Free Nitrous Acid (FNA) pre-treatment is a promising technology demonstrated effective in improving waste activated sludge degradability and anaerobic digestion (AD) performance. Pre-treatment conditions including FNA concentration and treatment duration determine operational and capital cost of full-scale implementation, which have not been studied in long-term experiments. The knowledge of FNA pre-treatment conditions improving the AD performance is urgently required to determine suitable conditions for the technology implementation. In this work, five different FNA concentrations (2.2, 4.4, 7.2, 12 mgN/L and nitrite only without pH adjustment) and three treatment durations (8, 24 and 48 h) were studied in four lab-scale semi-continuous AD reactors for over 300 days. FNA pre-treatment was shown under all tested conditions effective in enhancing AD performances, while its effectiveness and resulted benefits varied substantially amongst different pre-treatment conditions. The long-term experiment demonstrated that the methane production, sludge reduction and digested sludge viscosity of AD are positively correlated with the FNA concentration and durations, until an optimal condition is reached, which was identified in this work to be FNA concentration of 7.2 mgN/L and treatment duration of 24 h. Microbial community changes supported the apparent observation of enhanced sludge degradation at elevating FNA concentrations applied during pre-treatment. The short-term sludge solubilization results were inconsistent with the long-term AD performance, which was potentially caused by inhibitions from stringent FNA pre-treatment conditions applied (FNA = 12 mgN/L with 24-hour treatment & FNA = 7.2 mgN/L with 48-hour treatment). Overall, results suggested FNA pre-treatment at the optimized condition is highly beneficial to WWTPs and competitive with other pre-treatment technologies, e.g., thermal hydrolysis pre-treatment. This work comprehensively evaluated the key design parameters of FNA pre-treatment process, reached a major milestone in the development and applications of FNA technologies.

游离亚硝酸（FNA）预处理是一种很有前途的技术，已被证明可有效提高废物活性污泥的降解性和厌氧消化（AD）性能。包括 FNA 浓度和处理持续时间在内的预处理条件决定了全面实施的运营和资本成本，这在长期实验中尚未得到研究。迫切需要提高 AD 性能的 FNA 预处理条件的知识，以确定技术实施的合适条件。在这项工作中，在四个实验室规模的半连续 AD 反应器中研究了五种不同的 FNA 浓度（2.2、4.4、7.2、12 mgN/L 和亚硝酸盐，仅没有 pH 调节）和三种处理持续时间（8、24 和 48 小时）超过 300 天。FNA 预处理在所有测试条件下均显示可有效提高 AD 性能，而其有效性和产生的益处在不同的预处理条件之间存在很大差异。长期实验表明，AD 的甲烷产量、污泥减少量和消化污泥粘度与 FNA 浓度和持续时间呈正相关，直至达到最佳条件，在本工作中确定为 FNA 浓度为 7.2 mgN/ L 和处理持续时间为 24 小时。微生物群落的变化支持了在预处理期间应用的提高 FNA 浓度时污泥降解增强的明显观察结果。短期污泥增溶结果与长期 AD 性能不一致，这可能是由严格的 FNA 预处理条件的抑制引起的（FNA = 12 mgN/L，24 小时处理，FNA = 7.2 mgN/L，48 小时处理）。总体而言，结果表明优化条件下的 FNA 预处理对污水处理厂非常有利，并且与其他预处理技术（例如热水解预处理）竞争。这项工作全面评估了FNA预处理工艺的关键设计参数，达到了FNA技术发展和应用的一个重要里程碑。热水解预处理。这项工作全面评估了FNA预处理工艺的关键设计参数，达到了FNA技术发展和应用的一个重要里程碑。热水解预处理。这项工作全面评估了FNA预处理工艺的关键设计参数，达到了FNA技术发展和应用的一个重要里程碑。