RO BASED DESALINATION PLANT

FOR

COAL BASED THERMAL POWER PLANT

NEAR SEA

1.0 PURPOSE

This write up describes the details of Water Treatment System required to supply fresh water to meet plant service water requirements, potable water requirements for plant and personnel usage and demineralized water for the boiler cycle makeup for a Coal Based Thermal Power Plant. The Plant is considered at Sea Shore and raw water is taken from the sea. The author is having the hands on experience in detailed design of the various sea water desalination plants. The details of RO based desalination plant described here are of international standard and based upon the previous installed and designed project for the Coal based thermal power project.

2.0 SYSTEM DESCRIPTION

The source of water to the Power Plant is taken from the intake structure which is located near sea shore. Considering the plant is near sea then Sea water makeup pumps, which shall be located in the intake structure, supply sea water to the Cooling Tower (as makeup), Hypo Chlorination Plant, Ash Water Booster Pumps apart from the Water Treatment System.

In the intake structure, chlorine solution shall be dosed and hence chlorinated sea water will be delivered to the inlet of Water Treatment System from the common discharge header of the Sea Water Makeup Pumps.

2.1 Pretreatment System

The sea water passes through the Pretreatment system consists of Dual Media Filters (DMF) followed by Polishing Sand Filters (PSF) for the removal of particulate organic and colloidal matter from the incoming sea water. The upper layer of anthracite removes large suspended particles which otherwise may blind the bottom layer of sand. Before the sea water is passed through the Pressure Filters (DMF & PSF) it is dosed with coagulant (FeCl3) and polymer so as to prevent colloidal fouling in the RO membrane surface. Coagulant agglomerates small particles to form larger particles, which are more easily removed in the Pressure Filters.

Figure: Dual Media Filters (DMF) & Pressure Sand Filters (PSF)

The filtered water will be collected in Filtered Water Basin. An underground /above ground Pump House will be provided where the Filtered Backwash Pumps and SWRO Feed Pumps will be located.

Backwashing of the Pressure Filters will be done with the Filter Backwash Pump and the water will be supplied from Filtered Water Basin. The backwash waste water of the Pressure Filters will be disposed off adequately to the sea water outfall or some other discharge channel. Back wash waste water can be disposed off to the seawater outfall by gravity or by using Filtered Backwash Waste Water Sump and associated Filter Backwash Waste Water Transfer Pumps.

Normally Pressure Filters will be operated simultaneously and one (1) Pressure Filter will be on stand-by mode. The filtered sea water from the Pretreatment System will be fed to the Desalination System for producing treated water for various applications.

2.2 Desalination System

The prime purpose of Desalination System is to remove majority of the dissolved solids from filtered sea water and make it fit for Potable Water usage and feed for Demineralization System.

Filtered water from Filtered Water Basin shall be pumped to SWRO Unit via SWRO Feed Pump. The water will be first passed through SWRO Micro Filters where suspended solids above 5 micron (nominal) size will be removed. These filters also protect SWRO High Pressure Pumps in event of media carry over from Polishing Sand Filter.

Various chemicals such as acid (HCl), SHMP and sodium bi-sulphite will be dosed prior to SWRO Micro Filters so as to optimize the water parameters before feeding to SWRO membranes.

Acid (HCl) will be dosed so as to aid in pH control and ensure that calcium carbonate does not precipitate in the reject brine stream within the membranes. Acid converts carbonates to carbon dioxide gas thereby reducing the carbonate concentration and the probability of precipitation of calcium carbonate in the reject brine stream.

Anti-scalant sequestrates are used for maintaining anti-scalant property in the RO system. Technical grade SHMP or any commercial inhibitor products will be used.

Prior to the SWRO Micro Filter, the feed water is de-chlorinated by the addition of SBS, thus preventing chlorine attack on the membranes, which may otherwise cause irreversible damage to membranes. A residual chlorine analyzer is located downstream of the cartridge filter to control of SBS dosing rate.

After this, the water should meet the guidelines of feed to the RO membranes such as SDI< 5, pH, residual chlorine, etc.

If the incoming sea water to SWRO Unit does not meet the guidelines of feed to the RO Unit, the SWRO Units will be automatically shut down and this off-spec water will be dumped to the seawater outfall/other discharge channel until stable operation of the Pre-Treatment System produces feed water of acceptable quality.

