SEASONAL MICROALGAE VARIATION IN A SUBARCTIC WASTEWATER STABILIZATION POND USING CHEMICAL PRECIPITATION

Surface water microalgae samples were collected during the ice-free period in a small subarctic wastewater stabilization pond system, complemented with chemical precipitation during the winter period (fellingsdam), and serving 310 persons. In the primary pond microalgae dominance alternated between the Cryptophyte
Cryptomonas and green algae (Chlorophyta). In the second and third pond the general pattern was that Cryptomonas dominated during summer but was replaced by green algae in autumn and the following spring. Estimations of the microalgae part of the effluent COD and phosphorus showed that microalgae dominated these parameters for only 3–4 weeks of 12 evaluated. This does not support the reasoning behind the European
Union directive of the use of filtered samples for effluent BOD, COD and SS from stabilization ponds, in contrast to other wastewater treatment methods. The reasons behind the EU’s procedure for ponds are based on the assumption that stabilization ponds convert “sewage BOD” to “algal BOD”. The results of this study suggest that further investigations of the microalgae function in subarctic wastewater stabilization ponds and
fellingsdams should be conducted, before implementing the EC directive into Swedish law, or into similar laws in other countries with subarctic regions.