Ketchup is highly viscous and contains high concentrations of carbohydrates. These properties make waste ketchup more costly to treat prior to disposal. In this study ketchup was used to grow Ochromonas danica, a phagotrophic microalga that can produce and accumulate intracellular lipids using soluble or particulate organic substrates. Effects of the following factors on high-density O. danica fermentation were evaluated: nitrogen source, pH, temperature, agitation speed, aeration method, dissolved oxygen concentration, and nutrient feeding strategy. Under the optimized conditions the alga grew with a doubling time of about 10 h. Cell yield and intracellular lipid yield from the consumed ketchup carbohydrate were 40% and 18%, respectively. The intracellular lipid content could be raised to 40%. The developed process may also be applied to effectively convert other organic wastes to algal lipids, for use as, e.g., biofuel feedstock. (C) 2014 Elsevier Ltd. All rights reserved.