1. Fill the cleaning tank with RO permeate water. The volume of cleaning solution should be sufficient to fill all the pressure vessels and pipe lines. Add the calculated amount of the cleaning chemicals to the tank. Use a mixer or recirculate the solution with the transfer pump to ensure that all chemicals are dissolved and well mixed before circulating the solution to the elements.
2. Drain most of the water from the RO system to prevent the dilution of the cleaning solution by water within the RO system.
3. Heat the solution to the temperature recommended by the manufacturer to improve cleaning effectiveness.
4. Pump the preheated cleaning solution to the vessel at conditions of low flow rate (About half of that shown table 1) and low pressure to displace the process water remaing in the vessel. Pump the displaced water until the presence of the cleaning solution is evident in the RO concentrate system or in the return pipe  indicated by the pH and temperature of the cleaning solution. Adjust flow role and pressure according to the Table. Open the RO concentrate throttling valve completely to minimize operating pressure during cleaning. Use only enough pressure to recirculate the cleaning solution without permeate coming out.

Element Diameter (in)	Maximum Feed (gpm)	Flow Rate (m3/h)
2.5	5	1.1
4	10	2.3
8	40	9

5. Recycle the concentrate to the cleaning solution tank until the desired temperature is maintained throughout the system. Observe any increase in the turbidity to judge  efficiency of the cleaning solution, especially in the case of an alkaline cleaning solution or detergent solution. If the cleaning solution becomes turbid or colored, drain the solution and restart with a freshly prepared cleaning solution. Check the pH during acid cleaning. The acid is consumed when it dissolved inorganic precipitates, if the pH increases more than 0.5 pH unit, add more acid.

6. Turn the pump off and allow the elements to soak in the cleaning solution. Sometimes a soak period of about 1 hour is sufficient. For severe fouling, an extended soak period is necessary: soak the elements for 10-15 hours. To maintain a high temperature during an extended soak period, use a slow recirculation rate (about 10% of that shown in Table)

7. Circulate the cleaning solution at  the rates shown in Table 1 for 30 - 60 minutes. The high flow rate flushes out the flushes loosened from the membrane surface by the cleaning. If the elements are heavily fouled, a flow rate 50% higher than shown in Table 1 may aid cleaning. At higher flow rates excessive pressure drop may be a problem. The maximum recommended drops are 1.4bar (20psi) per element or 4.1 bar (60psi) per multi-element vessel. The direction of flow during cleaning must be the same as during normal operation to avoid telescoping of the elements.

8. Drain the used cleaning solutions out of the system. Analyse a sample of the used solution to determine the types and the amount of substances (fouling materials) removed from the membrane elements. The results could tell the degree of cleaning and the causes of fouling.

9. RO permeate or good quality water (filtered. SDI<3), free of bacteria and chloine, conductivity <10,000 s/cm is used for flushing out the residual cleaning solution. The minimum flush out temperature is 20⁰C to prevent precipitation.

 10. The RO Plant is started up again resuming normal operating conditions. However, the permeate must be drained until conductivity and pH returns to normal. And also the permeate side draining is necessary when another cleaning cycle with another cleaning chemical is to follow. During the rinse out step the operating parameters should be noted to judge the cleaning efficiency and to decide if another cleaning is required. If the system has to be shutdown after cleaning for longer than 24 hours, the elements should be stored in a preservation solution such as 1% sodium bisulfite and 0.5% formaldehyde. For multi-array systems, cleaning should be carried out separately for each array. This can be accomplished either by using one cleaning pump and operating one at a time, or using separate cleaning pump for each array.

  Cleaning Chemicals

Choosing right cleaning chemicals is important since harsh and frequent cleaning will shorter the membrane life, and sometime a wrong choice of cleaning chemicals cab worsen the fouling situation. The cleaning will be more effective if it is tailored to the specific fouling problem. Therefore the type of foulants should be determined prior to cleaning. There are helpful ways to determine the type of foulants as shown below.Analyse the plant Performance data:-

Analyse the feed water to find potential fouling substances:-

Check the analytical results of previously spent cleaning solutions, which may indicate specific fouling substances:-

 Analyse the foulants collected with a membrane filter used for SDI measurement.

 Analyse the deposits on the cartridge filter.

