Procédé de récupération du fer de boues de station d'épuration d'eaux usées et son utilisation pour le recyclage notamment du chlorure ferrique

Procédé de récupération du fer de boues de station d'épuration d'eaux usées obtenues après précipitation par des sels de fer, caractérisé en ce qu'il comprend une étape unique de re-dissolution du fer desdites boues par voie biologique, suivie d'une étape de séparation du fer de la fraction liquide obtenue à l'issue de l'étape biologique de dissolution du fer. L'étape biologique comprend une étape unique de fermentation desdites boues en présence de substrats riches en carbone facilement biodégradable, à une teneur supérieure ou égale à 0,3 g de DCO/ MV, conduisant à la libération du fer desdites boues et à sa mise en solution. Utilisation du procédé pour le recyclage d'au moins une partie des sels de fer, tel que ceux employés dans les stations d'épuration d'eaux usées, par exemple mettant en oeuvre une étape de précipitation par le chlorure ferrique

A new strategy to increase phosphorus recovery yield from wastewater sludge

International audiencePhosphorus (P) recovery as struvite from wastewater activated sludge (WAS) is a developing technology usually applied to recycle dissolved P by adding magnesium (Mg) and increasing pH in the liquid effluent from the digestate dewatering step after anaerobic digestion (AD) of WAS. Consequently, the P recovery yield is depending on the dissolved P concentration in digestate which is usually only a small fraction of the P in WAS. The aim of this study was to increase the P recovery yield by favouring P dissolution by a biological way, not after but just before the AD of WAS as described in figure 1. Recovering P before AD is also a way to prevent struvite crystallization on pipes and equipment, reducing maintenance cost in AD plants. For that, acidogenesis was favoured by adding several different wastes as co-substrate in thickened WAS

Fate of phosphorus from biological aerobic treatment of pig slurry. By-products characterization and recovery

International audienceThe fate of phosphorus distribution in the products obtained from biological aerobic treatment of pig slurry, e.g. separated solids, liquid effluent and sludge, was monitored in three different farm-scale units. Samples of raw slurry, solid products, aerated slurry, liquid effluent and sludge were characterised and analysed for their concentration in total phosphorus, nitrogen content and heavy metals (Cu and Zn). At each treatment stage, nitrogen, phosphorus and heavy metals mass balance between input and output was established. Moreover, liquid products were characterised and analysed both for their total and dissolved ortho-phosphate content. Separated solids, sluge and liquid effluent represented 5%, 15-40% and 75-83% of the mass of the raw slurry, respectively. A mechanical separation step prior to aeration allowed the export of 25-30% of total phosphorus for further use as organic fertiliser. A large amount of total phosphorus (e.g. 60-70%) was located in sludge while phosphorus remaining in liquid effluent was about 15-25%. Raw slurry separation and sufficient aeration allowed phosphorus to concentrate in the sludge. Insufficient aeration resulted in the release of phosphorus as dissolved ortho-phosphate within the liquid effluent. Finally, relevance of the agronomic use of the products was discussed and improvements of biological aerobic treatment to enhance phosphorus removal and/or recovery were considered

Phosphorus recovery from pig slurry through biological acidification and re-crystallization as struvite

