Improving phosphate removal of sand infiltration system using alkaline flyash

Septic tank effluent is customarily disposed of by soil infiltration. Coars e, sandy soil such as those found in Perth, Western Australia, exhibit low attenuation capabilities for phosphate (PO43-) during effluent infiltration . Amendment of such soil with different amounts of alkaline precipitator an d lagoon fly ashes was investigated as a means of reducing phosphorus (P) l eakage to ground water. Alkaline precipitator fly ash possessed the highest P sorption capacity in terms of its Langmuir and Freundlich isotherm param eters during initial batch tests. The test materials were repeatedly contac ted with fresh PO43- solutions over 90 contacting cycles to gain a better i ndication of long-term P sorption capability. Again, precipitator fly ash e xhibited higher P sorption capacity than lagoon fly ash and Spearwood sand. Column studies assessed the influence of various application rates of alka line precipitator and lagoon fly ashes on the P removal of septic tank effl uent. Septic tank effluent was applied at the rate of 4 cm/day to the colum n for 12 weeks. Concentrations of P were monitored in the column effluent. All the fly ash columns were more efficient in reducing P migration compare d to the sand column. Increased levels of fly ash in the soil columns resul ted in increased P attenuation. Lagoon fly ash was inferior to precipitator fly ash for P removal; high application rates of fly ash caused clogging o f the infiltration bed apparently due to their lower permeability. It is re asoned that 5-15% precipitator fly ash, and less than 30% lagoon fly ash co uld be added to coarse sands to produce an infiltration bed, which would re sult in a better quality effluent than can be obtained with untreated sand alone. (C) 2000 Elsevier Science Ltd. All rights reserved.