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University of Cambridge >  Nanoscience Laboratory Intranet > Cleanroom & Lab Information > DI Plant
 

Deionised / Demineralised Water Plant

Week beginning 14th November 2016

  Total Chlorine
ppm
Free Chlorine
ppm
Total hardness (CaCO3)
ppm
Total alkalinity (CaCO3)
ppm
pH Resistivity
Mains water 0 0 250 240 6.8  
DI produced 0 0 0 0 6.8 16.3
DI building loop 0 0 0 0 6.8 16.3

CONTENTS

Schematic of DI plant

TECHNICAL DESCRIPTION & DATA

The complete water treatment system has been designed to produce 150 l/h of humidifier water or 150 l/h of deionised water for the high purity loop, with an ultimate treated loop flow of 500 I/h. The loop draw-off was designed for 25 l/h and 500 l/day.

WATER ANALYSIS

Raw Water Quality

Cambridge Water Company supplies the raw water supply to Cambridge University. It has historically been a relatively high hardness water (275 mg/l CaCO3) with a small amount of organic matter being present.

 

Average Water Samples
for 2000

as mg/l

pH
7.28
Conductivity - µS/cm
543
TDS @ 180°C
380
Total Hardness as CaCO3
275
Alkalinity as HCO3
271
Ammonium
<0.03
Calcium
108
Chloride
20
Chlorine - residual
0.05
Fluoride
<0.1
Magnesium
14
Nitrate
38.7
Potassium
1.8
Sodium
<1.0
Sulphate
26

The mineral content of the water is generally medium, with an average total dissolved solids (TDS) concentration of 380 mg/l.

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Final Water Quality

The quality of the humidifier water being blended after the reverse osmosis plant will be approximately 90 µS/cm. The quality of the high purity loop is as detailed below.

 
E1 Standard
maximum as µg/l
Resistivity - megohm.cm @ 25°C
>18
TOC
25
Particles per millilitre
1
Bacteria per 1000 millilitre
1
Chloride
1
Copper
1
Nickel
0.1
Nitrate
1
Phosphate
1
Potassium
2
Silica
5
Sodium
0.5
Sulphate
1
Zinc
0.5
Endotoxins (Eu/ml)
0.03

TREATMENT REQUIREMENTS

To obtain the required treated water quality the removal of the total dissolved solids (TDS) is significant, which is obtained by use of reverse osmosis followed by mixed bed ion exchange resins.

Some organic material is present. Since organic matter has a fouling effect on the surface of membranes in the reverse osmosis process, it is essential that steps are taken to reduce this objectionable material as early in the process as possible.

The presence of hardness causes deposition of calcium and magnesium carbonate (chalk) and sulphate when the concentration of the salt takes place, and is aggravated by increase in temperature, increase of pH and increase of salt concentration.

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PROCESS DESCRIPTION

Hardness Removal/Softening

The hardness in the feed water will foul the reverse osmosis membranes. To prevent this calcium and magnesium ions are exchanged for sodium that will form more soluble salts in the water that will not scale the membrane surface. This softening operation is undertaken in the softener, which uses a cation ion exchange resin.

This process has been specifically designed and developed to provide softened water with regeneration of the softener taking place at night.

All the operating phases of the unit are controlled by an electronic timer.

The softener consists of:

  • 1 vertical vessel, in glass reinforced plastic, 1 valve harness, 1 electronic control system for the automatic operation of the unit.
The softener comes complete with a brine tank.



Technical Specifications  
Maximum daily capacity
(treating water with 275 mg/l as CaCO3)
: up tp 9 m3/d
Resin type : strong acid cation
Resin identifier : purolite C100
Relevant salt consumption : 7.5 kg/d continuous operation
Maximum fiowrate : 4.5 m3/h
Pressure loss (maximum flowrate) : 0.8 bar
Maximum operating pressure Minimum : 6 bar
Operating pressure : 1.8 bar
Vessel maximum operating pressure : 8 bar
Power supply : 240 Vac-50 Hz single phase
Power consumption : 10 watt
Injector No. : 4
Drain line flow : 26.5 l/min (7 usgpm - 1.6 m3/h)
Brine line flow : 3.8 l/min (1 usgpm - 0.23 m3//h)
   
Pipe Connections  
Inlet : 1" female threaded
Outlet : 1" male threaded
Drain : 10 mm hose connector
   
Dimensions  
Softener vessel height : 1,100 mm
Softener vessel diameter : 350 mm
Resin volume : 75 litres
Brine tank height : 760 mm
Brine tank diameter : 510 mm
Brine tank capacity : 150 litres

The 240 Vac power supply for the softener controller is provided by others.

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Chlorine Removal/Activated Carbon Filters

Chlorine that has been dosed into the drinking water supply will over a period of time adversely affect the reverse osmosis membranes. Therefore, a chlorine removal stage is required, which is provided by the activated carbon filters. These filters also provide some particle filtration down to 5 microns and are 10" long.

The filter elements are installed in two new housings.

The frequency of changing these activated carbon filters will depend on the incoming chlorine concentration. If the chlorine level after these activated carbon filters exceeds 0.1 mg/l then the activated carbon filters must be changed. They should be also changed when the pressure drop across the filters exceeds 1 bar.

Micron Filtration

To remove finer material that could block the reverse osmosis membranes a 5 micron filter element, 10" long, is used after the activated carbon filters.

The filter element is installed in a new housing.

This filter should be changed when the pressure drop across the filter exceeds 1 bar.

Reverse Osmosis Plant

Reduction of the mineral content of the softened and filtered water is achieved by passing the softened and filtered water through a reverse osmosis process.

This consists of passing the water across a semi-permeable membrane at high pressure whereby a proportion of water passes through the membrane whilst the remainder continues along the membrane surface.

