Chemical Cleaning
Ultrafiltration (UF) systems may be fouled by various impurities in the feed water, such as suspended solids, colloids, organic matter, microorganisms, and hydrated metal oxides. Fouling refers to various deposits covering the membrane surface and adsorbing in the membrane pores, including scaling substances in water.
The purpose of pretreatment is to minimize impurities that cause membrane fouling. This goal can be achieved by installing appropriate pretreatment equipment (e.g., pre-filters, coagulation/ clarification or filtration devices) and selecting proper operating conditions.
UF membrane fouling is generally considered a combination of one or more of the following types:
▬Inorganic fouling/scaling
▬Particulate/colloidal fouling
▬Microbiological/biofouling
▬Organic fouling
▬The possible causes of the above fouling issues include:
▬Inadequate pretreatment system
▬Abnormal operation of the pretreatment system
▬Changes in feed water composition or other conditions
▬Improper operation and control
▬Long-term accumulation of sediments on the membrane surface
▬Seasonal algal bloom fouling
▬Malfunction of chemical dosing systems
▬Improper backwashing and chemically enhanced backwashing (CEB)
▬Improper system shutdown procedures and preservation measures
▬Inappropriate material selection for the system (e.g., pumps and pipelines)
Membrane fouling will lead to the deterioration of system performance, such as decline in water production and flux, increase in transmembrane pressure difference (TMP), as well as higher chemical and energy consumption.
Chemical cleaning is the most effective method to solve membrane fouling problems. Only by adopting corresponding cleaning methods for specific fouling types can optimal results be achieved. Incorrect selection of cleaning chemicals and methods may sometimes worsen the situation. Therefore, it is necessary to determine the type of foulants on the membrane surface before cleaning. The following analytical methods are usually used for this purpose:
▬Analyze system performance data; refer to the detailed troubleshooting instructions in the previous issue.
▬Analyze feed water composition; the possibility of fouling contaminants can often be identified clearly by reviewing the raw water quality report.
▬Check previous cleaning records and their effectiveness.
▬Analyze substances retained on the filter paper used for SDI (Silt Density Index) measurement of feed water.
▬Inspect contaminants in the wastewater discharged from UF air scrubbing and backwashing processes.
▬Examine contaminants at the feed end of membrane modules: a reddish-brown color indicates possible iron fouling; sludge-like or colloidal deposits usually indicate microbiological or organic fouling.
Conditions for Chemical Cleaning
During the normal operation of a UF system, the surface of UF membrane fibers may be fouled by suspended particles, colloidal particles, microorganisms or insoluble organic matter. When such fouling accumulates continuously and cannot be reversed by routine backwashing or chemically enhanced backwashing, it will result in the decline of normalized water production and the increase of normalized transmembrane pressure difference. Membrane modules need to be cleaned to restore system performance when any of the following conditions occur:
▬Normalized water production decreases by 25%.
▬Normalized transmembrane pressure difference increases by 1.0 bar.
▬Operating transmembrane pressure difference rises to the maximum value of 2.1 bar.
▬If you have not normalized your operating data, refer to the above values to decide whether chemical cleaning is required.
It is imperative to strictly monitor the operating performance of the UF system during daily operations, including operating pressure difference and water production flow rate. As membrane fouling progresses, the pressure difference will increase and the water production flow rate will decrease. It should be noted that a decrease in UF membrane water production flow rate caused by a drop in feed water temperature is a normal phenomenon and not an indication of membrane fouling, meaning the UF membrane may not need cleaning under such circumstances.
Selection of Chemical Cleaning Schemes
Acidic and alkaline cleaning agents are widely used cleaning chemicals. The appropriate cleaning scheme should be selected based on the type of foulants.
3.1 Acidic Cleaning Scheme
Acidic solutions are used for cleaning UF systems when the content of Fe or Mn in feed water exceeds the design standards, or when the suspended solids content in the feed water of UF membrane modules is extremely high, leading to inorganic fouling on the feed side of the membrane.
3.2 Alkaline Cleaning Scheme
Alkaline oxidant solutions are adopted for cleaning UF systems when the organic matter content in feed water is high, which may cause organic fouling of the membrane. Additionally, under conditions favorable for microbial growth, some bacteria and algae will reproduce in UF membrane modules, resulting in biofouling.
