PRINCIPLE OF OPERATION
Pretreated water is pumped into the membrane housings along the membrane surface. Pure water is permitted to pass through the membrane while ionic, organic, colloidal and bacterial contaminants are swept away in a concentrated solution. Consequently, a reverse osmosis system always creates two continuous exit streams: pure water (permeate) and brine (concentrate). Normally 50 to 75 percent of the feed water can be recovered as permeate.

DEMINERALIZATION
Salts are repelled from the surface of the membrane while water molecules are allowed to diffuse freely through the membrane creating a purified product stream. Higher valence ions (salts) are rejected to a greater degree. Average rejection of dissolved salts ranges between 95 and 99 percent.

ULTRAFILTRATION
The mechanism for organic retention is different from the mechanism for salt rejection. Dissolved organic materials with a molecular weight over 200 daltons but also colloidal matter, bacteria, pyrogens and virus are widely removed because of their size and geometry, i.e. they are physically too large to pass through the pores of the membrane.

PRETREATMENT
Proper pretreatment of the feed water to a RO system is an essential factor for smooth long-term operation. Clogging of the membranes by suspended solids, scale-forming minerals and other impurities has to be prevented by appropriate measures. Hardness minerals can be removed by a softener, suspended solids in a mechanical filter, while free chlorine can be removed in an activated carbon filter.

SYSTEM DESIGN
In order to protect the RO membranes against foulants a complete system usually consists of a softening plant and a reverse osmosis unit equipped with a prefilter. An activated carbon filter has to be added if free chlorine is present in the feed water. To further improve the water quality in terms of conductivity the permeate can be polished downstream.

POLISHING
Polishing of the permeate in a mixed-bed unit (cf. special brochure) is normally required when high purity water is needed.

PROJECTING
Besides the reverse osmosis unit, INSTRUMECH has a complete programme for pretreatment of RO feed water and post-treatment of permeates. Based on a water analysis or on measurements on the spot, a complete system can be designed.

CONTROL PANEL
The electronic control panel can be programmed according to actual demand. The duration of pre-rinse, quality rinse, and post-rinse can be set individually. The conductivity meter continuously indicates the quality of the permeate measured in p5/cm. Special features include various control and alarm lamps as well as potential-free contacts for external signals and for remote alarm. The control panel operates on low voltage (220V)

QUALITY RINSE
After a period of standstill, the conductivity of the water in the membranes increases, i.e. the quality decreases. A restart of the unit triggers a quality rinse to drain until the permeate has reached the present quality limit. When the desired conductivity is reached the unit automatically switches to service.

POSTRINSE
When water consumption is interrupted the high pressure pump stops working and the membranes are rinsed with pretreated water at normal water works pressure for a preset number of minutes. This post-rinse reduces the risk of insoluble salt precipitations and minimizes algae and bacteria growth during standstill.

CAPACITY
The unit capacity depends upon pressure, salt content and temperature of the feed water. The capacity increases with increasing pressure and temperature and decreasing salt content. Capacities for individual membrane elements may vary by +/- 15%.

QUALITY
An Instrumech reverse osmosis unit will usually retain 95 to 99% of all inorganic dissolved solids and more than 90% of all organic contaminants. Carbonic acid, however, will penetrate through the membrane into the permeate.

TAP WATER MEMBRANES
These membranes operate at pressures below 21 bar and are used in series ROI 1.0. They are most economically utilized for relatively low salt contents and flow rates below 1 m3/h.

BRACKISH WATER MEMBRANES
These membranes stand pressure above 21 bar thus allowing higher flow rates per membrane. Otherwise the two membrane types mentioned are identical. Brackish water membranes are preferably used for major plants.

SERIES ROI 1.0 MEMBRANES
The membranes are mounted in vertical pressure vessels made of stainless steel (AISI 304). Each membrane is housed in one vessel giving a compact, space saving design. To reduce the risk of fouling the membranes, the unit is equipped with an adjustable recirculation facility.

SERIES ROI 2.1-5 MEMBRANES
This series has principally the same design as units of series 01 but uses brackish water membranes which can be operated at higher pressures. Units of series 02 are designed for feed waters with max. salt content up to 2000 ppm.