Tuesday, November 6, 2007
222-1
Nutrients and Heavy Metals Distribution in Thermally Treated Pig Manure.
Dewatered pig manure can be used as a fuel for energy production; however in order to recycle the ash for instance as fertilizer, it must meet environmental standards with respect to nutrients and heavy metals. In this paper, ashes from pig manure treated by incineration and thermal gasification are characterized and compared in terms of nutrients (potassium K and phosphorus P) and heavy metals (cadmium Cd, chromium Cr, copper Cu, nickel Ni and zinc Zn) contents. The advantage of thermal gasification over incineration is highlighted. The ashes were separated into a number of particle size fractions using wet sieving and vacuum filtration. Ash particle size varied between 1 to 250 μm for gasified pig manure ash. Extractable concentrations of the above mentioned elements were measured in raw ashes, ash fractions and untreated dewatered pig manure (fuel) using extraction with water, HNO3 and H2SO4 in combination with flame atomic absorption spectrometry (AAS) and colorimetric spectrometry. Total concentrations were measured using X-ray Fluorescence (XRF). Incinerated pig manure ash contained the highest heavy metal concentrations (with Cd exceeding 115ppm), 55500 ppm P and 10000 ppm K. Respectively from secondary and tertiary cyclones, thermally gasified pig manure ash contained 49000 ppm and 59000 ppm P and 13400 and 13800 ppm K. Cd content in both ashes did not exceed 75 ppm. Solubility of K was around three times higher in gasified than in incinerated pig manure, but incinerated pig manure shown much higher water extractable P (around 25 times more). Ni, Cr and Cd were found in the highest concentrations in the fine ash fractions (<30μm of particle diameter), whereas K, P, Zn and Cu dominate in coarser fractions (>30μm of particle diameter). All values are as extracted using HNO3.
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