to turbidity, which absorbs the radiation.
The backwash line is treated as part of the scheme, not as an auxiliary drain. Its flow is calculated, pipework of a matching diameter is provided and a discharge point is chosen; backwash water carries the retained precipitate and is not returned to a recirculating system without settling.
On sites with continuous draw-off a single unit stops the supply while it backwashes. This is decoupled either by a storage tank downstream or by two vessels working alternately; the choice between them is a matter of flow, space and how acceptable an interruption is, and it is settled at the scheme stage.
Scheme
Operation
Energy
The unit consumes little of its own: electricity goes only to the control valve drive, which runs for minutes a day, and a manual version needs no power at all. The significant running item here is a different one — backwash water. Depending on the size and the frequency of backwashes it amounts to anything from a few per cent to a tenth of the volume passed, and on sites with a limited source yield it is this consumption that decides the scheme.
Consumables
What goes to drain
What is removed leaves with the backwash water: iron and manganese hydroxide sludge, retained suspended solids and some attrited media. Backwash water is turbid, coloured and discharged in a surge — over a short time a volume many times the service flow goes to drain. Where the discharge goes to a septic tank or a drainage field this is accounted for separately: a surge stirs up the sludge in the septic tank and carries it further down the scheme. The sludge is not toxic, but its volume and turbidity call either for a discharge point of its own or for a receiving chamber with settling.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
Because iron in borehole water is dissolved, not suspended. In that state it passes any granular bed and any cartridge: the particles that could be retained do not yet exist. The precipitate forms later, when the water takes up oxygen — in the tank, in the bucket, in the pipework. The purpose of the stage is precisely to make the oxidation happen inside the filter rather than after it, and that is achieved with catalytic media, with aeration ahead of it where required.
None to the quality of the filtrate, as long as the backwash happens on time. The difference is in who is responsible for that. An automatic valve counts by itself, by timer or by water meter, and does not depend on anyone being present; a manual one costs less and works without power, but a missed backwash damages the bed irreversibly — a blinded layer is not restored to its original state even by several backwashes in a row. A manual version makes sense where the flow is predictable and servicing follows a routine.
Catalytic beds last years, and the life is decided not by the calendar but by the working conditions: grain attrition during backwashes, the actual iron and manganese load, how complete the backwashes are. The approach of the limit shows in advance — backwashes become more frequent, the pressure drop rises, and filtrate quality at the same regime starts to drift. Replacement is planned on those signs rather than waited for through worse water at the tap.
No. Calcium and magnesium pass the bed unchanged, and the scale after filtration is the same as before. Hardness is removed by ion exchange or by reverse osmosis, and that is a separate stage installed after the filter rather than instead of it. Substituting one for the other is a common reason for water that has become clear while the kettle still furs up.
The backwash flow is set by the cross-sectional area of the vessel and comes to several times the service flow of the unit; a backwash lasts from ten minutes to half an hour. Against the volume passed that works out at anything from a few per cent to a tenth — the figure depends on the size and on how often the bed has to be washed with the water in question. The requirement falls on the source: the borehole has to deliver that flow, otherwise the backwash will be incomplete.
It can, if the oxygen dissolved in the water is enough to oxidise all the iron and manganese and their content is moderate. That is settled by an analysis, not by a general rule: deep borehole water usually has almost no oxygen at all. Where there is not enough, the catalytic media works below its capability, some of the iron passes through and precipitates beyond the filter, and backwashes grow more frequent with no improvement in the result.
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