In SWRO Unit, the water is boosted up to the desired pressure by high-pressure pumps and is delivered to the RO permeate and reject streams by pressure. 35% of the feed sea water and a small quantity of total dissolved solids are transported across the membranes to the permeate stream( In some cases it may go up to 40%). The remaining 65% concentrated sea water flows out through the brine out-fall line to the energy recovery turbine, which is provided to recover the pressure energy available in the reject brine stream.

The reject water of SWRO Unit can then be disposed directly to the sea water outfall /discharge channel therefore the water coming out from energy recovery turbine should have sufficient pressure (i.e. 6 kg/cm2 a), which is required to dispose to the sea water outfall.

Figure: SWRO(RO I) & BWRO (RO II) Configuration

The permeate water of SWRO Unit is monitored in terms of conductivity and if the water is not of acceptable quality, it will be checked by the operator and then troubled SWRO Unit will be manually shutdown by the operator.

The permeate water of SWRO Unit, which is collected in the Brackish Water Storage Tanks will be delivered to BWRO Unit via BWRO Feed Pumps to get

the desired quality of permeate water. This good quality water will then be collected in the downstream Fresh Water Storage Tank prior to being transferred to the Demineralization System and Potable Water Usage. Permeate water from Fresh Water Storage Tank will then be transferred via Permeate Water Pump to the downstream Mixed Bed Polishers (MBP).

The recovery ratio in BWRO Unit will be more than 85%. The reject of BWRO Unit, being good quality water will be taken to the Filtered Water Basin so that it can be reused.

Chemical cleaning and flushing system will be provided in order to maximize the membrane efficiency in the long run. During normal operation with proper pretreatment, deposits of mineral scale, biological matter, silt and insoluble organic constituents build up on the membrane surface at a very slow rate.

Figure: Chemical Cleaning System

Frequent chemical cleaning is not required for properly operated RO systems.

However, it is anticipated that cleaning will be carried out every three (3) ~ four (4) months, this will vary according to the feed water characteristics. The cleaning system designs offered will minimize the use of operating chemicals and maximize the membrane efficiency in the long run.

The system is flushed back through any other method in order to replace the salinity water with product water having low total dissolved solids content. This prevents precipitation of sparingly soluble salts inside the membranes. (The stand-by cleaning pump will also be a common standby pump for flushing of RO Units.)

The waste water generated during chemical cleaning of RO Units, will be transferred to the local neutralization pit (of Mixed Bed Polisher) by gravity, where it will be neutralized prior to disposal.

The permeate water of SWRO Unit will be transferred to Brackish Water Storage Tanks and not directly to the BWRO Unit. From this tank only, some part of the permeate water is transferred to the BWRO Unit via the BWRO Feed Pumps.

On the other hand, the majority of the service water which is being collected in the Brackish Water Storage Tanks will be directly transferred for the following applications:

i) Service water application using dedicated Service water pumps

ii) Dust suppression application for Coal Transfer Points using dedicated Dust

Suppression pumps

iii) Dust suppression application for Coal Pile Area using dedicated Coal Pile spray pumps

2.3 Demineralization System

The prime purpose of Demineralization System is to treat the permeate water of BWRO Unit and produce demineralized water for boiler feed application.

The permeate water of BWRO Unit, which is collected in the Fresh Water Storage Tank is not good enough to be used in the steam/water cycle and it needs to be treated further by passing through a Mixed Bed Polisher (MBP) so as to reduce the total dissolved solids.

The permeate water from Fresh Water Storage Tank will be transferred via Permeate Water Pump to the downstream Mixed Bed Polisher (MBP), which contain strong acid cation (SAC) resin and strong base anion (SBA) resin. Here the remaining cation and anions are reduced so as to produce stringent quality demineralized water at the outlet.

Figure: Mixed Bed Polisher

The Mixed Bed Polisher (MBP) is essentially a carbon steel rubber lined pressure vessel. It is externally fitted with rubber lined pipe work, diaphragm valves, pressure gauges and sampling valves at the inlet and outlet. Differential pressure switch is provided across the resin trap and the resin bed.

