Pressure from worldwide population growth is placing increasing demand on phosphorus supply and production but resources are not limitless and no synthetic substitute exists. Nick Michell reveals the newest technologies being used to extract phosphorus from wastewater and asks whether the process is commercially viable
rimarily produced by mining, phosphorus, a component of DNA, is an essential nutrient for the development of every living being. It is an important ingredient in the fertilisers used in agriculture, and is found in the wastewater generated by human activities.
Controlling phosphorous discharged from municipal and industrial wastewater treatment plants is a key factor in preventing eutrophication of surface waters and is the most efficient way to avoid uncontrolled algae bloom. Its presence causes many water quality problems including increased purification costs, decreased recreational and conservation value of impoundments, loss of livestock and the possible damaging effect of algal toxins on drinking water.
“Not recovering phosphorus in wastewater treatment plants allows the phosphorus to reach water bodies, causing eutrophication, and to be lost in the sea, where it is not recoverable,” says Mathieu Delahaye, Innovation Manager, Treatment Infrastructure, Suez Internatonal. “The European Commission has added phosphate rock to the list of 20 Critical Raw Materials, for which supply security is at risk and economic importance is high. Therefore we should recover as much phosphorus as we can.”
Up until a few years ago, priority was given to solving recurrent and uncontrolled clogging due to struvite in wastewater treatment plant pipes. Now, with an increased awareness about the circular economy, struvite recovery processes have been designed and installed, operating at full scale.
Veolia Water Technologies has developed Struvia, a solution for recovery of phosphorus from effluents produced by various industrial and agricultural activities, or from municipal waste facilities, as struvite crystals. This opens the way to local reuse of phosphorus especially in agriculture.
“In the past, closing the loop of the phosphorus cycle was done by landspreading dewatered sludge from wastewater treatment plants for agricultural purposes,” says Ludovic Renoux, Market Manager, Veolia Water Technologies. “Efficiency varied a lot, depending on the soil’s phosphorus demand, and the way this process was managed by farmers. We felt that we needed a more robust, simple and cost-effective process to better address the industrial market segment as well as the smaller municipal installations. Struvia was born from several years of experience in phosphorus recovery from wastewater.”
With Europe having a dependency on imported phosphorus, and natural reserves not being limitless, some countries, like Sweden, Switzerland, the Netherlands and Denmark, are moving towards applying new regulation concerning the recycling of this valuable fertiliser.
“All technologies that allow resource recovery are highly appreciated,” states Willy Verstraete, Head of the Laboratory of Microbial Economy and Technology, Ghent University. “They open the door to integrating the concept of a cyclic economy into the domain of water treatment, although I have a feeling that the phosphorus issue may have been hyped somewhat. The figures on the reserves vary considerably but of course they will not last forever. However, if one compares the emphasis on phosphorus recovery with that of the better use of phytates (saturated cyclic acid) one wonders. Indeed, both in our gut and in the soil, phytates are not used very efficiently. We should address that issue in parallel with that of recovery of phosphorus from used water.”
Effluent containing high concentrations of phosphorus, for example centrates generated from digested sludge dewatering, are fed to a continuous stirred tank reactor where rapid mixing is achieved using a special mixing system: Turbomix. Struvite precipitation is initiated by increasing the pH and by the addition of a magnesium salt.
An integrated lamella settler ensures the separation of the produced struvite prills and the treated effluent. Typically in municipal applications the treated effluent is returned to the head of the treatment works.
For industrial applications Struvia is applied on the waterline. The effluent can be discharged directly or further treated if required. The struvite prills are then pumped and directed to a draining and storage facility before they are sent to the packaging unit.
Phosphorus removal usually requires the addition of ferric or aluminum salts at the wastewater treatment plant, which is costly and has a negative carbon footprint. It is important to recover phosphorus in wastewater treatment plants as it reduces maintenance costs by preventing uncontrolled struvite scaling in pipes, equipment and digesters. Although struvite is a plague to plant operators, it is also an excellent slow dissolving fertiliser.
The quantity of phosphate used as fertiliser is expected to reach 52.9 million tonnes by 2030, and the United Nations predicts an increase of 70 percent by 2050. Despite its importance, decreasing resources and the fact that 20 percent of the current world demand of phosphorus could be covered by reclamation from wastewater, phosphorus recovery has not been implemented on a large scale in wastewater treatment plants in the past.
“Nowadays the limitation is very much linked to the regulation, as recovered struvite is not approved for sale in all countries,” explains Mathieu Delahaye. “For example, it is an approved fertiliser in Denmark and the Netherlands but not yet in France or Spain. The regulation in Europe might change soon to widen the use of struvite. Suez is actively involved in work groups at EU level to make regulation change to reach consistent policies among European states.”
