Pre-treatment issues: reverse osmosis

Table 33. Troubleshooting reverse osmosis (RO) issues

Probable causes	Preventive measures	Checking preventive measures		Corrective action
		What to check	Signs that action is needed
Observation: Low product water or accelerated differential pressure build-up due to turbidity Possible hazards: Increases scale build-up
Excessive turbidity to the unit.	Check filters are working optimally Increase the efficacy of pre-treatment Regular membrane cleaning (usually RO units have an automatic cleaning)	Turbidity and colour of effluent from filtration system (i.e. going into RO unit)	High turbidity and colour inlet water Membrane fouling Pressure increase	Turbidity should be limited to below 1 NTU to RO unit.
Filter media breakthrough or cartridge filter failure	Monitor water quality going to RO unit Add additional pre-treatments Increase the efficacy of pre-treatment	Turbidity and colour of effluent from the filtration system. (i.e. going into RO unit)	High turbidity and colour inlet water Membrane fouling Pressure increase	Correct filtration problems
Inadequate cleaning of the membrane	Increase frequency of membrane cleaning to a rate that resolves the problem	Cleaning cycle (initiated when the flux is ± 15% of rated value)	Membrane fouling Pressure increase	Initiate more frequent cleaning of the membrane
Fine suspended solids on the face of membrane and feed concentrate mesh spacer	Add additional pre-treatments Increase the efficacy of pre-treatment	Turbidity and colour of effluent from the filtration system. (i.e. going into RO unit) Perform silt density index (SDI) and modified fouling index (MFI)—these should be between 0 and 2.	High turbidity and colour inlet water Membrane fouling Pressure increase	The system will require a higher level of turbidity removal
Observation: Low product water or accelerated differential pressure build-up due to minerals. Possible hazards: Increases scale build-up
Mineral scaling from carbonates, sulphates, silica, ferric (Fe⁺⁺⁺), or H₂S.	Additional pre-treatments	Hardness, iron or manganese and hydrogen sulphide	Membrane fouling Increased pressure	Convert carbonate hardness to CO₂ by adding acid. Remove hardness by ion exchange.
Elemental sulphur and iron oxide deposits on the membrane.	Increase frequency of membrane cleaning to a rate that resolves the problem Additional pre-treatments	Perform water quality tests for: hardness (temporary and permanent), iron or manganese and hydrogen sulphide.	Membrane fouling Increased pressure	Add polyphosphates or silicates to prevent scaling. Remove iron or manganese by greensand, chlorination or dichlorination (note, it is very important to remove residual) (no aeration.) H₂S can be removed by degasification. Initiate more frequent cleaning of the membrane using citric acid.
Observation: Low product water flow rate, higher salt rejection, high operating pressure Possible hazards: Low flow
Membrane compaction	Monitor pressure Ensure membrane installation is done correctly	Operating pressure should be less than 500psi	Low flow rate	Replace membrane.
Observation: High product flow rate with lower salt rejection Possible hazards: High salt
Membrane hydrolysis	Identify the cause of membrane hydrolysis and optimise system Add additional monitoring points for pH and temperature	Check pH and temperature to determine if they are within operating limits	High flow and high salt	Injection of colloid 189 will sometimes extend membrane life Replace membrane

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