Conductivity meter and pH analyzer will be provided at the outlet of each Mixed Bed Polisher (MBP) whereas a shared silica analyzer will be provided, common for both Mixed Bed Polishers.

2.4 Potable Water System

The prime purpose of Potable water system is to treat the permeate water of BWRO Unit (available from the discharge side of Potable Water Pumps, which are located on the downstream side of Fresh Water Storage Tanks) so that it is suitable for potable water consumption as per WHO requirements.

Potable Water Pumps, which will take feed from the common suction header of Permeate Water Pumps. It is recommended to consider suitable chlorine dosing system using static mixer and remineralization facility (if required) so that the permeate water from Fresh Water Storage Tank (available from the discharge side of Potable Water Pumps) is suitable for potable water usage as per WHO requirements. The dosing of chemicals will be adjusted automatically/manually based on the flow rate of Potable Water Pump.

3.0 GUARANTEED WATER QUALITY REQUIREMENTS

Sr. No.

Item Unit Guaranteed Value

1 Pretreatment System (Discharge of PSF)

Turbidity NTU <2

2 SWRO Inlet

Silt Density Index (“SDI”) <5

3 SWRO Unit

TDS of permeate water mg/l <500

4 BWRO Unit

TDS of permeate water mg/l <15

5 Mixed Bed Polisher (MBP)

Conductivity (at 25 degree C) Micro-siemens/cm

< 0.1

Total Silica mg/l as SiO2 <0.01

Total Solids mg/l 0.05

pH 6.0 ~8.0

4.0 SPECIFIC DESIGN REQUIREMENTS

4.1 Chemical Storage System

i) The storage capacity of chemical storage system (acid and caustic) will be sufficient for 1 month operation of the Water Treatment Plant.

ii) The chemical storage tanks for the storage of hydrochloric acid and caustic soda solution will be of the horizontal or vertical cylindrical type.

iii) The capacity of Coagulant, Acid and Caustic storage tank will be sufficient for one month usage of chemicals in Water Treatment System.

4.2 Chemical Dosing System

i) The dosing tank and pump for each kind of chemical will be skid mounted, including piping, support, electrical and instrumentation, etc.

ii) The agitator will be provided with proper supports.

iii) The dosing pump will be provided with the single-acting, internal self relief, positive-displacement diaphragm type with all accessories for proper manual operation.

iv) The capacity of all chemical dosing tanks i.e. Polymer / SHMP / Acid / SBS dosing tanks in the Water Treatment System will be sufficient for one day usage minimum.

4.3 Dual Media Filters

i) Dual Media Filters will consist of anthracite and sand other than gravel as filter media.

ii) The service flow rate of Dual Media Filters will be typically in the range of 10 ~ 12 m3/m2/hour. The back wash flow rate with air scouring will be considered in the range of 24 ~ 30 m3/m2/hour typically.

iii) The filter will be designed as per ASME VIII, Division I.

iv) The internal lower distributors will be constructed of ABS.

v) The filter will be equipped with all automatic valves required to auto operation.

vi) For filter backwashing, differential pressure indication transmitter will be equipped.

vii) The flow indicator will be equipped at inlet of each DMF.

viii) The diaphragm type pressure gauge will be equipped at inlet and outlet pipe of each filter.

x) The ladder, platform, handrail, etc. will be provided for operating and maintenance.

4.4 Polishing Sand Filters

i) Polishing Sand Filters will consist of sand other than gravel as a filter media.

ii) The service flow rate of Polishing Sand Filters will be typically 10m3/m2/hour. The back wash flow rate with air scouring will be considered in the range of 24 ~ 30 m3/m2/hour typically.

iii) The filter will be designed as per ASME VIII, Division I.

iv) The internal lower distributors will be constructed of ABS.

v) The filter will be equipped with all automatic valves required to auto operation.

vi) For filter backwashing, differential pressure indication transmitter will be

equipped.

vii) The diaphragm type pressure gauge will be equipped at inlet and outlet pipe of

each filter.

ix) The ladder, platform, handrail, etc. will be provided for operating and maintenance.

