Leachability and leaching patterns from aluminium-based water treatment residual used as media in laboratory-scale engineered wetlands

Concept and purpose Virtually all water treatment facilities worldwide generate an enormous amount of water treatment residual (WTR) solids for which environmentally friendly end-use options are continually being sought as opposed to their landfilling. Aluminium-based WTR (Al-WTR) can offer huge benefits particularly for phosphorus (P) removal and biofilm attachment when used as media in engineered wetlands. However, potential environmental risks that may arise from the leaching out of its constituents must be properly evaluated before such reuse can be assured. This paper presents results of an assessment carried out to monitor and examine the leachability and leaching patterns of the constituents of an Al-WTR used as media in laboratory-scale engineered wetland systems. Main features, materials and methods Al-WTR was used as media in four different configurations of laboratory-scale engineered wetland systems treating agricultural wastewater. Selected metal levels were determined in the Al-WTR prior to being used while levels of total and dissolved concentration for the metals were monitored in the influent and effluent samples. The increase or decrease of these metals in the used Al-WTR and their potential for leaching were determined. Leached metal levels in the effluents were compared with relevant environmental quality standards to ascertain if they pose considerable risks. Results Aluminium, arsenic, iron, lead and manganese were leached into the treated effluent, but aluminium exhibited the least leaching potential relative to its initial content in the fresh Al-WTR. Levels of P increased from 0.13 mg-P/g (fresh Al-WTR) to 33.9-40.6 mg-P/g (used Al-WTR). Dissolved levels of lead and arsenic (except on one instance) were below the prescribed limits for discharge. However, total and dissolved levels of aluminium were in most cases above the prescribed limits for discharge, especially at the beginning of the experiments. Conclusions, recommendations and perspectives Overall, the study indicates that leaching is observed when Al-WTR is beneficially reused for enhanced P removal in engineered wetlands. In particular, levels of aluminium in the treated effluent beyond the prescribed limits of 0.2 mg/l were observed. However, since the results obtained indicate that aluminium leached is mostly associated with solids, a post-treatment unit which can further reduce the level of aluminium in the treated effluent by filtering out the solids could serve to mitigate this. In addition, plants used in such wetland systems can uptake metals and this can also be a potential solution to ameliorating such metal releases. Periodic monitoring is thus advised. Notwithstanding, the use of Al-WTR as a media in engineered wetlands can serve to greatly enhance the removal of P from wastewaters and also serve as support material for biofilm attachment.