 Inspect the inner surface of the feed line tubing and the feed end scroll of the RO element. If it is reddish-brown, fouling by iron is possible. Biological fouling or an organic material deposit is often slimy or gelatinous.

Table lists suitable cleaning chemical depending on the type of foulants. The acid cleaners are to redissolve inorganic deposits including iron, while the alkaline cleaners are to remove organic fouling including biological matter. Sulphuric acid should not be used for cleaning because of the risk of calcium sulfate scaling.

For the preparation of the cleaning solutions, RO permeate is preferred, but prefiltered raw water may be used. The raw water could have some buttering ability, so more acid or hydroxide may be needed to reach the desired pH level, which is about 2 for acid cleaning and about 12 for alkaline cleaning at 300C, respectively. At 350C, the pH unit is in the range of 2 to 11 and at 500C the allowable pH range is 3 to 10.

Foulant	Cleaning Chemicals	Comments
Inorganic salts CaCO3, CaSO4, BaSO4	0.2% Hydrochloric Acid 0.5% Phosphoric Acid 2.0% Citric Acid	Best O.K. O.K.
Metal Oxides (Iron)	0.5% Phosphoric Acid 1.0% Sodium Hydrosulphite	Good Good
Inorganic Colloids (Silt)	0.1% Sodium Hydroxide (NaOH), 300C 0.025 Sodium Dodecylsulphate 0.1% NaOH, 300C	Good Good
Biofilms	0.1% Sodium Hydroxide, 300C 1.0% Sodium Ethylene Diamine Tetra Acetic Acid (Na, EDTA) And 0.1% NaOH, 300C	Best Best when biofilm Contains inorganic
Organics	0.025% Sodium Dodecyisulphate 0.1% NaOH, 300C 0.1% Sodium Triphosphate 1.0% No4 EDTA	Good Good
Silica	0.1% Sodium Hydroxide, 300C 1.0% Sodium Ethylene Diamine Tetra Acetic Acid (Na4 EDTA) and 0.1% NaOH, 300C	O.K. O.K.

Table shows the working formula for cleaning solutions, but brand name cleaning chemicals are frequently used in the field rather than self-made formulations. Most of the brand name chemicals are compatible with membranes in short term test. The long term compatibility test including cleaning efficacy test should be carried out. In the mean time, they can be used as long as the membrane performance is carefully monitored to detect any long term effects at an early stage. In any event, make sure that the brand name chemicals do not contain cationic and non-ionic surfactants, and the pH of the cleaning solution from the chemicals does not exceed the limits at the specified temperature.

 If the RO system suspected to be infected by bacteria or mold, e.g. slimy deposit or rotten smell, a disinfection should be performed after the cleaning. The procedure is the same as for cleaning, except the high flow pumping step.

Commonly used disinfectants are formaldehyde, hydrogen peroxide, peracetic acid and chlorine. Quarternary ammonium disinfectants iodine, and phenolic compounds should not be used because they cause flux losses.

 The effective concentration of formaldehyde is in the range of 0.5 to 3% Care should be taken in handling this chemical since it is considered a carcinogen.

 A 400ppm peracetic acid solution (also containing 2000ppm of hydrogen peroxide) Can be used to disinfect the RO system. The biocidal efficacy of peracetic acid is much higher than hydrogen peroxide. Care must be exercised not to exceed the 0.2% concentration as a sum of both compounds.

However when hydrogen peroxide is employed alone upto 0.2% concentration, the pH of the solution is preferably adjusted to be 3. This will ensure optimal biocidal effect and minimum damage to the membrane. If an alkaline cleaning has preceded disinfection,  acid rinsing is recommended for both sides of the membrane. Additionally, hydrogen peroxide can attack the membrane more aggressively at temperature above 25⁰C and in the presence of transition metals such as iron and manganese.

Membranes and withstand short-term exposure to free chlorine (hypo chlorite). However, eventual degradation may occur after 200-1000 hours of exposure to one ppm chlorine, depending on feed water characteristics, e.g. pH and the presence of heavy metals. Thus Chlorine is not recommended for disinfecting the membrane, but can be used in the pre-treatment prior to the RO elements.

Disinfection using chloramines, Chloramines-T and N-chloroiso-cyanurate is not recommended, since their effectiveness as disinfectants at low concentration (,3mg/) is limited and the compound can also slowly damage the membranes.