# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 5 Methodologies for Membrane Fouling Prevention and Reduction
### Membrane Cleaning  
Unlike the use of pretreatments and antiscalants that reduce to the maximum the fouling phenomena, cleaning of fouled membranes is other of the explored strategies to extend the membrane life. Generally, after rinsing the membrane with distilled water, the used chemical cleaning agents include acids, alkalis, surfactants, enzymes and metal chelating agents (i.e., organic compounds that form complexes with metal ions) [15; 17; 156]. Research studies used different strong and weak acids to clean scales such as HCl, which is particularly effective to remove basic crystal salts (e.g., CaCO\({}_{3}\)) by dissolving the deposit from the membrane surface [14; 49; 69]. According to [PERSON] [49], rinsing the membrane module by 3 wt % HCl allowed to dissolve the CaCO\({}_{3}\) scale on the PP hollow fiber membrane and restore the initial membrane permeability. In another study, [PERSON] [69] used 2-5 wt % HCl solution to rinse the membrane module every 40-80 h of DCMD operation. As a result, the formed deposit, which was mainly CaCO\({}_{3}\), was removed and the initial efficiency of the membrane module was recovered. Similarly, [PERSON], et al. [157] used two steps to clean a fouled membrane with HA and CaCO\({}_{3}\) by using citric acid aqueous solution at pH 4 for 20 min followed by 0.1 M NaOH aqueous solution for 20 min. This cleaning procedure also allowed the complete recovery of the permeate flux and the hydrophobicity of the membrane. The high efficiency of basic cleaning procedures against the deposit of HA due to its good dissociation and dissolution at high pH values was also confirmed by [PERSON], et al. [42] who reported that rinsing with distilled water the fouled membrane by HA containing CaCl\({}_{2}\), for 2 h followed by 20 min recirculation of 0.1 M NaOH aqueous solution resulted in 100% permeate flux recovery. [PERSON], et al. [111] studied membrane fouling and cleaning of solar MD plant following different cleaning strategies to remove the fouling layer and restore the membrane properties (i.e., distilled water at pH = 6.15, 5 wt % sulfuric acid at pH = 1, 5 wt % citric acid at pH = 1.77, 5 wt % formic acid at pH = 1.72, 0.1 wt % Na\({}_{5}\)P\({}_{3}\)O\({}_{10}\) + 0.2 wt % EDTA at pH = 6.67 and 0.1 wt % oxalic acid + 0.8 wt % citric acid at pH = 2.2). The use of 0.1 wt % oxalic acid and 0.8 wt % citric acid solution was found to be the most suitable cleaning protocol as it was able to remove a great part of the scaling layer, formed mainly by NaCl, Fe, Mg and Al oxides. After the second cleaning procedure, the distillate quality was improved (i.e., salt rejection factor was 85%). [PERSON] and [PERSON] [82] developed an effective cleaning procedure for PTFE membranes fouled after 2 h of OD processing with 21.5% of tomato puree. The permeate side of the membrane was flushed with fresh water, whereas the feed side of the membrane was rinsed with water at 40-50 \({}^{\circ}\)C for 10 min and then the cleaning agents were circulated at the same temperature range for 60 min. The cleaning agents included water, NaOH, HNO\({}_{3}\) and enzymes (lipolase, alcalase, palatase and pectinex) with different concentrations. The enzyme/surfactant cleaning was performed using P3 Ultras1 25, P3 Ultras1 30, P3 Ultras1 53, P3 Ultras1 60A and 1% P3 Ultras1 75. It was observed that acidic and enzymatic cleaning agents were ineffective cleaners. However, 1% NaOH was the most effective cleaner for membranes with a surface tension greater than 23 mN/m and 1% P3 Ultras156 was the best cleaner too for membranes with a surface tension less than 23 mN/m as it was able to maintain the water permeate flux in addition to the hydrophobicity of the membrane. [PERSON], et al. [103] investigated the effect of consecutive cleaning of MD membranes used for the treatment of model manure solution by using distilled water, alkaline/acid and Novadan cleaning agents.