Optimization of struvite precipitation in synthetic biologically treated swine wastewater - Determination of the optimal process parameters

A sustainable way to recover phosphorus (P) in swine wastewater involves a preliminary step of P dissolution followed by the separation of particulate organic matter. The next two steps are firstly the precipitation of struvite crystals done by adding a crystallization reagent (magnesia) and secondly the filtration of the crystals. A design of experiments with five process parameters was set up to optimize the size of the struvite crystals in a synthetic swine wastewater. More than 90% of P was recovered as large crystals of struvite in optimal conditions which were: low Mg:Ca ratio (2.25:1), the leading parameter, high N:P ratio (3:1), moderate stirring rate (between 45 and 90 rpm) and low temperature (below 20°C). These results were obtained despite the presence of a large amount of calcium and using a cheap reactant (MgO). The composition of the precipitates was identified by Raman analysis and solid dissolution. Results showed that amorphous calcium phosphate (ACP) co-precipitated with struvite and that carbonates were incorporated with solid fractions

Analyse des transformations du phosphore lors du stockage et du traitement biologique et physico-chimique des lisiers de porc en vue d'un recyclage

Pour favoriser le recyclage du phosphore des lisiers de porcs sous la forme d'engrais chimique, facilement exportable et valorisable en dehors des zones de production animale intensive, il est nécessaire de séparer le phosphore de la matière organique. Le phosphore des lisiers étant initialement à plus de 60% sous une forme minérale solide mélangée à la matière organique, nous avons conçu un procédé en 3 étapes : (1) solubilisation du phosphore, (2) séparation solide/liquide puis (3) précipitation. La solubilisation constitue le principal verrou technologique à la mise en place de ce procédé. Nous avons donc étudié trois stratégies/possibilités de solubiliser le phosphore et d'augmenter ainsi la proportion de la fraction dissoute du phosphore. La première stratégie évaluée est l'ajout d acide benzoïque à la ration des porcs et la réduction du taux de protéines. Cette modification de la ration alimentaire réduit le pH des lisiers d une unité environ, augmentant ainsi la proportion de phosphore dissous dans le lisier stocké jusqu'à 20% du phosphore total. Cette augmentation reste toutefois insuffisante pour envisager un procédé de recyclage performant. La seconde stratégie est l'optimisation du relargage de phosphore lors du traitement biologique de l'azote des lisiers par aération intermittente. Les résultats obtenus au cours de ce travail ont montré que les métabolismes de relargage/accumulation du phosphore sont masqués par l'effet du pH lors de ce traitement biologique. En effet, le pH élevé provoque la précipitation du phosphore relargué pendant l'anaérobiose alors que, au contraire, l'acidification du milieu induite par la nitratation, provoque la dissolution des formes minérales pendant l aérobiose. Cette dissolution masque l'accumulation du phosphore par la biomasse pendant l aération. Une augmentation significative de la concentration en phosphore dissous est obtenue en augmentant la concentration d azote nitrifié par cycle (augmentation de la durée des cycles et réduction du temps de séjour). Cependant, la fraction de phosphore dissous par ce biais ne dépasse pas 20-25% du phosphore total. La troisième possibilité est l utilisation de réactifs acides. Dans ce cas, la réduction du pouvoir tampon des lisiers, obtenus avec les régimes alimentaires acidifiants ou après traitement biologique permet d'envisager une forte réduction de réactif lors d'un procédé de dissolution par ajout d'acide puis de précipitation. Suite à la dissolution du P par ajout d'acide, des essais de précipitation ont été effectués avec ajout de soude, de bicarbonate de soude ou de magnésie. La précipitation de la totalité du P a été obtenue a partir de pH 7 et les principales formes obtenues et identifiées sont la struvite et une forme non cristalline de calcium phosphate. Enfin, le logiciel de modélisation géochimique PHREEQCI a été évalué au cours de ce travail et permet de simuler l'effet de l ajout d'acides ou de bases dans le milieu et de donner une description qualitative des phases solides obtenuesTo recycle phosphorus from pig slurry, as a chemical fertilizer which could be easily transported and used far from intensive livestock areas, it is necessary to remove phosphorus from organic matter. Since pig slurry has more than 60% of its phosphorus in solid mineral form we have designed a three step process of solubilisation, solid/liquid separation and precipitation to realise such a reduction. The solubilisation was the main obstacle in the development of this process. We have studied three means to increase the ratio of dissolved phosphorus. The first one was to add benzoic acid and to reduce the amount of protein in the pig feed. The pH was reduced by 1and the dissolved phosphorus ratio increased up to 20% after storage, but it was not sufficient for the recycling process. The second one was to optimise phosphorus release during the biological treatment of nitrogen by sequential aeration. Both biological phosphorus release and uptake were concealed by the effect of pH. The high level of pH led to the precipitation of released phosphorus during the anaerobic stage. Contrary to this, acidification induced by nitrification led to solubilisation of mineral forms of phosphorus which concealed the uptake of phosphorus during the aerobic stage. A significant increase of dissolved phosphorus was obtained by increasing the amount of nitrogen nitrified in one cycle, which was accomplished by increasing the cycle duration and decreasing the hydraulic retention time. However, the ratio of dissolved phosphorus was always less than 20-25%. The third way was to use an acidic reactant. This could be viewed only if the buffer effect was previously reduced by feeding the pigs with the acidifying diet or by biological treatment. After acidic solubilisation, phosphorus was precipitated as struvite and amorphous calcium phosphate by adding either sodium hydroxide, sodium carbonate or magnesium oxide. The geochemical modelling tool PHREEQCI was successfully used to simulate acid or base addition in slurry and was helpful to give a description of the solid phase compositionTOULOUSE-INSA (315552106) / SudocSudocFranceF

