1, RO water treatment introduces
Reverse osmosis is the water treatment technology, widely use in the industrial for the water treatment
Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline solution will tend to migrate to a strong saline solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys absorb water from our blood.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration , through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications.
Figure 1 Osmosis process
Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to remove ions , unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended chemical species as well as biological ones (principally bacteria) from water, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be “selective”, this membrane should not allow large molecules or ions through the holes, but should allow smaller components of the solution (such as solvent molecules, i.e., water, H2O) to pass freely.
Figure 2 Reverse Osmosis process
2, ESEC RO installation
RO supply water is from city multiply water, pre water treatment install Ultra filter , RO system in ESEC is 42m3 /hr permeate (1008m3 /day) (2x 21m3/hr Trains) Double Pass Reverse Osmosis (RO) System, which was submitted by H2O Innovation USA, The RO system (also known as the Demineralizer System) is for providing high quality process water. The RO system is designed to process a reclaimed wastewater supply containing approximately 1000 mg/l total dissolved solids (TDS) providing a high-quality product output of 42 (combined) m3 /hr of permeate at less than 1.00 mg/L TDS at an overall recovery of 72% water recovery.
The overall RO system includes scale inhibitor (for scale control) and sodium bisulfite addition (for chlorine scavenging) along with 5.0 micron cartridge filtration as pretreatment, high-pressure pumping and then two passes of permeate staged reverse osmosis desalination. Sodium Hydroxide is added between passes to elevate the pH aiding in the TDS rejection of the second pass. The second pass permeate is sent to the permeate tank for storage where it is then pumped through portable demineralizers as part of the final treatment step. A single centralized control panel containing the process controller (PLC) as well as a Human Machine Interface (HMI) is included which provides control of the provided equipment as well as monitoring of some remote items critical to the operation:
Figure 4 train #1,#2 P&ID
Figure 5 train #1,#2 first pass
Figure 6 train #1,#2 second pass
3, Operation problem
3.1 RO fouling
RO fueling is most challenge problem during the pass 6 years operation.RO fueling can classily to for category: scale, silt (particular), bacteria (bio fouling, growth of bacteria) and organic fouling (oil, grease).
Scale fouling is normal at last stage due to membrane in highest concentration, from autopsies SEC membranes were showing no scale on the last stage , when the SDI<3 the chance of the fouling due to particular is small which the pre treatment and ultrafilter will catch all particular below 5 um.
Bio fouling is most important cause for the membrane fueling, Bacteria are microscopic, single-celled organisms that exist in their millions, in every environment, both inside and outside other organisms, they grow up very quickly, after the dechlorane (SBS) add to the feed water, inside of the membrane which is very good environmental for the bacteria grow up, the bacteria will become the bio filming and to plug the membrane bore to decrease the permeant flow and increase the differential pressure between feed and rejection.
3.4 mix bed throughput low
The design of the RO train double pass plus mix bed as polisher can provide high quality water, the mixed bed, the mixed bed filling the cation and anion resin to remove the cation and anion ion inside of the process water, use resistivity and silica to monitory the output of the water quality. When silica high or resistivity low, then operator can make a decision mix bed exhausted, then replace it and to keep high quality of the water. The average throughput to replace mix bed less than 7000 cub meters.
3.2 RO trip
At first several years operations, the RO train #2 frequency trip due to the pass 2 feed valve not responsible fast when the fixed speed pump starts up to cause the pass permeant pressure high trip, also when first pass RO membrane start fouling, second pass feed pump is fixed speed , the RO feed control valve adjust the position not first enough to match the first pass permeant flow to cause the first pass permeant pressure low trip
3.3 RO leaking
RO first pass installed a conductivity probe to detect the conductivity of the RO first pass output at first pass header pipe, if the conductivity is high than normal, use hand meter to test individual array at outlet of each array middle valve to test the conductivity then can find the which array is leaking, it was happened several time.
3.4 RO membrane life far less than design
Most recently just change the RO train #1 membrane, which operation less than 15 month:
| New | replace | |
| 2019-05-29 | 2020-09-21 | |
| Operation days | 481 | Days |
| total throughput | 102926.3936 | Cubic meters |
| total Operation hours | 4231.74083 | hours |
Table 1 Train #1 membrane service period
Cleaning history:
From the blow table is show the train #1 membrane completed 15 cleaning.
