Heavy metal and phosphorus content of fractions from manure treatment and incineration

Alternative uses of pig manure are being considered, including separation and eventual incineration of the solid fraction to produce energy and ash. The efficiency of a screw press, a decanting centrifuge and chemical treatment in transferring N, P and heavy metals from slurry to a solid fraction were compared. Chemical treatment by coagulants and flocculants removed heavy metals most efficiently; they were transferred to the solid fraction in the order Zn > Cu > Cd by all three types of equipment. With centrifugation the heavy metal load on land where the solid fraction was applied was very low, whereas on land where the liquid fraction was applied it was only slightly less than that from un-separated manure. Conversely, chemical treatment resulted in a heavy metal load similar to that from un-separated manure with the solid fraction, while with the liquid fraction it was reduced to 20% of that from un-separated manure. Incineration of the solid fraction produces bottom ash and fly ash containing high levels of P. Most of the P and less than 10% of Cd is present in the bottom ash, producing an ash low in Cd content and a fly ash high in Cd. However, Cu and Ni tend to accumulate in the bottom ash. Chemical extraction procedures revealed that P-availability was high in all liquid and solid fractions except the bottom ash from incineration where approximately 80% of the P was transformed into a form of apatite. Since more bottom ash than fly ash is being formed, significant amounts of P may be immobilized by incineration of solid fractions.