The phosphorus
release in the anaerobic zone increased from run 1 to
run 3, but it decreased in run 4. Compared with 3.5
and 3 mg·L−1 of the anaerobic phosphorus release in
run 3 and run 4 respectively, those in run 1 and run 2
were negligible (0.2 and 0.3 mg·L−1 respectively). In
run 1 or run 2, there was phosphorus uptake in the
pre-anoxic zone, whereas in run 3 or run 4 there was
phosphorus release in the pre-anoxic zone. Furthermore,
run 4 had more phosphorus release in pre-anoxic zone