Background

The management of sludge from municipal wastewater treatment plants (WWTPs) is a global issue, but also an opportunity for circular reuse. Recent data show that sludge reuse in agriculture has the highest share of all utilization routes in the EU. Council Directive 86/278/EEC on the spreading of sludge in agriculture, adopted more than 35 years ago, is still relevant, notwithstanding the discussion on the need to update it. Extracting critical, strategic, and precious metals and metalloids from sludge is an alternative for sludge reuse, which offers several benefits, such as avoiding the high environmental and health risks associated with using sludge directly in agriculture. Additionally, it allows for the recovery of metals, including those listed as Critical Raw Materials by the European Commission. To implement this alternative, it is necessary to first assess the metal content in the sludge and then develop economically and technically viable technologies. In this study, the content of chemical elements in the sludge of eight full-scale WWTPs in Bulgaria is analysed with focus on: (1) assessing the suitability for agricultural application by evaluating the content of macro- and micronutrients and hazardous metals; (2) assessing the possibility of using the sludge as a source of critical and precious metals.

Results

For the main nutrients, the following contents as a percentage of the sludge dry weight (DW) were recorded—2.06% to 6% for N, 1.52% to 2.67% for P and 0.47% to 0.81% for K, which are in line with case studies of successful sludge application in agriculture. Only sludge samples from two WWTPs exceeded the permitted limit for hazardous metals and metalloids. On the other hand, of the 21 metal and metalloid constituents listed in the EU Critical and Strategic Material (CRM) list, at least one of the examined samples has a content above 10 mg/kg for 15 elements. The average contents in mg/kgDW of Au (1.1), Al (19,272.9), Mg (6677.6), Ti (1730.9), Ga (20.9) and As (16.6) measured in the investigated WWTPs are among the highest or second highest reported in other countries.

Conclusions

The results of the study show prospects for optimising and improving the reuse of sewage sludge in Bulgaria. Sewage sludge from most WWTPs has potential for agricultural application due to its high nutrient content. Large amounts of accumulated critical and strategic metals, gold and silver are trapped in Bulgarian sewage sludge, indicating that sewage sludge could be considered an alternative source with high potential for these valuable elements.

中文翻译：

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八个城市污水处理厂污泥中的有价值元素及其回收潜力

背景

城市污水处理厂（WWTP）污泥的管理是一个全球性问题，但也是循环利用的机会。最近的数据显示，农业污泥再利用在欧盟所有利用途径中所占比例最高。尽管存在关于是否需要更新的讨论，但 35 年前通过的关于农业污泥扩散的理事会指令 86/278/EEC 仍然具有相关性。从污泥中提取关键的、战略性的、贵金属和非金属是污泥再利用的一种替代方案，它具有多种好处，例如避免直接在农业中使用污泥带来的高环境和健康风险。此外，它还可以回收金属，包括被欧盟委员会列为关键原材料的金属。为了实施这种替代方案，有必要首先评估污泥中的金属含量，然后开发经济和技术上可行的技术。本研究对保加利亚八个大型污水处理厂污泥中的化学元素含量进行了分析，重点是：（1）通过评估常量和微量营养素以及有害金属的含量来评估其农业应用的适宜性；(2)评估利用污泥作为关键金属和贵金属来源的可能性。

结果

对于主要养分，记录了以下含量（占污泥干重（DW）的百分比）：N 为 2.06% 至 6%，P 为 1.52% 至 2.67%，K 为 0.47% 至 0.81%，均符合要求。污泥在农业中成功应用的案例研究。只有两个污水处理厂的污泥样品超出了有害金属和类金属的允许限值。另一方面，在欧盟关键和战略材料（CRM）清单中列出的 21 种金属和准金属成分中，至少有一个检测样品的 15 种元素含量超过 10 毫克/千克。在调查的污水处理厂中测得的 Au (1.1)、Al (19,272.9)、Mg (6677.6)、Ti (1730.9)、Ga (20.9) 和 As (16.6) 的平均含量 (mg/kgDW) 属于报告中最高或第二高的水平在其他国家。

结论

研究结果显示了保加利亚优化和改进污水污泥再利用的前景。大多数污水处理厂的污水污泥由于其营养成分高而具有农业应用潜力。大量积累的关键和战略金属、金和银被困在保加利亚的污水污泥中，这表明污水污泥可以被视为具有这些有价值元素的高潜力的替代来源。