TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Process: The acid water is preferably a heavy metal laden acidic effluent where the addition of the alkaline water causes the precipitation of the majority of the metals as hydroxides. The algal treatment of the water component is preferably effected in a vessel fitted with a mixing device to keep the algae in suspension and ensure an even distribution of light. Some of the algae-containing aqueous component may be withdrawn from the vessel and fresh or make-up aqueous component may be added to the to the vessel. The aqueous component may be normal water not contaminated with unwanted components, the water being optionally supplemented with phosphate and/or nitrate salts to promote algal growth. Alternatively it may be an effluent which does not retard algal growth. A non-sulfate or low sulfate containing source stream may be fed into an anaerobic digester together with CO2 to generate bicarbonate ions and the obtained bicarbonate rich stream may be fed to the algal growth vessel as part or all of the aqueous component.When mixing the alkaline and acidic water, sludge is preferably removed from the bottom of the reaction vessel and purified water is also removed from the vessel, the latter being optionally further treated with a biosorbent. If the purified effluent has a high sulfate loading, it is subjected to sulfate removal by passing through an aerobic digester with a sulfide-containing stream optionally being passed from the digester to the reaction vessel.    TECHNOLOGY FOCUS - BIOTECHNOLOGY - The disclosure states that the algae may be any C-assimilating type, especially unicellular algae or cyanobacteria, such as Spirulina. EXAMPLE - In a pilot plant, a metal-containing effluent in a reaction tank was treated using a filtered overflow from a Spirulina culture tank and then passed to an anaerobic sulfate digester with periodic sludge removal from the digester. The overflow from the digester had the following characteristics (figures in brackets are for raw effluent); all concentrations are in mg/l:    pH 8.18 (1.88); sulfate content 295.83 (4415.51); copper 0.1 (3.49); zinc 0.221 (7.16); lead 0.057 (2.35); and iron 0.263 (98.95).