Mother Nature supplies us with water, but it may be contaminated with substances that make it unfit for human consumption, including small creatures known as algae.
According to Global Water Intelligence (link is PDF), “of all the pretreatment issues” facing treatment plant operators, “the ability to effectively deal with harmful algal blooms (HABs) remains the most challenging.”
The blooming algae phenomenon is commonly known as red tide because some of the phytoplankton in the water may appear to turn it red when they bloom. Many scientists use the term “harmful algal blooms,” in part because some blooming incidents don’t involve species that discolor the water. The algae can secrete toxins and otherwise taint water.
The condition that precipitates red tides is known as eutrophication. In this state, fresh water that is too nutrient-rich — either from naturally occurring nitrates or phosphates, or those introduced from fertilizer runoff or sewage discharge — becomes a breeding ground for toxic algae blooms.
According to Dr. Aleksandra Drizo of the University of Vermont, there are 415 hypoxic coastal zones in the world. “The situation is even more alarming for freshwater resources with 54 percent of lakes in Asia, 53 percent in Europe, 48 percent in North America, 41 percent in South America and 30 percent in Africa being identified as eutrophic,” she says.
And although these algae bloom on the surface, the biomass can migrate throughout a water column. This means that plant operators can’t avert the problem even when they institute a deep-water intake. Algae blooms have, for example, caused numerous high-profile plant shutdowns in Oman since 2008.
In South Africa, the problem is particularly acute. More than 40% of the nation’s dams are reportedly affected by eutrophication. To treat digested sewage effluent rather than release it back into the environment, one new approach takes the algae and combines it with oxygen, transforming it into a slurry that can be used as fertilizer. The remaining water moves from that pond to another stocked with fish that are able to feed on any remaining algal content. More of the algae-based fertilizer is extracted, then the pond water can be used for irrigation.
In a February 2012 workshop held in Oman and devoted to red tides, David Furukawa, chief scientific officer of the Australian National Centre for Excellence in Desalination, stated that the toxins created by the life found in red tides are not the critical issue for desalination plant operators, but the extensive biomass found in them is. Among the mechanical problems is membrane clogging.
During the workshop, Don Anderson, a senior scientist at Woods Hole Oceanographic Institution, reportedly characterized the problem as an expanding threat to public and ecosystem health.
Furukawa said there are several challenges facing the industry. Namely, because there are “many varieties of ‘critters’ involved in the problem,” he says, “it is a challenge to identify those which are of most interest.”
An effective real-time monitoring tool for detecting algal blooms is sorely needed by the industry, says Furukawa. More research is needed, particularly published data related to the real operation of desalination plants during algal blooms.
“HABs are diverse phenomena, caused by many different species, producing different toxins and other compounds, in different hydrographic environments,” stated Anderson. “Methods that might mitigate the impacts of one type of HAB may not work for others. The threat from HABs to desalination plants is real, and deserves attention through targeted research.”
However, one industry expert surveyed by Global Water Intelligence noted there was an easy solution to the problem: simply shut a plant down during a severe bloom.