The water treatment sludge (WTS), a byproduct of coagulation-flocculation-sedimentation process in drinking water treatment contains significant concentration of aluminium (Al). The WTS is mostly disposed on land or in sewers without treatment. This practice not only poses a serious threat to environment but also results in permanent loss of metals like Al. In order to prevent the adverse effects, studies are being conducted to find alternative use of WTS, and one of the focus areas is to recover coagulant (a solution containing Al) from the WTS by leaching with high strength acid and reuse it as coagulant for treating industrial wastewater. However, the use of the recovered coagulant (RC) for treatment of drinking water is not yet reported. The main concern being the presence of other metals in the RC along with Al. The RC if added to remove turbidity from drinking water may result in elevated concentration of metals in treated water which is objectionable.

In order to address this issue, the present study proposes a method to recover coagulant from WTS, and reuse it safely as a coagulant in drinking water treatment without hampering the treated water quality. Specifically, the study used low strength nitric acid to leach Al from WTS. The leaching studies were performed using a Box-Behnken Design (BBD) with three factors: acid strength (M), reaction time and sludge dose. Each factor was set at three levels: acid strength (0.025 M, 0.1625 M and 0.3 M), reaction time (30 min, 75 min and 120 min) and sludge dose (10 g/l, 12.5 g/l and 15 g/l). As Al is the main element responsible for coagulation, leaching efficiency of Al (AlLE) was fixed as the response variable. Performance of the RC in removing turbidity was evaluated on synthetic turbid water and was compared with turbidity removal obtained with 1% (w/v) solution of commercial alum.

The results indicate that a maximum concentration of 432 mg/l of Al could be achieved in the RC with optimum factor settings (obtained from statistical analysis) of 0.25 M acid strength, 11.17 g/l sludge dose and 120 min reaction time. The addition of 1 ml/l of the RC in synthetic turbid water resulted in 74% removal of turbidity (with effluent turbidity concentration < 20 NTU) which was similar to removal efficiency obtained with addition of 0.6 ml/l of 1% (w/v) commercial alum solution.

The environmental significance of the study is that, it shows that the RC has potential to be used as a coagulant in drinking water treatment. This will not only prevent the permanent loss of Al due to disposing the WTS in landfills but also minimize the requirement of fresh Al salts for coagulation thus resulting in conservation of a natural resource.

水处理污泥 (WTS) 是饮用水处理中混凝-絮凝-沉淀过程的副产物，其中含有大量的铝 (Al)。WTS 大多未经处理就弃置在陆地或下水道中。这种做法不仅对环境构成严重威胁，而且会导致铝等金属的永久性损失。为了防止不利影响，正在进行研究以寻找 WTS 的替代用途，其中一个重点领域是通过用高强度酸浸出从 WTS 中回收混凝剂（含铝溶液）并将其重新用作混凝剂处理工业废水。然而，尚未报道使用回收的混凝剂（RC）处理饮用水。主要问题是 RC 中除了 Al 之外还存在其他金属。

为了解决这个问题，本研究提出了一种从 WTS 中回收混凝剂的方法，并将其安全地用作饮用水处理中的混凝剂，而不会影响处理后的水质。具体来说，该研究使用低强度硝酸从 WTS 中浸出铝。浸出研究使用 Box-Behnken 设计 (BBD) 进行，具有三个因素：酸强度 (M)、反应时间和污泥剂量。每个因素设置为三个水平：酸强度（0.025 M、0.1625 M 和 0.3 M）、反应时间（30 分钟、75 分钟和 120 分钟）和污泥剂量（10 g/l、12.5 g/l 和 15 g/ l)。由于铝是造成混凝的主要元素，铝的浸出效率（AlLE）被固定为响应变量。RC 在去除浊度方面的性能在合成浊度水上进行了评估，并与 1% 的浊度去除率进行了比较（w / v ) 商业明矾溶液。

结果表明，在 0.25 M 酸强度、11.17 g/l 污泥剂量和 120 分钟反应时间的最佳因子设置（从统计分析中获得）的 RC 中，可以实现 432 mg/l 的最大浓度。在合成混浊水中添加 1 ml/l RC 可去除 74% 的浊度（出水浊度浓度 < 20 NTU），这与添加 0.6 ml/l 1% ( w / v ) 商业明矾溶液。

该研究的环境意义在于，它表明 RC 具有用作饮用水处理中的混凝剂的潜力。这不仅可以防止由于在垃圾填埋场中处理 WTS 而造成的 Al 的永久性损失，而且还可以最大限度地减少对新鲜 Al 盐进行凝结的需求，从而保护自然资源。