TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The second effluent is alkaline and/or acid bleaching filtrate from the pulp mill. The pH is controlled during the fermentation by the second effluent. The pH of the first effluent is controlled before the hydrolysis and/or fermentation by the second effluent. Water is removed from the first effluent to provide a concentrate. The first effluent is filtered to provide a concentrate. The first effluent is wastewater from the pulp mill. The hydrolysis and fermentation are carried out at the same process stage. The hydrolysis is chosen from acid hydrolysis, alkaline hydrolysis, enzymatic hydrolysis and their combinations. After the fermentation, a suspension containing the hydroxycarboxylic acid is concentrated to form the hydroxycarboxylic acid product. After the fermentation, the suspension containing the hydroxycarboxylic acid is filtered for separating fermentation residue from the hydroxycarboxylic acid suspension. The fermentation residue is treated by an anaerobic treatment, burning and/or lignin purification. After the fermentation, the suspension containing the hydroxycarboxylic acid is concentrated and the hydroxycarboxylic acid is separated.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Device: The hydrolysis and fermentation unit comprises a hydrolysis device (11) and a fermentation device (12). The pH controlling unit is arranged to control the pH during the fermentation of the saccharides. The pH controlling unit is arranged to control the pH of the first effluent before the hydrolysis and/or fermentation. The apparatus includes a dewaterer for removing water (17) from the first effluent to provide a concentrate. The apparatus includes a first filtering device for filtering the first effluent to provide a concentrate. The apparatus includes a second concentration device (15) for concentrating a suspension containing the hydroxycarboxylic acid to form the hydroxycarboxylic acid product. The apparatus includes a second filtering device (14) for filtering the suspension containing the hydroxycarboxylic acid after the fermentation to separate lignin and fermentation residue from the hydroxycarboxylic acid suspension. The second concentration device is a suspension melt crystallization device. The second effluent used in the pH controlling is a bleaching filtrate. EXAMPLE - No suitable example given.