MechChem Africa

In this edited white paper, Uli Dölchow, Julien Ogier and Jens Lipnizki from chemical technology specialist, LANXESS, strive to better characterise the performance of reverse osmosis (RO) membranes.

Using appropriate design software to simulate the performance of RO elements in advance of practical implementation at a plant is common practice and generally useful. The actual operational performance of RO elements, however, depends on a whole range of different parameters, such as the temperature, pH and salt concentration of the water to be filtered. While these are taken into account by design programs, the calculations are based on pre-defined performance parameters such as permeate flow and salt rejection, which are determined under standard test conditions.

Lanxess membrade technology Lewabrane1These standard conditions are defined according to product classes, which might include: standard brackish water elements, low-pressure elements or other similar groups. The outlined test conditions generally define values for the operating temperature, pH, inflow pressure, recovery and the concentration of table salt (NaCl) in the feed water.

Natural water sources and industrial and municipal wastewater, however, generally contain a variety of salts and substances, which creates greater complexity. When considering natural water sources, which by their nature have a very diverse composition, operating conditions can have a huge impact on rejection from RO elements and the substances that remain dissolved in the water. The NaCl rejection figure given on data sheets cannot, therefore, be seen as a definitive value.

In addition, practical operating conditions also frequently differ from standard test conditions, primarily in terms of temperature and pH.

In this investigation, multi-component inflow water containing a variety of common substances was used, the goal being to apply statistical methods to examine the impact of temperatures and the pH on the permeate flux and the rejection of various dissolved substances. The original intention was to identify a new way of characterising membrane performance, but these results also offer a valuable contribution to optimising future engineering simulation software.

Surface effects, most notably, membrane charge, also play a role in relation to the analysis of rejection results, as they have a significant impact on salt rejection. Test results were used as a basis for examining whether a relationship could be established between the different performance characteristics of various membranes and a range of membrane structures (Figure 1).

The surface charge is determined by varying degrees of crosslinking during the polymerisation of the polyamide coating. The two components TMC (trimesoyl chloride) and m-PDA (m-phenylenediamine) make up the polyamide, which contains the structural elements 1 and 2 (Figure 2). Depending on how the polymerisation process is controlled, the result is either a highly crosslinked RO membrane with less surface charge, or one with less crosslinking and a more pronounced negative surface charge.

Test procedure, material and methods

The tests were conducted using 4-inch low-pressure 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% were used. The temperature was varied in the range of 15 to 35 °C, and the pH between 3 and 11. Compositions of the multi-component inflow water used in the tests are listed in Table 1.

In each area of testing, test conditions were varied in line with the design of experiments. The substances used were selected for their relevance to various applications.

Design of experiments

The experiments were planned and conducted in line with the design of experiments (DoE) methodology, which offers advantages when examining the effect of two or more parameters (factors) on one particular target value.

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