With drought, fires & warm weather impacting so many Australian drinking water catchments, much interest has turned to potential blooms of algae and cyanobacteria in drinking water supplies. So here I present an overview of the key issues (thanks @Gergyl for suggesting it). 1/21 
Until 1974, they were called “blue-green algae”, but cyanobacteria are very different organisms to algae, separated by millions of years of evolution. Cyanobacteria are prokaryotes (their cells do not contain a nucleus) and algae are eukaryotes (cells do contain a nucleus). 2/21
The class of “cyanobacteria” includes about 2000 known species. Among the most significant groups (“genera”) of cyanobacteria in Australian freshwater systems and drinking water are Microcystis, Anabaena and Cylindrospermopsis. 3/21
Some circumstances lead to increased waterway concentrations of nutrients, including nitrogen & phosphorous. Sources can include sewage effluents, agricultural fertilisers, manure, mining and bushfire ash. Increasing nutrient concentration is referred to as "eutrophication". 4/21
When the circumstances are right for cyanobacteria to grow, they can grow very rapidly and the result is called a cyanobacterial bloom. The ideal circumstances usually include plenty of nitrogen and phosphorous (i.e. eutrophication), abundant sunlight and warm temperatures. 5/21
Sydney experienced a major cyanobacterial bloom in 2007. Nutrients (N & P) had accumulated within the catchment during the millennium drought, including from bushfires in 2006. Rainfall in June 2007 then washed these nutrients into the reservoir, producing a bloom in August. 6/21
The Warragamba bloom was initiated in August with cyanobacterial cell counts increasing to >100,000 cells/millilitre by September and peaking at 745,000 cells/ml in October. The bloom began to decline during November, but with low levels persisting through until March 2008. 7/21
As they grow, cyanobacteria produce chemicals in the water including geosmin and 2-methylisoborneol (MIB). These two chemicals give the water are musty/earthy smell and flavour and commonly lead to consumer complaints about taste & odour of drinking water. 8/21
Some cyanobacteria also produce toxic chemicals, known as cyanotoxins. These are chemically diverse, but an important group are known as "microcystins" (produced by mirocystis and other cyanobacteria). Various microcystins are known as LR-microcystin, RR-microcystin, etc. 9/21
Due to the possible presence of cyanotoxins, cyanobacterial blooms in drinking water supplies can be very disruptive. A 2014 bloom on Lake Erie led to a State of Emergency in Toledo, Ohio, with 400,000 residents warned not to drinking or brush teeth with tap water. 10/21
The cyanobacterial bloom in Warragamba (Sydney) resulted in customer taste and odour complaints during August 2007, at around 100-1000 times normal call levels for a single month. But much more major disruptions from cyanotoxins were successfully avoided by good management. 11/21
Since many cyanobacteria require sunlight to grow, they tend to predominate close to the surface in deep reservoirs. The most impacted water in Warragamba was avoided by lowering the offtake for drinking water from ~15 below the surface to ~25m below, and then later to 40m. 12/21
Water from Warragamba is treated at 3 water filtration plants, each with slightly different processes. They may all cope slightly differently when challenged with cyanobacteria-impacted water. But the key processes for each include coagulation, filtration and disinfection. 13/21
Coagulation is an important process for cyanobacterial cell removal in many treatment plants. The coagulated cells are settled to the bottom of a tank and removed. Ideally, this is done gently enough to prevent cell rupture, which could release toxins to the water. 14/21
However, the process adopted in Sydney is "direct filtration", meaning that the coagulated water is not settled, but applied directly to sand filtration processes. This might limit opportunities to control cyanobacterial cell rupture (just my opinion!). 15/21
A challenging scenario would occur if water with high concentrations of cells (or other suspended material, such as ash) entered the plant. These would be captured on the filters, but would require more frequent filter backwashing, slowing down drinking water production. 16/21
A similar situation occurred in Brisbane, following major flooding in 2013. Flood waters entering the water filtration plant contained high sediment loads due to erosion caused by the flood. Increased filter backwashing rates almost led to Brisbane running out of water. 17/21
Many cyanotoxins can be degraded by reaction with strong chemical oxidants, including chlorine (but not chloramines). This is an opportunity for Australian water treatment plants since chlorination is universal. Close monitoring would be required to ensure effectiveness. 18/21
More effective removal of cyanotoxins, as well as taste & odour chemicals, could be achieved using activated carbon. Granular activated carbon (GAC) can be used as a filtration medium or powdered activated carbon (PAC) can be dosed prior to settling or filtration. 19/21
Activated carbon treatment is not common at Australian water treatment plants, mostly due to costs. Sydney Water's Orchard Hills plant has installed capacity for emergency ('on-demand') PAC dosing. But (as far as I know), its not available at the Prospect plant. 20/21
This summer, major bushfires have severely impacted Australian drinking water catchments including Sydney's. Cyanobacterial blooms seem likely following heavy rain. Taste & odour issues may ensue, but good management should maintain safe drinking water, at least in Sydney. 21/21
Afterthought (1): I didn't explain how activated carbon works. Activated carbon (GAC or PAC) works by some chemicals 'adsorbing' to it, which means they stick to it. So the chemicals are removed either by slow biodgradation on the carbon surface, or by removing the carbon itself.