| RO cleaned time | Feed pressure | Feed pressure | |
| Before cleaning | After Cleaning | ||
| kPa | kPa | ||
| New membrane | 2019-05-29 | 1056 | |
| 1 | 2019-08-20 | 1818 | 1446 |
| 2 | 2019-10-16 | 1902 | 1428 |
| 3 | 2019-11-19 | 1902 | 1558 |
| 4 | 2019-12-16 | 1900 | 1644 |
| 5 | 2020-01-07 | 1900 | 1570 |
| 6 | 2020-02-11 | 1723 | 1584 |
| 7 | 2020-03-18 | 1925 | 1618 |
| 8 | 2020-04-16 | 1926 | 1570 |
| 9 | 2020-05-08 | 1689 | 1525 |
| 10 | 2020-06-10 | 1963 | 1754 |
| 11 | 2020-07-07 | 1958 | 1768 |
| 12 | 2020-07-31 | 1879 | 1777 |
| 13 | 2020-08-09 | 1865 | 1840 |
| 14 | 2020-08-25 | 2062 | 1864 |
| 15 | 2020-09-07 | 2046 | 1835 |
Table 2 Train #1 membrane cleaning history
4, The measure for the RO system
To overcome the above problem, several measures was implicated to change the situation for the RO operation to minimize the RO fouling and improve operation reliability.
4.1 Install biocide injection system
Install biocide injection system. The logical set up inject 25 seconds injection DBNPA for each time RO stop to inject it to flush piping to flush the RO membrane to help kill bacteria then flush it out with Demin water.
Figure 7 train #1,#2 biocide injection skid
thin film composite polyamide membranes have limited resistance to chlorine-based oxidants. the biocide is use DBNPA which is a fast acting, non-oxidizing biocide which is very effective at low concentrations in controlling the growth of aerobic bacteria, anaerobic bacteria, fungi and algae. The chemical formula of DBNPA is:
DBNPA is an advantageous disinfectant since it also quickly degrades to carbon dioxide, ammonia and bromide ion when in an aqueous environment. This allows the effluent to be safely discharged even in sensitive water bodies:
4.2 Install permanent CIP piping
Install permanent CIP piping between CIP tank with RO train first pass to minimize the operator works to connect the temporary hose for each CIP , the permanent hard pip to prevent the connector leaking, then no environmental impact at CIP time
Figure 8 train #1,#2 CIP skid
4.4 Caustic pump automatic
Caustic pump set up automatic to adjust the output to help the stabilize the caustic injection, also stabilize the PH level, keep second pass PH at 8.4 is good practice to extend of the mix bed life, after caustic set up automatically and the PH set point increase to 8.4 from 7.0, the mix bed through put is largely increased to average 15,000 cube meters for each change . Caustic pump automatic operation to stable the RO first pass permeant PH to neutralize the bicarbonate acid CO2 obviously extend the mixed bed life to three times longer.
4.3 Increase Chlorine level to improve the pretreatment
The service tank chlorine level increase from 1.0 pp, to 2.0 ppm to keep service tank chorine level and kill the bacteria in the service tank and also keep the small drain line open at prefilter house to prevent the bugs accumulated at the prefilter.
4.4 install VFD for second pass pump
Install VFD to adjust the pump speed to match the first pass permeant flow and control the first pass permeant pressure to avoid the RO tripped due to the high or low RO first pass permeant pressure. Also, to keep the second pass feed valve 100% to save the energy to eliminate the second pass valve response issue successfully to avoid the RO trip due to second valve response issue.
To maintain the first pass permeant pressure, along with RO operation, the contamination for the membrane will cause the first feed valve gradually open and second pass pump speed gradually reduce, the permeant flow will gradually decrease.
4.5 Install reject pressure transmitter at first pass stage 1 and 2
At train #1,2 install 4 pressure transmitters to measure the pass 1 stage 1 and stage 2 reject pressure then connect to DCS to provide the DP for the first pass membrane to help calculation normalized flow and DP as the performance monitory measure.
4.7 Relocate the SBS (sodium bisulfite)
The design SBS injection is before the pretreatment cartridge filter, then bugs grow up quickly at the filter chamber due to it is free chlorine environmental, relocate it to after cartridge filter to mimimaze the space for the bacteria grow up.
