Nutrient recovery and improvement of anaerobic digestion process by low grade magnesium oxide application

[spa] Diferentes esfuerzos han sido desarrollados para reducir la inhibicion por nitrogeno amoniacal en la DA. Entre ellos, la adicion de materiales con capacidad de intercambio cationico (por ejemplo: bentonita, glauconita, fosforita y zeolitas) o materias con capacidad de adsorcion (ej. arena, oxidos de magnesio, sepiolitas y zeolitas) han mostrado buenos resultados. Igualmente, en anos recientes la posibilidad de acoplar la DA y la precipitacion de estruvita (MgNH4PO4·6H2O) en el mismo reactor han llamado la atencion. La precipitacion de estruvita ocurre de manera natural cuando la concentracion de 2+ + 3- Mg , NH4 y PO4 excede el producto de solubilidad de la estruvita. Con lo cual, la precipitacion de estruvita en muchos sistemas de DA requiere la adicion de compuestos 2+ 3- + quimicos, ya que la concentracion de Mg y PO4 es tipicamente inferior a la del NH4 . El objetivo de este estudio es comparar el desempeno de cinco diferentes fuentes de magnesio (ej. MgCl2, Mg(OH)2, HG-MgO, LG-MgO y SA) en reactores, simultaneando los procesos de la DA y la precipitacion de estruvita dentro del mismo reactor. La adicion de agente estabilizante al purin de cerdo presento una maxima eficiencia de remocion de nitrogeno amoniacal total del 80%. La operacion de los -1 digestores mostro que la adicion de agente estabilizante de 5 y 30 g L al digestor resulta en un incremento del 25 y 40% en la produccion de metano, respectivamente, comparado con el reactor de referencia. Este resultado puede estar relacionado con la reduccion de la concentracion de amonio y el incremento de la concentracion de magnesio y concentracion de particulas en el medio de digestion. Adicionalmente, los resultados de alcalinidad y acidos grasos volatiles muestran que la adicion de agente estabilizante no presenta un efecto negativo en los microorganismos anaerobicos. [eng] Anaerobic digestion is a worldwide technology to treat organic waste streams, primarily due to its capacity to produce methane as renewable energy. However, there is an increasing interest on nutrient recovery (N and P), which from both environmental and economic reasons have been identified as key feature in anaerobic digestion plants. Controlled struvite formation has been attracting increasing attention as a near mature technology to recover nutrients from anaerobic digestion. However, struvite feasibility is generally limited by the high cost of chemical reagents. Because the economic feasibility of struvite is heavily influenced by reagent cost, several authors have trialled lower-cost magnesium sources. Studies to date on struvite precipitation with MgO have largely focused on the aqueous phase, with little attention given to the preceeding MgO dissolution. However, the observations noted above suggest that there may be opportunity to better understand, intervene and improve dissolution and access to MgO. The present study uses experiments and chemistry modelling to evaluate and better understand TAN removal from pig manure using MgO. Tests were performed with four industrial magnesium oxide (MgO) from natural magnesite calcination: a commercial high grade MgO (HGMgO) as well as a number of low-grade MgO (LGMgOs) by-products. All these MgO reagents were also pre-treated with phosphoric acid and tested separately after pre-treatment. The study focussed on the underlying chemistry to show how reactivity and dissolution of the various magnesium by-products influenced struvite precipitation and TAN removal performance. Moreover, another option to minimise struvite precipitation cost is to combine struvite precipitation and AD in the same reactor. Several authors have studied the addition of magnesium reagents (MgCl2 and Mg(OH)2) to precipitate struvite during anaerobic digestion, causing in some cases inhibition by pH or cation toxicity. However, to our knowledge, no references bave been found evaluating the utilization of magnesium by- products within the reactor to precipitate struvite during anaerobic digestion. In this study struvite precipitation and pig manure anaerobic digestion were coupled in the same reactor in order to mitigate the inhibitory effect of free ammonia and avoid precipitator costs. The stabilizing agent used to facilitate struvite precipitation was formulated with low-grade magnesium oxide by-product; an approach that would notably reduce struvite processing costs. Therefore, the feasibility of coupling anaerobic digestion and struvite precipitation in the same reactor was evaluated to enhance manure anaerobic digestion methane yields through ammonia inhibition mitigation. Five different magnesium sources were tested as struvite (ammonia sequestration agent) precursor, i.e. MgCl2, Mg(OH)2, two industrial by-products rich in MgO but with different reactivity, and a stabilizing agent. The latter was formulated in advance with the low reactivity industrial by-product and phosphoric acid. The effect of each magnesium source on anaerobic digestion as well as its struvite precipitation capacity was evaluated through a series biomethane potential test. However, a long term anaerobic digester operation was required to assess the feasibility of the process and to ensure that the stabilizing agent does not introduce any harmful compound for the anaerobic biomass. In this vein, the -3 addition of 5 and 30 kg m of the stabilizing agent in a pig manure continuous digester 3 -1 3 -1 resulted in a 25% (0.17 m kg ) and a 40% (0.19 m kg ) increase in methane production per mass of volatile solid, respectively, when compared with the reference digester (0.13 3 -1 m kg ). Moreover, the stability of the process during four hydraulic retention times guarantees that the stabilizing agent did not exert a negative effect on the consortium of microorganisms.