Deposits of phosphorus are unequally distributed throughout the world and are sources of important geopolitical issues, through market volatility and the upward trend of phosphate prices. Phosphate ore prices have increased significantly over the last 10 years, and are now three times more expensive than in 2006.
Although a significant number of laws and regulations apply, which complicates the phosphorus recycling projects, Mark van Loosdrecht, Professor at Delft University of Technology, believes this can too easily be used as an excuse for the slow uptake of the recovered product.
“Usually the recovered material has a phosphorus content, but has no good product specifications,” he states. “It is like harvesting sugar beets and claiming you have produced sugar. There are steps in between a raw resource (recovered phosphorus at a wastewater treatment plant) and a product. I do not think that legislation is the true limitation. In my view this is too easily used as a scapegoat. The volumes are simply small and the quality poor.”
The circular economy within the water sector is expanding, but at a slow pace. There are several reasons for this, one being that the price of the converted material is often too high in comparison with the raw material. Another reason is that industrial companies can be reluctant to use recycled materials, which they regard as second choice. To overcome their reservations, they have to be given the same guarantees of quality, continuity and regularity of supply, as for virgin materials.
“Resource recovery from wastewater will not just be down to water firms: it is about innovation, but also cooperation,” says Ludovic Renoux. “Innovation, to allow us to put more used materials back into the production chain, and cooperation, because simply converting waste is not enough; it is essential to also develop relations with customers willing to buy it in the form of secondary raw materials.”
Is there a market?
A Veolia Struvia prototype unit was successfully implemented for the first time at the Brussels-North wastewater treatment plant in 2013 and 2014, as part of the P-Rex programme, a European initiative for the reuse of phosphorus from wastewater. It has reached phosphorus removal efficiency levels higher than 85 percent when treating centrate generated from digested sludge dewatering.
The technology has been developed to facilitate the recovery and reuse of phosphorus. This allows municipal and industrial clients to turn a cost (purchase of metal salts and sludge disposal), into a benefit (sale of struvite as a fertiliser), however some have argued that the recovered phosphorus is difficult to handle and unable to fetch a real market price.
“The main bottleneck is not the recovery technology but finding a suitable market for the recovered material, and I say material not product,” explains Mark van Loosdrecht. “More attention is required in finding a specialised market or other application, and maybe in that context it is more interesting to recover iron- or calcium-phosphate. One might think of the application in composite materials (mortars or even batteries), instead of bringing it to a value chain where food is involved. There is a lot of creativity in making technology and marginal creativity in marketing and product development.”
Suez claim that by implementing their Phosphogreen technology in a wastewater treatment plant the customer would typically get a five to 10 year return on investment. The main incomes or savings come from the reduction of chemical consumption for phosphorus physicochemical treatment, less energy consumed for biological treatment, increase of equipment service life thanks to controlled precipitation of struvite, reduction of maintenance costs, and reduction of sludge disposal costs by minimisation of sludge volumes.
The Phosphogreen technology is currently running in two plants in Denmark (Herning and Aaby–city of Aarhus), and while Mathieu Delahaye admits that the market needs to be made more visible, he does believe one exists for recovered phosphorus.
“The struvite is sold in Denmark through a fertiliser company and the product is marketed under the brand name Phosphorcare,” says Delahaye. “There is no problem in terms of quality and acceptance by the fertiliser company. The market already exists but has to be made more visible. In Denmark for example, utilities that are recovering phosphorus in their plants are joining forces in order to increase struvite production and better market and negotiate with resellers from the fertiliser industry.”
Phosphogreen is a phosphorus recovery process based on a precipitation-crystallisation reaction. Phosphorus is recovered from phosphorus-rich effluents. They are introduced into the Phosphogreen reactor where carbon dioxide is degased by air injection. The pH is measured and adjusted if necessary, by adding sodium hydroxide to optimise the reaction conditions.
The heart of the process consists of injecting magnesium chloride to obtain precipitation-crystallisation of struvite. Struvite is then extracted at the bottom of the reactor, washed, drained and dried before being packaged.
The minimum phosphorus concentration in the effluent for the technical and economic viability of the process is 70 mg/l. It is therefore often necessary to have a biological phosphorus removal on the water-line.
It is clear that more value is now being extracted from wastewater than in previous years and recovering phosphorus can make a contribution to the agricultural sector, in the form of fertilisers. An energy-neutral or even energy-positive wastewater treatment plant is already achievable. However, there is still some way to go to reach income positive plants unless external materials are brought to the plant for codigestion, but developments are ongoing to reach this goal.
“The main issue is that the volume produced of most recovered resources are small compared to market size, and the product cannot be easily be taken up in the current value chain,” concludes van Loosdrecht. “What should be done? Find products that when recovered bring advantage for the primary task of a wastewater treatment plant (struvite recovery), set-up a marketing company that can function with marketing as the only task and is not linked to a technology supplier, but is maybe linked to public utilities who guarantee to supply the raw material for a certain time.”