Thesis Archive
Phosphorus Recovery from Septage Treatment Plant Sludge by Struvite Formation
Nochefranca, Cristine Khay L.
Oyson, Zcelina Kristle C.
San Pedro, Anna B.
Abstract:
Phosphorus derived from phosphate rock which is a limited natural resource is expected to be depleted within the next few decades especially since these are mostly used as a fertilizer in the agricultural industry. Given the dependence of the Philippine agricultural sector on the importation of fertilizer, the recovery of N and P in the form of a fertilizer is seen as an attractive option. Hence, this study aimed to determine the potential of septage sludge in Metro Manila as a source for phosphorus recovery through the precipitation of struvite (MgNH4PO4·6H2O), a slow release fertilizer. The septage sludge used for struvite crystallization was characterized in terms of its magnesium, ammonium, and phosphate content. The septage sludge was first characterized, having an average TP of 235.05 mg/L and an average NH3-N of 74 mg/L. The effects of metal ions such as Ca2+, Al3+ and Fe3+ competing with Mg2+ in the precipitation process were also assessed. To maximize the recoverable nutrients in the feed, the ability of acid and alkaline hydrolysis on the release of ammonium and phosphate was investigated and compared. After alkaline and acid hydrolysis, the phosphate concentration increased from 17.05% to 46.88% and 5.28% to 73.81%. In the precipitation proper, the pretreatment utilized was acid hydrolysis due to its higher phosphorus release compared to alkaline hydrolysis. To determine the optimum conditions for struvite formation, lab-scale experiments were conducted to assess the effects of the parameters pH, Mg:P molar ratio and residence time on percent phosphorus recovery. Box-Behnken response surface methodology was performed in the Design Expert 7.0 software to determine the parameter levels that would result to the optimum percent phosphorus recovery. Upon the addition of MgCl2·6H2O the Mg2+ source in struvite precipitation, XRD and SEM results showed that the samples precipitated are amorphous forms of phosphates with diameters ranging only from 1-6.5 μm. Such results are due to the possible decomposition of struvite at a drying temperature of 70ºC and the high amount of metal ions.
Adviser:
Orbecido, Aileen H.