FWG plant membrane type

Membrane type RO FWG plants mainly uses on board of cruise ships first of all due to their high capacity and energy efficiency (production capacity 10…3000 m³/d/vessel, energy consumption 2,4…4,0 kWh/m³ of freshwater) compared to conventional freshwater systems. One of the world´s leading manufacturers of reverse osmosis systems for cruise ships is WatMan. Below provides description of working principle and figures of lay out and process diagram based on WatMan RO 45000 SW 2-pass reverse osmosis FWG plant.

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 Lay out of WatMan RO 45000 SW 2-pass reverse osmosis FWG plant, up view (a) and side view (b)

1 – heating water circulating pump, 2 – heat exchanger, 3 – energy recovery turbine, 4 – 2-pass RO unit, 5 – RO unit elements (1^st and 2^nd pass membranes), 6 – multimedia (sand) filters

WatMan RO 45000 SW 2-pass reverse osmosis FWG plant double-pass plant with energy recovery by turbine. Its capacity is 1070 m³/day (44,6 m³/hour). The plant consist of one seawater centrifugal pump, six WatMan MLFA 48 Multimedia filters, two sets chemical dosing units, one heat exchanger (plate type, heating water / seawater (hot/cold)) unit, one 2-pass RO unit (First pass: spiral wound thin film composite  low energy membranes Seawater SW30 by Filmtec or equivalent - 84 pcs; Second pass: Spiral wound thin film composite membranes Brackish water low energy membranes by Filmtec or equivalent - 42 pcs, hydraulic pressure booster, high pressure feed pump, flow meters, pressure gauges, conductivity controller, automatic fresh water flushing system, draw-back tank) and one electrical control cabinet.

Seawater pump supply raw water to the set of WatMan MLFA 48 Multimedia filters (six filters to be installed in parallel). Multimedia filter filings shown in the figure below. Then after filters water going to heat exchanger which maintain appropriate temperature before RO unit. Meanwhile one chemical dosing unit automatically adds (before multimedia filters) NaHSO3 for disinfection of raw water and another one adds (between heat exchanger and RO units) Ameroyal 642 or equivalent for RO membrane antiscaling.

Multimedia filter filings

1 – support layer of gravel (Ø 2…3 mm / 300 l), 2 – 1^st filter layer of sand (Ø 1…2 mm / 100 l), 3 – 2^nd filter layer of sand (Ø 0.5…1.0 mm / 300 l), 4 – 3^rd filter layer of aqua-volcano (Ø 1.5…2.5 mm / 600 l), 5 – empty space

 

In RO unit water passes membranes of 1^st pass and after 2^nd pass (see figure below). 1^st pass high pressure pump increases pressure up to 30…40 bar before RO unit 1^st pass. Then water flow through hydraulic pressure booster (which boost pressure up to 50 bar by recover energy of reject flow) going to 1^st pass membranes. After 1^st pass 60 % of incoming water rejects by membranes (through HBP to overboard) and 40 % water (TDS 200…400 mg/l) supplies by high pressure pump under pressure up to 16 bar to the 2^nd pass. After 2^nd pass the freshwater with quality TDS < 20 mg/l going to FW tank.

Source: https://www.watman.fi/pages/en/sea-water-desalination

 

WatMan RO 45000 SW 2-pass reverse osmosis FWG plant process diagram

 

Figures below illustrates a simplified spiral wound RO membrane element. The spiral wound RO membrane is constructed of one or more membrane envelops wound around a perforated central tube. The permeate passes through the membrane into the envelop and spirals inward to the central tube for collection.

 

 

Simplified spiral wound RO membrane element

 

Cross section of a simplified membrane element

 

 

Transversal cut of a Filmtec spiral wound RO membrane element