Water System Issue In The Ship'S Main Engine

Overview of the Process for Producing Ultrapure Water from Seawater

I. Core Definitions

Seawater Desalination: The removal of large amounts of salt, suspended solids, and microorganisms from seawater to produce low-salinity fresh water.
Ultrapure Water: Water from which almost all ions, particulates, organic matter, and bacteria have been removed; it has a resistivity of ≥15–18 MΩ·cm and meets water quality standards for laboratories, electronics manufacturing, boilers, and high-end marine equipment.
Seawater cannot be converted directly into ultrapure water; it requires multi-stage processing. The mainstream process flow is: Pretreatment + First-stage Seawater Reverse Osmosis (SWRO) + Second-stage Reverse Osmosis + EDI (Electrodeionization) for deep desalination (the most common integrated solution).

II. Complete Process Flow

1. Seawater Pretreatment (Upstream filtration to protect membrane elements)

Seawater contains numerous impurities and has a salt content of 30,000–40,000 mg/L; purification is essential:

1. Water Intake & Sedimentation Tank: Settles out silt, sand, seaweed, and floating debris.
2. Dosing System: Adds antiscalants, biocides, and flocculants to inhibit marine biological growth and calcium carbonate scaling.
3. Multi-media Filter + Activated Carbon Filter: Removes colloids, organic matter, and residual chlorine.
4. Precision Security Filter (5μm): Traps fine particles to prevent scratching of the reverse osmosis membranes.

2. First-stage Seawater Reverse Osmosis (SWRO – The core of desalination)

A high-pressure booster pump pressurizes the seawater to 55–70 bar, forcing it through specialized seawater reverse osmosis membranes:

- Permeate (Product Water): Desalinated water with a salt content of 300–500 mg/L.
- Concentrate (Brine): High-salinity brine; can be discharged into the sea or processed for salt recovery.
Function: Removes over 99% of sodium chloride, completing the basic desalination step; at this stage, the water is merely desalinated water and does not yet meet ultrapure standards. 3. Secondary Reverse Osmosis (RO – Preliminary Purification)

Permeate from the primary stage enters the secondary low-pressure RO unit to further remove residual calcium and magnesium ions as well as trace salts:
Product water conductivity is reduced to below 5 μS/cm, significantly lowering ion content and serving as the feed source for the ultrapure water stage.

4. EDI (Electrodeionization) Deep Refining (Core Ultrapure Water Stage)

Continuous deep desalination without the need for acid/alkali regeneration:
Secondary RO permeate flows into the EDI module, where trace anions and cations are separated under the influence of an electric field;
Output water resistivity stabilizes at 15–18.25 MΩ·cm, meeting standard ultrapure water specifications.

5. Terminal Polishing Treatment (Optional for High-End Ultrapure Water)

Polishing mixed-bed resin, ultrafiltration, UV sterilization, and microporous terminal filtration: removes trace TOC, bacteria, and micro-particles; suitable for chip manufacturing, laboratories, and medical applications.

III. Key Equipment Parameters (Standard 20 tons/day Marine/Land-based Model)

1. Feedwater: Seawater TDS 35,000 mg/L;
2. Primary seawater desalination rate: ≥99.2%;
3. Secondary RO desalination rate: ≥98%;
4. EDI output water resistivity: 15–18.2 MΩ·cm;
5. System recovery rate: 40%–55% overall;
6. Operating energy consumption: Higher energy demand in the seawater stage; power consumption is 4–6 kWh per ton of product water.

IV. Application Scenarios

1. Cooling water for high-end marine boilers and precision instruments;
2. Process water for island-based photovoltaic facilities and small-scale semiconductor plants;
3. Water supply for offshore scientific research laboratories and medical equipment;
4. Makeup water for large high-pressure boilers on islands. V. Process Advantages

1. Employs a fully physical separation process; generates no large volumes of acidic or alkaline waste liquid; environmentally friendly and easy to maintain.
2. Modular design; customizable for capacities of 5, 20, or 50 tons per hour.
3. Fully automated operation with automatic membrane module flushing to minimize scaling and fouling.
4. Lower energy consumption and a smaller footprint compared to conventional distillation methods.