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CO2 concentration in activated sludge in the presence (circles) or absence (triangles) of ammonium sulfate over 10 h. Closed symbols show the CO2 concentration in samples pulsed with [12C]bicarbonate, while the open symbols show the CO2 concentration in the treatments with [13C]bicarbonate. Arrows show the three time points at which the labeled pulses were added. The data were gathered from a single reactor per treatment.
