Struvite precipitation as a Technology to be integrated in a farm scale anaerobic Digestion-composting Treatment plant

Manures which are potentially fertilizers or soil conditioners must be considered as resources that should be managedadequately. The farm SAT Sant Mer decided to include a biogas/composting plant to manage the excess of manureproduced. In order to improve nutrient management and to reduce cropland requirements, the inclusion of a struviteprecipitation process was assessed in lab-scale and pilot-scale experiments.A serial of batch and continuous experiments were conducted to assess the influence of operational parameters onstruvite crystallization (pH, temperature and alkalinity).

Further Authors:
J.Vicens - Aprofitaments Energètics Agrícoels
M. Cerrillo - IRTA. GIRO Joint Research Unit IRTA-UPC

A first group of sixteen batch assays were performed usingsynthetic liquor. The effect of variation in reactor mixing velocity and the effect of the introduction of CO2 strippingwere investigated in a second group of three experiments, carried out with the same synthetic liquor in a continuousreactor. Finally, a third group of three assays were carried out in the continuous reactor with digested swine manure, toinvestigate the effect of total solids and organic matter contentIt has been observed that as the pH of the medium increased, the percentage of ion removal in the supernatant graduallyincreased. An optimum, in terms of process efficiency, could be set around pH 9 (99’86%±0’01 for phosphate,95’49%±0’88 for magnesium and 89’43%±0’60 for ammonium removal). The effect of temperature on ammonia,magnesium and phosphate recovery as struvite was assessed also at 36ºC (simulating the outflow conditions from theanaerobic digestion process). No differences were observed in terms of process efficiency between 25ºC and 36ºC. Theeffect of the system buffer capacity and presence of other cations was assessed at equivalents alkalinities of 5.125,10.25 and 20.50 gCaCO3/L with CaCO3 or Na2CO3. When sodium carbonate (Na2CO3) was used, the percents ofelimination were similar to the assays without added buffer. When calcium carbonate was used (CaCO3), the percent ofmagnesium and ammonium removed decreased, while the percent of phosphate removed was not affected. This effectcan be explained by the formation of other salts as dolomite (CaMg(CO3)2), hidroxyapatite (Ca5(PO4)3OH) and calcite(CaCO3).Continuous experiments with the synthetic solution, performed in a lab-scale reactor of 20 L with a mixing velocity of100 rpm, showed an ammonium removal efficiency of 66.03%±7.69. The removal efficiency was improved with amixing velocity of 200 rpm (78.26%±0.14), although the size of the formed crystals was reduced. When the reactor wasequipped with an aeration system for CO2 stripping, despite a slightly lower ammonium removal efficiencies reported(71.93%±9.67) a reduction in reagents (NaOH) was obtained (15 – 28%). Continuous experiments with the digested pigslurry, performed with a mixing velocity of 100 rpm showed an ammonium removal efficiency of 85.18%±0.75. Theremoval efficiency was of 84.63%±0.91 when the mixing velocity was raised to 200 rpm. As expected, higher organicmatter resulted in poor quality of the struvite in terms of organic matter contamination, and crystals size.These results pointed struvite precipitation as a good alternative to improve nutrients management. Laboratorypreliminary results were confirmed at farm-scale pilot plant. Nevertheless, results should be contrasted in order to establish/adjust operation parameters, operational cost, and quality of produced struvite.



Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2012 (Juni 2012)
Seiten: 8
Preis inkl. MwSt.: € 8,00
Autor: Jordi Palatsi
August Bonmatí

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