Chemical foam cleaning as an efficient alternative for flux recovery in ultrafiltration processes

Foam cleaning of membranes is a promising new approach with high potential to reduce the required amount of chemical cleaning agents. The focus of this study is to utilize this foam cleaning method in a cross-flow ultrafiltration of sludge from an anaerobic digester to increase the competitiveness of anaerobic MBR by reducing down times of plants during cleaning and costs for chemicals. Therefore, 2 h filtration tests with subsequent cleaning were conducted in a laboratory scale cross-flow mode filtration apparatus. The effectiveness of foam cleaning was compared to a reference 2 step cleaning procedure, using solutions of hypochlorite (1000 mg L−1) and citric acid (5 g L−1). Foams were prepared from commercially available agents with similar active components: Calgonit CF 314, CF 210 and SF 5501 (Calvatis GmbH, DE). In the first part of the study, foam density, foam quality and foamability were determined as parameters of foam properties. Furthermore, it was demonstrated that the performance of ceramic ultrafiltration membranes could be efficiently restored using the proposed foam cleaning technique. Beyond performance, reaction time was reduced from 2 h to 30 min and even lower membrane resistances were measured when using foam cleaning compared to the reference method. Also the concentrations of active compounds in the foam cleaners were much lower (CF 210: 93.6 mg L−1 active chlorine; SF 5501: 900 mg L−1 citric acid). Even after repetitive filtration and cleaning experiments, full flux recovery was obtained. Additional scanning electron microscopy (SEM) analyses including identification of surface elements (EDX) were conducted to confirm the removal of fouling layers. Results show that by means of foaming, the amount of cleaning agents required per filling volume can be reduced approximately 4–10 times. Thus, foam cleaning may contribute to reduce the environmental impact of chemical cleaning and chemical costs in diverse UF applications such as AnMBR.