Achieving Less than 0.050 mg P/L Reliably with Active Chemical Sludge

A pilot demonstration unit was operated at the City of Coeur D'Alene, ID wastewater treatment plant to meet future low level phosphorus limits (<0.050 mg P/L). To meet such low levels, the City operated different technologies that investigated a combination of biological P removal complemented with chemical P removal. This study focuses on the latter with emphasis on the use of active chemical sludge to remove P upstream of filtration in a mixing tank.Conventional filtration practice wastes the metal salts solids by-products with each backwash cycle. Rather than wasting the sludge, the pilot demonstration unit discovered the potential to buildup the active chemical sludge to remove soluble orthophosphate (sOP). The active chemical sludge is recycled to a mixing tank upstream of the filtration unit. This operational strategy leverages any remaining sOP removal potential in the active chemical sludge. By removing additional sOP, alum dosing can be reduced and in turn minimize chemical costs.The study's objectives were to build on the pilot results by determining the active chemical sludge sOP removal at different temperatures and P levels. Jar tests were carried out at low and high levels for both temperature (4 and 15 degrees C) and sOP levels (5 and 50 mg P/L spike). At the lower dose level (5 mg P/L), there was no significant difference in sOP removal rates at the low and high temperature (averaged 0.33±0.01 lb P removed/lb sludge/d). In contrast, the high dose sOP (50 mg P/L) found that active chemical sludge removes more sOP at lower temperatures (0.44 versus 0.27 lb P removed/lb sludge/d) and the sludge was exhausted after a single dose of sOP (50 mg P/L).Overall, the pilot and jar test results are promising for reducing chemical costs while meeting low level P limits. Future work is required to evaluate the chemical demand reductions associated with this approach and the scale-up to full-scale applications.