4.8 Use software to monitory performance
Use the membrane manufacture provide free software to set up it to connect to PI for raw data input, the performance monitory software has function for normalized parameter calculation and trend it, the software use the new membrane operation data as bias line to compare with operation data to calculation the normalized data to provide the flow, DP and salt passage change, it is a good performance monitory tool for RO membrane fouling analysis and cleaning performance improvement judgement
5 RO cleaning
Normally RO has two kinds of the cleaning which is high PH cleaning and Low PH cleaning, for the Low PH cleaning is effective for the scaling, for the High PH cleaning is more effective for the bio fouling,
Cleaning strategies must be able to remove biomass from spiral-wound membrane modules. If the feed channel is completely blocked by biomass, this can limit the transport of the cleaning chemicals to the blocked spacer and restrict the removal of biomass from the membrane module. Therefore, early cleaning of partially fouled membranes and isolating the lead membrane modules from the installation is vital, if the membrane flow decrease 30% to 50% it is hard to bring RO back to base line performance
SEC RO cleaning use Chemetrate provide RL1500 (Ethylenediaminetetraacetic acid (EDTA), tetrasodium salt, sodium hydroxide) and RL 5000 (sodium percarbonate, sodium carbonate, silicic acid, disodium salt), through CIP pump cycling to membrane and pressure soak membrane 8 to 12 hours. Add extra caustic to keep solution PH above 12.
6 Discussion
The modification on the RO system is largely improve the RO system operation reliability, improve the capability to monitory the RO performance, increase RO chlorine level to prevent the bug grow up in service tank, second pass pump install VFD to adjust the flow automatic to avoid RO frequency to avoid trip due to high or low first pass permeant pressure, Hard pipe install between the CIP tank and RO membrane to help CIP cleaning, Biocide injection to membrane after the RO stop to kill the bacteria during the RO idle time.
RO bio-fouling is worldwide issue, especially to use reclaim water as water treatment source, the below table is showing after new membrane installed the normalized data change vis, the operation hours.
Normalized membrane data
| Operation time (hours) | % Salt Passage | Permeate Flow (t/h) | Flow Change | Differential Pressure(bar) | DP Change |
| 0 | 0.65 | 25.60 | 0.00% | 1.23 | 0.00% |
| 10 | 0.58 | 24.72 | -3.44% | 1.41 | 14.32% |
| 20 | 0.63 | 24.49 | -4.51% | 1.51 | 22.29% |
| 30 | 0.63 | 23.36 | -9.15% | 1.61 | 30.62% |
| 40 | 0.67 | 24.13 | -6.30% | 1.73 | 39.87% |
| 50 | 0.63 | 22.57 | -12.57% | 1.75 | 41.71% |
| 60 | 0.63 | 22.34 | -14.44% | 2.24 | 81.24% |
| 70 | 0.64 | 22.07 | -15.80% | 2.38 | 92.54% |
| 80 | 0.69 | 22.06 | -16.06% | 2.61 | 111.06% |
| 90 | 0.66 | 21.63 | -18.00% | 3.09 | 150.22% |
Table 3 membrane bi-fouling vs operation time
From above table is showing the normalized flow decrease and DP increase very fast, recovery is necessary at least every 100 hours operation. After the 8 hours CIP cleaning, the flow increased 15.23% and DP decreased 188%, very close to new membrane situation
| Normalized Fow | Increased( compare with before clean) | Change | ||
| m3/h | m3/h | |||
| 2020-09-29 19:15 | Before cleaning | 20.42 | ||
| 2020-09-30 3:50 | After cleaning | 23.83 | 3.15 | 15.23% |
Table 4 after 90 hours clean flow results
| Normalized DP | Decreased (compare with before cleaning) | Percent |
| Bar | Bar | |
| 3.77 | ||
| 1.40 | 2.53 | 188.63% |
Table 5 after 90 hours clean DP results
A lot of discussion and study for the RO bo-fouling in the world, some very good practice is successfully realized in different plant and very good operation results and extend RO membrane life, below is some results:
6.1 Continuously add biocide after dechlorane injection.
As composite polyamide structure is vulnerable for the oxidant which will burn a hole at membrane, One of the chief concerns is the effect of free chlorine, which can lead to chlorination of the aromatic ring and greatly reduce rejection, so SBS injected to keep the feed water free chlorine level to 0.
RO membrane recommended several products can compatible with membrane which can continuously inject it to membrane to prevent the bio-fouling, keep 1 to 2 ppm concentration is good practice to stop bio-fouling , DBNPA, Isothiazolin, Sodium Bisulfite.
Membrane manufacture recommended for continuous dosing during service operation, between 0.5 to 2 ppm of active ingredient is recommended to maintain a biostatic environment. Continuous dosing can be significantly more expensive in terms of operating costs .
The one exception which has been proven successful is the use of chloramines, particularly for wastewaters. Most wastewater RO plants run with 2-4 ppm of chloramines and can achieve 5 years or more of stable membrane performance. Chloramines is cheaper than DBNPA.
Continuously add membrane compatible biocide to stop the bi-fouling is best solution, but it is expensive solution which to do economic analysis to compare with predictive CIP method.
