Understanding iron chemistry helps us understand chemical well clogging and iron removal in drinking water preparation

KWR researcher Kees van Beek studied iron chemistry after he retired because iron chemistry is not only relevant for the occurrence of chemical well clogging but also for the removal of iron in the preparation of drinking water. Three processes are distinguished in iron removal: homogeneous, heterogeneous and biological iron oxidation. When groundwater has a higher pH, as at the dune waterworks, homogenous oxidation predominates. Homogenous oxidation is a slow process and therefore leads to blockage, particularly in field lines, conveying the abstracted groundwater from the well to the treatment plant. At lower pH levels, as in most other groundwater abstractions, heterogeneous oxidation and biological oxidation are relevant, resulting in serious chemical and biological well clogging. Van Beek’s research not only provided an explanation for the observed well clogging under various conditions but also applies to a vital process in the preparation of drinking water, removal of iron from groundwater – also an iron oxidation process. Whereas in well clogging conditions are fixed, in iron removal during drinking water treatment, conditions may be optimised. Van Beek: “I got a lot of help from technologists from the drinking water sector and from KWR. By comparing their experiences with iron removal during drinking water preparation with the occurrence of chemical well clogging and comparing these experiences with the chemistry of the oxidation of iron by oxygen, it was possible to get a better understanding of the iron removal process. This resulted, for example, in the suggestion to remove iron in such a way that the quality of the produced iron sludge better suits further use as raw material.

Kees van Beek: “Chemical well clogging and iron removal in drinking water preparation are identical processes.”

Kees van Beek officially retired in 2006, after 33 years of applied research at KWR. He was a specialist in well clogging and, among other things, made a significant contribution to the prevention of mechanical well-clogging by switching wells on and off regularly. This was, amongst others, discussed at the 2017 Well Management symposium. Switching wells regularly on and off prevents accumulation of small particles on the borehole wall. He also wanted to get a better understanding of the causes of chemical well-clogging, another phenomenon that causes problems in many well fields. That is why, after his PhD thesis in 2010, he studied the chemistry of iron underlying chemical well clogging and iron removal. For this research, he could retain his position at KWR and take advantage of the KWR facilities, much to our mutual delight. But now it has been enough: it is time for Van Beek to retire for real. He would like to share his findings from this last period with the drinking water sector. The six papers in which he summarised his findings over the last few years have been put together and combined with this summarising article on the KWR website,. Van Beek’s “digging” into the iron chemistry has contributed significantly to the explanation of experiences and observations of various colleagues at KWR and at drinking water utilities.

“Van Beek: “Comparing experiences and observations with chemical well clogging and with iron removal by aeration and precipitation provides useful information for both applications.”

3 types of iron oxidation

Kees van Beek: “Three processes may be distinguished in the oxidation of iron(II) by oxygen. Firstly, homogenous oxidation occurs when ferrous water is aerated or when ferrous water is mixed with oxygenated water. A great deal is known about this. Homogenous oxidation produces flocs offerric hydroxide. Then there is heterogenous oxidation: oxidation of iron(II) after adsorption on the surface of previously precipitated iron(III) hydroxides. Less is known about this process. Then there is biological oxidation, about which we know the least. The iron-oxidising Gallionella bacteria thrive under conditions where iron(II)-containing water and oxygen-containing water are mixing, deriving their energy from this mixing process. In order to use the released energy to their advantage, Gallionella bacteria possess excellent adhesive properties: they “survive” flow in well screens and backwashing of sand filters. As soon as both water flows or types are mixed, growth conditions for Gallionella bacteria are over, and only homogeneous oxidation will occur.”

Explaining removal of iron from groundwater and chemical well clogging

Oxidation of iron(II) to iron(III) plays a role in several parts/steps in the public drinking water supply. As mentioned above, chemical well clogging is caused by iron oxidation. Understanding iron chemistry helps to explain under which conditions and at which locations well clogging will or will not occur. At the same time, iron chemistry provides a better understanding of the removal of dissolved iron in the abstracted groundwater and eventually into more optimal methods for iron removal. “The processes relevant in well clogging and in iron removal are essentially the same,” says Van Beek.

