Blended high performance coagulant for phosphorus removal at primary stage
Gerardo, M., Gaskin, L. and Sullivan, M., Dwr Cymru Welsh Water, UK
(free)
This study focused on testing several commercially available coagulants for TP removal at primary stage including PIX122 (12.5% Fe3+), PIX511 (13.8% Fe3+), PAX18 (9% Al3+) and PAX-XL60 (7.5% Al3+). The jar-test highlighted that all coagulants were highly suitable for TP removal with up to 99% removal.
At certain dose rates of iron based coagulants, spikes of residual iron were measured as 65 mg/L when using PIX122. The comparison between PIX122 and PIX511 showed very similar profile for TP removal and residual iron, with PIX511 achieving slightly higher TP removal and lower residual iron around the 50 mg Fe3+/L dose rate. Both coagulants were shown to deplete pH and to require high dose rates of 120 L/h per 100 L/s crude sewage. Both Al coagulants showed very analogous TP removal and residual Al profile. Highest residual Al was 9 mg/L at dose rate of 11 mg Al3+/L with final TP measured as 0.3 mg/L for both PAX18 and PAX-XL60. Optimal dose rate of either product was determined to be 64 L/h for 100 L/s incoming flow. Poly-amide (PA) was tested as a co-coagulant with PAX18 at blends of 8% (PAX-XL3103E), 10% (PAX-XL3103F) and 15% (PAX-XL3103G).
The introduction of PA showed a substantial reduction in the residual Al particularly with a 250 mg/L dose rate. Removals using 250 mg/L of PAX-XL3103G were 93% TP, 95% ortho-P (OP), 67% COD, 66% BOD and 95 % solids. For any of the dose rates of PAX-XL3103G tested, residual Al was consistently below 2 mg/L. The performance of PAX-XL3103G at full scale was lower owing to conservative dose rates (55 L/h) however variable flow and load were likely to cause significant variation in the coagulation efficiency and increased solids carryover.
The financial assessment demonstrated that inclusion of PA increases costs significantly in contrast with PIX122 and PAX18, yet with much lower risk of residual Al carryover. Overall, PAX18 was determined to be the most cost effective solution however a significant risk of Al carryover was identified during jar-testing for dose rates below 21 mg Al3+/L.
Keywords: blend, coagulation, phosphorus removal, poly-aluminium chloride, polyamide,
