Algae are common, normal inhabitants of surface waters and are beneficial to the health of a water body. Low concentrations of most algae are an asset rather than a liability in a raw water source. Algae are important as primary producers of organic matter at the base of the food chain and are useful indicators of pollution. They help remove excess nutrients, produce oxygen and provide spawning habitat for fish. Algae also constitute the raw material to manufacture agar, iodine, diatomaceous earth and various food products.

In high concentrations, algae can become a nuisance to water system operators, causing filters or screens to become clogged or creating taste and odor problems. 

Blue-green algae and some species of diatoms are more often associated with taste and odor problems in water. Algal blooms can modify the pH, color and turbidity of the source water. Diatom blooms may lead to filter clogging within the treatment facility.  With time and experience, operators can identify a biological problem caused by algae and treat it effectively. 

Consult an aquatic biologist or a phycologist to help with identification. State environmental agencies or universities have aquatic biologists on staff who can assist with identification. 

Nuisance algae blooms occur when growth is stimulated by an enrichment of nutrients or other enhanced, optimal environmental condition that results in the production of accelerated growth. We typically associate algae blooms with warm, summer months, but this is not always the case. As source waters continue to become nutrient enriched due to urbanization of natural watersheds, an increase in nuisance algal blooms is anticipated.

Proper identification of nuisance algae is important to planning treatment and identifying potential health hazards. Algae can behave in mysterious ways. For example, the blue-green algae Oscillatoria rubescens (pictured below), which can appear pink in winter, could easily be mistaken for a chemical spill.

Close-up image of Oscillatoria	Oscillatoria bloom in late winter	Image of diatoms

Blue-green algae, or diatom blooms (pictured above), are frequent causes of taste and odor problems encountered by a water system. The two most common taste and odor compounds are methylisoborneol (MIB) and geosmin. Both produce a grassy, musty-type odor. 

Some algae species produce taste and odor as a natural part of cell growth and division and as decaying vegetation. As fungi and bacteria decay or decompose the dead algae, substances are synthesized that cause the odor problems.

Odor-producing species (sp.) may be benthic or pelagic. Benthic species occur at the bottom of a body of water while pelagic species are found floating near the surface of the water. The inability to immediately locate benthic blooms increases the difficulty of an early warning modeling. Examples of benthic odor producers are Oscillitoria sp. (pictured above), Aphanizomen sp. and Phomordium sp. Some pelagic nuisance taste and odor producers include Microcystis sp. and Anabaena sp.  

Algae blooms can create very large quantities of organic matter in source water. This will substantially increase the total organic carbon (TOC) content, may effect TOC compliance and, subsequently, may require modifications to treatment. Studies have related increases in algal production to increases in disinfectant by-product formation.

Small numbers of some diatoms, such as Tabellaria, Synedra and Melosira, will almost invariably reduce the length of filter runs. The rigid walls of diatoms may hasten the clogging of rapid filters.

Tabellaria is the diatom causing most short filter runs. It is likely to be present in considerable numbers all year, except January and February. This is a free-floating diatom most abundant in water having little turbidity. When coagulated with aluminum sulfate, it does not settle rapidly. The cells are generally united in zigzag chains by gelatinous cushions at the corners. The zigzag arrangement and the gelatinous cushion, which can be stretched, make the length of the chain flexible. This helps prevent the cells from breaking on the filter surface. 

Culture Collection of Algae, University of Texas at Austin. 

Ohio University Directory of Algal Images.

Reducing Taste and Odor and Other Algae-Related Problems:  An Interactive Taxonomic Guide, Arizona State University.

The Centers for Disease Control (CDC) discusses harmful algal blooms.     

The Phycological Society of America promotes research and scientific exploration of algae.

Water operators can learn more about how to control taste and odor problems in Standard Methods for the Examination of Water and Wastewater, including how to construct continuous and batch-type odor monitoring devices.