4.5 SWRO Unit

i) The recovery ratio of SWRO Unit will not be less than 35% of feed sea water. Salt reject rate of SWRO membranes will not be less than 97%. SWRO train will be capable of operating individually or simultaneously in accordance with the water consumption.

ii) Energy Recovery Turbine will be designed such that the outlet water will have

sufficient discharge pressure so that it can be disposed off accordingly to the outfall.

iii) The high-pressure pump will be provided with sufficient capacity and pressure.

iv) The membrane lifetime will be guaranteed for more than 3 years.

v) SWRO Unit will be skid mounted, including piping, support, electrical and

instrumentation etc.

vi) The SWRO train will be capable of operating individual or simultaneously in

accordance with water consumption.

vii) Flushing system will be provided for SWRO.

viii) The ladder, platform, handrail etc. will be provided for operating and maintenance.

Figure: Skid Mounted RO Plant

4.6 BWRO Unit

i) The recovery ratio of BWRO Unit will not be less than 85% of feed water. Salt reject rate of BWRO membranes will not be less than 95%. BWRO train will be capable of operating individual or simultaneously in accordance with water consumption.

ii) The high-pressure pump will be provided with sufficient capacity and pressure.

iii) The membrane lifetime will be guaranteed for more than 3 years.

iv) BWRO Unit will be skid mounted, including piping, support, electrical and instrumentation etc.

v) The BWRO train will be capable of operating individual or simultaneously in accordance with water consumption.

vi) Flushing system will be provided for BWRO.

vii) The ladder, platform, handrail etc. will be provided for operating and maintenance.

4.7 Mixed Bed Polishers

i) Mixed Bed Polishers will be vertical, cylindrical pressure vessels and will be designed with a design pressure of 9 kg/cm2a or permeate water transfer pumps shut off heads, which ever is greater.

ii) The inside chemical distribution and under drain system will be of acid and alkali resistant material.

iii) The inside chemical distribution and under drain system will be acid and caustic resistant.

iv) Each Mixed Bed Polisher Vessels will be provided with the necessary flange connection and manholes.

v) All vessels will be provided with three observation windows.

vi) The Mixed Bed Polishers will be supplied with all internal piping prior to shipment.

All internal piping systems of the hub or header-lateral type will be installed in the Mixed Bed Polishers using flanged gasket connections where attached to the vessel nozzles. All piping will be rigidly supported and braced to withstand motion of water and resin beds. The vessel internals will be arranged so that no sub fill will be required to support the media.

vii) Design of the internal piping will provide uniform collection or distribution of flow throughout the entire exchange bed volume avoiding excessive velocity of channeling in any part. The under drain system will provide uniform collection of effluent at rated capacity and will be designed to distribute backwash water uniformly through the bed. The backwash collector system will be designed to remove effectively the maximum expected quantity of backwash water when backwashing with water having the maximum expected temperature without loss of serviceable material.

ix) All resin will be new, unused material received in manufacturer's original unbroken containers.

xi) Additional auxiliaries will comprise inlet and outlet pressure gauges, automatic sequence, conductivity controllers, sampling valves on inlet and outlet connection, etc.

4.8 Regeneration System

i) All resins will be regenerated in the Mixed Bed Polishers. The regeneration system will perform all regenerating functions. The regeneration system consists of Acid and Caustic Storage Tanks, Chemical Injection Pumps and the necessary distribution system. Both the chemical solutions will be diluted by mixing in line.

ii) Both the hydrochloric acid and caustic soda solutions are to be diluted by mixing in-line.

4.9 Neutralization System

i) All effluent going to waste from the Water Treatment System will have a pH-value between 6 ~ 8. Therefore, when using acid or caustic for regeneration of the Mixed Bed Polishers, the effluents will pass through a Neutralization Pit before being discharged at outfall channel to sea.

ii) The Neutralization Pit will be fitted with a complete mixing system with water jets fed through the pressurized circulating water line of the Neutralization Pumps or Mixing Blower.

iii) The Supplier will furnish all equipment required for proper operation of the neutralization system including the chemical injection system.

iv) The capacity of the Neutralization Pit will be designed so that the effluent from the Water Treatment System can be taken up and neutralized. One of Mixed Bed Polisher’s regeneration will be executable within 24 hours.

v) Two level switches mounted in the Neutralization Pit will be used for automatic control of the main pump (selected by means of a selector

switch). The main pump will start up when reaching the high level switch and will stop when the Neutralization Pit level has reached the low level switch.

vi) The neutralization pit will be emptied in one hour with one Neutralization Pump in operation.

