The main source of off-farm recycled P is wastewater streams from sewage treatment plants. Other sources are abattoirs / animal by-products and manures, meat and bone meal ash (MBA), food, beverage and dairy processing and other food wastes (ESPP, 2021).

The wastewater streams are separated into liquid and solid fractions, and the solid fractions are either composted or, increasingly, treated by anaerobic digestion (AD), to sanitise them. Anaerobic digestion is being used more widely because it enables the chemical energy of the material to be ‘tapped’ as biogas by mutual degradation of the organic matter. This can then be either sold directly or used to generate electricity. As a consequence, the volume of the waste material is reduced, meaning lower handling or disposal costs.

Average figures for the nutrient content of digested or composted sewage sludge, or biosolids, are c.11 kg N / tonne (thermally dried sludge contains c.40 kg N / tonne), composted sludge typically contains 3 kg available P₂O₅ / tonne, digested sludge contains c.9 kg available P₂O₅ per tonne, while thermally dried sludge contains c.35 kg available P₂O₅ / tonne (AHDB, 2020). But note the comment above that actual nutrient content can vary markedly around these averages.

Note also that the availability of P varies significantly between different types or sources of sewage sludge and derived ash. This is also affected by the processes used to remove the P in the sewage treatment plant e.g. biologically or chemically.

Historically, in some countries, the processed sewage sludge has been applied straight to land. However this practice is now being questioned in many countries, due to concerns about contamination with substances such as heavy metals, PFAs, pharmaceuticals and microplastics, which cannot be removed by the water treatment plants. It should also be noted that, during the separation of the liquid and solid phases in the waste water treatment plant (WWTP), most pollutants leave the liquid phase and are concentrated in the sludge.

Downstream from this point in the process there are several different technologies and processes being used to produce a material suitable for use as a crop nutrient. Approaches to processing digestates (whether from water treatment works or food waste), which various organisations are investigating the viability of, include:

• drying and pelletising.

• phosphorus precipitation, producing materials such as struvite or calcium phosphate (brushite).

• membrane separation / nano-filtration.

• nitrogen stripping / ammonia salt recovery.

• osmosis.

• evapo-concentration.

• ion exchange, used with municipal wastewater after secondary treatment.

• pyrolysis / biochar

• incineration to ash.

One of the higher profile approaches is to produce Struvite (magnesium ammonium phosphate). Good quality struvite has demonstrated high fertilising efficiency, and there are currently (first half 2022) c. 80 full-scale struvite recovery plants operating worldwide. Three quarters of these are in municipal sewage works, with others in food processing plants, and manure digesters. The establishment of these plants is mainly driven and paid-back by operational improvements/savings, as the process avoids scaling in digesters and pipes and improves the dewaterability of the sludge. The P recovery rate is 8-15% of the WWTP inflow, with up to 40% recovery achievable with sludge pre-treatment. The largest plant to date is in Chicago, with an output of 9,000 t/y of struvite. The process is currently only applicable to enhanced bio-P removal, which limits it to c.10% of WWTPs in the EU. Further, the total potential production of struvite from WWTP’s will only account for a small proportion of the total P applied in fertilisers.

There is increasing interest in the production of P-rich ash, produced as a secondary material, because this can be used to replace P-rock in existing industry process or specific plants. Ash can be produced from the incineration of both sewage sludge and animal by-products (Meat and Bone Meal MBM). The ash can be introduced into the fertiliser production process at the rock acid attack or acidulation stage, which improves the plant availability of P in the ash.

However, any heavy metals that are present must be extracted to the level compliant with set legal limits, if the ash is to be blended into fertilisers. Only a small number of ashes currently being produced will be allowed to be used in their pure form. The majority will need to be further processed to separate out the heavy metals to the legal limit for ashes (EC, 2019. Strubias section CMC 13, as an example).

