Study on the effects of phosphate concentration, pH, molar ratios and pre-treatment on phosphorus recovery from biosolids by a fluidized-bed crystallizer (FBC)

Autor: Nan-Hung Lin, 林南宏
Rok vydání: 2018
Druh dokumentu: 學位論文 ; thesis
Popis: 106
Phosphorus is a life element and also a nonrenewable-limited resource. Sewage biosolids contain rich phosphorus, how to reuse it efficiently will be an important task in the future. This study aims at phosphorus recovery from the synthetic wastewater and dewatering filtrate of Futien Water Resources Recycling Center in Taichung City by fluidized-bed crystallizers (FBC). The results of the crystallization tests showed that phosphate concentration was the greatest among the parameters on removal efficiency of phosphate (RP) and crystal efficiency of phosphate (CP), pH value was next, Me/P molar ratio was next again, and hydraulic retention time (HRT) had little effect. A synthetic wastewater PO4-P concentration of 100 mg/L at pH=8.5 and Mg/P=1.0, heterogeneous nucleation (with seed) was compared with homogeneous nucleation (without seed) using laboratory FBC. The heterogeneous crystallization was controlled under the metastable zone. After 4 hours, the average RP and CP were 75.3% and 68.2%, respectively. The homogeneous crystallization must first be controlled under a supersaturated state. The crystals grew and reached the maximum after 48 hours. The average RP and CP were 69.3% and 66.3%, respectively. The heterogeneous and homogeneous crystallization were described by the first-order kinetic model. The reaction constants of heterogeneous crystallization were greater than those of homogeneous crystallization, indicating that the rate of heterogeneous crystallization was faster than that of the homogeneous crystallization. In order to increase orth-phosphate content, the concentrated and digested sludge were pre-treated in the laboratory, including alkaline hydrolysis, ultrasonic hydrolysis and alkali combined ultrasonic hydrolysis. Phosphorus release degree (PRD) and phosphorus release rate (PRR) were evaluated for the perforrmance of hydrolysis. The results of tests found that the efficiency of alkali hydrolysis increased with increasing pH and pH of 10 was more efficient. The efficiency of ultrasonic hydrolysis was proportional to the operation time. Operation for 10 minutes was more efficient. For digested sludge, the performance of alkali (pH9) plus ultrasonic (US10min) hydrolysis was slightly better than that of alkali (pH10) plus ultrasonic (US5min) hydrolysis. Dewatering filtrate under PO4-P concentration of 50 mg/L, pH=8.6, and Ca/P=1.75 was conducted with HAP heterogeneous crystallization by laboratory FBC for 4 hours. The average RP and CP were 68% and 61%, respectively. The heterogeneous crystallization was described by the first-order kinetic model. In addition, dewatering filtrate PO4-P concentration of 25 and 100 mg/L were conducted with HAP heterogeneous crystallization by FBC module in actual plant at pH of 9.0 and Ca/P of 2.0. The average CP were 46.9% and 71.2%, respectively. However, they performed worse than the Lab. did at the same condition. Due to high concentration of SS in the dewatering filtrate of real plant, it interfereed withof the HAP crystallization process. According to the characteristics analysis of MAP and HAP heterogeneous crystal products, the results showed that the water content was less than 10%, the particle size was about 0.5-3.0mm, MAP and HAP were confirmed by SEM, EDS and XRD. By ICP analysis, the phosphorus contents were above 32,000 mg/kg and the heavy metal levels were below the regulatory limit for fertilizer usage. Therefore, recovery of phosphorus from biosolids in municipal wastewater shows significant potential in the future.
Databáze: Networked Digital Library of Theses & Dissertations
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