6. REVERSE OSMOSIS (RO)

6. REVERSE OSMOSIS (RO)

RO SET UP:

The RO system is designed to recover 65-70% of the manure as clean water while concentrating the remaining dissolved solids as a nutrient fertilizer.

The RO pressures are set by closing the concentrate valve halfway and allowing the system pumps to ramp up to operating speed. Once the pumps are running at speed, the concentrate valve can be closed slowly to increase the pressure across the entire system. Don't allow the concentrate flow to drop to zero.

On the first RO: Once the pressure is set, fine adjustments can be made to the VFD speed to balance the flows through each bank of the RO.

RO MONITORING:

There are a number of parameters that need to be monitored to ensure the RO is functioning correctly and that loss of performance isn’t occurring. These parameters are the RO recovery and the differential pressures across each RO bank in the system. Rapidly decreasing recoveries or rapidly increasing differential pressures are indicators that the RO performance is dropping either to scale precipitation or biological fouling. Take note of the TSS, pH, and Conductivity of the RO Feed Water, and the cartridge filter differential pressure. Adjustment of the upstream equipment might be required.
The recovery of the RO is determined by dividing the permeate flow rate(s) by the total feed flow rate.

The first RO recovery is calculated by adding the two permeate flow rates together and dividing by the sum of permeate and concentrate flows.

% Recovery = [(FQIT-3007A + FQIT-3007B)/(FQIT-3006 + FQIT-3007A + FQIT-3007B)] * 100.

The differential pressure of the RO banks is determined by subtracting the feed pressure by the discharge pressure for each bank.
The RO#1 Stage#1 differential pressure = PI-2902 – PI-3002.
The RO#1 Stage#2 differential pressure = PI-3003 – PI-3005.

WarningNOTE: The differential pressures of RO stages are critical to the membrane integrity. If system is shut down on high differential pressure alarms, operator MUST NOT press reset button immediately and repeatedly for the purpose of continue operation. Operator shall start a RO cleaning process right away to resolve high differential pressure situation.


RO OPTIMIZATION:

The RO system pressures and flow rates above were developed with modeling software. The actual pressures and flows of the RO may differ slightly from the above settings. Once the system is set initially, the RO pressures and flow rates should be monitored for changes. The feed pump VFD speeds can be adjusted to properly balance the flow rates and achieve the desired recovery. The recovery of the RO system should never exceed the recoveries listed above or rapid scaling of the membranes may take place.

RO PERMEATE QUALITY:

The RO permeate quality should be checked on a weekly basis or if V-310 tank fills with turbid water. The RO permeate water should always be clear with a TSS measurement under 10ppm when the RO is running (measurements taken with the RO feed pump not running may read a higher TSS). If the permeate water is turbid or dark, then samples of the permeate water should be taken at each individual pressure vessel to determine which vessel has O-rings that are leaking.

In order to troubleshoot and fix the color issue, the following will need to be completed:
  1. Shut off the RO feed pumps.
  2. Allow the system to depressurize.
  3. Remove the retaining ring from the pressure vessel.
  4. Pull the end cap out of the pressure vessel. Take care not to damage the end cap or any of the piping.
  5. Pull the RO membrane elements out of the pressure vessel, the whole stack should come out connected. Note if the interlock between the two nearby elements broke off.
  6. Inspect the O-ring that seals around the permeate connection on the RO element.
  7. Inspect the O-ring that seals around the permeate adapter on the end cap.
  8. Inspect adapters and permeate ports for cracks/damages;
  9. Replace any O-ring that has been rolled, cut or missing. Once the O-ring is in place apply a small amount of Dow Corning 111 Valve lubricant to the O-ring surface.
  10. Replace any damaged adapters or permeate ports.
  11. Reinstall the RO elements.
  12. Reinstall thrust cone and end cap on the downstream side of vessel and reassemble the piping.
  13. Properly seat the element stack on thrust cone. Shim and reinstall upstream end cap.
PROBING THE RO PRESSURE VESSEL:

If permeate color still present after the above procedure, probing of the pressure vessel is required to find out the leaky point where permeate is contaminated from. A probing tool consists of:

1” x ½” SS reducer bushing;
½” SS Close Nipple;
½” SS Ball valve Full Port;
½” NPT x ¼” Compression tube connector (to be drilled through);
25ft ¼”OD HDPE tubing.

