Demineralization is the removal of minerals and nitrate from the water. The three that we will be discussing in the lesson are ion exchange, reverse osmosis and electrodialysis. These methods are widely used for water and wastewater treatment. Ion exchange is primarily used for the removal of hardness ions like magnesium and calcium and for water demineralization. Reverse osmosis and electrodialysis, which are both membrane processes, remove dissolved solids from water using membranes.
How each Process Works
The ion exchange units are used to remove any charged substance from the water but are mainly used to remove hardness and nitrate from groundwater.
The water is pretreated to reduce the suspended solids and total dissolved solids load to ion-exchange unit.
The methods used for pretreatment include:
Cold lime without soda ash
Hot lime with or without soda ash
Coagulation and filtration
Evaporation or distillation
In this type of process, 90% of barium, arsenic, cadmium, chromium, silver, radium, nitrites, selenium and nitrates can be effectively removed from water. Ion exchange is one of the choices that is better for small systems that need to remove radionuclides.
Advantages of Ion Exchange
Ion exchange can be used with fluctuating flow rates.
Makes effluent contamination impossible
Resins are available in large varieties from suppliers and each resin is effective in removing specific contaminants.
Limitations of Ion Exchange
Pretreatment is required for most surface waters
Waste is highly concentrated and requires careful disposal
Unacceptable high levels of contamination in effluent
Units are sensitive to the other ions present.
Ion Exchange Process
Inorganic removal is accomplished through the adsorption of contaminant ions onto a resin exchange medium. One ion is substituted for another on the charged surface of the medium that is usually a plastic resin. This type of surface is designed as either cationic or anionic-negatively charged. The medium is saturated with exchangeable ions before the treatment operations.
The contaminant ions during ion exchange, replace the regenerant ions because they are preferred by the exchange medium. When no ions are left to take the place of the contaminant ions, the medium is regenerated with a suitable solution that saturates the medium with the appropriate ions. Since there is a required down time, the regeneration cycles are done only once per day.
For resin exchange, capacity is expressed in terms of weight per unit volume of the resin used. Calculation of the breakthrough time for an ion exchange unit requires knowing the resin exchange capacity, influent contaminant concentration and the desired effluent quality.
Ion Exchange Equipment
The ion exchange unit contains prefiltration, disinfection, ion exchange, storage and distribution elements.
To regenerate the exchange medium in ion exchangers, the chemical, sodium chloride is often used because of its low cost. When using this chemical one must remember that high sodium residual will result in the finished water which may cause it to be unacceptable for individuals with a salt restricted diet. To avoid this problem, other regenerant materials can be used such as potassium chloride.
In a reverse osmosis system, the water is put under pressure and forced through a membrane that filters out the minerals and nitrate. These systems are compact and easy to operate and require minimal labor, which make them suitable for small systems and for systems where there is a high degree of seasonal fluctuation in water demand.
Reverse osmosis effectively removes nearly all the inorganic contaminants from water. This process removes over 70% of the following:
• Selenium-4 and selenium-6
When the units are operated properly, ninety-six percent removal rates will be attained. Reverse osmosis also affectively removes natural organic substances, pesticides, radium and microbiological contaminants. To work effectively, reverse osmosis should be used in series. Near 0 effluent contaminant concentrations can be achieved by water passing through multiple units.
Advantages of Reverse Osmosis
Nearly all contaminant ions and most dissolved non-ions are removed
Suitable for small systems with a high degree of seasonal fluctuation in water demand
Insensitive to flow and TDA levels
Operates immediately without any minimum break-in period
Possible low effluent concentrations
Removes bacteria and particles
Simplicity and automation operation allows for less operator attention which makes them suitable for small system applications.
Limitation of RO
High operating costs and capital
Potential problem with managing the wastewater brine solution
Pretreatment at high levels
Fouling of membranes
In this process, contaminants are removed from water by using a semi-permeable membrane that permits only water, not the dissolved ions, to pass through its pores. The contaminated water is subjected to high pressure that forces the pure water through the membrane, leaving the contaminants behind in a brine solution. The membranes come in a variety of pore sizes and characteristics.
RO units include the following:
Raw water pumps
Units are able to process any desired quantity or quality of water by configuring units sequentially to reprocess waste brine from earlier stages of the process. Principle design for reverse osmosis units are:
Membrane type and pore size
Product conversion rate
Electrodialysis is effective in removing fluoride and nitrate from water. This process also uses membranes but direct electrical currents are used to attract ions to one side of the treatment chamber. This system includes a source of pressurized water, direct current power supply and a pair of selective membranes.
Advantages of Electrodialysis
All the contaminant ions and many of the dissolved non-ions are removed
Insensitive to flow and TDS levels
Possible low effluent concentrations
Limitations of Electrodialysis
Operating costs and capital are high
Level of pretreatment required is high
Twenty to ninety percent of feed flow is rejected stream
Replacement of electrodes
In this process, the membranes adjacent to the influent steam are charged either positively or negatively and this charge attracts counter-ions toward the membrane. These membranes are designed to allow the positive or the negative charged ions to pass through the membrane, where the ions move from the product water stream through a membrane to the two reject water streams.
The electrodialysis system has three essential elements:
Source of pressurized water
Direct current power supply
A pair of selective membranes
The average ion removal rate varies from one-fourth to two-thirds percent per stage. Using multiple stage units can increase the efficiency of removal. The membrane pairs are stacked in the treatment vessel.
The amount of water treated may be limited by the fouling of the membranes. Fouling is caused if the membranes pores become clogged by salt precipitation or by the obstruction of suspended particulates. The particulates that are suspended in the water can be removed in pretreatment but the salts that exceed the solubility product at the membrane surface has to be controlled chemically by pH reduction or chelation of the metal ions by using phosphate. Reversal of the charge on the membranes, electrodialysis reversal (EDR) helps flush the attached ions from the membrane surface which helps extend the time between cleanings.