DESALINATION

It would not be exaggeration to say that success of the project bidding substantially depends upon how complete the input data are. Below is a roadmap to these data collection with some brief explanation of each milestone contents and/or necessity.

tickProject contract type

In the not distant past 2 types of the project contracts dominated: EPC and BOO. The EPC contract scope includes Engineering, Procurement and Construction phases. The BOO (Build, Own, Operate) contracts cover the plant life cycle. Accordingly, the EPC contracts focus on the minimum initial costs, while BOO ones consider the life-cycle costs.
Lately substantial rise is observed of the EPS contracts, which include only part of the construction phase - supervision.

Battery limits

Battery limits specify interaction between the plant and the client mains or infrastructure. Environmental issues (like the maximum noise level) shall be discussed as well.

Plant plot and layout

Plant allocated area and layout may impact the selection of the pretreatment scheme as, for example, the gravity filters require 4 times more area than the ultrafiltration plant. As a rule of thumb, the required total plant area may be calculated based on the specific production of 4.5 – 5.5 (m3/h)/m2.

Seawater intake

Seawater intake questionnaire should have at least the following subsections.

  1. General data
  2. Water/shore levels
  3. Construction materials
  4. Design constraints
  5. Intake/outfall systems basis of design details
  6. Outfall diffuser system details

Seawater analysis data over the time of the year

Seasonal swings in salinity and temperature may lead entirely different implementations of SWRO trains. If the analysis in not available, the database on the existing desalination plants shall be used. Seawater salinities and temperatures may be obtained from NOAA or from this salinity map.
Equally important are the lowest and the highest temperatures durations. The lowest temperatures define the plant overall production, while the highest ones the selection of the materials withstanding corrosion attacks and the biofouling probability.

Worst case scenario (design conditions)

Special attention shall be given to the seawater TSS and turbidity during stormy weather as generally the plant is engineered for the worst case scenario defined by the maximum TSS and/or Turbidity, the lowest temperature, maximum RO membranes fouling and requirements for uninterrupted water supply. The probability of jellyfish inrush and algae blooming shall be analyzed as well.

Contracted water loads

Constant feed pressure and flowrate

At this operation mode the seawater pressure before RO membranes is always constant. All pumps are kept operated at the same operation point around-the-clock.

Constant demand

Constant production mode is often specified by the client in cases when information about actual water consumption and its predicted growth is not available.

Constant monthly demand

This mode defines the monthly quantities irrespective of the daily production rates. It allows the plant designers to relax the requirements for the equipment availability and maintainability and to use more flexible Preventive Maintenance Plans (PMP) as regards the pumps overhaul, CIP and the RO membrane replacement.

Varying demand

It is expected that the varying demand absorb seasonal variations and the consumption growth over time.

Varying daily demand

The seawater desalination plant may be effectively engaged in the peak shaving - the reduction of the amount of electricity drawn from a power utility during utility designated peak time periods. In this mode of operation the plant doesn't produce water during noon peak hours when the electricity tariffs are the highest.

Production availability

As a default, the actual availability of the plant shall not less than 95%. If uninterrupted water supply is requested (for technological processes), the plant shall be equipped with a surge tank with the volume sufficient for 4 hours of the nominal water supply.

Water storage

Unlike electricity, water may be stored. Plants with water storage capacities above 4 hours have substantially higher reliability as most equipment and instrumentation failures may be repaired.

Product water quality

Product quality falls within 3 categories

  1. Process water
  2. Potable water
  3. Potable low-Br water

For potable water grade the client requirements shall be checked against the national and international standards (WHO standard, European standard etc.)
The process water requirements shall be accompanied by the target process or equipment specification like the boiler, ion-exchanger, or electro-dialysis unit.
Provision shall be added for allowable spikes in LSI, hardness, alkalinity, TDS, pH, and Boron content.

Power supply

Major part of the desalination market is powered by 50hz voltage. The 60hz pumps, motors and VFDs require more attention in terms of the reliable operation range.
The AC motors above 1600 kW and VSDs above 1200kW require transition to medium voltage.
Power supply reliability shall be tied to the plant general reliability.

Brine disposal

It may be required to dilute the brine flow before disposal. Another critical point is whether the backwash water flow from the multi-media filters, ultrafiltration units and the lime stone reactors shall be disposed separately or not.

Sludge treatment

Sludge treatment does not tolerate long pipe runs and batch operation: it shall be equally distanced from the pretreatment and posttreatment areas. The best dewatering technology shall be selected based on the final sludge cake dryness.

Chemical reagents sources and pricing

Chemical reagents availability, sources location, quality and pricing shall be taken into account in optimizing the pretreatment and posttreatment processes. As a default, the 2-week storage systems shall be considered.

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