Water & Wastewater Asia May/June 2019

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46 | INSIGHT and a more pronounced negative surface charge. TEST PROCEDURE (MATERIAL AND METHODS) The tests were conducted using 4” lowpressure RO elements on an automated laboratory test bench. The test compared the performance of a highly crosslinked Lewabrane ® membrane with that of a membrane that differs in only minor ways in terms of data sheet specifications. A test pressure of 10.3 bar and a recovery of 15 per cent were used. The temperature was varied in the range of 15-35°C, and the pH between 3 and 11. The composition of the multi-component inflow water used in the tests is listed in Table 1. In this investigation, the responses are the permeate flux, the rejection of the total dissolved solids in the water (TDS rejection) and the rejection levels for nitrate, boron and silicon dioxide. The factors are always the temperature of the multi-component feed water and the pH. Statistical evaluations are used to determine which of the terms in equation 1 are statistically significant. Statistically significant factors are then labelled “effects.” The term with the product of x1 and x2 is called the “interaction.” DoE allows the opportunity to identify interactions. Where there is an interaction, the impact of one factor depends on the value at which the other is set. When the model functions have been determined, they can be used to calculate membrane performance. This happens across the entire test range, even for points that have not been measured explicitly. RESULTS Fig. 3 shows the permeate flux in the form of the flow rate per membrane area of the Lewabrane ® membrane and the comparison membrane. The flux increases as the temperature rises, which can be explained by the reduction in water viscosity. The pH has no significant impact on the flux. Fig. 4 compares TDS rejection for the various membranes. Rejection in both membranes drops significantly at extreme pH, which can be attributed to the dissociation of the various dissolved substances. The charge of the membranes is also important and varies with the pH. Notably, the Lewabrane ® membrane exhibits excellent rejection across a larger range and the drop in rejection is less pronounced at higher and lower pH values. Table 1: Information about multi-component inflow water DESIGN OF EXPERIMENTS DoE offers advantages when an investigation aims to examine the impact of two or more test parameters, or factors, on one particular target value. DoE is based around the principle that the settings of the various different factors can be changed simultaneously, whereas conventional investigations change one factor at a time. Fig. 3: Flux in relation to pH value and temperature To assess which factors impact on the target value, regression analysis is used to adapt a model function to the test results. This means trying to describe the target values (y) as accurately as possible as a function of the factors (e.g. x1, x2) using a quadratic function, for example. The target values are also called “responses.” y=β_0+β_1*x_1+β_2*x_2+β_11*x_1^2+β _22*x_2^2+β_12*x_1*x_2 (1) Fig. 4: TDS rejection in relation to pH and temperature Water & Wastewater Asia • May / June 2019
INSIGHT | 47 Fig. 5 shows the results for boron rejection. When the pH is high, boric acid is primarily present in ionised form, so the negatively charged membrane rejects it very efficiently. This is no longer the case when the pH falls below 9. The Lewabrane ® membrane is able to maintain greater rejection at lower pH values, which can be attributed to the highly crosslinked and less charged membrane. The level of boron rejection is also influenced by the temperature. Higher temperatures improve boron permeability, which leads to a slight reduction in boron rejection. Nitrate rejection is also important for the production of drinking water. Analysis shows that both RO membranes achieve outstanding nitrate rejection at pH values greater than 7, even though there is a very high nitrate concentration in the feed (200 mg/l) (Fig. 6). Fig. 5: Boron rejection in relation to pH and temperature At lower pH values, the membranes have a lower negative charge, or are even positively charged, as soon as their isoelectric point is reached. The change in the membrane charge affects nitrate rejection, as nitrates can pass through the membrane more easily at lower pH levels, leading to diminishing rejection. Here, the reduction in nitrate rejection is less pronounced in the Lewabrane ® membrane and rejection falls as the temperature rises. Silicon dioxide and silica rejection is important for treating boiler feed water. Removing silica can extend the service life of the ion exchanger as an additional treatment, resulting in fewer regenerations. Analysis of the silicon dioxide rejection (Fig. 7) shows that rejection using the Lewabrane ® membrane is higher, especially at mid-range pH values. This highlights the outstanding effectiveness of the highly crosslinked Lewabrane ® membrane. Fig. 6: Nitrate rejection in relation to pH value and temperature At present, ammonium rejection cannot be clearly described using a quadratic model. Experiments are currently being conducted to examine whether non-linear regression can describe this behaviour better. The resulting innovative findings can further make a valuable contribution to optimising engineering software in the future. All images are credited to LANXESS. Fig. 7: Silicon dioxide rejection in relation to pH and temperature Water & Wastewater Asia • May / June 2019
Page 1 and 2: MICA (P) No: 120/05/2018 • ISSN:
Page 3 and 4: spotlight 2019 SINGAPORE INTERNATIO
Page 6: EDITOR’S NOTE SUSTAINABILITY AND
Page 9 and 10: NEWS | 7 Water & Wastewater Asia
Page 11 and 12: NEWS | 9 Introducing an online comm
Page 13 and 14: Drinking Water Disinfection for Chi
Page 15 and 16: GRUNDFOS SPECIAL | 13 to optimise p
Page 17 and 18: GRUNDFOS SPECIAL | 15 Grundfos Dema
Page 19 and 20: MYANMAR WATER PORTAL | 17 Working t
Page 21 and 22: IN PERSON | 19 commercial feasibili
Page 23 and 24: IN PERSON | 21 At the recent IE Exp
Page 25 and 26: IN PERSON | 23 Danfoss Pumps a show
Page 27 and 28: FROM THE GROUND | 25 Water & Wastew
Page 29 and 30: FROM THE GROUND | 27 Mueller’s Mi
Page 31 and 32: FROM THE GROUND | 29 The United Nat
Page 33 and 34: FROM THE GROUND | 31 In Houston, wh
Page 35 and 36: Based on the Kaplan turbine design
Page 37 and 38: INSIGHT | 35 By Roger Lah, Cla-Val
Page 39 and 40: INSIGHT | 37 In effect, the hydraul
Page 41 and 42: INSIGHT | 39 With the opening of th
Page 43 and 44: INSIGHT | 41 Utilisation of biogas
Page 45 and 46: INSIGHT | 43 IntelliSAW is well kno
Page 47: INSIGHT | 45 Using DoE to improve t
Page 51 and 52: INSIGHT | 49 WWA: How is process sa
Page 53 and 54: OPINION | 51 Thankfully, response t
Page 55 and 56: OPINION | 53 Termed as low impact d
Page 57 and 58: MANILA BUSINESS MISSION 2019 18 to
Page 59 and 60: SWA WELCOMES NEW MEMBERS (joined fr
Page 61 and 62: SHOW PREVIEW | 59 Pump & Valves Ind
Page 63 and 64: SHOW REVIEW | 61 GOBLUE4SG The GoBl
Page 65 and 66: EVENTS CALENDAR 2019 JANUARY WFES W

Water & Wastewater Asia May/June 2019

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