How Algal Toxins Impact Water Treatment (And What To Do About It)

Water treatment plants (WTPs) that experience harmful algal blooms (HABs) in their source water have three critical opportunities to minimize the risk of cyanobacteria (blue-green algae) and cyanotoxins reaching their distribution systems: 1) quantifying the true level of risk, 2) adjusting treatment to minimize cyanotoxin exposure, and 3) removing the toxins. Here is how the right in-house detection technology can help them achieve all three.

Detecting The Degree Of Cyanotoxin Exposure

It is known that the degree of algal-cell presence does not necessarily correspond to the degree of cyanotoxin threat. Still, it is important to know the total concentration of those toxins — intracellular (still contained within the cell walls of living or dead algae) plus extracellular (released from whole algal cells or cells lysed [ruptured] by WTP processes).

Intracellular toxins can be removed by capturing intact algal cells, taking care to avoid physical or chemical impacts that could cause them to lyse and release their toxins. Extracellular toxins require the use of more advanced treatment technology to neutralize them. Making the right process decisions depends upon quantifying cyanotoxin presence accurately through in-house or third-party testing (Figure 1).

Photo courtesy of LightDeck

Figure 1. Sensitive in-house testing that uses laser light to detect the fluorescence levels of reagent-prepped water samples can identify cyanotoxin levels for common blue-green algae such as Microcystis and Cylindrospermopsis in as little as 10 minutes. Quick turnaround gives WTPs more timely control over critical treatment decisions at a lower cost per test (after equipment purchase) vs. sending samples out to third-party labs.

Whether a WTP has had cyanobacteria/cyanotoxin issues in the past or not, it is important to keep the following factors (and the 7 Steps Toward Cyanotoxin Risk Reduction, below) in mind:

• Be Proactive. An important aspect of any cyanobacteria/cyanotoxin management strategy is establishing source water baselines of cyanotoxin presence (microcystins, cylindropspermopsins, etc.) across different seasons of the year and different levels in the water column. With or without cyanotoxin detection capabilities, though, WTPs can also look for corresponding indicators of algal growth — e.g., departures from baseline norms in terms of increasing turbidity, total organic carbon, or sharp diurnal changes in oxygen levels caused by photosynthetic cyanobacteria.

• Monitor Toxin Levels Closely. While monitoring conditions related to algal growth can be helpful, detecting the presence of specific cyanotoxins — intracellular or extracellular — is essential. In-house testing is the quickest and easiest way to identify and monitor cyanotoxin presence. New in-house, 10-minute, cyanotoxin testing capabilities make detection and quantification more practical and cost-effective than ever. They do not require expensive liquid chromatography or mass spectrometry instrumentation, personnel with specialized training, or the costs/delays of shipping water samples to third-party laboratories.

• Avoid Releasing Intracellular Toxins. If toxin-bearing algae are present in the source water, any pre-oxidation treatments (chlorine, ozone, potassium permanganate, etc.) used for other purposes (zebra mussel protection, iron/manganese removal, etc.) can lyse algal cells and release intracellular toxins (Figure 2).

• Plan Ahead For Extracellular Toxin Removal. Whether extracellular toxins are present in the original source water or released due to lysing of algal cells during treatment, WTPs have multiple options for neutralizing them with existing or additional treatment capabilities.

Source: EPA 810-B-16-007 Water Treatment Optimization for Cyanotoxins

Figure 2. This graphic shows multiple treatment considerations to take into account throughout the entire water-treatment train — especially at the pre-oxidation phase — when dealing with intracellular cyanotoxins. The EPA document it comes from also outlines steps to deal with extracellular-cyanotoxin removal. 

Stay Mindful Of Hidden Dangers

One of the biggest dangers with cyanotoxins is the risk that ‘out of sight is out of mind.’ That can happen when a first-time event catches a WTP by surprise or out-of-season events occur at times when experienced WTPs are not actively monitoring for cyanotoxins. It can occur if algal blooms occur near a source water intake far below the surface, out of sight from visual observation. It can happen if extracellular toxins are in the WTP intake without a visible mass of algal cells, or if in-plant treatment processes cause toxin release from algal cells before they are removed from the treatment train.

For utilities that are unfamiliar with the challenges, this in-depth U.S. EPA document on Water Treatment Optimization For Cyanotoxins and this Water Utility Manager’s Guide To Cyanotoxins provide more background on the entire process.

7 Steps Toward Cyanotoxin Risk Reduction

Once the WTP knows of the presence of cyanobacteria/cyanotoxins in the water treatment train, there are multiple steps that can be taken to neutralize their threats early in the water treatment cycle and at final disinfection.  

• Stopping Risky Behavior. Shutting down pre-oxidation steps at the first sign of algal presence is a key to minimizing cell lysing that can release intracellular toxins into the water stream.

• Choosing Alternate Water Sources. Avoiding cyanotoxin problems can be as simple as drawing water from another reservoir intake level or alternate source — e.g., groundwater, neighboring water system.

• Carefully Removing Whole Algal Cells. With careful coagulation, flocculation, and/or dissolved air flotation (DAF) technology, a WTP can remove a large volume of algal cells with their intracellular cyanotoxins still intact. 

• Adding Powdered Activated Carbon (PAC). Dosing with PAC before coagulation/flocculation removes cyanotoxins as the PAC settles and is drawn off during the flocculation/sedimentation phase. PAC can also be used after filtration to remove any remaining toxins. It gives a WTP the ability to turn dosing on and off as seasonal problems vary.

• Getting Granular Activated Carbon (GAC) Contact Time (CT) Right. For WTPs that use fixed-bed GAC filtration, knowing the degree of cyanotoxin levels can help to establish the appropriate CT for cyanobacteria/cyanotoxin removal. Conducting bench-scale tests or pilot-scale tests of GAC/PAC treatment is helpful for establishing the appropriate CT for cyanotoxin removal — particularly when HABs are a new experience for the utility.

• Exploring Filtration Options. Microfiltration and ultrafiltration processes can stop algal cells but not cyanotoxins. Nanofiltration and reverse osmosis can filter out cyanotoxins as long as the membranes are not compromised.

• Testing Early. Testing Often. Having affordable in-house cyanotoxin testing capabilities that deliver results in about 10 minutes makes it easier to monitor WTP processes on demand. Conducting spot tests at multiple stages in the treatment process — e.g., at intake, post-GAC, post-sedimentation, final disinfection, etc. — can indicate whether to adjust prior or subsequent stages of treatment. For example, getting elevated readings later in the treatment process can indicate that lysing of whole cells might be releasing additional toxins.

Finally, it is important to test after the final disinfection stage — whether that involves ozone, chlorination, chloramination, etc. — to determine if cyanotoxins are still present. Take note of how widely the effective CT can vary by water temperature, pH, and cyanotoxin concentrations (Figure 3). 

Source: EPA 810-B-16-007 Water Treatment Optimization for Cyanotoxins

Figure 3. This chart underscores the significant impacts water temperature, pH, and cyanotoxin concentrations can have on the contact time required to reach targeted levels of toxin reduction.