Phosphate requirement for anaerobic fixed film treatment of landfill leachate

Three laboratory model anaerobic fixed film reactors, AFFR-A, B, and C, fed by a pretreated leachate, were monitored at 35 ± 5 °C for 10 months to estimate the effects of different concentrations and the forms of phosphate (ortho, organic, or condensed) on performances at 1.2–1.8 g COD/(d∙L) of reactor volume. Ortho phosphate (Na3PO4) supplement was added to the feed of AFFR-A, organic phosphate (sodium glycerophosphate: C3H7Na2O6P∙5H2O) to AFFR-B, and condensed phosphate (Na4P2O7) to AFFR-C at a feed COD/P value of about 6100 for 23 weeks (Phase I). When no PO4 deficiency was observed, the value was increased to 7700 in reactors A and B, but the PO4 supplement was terminated for reactor C, resulting in a value of 64 300 (Phase II). The average COD of C effluent was 599 mg/L as compared to 451 and 442 mg/L for reactors A and B, respectively, suggesting that a COD/P of 64 300 was too high. During Phase III, the COD/P ratios were changed in reactors A, B, and C, respectively, to 10 200, 15 200, and 34 300. The results indicate that the "optimal" ratio of COD/P lies perhaps between 15 000 and 34 300. Anaerobes did not prefer any one form of PO4 over the other two. Key words: anaerobic treatment, phosphate requirement, ortho phosphate, condensed phosphate, organic phosphate, leachate treatment, fixed film reactor.