Vitens extracts groundwater containing methane and iron at its Hammerflier production site. The removal of methane and iron by conventional means yields a ten percent backwashing loss, while the methane is released into the atmosphere. Thanks to the application an innovative new technology, however, Vitens can now significantly reduce both the backwashing loss and the CO₂ footprint of the purification process. It offers the added advantage of improved efficiency, which implies that a smaller system now suffices.
“To produce clean and safe drinking water, we apply sand filtration to remove numerous substances from the groundwater, such as iron, manganese and ammonium,” process technologist Abel Heinsbroek explains. “Before we can do that, we have to remove the dissolved methane. After all, one of the side-effects of Methane is that it disrupts the operation of sand filters. Obligate aerobic bacteria grow very fast on methane, ultimately clogging the filter. Furthermore, methane can cause unwanted bacterial growth in the remainder of the purification system and the distribution network.
Plate aerators
“The conventional method of methane removal is aeration using plate aerators, whereby the methane is expelled from the water as it were. This works highly favourably, achieving around 99% removal. There are nevertheless several drawbacks to the method. For example, some fifty cubic metres of air are required per cubic metre of water, which involves considerable energy consumption. Moreover, the methane – which is a highly potent greenhouse gas – becomes greatly diluted by this large quantity of air. The methane therefore cannot be captured and is released into the atmosphere with the ensuing harmful consequences.”
“This method of methane removal is also disadvantageous for the subsequent stages of the purification process. The intensive aeration creates an overabundance of oxygen in the water, which implies that components including the plate aerators soon become soiled with iron and bacteria deposits and therefore have to be frequently cleaned. Furthermore, the air removes not only methane, but also a large proportion of the CO₂ dissolved in the water. This raises the pH level, which has a detrimental effect on the deferrisation process. The high oxygen concentration and pH cause the iron to react with the oxygen too quickly, thus clogging the filters and obliging us to backwash more often.”
Deferrisation
“Our treatment plants apply two different mechanisms for the deferrisation of water,” says fellow process technologist Frank Schoonenberg. “We generally use fast sand filtration combined with aeration. We oxygenate the water, which causes the iron to oxidise and precipitate in the form of flakes. This is known as flocculent deferrisation. The iron flakes become trapped in the filter material, which gradually clogs the filter. Once the permeability becomes too low, we backwash the sand filter from beneath and drain the water containing the iron sludge.”
“One of the benefits of adsorptive deferrisation is that we need to backwash far less frequently, which implies that the water loss is much smaller.”
“Adsorptive organic deferrisation is the other mechanism. This technique also uses rapid sand filtration and works well with groundwater with a pH of 7 or lower. The secret of this mechanism is that it limits aeration. As a result, the iron bonds to the grains in the sand filter forming a thin layer, thus sharply reducing the amount of iron sludge. What are known as iron-oxidising bacteria also play a role in this deferrisation mechanism. One of the benefits of this method of deferrisation is that we need to backwash far less frequently, which implies that the water loss is much smaller.” Another advantage is that the groundwater can flow through the sand filter twice as fast as it would with the conventional approach that involves more intensive aeration.”
Membrane degasification
Abel adds: “The groundwater that we extract at Hammerflier is suitable for adsorptive deferrisation in terms of its pH. In removing methane using plate aerators, however, we also expel some of the dissolved CO₂ from the water. This alters the pH level to such an extent that both deferrisation mechanisms occur simultaneously, so that neither proceeds optimally nor efficiently. The result is that we incur substantial backwashing losses and have to replace the filter material annually.”
“In view of all these drawbacks, we began investigating the possibility of removing the methane by some other means. After careful consideration, we came up with vacuum degasification by means of hollow fibre membranes, using a small quantity of nitrogen as the ‘stripping gas’. In addition, we dose a minute quantity of CO₂ to ensure that the acidity remains stable. This method also enables us to remove 99% of the methane, while consuming approximately two thousand times less gas than with plate aeration.”
“This alternative method of methane removal enables us to adequately capture it for reuse”
“This alternative method of methane removal offers all sorts of advantages. For instance, we can now adequately capture and reuse the methane, as it is hardly diluted. This reduces the CO₂ footprint of the treatment process considerably. Furthermore, as we can now remove the iron using the adsorptive organic mechanism, we hardly need to backwash the sand filter. And because adsorptive organic deferrisation is far more efficient than flocculent deferrisation with plate aeration, we can purify the same amount of water using only half the surface area.”
“The backwashing loss has been reduced from around 10% to 2.5%”
Highly stable
“We built a demonstration plant at Hammerflier to carry out trials with this alternative method, which has already purified over 400 thousand cubic metres of water. The process remains highly stable, while the backwashing loss has been reduced from around 10% to 2.5%. We have nevertheless noticed that the membranes are gradually becoming fouled. We therefore predict that we will have to clean them chemically every six months. We will continue to run the plant during the period ahead, to establish how it performs in the longer term. Given the favourable results, we have commenced with the construction of a full-scale plant on Terschelling.”
Acidity of water
The pH level of water indicates how acidic it is. Water with a pH of exactly 7 is known as neutral. Water with a lower pH (below 7) is acidic, and that with a higher pH (above 7) is alkaline.
World first
The removal of methane from groundwater using membrane degasification, as carried out at Vitens’ Hammerflier production site, is a world first. The membranes that Vitens uses for this purpose are nevertheless utilised in other industries. The food industry already uses membrane degassers to remove oxygen from soft drinks and wine, while the printer industry applies the method to produce ink devoid of gas bubbles.
Inspired?
For further information or a partnership, please contact us through abel.heinsbroek@vitens.nl or frank.schoonenberg@vitens.nl
