Looking to purify lake water from cyanobacteria and PFAS? Consider these two options.
Special Contribution By Dr. Bruce Capron
Folks of a certain age remember pulling drinking water directly from the lake with nary a care. Most homeowners with private drinking water systems have since added UV lights and filters.
New threats — cyanobacteria and PFAS chemicals — require homeowners to consider updated water purification. Chemical engineer and longtime South Bristol resident Dr. Bruce Capron defines these threats and suggests ways to help address them.
Until recently, living on Canandaigua Lake and drinking its water, it was considered sufficient for a private water system to filter sediment and particles and have a method to kill microbes such as E.coli.
Today, homeowners must go a step farther to remove toxins such as blue-green algae and plasticizer chemicals. This requires considering updated water purification for potential contaminants.
What is the threat posed by blue-green algae?
Let’s look first at blue-green algae. Because they can cause illness to people and pets, CLWA volunteers monitor around the lake and CLWA disseminates bloom reports each summer. These algae are a group of photosynthetic bacteria called cyanobacteria. Different species of these bacteria can produce cyanotoxins. Different cyanotoxins can damage the liver (hepatotoxins), the nervous system (neurotoxins), or can impact the functioning of cells (cytotoxins).[1] For freshwater, the most common cyanotoxins are microcystins (hepatotoxins) and anatoxins (neurotoxins). [2]
There have been over 250 microcystin identified. The primary toxin identified and studied is Microcystin-LR. This is a molecule of seven amino acids and has a molecular weight of 995 g/mole. The size of this molecule has been estimated at 1.2nm to 2.6nm. [3]
Literature studies indicate the anatoxin are not as widespread as microcystins and do not pose the same level of risk. [4] However, the anatoxin molecules have a molecular weight of 254 g/mole and are assumed to be smaller in size than the microcystin molecules.
Although the size of cyanobacteria is in the range of 2 to 7 microns, the size of the cyanotoxin molecules found within these bacteria are more than 1000 times smaller (1 nanometer = 0.001 microns).
Even though the bacteria can be captured by relatively large filter media (e.g., 1 micron filter), it is important to note that the cyanotoxin molecules can be released outside of the bacteria’s cell membrane in living bacterium or released when the cell membrane is broken when the bacterium dies or during a water treatment process.
Blue-green algae require nanofiltration.
General filtration with cartridges removes particles by forcing the water through a porous media (e.g., paper) where most particles of a larger size are prevented from passing through the filter media.
This type of filtration is commonly used to remove sediment and other contaminants that are 1 micron or larger in size. As noted above, the toxin molecules and molecules commonly associated with plastics are nearly 1000 times smaller. Therefore, some form of nanofiltration is needed to remove these molecules.
What chemicals associated with plastics are threats and how can they be removed?
The news is often filled with the term “microplastics”. These are small particles of various plastics found in everyday items. Although the health risks of microplastics are under debate, a more serious concern are families of chemicals that are associated with plastics, food containers, cookware, and food packaging.
Bisphenols: The most common bisphenol is BPA [5]. This is often used to make plastics harder and more durable. The BPA molecule (C15H16O2) is weakly soluble in water and tends to form colloidal particles that are greater than 90nm in size. Studies have shown that reverse osmosis and nanofiltration membranes are effective in removing BPA from water. [6]
PFAS: Polyfluoroalkyl substances (PFAS) are a large family of Polyflouro alkayline chemicals. Consumer Reports list some of the most common PFAS chemicals — PFOA, PFOS, GenX, and PTFE (Teflon). These are chemicals used as surface treatments for plastics, cloth, paper, cardboard, and metal. [7]
The EPA indicates that both activated carbon filtration and reverse osmosis systems are effective in removing PFAS chemicals. [8]
Phthalates: Phthalates are groups of chemicals used as plasticizers to make various plastics more flexible. Certain phthalates have been shown to be endocrine disrupters. One common phthalate is DEHP. (Consumer Reports, May 29, 2024). DEHP has a chemical formula of C24H38O4 and a molecular weight of 390.5 g/mole.
What home water treatment options can remove these chemicals?
Removing these chemicals from water requires some form of nanofiltration. The two most common forms are the use of activated carbon filtration and reverse osmosis.
Activated Carbon Filtration: Granulated Activated Carbon (GAC filters are typically manufactured by heating wood or coconut husks to create millions of small pores with an extremely large surface area.
A single gram of GAC material may have over 1000 square meters of surface area made containing small pores ranging from 0.5nm to 1000nm. [9] These filters remove contaminants from water when a contaminant molecule prefers to “stick” to the carbon material rather than remain in solution in water. This process is called adsorption.
For these filters to function, the water must spend sufficient time in the filter media for contaminant molecules to find an active site within the carbon matrix. Also, the effectiveness of these filters decreases with use as active adsorption sites are filled with contaminant molecules.
Activated carbon filters are commonly used in municipal water treatment facilities and many home systems. The challenge for residents that use lake water is to determine what size filter to use, how often to change the filter, and what level of the contaminants being discussed in this article are removed. (These questions might be answered if the text of the NSF 42 standard were available.)
