By Simon Breese, P.Eng., AECOM
Algae – Bane of Our LakesEnvironmental Water & Wastewater algae
Algae blooms are spoiling our lakes and clogging our water treatment plants. Here are a range of techniques to deal with the problem.
From the June-July 2015 print issue
Algae have always posed a challenge to water treatment plants, either through their tendency to clog filters, or due to issues such as taste and odour which can be challenging to address.
But the frequency and magnitude of algae blooms appears to be on the rise. This tendency is driven in part by global warming, and by more intensive land use around our watersheds, which increases their nutrient loading.
Of particular concern is the apparent global rise in harmful algae blooms caused by some species of algae, notably cyanobacteria, which can be toxic to humans and aquatic life.
This article presents an overview of a wide variety of techniques that are presently employed in the water supply and treatment industry to address algae, either by managing algae growth at the source, or by removing algae or its metabolites at the water treatment plant.
As a general rule, given that the compounds that cause taste and odour (T&O) and toxins are initially formed as metabolic by-products within the algae cell, but become an issue primarily when released into the water, either by being excreted by the cell or through the death of the cell, it is almost always preferable to avoid techniques that kill live algae cells, as this can exacerbate the issue.
The problems posed for a water treatment plant by algae can therefore also change seasonally. For example, early in the algae season, filter clogging due to live algae cells may be the primary concern, whereas later in the season the decline of the algae bloom and the resulting death of large populations of algae can result in taste and odour or toxicity issues.
Managing algae growth
at the source
While it is always preferable to undertake strategies preventing algae growth at the source, this is often difficult to achieve in a complex lake ecosystem where the water purveyor such as a municipality may not have complete control over the watershed.
A wide variety of techniques have been used, including:
• selecting an alternative intake location: use of an intake designed to allow withdrawal of water at different depths, to allow the best quality water to be drawn;
• reducing nutrient input from point and non-point sources;
• precipitation of nutrients within the water body by the addition of chemicals;
• dredging of lake sediments to physically remove nutrients;
• de-stratification of the water body using artificial circulation;
• aeration of deeper waters in stratified lakes to allow the use of these deep waters as a source instead of waters nearer to the surface prone to algae growth;
• use of algaecides such as copper sulphate to prevent the onset of algae blooms;
• bio-manipulation: seeding of the water body with species of fish or other aquatic life that feed upon the problem algae species;
• sonication: use of ultrasonic irradiation to disrupt algae and retard their proliferation.
Given the complexity of lake ecosystems and the enormous variety in the number of species of algae, it is difficult to point to any one of these techniques as universally successful. When considering any of them, care is needed to ensure that they do not give rise to unintended consequences. Careful liaison with all relevant regulatory authorities is also essential.
Managing algae and its metabolites at the plant
If algae cannot be managed within the source itself, a variety of techniques can be employed, either to deal with the removal of live algae, or to deal directly with their metabolites.
Conventional water treatment plants are capable of 90% or higher physical removal of live algae cells, given that algae generally become captured within coagulant floc. For source waters prone to algae, it is preferable to rely upon clarification processes such as sedimentation or dissolved air flotation (considered the best available technology for algae removal) to remove the majority of the algae. Although filtration also is very effective, it will typically come at the price of significant reductions in the plant’s productivity due to filter clogging.
Treatment techniques for addressing algal metabolites generally use either oxidative or sorptive techniques. Where oxidation is used, care must be taken to remove live algae as far as possible before applying the oxidant to avoid killing them. Commonly employed oxidation techniques include:
• Chlorination, which can be effective against many T&O-causing compounds and some algal toxins, but can result in disinfection by-product formation;
• Potassium permanganate;
• Ozone, which is highly effective against most algae related T&O, as well as algal toxins;
• Advanced oxidation, which typically employs a combination of ultraviolet irradiation of hydrogen peroxide or chlorine, or ozone oxidation of hydrogen peroxide, to form hydroxyl radicals extremely effective in the destruction of T&O or algal toxins;
Sorptive techniques that are commonly used include:
• powdered activated carbon (PAC), which is highly effective for the removal of both T&O and algal toxins if applied in sufficient dosages. PAC can become cost prohibitive unless the algae season is comparatively short in duration;
• granular activated carbon (GAC), where coarse activated carbon is placed as a sorptive medium as part of a filter bed. GAC is also highly effective as long as the GAC retains sorptive capacity. The effective life of the GAC can vary from a few months to several years, depending upon source water quality;
• Biological filtration, whereby the growth of biomass on a porous filter media such as GAC is actively encouraged, commonly through the use of ozone upstream of the filter. This process cleaves long chain organic molecules into short chain assimilable organic carbon, which is useable as a food source by the biomass. A combination of ozone with biological filtration provides a dual barrier against T&O and algal toxins, although the removal efficiency in typical Canadian plants can vary seasonally, as biological activity slows dramatically in cold waters.
In summary, addressing algae related issues in a water treatment plant is a highly complex issue and the appropriate solution not only is highly case specific, but also can vary seasonally. Given this complexity, pilot testing is almost always recommended to ensure the long term sustainability of any approaches. cce
Simon Breese, M.A.Sc., P.Eng. is technical director of water treatment with AECOM in Kitchener, Ontario.