Phosphorus recycling from agro-industrial effluents:
from labscale to pre-industrial pilots.
AVOIDING FRESH WATER EUTROPHICATION AND SAVING LIMITED RESOURCES BY RECYCLING P AS MINERAL FERTILIZER
P is an essential component for life. P is provided to plants and animals as mineral form, resource whose mining reserves are estimated between 100 and 250 years. P is badly assimilated and partly excreted in the environment. An excess of P in surface water leads to eutrophication, degrading the biological status, compromising fishing, fish farming or leisure and increasing the drinkable water production cost. We are facing a paradoxical situation with an excess of P whose treatment is expensive, on one hand and the need for importing phosphate-enriched fertilizers whose reserves are limited, on the other hand. The main sources of P which can be recycled are livestock and agro-industrial effluents. This project made it possible to acquire knowledge on the extraction and the crystallization of P from these effluents. This knowledge will be used to develop two new processes. The phosphate-enriched fertilizers, containing struvite, produced from effluents is as effective as phosphate-enriched fertilizers from mines, reducing the agricultural and industrial impacts on ground and water while preserving the resource.
A SCIENTIFIC APPROACH SHARED TO PRODUCE KNOWLEDGE USED TO DEVELOP TWO PILOTS AT PRE-INDUSTRIAL SCALE.
The first process (P1) is designed for the effluents in which P is present mainly in particulate mineral form and the other (P2) for the effluents whose P is mainly dissolved. In both cases, P is recycled as struvite and/or calcium phosphate in a medium containing a high amount of organic matter. Batch tests were carried out to confirm or implement knowledge on P crystallization in organic medium (influence of the process parameters, ionic interactions, role of the organic matter). A Raman probe, innovating technical to following-up in situ crystallization, was used to study the influence of various factors on crystallization and to determine the thermodynamic constants and kinetics of the models. The labscale pilots allowed to study the influent parameters and to design the preindustrial pilots tested on effluents, in situ. The P fertilizing value of the products from recycling processes was tested by pot tests with 32P. Data obtained made it possible the economic and environmental studies aiming at comparing the innovating processes with references. At the end of the project, a P2 process is in construction on an industrial scale. The P1 pilot is effective and the product, composed mainly of struvite, is a good substitute of the chemical fertilizers but it appears not very competitive on the economic point of view. So, new works are in going to improve it.