Reverse Osmosis (RO): Reverse osmosis systems remove contaminants from water by using pressure to force water through a fine membrane. Of all the water filtration options, RO membranes have the smallest pore size distribution ranging from 0.1nm to 1nm, which is small enough to remove the contaminants discussed in this article.
Water and contaminants that do not pass through the membrane are rejected and go directly to the drain. Only the “permeate” stream is used as drinking water.
Most RO systems available to homeowners include a series of prefilters to remove sediment or contaminants that could load the RO membrane and reduce its useful life. These systems commonly include an activated carbon filter as one of the pre-filters.
There are two reverse osmosis systems to consider.
In the most common, the RO system purifies water and sends it to a storage tank. When the faucet is open, the water in the tank is available and will flow. As the water is used and the pressure in the storage tank drops, the RO system will begin refilling the tank.
The time required to fill the tank will depend on the pressure in the feed water line and size of the system. Homeowners who need more instantaneous capacity can install a larger storage tank or run a second tank in parallel.
The second type of RO system is a continuous flow system. These systems may include an integrated booster pump and a larger membrane to provide water continuously when the tap is open.
One metric of a RO system is the ratio of purified water to wastewater. This can range anywhere from generating two gallons of wastewater per gallon of purified water to 0.5 gallons of wastewater to purified water. Some systems are installed with a permeate pump. This is a hydraulic pump that uses the pressure in the wastewater stream to help pump the purified water into a storage tank.
Homeowners should also be aware that many manufacturers develop the specifications for their systems based on assuming a constant water pressure of 60 psi. Homeowners with submersible pumps will likely have higher pressure in their cottages than homeowners using a “jet” pump. For those cottages with low water pressure, booster pumps can be installed in the feed water line to the RO system to increase the pressure across the RO membrane and improve the recovery time and flow rate.
As cyanobacteria and PFOS impact the water quality of the Finger Lakes, homeowners with private drinking water systems have options for upgrading to remove these new threats.
Bruce Capron earned his Ph.D. in Chemical Engineering from the University of California, Berkley. He has worked in engineering and business management positions in the optoelectronics, optics, laser, and medical device industries. Inspired by his work as a school board member, from 2001 to 2009. Capron earned a master’s in educational administration. He retired as Assistant Superintendent for Business and Operations, Honeoye Falls-Lima School District in October 2022. He is adjunct professor at SUNY Brockport, where he teaches data analytics to school administrators.
Editor’s Note: Several area companies have kept pace with new technologies for the systems described, but CLWA does not recommend specific manufacturers or installers. We do encourage you to use the same common-sense approach you’d use to make any large purchase for your home: Do your due diligence.
- Draw on the technical resources that Dr. Capron has appended to this article.
- Monitor guidance from the Centers for Disease Control, Environmental Protection Agency, and National Science Foundation relating to water purification systems. They are updated often.
- Discuss what capacity you need. Do you want to purify your drinking water, or do you want capacity for showers, dishwashing and other uses?
- Talk with neighbors and friends who have these systems. Ask what they cost to install and maintain. Ask who installed and were they satisfied with the job? Is there an annual service contract? And so on.
- Interview installers and ask for references.
- Get competitive bids.
- If you are as skilled and intrepid as the author, consider installing a system yourself. Now retired, Dr. Capron is self-installing a home system and would be happy to share what he’s learned. Contact him at brucecapron@gmail.com.
REFERENCES:
[1] A. Cohen, Blue-Green Algae Poisoning: Cyanobacteria toxicosis
[2] EPA, Drinking Water Health Advisory for the Cyanobaterial Microsystin Toxins, EPA – 820R15100, June 2015
[3] ScienceDirect.com/topics/chemistry/microcystin-lr
[4] Cyanobacterial toxins: Microcystin-LR in Drinking-water, WHO 2003
[5] PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanoic sulfonate) are two of the most common PFAS chemicals. Although not currently manufactured in the U.S., they may occur in the environment and chemically similar molecules are still used.(American Cancer Society web page: https://www.cancer.org/cancer/risk-prevention/chemicals/teflon-and-perfluorooctanoic-acid-pfoa.html)
[6] https://www.sciencedirect.com/science/article/abs/pii/S0304389413003361#:~:text=The%20polyamide%20based%20membranes%20exhibited,XLE%20BWRO%20and%20AD%20SWRO
[7] PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanoic sulfonate) are two of the most common PFAS chemicals. Although not currently manufactured in the U.S., they may occur in the environment and chemically similar molecules are still used.(American Cancer Society web page: https://www.cancer.org/cancer/risk-prevention/chemicals/teflon-and-perfluorooctanoic-acid-pfoa.html
[8] https://www.epa.gov/sciencematters/reducing-pfas-drinking-water-treatment-technologies )
[9] Water Quality Association, Granular Activated Carbon Fact Sheet, www.wqa.org