International audiencePig slurry is a phosphorus-rich waste stream that requires costly physical/chemical and-or biological treatment, particularly in areas vulnerable to eutrophication. Swine slurry commonly undergoes a solid-liquid separation: the liquid fraction is spread in the neighbouring fields while the solid phase has to be exported, often as compost, to phosphorus-depleted regions. There is currently a growing trend towards phosphorus recovery under a purified, marketable form such as struvite, a slow-release mineral fertilizer. Recycling phosphorus as struvite could reduce the treatment costs of pig slurry and provide a renewable phosphorus source for agriculture. Indeed, the phosphate rock used in the fertilizer industry is a non-renewable resource whose decreasing quality and progressive depletion could threaten the fertilizer-based agricultural model and harm global food supply. Phosphorus in swine slurry mostly exists as mineral solids. It is now possible to extract this phosphorus as ortho-phosphate by acidification and recover it as struvite by adding magnesia (MgO). Biological acidification of swine slurry using waste-type co-substrates would be more environment-friendly than chemical acidification and save the costs of chemical addition. Sucrose was used as a practical co-substrate in order to realize a preliminary study of the biological and chemical reactions taking place during the acidification of pig slurry. A continuous reactor and batch tests were used to ascertain the link between the amount of sucrose added, the type/amount of volatile fatty acids (VFA) produced, the resulting pH drop and the dissolved phosphorus obtained. The batch tests consisted in 12 flasks, each containing raw swine slurry and four different amounts of sucrose in triplicate (0, 15, 30 and 60g/L). At maximum initial sucrose concentration the pH dropped from 7.5 to 5.5 after 2 days, while dissolved phosphates reached 800 mgPO4-P/L from 50 mgPO4-P/L initially. Lactate was the main VFA all along (70-95%). At lower sucrose concentrations, the pH either increased or dropped slightly before re-increasing. At 30g-sucrose/L, 420 mgPO4-P/L were measured after 2 days when a pH of 6.2 was reached. However those phosphates re-precipitated when the pH increased to 6.5 after 60 hours. Lactate was the main VFA during the first 40 hours but disappeared afterwards, simultaneously with the pH re-increase. The pattern in term of VFA production at the highest sucrose concentration was comparable to what occurs in successful silage. The initial amount of water soluble carbohydrates (WSC) (i.e. sucrose in this case) was high enough to give a competitive advantage to Lactic Acid Bacteria (LAB). They were able to acidify the slurry down to a pH too low for other microorganisms to metabolize the lactate into less acidic VFAs or other end-products. This resulted in a stable, low pH and a constant VFA concentration and composition. On the other hand, at sucrose concentrations lower than 60 g/L, the pattern of VFA production was similar to what occurs in spoiled silage. At such WSC concentration, the initial lactic acid production was not sufficient and other microorganisms were able to consume the lactic acid, resulting in a pH increase and a decrease in VFA concentration (possibly due to the beginning of methane production). The continuous reactor was maintained at 36°C and fed with sucrose and the supernatant of digested, centrifuged pig slurry, at a fixed hydraulic retention time of 4 days, during 30 days. The reactor stabilized after 4 days at a pH of 4.8 while the sucrose concentration in the influent was decreased from 150 to 40g-sucrose/L in the feed. Sucrose was mostly converted to lactate (60-95% of removed sucrose) throughout the study, indicating that the biological process similar to ensiling could be maintained during 30 days in a continuous reactor. The results obtained indicate that the biological acidification of pig slurry with high sucrose concentration (40-60g/L), leads to the dissolution of most of the phosphorus (700-900g/L, 70-80% of TP). The critical pH range in which phosphorus got dissolved was between 5 and 6. The biological process taking place had many similarities with ensiling, with lactic acid being produced predominantly. This ensiling-like process could be maintained in a continuous reactor for 30 days. Based on these findings, the choice of real co-substrates for biological acidification of swine slurry should be oriented towards WSC-rich wastes, like fruits, vegetables, grass, or crop-residues

The effect of phytase in pig diet and solid/liquid separation of pig slurry on phosphorus, calcium, and magnesium fractionation

International audienceIn some intensive animal production areas, the accumulation of nutrient surpluses (N, P, etc.) from livestock effluents has led to severe pollution problems (water, air, soil). The control of this potential pollutant load requires the development of processing methods to remove the excess nutrients. In France, biological treatment based on aeration (nitrification/denitrification)is the most widespread technology on farms. This treatment must now be adapted to include phosphorus removal as well as nitrogen removal. For this purpose, the characteristics of pig slurry from different farms (with or without phytase in pig diets) were studied through phosphorus, magnesium, and calcium fractionation including ortho−P, organic dissolved P, precipitated P, biomass P, and residual P. Moreover, the influence of mechanical separation (press auger and centrifugation)was studied. In raw slurry, 4% to 10% of phosphorus was soluble, 60% to 85% was precipitated, and 3% to 20% was phosphorus linked to the biomass. The total phosphorus concentration decreased slightly when diets with phytase were used (12%). Without phytase, around 20% of total phosphorus was "residual" (i.e., in a very insoluble form, probably as calcium phytate). Up to 50% of this form remained after the separation step. No residual phosphorus was found with phytase in the diet. Both separators studied (press auger and centrifugation) did not affect the concentration of soluble compounds in the separated slurry. When the TSS concentration in the raw slurry was high (>3.5%), the abatement of the TSS concentration was similar with centrifugation or press auger. In contrast, centrifugation and press auger decreased the total phosphorus concentration in the effluent by up to 50% and 15%, respectively. Most of the phosphorus removed by centrifugation is precipitated phosphorus. The difference between the amount and the quality of the phosphorus present in the effluents for each type separator should be considered in proposing a relevant dephosphorization strategy for pig slurry

Biological aerobic treatment of pig slurry in France: nutrients removal efficiency and separation performances