If ashes are blended in, they need to fulfil the requirements of the respective fertiliser regulations, otherwise they will need to be decontaminated. Another route is to extract the P out of the ashes in purified and refined forms, providing a raw material for fertiliser production, or even a ready to use fertiliser. Heavy metals and iron (from sewage sludge) are (partly) removed by solvent extraction and/or ion exchange and/or selective precipitation. A P-recovery rate of over 85% can be achieved. At present the material is being used in high value ‘commodity’ products such as Dicalcium phosphate (DCP), and mineral fertilisers. Some processes recover Fe/Al salts for P-removal in sewage works and/or silicates for use in cement production and / or gypsum.

There is a variety of alternative approaches to P-recovery being developed and tested; it remains to be seen which ones prove to be viable, practical, and economic on an industrially meaningful scale.

All the existing approaches are still part way towards full scale commercial development, at a level that would make a significant impact on overall nutrient flows. The situation in Europe is monitored by the European Sustainable Phosphorus Platform (ESPP 2022). This provides details of the following data for each P-recovery plant known to be operating within Europe:

• Process & contact

• Input materials

• Output products

• Process description Operating status, including plant output capacity.

References

Agricultural and Horticultural Development Board. (2020). Nutrient Management Guide (RB209).

European Commission. (2019). Regulation (EU) 2019/1009, Fertilising Products Regulation.

European Sustainable Phosphorus Platform. (2021). Opportunities for phosphorus recycling in Europe, today and tomorrow.

European Sustainable Phosphorus Platform. (2022). Phosphorus recovery technology catalogue

Links to related IFS Proceedings

591, (2006), Removing Phosphorus from Municipal Waste Water before Its Discharge to Watercourse, P Balmér

638, (2008), Phosphorus Imports, Exports, Fluxes and Sinks in Europe, I R Richards, C J Dawson

668, (2010), The Phosphate Life-Cycle: Rethinking the Options for a Finite Resource, J Hilton, A E Johnston, C J Dawson

680, (2010), Treatment and Use of Wastewater for Agricultural Irrigation, J Hagin, M Khamis, A Manassra, J Abbadi, L Al Hadidi, O Blonder, A Bulad, C Dosoretz, I Katz, M Qurie, R Semiat, A Shaviv

685, (2010), Future Supply of Phosphorus in Agriculture and the Need to Maximise Efficiency of Use and Reuse, A Rosemarin, J J Schröder, L Dagerskog, D Cordell, A L Smit

717, (2012), Phosphorus Fertilisers from By-Products and Wastes, O Oenema, W Chardon, P A I Ehlert, W Rulkens, O Schoumans, K van Dijk

727, (2013), Phosphate Recycling in Mineral Fertiliser Production, C P Langeveld, K W ten Wolde

732, (2013), Phosphate-Containing Waste Ash Process for Producing Mineral Fertiliser, L Hermann

763, (2015), Review of Promising Methods for Phosphorus Recovery and Recycling from Wastewater, C Kabbe, C Remy, F Kraus

765, (2015), Recovery of Phosphate as Struvite from Wastewater Streams, A T Britton, F P Abrary

832, (2019), Production of Clean Phosphorus Products from Sewage Sludge Ash using the Ash2phos Process, Y Cohen, P Enfält, C Kabbe

833, (2019), Realising Phosphorus Recycling, S Brandjes

847, (2020), Production of Phosphorus Fertiliser from Abattoir and other Industrial Wastes. M.S.A. Blackwell, T.S. Darch, R. Dunn

Links to external information sources

Kabbe, C. (2019). Global Compendium on Phosphorus Recovery from Sewage/Sludge/ Ash, Global Water Research Coalition.

Phosphorus removal and recovery technologies, S.Brett, J Guy, G.K. Morse and J.N.Lester, Publications Division Selper Ltd 1997.

Phosphorus: polluter and resource of the future, Christian Schaum, IWA publishing 2018.

Phosphorus recovery and recycling, Hisao Ohtake and Satoshi Tsuneda, Springer 2019