Chlorine Dioxide for Reverse Osmosis
Chlorine dioxide (Neox)
Chlorine dioxide products are ideal for use in reverse osmosis systems to meet the microbial and oxidative challenges of today’s environmentally concerned world. It is an ideal replacement for chlorine, providing all of the benefits of chlorine and more, but without any of its weaknesses and detriments. Chlorine dioxide is a broad spectrum biocide with 2.6 times the oxidizing capacity of chlorine. It is a selective oxidizer that is effective across a broad pH range. Chlorine dioxide is an effective tool for the treatment of raw water. It is a powerful disinfectant that nicely balances purification performance against disinfection by product formation. It is one of EPA approved disinfectants for drinking water with CT values second only to ozone in biocidal efficacy but without the zonation by-products or high capital expense.
Chlorine dioxide is a highly effective biocide that carries a number of important regulatory approvals including US EPA, FDA and UK Government for many of its uses. It is effective at concentrations as low as 0.1 ppm and will eliminate both planktonic and sessile bacteria.
(Neox) Chlorine Dioxide has several advantages over chlorine, bromides and ozone.
- It is more effective as a disinfectant than chlorine in most circumstances against water borne pathogenic microbes such as viruses, bacteria and protozoa – including the cysts of Giardia and the oocytes of Cryptosporidium.
- Fully operational on pH levels between 4-10
- Temperature Independent
- Very Low CT Value which ranges from 0 to 10 min, therefore much less time is needed to treat water in large production facility
- Very low dose rate about 0.1 ppm, so 20 gram is sufficient for 20,000 liters of raw water before RO
- Long term residual disinfection effectiveness
- No reaction with ammonia, thus no release of THM which are carcinogenic in nature. Highly recommended for human consumption
- Fully effective to remove biofilm in RO Plants
- No corrosive effects
- Does not change the smell, taste and colour of drinking water
- Very flexible in dosing rates and combined disinfection
- No release of free chlorine, chlorite or chlorate.
- Only limited investment cost
- Easy to transport
Application of Chlorine Dioxide in RO Plants
The function of adding Neox after the active coal and before the membrane is simple by adding Neox in low dosage rates the water contains disinfection power at low dosage contributing positively to face the undesired fouling of RO systems. The degree and frequency of fouling varies widely from one membrane system to another. Fouling to the point of cleaning begin required can occur as limited as once per year or as frequently every day. The foul-ants can be classified into four main categories: dissolved solids, suspended solids, biological, and non-biological organics.
Biological fouling continues to be a major unresolved problem for membranes and systems as the most common RO-membrane types in use today are attacked and degraded by chlorine and according to public literature by other oxidizing agents. Chlorine is commonly used as a feed water disinfectant. However it must be removed from the feed water prior to entering the RO system. Without a disinfectant present in the water, microorganism colonizes and forms a biofilm in the RO system. Ultimately the RO membranes have to be removed from service and cleaned. Thus the biofilm causes a reduction in membrane performance and membrane damage leading to higher maintenance and system operating cost.
The main object of Neox is to efficiently treat a membrane separation system to control microbes and biofilm formation by extremely low levels of Neox solution by not adversely affecting the RO membrane.
The minimization of the biofilm in the membrane system includes the biofilm formation on the surface of the membranes. By dosing extreme low levels of Neox solution in the feed water the depositing and growing of biofilm on membranes can be significantly reduced without damaging the membranes leading to increased salt passage.
The dosage rate of Neox to the feed water will vary per RO system depending on the available biofilm in the RO system as well as the composition of the feed water (e.g.enzymes)
In various cases we have seen that a dosage rate of 5 ppb of Neox provided a desired result.To avoid the growth of biofilm a dosage rate of 100 ppb or 0.1 ppm on average as proven to be effective.
Further a dosage rate above 500 ppb will provide a critical level of Neox where the membrane possibly might be affected by the oxidizing power of Neox (this variation depends on the feed water composition and quality and therefore an appropriate dosage rate must be determined by test).
- A dosage rate of Neox should never exceed the level of 0.1 ppm and have a minimum of 50ppb.
- While using Neox the salt passage will not increase (a true and valid indicator that the membrane is not damaged)
- An appropriate sensor should control the Neox level in the system (chlorine dioxide sensor or potentiostatic analyzer) at ppb level.
- The dosing system that injects Neox in the feed water should be controlled by the monitoring sensor and stop dosing if levels exceed the maximum value to avoid damage to the membrane.
- Appropriate testing by the customer is advisable.
- You can also dose in after the RO system. A lot of problems occur in the waterlines after the RO system as biofilm will still be present.
Further Neox can be used on the permeate for normal disinfection control and replace existing disinfectants (e.g. chlorine). Standard dosing rates are around 0.06ppm.
To sterilize an RO system a dosage a dosage rate of 50-100ppm Neox is required with a circulate sterilization solution for 10-15 minutes. After that the system needs to be rinsed several times with tap water. The tap water needs to be renewed between rinses. After that the system needs to be rinsed 3-4 times with de-ionized water. Once drained, the system is ready for use and the membrane can be installed.
Reverse Osmosis De-Chlorinators needed for chlorine
Reverse Osmosis De-Chlorinators – Chlorine (Na/Ca hypochlorite, bleach or gas) can be dosed to control biological fouling of a reverse osmosis pre-treatment system. If biological contamination is an issue, chlorine can be dosed prior to the pre-treatment system to give a free chlorine residual of 0.2 – 1.00 ppm depending on severity of contamination.
It is important to note that chlorine will destroy polyamide thin film composite membranes. It is therefore essential that ALL chlorine be removed from the feed water prior to entering the membranes (CA membranes can tolerate up to 1ppm free chlorine) Even trace amounts of free chlorine can cause oxidation damage especially in the presence of metals such as iron.
Chlorine can be removed by bisulfite/metabisulfite addition or by the use of carbon filters.