A pilot-scale experimental high-rate algal pond (HRAP) was investigated in the subarctic mid-Sweden region, at latitude 63°N. During autumn 2002, conditions included temperatures below 10°C and photosynthetic active radiation below 200 µE/m2·s. Biochemical
oxygen demand was reduced by approximately 90% (approximately 40 g/m3), chemical oxygen demand by 65% (approximately 80 g/m3), total phosphorus by 20% (approximately 1 g/m3), and total nitrogen by 46% (approximately 15 g/m3),
at a retention time of approximately 2.5 days. During autumn 2003, the performance of the HRAP appeared better with a more dense microalgae culture; however, as a result of poor settling of the microalgae, the reduction was considerably lower. A major difference between the years was
the microalgae composition. In 2002, the large green algae Coelastrum dominated with Chlamydomonas, Scenedesmus, Lagerheimia, and the Cryptophyte Rhodomonas. In 2003, there was a total dominance of the very small green algae Chlorella, known to be
difficult to settle. In batch growth experiments during spring 2002, doubling times of 4 to 6 days were achieved. The period of temperatures above 10°C and an insolation of more than approximately 270 uE/m2·s (125 Langleys), which is well-documented as appropriate
for HRAP function (Oswald, 1988a, 1988c), were measured to last for 4 to 4.5 months from early May to late September. However, the growth
and treatment performance experiments indicated that a longer season may be possible—6.5 to 7 months, at best—from early April to late October.
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