Mechanisms for removing phosphorus species through sequential coagulation using inorganic coagulants and organic polymers

The need for stringent phosphorus removal from domestic wastewater is increasing to mitigate eutrophication, while efficient phosphate reuse is critical due to the global phosphate crisis. Combining aluminum sulfate (ALS) with high molecular weight organic polymers achieved 95-99% removal of particles, turbidity, and phosphates, reducing ALS usage by 40%. We propose mechanisms to explain the enhanced treatment efficiency. Particle and turbidity removal is more influenced by polymer charge density than molecular weight, while orthophosphate (OP) removal is linked to a change in zeta potential from negative to positive, allowing additional OP binding through complex formation with hydrolysis products and polymers. Enhanced phospholipid (PL) removal likely results from adsorption and neutralization of micelle PL charges by intermediate positively charged aluminum hydroxyphosphate ions. Higher PL removal with low ALS doses is attributed to a two-stage dosing process that optimizes coagulant and polymer dosages. The combined removal of OP and PL improves phosphorus bioavailability, increasing the sludge's fertilizer value.