Relevance of a perchloric acid extraction scheme to determine mineral aud organic phosphorus in swine slurry

International audienceTo increase the phosphorus recycling potential from swine slurry, mineral phosphorus products which could be used as fertilizers should be obtained and new processes need to be investigated. A routine method is needed to better evaluate the dissolved and solid mineral phosphorus in swine slurry. Cold perchloric acid extraction method previously developed for wastewater or sludge analysis was adapted. Ionic chromatography was used to measure orthophosphate in extracts. Only one extraction step was needed to distinguish between mineral and organic phosphorus in slurry. Reproducibility of the method was high (less than 5% of variation on the measured fractions). Selectivity was assessed by adding several organic and mineral phosphorus sources in the slurry. Cold perchloric extraction followed by ionic chromatography was very selective in quantifying both the mineral and organic forms of phosphorus in swine slurry. (C) 2007 Elsevier Ltd. All rights reserved

Phosphorus recycling potential assessment by a biological test applied to wastewater sludge

Phosphorus (P) recycling as mineral fertilizer from wastewater activated sludge (WAS) depends on the amount that can be dissolved and separated from the organic matter before the final crystallization step. The aim of the biological phosphorus dissolution potential (BPDP) test developed here was to assess the maximum amount of P that could be biologically released from WAS prior that the liquid phase enters the recovery process. It was first developed for sludge combining enhanced biological phosphorus removal and iron chloride. Because carbohydrates are known to induce acidification during the first stage of anaerobic digestion, sucrose was used as a co-substrate. Best results were obtained after 24-48 h, without inoculum, with a sugar/sludge ratio of 0.5 gCOD/gVS and under strict anaerobic conditions. Up to 75% of the total phosphorus in sludge from a wastewater treatment plant combining enhanced biological phosphorus removal and iron chloride phosphorus removal could be dissolved. Finally, the test was applied to assess BPDP from different sludge using alum compounds for P removal. No dissolution was observed when alum polychloride was used and less than 20% when alum sulphate was used. In all the cases, comparison to chemical acidification showed that the biological process was a major contributor to P dissolution. The possibility to crystallize struvite was discussed from the composition of the liquids obtained. The BPDP will be used not only to assess the potential for phosphorus recycling from sludge, but also to study the influence of the co-substrates available for anaerobic digestion of sludge

Reducing ammonia volatilisation from pig slurry through the reduction of dietary crude protein and the incorporation of benzoic acid

O-1International audienc