The work plan comprised two batches of four experiments. The experimental conditions are summarised in Table 1. Algal paste was re-suspended with de-ionized water and then the mixed liquor was evenly distributed between four beakers (reactor volume 500 ml) for FNA pretreatment Pre-determined amounts of sodium nitrite stock solution (30 g N/L) were added to the batch reactors in different volumes at the beginning of each experiment, which resulted in initial concentrations of nitrite varying between 0 and 1200 mg NO2–N/L, as summarized in Table 1. Batch 1 was treated for 48 h and batch 2 was treated for 60 h. pH was kept approximately constant during the whole pretreatment at 5.0 ± 0.2 or 6.0 ± 0.2 (according to Table 1) by manually adding 0.5 M HCl. All reactors were well mixed by magnetic stirrers at a constant speed of 350 rpm.