The fate of crop nutrients during digestion of swine manure in psychrophilic anaerobic sequencing batch reactors.

ABSTRACT

The objectives of the study were to measure the levels of manure nutrients retained in psychrophilic anaerobic sequencing batch reactors (PASBRs) digesting swine manure, and to determine the distribution of nutrients in the sludge and supernatant zones of settled bioreactor effluent. Anaerobic digestion reduced the total solids (TS) concentration and the soluble chemical oxygen demand (SCOD) of manure by 71.4% and 79.9%, respectively. The nitrogen, potassium, and sodium fed with the manure to the PASBRs were recovered in the effluent. The bioreactors retained on average 25.5% of the P, 8.7% of the Ca, 41.5% of the Cu, 18.4% of the Zn, and 67.7% of the S fed to the PASBRs. The natural settling of bioreactor effluent allowed further nutrient separation. The supernatant fraction, which represented 71.4% of effluent volume, contained 61.8% of the total N, 67.1% of the NH4-N, and 73.3% of the Na. The settled sludge fraction, which represented 28.6% of the volume, contained 57.6% of the solids, 62.3% of the P, 71.6% of the Ca, 89.6% of the Mg, 76.1% of the Al, 90.0% of the Cu, 74.2% of the Zn, and 52.2% of the S. The N/P ratio was increased from 3.9 in the raw manure to 5.2 in the bioreactor effluent and 9.2 in the supernatant fraction of the settled effluent. The PASBR technology will then substantially decrease the manure management costs of swine operations producing excess phosphorus, by reducing the volume of manure to export outside the farm. The separation of nutrients will also allow land spreading strategies that increase the agronomic value of manure by matching more closely the crop nutrient requirements.