Granular stability, nitrogen and phosphorus removal pathways of aerobic granular sludge treating real municipal wastewater at different temperatures

Abstract

Variations in temperature (T) can affect the activity of microorganisms in aerobic granular sludge (AGS) and biological wastewater treatment. Seasonal or diurnal fluctuations specifically low T can affect the overall wastewater treatment performance. Here, the short-term response of AGS to variation in T was investigated in terms of granular stability, sewage treatment and biological nutrient removal (BNR) pathways. AGS cultivated under tropical climate at T ≥ 30 ℃ was incubated at 15–25 ℃ by decreasing either in 10–15 °C steps or in 5 °C steps. Ammonium (74 % at 15 ℃; 94 % at 20 ℃), total inorganic nitrogen (46 % at 15 ℃; 50 % at 20 ℃), and phosphorus (38 % at 15 ℃; 51 % at 20 ℃) removals were significantly lower at 15 ℃ and 20 ℃ than at 30 ℃ (ammonium: >99 %; total inorganic nitrogen: 99 %; phosphorus: 76 %). The effect of lower temperature on BNR was less prominent when T was gradually decreased in 5 ℃ steps from 30 to 15 ℃. Moreover, the impact of lower temperature was quickly recovered upon reverting to 30 ℃. Ammonium was removed by partial nitrification and denitritation pathway. While, phosphorus was removed through enhanced biological phosphorus removal (EBPR) via polyphosphate accumulating organisms (PAOs). About 2–4-fold difference in the gene copies of ammonium oxidizing bacteria (AOB), denitrifying bacteria, and PAOs between 5 ℃ and 10 ℃−15 ℃ changes, respectively, explained operation of partial nitrification-denitritation and EBPR mechanisms. This study shows that a gradual decrease in T from 30° to 15°C in 5 ℃ steps can avoid inhibition of BNR pathways in AGS reactors. The granules were found to be stable with excellent settling properties. However, the sludge was dominated by smaller granules at lower temperatures of 15 ℃ and 20 ℃ as compared to 30 ℃ warranting long-term studies for evaluating the potential impact of T on AGS process.