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Optimizing hydrolysis and acidogenesis in order to dissolve and recover phosphorus in organic effluents upstream from methane production

Abstract : Phosphorus is a crucial nutrient for life, implicated in cellular bioenergetics as well as storage and processing of genetic information. It is also one of the limiting nutrients in agriculture with nitrogen and potassium. Since the green revolution in the middle of the 20th century, agriculture has relied on increasing amounts of cheap mineral P-fertilizers produced from a fossil resource to improve crop yields and sustain population growth. However, the resource is depleting and its use efficiency is poor: less than 20% of extracted P is actually consumed in food. One of the reasons for this is the specialization of entire regions into on type of agricultural production or another. Thus, regions focusing on high yield crops require large applications of fossil mineral fertilizers while intensive livestock breeding areas cannot find an output for their P-rich manure due to the distance with crop fields in need of P fertilization. Over application of animal manure in Brittany is the main cause of eutrophication in the region. Phosphorus could be recovered from pig manure as struvite, a concentrated, slow-release mineral fertilizer easily transported to crop-oriented regions in need of P fertilization. P in pig slurry is mostly under a solid inorganic form, requiring dissolution prior to precipitation as struvite. Because chemical acidification is too expensive and harmful to the environment, the process developed in this PhD relied on acidogenesis, a biological process in which organic matter is converted to organic acids under anaerobic conditions, thus naturally acidifying the swine slurry. Various organic wastes were tested as organic co-substrates on raw and digested pig slurry, leading to lactic acid fermentation when the co-substrate had a high content in easily biodegradable carbohydrates and a fermentation with diverse organic acids produced at low content in easily biodegradable carbohydrates. Lactobacillus was the genus responsible for lactic acid fermentation and various Clostridiales dominated otherwise, producing acetate, propionate, butyrate and valerate. A reactor was operated with semi-continuous feeding of raw swine slurry and carrot/pea, leading to the dissolution of 50% total-phosphorus or 750 mg-P/L. After centrifugation, struvite was precipitated in the supernatant by adding magnesium hydroxide to increase the pH to 8. 99% of dissolved P precipitated. The solid recovered contained 70% of struvite, a slight excess of P and Mg as well as organic matter. Because hydrolysis of organic matter and production of organic acids occurs during acidogenesis, the process could be implemented in the many anaerobic digestion units installed in Brittany treating animal manure and agricultural, industrial and municipal organic waste. The struvite recovered could be sold to regions in need while the digestate impoverished in P and rich in organic matter could be kept locally. Such process would reduce eutrophication due to over application of pig manure and also reduce the reliance on fossil P fertilizer by offering an alternative source with equivalent fertilizing performances.
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Simon Piveteau. Optimizing hydrolysis and acidogenesis in order to dissolve and recover phosphorus in organic effluents upstream from methane production. Organic chemistry. Université Rennes 1, 2017. English. ⟨NNT : 2017REN1S123⟩. ⟨tel-01800084⟩

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