The probing should be carried out as per following procedure:

  1. Make sure that the RO is not in operation and RO vessels are de-pressurized;
  2. Go to the RO vessel in question, remove the PVC plug on the permeate port from the upstream end cap;
  3. Install inspection probe fittings on the permeate port;
  4. Mark the probe tubing on 20 inches from the outside end, insert the other end of probe tubing all the way(25ft) into the permeate passage inside the RO vessel;
  5. Start RO and let it run for 5 minutes;
  6. Collect water sample from the tubing end of the probe;
  7. Pull out the tubing for 20 inches in length (by the marks);
  8. Wait for 2 minutes before collecting water sample from tubing end of the probe;
  9. Repeat steps 7 and step 8 five more times;
  10. Measure TSS/conductivity of each sample;
  11. Plot the TSS/conductivity measurements to build the profile, the point where the measurement spikes is where the leak is;
  12. Remove the probing tool and reinstall PVC plug on end cap;
  13. Fix the leak accordingly

RO CLEANING REQUIREMENT:

The RO Membrane System will require periodic cleaning as the RO membranes become fouled from biological matter, colloidal particles, and possibly mineral scale. The elements should be cleaned when one or more of the below parameters are applicable:

  1. The normalized permeate flow drops 10%.
  2. The normalized salt passage increases 5 – 10%.
  3. The normalized pressure drop (feed pressure – concentrate pressure) increases 10 – 15%.

It should be noted that permeate flux will drop if feed water temperature decreases. This is normal and does not indicate membrane fouling. Each RO stage should be cleaned separately to prevent contaminants from one stage being pushed into another stage.

Always complete the cleanings in the following steps: Flush Membranes, Alkaline Cleaning, Flush Membranes, followed by Acid Cleaning. Alkaline cleaning solution should be a 0.1% NaOH solution with a pH of 12. Acid cleaning solution should be 0.2% HCl or Citric Acid solution with a pH of 1 – 2. All cleaning solutions should be prepared with RO permeate water or a clean source of water.

Always measure the pH during cleaning. If the pH increases more than 0.5 pH units during acid cleaning, more acid needs to be added. If the pH decreases more than 0.5 pH units during alkaline cleaning, more caustic needs to be added.

Turbid or strong colored cleaning solution should be replaced. The cleaning is repeated with fresh cleaning solution.

There are three types of cleanings available for an operator, an automated daily cleaning which soaks and flushes membranes with cleaning solution, a manual cleaning with 45 minute soak of cleaning solution and requires operator to input of pH and a MANU-RENEW cleaning with circulation of cleaning solutions. All cleanings are triggered by the operator. The automated cleaning can be set by operator to be started at any chosen time. If the recovery of the RO doesn’t return back to 65-70% after completing the automated cleaning then the operator must complete the full manual cleaning described below.

RO AUTOMATIC MEMBRANE CLEANING:
  1. Go to the “RO#1 CIP” screen on the touch screen and press on P360-1 symbol.
  2. Press “AUTOCLEAN OFF” button to turn it into “AUTOCLEAN ON” ;
  3. The system will automatically start a sequence of RO cleaning steps at a pre-determined time. The steps are:
    1. System pump down V350 if water level inside V350 is high;
    2. Prepare two tanks base solution in V350 at pH 12.5;
    3. Pump the base solution through RO system;
    4. Soak it for 45 minutes;
    5. System runs through one tank of water to flush base solution out;
    6. Prepare two tanks acid solution in V350 at pH 1.2;
    7. Pump the acid solution through RO system;
    8. Soak it for 45 minutes;
    9. System runs through one tank of water to flush acid solution out.
  4. Time for Autoclean process to start can be reset as:
    1. Press “TITLE” button to get to Title screen;
    2. Press and hold the top right hand corner of the screen for 3 seconds.
    3. That will open the PLC setting screen.
    4. Adjust the RO#1 AUTO CLEAN HR and AUTO CLEAN MIN in 24 hour format to set auto clean start time;
    5. Press “EXIT” to return to Title screen.
RO MANU-RENEW MEMBRANE CLEANING:

