Phosphorus Recovery From Sewage Water By Layered Double Hydroxides As An Adsorbent And By Vivianite Precipitation

Two innovative technologies were delevoped in this study for recovering phosphate from municipal and rural sewage respectively based on their different characteristics and treatment methods.For the phosphate-rich sludge filtrate in municipal wastewater treatment plants, ZnAl layered double hydroxides (LDHs) were prepared and employed for phosphate removal and recovery. Synthesis conditions of LDHs, adsorption parameters and La-amended LDHs were investigated in order to improve the phosphate adsorption capacity of the ZnAl LDHs. Methods of phosphate desorption and recovery were then studied for the phosphate-loaded LDHs. In addition, the relationship between LDHs structure and the phosphate adsorption capacity, phosphate adsorption kinetics and isotherms were investigated to explore the mechanisms of phosphate uptake by the LDHs.The results indicated that phosphate adsorption capacity of LDHs was greatly affected by the compositon of metal ions in LDHs and the synthesis conditions. Among the different LDHs prepared, the highest phosphate adsorption capacity was achieved by the ZnAl with a Zn:Al molar ratio of 2:1, when the adsorbent was synthesized at 70°C, using 20% NaOH as the coprecipitant, aged for 6 h and then calcined at 300°C. ZnAl-300 had a good stability and a high phosphate uptake in the water samples at pH 6~11. The amount of the adsorbed phosphate per gram of ZnAl was increased with an increase in the initial phosphate concentration and was decreased with an increase in the LDHs dosage. Agitation speeded up the phosphate uptake by the ZnAl-300. And anions in the sewage sludge filtrate imposed an interfering effect on the phosphate adsorption, especially at high concentrations.The phosphate uptake in 24 h by the ZnAlLa LDHs prepared at a Zn:Al:La molar ratio of 2.0:0.9:0.1 was 35.15 mg P·g-1, which was 41.9% higher than that by the ZnAl. XRD results showed that the addition of La lowered the crystallinity of the LDHs compound and increased the irregularity of the surface thus enhanced the phosphate adsorption. La atoms with big radius increased the interlayer distance of the ZnAl, promoting phosphate ions to enter the interlayer space for ion exchange. After calcination at 300°C, the phosphate uptake by the ZnAlLa in 24 h was 56.92 mg P·g-1, which was 1.48-fold higher than that by the uncalcined. The selectivity of phosphate adsorption by ZnAlLa-300 was marked improved compared with that by the ZnAl-300.5% NaOH could be a suitable solution for phosphate desorption from the used LDHs; the desorption efficiency was ~80.57%. Adding CaCl2 to precipitate phosphate in the alkaline desorbent as hydroxyapatite [Ca5OH(PO4)3] was an effective method to recover the phosphorus. The experimental results showed that 90% of the phosphorus was recovered when the pH, Ca:P molar ratio and reaction time were 10.5, 2.0 and 30 min, respectively.The kinetics of phosphate adsorption by all the ZnAl series of LDHs showed a good fit with pseudo-second-order equations. The adsorption kinetics suggested that the addition of La increased the affinity between phosphate ions and LDHs. The adsorption isotherms followed a Langmuir-type model and indicated that the phosphate adsorption by the ZnAlLa-300 was an endothermic process. By studying the relationship among the LDHs structure, the phosphate adsorption capacities and the adsorption characteristics, it was concluded that the phosphate adsorption by the ZnAl-300 was attributed to a combination of different mechanisms, e.g., surface adsorption, ion exchange, surface complexation and ion incorporation during the LDHs rehydration, etc.For the rural sewage in this study, the feasibility of phosphorus removal and recovery in septic tanks by iron reduction-induced vivianite precipitation was investigated. In an anaerobic sequencing batch reactor (ASBR), effects of operating paraments on the continous phosphorus removal were then evaluated. In addition, the subreactions involved in the phosphate removal was investigated by batch assays, the precipitates in the ASBR were analyzed and the mass balance of iron and phosphate was established to explore mechanisms of the iron reduction and the subsequent phosphate removal.The results showed that supplementation of the amorphous FeOOH (A-FeOOH) effectively removed soluble PO4-P from the septic wastewater. The phosphate removal was closely iron reduction-related, increased with an increase in the A-FeOOH dosage. The ASBR for continuous phosphate removal could be started up in around 10 d. At the Fe:P (molar ratio of the supplementated Fe(III) to the soluble phosphate in the influent) of 1.5, 45% of the soluble PO4-P was removed after the ASBR got stabilized; when the Fe:P went up to 3, the P removal efficiency reached 96%. During the periods with agitation rates of 100 and 50 rpm, the soluble PO4-P in the ASBR was efficiently removed with the iron reduction. When the agitation was stopped, both the iron reduction and the phosphate removal were incomplete. The iron reduction and the phosphate removal kept stable in the periods with the cycle time varying from 1~3 d. When the cycle time was shortened to 0.5 d, a slight influence on the iron reduction and the phosphate removal was observed. Decreases in the temperature to 18°C and in the organic loading rate to 0.2 kg·d-1·m-3 didn't affect the ASBR system.Studies on the subreactions involved in the anaerobic phosphate removal revealed that the soluble PO4-P could be adsorbed by both the A-FeOOH and the anaerobic sludge in the ASBR, but the adsorption was not responsible for the continuous phosphate removal in the system. The transformation of A-FeOOH to soluble Fe(II) was a biological reduction, and the phosphate removal in the ASBR was attributed to the biological iron reduction.A certain amount of ellipsoidal precipitates with dark-blue particles on the surface and scattered small dark-blue precipitates were collected on the bottom of the ASBR after 208 d of operation. The results of SEM-EDS and XRD analyses revealed that the dark-blue precipitates were vivianite crystals as a new solid phase in the reactor. By establishing a mass balance in the ASBR, the P and Fe accumulation calculated through the difference between their concentrations in the influents and in the effluents were close to the one by the HCl extraction and measurement.