Bromley on Membranes: Assessing Membrane Technology
The membrane industry continues to drive the use of membranes for all- encompassing applications. Although there are numerous benefits to using membranes for ultrapure water applications, membranes diminish cost efficiency when used as the first barrier. In particular, they are not well suited for microfiltration applications.
Scaling and biofouling problems continue to plague this technology. When the membrane acts as the initial particle separation step, these fouling problems are even more exasperated.
Recent literature is now very clear on this problem. Frost and Sullivan reported in their recent 2006 assessment of the membrane market that:
- Membrane technology supporters must address the technical and cost issues associated with membrane use. Of these issues, the fouling of membranes by rejected chemicals and microbes continues to demand considerable attention from the research community.
- Recovery rates are approximately 80%, and membrane systems that tout higher recovery rates end up with more fouling issues due to the higher concentration of solids in the concentrate.
- Significant research is being done to understand membrane fouling though several autopsies, which suggest there are a number of factors contributing to membrane fouling.
- Typically, over the life of a membrane system, operating costs become a greater expense than the purchase price.
A 2004 paper authored by Li and Elimeilech in Environmental Science and Technology showed that natural organic material (NOM) is considered a major component of ambiotic membrane fouling in water separation applications. Typically, the negative charge of most membranes repels like-charged organic substances; however, calcium greatly enhances NOM fouling. The size of NOM compounds can also affect fouling. For low pressure membranes (microfiltration membranes), inorganic particles such as silica will foul membranes. Many manufacturers have recognized this for years, yet the lack of understanding of the chemical and physical interactions simply has not allowed a clear view of membrane fouling.
A detailed 2005 review of membrane fouling was prepared by the American Water Works Association’s Membrane Technology Research Committee. The paper, entitled “Recent Advances in Research Needs in Membrane Fouling,” identified a number of problems with the fouling of membranes, including:
Scaling and biofouling. Biofouling is a major problem because it leads to higher operating pressures, the need for frequent chemical cleaning, membrane deterioration and compromised product water quality.
Poor cleaning techniques. These are developed more by trial and error than on a research basis. Use of air is common. There have been attempts to use vibratory or ultrasonic cleaning of membranes, but the polymeric materials have not held up well.
Soluble microbial byproducts. These have a profoundly negative effect on membrane performance and should be removed prior to membrane use to reduce membrane fouling. One of the methods is chlorination, but typically many membranes, particularly thin-film composite membranes, have a low tolerance to oxidants. The other approach is to use monochloramine, but it can be a problem with membranes in the presence of ferrous iron.
Incomplete understanding of the fouling process. The nature of the foulants differs not only with the source water, but also with the type of membrane process applied (low versus high pressure). The article goes on to say that the critical flux concept remains relatively unproven under practical operating conditions in low pressure systems, and the introduction of submerged membrane technology raises new research questions with regards to the use and optimization of air sparging systems.
Another article by Howe and Clarke (AWWA Journal, April 2006) focused on pretreatment through coagulation. This article confirms the need for high dosages of coagulant to improve membrane performance. Dosages as high as 30 to 210 mg/L for alum were found to improve membrane performance. This information coincides well with the need to manage total organic carbon (TOC).
If the objective is to remove TOC, high levels of coagulant are recommended.
Nonetheless, there are two fallacies in the rationalization to use more chemistry. First, if we use a disinfectant such as UV, the need for TOC removal is not as important and increased chemistry is not necessary.
So who is driving these solutions? For years, we were told as professionals that one of the benefits of membranes was the reduced use of chemistry. Yet chemistry usage is much greater with membrane technology, either through the cleaning process, the reduction in biofouling or the need to pretreat to improve membrane performance.
When membrane manufacturers are questioned on this fact, they divert to the suggestion that increased chemistry is a result of the disinfection byproducts rule (i.e., there is a need to reduce TOC with chemistry to satisfy the need to reduce disinfection byproducts such as trihalomethanes). To put it succinctly, to ensure good membrane performance, TOC has to be reduced via the use of high amounts of chemistry (known as enhanced coagulation).
The Howe Clark study revealed that the use of alum with prefiltration provided excellent pretreatment for membranes. In addition, they also found that the use of a cationic polymer as a coagulant aid plus alum improved permeability significantly. Finally, the other significant observation was that “in nearly every experiment with the raw water, the ability to remove particulate matter reduced the fouling potential.”
If you intend to use membranes for microfiltration in such applications as potable water treatment or prefiltration for nanofiltration and reverse osmosis applications, expect to use high levels of chemistry. You will need enhanced coagulation levels of chemistry for pretreatment to prevent organic fouling (i.e., 30 to 200 mg/L of alum or ferric sulphate) and significant clean-in-place chemistry. In addition, do not be surprised with short time frames between membrane module replacements.