open access

https://doi.org/10.14943/doctoral.k12909

https://hdl.handle.net/2115/71371

正浸透法による加水分解尿からの栄養塩回収システムの開発

英語

2017-09-25

Hokkaido University

91p

Human urine is nutrients-rich resource as it contains the major part of nutrients e.g. nitrogen, phosphate, potassium found in domestic wastewater therefore, urine has the potential to be reused in agriculture as a liquid fertilizer. Large quantities of urine are produced continuously, especially in populated areas, making available a continuous supply of nutrients. However, urine contains 95% of water giving bulky volume and low concentration of nutrients. So, urine storage and transport prior to application in farmland is non-economically competitive with chemical fertilizers and renders its reuse challenging. Hence, urine volume reduction and nutrients concentration appear to be necessary. Earlier studies suggested 80% volume reduction as a minimum requirement tobe cost effective. Several volume reduction techniques were reported in the literature. However, they are all energy demanding processes, making the concentration of urine non-advantageous. Forward osmosis process is an emerging Technology used in several applications for volume reduction and concentration and is reported to consume less energy than other concentration alternatives e.g. reverse osmosis, evaporation… In this study, we propose a nutrients recovery system with urine volume reduction by forward osmosis process. The FO volume reduction technic is an osmotic pressure driven process where water molecules move across a semipermeable membrane from a feed solution fo low solute concentration to a draw solution of high solute concentration. Moreover, solutes in the solutions can diffuse from one compartment another by their concentration differences. The objectives of this research were to develop a mathematical model for water and solutes flux estimation during urine volume reduction and to design a nutrient recovery system with forward osmosis concentration process. In chapter 1, the problems on sanitation all over the world, potential of urine as a fertilizer, crisis of natural resources, resource oriented sanitation, forward osmosis applications and the phenomena involved in the membrane separation process were reviewed. The issues that should be assessed were identified and the objectives of the thesis were summarized. In chapter 2, the phenomena that occur during forward osmosis process were studied. Experiments were carried out 1) to assess water flux performances of real and synthetic hydrolyzed urine, 2) to evaluate solutes diffusion, 3) to assess the adequacy of solutes activity for the calculation fo water flux, and 4) to identify the major solutes for water flux estimation during the volume reduction process. Hydrolyzed synthetic urine and hydrolyzed real urine were used as feed solutions, and sodium chloride solution with concentration range of 3-5 mol/L was used as draw solution. The solutes activities were calculated with PHREEQC from the molar concentrations. As a result it was found that: 1) the volumes of real and synthetic hydrolyzed urine could be concentrated to 2-5 times with 3-5 mol/L sodium chloride solution, 2) ammonia and the inorganic carbon in urine easily diffused to draw solution through the membrane, 3) solute activities in the feed and the draw solutions were suitable for the estimation of the osmotic pressure, and 4) the organic matter presented in real hydrolyzed urine had a negligible effect on the osmotic pressure variation. In chapter 3, a multicomponent mathematical model was developed to describe the phenomena occurring during forward osmosis process. The model considered the advection, diffusion and activities of the solutes in feed and draw solutions through a semi-concentration variation across the membrane and in the bulk solutions. The finite difference approximation of the partial derivatives was applied to numerically solve these equations, then the differential equations were discretized with Crank Nicholson scheme. The obtained systematic non-linear equations were solved with the Newton-Raphson method at each time step. The solutes diffusivities and pure water permeability of the membrane were required for the simulation. These parameters were calibrated by the experimental data with single salt solutions as draw solutions and pure water as feed solution. The experimental conditions were simulated and compared with the experimental results. Least square method with Nelder mead algorithm was used to find the best fit of the volume and concentration curves for the diffusivities estimation. The model was later validated by comparing simulated and other forward osmosis experiments results using synthetic hydrolyzed urine and sodium chloride draw solution. The important outcomes of this research are that: 1) the simulation of the model was succeeded to estimate the evolution of volume and solute concentrations in both solution, 2) ammonia can diffuse from urine to draw solution and presented a lower concentration factor than feed solution volume reduction factor, and 3) the nutrients concentration profile inside the membrane was calculated to show the effect of the internal concentration polarization which reduced the osmotic pressure at the active layer surface to 35% of its initial value. In chapter4, a forward osmosis unit to be implemented for urine concentration was designed. The developed model was used to evaluate the required membrane area to concentrate hydrolyzed urine into 1/5 of its initial volume. To propose the design parameters the following points were assessed: 1) the required membrane area decreases with the increase of the initial draw solution volume set exceeds urine initial volume, 2) Ammonia concentration factor slightly increase with the importance of the initial draw solution concentration. At 5 times volume reduction levels of urine, 1.1 to 1.4 concentration factor of ammonia were obtained and 22.7 - 27.5% of ammonia could be recovered with 300 cm2 membrane areas. 3) The reduction of the area from 342 to 56 cm2 enhanced the concentration factor of ammonia that increased from 1.4 to 3.9. To reduce 5 liters of urine to 1/5 in 12 hours operation we suggested a membrane area of 280 cm2 and a draw solution volume of 5 L with the osmotic pressure of 32.6 MPa. In chapter 5, the main findings and recommendations related to the application of forward osmosis process for a concentration of urine liquid fertilizer in agriculture were summarized.

2017-09-25

甲第12909号

博士(工学)

(主査) 特任教授 船水 尚行, 特任教授 高橋 正宏, 教授 五十嵐 敏文, 助教 伊藤 竜生

工学院（環境創生工学専攻）

博士論文

Version of Record