This post is a review of a paper reviewing the costs of membrane bioreactor (MBR) and reverse osmosis (RO) technology and how research could positively influence those costs.
"Membrane technology costs and me", by Dr. Simon Judd was published in Water Research in 2017. Before I discuss it, here is the abstract:
A reflection of the place cost analysis holds in membrane process technology research and development is provided. The review encompassed two membrane processes and applications: (a) reverse osmosis (RO) for seawater desalination, and (b) membrane bioreactor (MBR) technology for wastewater treatment. The cost analysis undertaken extended to (i) the determination of operating expenditure (OPEX) trends using simple analytical expressions, (ii) the subsequent estimation of the sensitivity of OPEX to individual system parameters, and (iii) published data on CAPEX for individual full-scale installations or from cost analyses. An appraisal of the peer-reviewed literature through a survey of a leading scientific database was also carried out. This bibliometric analysis was based on authors’ keywords; it aimed to establish the profile of process cost for each of the two applications when compared with other popular research topics.
The OPEX analysis, ostensibly through a consideration of specific energy demand in kWh per m3 permeate, revealed it to relate primarily to hydrodynamics in the case of RO, and to both membrane fouling and air scouring for MBRs. The bibliometric analysis of research trends revealed a marked difference in emphasis on cost aspects between the two research areas, with the focus on cost specifically being 16 times greater for RO desalination of seawater than MBR treatment of wastewater. MBR research appears to be dominated by fouling and foulant characterisation, making up almost a quarter of all studies, notwithstanding evidence from practitioners that other process parameters are as important in determining MBR process OPEX and operability.
Using the first person pronoun in the title of the paper is rare, but appropriate for someone who has contributed so much to the literature on water treatment with membrane technology. Dr. Judd is in good company with this personalized article on nitrification by Dr. W. Gujer, for example.
The paper starts with a brief overview of the history of membrane technology for water treatment. The introduction also defines operating expenditure (Opex) and capital expenditure (Capex). The purpose of the paper is to "consider the extent to which the costs of implementing and operating membrane process plants in water and wastewater treatment have been or are likely to be impacted by scientific, technological and commercial development." To address this objective, the paper has sections looking at trends in costs of MBR and RO technology, trends in research on these technologies, and a final section on how these trends relate to each other.
The trends in costs include both Capex and Opex; however, the author explains that Capex is difficult to analyze since it includes location-specific factors like the cost of land, and is not generally reported or itemized in a standard way. RO is compared to thermal desalination and MBRs are compared to conventional activated sludge (CAS) wastewater treatment systems. For both MBR and RO technology, "the decline in OPEX since original implementation has been primarily through improved energy efficiency," so the Opex analyses focus on energy demand (though other factors such as chemical consumption, membrane replacement, and labour are also mentioned; for more on factors that go into wastewater treatment economics, check out the mindmap that I drew in this post).
For RO, sensitivity analysis on energy demand calculations showed that salt concentration and energy recovery (usually done with a turbine on the retentate stream) are important factors. The impact of salt concentration on energy demand implies that mitigating concentration polarization (the author recommends improving hydrodynamics of RO modules but doesn't go into detail) is a promising route for improving Opex.
Cost information for seawater desalination with RO is provided in Table 2. Capex generally appears to be less than $2/L/d of capacity and the cost of water produced is around $0.5/m3 with a couple of exceptions. The specific energy demand is around 4 kWh/m3.
For MBRs, sensitivity analysis on energy demand calculations showed that flux and the scour air required per unit area are important factors. Increasing the achievable flux (without shortening the lifespan) decreases the membrane surface area required, which makes replacing membranes less expensive and decreases the overall amount of scour air required. However, increasing flux is difficult since fouling increases exponentially at higher fluxes (see the paper for references on this). In contrast, reducing the scour air required per unit area of membrane surface has been more successful; Dr. Judd notes that, "Much of the commercial membrane development over the past 15 years has focused on maximising the mixing imparted by membrane air scouring while minimising the amount of air required for this, resulting in significant improvements in energy efficiency".
The section on trends in research on membrane technology is based on bibliometric analysis: summing up the incidence rate of papers in a database containing some relevant keywords. Dr. Judd has a blog post on his website that summarizes the findings, which is useful if you don't have access to the full paper. One key observation was that fouling studies have a high incidence rate among MBR research papers; energy demand, in contrast, did not appear to receive a lot of focus—although he does point out that reducing fouling would facilitate higher fluxes, indirectly improving energy efficiency of MBRs.
The final section of "Membrane technology costs and me" ties together the trends in research with the trends in costs. The author points out that the published literature on MBRs doesn't necessarily match the highest priority issues faced in real systems. Here are some excerpts:
In the case of MBRs a disproportionate amount of research appears to have been dedicated to the study of MBR fouling ([incidence] = 24% on average over the period considered) and its characterisation, especially in comparison to RO seawater desalination for which [incidence] = 10% for fouling studies. The emphasis on characterisation is evidenced by the preponderance of the term EPS (extracellular polymeric substances)... Notwithstanding this large body of research, all practical evidence from full-scale operation suggests that surface fouling is predominantly successfully mitigated by chemical cleaning with a combination of hypochlorite and citric acid (Wang et al., 2014a,b; Judd, 2010; Brepols et al., 2008). Whilst fouling impacts directly on cost (Table 3), it is by no means the only parameter doing so and is not necessarily the most logical system facet to target given that it is determined by the microbiology which, by its nature, is not readily controlled.
Most crucially, there is a marked difference in emphasis on cost between the two research areas. Whilst cost sensitivity seems to be
considered a valid subject for research in desalination technology, and perhaps other water/wastewater treatment areas (such as algal technologies), this does not seem to be the case for MBRs... Moreover, anecdotal evidence from surveys (Judd, 2016) suggests that issues such as membrane channel clogging, energy demand and, for some sites, foaming feature as prominently as membrane fouling amongst the concerns of MBR practitioner community (in addition to cost). There is no evidence of the incremental improvements in cost effectiveness achieved over the past 25 years being in directly attributable to the considerable scientiﬁc research effort in fouling and its characterisation. In practice, the improved energy efficiencies and decreased membrane costs that have combined to make MBRs more cost competitive appear to have arisen from innovations in engineering design and manufacturing respectively.
... Whilst much research has been conducted on the science of the two processes and applications considered here, the reality is that actual practical developments that have led to cost reductions owe more to engineering. This dislocation between the research and practitioner communities is hardly a new observation, but is particularly apparent in MBR research where the focus on practically relevant aspects has been less apparent than in the case of RO seawater desalination.
This type of research is supposed to be applied (rather than "blue sky") so it would probably be beneficial if researchers consulted more with practitioners to ensure that their research can end up being applicable.
I found this paper by Dr. Judd to be a good read for understanding trends in MBR and RO research, cost, and energy efficiency. I also thought that he has some important things to say about bridging the gap between research and application in the conclusion excerpted above.
Disclaimer: This is just a reminder that the views expressed in this blog are mine alone; I write it in my own capacity and not as part of my job.
Finally, I read an article this week about the value of writing in the business world. It reflects some of my own thoughts about why I practice writing a blog and venture into topics related to what I do as a profession sometimes.