5.0 BILL OF QUANTITIES:

5.1 Mechanical Equipment

A Pretreatment System

(i) Sea Water Supply Pumps

(ii) Static Mixer,

(iii) Dual Media Filters (DMF),

(iv) Media Traps (DMF),

(v) Polishing Sand Filters (PSF),

(vi) Media Traps (PSF),

(vii) Filter Backwash Pumps,

(viii) Scour Air Blowers (for Air Scouring of Filters),

(ix) Coagulant (FeCl3) Unloading cum Transfer Pump,

(x) Coagulant (FeCl3) Storage Tank,

(xi) Coagulant (FeCl3) Dosing System consisting of Dosing Tank with Agitator and Coagulant (FeCl3) Dosing Pumps,

(xii) Polymer Solution Preparation and Dosing System consisting of Dissolving Tank with Agitator, Dosing Tank with Agitator and Dosing Pumps.

B Desalination System

(i) SWRO Feed Pumps,

(ii) Static Mixer,

(iii) SWRO Micro Filters,

(iv) SWRO High Pressure Pumps with Energy Recovery Turbine,

(v) SWRO Units,

(vi) BWRO High Pressure Pumps,

(vii) BWRO Units,

(viii) Chemical Cleaning System comprising of Cleaning Tank with all accessories such as electric heater and agitator (if required), Cleaning Micro Filter and Cleaning Pumps,

(ix) SBS Solution Preparation and Dosing System consisting of Dissolving Tank with Agitator, Dosing Tank with Agitator and Dosing Pumps,

(x) Anti-Scalant (SHMP) Preparation and Dosing System consisting of Dissolving Tank with Agitator, Dosing Tank with Agitator and Dosing Pumps,

(xi) Acid (HCl) Dosing System (RO) consisting of Dosing Tank and Dosing Pumps,

(xii) Caustic (NaOH) Injection System (for dosing caustic to downstream side of SWRO Units) consisting of InjectionTank, Agitator and Injection Pumps*,

Note: * It is expected that the pH of permeate water of SWRO Unit will be acidic in nature as acid may be dosed prior to the SWRO Units. Therefore it is recommended to provide suitable Caustic Injection System for dosing caustic at the downstream side of SWRO Units (into the SWRO permeate water), i.e. prior to entry in to the Brackish Water Storage Tanks so that the pH of permeate water is neutral. It is also to be ensured that the TDS of the pH corrected water entering in to the Brackish Water Storage Tanks is < 500 mg/l. The design of the BWRO Unit shall be considering this pH corrected water quality.

C Demineralization System

(i) Permeate Water Pumps,

(ii) Mixed Bed Polishers (MBP’s),

(iii) Resin Traps (MBP),

(iv) Regeneration Pumps,

(v) Acid (HCl) Injection System consisting of Injection Tank and Injection Pumps,

(vi) Caustic (NaOH) Injection System consisting of Injection Tank and Injection Pumps,

(vii) Neutralization Pumps,

(viii) Priming Tank (for Neutralization Pumps),

(ix) Mixing Air Blowers (Common for MBP and Neutralization Pit) each with V-belt drive with belt guard, air filters, silencers, discharge snubber, noise hoods (if required), for supply of process air.

D Potable Water System

(i) Potable Water Pumps,

(ii) Hypochlorination (NaOCl) Injection System consisting of Injection Tank and Injection Pumps,

(iii) Static Mixer,

(iv) Remineralization System (If required)

E Common Equipments with all Accessories

(i) Acid (HCl) Unloading cum Transfer Pumps (Common for WTP & WWTP),

(ii) Acid (HCl) Storage Tank (Common for WTP & WWTP),

(iii) Fume Absorber (Common for Acid Storage Tank and Injection Tank).

(iv) Caustic (NaOH) Unloading cum Transfer Pumps (Common for WTP & WWTP),

(v) Caustic (NaOH) Storage Tank (Common for WTP & WWTP),

(vi) Drum Pumps.