Notes:
a）All cleaning agents must be fed into UF membrane modules from the feed side to prevent possible impurities in the cleaning agents from entering the interior of membrane fiber walls through the back side of the dense filtration skin layer.
b）Before conducting chemical cleaning on the UF system, repeated air scrubbing and thorough backwashing must be performed first.
c）The entire chemical cleaning process of the UF system takes approximately 2–4 hours; for severe fouling, the soaking time needs to be extended to more than 12 hours.
d）If the UF system will be shut down for more than three days after cleaning, it must be maintained in accordance with the requirements for long-term shutdown.
e）Cleaning solutions must be prepared using UF permeate or higher-quality water.
f）Possible contaminants in the cleaning agents must be removed before the cleaning solution is circulated into the membrane modules.
g）The temperature of the cleaning solution can generally be controlled within the range of 10℃–40℃; increasing the temperature of the cleaning solution can improve cleaning efficiency.
h）If necessary, multiple types of cleaning agents can be used, but the cleaning agents and disinfectants must not cause damage to the membrane and module materials. After each cleaning process, all cleaning agents should be drained, and the system should be thoroughly rinsed with UF or reverse osmosis (RO) permeate before another type of cleaning agent is used.
Chemical Cleaning Procedures
4.1 Acidic Cleaning
A 0.2% HCl solution or a 1–2% citric acid solution is suitable for iron fouling and carbonate scaling.
The basic procedure for acid cleaning of UF membrane modules is as follows:a) Preparation of the cleaning system;b) Circulation of the acidic cleaning solution in UF membrane modules;c) Rinsing of UF membrane modules and restoration to normal production operation.
1) Preparation Work
- a) Shut down the system in accordance with the shutdown procedure.b) Close all valves of the system.c) Prepare a 1–2% citric acid solution or 0.2% HCl solution in the cleaning solution tank, and stir thoroughly to ensure uniform mixing.
2) Cleaning Process
- a) Perform high-frequency, short-duration air scrubbing (generally 3–8 times, 10–15 seconds each time) followed by water backwashing; repeat this process several times until the discharge water from air scrubbing is basically clear. Drain all water from the membrane modules (Note: The cleaning solution should be pumped into the modules immediately after draining to prevent irreversible damage caused by membrane dehydration).
- b) Start the cleaning water pump, slowly open the cleaning pump outlet valve and the UF system cleaning solution inlet/outlet valves. Control the cleaning flow rate of each membrane module as required, allow the cleaning solution to flow into the membrane modules and return to the cleaning solution tank. The circulation cleaning time is 30 minutes.
- c) Turn off the cleaning pump and let the membrane modules soak statically for 60 minutes; extend the soaking time accordingly for severe fouling.
- d) After soaking, restart the circulation for another 30 minutes at the same flow rate.
- e) Drain the cleaning solution tank and cleaning filter, and rinse them thoroughly with clean water.
3) Rinsing of the UF System
The purpose of rinsing is to remove residual chemical solutions from the UF system.a) Open the UF system concentrate drain valve and permeate drain valve.b) Open the UF system feed water valve to allow feed water to pass through the membrane modules until the difference in conductivity between the feed water and the discharged water is within 20 μS/cm.c) Restore the system to normal production operation.
4.2 Alkaline Cleaning
A mixed solution of 0.2% NaClO and 0.1% NaOH is used for cleaning UF membrane modules fouled by organic matter and microorganisms.
The operating procedure for alkaline cleaning of UF membrane modules is as follows:a) Preparation of the cleaning system;b) Cleaning of membrane modules with alkaline oxidant solution;c) Rinsing of membrane modules and restoration to normal operating conditions.
1) Preparation Work
- a) Shut down the UF system in accordance with the shutdown procedure.b) Close all valves of the system.c) Prepare a mixed solution of 0.2% NaClO and 0.1% NaOH in the cleaning tank, and stir thoroughly to ensure uniform mixing.
2) Cleaning Process and 3) Rinsing Process
Same as the steps for acidic cleaning.