Mechanisms of nitrite addition for simultaneous sludge fermentation/nitrite removal (SFNR)

• Dosing nitrite substantially accelerates PS fermentation.
• SFNR process achieves high COD yields and VSS reduction at a short SRT.
• SFNR produces more soluble organics, but releases less nutrients and methane.
• SFNR improves sludge reduction and nitrogen removal simultaneously.
• SFNR process well integrates sludge treatment and wastewater treatment.

Simultaneous sludge fermentation and nitrite removal (SFNR) was investigated as a novel sludge/wastewater treatment process with high nitrogen concentrations. The results showed that introducing nitrite improved the primary sludge (PS) fermentation system by improving the chemical oxygen demand (COD) yields and the volatile suspend solid (VSS) reduction. At a nitrite dosage of 0.2 g g SS−1, the COD production was 1.02 g g VSS−1 and the VSS reduction was 63.4% within 7-day fermentation, while the COD production was only 0.17 g g VSS−1 and the VSS reduction was only 4.9% in the blank test. Nitrite contained in wastewater was removed through denitrification process in the SFNR system. The solubility of carbohydrate and protein was substantially enhanced, and their contents reached the peak once nitrite was consumed. In addition, the nutrient release and methane generation were inhibited in the SFNR system, which alleviated the environmental pollution. Unlike traditional fermentation systems, neither alkaline condition nor high free nitrite acid (FNA) concentration affected the PS fermentation in the SFNR system. Molecular weight distribution (MWD) and Live/Dead cell analysis indicated that the sludge disruption by nitrite and the consumption of soluble organic substances in sludge might play important roles in SFNR.

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