Application of out–in MF/UF-systems for drinking water treatment with air supported backwash — three case studies

Membrane filtration is increasingly accepted as a treatment process for drinking water supply in Germany as it guarantees an effective particle removal. However, there is still research and development required for the regular backwash and cleaning of the membrane systems. In this paper three case studies will be shown where MF/UFsystems with minimized chemical demand were operated. These systems use air-water-mixtures on the feed side of the membrane during regular backwash of the membranes with filtrate and thereby causing a mechanical cleaning of the membrane surfaces. The investigations showed that regular backwash with filtrate supported by air flush on the feed side is a very effective method to keep TMP low and minimize its increase. The use of air supported backwash helps minimizing the demand for wastewater. As for the regular backwash with filtrate no chemicals are required, the backwash waters may be discharged to the recipient or the sewer without any restrictions. The investigations showed also, that there still is the need for chemical cleaning, because some irreversible fouling always remains. The frequency of chemical cleaning however is very much dependent on the raw water quality. A specific throughput has been calculated which should help to compare different systems with each other.

The investigations showed that the tested MF/ UF-systems using air supported backwash were suitable to treat the different types of raw waters. The demand for backwash water and chemicals was rather low with filtrate qualities being stable over the period of investigation. Table 3 summarizes the results of the three case studies. Systems without air flush usually require a wastewater demand higher than 5% related to filtrate production. In order to be able to compare the performances of different membrane systems with each other a specific throughput (m3/m2/bar) is defined as the filtrate volume (m3) produced for a time interval between two chemical cleanings related to the effective membrane area (m 2) of the membrane system and the TMP increase (bar) during this filtration period (see bottom line in Table 1). The specific throughput is corrected for a temperature of 20°C. Small values for the specific throughput mean often chemical cleaning and large values for the specific throughput mean less chemical cleaning. While considering the transmembrane pressure module 1 chemical cleaning transmembrane pressure module 2 raw water turbidity raw water temperature raw water temperature 1.8 1.6 period of malfunction of back wash process 1.4 1.2 1.0 0.8 0.6 Flux: 70 L/m²/h backwash/airflush cycle: module 1: 30 min module 2: 66 min 0.4 0.2 0.0 operation time, weeks Fig. 3. Increase of TMP with time as a function of raw water turbidity and raw water temperature (case 3).