Abstract Phosphorus crystallization-filtration (PCF) was devised as a novel tertiary process for phosphorus removal from domestic wastewater. The results obtained showed that during the PCF process, high pH and excessive calcium dosage conditions were required to obtain effluents with total phosphorus (T-P) and suspended solid (SS) concentrations below 0.2 and 10 mg/L, respectively, within 2 h of operation. Phosphorus was precipitated during the pre-treatment step, and thereafter it crystallized on the surface of the fixed seed material in the PCF reactor. Furthermore, the addition of Ca2+ resulted in phosphorus removal efficiencies >95%, and pH, residual Ca2+, filtration depth, and linear velocity were identified as the main design and operation parameters of the PCF process. Following the pilot-scale PCF process, the average concentrations of T-P, PO4-P, and SS in the effluent were 0.05, 0.04, and 1.1 mg/L, respectively, corresponding to removal efficiencies of 90.9, 86.5, and 79.7%, respectively. The investigation of the backwashing sludge characteristics of the PCF process using SEM, FT-IR, EDS, and XRD analyses showed that owing to its high contents in calcite and hydrated phosphorus compounds, PCF sludge could be used as an alternative soil amendment resource.

The struvite crystallization process can recover struvite crystals as a valuable slow-release fertilizer from the side stream of wastewater treatment plants (WWTPs). The purpose of this study is to demonstrate the crystal growth characteristics and determine the appropriate recovery criteria for a struvite crystallization pilot plant. A pilot plant (8.6 m3/d) was designed with a feeding system of MgO (magnesium oxide), a pH controller, and a hydrocyclone for recovering struvite; the plant was operated for 42 hours at a pH range of 8.25–8.5. The removal efficiencies for PO4-P and NH4-N were 82.5–90.7% and 13.4–22.9%, respectively. The struvite recovered from the hydrocyclone was sifted using standard sieves and analyzed by SEM and XRD. The dry weight fraction of the precipitate in the 300–600 μm range increased gradually from 7% to 74% in 18 hours. The XRD analysis revealed that the crystalline structure of the precipitate in the 150–600 μm range indicates struvite without any peaks of MgO, Mg(OH)2, and MgCO3. This indicates that the critical conditions for recovering struvite from the side-stream of WWTPs are an operation period of 18 hours and a crystal size greater than 300 μm.

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In this study, we have made the annual total phosphorus (TP) mass balance diagrams for I wastewater treatment plant by utilizing the data of flow rate and TP of each process and tried to choose the optimum unit process empirically for phosphorus recovery. For the applicability evaluation, we have suggested several quantitative indices of flow rate, TP concentration, TP load and SS. Based on the analyses of TP mass balance, it became clear for reducing TP load of the wastewater treatment plant that it is efficient to recover phosphorus from the side stream in which the amount of flow rate is just 1/16, but TP concentration and load are 78 and 4.8 times larger than those of the influent of the plant. After the detailed applicability evaluation for the side stream, it could be concluded that the unit process of waste activated sludge thickener supernatant or dehydration filtrate are appropriate. Meanwhile, we did fundamental experiments utilizing the dewatering filtrate with TP concentration of 141.5 mg/L. After adjusting pH 10 and Ca 2+ concentration 250, 500, 1000 mg/L, it was stirred slowly. As a result, the PO4-P and TP removal efficiencies were above 95 percent; the results of the experiment imply the applicability of phosphorus recovery process in a wastewater treatment plant strongly.

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