In a world in which phosphates, needed for agricultural fertilisers, are increasingly recognised as the finite resource they are, there is growing interest in promoting a circular economy approach and looking for ways to recycle this essential nutrient.

Municipal sewage can be a significant source of phosphates, and there is a growing number of full scale phosphate recovery installations. This is particularly so for processes that recover the phosphate in the form of magnesium ammonium phosphate, or struvite. But with attention focused on struvite recovery as a clear current viable option, is research in other areas being neglected? Leon Korving is a wastewater expert at Dutch research organisation Wetsus. His answer to this question: yes.

Many sewage treatment plants are already set up to remove phosphates from effluent as part of efforts to reduce the pollution of surface waters. The phosphates are instead retained in the sludge residuals. One option for carrying out this removal is to exploit the potential for bacteria to accumulate phosphate in so-called enhanced biological phosphorus removal (EBPR) treatment plants.

‘A lot of research is being done over and over again. What we really need is some new, original research angles.’

It is these plants that present the main opportunity for struvite recovery, as Korving explains. Under the conditions created in such plants, phosphate is accumulated in the bacterial cells as polyphosphate and is then released as orthophosphate during processing of the sludge. ‘You get orthophosphate in quite high concentrations. That gives you the possibility to precipitate it as struvite and then recover it,’ he comments.

Wastewater treatment plant operators have a range of options for dealing with their sludge, which include applying it to agricultural land, providing a route back to agriculture for phosphates. This approach faces practical limits in terms of the distance from treatment plant to application site that is viable, and seasonal demand poses issues, making conversion into a recovered product more attractive, Koving notes. ‘There are some drawbacks there, so it would be preferable to have a phosphorus recovery product like struvite that has a higher concentration,’ he says.

Struvite limitations

Korving explains that far from all treatment plants use the EBPR needed to create the high concentrations of recoverable orthophosphate. Instead, most plants that remove phosphates do so using a chemical approach, especially precipitation using iron salts.

Accurate data about EBPR use is currently lacking for most countries, says Korving, but the situation in Germany, the United Kingdom and the Netherlands illustrates the point. Data from the UK dating from 2010 put EBPR as accounting for only 4% of phosphorus removal installations, the remainder using a chemical approach. Data from 2003 from Germany put EBRP at less than 10% of phosphorus removal, with around one third of plants using a combination of biological and chemical removal, with most plants using only chemical removal. Biological removal is higher in the Netherlands. ‘We use EBPR a lot, which is the reason you see a lot of struvite plants being built in the Netherlands,’ comments Korving.

This suggests that plants using chemical removal should be brought into play if phosphate recovery is to be maximised across the sector. ‘When you realise that struvite recovery is only applicable to 10-15% of the sewage treatment plants, you would say that we are overlooking quite a big potential for phosphorus recovery in sewage treatment plants,’ says Korving. ‘Even if you recover struvite, you only get about 20-30% of the influent phosphorus as struvite,’ he adds.

One problem though is that the binding of phosphate with iron during precipitation means it is not available for struvite recovery. The link between iron and phosphate is therefore key. ‘Iron plays an important role in sewage treatment. Already a lot of iron is being used for phosphorus removal, and in some countries it is the main means of phosphorus removal, so you cannot apply struvite recovery and need to find a way to recover phosphorus from these plants,’ Korving adds.

One option could be for more plants to switch to biological phosphorus removal, but Korving points out that there other factors driving things in the opposite direction. Another trend in the water sector to consider, also part of circular economy thinking, is the increasing interest in creating energy neutral or energy positive wastewater treatment plants. According to Korving, this interest means there is the prospect for greater use of chemical methods to remove phosphates since such plants are geared to maximising conversion of incoming carbon into the sludge stream. ‘In an energy positive plant, you want to transform your soluble COD in your sewage into sludge,’ says Korving. ‘In this energy factory process, you cannot apply EBPR, so your only alternative is to dose chemicals to remove the phosphorus. Iron-phosphorus chemistry is important in sewage treatment, and may even be more important in the future if we go to these energy producing plants.’

Iron – the missing link in wastewater research

Another point Korving makes is that there is often quite a lot of iron already in the incoming sewage flow at a treatment plant, which binds to phosphate and means this portion is unavailable even in an EBPR plant. ‘If you dose no iron, there is probably already a lot of iron in the sewage treatment plants for other reasons,’ he says.

Yet despite this importance, iron-phosphate chemistry is a neglected area of research, according to Korving. ‘Nobody has really looked at how iron and phosphate are present in sewage sludge,’ he says. Iron chemistry is diverse, so some basic research is needed before any practical solutions can be identified. ‘You need to understand the chemistry, and if you understand that chemistry then you can go to the next step of devising methods to recover the phosphorus using this diverse chemisty,’ says Korving.

‘Iron-phosphorus chemistry is important in sewage treatment, and may even be more important in the future if we go to energy producing plants.’

This suggests a need for a shift from a current overemphasis on struvite in research. ‘A lot of research is being done over and over again. You see that particularly for struvite recovery. What we really need is some new, original research angles,’ he says, adding: ‘A lot of research groups are publishing about struvite and struvite precipitation. If they would do a little bit less on that and focus more on this gap for phosphorus removal, I think that would be a good way forward.’

As well as iron-phosphorus chemistry, there are other research opportunities for progress with phosphates in sewage treatment, including looking at the possibilities for recovering the organically bound phosphorus in sewage treatment plants and also at the potential for the direct recovery of phosphorus using approaches such as adsorption. Having said that, Korving notes: ‘What we are talking about here is how, in a relatively short perspective, to introduce phosphorus recovery to existing sewage treatment plants or to sewage treatment plants of the near future.’ A real transformation to a circular economy will need more radical changes. This includes, for example, the recovery of urine as a nitrogen and phosphorus source, which could potentially see application as a means of avoiding the need to expand overloaded treatment plants. ‘When you really start rethinking the whole process, you start also thinking towards more decentralised approaches,’ concludes Korving.

More information

'The relevance of phosphorus and iron chemistry to the recovery of phosphorus from wastewater: a review.' P Wilfert, P Suresh Kumar, L Korving, G-J Witkamp and MCM van Loosdrecht. Environ. Sci. Technol., 2015, 49 (16), pp 9400-9414.