Drinking water is becoming increasingly scarce and polluted through anthropogenic influences, leading to more than one million deaths a year. Poor water quality is a problem in both developing and technologically advanced countries, such as the USA and China.
Water filters, like activated carbon or reverse osmosis membranes, could considerably enhance drinking water quality at the point of use. Those filters are often certified for a particular volume by certification agencies.
Actual filter lifespans, however, are based on several site-specific aspects and can deviate from certified lifespans. This article presents data from filter contamination experiments which found the breakthrough of organic contaminants after only 1/5 of the certified filter volume. The results also demonstrate that filter performance can be monitored in real-time with affordable sensor solutions.
Water Scarcity and Diminishing Quality
For at least one month every year, nearly 2/3 of the global population suffers from extreme water scarcity.1 With population growth and climate change, water shortages are further intensified.2 Water is not only becoming a scarce resource, but its quality of it is also quickly diminishing.
As a result, UNESCO identified water quality as one of the most significant societal challenges of the 21st century.3 Water quality is a problem in both developing countries and technologically advanced countries such as the USA and China.
Moreover, a recent study4 highlighted that between 1980 and 2015, violations of the U.S. Safe Drinking Water Act more than doubled. The research also notes that in 2015, nearly 21 million US citizens were exposed to municipal drinking water that breached health-associated quality standards.
Common Water Pollutants
What are some of the most common pollutants that make rivers, seas, groundwater, lakes, and tap water toxic—or even unsuitable for use? As most pollutants arise from human activity, this differs highly from area to area and is based on environmental and anthropogenic influences and factors.
Bacteria, fertilizers, viruses, herbicides and pesticides, by-products of fossil fuel processing, disinfectants, pharmaceutical products, and heavy metals are some of the most common pollutants. Although some of these contaminants only have small effects on human health, others can be extremely toxic or even deadly.5
Unsafe water sources are also estimated to be accountable for over one million deaths annually.6
Water Filters Help Improve Water Quality
Ideally, appropriate measures should be taken to prevent drinking water from becoming polluted in the first place. This will, however, represent a huge challenge in the coming decades. For now, water filtration systems represent an effective alternative. In many countries around the world, water is filtered and disinfected at the site of the municipal water supply.
Residential filtering solutions usually need to guarantee safe drinking water in the absence of a clean municipal water supply, to decrease contaminants that go through the supply lines from the water supply facilities to the end-user, or to eliminate disinfectants. Residential filtering systems are classified as point-of-entry filters (e.g., filtering the main water supply line of a house) or point-of-use filters (e.g., under the sink or in a refrigerator) (refer to Figure 1).
The most extensively employed residential treatment technologies are activated carbon filters (AC), reverse osmosis membranes (RO), or a combination of both.
Figure 1. Schematic of water supply and point-of-use and point-of-entry filters. Image Credit: Sensirion AG
Activated carbon filters are based on the process of adsorption when contaminated water flows through the porous structure of the carbon material (see Figure 2A). As the attracting Van der Waals forces between the non-polar pollutants — like organic compounds — and non-polar carbon particles are more powerful than the forces that retain the pollutants in solution, the pollutants get adsorbed to the carbon particles’ surface and leave clean water behind.
A large surface area is needed for effective filtering as Van der Waals forces are only efficient over very short distances. For this reason, the filter material is treated (activated) with hot gas to purify the charcoal and create microscopic pores within it, thereby creating very large surface-area-to-weight ratios.7
By-products of disinfection and several chemicals, including volatile organic compounds, can be removed successfully by AC filters. The charcoal’s special treatment also allows other pollutants, like heavy metals, to be eliminated.8
Figure 2. Activated carbon (A) and reverse osmosis (B) filters. Image Credit: Sensirion AG
Filters based on reverse osmosis apply pressure (e.g., with a pump or piston) to the contaminated water to overcome the osmotic pressure, thereby forcing the solvent (water molecules) to flow from the concentrated side of a fine-meshed, semi-permeable membrane to the “pure” solvent side, leaving behind the contaminants (Figure 2B).
Due to their fine mesh, RO systems successfully eliminate different organic and inorganic pollutants, along with bacteria like E. coli, at sufficient pressure.8 Although RO systems are usually more efficient than AC filters at filtering pollutants, they are generally more expensive and need pre-filtering with AC filters and sediment filters to prevent membrane fouling and clogging.
