Reverse osmosis system pretreatment
The type of water
Life is the main source of drinking water. Surface water and ground water two kinds, including the use of fresh water is the main type of water.
- Surface water
Surface water is rain and snow, rivers, lakes, reservoirs and marine water, these are the characteristics of the water with them is closely related to the formation process.
- Rain and snow
Rain and snow in the landing process of a solution are a certain number of impurities, such as oxygen, carbon dioxide, nitrogen, etc., may also be mixed with industrial cities in emissions, dust and other harmful gases. This part of the whole surface water impurities and less low salt content, the average salinity of only about 40 milligrams per liter, the hardness is also very low, only an average of about 2.5 mg per liter, a soft water.
- River water
River water through surface runoff from the pool, it contacts the weathering of rocks and soil, into some salts, the salt content higher than rain and snow. China's river water, the average salt concentration is 166 mg/L. Because of its river water also washed away, with a large number of volumes of silt, clay and other suspended material, the water contains high suspended solids and colloid, high water turbidity. River water by the geographical environment and the impact of climate conditions vary with the seasons and fluctuations, but also the hydrological and meteorological conditions of the impact of changes in water easily, unstable and more vulnerable to agricultural and industrial wastewater, sewage and other pollution.
- Lakes and reservoirs of water
Lakes and reservoirs because water lake wide, slow flows, long-term natural precipitation to the suspension of water smaller. If the inflow and water from the lake are greater, so the lake's relatively small amount of evaporation, thereby maintaining a lower salinity and a fresh water lake, otherwise, if most of the inflow of water evaporation, water and enrichment Salinity increased, it turned into lagoons, or salt lakes, and lakes due to the light of the mobility of the small, and is conducive to microbial growth and reproduction of algae, thus the higher the water content of humus, and also easier to be man-made pollution.
- Ocean water
Ocean water evaporation of the annual 40 x 10000km 3, from the dissolved salts into the river each year are 3.85 x 10 (9) 1986, and accumulated over many years of evaporation the salt content in sea water concentrations as high as 35000 mg per liter. Sea salts in the weight ratio of basic stability, a high chloride content, chloride ion of about 55 percent of the total, followed by sodium ions, about 30 percent, and other salts is mainly potassium ions, calcium, sulfate ions, re-carbonate ions, ion bromine, there are trace elements, such as dissolved gases and organic matter. Ocean water only in exceptional circumstances were not used as domestic water resources of water.
- Ground water
Ground water in rain water through the soil and the penetration of mobile formed by water, in its lengthy process and extensive contacts, into more soluble minerals, thus the hardness of underground water, salt content, usually higher than that of iron High groundwater. On the other hand, groundwater and soil formation due to the layers of filtering, colloidal suspension of the content and very little water clear and transparent, low turbidity. As groundwater dissolved oxygen is very low, not even bacteria cannot survive. The price of ferrous ions and stability can exist in water, groundwater and air contacts when the price of iron oxide into a quick three highest bidders iron, and forming a flock of precipitation.
Overall, the quality of groundwater is superior to surface water quality, as long as the appropriate treatment can be as drinking water, but with the ground pollutants and increase air pollution, will be on the shallow groundwater pollution, should be in use Attention.
- Tap water
Tap water is purified through the water treatment plants, produced after disinfection of drinking water in line with national standards for people's lives, production use of water. It is mainly through the Waterworks water pumping stations and underground water from rivers and lakes (that is, these two types of water), and the precipitation, disinfection and filtration process, with final adoption of pump stations transported to various users. Pump through the water by pipeline transmission and distribution of water supply users. Must meet the national drinking water health standards.
Reverse osmosis water chemistry
Color: non-precision test parameters, based on different color from organic matter the size, color can be said that the size of the water content of organic compounds and the use of platinum as a standard APHA units.
Conductivity of the water soluble ions that are conductive capability of indicators, in units of micro-Siemens / cm (us / cm), it is relatively intuitive reaction of the ion content in the water;
TDS (total dissolved solids) is filtered and colloidal suspension of evaporation and water out all the remaining minerals, in units of mg / L; it is relatively intuitive reaction of the salinity of the water; water through the TDS Conductivity converted out, a rough method is: the reference to sodium chloride solution, every 1 mg / L TDS value of the corresponding 2 us / cm conductivity;
BOD (biological oxygen demand) that the water degradable organic matter content of oxygen to the MG / L value of units;
TOC (TOC) is the carbon MG / L value of unit testing and integration of organic carbon in the total; organic matter does not include carbon dioxide, bicarbonate and carbonate, carbon compounds. The typical natural organic matter, including negatively charged colloidal, SS, tannic acid, lignin, the decay of plant-generated water-soluble humus acid mixture, decomposed plant generated organic acid, and so on. RO systems can effectively remove the organic matter, the molecular weight of more than 200 organic compounds can be removed more than 99 percent, less than 200 according to the molecular weight, shape, charged different, their removal is also different;
COD (chemical oxygen demand) for the determination of biodegradable and non-degradable organic matter content;
Alkalinity mainly refers to carbon dioxide, bicarbonate, carbonate and hydroxides. Alkalinity (especially for the boiler water chemistry) can be expressed as P M alkalinity and alkalinity. M alkalinity refers to the calcium carbonate of Mg / L values that the total alkalinity of the water; P alkalinity measurements bicarbonate, carbonate hydroxide and the volume;
Turbidity of the water difficult to precipitate the tiny colloidal suspension of testing indicators to NTU as a unit; RO membrane components in the regular operating limits: the largest water turbidity to 1.0 NTU.
