Fouling control in ceramic nanofiltration membrane by pre-coating with calcium carbonate

Abstract

Water scarcity and uneven distribution of water resources pose a significant challenge globally. Searching for alternative water resources could alleviate this issue. Municipal sewage reclamation with ceramic NF membrane has gained momentum nowadays. However, the inevitable fouling, especially organic fouling during membrane filtration, is the major limitation of the application. To mitigate the fouling issues and protect the membrane from frequent sodium hypochlorite cleaning, a reaction-based CaCO3 pre-coating method, which could prevent the direct contact of foulant and ceramic NF surface was developed. Acid cleaning was applied to initiate the reaction between CaCO3, which was attached with foulants (sodium alginate) and acid. Lastly, forward flush was implemented to remove the loosened CaCO3 and sodium alginate layers.

The effectiveness of hydrochloric acid cleaning, formic acid cleaning and citric acid cleaning was studied in this paper. Citric acid was found to be the most effective way of cleaning because of the highest permeability recovery rate obtained and the lowest consumption rate of the pre-coating layer. This could be ascribed to the carboxyl groups chelation with calcium ions and the ‘peeled’ chelates adsorbed with more foulants were flushed away. Formic acid was less efficient showing moderate efficiency. Besides, HCl cleaning restored the lowest extent of membrane permeability. Additionally, experiments of increasing the pre-coated CaCO3 amount (7655 mg/m2) to apply for more filtration/acid cleaning cycles were executed. The effectiveness increases in the order of HCl, formic acid and citric acid. The pre-coated membrane cleaned with citric acid could last for the whole six cycles, while the membrane cleaned with HCl only worked in the first three cycles. Lastly, the effect of bubbles generated during the reaction was explored using CaHPO4 as a pre-coating layer. However, the hypothesized positive impact of bubbles was not verified.