Limitations of Current Filtration Systems
Unfortunately, not everyone can afford such a filtering solution — or is even aware of the poor quality of their drinking water. On the other hand, residential filtering solutions in China and the USA are very famous due to a combination of mistrust in municipal water supplies and poor drinking water quality.
In a newly conducted survey by the US public health organization NSF International,9 58% of the respondents stated that they utilize water treatment systems.
However, most of those surveyed depend on filters available in their sink, house, or refrigerator. This is concerning as filters are developed to decrease particular pollutants and must be chosen accordingly.
Filter vendors can have their filters certified by an accredited organization such as NSF International, the Water Quality Association,10 or Underwriters Laboratories,11 who challenge the filters with water containing a defined mixture of contaminants while monitoring the filter’s efficiency over time.
This produces a capacity or volume rating (in liters or gallons) for all verified filters. This implies the volume after which the end-user has to change the filter cartridge. While this might be a valuable indicator of when the filter should be changed in some cases, it is also questionable in many others, as the lifetime of the filter is very site-specific.
Based on the concentration and type of pollutants, temperature, and pH value, the efficiency of the same filter can highly differ. As such, filters are likely to be changed too early (uneconomical) or too late (unsafe) and not at the right time.
The Solution: Real-Time Filter Monitoring and Smart Filters
Smart filters are an ideal, simple solution that tracks water quality to alert the end-user when pollutants have breached the filter and when the filter is approaching the end of its life. This concept is experimentally demonstrated by equipping an activated carbon filter with water quality sensors before and after the filter (refer to Figure 3).
Such sensors quantify total organic carbon (TOC), a common measure of the quantity of carbon present in (harmless and harmful) organic compounds in the water flowing out of and into the filter.
Water containing high levels of organic contaminants is constantly fed into the filter. Initially, the sensor installed behind the filter detects clean water, free of organic contaminants (i.e., if the filter is working correctly).
During the experiment, the sensor behind the filter detected a sudden rise in organic contaminants after approximately 220 L, indicating that the filter was saturated and not working.
Although this filter is certified for a volume of approximately 2,400 L of water, it can still fail after only 220 L as contamination levels in the water are relatively high (5 mg/L, corresponding to roughly 1–10x the expected concentration in municipal drinking water). This experiment proves two crucial factors:
- The certified filter volume is actually only a very vague indicator of the lifetime of a filter. The real lifespan will vary and is specific to sites.
- A TOC sensor can be employed to track filter capacity and alert an approaching failure in real-time.
Unfortunately, instruments and sensors for tracking contamination levels and water quality parameters are very bulky and costly, rendering them impractical for application in residential filter monitoring. Some filtration systems use timers, flow meters, and pressure sensors with some success to try and derive filter efficiency from these surrogate parameters.
However, no smart residential water filters are available at present, which would enable real-time performance tracking, as is general practice with ventilation and air quality monitoring in other home appliances.8
Figure 3. Data from filter intrusion experiment. Data from the sensor installed in front of the filter is denoted in black, while data from the sensor installed behind the filter is shown in green. Image Credit: Sensirion AG
Figure 4. Schematic of traditional filter versus smart filter. Image Credit: Sensirion AG
Sensirion’s Contribution to Smart Water Filters
The new SWT50 sensor (see Figure 5) from Sensirion is the first product in the series of water quality sensing. The sensor employs a UV-absorption-based measurement principle to quantify TOC in water. Its low cost and small shape factor make it suitable for applications like filter monitoring.
Figure 5. Sensirion SWT50 sensor. Image Credit: Sensirion AG
References
- Mekonnen and Hoekstra, 2016, Sci. Adv. https://www.science.org/doi/10.1126/sciadv.1500323
- https://www.unwater.org/water-facts/water-scarcity
- https://en.unesco.org/waterquality-iiwq/wq-challenge
- Allaire, Wu and Lall, 2018, PNAS. https://www.pnas.org/doi/full/10.1073/pnas.1719805115
- https://www.nrdc.org/stories/water-pollution-everything-you-need-know
- https://ourworldindata.org/water-access
- Dillon, Wilton, Barlow and Watson, 1989. Annals of Emergency Medicine. https://www.annemergmed.com/article/S0196-0644(89)80841-8/pdf
- Wu, Cao, Tong, Finkelstein and Hoek, 2021. npj clean water. https://www.nature.com/articles/s41545-021-00128-z
- https://www.nsf.org/
- https://wqa.org/
- 11 https://www.ul.com/
This information has been sourced, reviewed and adapted from materials provided by Sensirion AG.
For more information on this source, please visit Sensirion AG.