SDI (pollution index) is a membrane system for testing the water to the suspension of
colloidal particles with the blockage of the 0.45-micron filter paper aperture speed of test data. RO typical components of the use of the water requirements of the 15-minute maximum value of 4.0 SDI if the blockage 而使 SDI test because only five or 10 minutes, that water pollution of the RO system will be very serious. Slightly treatment or no pretreatment circumstances, deep well water the SDI value is equal to or less than 3, turbidity is less than 1. For surface water, to meet the requirements of SDI and turbidity, to be used to remove pre-treatment and the suspension of colloidal particles.
Brackish water, RO areas in the bitter and salty water can be defined as: TDS value in the salt content in the low-level (up to 10000-15000 Mg / L), and can use the largest water pressure of 600 psi RO membrane to bitter and salty water The reverse osmosis water treatment.
Pollution density index SDI's determination
SDI pollution density index value of reverse osmosis system is characterized an important indicator of water quality.
SDI value of the original is the basic measurement in the 2.1 bar (30PSI) under pressure of water supply by 0.45 μ m membrane filters a certain amount of raw water by the time required.
The assembly and test equipment
- Click an assembly of test devices.
- Test device connected to the water pipes into the RO system tube sampling point.
- Membrane will be installed in water pressure regulator to 30PSI. In the actual test, they should use the new the membrane. To obtain accurate test results;
Special attention should be paid the following:
- Ensure that the O-ring intact and install the proper cleaning;
- Hand to avoid touching the membrane;
- Washed prior testing devices, removing theλ pollutants in the system.
- Temperature record test. During the trail start to the end of the test of time, figure 1 SDI test setup.
- Temperature changes of not more than 1 ℃;
- Excluded pool filter the air pressure. According to the type of filter, the water supply valve to open in the circumstances, or open at the top of the filter exhaust valve, or screw-thread filter jacket, full vent or exhaust valve closed after fastening the filter jacket thread ;
- Using a scale of 500 ml graduated cylinder access filtering water through membranes to measure the volume of water;
- Full ball valve, measured from the valve full access to over 100 ml and 500 ml of water samples and record the time required;
- 5 minutes after the re-measurement collected 100 ml and 500 ml of water kind of time, 10 minutes and 15 minutes respectively after the same measurement;
- If access 100 ml water sample the time required for more than 60 seconds, it means that about 90% of the membrane filter area was blocked, this time has no longer need to carry out experiments;
- The end of the experiment and open the filter, will be the best preserved of the membrane, and to prepare for future reference
SDI - pollution density index
P30: In the water under the pressure of 30 psi plug the percentage of the membrane
Tt: The test of time, in units of minutes. Tt is usually for 15 minutes, but if within 15 minutes that 75% of the membrane area was blocked on the need to shorten the test time
Tf: 15 minutes (or less) after the sampling time
Ti: The first time sampling
Access 500ml water samples around the time required to access the necessary time to 100 ml of water five times. If access for 500 ml.
Time far greater than five times, in the calculation of SDI, access should be adopted by 100 ml of time.
In order to accurately measure SDI value, P30 should not exceed 75 percent, if the P30 more than 75 percent should be re-test in a relatively short period of time and access to Tf value.
Film is being dealt with pollution from particulates in solution, colloidal particles or solute interaction with the membrane, or because of the concentration polarization on the surface of certain substances in concentrations exceeding its solubility caused by these substances in the water channel, or membrane surface Hole absorption, sediment, causing water channels or channel smaller or blockage of the phenomenon.
- Suspended solids
Suspended solids commonly found in surface water, particle diameter> 1 μ m, such as sand, mud Nien, SiO2 particles, water in the state can stop sediment down. It is very easy to set up a system of reverse osmosis filter or fine sand filter medium filtered, after pretreatment, must meet the following indicators: turbidity <1 NTU, 15 分钟 SDI value of <5.
- Colloidal pollutants
Colloid of commonly found in surface water, mainly aluminum type of clay, such as silicate compounds, Fe-Al oxides, sulfides, tannic acid, humus, such colloidal particle size in the range 0.3-1.0 μ m, with negative charge, Single-use filters cannot be removed, the use of pool, filter or a DC coagulation ways in which colloidal particles increased to 10-20 μ m filter removed. When the SS, colloidal too much content, the need to unite, to clarify, filters.
