4.5.5.1 RO dosing unit
a. Anti-scalant dosing unit
To avoid carbonates, sulfates, and Ca2+ or Mg2+ chemical scales appearing in RO condensed water end, specially in the condensed water side of the last membrane element in RO pressure container, hence affecting the performance of the membrane elements under fairly high recovery rate, anti-scalant is needed to add to RO feed water. The unit adopts anti-scalant imported from America, which is dispersing maskant (参考：http://www.wanfangdata.com.cn/qikan/periodical.articles/hxtb2/hxtb99/hxtb9909/990914.htm), whose role is as follows:
•High anti-scale efficiency. The maximum allowable value of LSI index in original water is 2.8, and it can control inorganic scales effectively within this range.
•Its major components are small-molecular organic phosphorus compounds, which have the advantages of water-stability, uneasy to degrade, non-production of microorganism-nutritive n-phosphates, and non-pollution to the environment after discharging.
•It’s an excellent dispersant, which can control ferrous-, aluminum- and heavy metal pollution and disperse ferrous-colloids and tiny particles blocking micro-holes in the membrane.
The anti-scalant dosing unit mainly includes two America Milton Roy Company electromagnetic diaphragm meter pumps (1 in use & 1 for preparation) and a solution tank.
Anti-scalant dosing control adopts signal-output-coupled methods of dosing meter pumps and feed water online flow meter to reach to regulate dosage automatically.
b. Reducer dosing unit
RO compound membrane demands less than 0.1mg/l chlorine residue contained in the feed water, and the addition of reducers such as sodium hydrogen sulfite is to neutralize oxidizing components such as chlorine residue contained in the feed water, to avoid their damages to RO membrane element.
The dosing unit mainly includes two mechanically diaphragm meter pumps (1 in use & 1 for preparation) produced by America Milton Roy Company and a solution tank. Dosages can be regulated automatically according to 4-20mA signals of online Chlorine Residue (ORP) (自己再核实一下，这个很不准确), they also can be regulated manually.
4.5.5.2 RO unit
RO is a desalting technology used the widest in current water treatment systems having the advantages of extremely high desalting efficiency, safety and flexible running, and simple operation.
RO system includes safeguard filter, high pressure pump, RO membrane array, cleaning unit, washing unit, as well as control instrument.
a. Safeguard filter
Solid particles can damage membrane elements during their running, thus each set of RO is preset with a 5mm safeguard filter, whose role is to prevent particles more than 5μ size in pre-processing water outlet from entering into RO system. These particles may puncture RO membrane module after pressuring by the high-pressure pump and results in salt leakage and/or scratch the impellers of the high-pressure pump. Safeguard filter is made of 316 stainless steel material and adopts melt-blown polypropylene filtering element produced by America OSMonICS Company, and it has the advantages of deep bed filtration, large contamination capacity, long life, and easy to be replaced. When the inlet/outlet pressure difference (generally 0.07-0.1MPa) of the filter reaches to more than the setting value, the filtering element should be replaced.
b. High pressure pump
High pressure pump provides RO membrane arrays with enough feed water pressure, and maintain the normal running of RO membrane. Every RO inlet in the unit is equipped with a horizontal multi-stage centrifugal pump produced by America Pentair Company as well as a pump for preparation every four sets of RO units.