Stricter nutrient discharge limits at wastewater treatment plants (WWTPs) are increasing the demand for external carbon sources for denitrification, especially at cold temperatures. Production of carbon sources at WWTP by fermentation of sewage sludge often results in low yields of soluble carbon and volatile fatty acids (VFA) and high biogas losses, limiting its feasibility for full-scale application. This study investigated the overall impact of thermal hydrolysis pre-treatment (THP) on the production of VFA for denitrification through the fermentation of municipal sludge and digestate. Fermentation products and yields, denitrification efficiency and potential impacts on methane yield in the downstream process after carbon source separation were evaluated. Fermentation of THP substrates resulted in 37–70 % higher soluble chemical oxygen demand (sCOD) concentrations than fermentation of untreated substrates but did not significantly affect VFA yield after fermentation. Nevertheless, THP had a positive impact on the denitrification rates and on the methane yields of the residual solid fraction in all experiments. Among the different carbon sources tested, the one produced from the fermentation of THP-digestate showed an overall better potential as a carbon source than other substrates (e.g. sludge). It obtained a relatively high carbon solubilisation degree (39 %) and higher concentrations of sCOD (19 g sCOD/L) and VFA (9.8 g VFACOD/L), which resulted in a higher denitrification rate (8.77 mg NOx-N/g VSS∙h). After the separation of the carbon source, the solid phase from this sample produced a methane yield of 101 mL CH4/g VS. Furthermore, fermentation of a 50:50 mixture of THP-substrate and raw sludge produced also resulted in a high VFA yield (283 g VFACOD/kg VSin) and denitrification rate of 8.74 mg NOx-N/g VSS∙h, indicating a potential for reduced treatment volumes. Calculations based on a full-scale WWTP (Käppala, Stockholm) demonstrated that the carbon sources produced could replace fossil-based methanol and meet the nitrogen effluent limit (6 mg/L) despite their ammonium content. Fermentation of 50–63 % of the available sludge at Käppala WWTP in 2028 could produce enough carbon source to replace methanol, with only an 8–20 % reduction in methane production, depending on the production process. Additionally, digestate production would be sufficient to generate 81 % of the required carbon source while also increasing methane production by 5 % if a portion of the solid residues were recirculated to the digester.

中文翻译：

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热水解对污泥和沼渣发酵反硝化过程中基于 VFA 的碳源生产的影响：实验和放大影响


污水处理厂 （WWTP） 更严格的营养物质排放限制增加了对反硝化外部碳源的需求，尤其是在低温下。污水处理厂通过发酵污水污泥生产碳源通常会导致可溶性碳和挥发性脂肪酸 （VFA） 产量低以及沼气损失高，从而限制了其全面应用的可行性。本研究调查了热水解预处理 （THP） 对通过城市污泥和沼渣发酵生产用于反硝化的 VFA 的总体影响。评估了碳源分离后发酵产物和产量、反硝化效率和对下游工艺中甲烷产量的潜在影响。THP 底物的发酵导致可溶性化学需氧量 （sCOD） 浓度比未处理底物的发酵高 37-70%，但发酵后 VFA 产量没有显着影响。尽管如此，在所有实验中，THP 对反硝化速率和残余固体部分的甲烷产率都有积极影响。在测试的不同碳源中，由 THP 沼渣发酵产生的碳源总体上比其他基质（例如污泥）具有更好的碳源潜力。它获得了相对较高的碳溶解度 （39%） 和较高浓度的 sCOD （19 g sCOD/L） 和 VFA （9.8 g VFACOD/L），从而导致更高的反硝化率 （8.77 mg NOx-N/g VSS∙h）。分离碳源后，该样品的固相产生 101 mL CH4/g VS 的甲烷产率。 此外，对 THP 底物和产生的原始污泥的 50：50 混合物进行发酵也导致高 VFA 产量 （283 g VFACOD/kg VSin） 和 8.74 mg NOx-N/g VSS∙h 的反硝化率，表明处理量可能会减少。基于全尺寸污水处理厂（斯德哥尔摩 Käppala）的计算表明，产生的碳源可以替代化石基甲醇，并满足氮排放限值 （6 mg/L），尽管其铵含量很高。到 2028 年，Käppala 污水处理厂发酵 50-63% 的可用污泥可以产生足够的碳源来替代甲醇，而甲烷产生量仅减少 8-20%，具体取决于生产工艺。此外，如果将一部分固体残留物再循环到消化池，沼渣的生产将足以产生 81% 的所需碳源，同时还将甲烷产量增加 5%。