  1. Go to the RO# CIP screen on the touch screen and press on P360-1 symbol.
  2. Press “MANUAL CLEAN” button then “TRIGGER MAN. CLEAN” button, select “RO#1 MANURENEW CLEAN” button;
  3. Press “MANURENEW CLEAN OFF” button;
  4. When “MANURENEW CLEAN OFF ” button is pressed, It turns into “MANURENEW CLEAN ON”, and the following would happen in sequence:
    1. System pump down V350 if water level inside V350 is high;
    2. System prompt operator to put 1000 grams (about 1000mls of powder) of cleaning detergent into V350. When operator acknowledges this, system fill V350 with water then pump the cleaning solution through RO.
    3. System repeat the above process to pump another tank of cleaning solution through RO;
    4. System prepares another tank of cleaning solution then prompt operator to close BV2, BV3 and BV6, open BV7, BV8, BV9 and BV10. When operator acknowledges this, system start P-360 circulating cleaning solution. At the same time starts a 60 minute countdown;
    5. Circulate the cleaning detergent for 60 minutes, display a countdown clock so that operator is aware of the time while he/she attends other needs and return at the end of circulating time;
    6. When timer countdown finishes, system prompt operator to open BV3 and BV6, close BV8, BV9 and BV10;
    7. When operator acknowledges valve changes, system pump down V350 and run through one tank of water to flush out the cleaning solution;
    8. System starts to prepare 500 gallons of 0.2% acid solution (pH1.2) and pump the solution through RO;
    9. Prompt operator to check pH of RO effluent and input the pH measurement at the end of pumping cycle;
    10. When operator confirms that the pH of RO effluent is pH1.2, system prepares another tank of acid solution then prompt operator to close BV3 and BV6, open BV7, BV8, BV9 and BV10. When operator acknowledges this, system starts P-360 to circulate acid solution and a 60 minute countdown at the same time;
    11. Circulate the acid solution for 60 minutes, display a countdown clock so that operator is aware of the time while he/she attends other needs and return at the end of circulating time;
    12. When timer countdown finishes, system prompt operator to open BV2, BV3 and BV6, close BV8, BV9 and BV10;
    13. When operator acknowledges valve changes, system pump down V350 and run through one tank of water to flush out the cleaning solution;
    14. Prompt operator to replace cleaning bag filter;
    15. RO MANU-RENEW cleaning is completed.
RO MANUAL MEMBRANE CLEANING WITH 45 MINUTE SOAKING:

  1. Go to the RO# CIP screen on the touch screen and press on P360-1 symbol.
  2. Press “MANUAL CLEAN” button then “TRIGGER MAN. CLEAN” button
  3. Select the type of cleaning required – Base, Acid, or Water Flush.
  4. When “Base Clean” is selected:
    1. This will initialize the process of preparing of 500 gallons of 0.2% base solution;
    2. Once the solution is prepared, prompt operator to measure the pH of the solution and input it in the touch screen;
    3. Once the pH is input into the touch screen, close the valve on permeate lines then start p-360 to pump the solution into RO;
    4. Prompt operator to check pH of RO effluent and input the pH measurement at the end of pumping cycle;
    5. If the measurement matches the solution measurement in step 2, go to step 6. If the measurement is lower than that of step 2, repeat steps from 1 to 4 until pH measurements matches each other;
    6. Soak the system for 45 minutes, display a countdown clock so that operator is aware of the time while he/she attends other needs and return at the end of soaking time;
    7. Flush system with 500 gallons of water after soaking;
    8. Open the valve on permeate lines;
    9. Operator is required to press “Base Clean ACK” button to acknowledge that base cleaning has finished.
  5. When the “Acid Clean” is selected:
    1. This will initialize the process of preparing of 500 gallons of 0.2% acid solution;
    2. Once the solution is prepared, prompt operator to measure the pH of the solution and input it in the touch screen.
    3. Once the pH is input into the touch screen, close the valve on permeate lines then start p-360 to pump the solution into RO;
    4. Prompt operator to check pH of RO effluent and input the pH measurement at the end of pumping cycle;
    5. If the measurement matches the solution measurement in step 2, go to step 6. If the measurement is higher than that of step 2, repeat steps from 1 to 5 until pH measurements matches each other;
    6. Soak the system for 30 minutes, display a countdown clock so that operator is aware of the time while he/she attends other needs and return at the end of soaking time;
    7. Flush system with 500 gallons of water after soaking;
    8. Open the valve on permeate lines;
    9. Operator is required to press “Acid Clean ACK” button to acknowledge that acid cleaning has finished.
  6. When “Water Flush” is selected:
    1. This will initialize the process of flushing the RO with 500 gallons of clean water;
    2. Operator is required to press “Water Flush ACK” button to acknowledge that water flush has finished
RO WATER FLUSH UNDER ABNORMAL CONDITION:

The System has built in the control program to prevent process water from sitting in RO for longer period of time. Sitting of process water tends to speed up scaling/fouling on the membrane surfaces.

