Dairy operations in Florida face the dual problem of water pollution and air pollution (odors) as a result of the large amounts of manure produced on the farms. Ground and surface waters are contaminated by nitrogen present in seepage and runoff. A low-cost method of treatment of dairy wastewater is to convert the dissolved nutrients to microalgae biomass in engineered ponds designed to maximize photosynthetic production through solar energy. Laboratory experiments conducted on effluent from an anaerobic lagoon of a modern dairy showed that cyanobacteria (= blue-green algae) grow well on dairy wastewater and that nitrogen removal is rapid and complete. Ammonia nitrogen concentrations were reduced from 100 mg l(-1) to less than 1 mg l(-1) in seven days. Maximum removal rate was 24 mg l(-1) per day. Prospects for nitrogen recycling are considered. Copyright (C) 1996 Elsevier Science Ltd.