The water passing through the membrane is referred to as PERMEATE (or product) whilst the remainder is referred to as CONCENTRATE (or reject).

The membrane only allows a minute fraction of the salts in the feed water to pass through and consequently the conductivity of the PERMEATE is very low whereas the CONCENTRATE is very high in dissolved salts.

Typically 95 -99% of the salts in the feed water are rejected to the concentrate and normally about 50% of the feed water is recovered as permeate for this type and size of plant.

One stream is provided capable of providing the design output of 0.15 m3/h.

No matter how effective the pre-treatment is, it can never be 100% as water characteristics can change daily.

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Membranes  
Membrane supplier : Dow or Fluid Systems
Membrane identifier : Filmtec TW30-4014 or Koch TFC-HF
Membrane size : 4" diameter and 14" long

The 240 Vac power supply for the RO plant is provided by others.

RO Cleaning

There is a need to clean the membranes from time to time. Generally membrane cleaning is required when the permeate flow reduces from normal by 25%.

Membrane cleaning is undertaken as a specialist operation. The nature of the cleaning chemicals and frequency is determined by operating conditions and therefore procedures and recommendations given in this manual are for typical only and will require variation in the light of operating experience.

No membrane cleaning system has been incorporated into the RO process.

Humidifier Water Storage

RO permeate is blended with towns water and is stored in a 500 litre storage tank. This tank provides a buffer storage capacity. It also feeds the humidifier supply pump that will transfer water into the humidifiers.

The storage tank is complete with two sets of level controls. another set inhibits the operation of the humidifier supply pump.

Humidifier Supply Pump

This is a self-contained centrifugal pump. The pump is complete with non-return valve, pressure switch and pressure gauge.

The 240 Vac supply to the pump motor is routed via the motor control centre.

The pump is inhibited from operating if there is a low level in the humidifier water storage tank, via a Kasuga level switch. The 110 Vac controls voltage is provided by the motor control centre.

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Duty Summary  
Fluid : RO permeate
Specific gravity : 1.0
Solids content : nil
Viscosity : 1cP
Temperature : 25°C
Flowrate : 0.15 m3/h
Suction condition : Flooded
   
Pump Set Description  
Model : Lowara BGM 3 stainless steel, self priming centrifugal pump
Flowrate : 0.6 m3/h - 3 m3/h
Motor details : 0.37 kW, 220 Vac, 50 Hz
Motor protection : IP55
Pressure switch setting : 2.5 - 3.5 barg

High Purity Water Storage

RO permeate is stored in a 500 litre storage tank. This tank provides a buffer storage capacity and also acts as the supply tank for the high purity loop. The high purity loop pump pressurises the water through the mixed bed ion exchange resins, subsequent filtration and ultraviolet irradiation.

The storage tank is complete with two sets of level controls. One set controls the RO plant, another set inhibits the operation of the high purity loop pump.

High Purity Loop Pump

This is a stainless steel, multi-stage, vertical, centrifugal pump.

The 240 Vac supply to the pump motor is routed via the motor control centre.

The pump is inhibited from operating if there is a low level in the humidifier water storage tank, via a Kasuga level switch. The 110 Vac controls voltage is provided by the motor control centre.

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Duty Summary  
Fluid : High purity water
Specific gravity : 1.0
Solids content : nil
Viscosity : 1 cP
Temperature : 25 °C
Flowrate : 0.5 m3/h
Suction condition : Flooded
   
Pump Set Description  
Manufacturer : Lowara
Type : Vertical multi-stage
Pump wetted parts : SS304
Seal : Single special mechanical seal
Model : SV206F07MSP
Flowrate (each) : 0.5 m3/h in service
Head : 6 barg
Inlet : 25 mm
Outlet : 25 mm
Speed : 2,900 rpm
Power : 0.75 kW
Voltage : 240 Vac, 50 Hz, single phase

Mixed Bed Ion Exchangers

The permeate from the reverse osmosis plant is pumped through two mixed bed units which contain replaceable ion exchange resin. The resins are a mixture of cation and anion resins. These mixed bed resins will remove the remaining ions and produce the required water quality. The vessels will operate in a duty/standby mode with one vessel being on-line at anyone time.

The mixed bed vessels are 10" diameter and contain 25 litres of mixed bed ion exchange resin.

Two resistivity meters are provided, one for each outlet of the mixed beds.

The resins should be replaced when the water quality falls below the required levels and the second vessel brought into operation.

0.45 Micron Filtration

These filters will remove any large bacteria and particles that may gain access into the high purity tank. The filter element is sized at 0.45 microns and is 10" long.

The filter element is installed in a new stainless steel SS316L housing.

The filter element should be changed when the pressure drop across the filter exceeds 1 bar.

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Loop UV Irradiator

To safeguard the water going out to the loops from bacteria, a UV irradiator has been provided and is installed between the 0.45 and 0.2 micron filters. The irradiator's details are:-

Model : General Waters 0030
Maximum flow : 2 m3/h
Number of lamps : 1
Power consumption : 40 W
Fittings : 1" BSPF threaded

0.2 Micron Filtration

This filter will remove any disrupted bacteria particles that may be produced by the UV irradiator. The filter element is sized at 0.2 microns and is 10" long.

The filter element is installed in a new stainless steel SS316L housing.

The filter element should be changed when the pressure drop across the filter exceeds 1 bar.

Motor Control Centre

A new motor control centre (MCC) has been provided for the control of the humidifier supply pump and the high purity loop pump. Both pumps are driven by single-phase motors.

110 Vac control voltage has been used for pump inhibition on low level and pressure switch operation.

Emergency stop push buttons located on the front of the MCC have been provided for each pump.

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last updated 28-09-2015