Stopping algae in its tracks before it disrupts your water treatment plant
May 08, 2020
Harmful algae blooms are a major environmental problem that can have severe impacts on water treatment plant operation and human health
Beaches are closed for a third straight day.?. Toxic algae?. Local economy??a hit from toxic algal blooms. These headlines are becoming all too familiar across the Great Lakes region¡ªand beyond.
Changes are occurring in our fresh water sources. Warmer water temperatures, coupled with increased development in our watersheds, has led to harmful algal blooms. In some cases, these produce cyanotoxins, affecting not only recreational activities but also water treatment plants (WTP) and drinking water safety.
Algae can disrupt the water treatment process in many ways. The most common problem is clogged filters and reduced water production. Algae cells are hard to filter, and they don¡¯t settle well. Algae can also cause bad taste and odor in the water. They increase the formation of disinfection byproducts, by elevating the level of reactive organic matter in the water. Unfortunately, many existing water treatment plants do not have processes in place to remove algae cells or treat their cyanotoxins.
Harmful algal blooms are overgrowths of algae in water. Some produce dangerous toxins in fresh or marine water, but even nontoxic blooms hurt the environment and local economies.
What¡¯s pristine today may not be tomorrow
Algae blooms may not just cause short-term operational challenges, they may also affect a treatment plant¡¯s long-term viability. The first thing to consider is that many pristine reservoir sources suitable for direct filtration and conventional sedimentation today may not be so pristine in 10 years as the levels of algae increase. Climate change is a key factor at play. Increases in population density in the watershed can also lead to higher nutrient inputs. Decreased water supply or increased water demands may also reduce the available depth of lakes and reservoirs, increase stagnation, and affect dissolved oxygen levels, therefore increasing algal blooms.
This presents a challenge to water treatment plant managers. How can they plan for a future with algal blooms when they aren¡¯t facing them yet?
?Unfortunately, many existing water treatment plants do not have processes in place to remove algae cells or treat their cyanotoxins.
The first step is to make an honest assessment of your reservoir or water source. Then you can begin to get an understanding of the risk for increased algal blooms. Is there a good chance that residential or agricultural development will increase in the watershed? Is it reasonable to assume demands on your source water supply will increase? Could your allowable water allocations be decreased in response to climate change or regional growth? Can any of these effects be mitigated by land-use regulations or pollution-control measures?
Stopping algae in its tracks
If source water management isn¡¯t enough, more direct measures may be necessary.
Consider hypolimnetic oxygenation. Low oxygen concentration at the bottom of a lake or reservoir can cause nutrients like phosphate to be released from sediments, accelerating algae growth. If that¡¯s the case, you can install a piping system that places oxygen at the sediment-water interface. It has been effective for other contaminants like manganese and iron, and it¡¯s gaining traction because of the problems with phosphorous and algae.
A dissolved air flotation (DAF) process can be added in front of existing direct filtration plants or retrofitted into existing sedimentation basins to mitigate the effects of algae. DAF systems are considered the best available technology for removal of algae in drinking water treatment plants, as they take advantage of the buoyancy of algae cells. It¡¯s a lot easier to make an algae cell float than it is to make it settle.
Furthermore, DAF removes algae from the water column gently, without breaking the cell wall. This ensures that any reactive DBP precursors or algal toxins stay in the cell and don¡¯t leach into the water. Look at the space available on your existing site and examine your hydraulic grade line to see if DAF could be incorporated in the future.
Circulation and oxygen¡ªa tale of two water bodies.
Making decisions when algae or algal toxins are detected
If an algae bloom is occurring, or if algal toxins are discovered in your source water, it¡¯s important to decide if the water source is safe to use. In many cases, water treatment processes can be adjusted to remove toxins, whereas in other cases affected source waters should be shut in. The appropriate response depends on which toxins are present, their concentration, what treatment processes are available at the facility, and if other source waters are available.?
We developed a decision tool that can assist operators with selecting the appropriate source water (if multiple sources are available) and/or modifying treatment conditions based on algae and toxin data collected by the lab. Treatment efficiency for individual water treatment processes can be based on removal values published in the literature or by site-specific bench- or full-scale data collected by the utility.?
For example, if lab data indicates that several cyanotoxins have been detected in the source water reservoir, this data is input into the tool to determine if operator action is required. As illustrated below, jar tests to simulate toxin removal in the WTP may be recommended when toxin concentrations exceed a predetermined threshold.
Jar testing is a bench-scale test that can be used to determine the optimal dose of treatment chemicals used in a water treatment plant.
The decision tool includes a series of unit-process toxin removal calculators that estimate the removal of various toxins through the WTP. For example, free chlorine CT values necessary to comply with regulatory maximum concentrations can be calculated based on individual toxin levels in the raw water.
The tool sums up the removal of each class of toxin by each water treatment process and provides the operator with a predicted toxin concentration in the treated water, as well as guidance on whether additional treatment or other actions are needed.
The decision tool can be run as a stand-alone application or integrated into a WTP¡¯s control system or laboratory information management system.?
An example from the WTP process calculator tool to help select or modify treatment conditions based on algae and toxin data collected by the lab.
Regulatory changes are on the horizon
As communities continue to grapple with the challenges of algal blooms, the U.S. Environmental Protection Agency (EPA)?introduced new science-based recommendations?for water quality in May 2019. State and local governments can adopt these recommendations into their own water quality standards or use them as a benchmark for determining the safety of swimming areas. The EPA will soon begin soliciting public comments that will influence later guidance on the implementation of these recommendations.
In Canada,??for cyanotoxins in 2018, setting a new maximum allowable concentration in treated drinking water.
Water monitoring programs can help to understand the conditions that cause algae blooms in a waterbody and prepare water utilities for remediation. Â鶹´«Ã½ offers a range of services for water utilities facing algae bloom concerns in their water supply.?