Notes:

(i) 33~35% HCl, 48% NaOH and 38~42% FeCl3 will be applied for the Plant.

(ii) Priming tank and Fume Absorbers will be provided with level switch and pressure indicator and isolation valves for instrument, service water and drain.

F Control & Instrumentation

(i) All equipments required for automatic control,

(ii) Local control panel with redundant structured PLC with video display unit (“VDU”) based operator stations

(iii) Solenoid valve box including operation switch for easy manual operation,

(iv) All required instruments and control valves with all accessories for proper automatic and/or manual operation,

(v) Programming devices for the supplied PLC,

(vi) Water quality analyzers such as turbidity at the inlet of the plant,

(vii) Water quality analyzers such as turbidity at the outlet of Polishing Sand Filters,

(viii) Water quality analyzers such as pH and conductivity at the inlet of SWRO Micro Filters,

(ix) Water quality analyzers such as residual chlorine, etc. at the inlet of SWRO Unit,

(x) Water quality analyzers such as conductivity at the outlet of each SWRO train and each BWRO train,

(xi) Water quality analyzer such as conductivity for each MBP,

(xii) Water quality analyzers such as pH and silica analyzer (dual sampling facility) on the common discharge header. Silica analyzer should be able to receive samples of both the MBP’s,

(xiii) Water quality analyzer such as pH at the outlet of neutralization pump,

(xiv) Temperature transmitter and indicator at the inlet of the Plant,

(xv) Water quality analyzers such as residual chlorine, pH and conductivity for checking the potable water quality,

(xvi) Instrument racks for mounting pressure and flow transmitters, pressure transmitters/switches, conductivity analyzers, pH analyzers, etc.,

(xvii) All instruments, process transmitters / switches, level transmitters / switches, control valves, limit switches for control valves and motor operated valves, solenoid valves, pressure gauges, etc., as required,

(xviii) Supply of all screened, instrument and special cables,

(xix) Pneumatic & process hook up tubing / piping.

G Electrical Equipment

(i) Island MCC

(ii) All drive motors / actuators associated with the system / equipment,

(iii) All junction and solenoid boxes,

(iv) Supply of all power and control cables including necessary cable trays (hot dipped galvanized), supports, etc.

(v) Supply of all control cables from field devices up to junction boxes,

(vi) All cable glands and lugs as required,

(vii) Grounding system up to the earthing box terminals.

H Civil Works

(viii) Anchor bolts and nuts for the equipment,

(ix) Embedded materials (such as steel plates, rebars, template, etc.) for the equipment / structures,

(x) Operating platforms, ladders, handrails, etc. associated with the equipment.

I Others

(i) Personal safety shower and eye wash station

(ii) Personal safety equipments,

(iii) All nuts, bolts, gaskets, fasteners, packing rings, etc. between components of the equipment,

(iv) All necessary piping, valves, fittings, etc. within the skid including their piping supports,

(v) All necessary interconnecting piping, valves, fittings, etc. within the battery limits of the plant and their piping supports,

(vi) All other materials required for installation, commissioning, operation and

(vii) maintenance of the supplied equipment or facilities,

(viii) Painting (including final painting) and suitable corrosion protection and preservation measures (Including paint material for final touch up at site),

(ix) Valve locking device for isolation valves,

(x) Name plates and tags

(xi) Safety guards for personnel protection from rotating parts,

(xii) Special tools, if any,

(xiii) Consumables such as lubricating oil, grease, chemical reagents for water quality analyzers, etc. for initial charge

(xiv) Start-up and commissioning spare parts

(xv) Shop testing and inspection,

(xvi) All required test equipment for commissioning and performance test.

(xvii) Portable calibrator for the related instruments (HART protocol),

(xviii) Recommended spare parts list

(xix) All external to the units.

(xx) DCS.

(xxi) Power Plant MCC (which will supply a feeder to the island MCC).

(xxii) Power supply and control cables

(xxiii) Grounding system (earthing mat).

(xxiv) Pipe rack and supports for piping

(xxv) Cable trays/ conduit pipes and their hangers / supports

(xxvi) Lifting hoist and monorail

(xxvii) Chemical resistant lining

(xxviii) Bulk chemicals (hydrochloric acid, caustic, ferric chloride, polymer, SBS, SHMP)