6.2 Reverse flushing
If the fouling is primarily on the front part of the system, consider reverse flushing/cleaning is benefit. This is a practice where the chemical or flush liquid is circulated in a direction opposite to the normal flow of feedwater. This is shown in a real plant experience, where this method was found to be more effective than using cleaning chemicals in a normal flow direction.
Additionally, trials with reverse flushing have been used in some plants. This cleaning procedure, where cleaning solution or flush water is flowed in the opposite direction of the normal feed flow, has shown a better propensity for removing the colloidal foulant from feed-end elements.
Figure 9 Reverse flushing
6.3 Air microbubbles generator
Study is showing there are encouraging results that the creation of micro-bubbles using air induction and specially formulated cleaning chemicals helps dislodge clay and biofilm foulants in the front elements of a membrane plant without damaging the polyamide layer.
Figure 10 Reverse flushing
micro-bubbles will create more turbulence on the membrane surface and to help remove the hard gel layer to recover the membrane performance.
6.4 cleaning chemical choice
Two different chemical cleaning is better results than only one chemical. SDS (Sodium dodecyl sulfate) mixed with chelating agents (EDTA) is sufficiently effective in removing organic fouling.
Disodium ethylenediaminetetraacetate (Na2-EDTA) as a metal chelating agent, sodium dodecyl sulfate (SDS) as an anionic surfactant, and NaCl as a salt cleaning solution.
Study is showing and also from manufacture suggestion, after the first batch cleaning use EDTA plus caustic to move the surface contamination and bio-fouling material, the second batch of cleaning chemical should use the SDS plus NaCl with caustic solution to do deep cleaning to remove the hard gel layer.
6.5 Cleaning time choice
Predicative clean is use manufacture software to online monitor the change of normalized flow and normalized DP between the feed pressure with reject pressure, and set up threshold point to trigger the CIP cleaning. Due to bi-fouling is very fast, if completely follow manufacture suggestion at 10% decrease normalized flow or 15% increase normalized differential pressure, form table it is requests to cleaning at 50 operation hours.
Table 3,4 is showing the results after 90 hours new membrane cleaning results, which flow and pressure almost reach new membrane conditions. Predictive cleaning can extend the membrane life and prevent the spacer channel plugged and bio-film is not become harder to dissolved to cleaning solution.
Reactive cleaning which is showing on the table 1 the history of train membrane reactive cleaning, at first cleaning is happen 3 month later, due to the new install membrane, it can keep longer operation to fouling the membrane until the feed pressure was reach upper limit, but after cleaning, the feed pressure can’t decrease to new membrane level, which lost almost 40% of membrane life, the hard bi-film can’t be remove due to long time no cleaning, after that each cleaning was waiting the feed pressure reach upper limits, until RO can’t safely run, the membrane life only last less than 15 month.
7.0 Cost analysis
Cost for replace membrane
Membrane replace cost, each membrane is $600.00 plus consumable, coupling, seal ring, replace labor etc . totally is $40,000 per each train.
Cost for each cleaning
| Chemical name/kw | consumption per cleaning | Price | Item price | |
| Kg | $/kg | |||
| Cleaning solution | RL1500 | 20 | 10.05 | $201.50 |
| Cleaning powder | RL5000 | 1 | 8.5 | $8.50 |
| PH enhancer | BL1301B | 10 | 3.6 | $36.00 |
| CIP pump (KW) | 11 | 6 | 0.033 | $2.18 |
| CIP Heater (kW) | 45 | 6 | 0.33 | $89.10 |
| Rise water | 20 | 2 | $40.00 | |
| Total | $377.28 |
Each cleaning the cost is less than $400, consider predictive clean plan, 90 operation hours for cleaning, one year cost will less than $10,000. Which can easy to extend membrane up to 3 ~4 years.
8.0 Conclusion
The capital investment for RO operation is success but not enough, from the most recently RO fouling speed (table 3) to demonstrate bio-fouling still a big challenge at SEC.
RO operation strategy is crucial for RO successfully operation, let it running to the upper limit feed pressure then to do reactively cleaning will short RO life.
Consider Continuously to inject the biocide to membrane to slowdown the bi-fouling speed to extend membrane life is valuable test try, set up 1 to 3 month injection to analysis the normalized differential pressure change to find bi-fouling speed to forecast membrane life and biocide cost.
In another hand, improve the RO cleaning techniques by add microbubble air generator and add back flush piping, use two step deep chemical cleaning, and also set up predictive cleaning schedule will largely improve the cleaning efficiency to extend the membrane life to payback the chemical cost and capital investment.
[Reference material will be provided later]