Fresh water production system

Reverse Osmosis fresh water system

Reverse Osmosis fresh water system

Reverse osmosis system gives instant water production and advantageous for the ships having longer anchorages outside 20 mile of port limit.

A) principle of reverse osmosis:- Reverse osmosis is a process by which a solvent passes through a porous membrane in the direction opposite to that for natural osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure.

• Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to remove ions, unwanted molecules and larger particles from drinking water.
• In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential differences of the solvent, a thermodynamic parameter.
• Reverse osmosis can remove many types of dissolved and suspended chemical species as well as biological ones (principally bacteria) from water. The result is that the solute(large molecules or ions) is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side.
• In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). 
• Reverse osmosis differs from filtration in that the mechanism of fluid flow is by osmosis across a membrane. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution's pressure and concentration. Reverse osmosis instead involves solvent diffusion across a membrane that is either nonporous or uses nanofiltration with pores 0.001 micrometers in size. The predominant removal mechanism is from differences in solubility or diffusivity, and the process is dependent on pressure, solute concentration, and other conditions.

 (B)system for producing fresh water from sea water:- 

The water pass through the membrane is called permeate or product water.
The equipment designed to reject 99% of the TDS or to allow 1% of the TDS of the SW to pass through into fresh water.
about 25% recovery is possible, rest of the feed water is used to flush away the solids.
the capacity of the system may vary seasonally with temperature. about 1.5% per one degree Celsius.

Components and their function:-
a). cyclone separator :- it is used to remove large suspended solids such as sand. it uses centrifugal force.
this is designed to induce a centrifugal flow.
b). sterilization:- sea water is sterilized to remove bacteria which would otherwise become resident in the filter. Chlorination used to kill bacteria.
c). de-chlorination:- it is done by activated carbon filter to increase life of the membrane, since chlorine adversely affect the life of membrane.
d). chemical treatment:- poly-electrolyte or flocculant are used to sequester undesired suspended compounds.
sodium-hexa-meta-phosphate is added to assist the wash through the salt deposit on the surface of element.
e). filters:- 20 micron and 3 micron replaceable cartridge filters are fitted in series to remove solids.
f). pump:- a HP pump with 60bar pressure to over come osmotic pressure.
thumb rules for pressure is for each 100milligram of seawater per liter require one Psi pressure.
to achieve high pressure a gear pump with relief arrangement is provided.
g). accumulator:- it is fitted to maintain flow regularities.

h). semipermeable membranes:- they are made up of film of polyamide or poly sulphonate sheets with backing material for strength.
for large amount of water, the membrane area must be large. to achieve washing action hollow fiber configuration or spirally wound cartridge is used. that keeps surface clear off the deposits.
membrane to be cleaned periodically by back flushing or using chemical cleaning agent.
i). output of the plant:- quantity depends on the number of cartridge in parallel and quality depends on the number of cartridge in series. single stage R.O plant produce product water with approximately 350ppm TDS.

Fresh water Generation using Reverse Osmosis
The generation of fresh water from saline water can be carried out by the process of reverse osmosis. The control system is designed, around a microprocessor, whose program includes a complete fail-safe operation with rinse procedure, circuit cleaning modes and fault indication with a sub-program for manual operation.
The first stage consists of the Pre-filter pump, pre-filters ( Sand filter and Cartridge filter), High pressure pump; Module containing the permeable membrane, controls and externally fitted De-acidification filter. Sea water is pumped into the unit by the sea water booster pump. The pre-filters remove particles down to 10 microns. The filtered water is pumped by the high pressure pump, which raises the pressure to 65 bar and delivers to the Module containing the Membrane. Inside the Module, about 30 % of the sea water is forced by the high p.ressure through the thin membrane to the fresh water recovery side. Salt ions and other impurities are rejected by the membrane, and are flushed out with the remaining sea water. The pressure on the membrane is automatically controlled by the pressure control motor valve.
The fresh water (permeate) is controlled by a conductivity meter. The concentration of salt ions present can thus be monitored. If the set-point is exceeded, there will be a fault indication, and the water will be diverted to the rinse tank. If the conductivity is low enough, the fresh water passes to the De-acidification filter, to produce drinking or potable water. If battery water is required, the fresh water is passed through the second stage, where the high pressure pump raises the water pressure to 65 bar before delivering to the second stage Module, where about 75 % of the water passes through the membrane to produce distilled water. Production rate depends on the temperature of the sea water, and drops when the sea water temperature deviates from the design value, being higher for higher values of sea water temperature. The production rate also depends upon the initial salinity of the sea water, the condition of the membrane, the scaling of the membrane and other operational factors.
Pumps
Pre-filter booster pump and filter pump are usually centrifugal pumps. The High pressure pump system consists of a slow revolution triplex plunger pump having attached motor and belt drive, a pulsation damper and a pressure relief valve.
Pulsation Damper
This is fitted after the high pressure pump, to dampen the pulsations which could damage the membrane. The Pulsation damper consists of a pressure vessel with a gas valve, the membrane with a plate valve and a filling connection for Nitrogen. The gas side is isolated from the liquid side by the membrane. The valve plate is vulcanised into the diaphragm bottom. It closes the fluid outlet in case of a sudden drop of system pressure or complete discharge, thus protecting the membrane from extruding through the fluid port. The damper is pressurised to 25 — 30 bar.
Filters
The Sand filter is designed for rapid filtration, with back-wash facility. The Cartridge filter is used to remove suspended matter, and reduce fouling of the membrane. Filter elements need to be replaced after pre-determined intervals, depending on the quality of raw water and pollution. The De-acidification filter prevents corrosion of the pipelines by the chemical reactions which neutralise any acids present.