טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAvraham Green Nazima-Neta
SubjectExamination of Backwash in Membrane Separation Processes for
Water and Wastewater Desalination
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Carlos Dosoretz
Professor Emeritus Raphael Semiat
Full Thesis textFull thesis text - English Version


Abstract

Membranes’ fouling is an unavoidable phenomenon in water and wastewater desalination. There are four main ways to reduce the fouling from the membrane surface: pretreatment, chemical treatments, physical treatments and physicochemical treatments. The aim of this research is to examine a new physical treatment - osmotic backwash for RO systems. The osmotic backwash process takes advantage of the high osmotic pressure in RO systems for the benefit of removing fouling from the membrane surface. The first part of the research included the examination of parameters which may be controlled in the RO and in the osmotic backwash process. The parameters which were examined are: feed concentration, RO operating pressure, feed flow rate, RO operating pressure during the osmotic backwash process and the application of mechanical pressure from the permeate side to the feed side of the membrane. Examination of the effect of feed concentration on the accumulated backwash volume shows that the accumulated backwash volume increases with the raise of the feed concentration until a certain concentration and then any rise in the feed concentration leads to a decrease in the accumulated backwash volume. The explanation for this behavior is probably the existence of a secondary boundary layer on the permeate side of the membrane. During the backwash process water transfers through the membrane from the permeate side of the membrane to the feed side of the membrane. The salt in the permeate side accumulates on the membrane surface creating a boundary layer which is concentrating throughout the backwash process. Examination of the influence of the operating pressure during the RO process showed that as the RO pressure increased the accumulated backwash volume increased slightly too. The changes in the feed flow did not show any influence along the flow range examined (200-600 L/hr). As the RO pressure during the osmotic backwash increased the accumulated backwash volume decreased. Examination of  the influence of pressurized permeate during the backwash process in addition to operation of RO pressure during the backwash process found that as the difference between the operating pressure difference and the osmotic pressure difference across the membrane was more negative - the accumulated wash volume increased.

The second part of the research examined the efficiency of removing scaling of CaCO3 and Mg (OH)2 from the membrane surface after operation of RO process. The osmotic backwash had a little success in cleaning the membrane.