Due to the increased concern on dissolved organic nitrogen (DON) in surface waters, it is necessary to understand the biodegradability and bioavailability of DON in point and non-point sources. In this study, algae and bacteria were applied under lab condition to undestand the impact of DON to water environment. Biodegradable DON (BDON) was determined using bacteria while bioavailable DON (ABDON) was determined using green algae Selenastrum capricornutum, Chlamydomonas reinhardtii, and Chlorella vulgaris and/or mixed culture bacteria in municipal and animal wastewaters. In both wastewater sources, ABDON efficiencies (%) for all three algae were not significantly different indicating that Chlamydomonas reinhardtii and Chlorella vulgaris can be used as a test species for nitrogen determination similar to Selenastrum capricornutum. Results showed that, the ranges of BDON and ABDON in municipal wastewaters were 50-60% and 30-77%, respectively, while the ranges of BDON and ABDON in animal wastewaters were 48-54% and 40-81%, respectively.