Spent bio-slurry from “Adani” rice husk mixed with graded levels of cow dung in bio-gasification: Heavy metals identification and potentials

Abstract

Heavy metals identification and potentials of spent bio-slurry from “Adani” powdered rice husk (PRH) mixed with graded levels of cow dung (CD) in bio-gasification process was investigated. The waste complements include; 90:10, 80:20, 70:30, 60:40 and 50:50 PRH: CD, respectively. The PRH and CD separately served as control. Bio-wastes validity for the anaerobic digestion (AD) process based on physico-chemical properties and calorific values were determined using standard laboratory methods. All the wastes were gasified at ambient conditions using biogas plants of similar volume (48 L) for 30 days. The spent bio-slurry was evaluated for heavy metals (Ni, Cr, Cu, Zn, Mn, Pb, Fe and Ag) and macro elements (Ca, Mg and K) using atomic absorption spectrophotometer. Nutrient contents of undigested wastes showed reliability for the bio-gasification and the waste systems yielded total gas volumes of123, 127, 125, 141, 127, 246 and 291 liters for 90:10; 80:20; 70:30; 60:40 and 50:50 PRH: CD, PRH and CD, respectively. Onset of flammable gas ignition by digester systems were recorded as 15, 15, 11, 9, 6, 8 and 4 days, for the composite and single wastes, respectively. Concentrations of the heavy metals in the spent bio-slurry composite (90:10, 80:20, 70:30, 60:40 and 50:50 PRH: CD) ranged from 4.28- 21.37(Ni); 0.00-4.86 (Cr); 0.00-2.37 (Ag); 0.00-2.70(Cu); 0.00-10.42(Pb); 0.00- 9.46(Mn), 61.14-1,589.65(Fe)and 1.80-17.32 (Zn) ppm/g of sample, respectively. Concentrations for Ca range from 111.12-333.36; K: 9.67-35.45; Mg: 127.14-238.52 ppm/g of sample. Overall results showed that partially decayed ‘Adani’ powdered rice husk could generate biogas energy in single and co-digested forms for cooking and lighting, among other benefits. Secondly, the co-digested spent slurries contained low amounts of the heavy trace metals identified and have great potentials for application to organic farming and horticultural practices.