Sequencing biological acidification of waste activated sludge aiming to optimize phosphorus dissolution and recovery

Phosphorus (P) recovery in wastewater treatment plants (WWTP) as pure crystals such as struvite (MgNH4PO4.6H2O), potassium struvite (KMgPO4.6H2O) and calcium phosphates (e.g. Ca3(PO4)2) is an already feasible technique that permits the production of green and marketable fertilizers and the reduction of operational costs. Commercial crystallizers can recovery more than 90% of soluble P. However, most of the P in WWTP sludge is unavailable for the processes (not dissolved). P solubilization and separation are thus the limiting steps in Pcrystallization. With an innovative two-step sequencing acidification strategy, the current study has aimed to improve biological P solubilization on waste activated sludge (WAS) from a fullscale plant. In the first step (P-release), low charges of organic waste were used as co-substrates of WAS pre-fermentation, seeking to produce volatile fatty acids to feed the P-release by Polyphosphate-accumulating organisms (PAO), while keeping its optimal metabolic pH (6 to 7). In this phase, milk serum, WWTP grease, urban organic waste and collective restaurant waste were individually applied as co-substrates. In the second step (P-dissolution), pH 4 was aimed as it allows the dissolution of the most common precipitated species of P. Biological acidification was performed by white sugar addition, as a carbohydrate-rich organic waste model, which was compared to chemical acidification by HCl (12M) addition. With short retention times (48-96 hours) and without inoculum application, all experiences succeeded on P solubilization (37-55% of soluble P), principally when carbohydrate-rich co-substrates were applied. Concentrations from 270 to 450 mg P-PO4 3-.L-1 were achieved.