When the system is shut down by operator or alarm condition for more than 20 minutes, a water flush would be carried out automatically. During which a “LT water flush ON” will be displayed below P360 symbol on RO CIP screen. The operator shall not interrupt the water flush process. System will be ready for start after the water flush is finished.

RO PRESERVATION (when RO will be shut down for less than 24 hours)

After cleaning, please follow the preservation steps below:
  1. Press “LT FLUSH OFF” button and turns into “LT FLUSH ON” which will flush RO every 12 hours using clean water;
  2. The RO elements do not dry out. Dry elements will irreversibly lose recovery
  3. When applicable, the system is protected against temperature extremes.
RO PRESERVATION (when RO will be shut down for more than 24 hours)

RO elements must be preserved any time the plant is shut down for more than a maximum of 24 h to prevent biological growth. After cleaning, the preservation should follow within the next 10 h as follows:
  1. Totally immerse the elements in the pressure vessels in a solution of 1% sodium bisulfite, venting the air outside of the pressure vessels. Use the overflow technique: circulate the SMBS solution in such a way that the remaining air in the system is minimized after the recirculation is completed. After the pressure vessel is filled, the SMBS solution should be allowed to overflow through an opening located higher than the upper end of the highest pressure vessel being filled.
  2. Separate the preservation solution from the air outside by closing all valves. Any contact with oxygen will oxidize the sodium bisulfite.
  3. Check the pH once a week. When the pH becomes 3 or lower, change the preservation solution.
  4. Change the preservation solution at least once a month.
Info
During the shutdown period, the plant must be kept frost-free, and the temperature must not exceed 113°F (45°C). A low temperature is desirable.

CIP pH Probe Calibration

Life of a pH probe is limited due to the degradation of the pH sensitive membrane. Over time, the probe will become less accurate, interfering with the effective cleaning of RO membranes and increasing risk of over-concentrating of the cleaning solution. To compensate, regular calibration against standard reference solutions needs to be done to maintain the integrity of the CIP process. The steps to do so are as follows:
  1. Prepare two 500mL beakers, one of a 4.0pH buffer and one of a 7.0pH buffer solution.
  2. Remove the pH probe from the CIP vessel arm.
  3. On the CM442 Liquiline transmitter, press the ‘Cal’ button. Select the ‘pH Glass’ or ‘pH ISFET’ option. Then select ‘2-pnt. Calibration’. This will lead to the calibration menu. Follow on screen prompts and instructions when applicable.
  4. Place the probe into the 7.0pH Buffer solution. Input a pH value of 7.0 for the buffer solution when prompted by the transmitter. Start the calibration.
  5. Once the measurement has stabilized, it will accept the value. Rinse the end of the probe. Do not dry the probe, a light shake to remove excess water is sufficient.
  6. Repeat steps 4 and 5 for the 4.0pH buffer solution.
  7. Once calibration is completed, the Liquiline display can be returned to displaying the process values. Return the pH probe to the CIP vessel arm. Dispose of the buffer solutions used for calibration.


RO VALVE SCHEDULE:

 RO#1 - Bank #1

RO#1 - Bank #2

Valve Number

Chemical Addition / Flush

Soak

Recirc

Chemical Addition / Flush

Soak

Recirc

RO Normal Operation

BV #1

Open

Open

Open

Open

Open

Open

Open

BV #2

Close

Close

Close

Open

Open

Open

Open

BV #3

Close

Close

Close

Open

Open

Open

Open

BV #4

Open

Open

Open

Close

Close

Close

Open

BV #5

Close

Close

Close

Close

Close

Close

Close

BV #6

Open

Open

Open

Close

Close

Close

Open

BV #7

Open

Close

Open

Close

Close

Close

Open

BV #8

Close

Close

Close

Open

Close

Open

Close

BV #9

Open

Close

Open

Close

Close

Close

Close

BV #10

Close

Close

Close

Close

Close

Open

Close

BV #11

Close

Close

Open

Close

Close

Close

Close

BV #12

Close

Close

Close

Close

Close

Open

Close

BV #13

Open

Open

Open

Open

Open

Open

Open

BV #14

Open

Open

Open

Open

Open

Open

Open

BV #15

Close

Close

Open

Open

Close

Open

Close

MOV2902

Close

Close

Close

Close

Close

Close

Open

MOV3008

Open

Close

Open

Open

Close

Open

Close

MOV3009

Open

Close

Close

Open

Close

Close

Close

MOV3010

Close

Close

Close

Close

Close

Close

Open


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