In this study an investigation was made on whether a petroleum based polyelectrolyte could be replaced by a biodegradable polymer. This was done to enable the use of sludge from the biogas plant of the dairy in Umeå, Sweden, as fertilizer. The properties and effectiveness of a starch based polyelectrolyte were compared to polyacrylamide in laboratory tests. Other biodegradable polyelectrolytes were also presented. The tests were made on sludge from the dairy. In tests with a waste water with a low dry substance content (<0,1 %) the dewatering effects were similar between polyacrylamide and the starch based polymer. Starch had a small effect on dewatering sludge with a higher dry substance content (4 %). Cellulose fibers in combination with starch increased the amount of dewatered sludge, but polyacrylamide was still the most effective polyelectrolyte. A starch based polymer could be an alternative to polyacrylamide, if cellulose fibers are added. Other biodegradable polyelectrolytes which could have sufficient dewatering effects are chitosan and tannins. There were methodical problems in this study with the accuracy of measuring dry substance content and a difference in dilution of sludge samples when adding chemicals. To proceed in the search for biodegradable alternatives to polyacrylamide at the biogas plant the first step is to develop reliable methods for evaluating the effects of different polyelectrolytes.   Key words: dewatering, sludge, polyelectrolyte, biodegradable