1. Methods for assessing biological contamination
Raw water generally contains microorganisms, namely bacteria, algae, fungi, viruses, etc. Bacteria are generally 1~3μm in size. Unlike inanimate particles, they have the ability to reproduce and form biofilms under suitable living conditions. When the thickness of the biofilm exceeds a certain limit, it will cause biological pollution.
Biofouling of membrane elements will seriously affect the performance of reverse osmosis systems, causing a rapid increase in the pressure difference between the feed water and concentrate, leading to the “telescope” phenomenon and mechanical damage to the membrane elements, resulting in a decrease in membrane permeate output. Sometimes, biofouling can even occur on the permeate side of the membrane elements, causing contamination of the system’s permeate.
(1) Cultivation method
The concentration of bacteria in water is a significant factor influencing the likelihood of biofouling. Total bacterial count (TBC) is a quantitative representation of the total number of microorganisms present in a water sample. Following the method specified in ASTM F60, a quantitative water sample is filtered using membrane filtration to determine the total bacterial count. Biological tissue trapped on the filter media surface is placed in a nutrient solution and cultured to form colonies. These colonies can be observed and counted using a low-powered microscope. The main advantages of this method are its simplicity and low cost; however, results are only available after one week. Culture techniques are an effective method for indicating the degree and trend of microbial contamination. They can be used to observe the situation in influent, concentrate, and permeate. An increase in the total bacterial count in the concentrate indirectly indicates biofilm contamination within the membrane element.
(2) Direct bacterial count
Direct counting techniques involve filtering a water sample and then counting the microorganisms trapped on the filter medium directly under a microscope. To make the microorganisms observable, they must be stained before being counted under a transmission fluorescence microscope.
This allows for immediate determination of the precise total number of microorganisms, and the types of microorganisms can be distinguished from the sediment particles. Using INT (Intensive Interference Microscopy), areas of reduced staining indicate enrichment by living cells. Phase contrast and differential interference microscopy is then used to differentiate living cells from dead cells. This method is faster and more precise than culture methods, making it the preferred primary approach.
(3) Biofilm detection
The concentration and type of nutrients, as well as operating parameters, indirectly determine the development trend of biofilms. In actual system operation, regularly and carefully inspecting the filter cartridges of the security filter and the inner surfaces of the inlet and concentrate pipes is also an effective practice; the presence of slime and odors indicates microbial contamination.
2. Biological pollution control
(1) Chlorine disinfection
Chlorine has long been used as a sterilizing agent to treat municipal and industrial water supply and wastewater because it can rapidly inactivate many pathogenic microorganisms. The efficiency of chlorine depends on the chlorine concentration, contact time, and pH value of the water; the sterilization effect can be confirmed by measuring residual chlorine levels.
Chlorine disinfection is typically used in the reverse osmosis pretreatment section to prevent microbial contamination, maintaining a reaction time of 20-30 minutes to keep the residual chlorine concentration at 0.5-1.0 ppm throughout the pretreatment pipeline. However, thorough dechlorination treatment is necessary before the chlorine enters the membrane element to prevent oxidative damage from chlorine.
(2) Chlorination reaction
Commonly used chlorine-containing disinfectants include chlorine gas, sodium hypochlorite, or calcium hypochlorite. In water, they rapidly hydrolyze into hypochlorous acid, which then decomposes into hydrogen ions and hypochlorite ions.
2NaClO3 +H2O2 +H2SO4 ↔2ClO2 +O2 +NaSO4 +2H2O
Chlorine reacts with ammonia in water to form chloramines. These chloramine compounds are called bound chlorine (CAC) or bound residual chlorine (CRC). The sum of residual chlorine and bound chlorine is called total residual chlorine (TRC).
(3) Ozone
It has a stronger oxidizing power than chlorine and can decompose rapidly. Therefore, a certain concentration needs to be maintained to kill microorganisms, while also considering the ozone resistance of the equipment used. To protect the membrane elements, ozone must be carefully and thoroughly removed.
(4) Ultraviolet irradiation
Ultraviolet light has been proven to have bactericidal effects and is already being used in small-scale water treatment plants. It does not require the addition of chemicals to the water and has low equipment maintenance requirements, only requiring periodic cleaning or replacement of the steam lamp tubes. However, it is not suitable for water sources with poor quality because colloids and organic matter can affect the light’s penetration.
