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Automatic Screen Filtration for Protecting Reverse Osmosis Membranes

why a screen filter is better than a cartridge filter

AMTA fitlersafe article

The following article was written by Fitlersafe staff engineers and published by the American Membrane Technology Association. It has been edited to make it easier to read. For a digital copy of the original article, please fill out the form on the bottom of the page. Click the following hyperlink to learn more about our activities in desalination

Introduction

There are a number of ways to pretreat water to make it suitable for drinking. This article will examine the use of an automatic screen filter versus the traditionally used cartridge filter for pretreatment of drinking water. 

Though commonly used, there are many negative economic costs associated with cartridge filters:

  1. The filter is made of non-biodegradable materials and must be disposed of at least every three months. 
  2. During this period, the cartridge has material that builds up in it and therefore requires more energy to continue filtering the water at the same pace (known as head loss). 
  3. When it’s time to replace the cartridges, they need to be replaced by hand, which involves completely stopping the filtration system, resulting in a loss of productivity of the system, in addition to labor costs (called OPEX). 

An automatic flushing cleaning sequence provides a reliable, and continuous, operation of pretreatment filtration for drinking water. The technology has been demonstrated to be successful with a very fine screen (<10µm) filter and provides an interesting alternative to the traditional use of disposable cartridge filters for reverse osmosis (RO) pre-treatment. 

As we’ll see below, pilot results from a seawater desalination plant in Israel show that an automatic self-cleaning screen alternative can achieve significant OPEX savings without compromising on filtrate quality. Analysis and breakdown of operation and maintenance costs of cartridge filters indicate a potential OPEX saving of 80-92% by the automatic alternative. In particular, 88% of the energy invested in pumping water through cartridges can be saved (the problem identified in #2 above), minimizing the environmental footprint of the desalination plant by hundreds of metric tons of CO2. Protecting RO membranes by a fine automatic screen is an economical and environmentally friendly alternative to the current standard.

Comparable filtration abilities

One of the most important things a cartridge does during pre filtration is removing silt and other tiny suspended solids that damage the RO membrane downstream (this damage is known as fouling). The automatic screens Filtersafe offers have a filtration grade of 10-20 microns, which has not traditionally been considered sufficient for successfully removing enough silt and other solids. 

However, the data collected by the pilot automatic screen test shows that the silt density index (SDI) reduction abilities of the two RO protection alternatives are comparable – that means the filter does as good a job as the cartridge for reducing SDI. Figure 1 demonstrates the seasonal variations and comparable SDI reduction in 2018-19.

Figure 1 demonstrates the seasonal variations and comparable SDI reduction in 2018-19.

SDI reduction by filter

Figure 1. SDI values from VMD, winter 2018 – summer 2019. Error bars = SE.

The automatic alternative for RO protection

The fact is, it is not the cartridges that are responsible for the actual pre-treatment and removal of suspended solids in the water – rather it is multi-media filters (MMF) or ultrafiltration (UF) which are responsible for most of the SDI reduction. Cartridges are being placed not for SDI reduction, but primarily to serve as a sentinel, which is a final barrier before the membranes, protecting them from large particles which can clog the front membranes and cause mechanical damages in energy recovery devices (ERDs). The automatic screen alternative, with a fine 10μm screen, can equally serve this purpose when placed between UF/MMF and the RO membranes, preventing solids from getting downstream. Examination of large particles captured by the pilot screen revealed escaped media, paint fractions, what seemed to be chunks of mineral scaling formed upstream and detached, and of course, biofilm chunks created upstream. 

Unlike the cartridges which accumulate solids on the surface for weeks, Filtersafe’s automatic screen flushes itself clean every few hours. A sensor senses the accumulation of solids on the screen surface and the fact that pressure in the filter is less than the pressure outside due to the accumulation of particles, and triggers the flush mechanism. An internal suction scanner with patented proximity nozzles (NozzleXTM) scans the screen surface, and within minutes the screen is cleaned, and its hydraulic properties are restored. During the flushing process there is no down-time, like when the system has to stop to replace a cartridge filter. The filter is continuing to work during the flushing sequence and only uses a very small amount (~5%) of the filtered water to clean itself, with no need for an external clean water tank or flush pumps.

Figure 2 illustrates how the screen protects the membranes downstream from small and large organic particles, including biofilm (which are microorganisms that stick together). On the left-hand side, a biofilm particle, almost 900μm in size, was removed by the 10μm screen. The exact source of this biofilm is unknown, but it is likely it was detached from a grown biofilm on piping/equipment upstream. The non-automatic alternative (cartridges) is certainly capable of capturing similar biofilm particles. However, while the automatic screen flushes and rejects captured particles within 3-6 hours, this biofilm can stay for weeks on cartridges, continuing to grow, and eventually releasing bacteria to the membranes downstream, contributing to their biofouling.

why a screen filter is better than a cartridge filter

Figure 2. RO protection by the two alternatives. (a) Close-up on un-flushed screen, covered by solids removed from the RO feed. A large biofilm particle is magnified. (b) Illustration, a used cartridge.

Minimized environmental footprint

The environmental footprint related to the production, transport, and disposal of the cartridges depends on the facility and can be difficult to quantify (though we are able to show some real live environmental savings in Table 1 below). However, the energy use of cartridges is well-known. Figure 3 illustrates the differential pressure profile of cartridges versus a screen filter. With the lack of self-flushing ability, cartridges typically have a significant reduction in the amount they can filter and an increase in the amount of energy it takes to push the water through it before being replaced. Ultimately, we can assume that 88% of energy-related costs and carbon emissions can be saved by using an automatic filter instead of a cartridge.

filter energy consumption by head loss chart

Figure 3. Head loss profiles of cartridges and automatic screen.

Five desalination plants in California were selected to demonstrate the potential minimization of environmental footprint and are presented in Table 1.

Table 1. Demonstrating minimized carbon footprint in real desalination plants in the USA. Data include open-source data for desalination production rate. Calculated values were made with typical parameters for pump efficiency, and feed-product ratios. Conversion factor by EPA website. 

PlantPlaceBW/SWProduce MGDMWh/y savingMetric Ton CO2 Saving
C. Marvin Brewer Desalter Treatment FacilitySan Francisco BayBW5166118
Richard A. Reynolds Brackish Groundwater Desalination FacilityChula Vista CA.BW10337238
Orange County GWRFountain Valley,  CA.BW722,5581,809
Charles Meyer Desalination Plant refitSanta Barbara, CASW2.312387
Claude "Bud" Lewis Carlsbad Desalination PlantSan Diego Pacific CoastSW502,4811,884

Summary

The conventional use of cartridges to protect RO membranes now has an alternative with automatic filters. The pilot results from a seawater desalination plant in Israel suggests that similar filtration performance can be maintained, with the additional benefits of reduced biofilm potential and a significant reduction in OPEX. In particular, the carbon footprint of desalination plants can be significantly reduced by using an automatic alternative that keeps head loss at a minimum. Filtersafe has developed automatic 10-20μm screen filters to keep up with the environmental and innovative needs of the industry.

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