Toxic Algal Blooms in Drinking Water Reservoirs


Algal blooms occur when the nutrients getting into surface waters cause rapid growth in the algae present in the water. Algal blooms have caused sporadic problems in water treatment processes for decades. When water abstracted for drinking water treatment contains algal blooms, blockages can occur in filters and odours may develop in the treated water. Toxic algal blooms, which cause toxins to be liberated in the water, are caused by the cyanobacteria species of algae. These are commonly referred to as blue-green algae.

This article reviews available information on the subject of toxic algal blooms and summarises the implications for water supply.


In 1989 the deaths of several dogs and lambs that had consumed algal-laden water from the margins of Rutland water storage reservoir raised the question of possible risks to health via drinking water. At that time cyanobacteria were known to produce a range of toxins, some of which are of concern for human health. The health risks arising from contact with toxic algal blooms via recreational activities were well understood but little was known about the persistence of the toxins in water or whether the toxins were removed or inactivated by water treatment processes. Suitable analytical techniques for drinking water analysis were not available and there was insufficient toxicological data to set health-based guidelines or standards for drinking water.

These knowledge gaps lead to a national algal toxins research programme. Managed by FWR and funded by the National Rivers Authority, UK Water Industry Research Ltd and the Department of the Environment, the programme addressed the following topics:

The outputs from the research programme are listed in the FWR Catalogue on this website under Algal Toxins

Development of Algal Blooms

Algal growth is influenced mainly by water composition, temperature and light intensity. Lowland water storage reservoirs, where the abstracted river water carries treated sewage effluent and agricultural run-off, contain relatively high levels of nitrogen and phosphorus compounds that promote the growth of algae. In many nutrient rich or so-called eutrophic waters, the growth potential of algae is determined by the dissolved phosphate concentration in the water.

A characteristic of algal blooms, which contributes to their nuisance value, is the tendency to form a surface scum. Cyanobacteria have a buoyancy control mechanism that maintains their vertical position in relation to the wind-induced circulation in the water column. Under calm conditions, when natural circulation within the water ceases, algae float and produce a scum that accumulates in the margins. This presents a possible health risk for recreational activities that involve water contact and for animals that drink from the margins of reservoirs.

Another significant adverse effect of algal blooms is their propensity to cause de-oxygenation. During daytime, algae use carbon dioxide from the bicarbonate ions in the water and respire oxygen into the water. At night this process is reversed and the carbon dioxide respiration can cause localised oxygen depletion and consequent fish mortalities.

Toxic Algal Blooms

Some species of cyanobacteria produce toxins within their cell structure. These toxins can be released when the algae die and decay. Toxin release can also occur during the growth phase of young algae. The implications of toxic algal blooms are significant and likely to become more so if global warming results in reduced flow in rivers with correspondingly less dilution for effluents and agricultural run-off. In its 2003 Background Document for the Development of WHO Guidelines for Drinking-Water Quality, WHO noted that recreational use of water contaminated with toxic algae had been linked to incidents of human illness in many countries. Symptoms ranged from sickness with diarrhoea to muscle and joint pains and irritation of the skin, eyes and throat. WHO also considered reports of illnesses associated with the consumption of contaminated drinking water reported in the USA, Australia, South Africa and China. In China incidence of liver cancer was related to consumption of contaminated water from ponds and ditches.

Control Measures

Possible short-term measures include dosing algicides such as copper sulphate or adding chemicals that adsorb or precipitate dissolved phosphates. These measures may be impracticable in the large reservoirs that are used for water supply purposes. Furthermore, where algicides have been used in water supply reservoirs there have been instances where rapid decay of an algal bloom has released toxins that have contaminated the water supply. In the long-term, control measures need to bring about a reduction in the nutrient concentrations and thus limit the growth of algae. In lakes and reservoirs where there is history of high nutrient concentrations, there may also be a need to control the recycling of nutrients that have built up in the sediments on the bed of the reservoir. De-silting and dosing of materials to form a barrier to release of nutrients have been used for this purpose.

The reduction of nitrogen and phosphorus inputs into water has been identified as one of the most important measures to be taken in the River Basin Management Plans (RBMPs) prepared for each River Basin District under the Water Framework Directive. In the UK the principle control measures in water supply catchment areas have been to reduce nutrient inputs to watercourses e.g. by applying phosphate removal processes in sewage treatment and modifying agricultural practices to reduce the run-off of nutrients. In large water supply reservoirs, physical measures are applied to reduce the growth of algae e.g. aeration by blowing compressed air through a network of perforated tubes laid on the reservoir bed, or re-circulating pumped water through inclined jets to induce mixing within the water column.

Reduction in the algal load on water treatment processes is also achieved by the use of multiple draw-off levels i.e. a number of outlet valves are provided at different depths on the draw-off tower. By monitoring algal concentrations against water depth, it is possible to ensure that high concentrations of algae are avoided.

Implications for Water Supply

The results of the national research programme provided much reassurance as to the possibility of risks to health via drinking water. Activated carbon treatment was shown to be very effective in removing the microcystins and anatoxins that are the major components of toxic algal blooms in the UK. Ozone treatment was also shown to decompose those toxins. In the lowland water sources where algal blooms tend to occur, the use of activated carbon (alone or in combination with ozone) is invariably incorporated into water treatment processes for pesticide removal and/or odour control. These treatment facilities, coupled with the measures to ensure that algal blooms are not drawn into the treatment process, ensure that even when toxic algal blooms occur, the treated water will not present any risk to health.

It is also worth noting that in the many cases worldwide where consumers have been exposed to algal toxins via drinking water, there was either no treatment, or the treatment provided was inadequate for the removal of algal toxins.


There may be a risk to human health at reservoirs where recreational contact with water is possible and animals may be at risk where footpaths and bridleways allow access to water. Where access to reservoirs is permitted, warning signs are now installed to alert the public to the risk of contact with the water, or allowing their animals to drink from or enter the water.

However, in respect of drinking water, the national research programme provided reassurance that, as long as algal loadings do not overwhelm treatment capabilities, exposure to algal toxins via drinking water is not an identifiable risk to health. This situation could change if water temperatures and nutrient inputs continue to increase.

In the longer term, algal growth potential may be reduced through the implementation of RBMPs in general and, in particular, through the various enviro-agricultural schemes and projects funded under the Water Framework Directive implementation, such as those mentioned in the 2010 Issue 2 of the FWR Newsletter.

Further Reading

Eutrophication of Freshwaters FR/R0002
Cyanobacterial Toxins in the Water Environment FR/R0009.