Title: Recovery of Valuable Resources from Wastewater using a Fluidized Pellet Reactor and Electrodialysis
Other Titles: Herwinning van waardevolle grondstoffen uit afvalwater door middel van een gefluïdiseerde pellet reactor en elektrodialyse
Authors: Tran, Thi Kim Anh
Issue Date: 26-Nov-2014
Abstract: Water scarcity and natural resource depletion are important issues for achieving sustainable development. According to the United Nations, 1.2 billion people live in a situation of physical water scarcity and 0.5 billion approach this situation. Two-thirds of the world will be in a condition of water stress by 2025. In addition, due to the increasing demand arising from population growth, natural non-renewable resources will be further depleted in the coming years. A number of technologies have been recently investigated to achieve the objective of valuable resources recovery from wastewater in order to reduce water scarcity, resource depletion, and preserve the environment and reduce the cost of wastewater treatment. However, the question remains whether these technologies are efficient enough. The aim of this thesis is to propose a feasible integrated system to reclaim water from reverse osmosis concentrate for water reuse, to regenerate acid/base from wastewater with a high scaling potential and to recover phosphorous as calcium phosphate from municipal wastewater to achieve sustainable development.At first, the feasibility of a hybrid system consisting of a fluidized pellet reactor and an electrodialysis (ED) unit was studied to treat reverse osmosis (RO) concentrates for water reuse. A fluidized pellet reactor was used to remove the scaling potential before ED treatment. The performance of the fluidized pellet reactor on lab-scale was evaluated at superficial velocities of 48 m/h, 61 m/h and 73 m/h with an initial pH 11. The efficiency of calcium removal was between 70% and 80% and up to 95% when the pH was adjusted to 11.5. The wastewater after pellet crystallization was treated by ED to achieve the required characteristics for reinserting the liquid to the UF-RO system. Without the fluidized pellet reactor and using RO concentrate as feed of ED in feed and bleed operation, the ED was rapidly scaled and reached the maximum voltage. By removing 80% calcium in RO concentrate in the fluidized pellet reactor, the ED system can be operated in a stable way with high current efficiency and without scaling. The product of this hybrid system can be reused for UF-RO. Secondly, acid/base regeneration by bipolar membrane electrodialysis (EDBM) from industrial wastewater treatment with high scaling potential was studied. Two kinds of industrial wastewater from a metal production company were tested; one with high calcium concentration and one with high concentrations of both calcium and nickel. The results showed that a high removal efficiency, i.e., 90% of calcium (for the former wastewater) and 73% of calcium and 74% of nickel (for the latter) was obtained by adding sodium carbonate and sodium hydroxide to achieve calcium carbonate and nickel hydroxide precipitation at the optimum conditions in the fluidized pellet reactor. Mineral scaling was reduced both on cation exchange membranes and on bipolar membranes when operating EDBM with pretreated wastewater. After pretreatment, it was feasible to regenerate mixed acid and base using EDBM. The current efficiency was 69% for the acid, and 80% for the base; the energy consumption was 5.5 kWh/kg acid and 4.8 kWh/kg base at a current density of 60 mA/cm2 and an initial concentration of 0.2 N. In a long-term experiment, the pretreatment increased the performance of the cation exchange membrane; a high concentration of acid and base up to 1.76 N and 2.41 N was produced with only slight scaling on the cation exchange membrane surface. Finally, a promising and sustainable renewable source of phosphate was obtained by an integrated selectrodialysis/crystallization process, in which selectrodialysis was first used to pre-concentrate the phosphate before crystallization in a fluidized pellet reactor. An increase in current density, initial pH of the product, and initial feed concentration of phosphate led to an increase in phosphate concentration in the product of selectrodialysis. The phosphate concentration in the product can reach 16 mM with a purity of 44%, the current efficiency to transport phosphate and chloride was 26.6% for a standard anion exchange membrane and 63% for a monovalent selective anion exchange membrane. The effect of competing ions (nitrate, bicarbonate and sulphate) in wastewater on the phosphate pre-concentrating efficiency by selectrodialysis was also demonstrated. An inhibition of these ions to the phosphate concentrating efficiency was found to occur initially, but if the system runs for a longer time, the effect disappeared. The phosphate concentrating efficiency in the product was significantly reduced from 188% to 78% after 210 min, but this could be recovered to 161% after 300 min. In addition, when operating selectrodialysis with synthetic municipal wastewater, phosphate was concentrated with a twofold concentration compared to the initial concentration in the wastewater. Scaling on the cation exchange membranes and monovalent selective anion exchange membranes was observed during the selectrodialysis process when operating with synthetic municipal wastewater, but the membranes were almost completely recovered after a simple acid cleaning with 0.1 M HCl solution for 1 h. After pre-concentrating phosphate by selectrodialysis, phosphate was recovered as calcium phosphate in the fluidized pellet reactor, with a precipitation efficiency of 82.7% at pH 11, an inlet concentration of phosphates of 2.5 mM, a Ca/P molar ratio of 1.5 and a superficial velocity of 61 m/h. From the results, it can be concluded that the fluidized pellet reactor combined with electrodialysis is an appropriate method to recover water, acid/base from salts, and nutrients from waste streams, which is technically feasible, reducing the cost of wastewater treatment, preventing water pollution and yielding valuable and sustainable resources.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Process Engineering for Sustainable Systems Section

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