Abstract:
The anaerobic digestion of swine waste is a widely used practice to reduce the pollutant
load and to abate the odour problems, while producing biogas for many farm
operations. The resultant liquor is rich in phosphate and ammonium as well. These
compounds are the principal pollutants associated with surface water pollution. The
reduction of phosphate and ammonium to the acceptable limits is a prerequisite for the
disposal of effluent. As both these elements are highly sought material in agriculture,
recovery of those not only prevents the pollution, but also reduces the excessive use of
natural resources. The potential of nitrogen and phosphorus recovery from swine waste
biogas digester effluent through pH and NH4+: Mg2+: P 0 43' molar ratio amendment was
studied. Precipitation experiments with swine waste biogas digester effluent were
conducted at pH 7.5, 8.0, 8.5 and 9.0 together with NH4+: Mg2+: P 0 43' molar ratios 1:
0.2: 0.08, 1: 1: 1, and 1: 1.5: 1.5. Chemical and X-ray diffraction (XRD) analysis were
done to determine the composition of the precipitate. Scanning Electron Microscopy
(SEM) was done to determine the crystal structure of the precipitated product. The
highest removal and recovery of NH4+ and P 0 43' were achieved at pH 9.0 in each
experiment. The elevation of pH to 9.0 alone could decrease the initial P 0 43'
concentration from 42 mg L'1 to 4.7 mg L'1 and 89.2% P 0 43' recovery was achieved.
The pH-molar ratio combination 9.0-1: 1.5: 1.5 reported 76.5% NH4+ and 68.5% P 0 43'
recovery. The molar ratio of 1: T. 1 together with pH elevation to 9.0 resulted over 70%
and 97% of the initial N H / and P 0 43' removal respectively and it was determined to be
the optimum combination for both NH4+ and P 0 43' removal. It is found that nitrogen
and phosphorus could be recovered from biogas digester effluent. The XRD patterns
and the SEM images proved that the precipitated product is struvite.