Groundwater wells regularly become clogged by the accumulation of deposits precipitated by the oxidation of iron from the groundwater.

PH-dependent well clogging

Chemical well clogging is a nuisance phenomenon, and there are hardly any possibilities for adjusting underground conditions. Understanding the conditions resulting in clogging is already a step forward. Van Beek: “In groundwater containing iron as well as oxygen, with a high pH – say, above 7.5 – homogeneous oxidation dominates. This high pH is prevalent at water utilities abstracting dune water and some outside well fields. Because the iron concentrations in the abstracted groundwater are low, and the homogenous oxidation reaction is relatively slow, there is hardly any clogging of the screen slots. Moreover, because the iron (II) concentrations are low, Gallionella bacteria probably cannot obtain enough energy for abundant growth. Observations on these well fields show that iron hydroxide flocs develop after the water has been underway for some time in the field lines. This continuous precipitation of iron(III) hydroxides may result, despite the low concentration of iron(II), in severe clogging of these field lines.”

Conditions are different at lower pH. “At a pH below 7.5, as in most well fields, heterogeneous oxidation and biological oxidation are dominant. In wells that abstract (separate) flows of oxygen-rich and of iron-rich water, conditions are ideal for Gallionella bacteria. To get the maximum energy from this mixing, they will move “upstream” to the slots of the well screen. Thanks to their excellent adhesive properties, they are able to maintain themselves there, resulting in severe chemical and biological clogging of the well screen. As soon as both water flows are completely mixed – after the (submersible) pump – the role of Gallionella bacteria is over. In the field /lines, clogging will be  negligible because, despite higher iron(II) concentrations, heterogeneous and biological oxidation will be (almost) absent.”

Iron removal: controlling through conditions

The same processes as in well clogging are active in the above-ground removal of iron(II) from groundwater by aeration and filtration, as in the preparation of drinking water. The best results are achieved by designing the process conditions in such a way that these conditions are optimal to one of the oxidation reactions, i.e. homogenous oxidation “only in the supernatant water” or heterogeneous oxidation “only in the (sand) filter”. The role of biological oxidation of iron(II) in this “classic” treatment is poorly known.

An already well-known method that is currently attracting renewed attention is removing iron(II) from groundwater through underground iron removal. Van Beek: “This involves infiltrating oxygen-rich water into the aquifer, after which a larger volume of iron-free groundwater can be abstracted discontinuously. The underlying process in this method is heterogeneous oxidation, in which the adsorption of iron on the surface of the soil particles determines the efficiency of this method.

Can the quality of iron sludge be controlled?

Different types of oxidation also result in different kinds of sludge with different physical properties. These properties may determine further possibilities for use as raw material, which means that the selection of the oxidation process may significantly influence further meaningful use of the sludge. “There may still be opportunities here,” Van Beek supposes. “

The results of research into underground iron removal are potentially also relevant for application in ‘classic’ iron removal. In the optimal application range of heterogeneous oxidation, it should be possible to remove iron in a fluidised bed, analogous to softening in pellet reactors. Iron removal in pellet reactors has many advantages: no backwashing, no iron flocs passing the filter during start-up after backwashing, high hydraulic load, residue with a high dry matter content and possibly a higher market value, et cetera. By varying operation, for example, between the continuous or discontinuous supply of oxygen, it may also be possible to vary the quality of the iron pellets between completely inorganic and inorganic/organic pellets and thus to produce a requested quality of iron pellets.”

Siderite determines concentration of iron(II) in groundwater

An interesting “by-catch” of Van Beek’s research was the observation that the concentration of iron(II) in Dutch groundwater is determined by the mineral siderite (FeCO₃). This mineral is formed in the subsurface as soon as groundwater containing iron(II) becomes supersaturated with respect to siderite. Iron(II) is released by various soil chemical processes, among others, by the reduction of iron hydroxides (iron skins on sand). The conversion of these iron skins into siderite precipitates also determines the concentration of trace elements in groundwater, such as arsenic, cobalt, nickel and zinc.