Recovery of phosphorus from chemical sludge
Ofeoritse, Ayaobu Cherish
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The shortage of pure phosphate rocks may be a limitation to the availability of food and other resources to man and animals. Understanding how phosphorus (P) can be recycled from chemical sludge to make-up for the shortage in availability of pure phosphate rocks is necessary as the world continues to survive. The inclusion of chemical sludge for nutrient utilization purposes in a well-drained Entisol (dark-grey sandy Gleyed Regosolic sand) was demonstrated in this work by applying dewatered sludge from Headingley Wastewater Treatment Plant, Canada. Growth room experiments were conducted to determine the effect of chemical sludge (alum phosphate sludge) as a P source to soil and this was compared to commercial fertilizer (monoammonium phosphate). Maize and switchgrass were examined for a 45- and 50-day cycle with 3 replications each. Low-P soil pots of 1.455kg were treated with different P application rates of 9.7, 19.4, 29.1 and 38.8 mg P kg-1 dry soil. For switchgrass, over the three growth cycles, sludge treated pots showed that P availability increased as the cropping cycle increased. The rate of P application to each pot showed no significant variations based on biomass generated in all cycles. Improved P availability was observed in sludge amended pots compared to monoammonium phosphate (MAP) treated and control pots. Switchgrass also experienced increase in aluminum uptake in the biomass as the cropping cycle increased. The application rate of 9.7 mg P kg soil-1 was identified as the best rate for switchgrass growth and P recovery in biomass. Low-P soil was also seeded with maize for four cropping cycles with each cycle lasting about 45 days. Soil pots were amended just once at the beginning of the first cycle and results were analyzed for dry matter yield, P uptake, aluminum uptake and phosphorus recovery efficiency at the end of each cycle. Results showed that sludge is as effective as MAP in P utilization for maize growth and P uptake was higher in sludge than in MAP. Biomass generated from switchgrass and maize could be harnessed as feedstock for biofuel, however, proper analysis of all forms of P and N in the sludge is necessary to prevent overdosing. Chemical sludge was also exposed to alkali solubilization where waste activated sludge (WAS) and dewatered sludge (DS) were treated with different strengths of sodium hydroxide (NaOH). Results highlighted the importance of floc content in the sludge as the solubilization is mostly dependent on the floc content. Statistical analysis showed that maximum sludge solubilization for P was achieved within the first 30min. Further alkali addition showed no significant release of P from the sludge however, P in resulting solution must be analyzed for its separation and recovery cost. All results indicate the potential possibility of reuse of chemical sludge to reduce the challenges around the availability of P rock and provides some insight into the usability of P in chemical sludge.
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