Algal treatments

Excessive algal growth in raw water reservoirs can cause severe problems with water supply, even in a traditional UK climate. Global warming means algal problems could become more frequent and acute in the UK, so effective control measures will become more important. This issue has been the subject of recent research by WRc through Toxic, a European programme supported by UKWIR.

The most common cyanotoxins produced by blue-green algae in European waters are microcystins and anatoxins. The World Health Organisation has set a drinking water provisional Guideline Value of 1µg/l for microcystin-LR, one of the most commonly occurring cyanotoxins. These toxins can occur within algal cells (intracellular or cell-bound toxin) or be released from cells (extracellular toxin). An algal bloom with 100,000 Microcystis cells per ml could contain 20µg/l microcystin.

The production of the taste and odour compounds geosmin and methyl isoborneol (MIB) is known to occur by many blue-green algal species in rivers and, more significantly, reservoirs

The musty taste and odour from such compounds in drinking water is detectable at very low concentrations of less than 10ng/l. Like cyanotoxins, geosmin and MIB can occur in both intracellular and extracellular forms. Algal cells can contain, for example, concentrations of 10-5ng/cell, so a relatively small algal bloom of 10,000 cells/ml would produce levels of 100 ng/l.

Prevention of algal problems in drinking water may require catchment management to prevent ingress of nutrients to water sources, reservoir management to remove nutrients or inhibit algal growth, or the installation of suitable treatment regimes to remove algal cells and their metabolites.

One of the key causative factors for algal blooms in reservoirs and rivers is nutrient concentration, particularly phosphate. Catchment management measures therefore aim to prevent excessive nutrient input.

Monitoring for changes in nutrient input from point or diffuse sources, arising primarily from sewage discharges (including septic tanks and cesspits) and agriculture should be in place to identify an increased risk of blooms. Many of the factors of importance in catchment protection are outside the direct responsibility of the water industry and catchment management therefore relies on effective liaison with other organisations to promote good practices.

Many reservoirs and feeder streams may already contain nutrient concentrations well above the threshold for algal growth and further control of nutrient input will therefore have minimal benefit. In these circumstances, the emphasis needs to be on reservoir management to control algal blooms. Reservoir management strategies can be either reactive or proactive in relation to algal growth. Reactive management in response to increased algal levels is limited to changing draw-off levels or moving the point of abstraction (if possible).

Proactive control is aimed at preventing or minimising algal blooms, including:

• Destratification/aeration; stratification of deeper reservoirs enhances algal growth in the warmer upper regions (epilimnion), and can cause other water quality problems as a result of low dissolved oxygen concentration in the colder deeper water (hypolimnion). mixing or aeration techniques, often proprietary, have been developed to prevent stratification by aerating the hypolimnion
• Use of barley straw, which decomposes to release compounds that inhibit algal growth. This is currently exempted from the EU Biocidal Products Regulations but only until 2008. Unless the mechanism can be clarified, it may be banned
• Nutrient removal by plants (e.g. reed beds, willow) situated around or at the inlet to the reservoir
• Chemical stripping of phosphate – historically this has used salts of iron or aluminium, already used as coagulants in water treatment, but new technologies are available, such as Phoslock, a proprietary material developed in Australia which is applied in a pelletised form to incorporate phosphate and make it unavailable for algal growth
• Chemical dosing of algicides – copper salts are used in some countries but this is not permissible in the UK
• Biological control, using micro-organisms which are pathogenic to algae or by manipulation of the animal populations within the algal food chain
• Ultrasound for disruption of algal cells

Some of these can also be reactive in relation to destroying existing blooms. The performance and acceptability of each approach depends upon the size and depth of the reservoir, the hardness of the water and the other uses of the reservoir. Some techniques are well established but others, such as ultrasound and biological control, have yet to be widely demonstrated at the scale of water supply reservoirs. Many control techniques have regulatory implications and approval would be needed.

Conventional water treatment using chemical coagulation can remove algal cells (and intracellular or cell associated products) but it is largely ineffective for removing extracellular toxins or taste and odour compounds. These require more extensive treatment using, for example, activated carbon or oxidants such as ozone. Some treatments, particularly using oxidants, can break down algal cells and release toxin or taste and odour compounds, and care is therefore needed in operating these processes to avoid creating additional treatment problems downstream. Concerns over by-product formation also needs to be taken into account in any treatment strategy using oxidants.

A number of approaches can be used to reduce treatment problems directly associated with the load of algal solids. These include:

• Pre-treatment using microstrainers or flotation to reduce algal loads to the main treatment units.
• Modification to the coagulation conditions to improve removal or reduce coagulant/algal solids loading.
• Use of an oxidant prior to coagulation to enhance its performance.
• Use of alternative filter media, such as dual media or multi-media, principally to increase filter run lengths.
• Use of pre-filters or polishing filters.

Ozonation is highly effective for cyanotoxin degradation. Oxidation using potassium permanganate is also effective but relatively high doses may be needed for raw waters compared with part-treated water within the treatment process. Chlorination under typical water treatment conditions is effective for microcystins but not for anatoxins. Low doses of oxidants to raw water can increase the concentration of extracellular cyanotoxin as a result of cell lysis.

Activated carbon adsorption is moderately effective for both microcystin and anatoxin, and removal by granular activated carbon (GAC) can be enhanced by biological activity in the bed. Performance is dependent upon the type of carbon. For powdered activated carbon (PAC), wood-based types are the most effective but even for these relatively high doses (>20mg/l) may be needed.

Biological activity in slow sand filters may give some removal of extracellular cyanotoxin, although there is little firm evidence for this. Use of a GAC layer within the sand, applied at some works for pesticide removal, could be used to give more reliable removal.

Of the oxidants used in water treatment, only ozone is effective for geosmin and MIB removal, although relatively high doses can be needed. Geosmin is more readily oxidised than MIB and is also more amenable to removal by activated carbon. GAC can be effective for taste and odour removal, even at relatively short empty bed contact times. However, longer contact times of 15 minutes or more can enhance removal by increased biological activity. Doses of PAC needed for geosmin and MIB are similar to those required for cyanotoxin removal.

It is important to have an effective algal monitoring strategy in place to control reservoir and treatment regimes. Management of large reservoirs can be highly demanding and monitoring technology may have an increasing role in future. Instruments are available which indicate the predominant algal types and the nature of the risk, based on fluorescence profiles. A hierarchy of monitoring procedures can then be identified, ranging from routine visual inspections to laboratory analysis to identify and manage cyanotoxin bloom formation. A suggested monitoring framework is illustrated in Figure 1.

Drinking water suppliers need a strategy to address the risks posed by algae and Drinking Water Safety Plans (DWSPs) are a key element. Introduced by WHO and adopted by the UK Drinking Water Inspectorate, DWSPs identify actual and potential hazards in the catchment and define measures to deal with them. WRc has been supporting UK water companies in developing DWSPs. Treatments using oxidants or activated carbon are typical, and a DWSP needs to show that design and operating conditions are appropriate in relation to doses of oxidant and PAC and GAC regeneration frequency. They also need to show that an appropriate monitoring and control strategy is in place. With global warming and increasing pollution, the frequency of blooms will increase. The processes and procedures currently employed should be sufficient for now, but the water sector needs to continue investing in research to meet new threats that will emerge in future.