International audienceFour types of biological aerobic treatment units were identified in France: (1) intermittent aeration without any separation, (2) intermittent aeration followed by sedimentation of aerated slurry, (3) mechanical separation of raw slurry followed by intermittent aeration of the liquid fraction and sedimentation of aerated slurry and (4) mechanical separation of raw slurry followed by intermittent aeration of the liquid fraction and mechanical separation of the aerated slurry. Among the 26 treatment units built in Brittany (June 1999), three were studied for 8 weeks. This monitoring period allowed to establish nutrient distribution for nitrogen, phosphorus, copper and zinc. Between 60 and 70 % of the nitrogen was removed in gaseous form. Mechanical separation of the raw slurry (press-auger) concentrated 25-30% of total phosphorus, 10% of total nitrogen and 8-12% of heavy metals (copper and zinc) into a solid fraction representing 4-5% of the raw slurry on a weight basis. Separation of the treated slurry using sedimentation or mechanical separation concentrated phosphorus and heavy metals in the sludge (60-90%). This separation could lead to a high content of copper and zinc in the sludge up to 160 and 340 g/ton, respectively. Consequently, the use of this product on the farm as organic fertilizer led to an overload with regard to agronomic requirements and export was difficult due to the high copper and zinc contents. The clarified supernatant obtained by separation of the treated slurry could be irrigated with a reduced environmental risk. Concerning the biological reactor, the variation of the volumetric load owing to the variation of raw slurry characteristics was identified as the main parameter influencing the treatment efficiency

Le recyclage du phosphore à partir de déchets peut-il conduire à une amélioration de la durabilité environnementale des filières ? Illustration avec le cas des fertilisants phosphatés boue-sourcés

National audienc

Biological aerobic treatment of pig slurry in France: nutrients removal efficiency and separation performances

International audienceFour types of biological aerobic treatment units were identified in France: (1) intermittent aeration without any separation, (2) intermittent aeration followed by sedimentation of aerated slurry, (3) mechanical separation of raw slurry followed by intermittent aeration of the liquid fraction and sedimentation of aerated slurry and (4) mechanical separation of raw slurry followed by intermittent aeration of the liquid fraction and mechanical separation of the aerated slurry. Among the 26 treatment units built in Brittany (June 1999), three were studied for 8 weeks. This monitoring period allowed to establish nutrient distribution for nitrogen, phosphorus, copper and zinc. Between 60 and 70 % of the nitrogen was removed in gaseous form. Mechanical separation of the raw slurry (press-auger) concentrated 25-30% of total phosphorus, 10% of total nitrogen and 8-12% of heavy metals (copper and zinc) into a solid fraction representing 4-5% of the raw slurry on a weight basis. Separation of the treated slurry using sedimentation or mechanical separation concentrated phosphorus and heavy metals in the sludge (60-90%). This separation could lead to a high content of copper and zinc in the sludge up to 160 and 340 g/ton, respectively. Consequently, the use of this product on the farm as organic fertilizer led to an overload with regard to agronomic requirements and export was difficult due to the high copper and zinc contents. The clarified supernatant obtained by separation of the treated slurry could be irrigated with a reduced environmental risk. Concerning the biological reactor, the variation of the volumetric load owing to the variation of raw slurry characteristics was identified as the main parameter influencing the treatment efficiency

Recyclage chimique du phosphore des lisiers de porcs

International audienceThe technical feasibility of a dissolution/precipitation chemical phosphorus recycling process from piggery wastewater was assessed. Several combinations of acidifying/precipitating reactants were evaluated for their impact on liquid effluent quality and solids formed. Chloride and sodium concentration in the liquid effluent, which could contribute to soil salinisation, were reduced by a third by using acetic acid/magnesium oxide when compared to hydrochloric acid/sodium carbonate. Finally more than 95% of the initial dissolved phosphorus from the acidified supernatant was recovered. Struvite crystals and amorphous calcium phosphate were the main components in the solid, identified by X-ray diffraction, optical and SEM-EDS microscopy. The size and the shape of struvite crystals were increased by increasing the magnesium or ammonium/phosphate ratio which made them more suitable for the filtration and drying steps required to export phosphorus as a dry mineral product

Fate of phosphorus from biological aerobic treatment of pig slurry. By-products characterization and recovery

International audienceThe fate of phosphorus distribution in the products obtained from biological aerobic treatment of pig slurry, e.g. separated solids, liquid effluent and sludge, was monitored in three different farm-scale units. Samples of raw slurry, solid products, aerated slurry, liquid effluent and sludge were characterised and analysed for their concentration in total phosphorus, nitrogen content and heavy metals (Cu and Zn). At each treatment stage, nitrogen, phosphorus and heavy metals mass balance between input and output was established. Moreover, liquid products were characterised and analysed both for their total and dissolved ortho-phosphate content. Separated solids, sluge and liquid effluent represented 5%, 15-40% and 75-83% of the mass of the raw slurry, respectively. A mechanical separation step prior to aeration allowed the export of 25-30% of total phosphorus for further use as organic fertiliser. A large amount of total phosphorus (e.g. 60-70%) was located in sludge while phosphorus remaining in liquid effluent was about 15-25%. Raw slurry separation and sufficient aeration allowed phosphorus to concentrate in the sludge. Insufficient aeration resulted in the release of phosphorus as dissolved ortho-phosphate within the liquid effluent. Finally, relevance of the agronomic use of the products was discussed and improvements of biological aerobic treatment to enhance phosphorus removal and/or recovery were considered