- Organic pollution
Organic pollution of the film is a complex, the main type of humic acid substances, clarification and cohesion are only activated carbon filters to remove part of organic matter; ultrafiltration can also be used to remove organic matter.
- Biological pollution
Such pollutants usually bacteria, biofilm, algae and fungi. Bacteria will be cellulose acetate for food and thus vulnerable to bacterial acetate film erosion of the membrane, is not susceptible to bacteria against, but the bacteria will cause mucosal membranes of the sewage blocked. A reverse osmosis system design technology, to control biological activity, the bacterial content of water in the 10000 cfu / cm above, we must consider measures to remove.
Fouling usually mixed, and pollutants are more of the mixture.
General pollution from the thick film components of the net at the water surface sampling and analysis of pollutants ingredients. Dense network of water at low velocity areas most vulnerable to larger deposition of pollutants, including CaCO3 grain, bio-film, mesh organic film, particulates, colloids and coagulant. These pollutants will lead to increased pressure on the system and reduce water production. Film on the surface of the pollutants are usually close attachment to the silicate compounds, sulphate, polymers, organic compounds, metal oxides and hydroxides, and other pollutants. These lead to blocked sewage water production and reduce the rate of decline in the rate of desalination.
RO pretreatment system
All the water they contained a certain concentration of suspended solids and dissolved substances. Suspensions are the main mineral, colloidal and micro-organisms, algae and other biological particles. Soluble material is soluble salt (such as chloride) and the insoluble salts (such as carbonate, sulfate and silicate) metal oxide, acid and alkali, and so on. In the reverse osmosis process, the reduction in the volume of water, suspended particles and the concentration of dissolved substances on the increase. Suspended particles will be deposited in the film, plugged into the flow, and increase frictional resistance (pressure drop).
In soluble salt in more than a total saturation limit, from the thick sediment from the water, formed on the surface scaling, reducing the RO membrane flux, increasing pressure on the operation of the pressure drop, and lead to deterioration of water quality, in this surface On the formation of sediment into the phenomenon known as fouling, fouling system performance is the result of the deterioration. The need of raw water into the system prior to the reverse osmosis membrane pretreatment, reverse osmosis membranes to remove possible pollution of suspended solids, dissolved organic matter and excessive salt insoluble components, reducing fouling tendencies. Pretreatment of the water objective is to improve water quality so that the RO membrane access to reliable guarantee for the operation.
Pretreatment of the raw water to reflect the effect of TSS, TOC, COD, BOD, LSI and iron, manganese, aluminum, silicon, barium, strontium and other pollutants reduce the absolute value of the water quality objectives. Characterization fouling tendency another important indicator is the quality of SDI. In addition to the above through the pretreatment indicators of reverse osmosis membranes to reduce the water requirements of the context, there are important point is to minimize the SDI, the ideal SDI (15 minutes) is less than 3.
Suitable for the programmed depends on the pretreatment water, raw water composition and application conditions, and mainly depends on the raw water sources, such as the well water, surface water and municipal wastewater to be treated differently. Under normal conditions, water quality and stability, the possibility of low-pollution, just a simple pretreatment, such as acid or set up and increase inhibitors and 5 μ m filter to security. On the contrary, surface water is a directly affected by the seasonal effects of water; microorganisms and colloid have occurred both the possibility of a high degree of pollution. The pretreatment than well water complex and require other steps, including the pretreatment chlorine disinfection, Flocculation / condensate help to clarify, multi-media filter, dechlorination, and acid or increase scale agent. Industrial and municipal wastewater containing more complex organic and inorganic elements, some organisms may seriously affect RO membrane; caused a serious decline in water production or degradation of the membrane, thus there must be more comprehensive design of the pretreatment.
Once identified by the optional water sources, must conduct a comprehensive and accurate analysis of all the raw water. It is establish appropriate programmers and RO pretreatment system are the basis for the design of the most critical.
The raw water quality analysis
A water sample performance of a specific water quality in a given time. Therefore, through a water sample and cannot fully understand the entire operation of the time, can affect the operation of the system trends or changes. In addition to a number of water samples collected to get a better understanding of water resources, but also the reasons for the changes in water quality analysis. Understanding of existing or potential sources of water of the correct distribution profile is a reverse osmosis system design and operation of the essential elements.
A suitable container to collect samples of water to ensure accurate analysis of the key. Must be considered because of pollution caused by containers. Regular use of plastic or glass containers with samples. Sample container must be cleaned before use, so as to avoid contamination samples. Sulfuric acid or bottles dichromate cleaning solution, to avoid this solution on the bottle of organic pollution particularly effective. Plastic containers can be used laboratory or scrub with detergent concentrated hydrochloric acid rinse, and then spent deionizer water to wash. The containers used for sampling the need for disinfection.
Brackish water treatment system is the main constraints of nature, namely, calcium carbonate or calcium sulfate compounds because of water constantly being concentrated, when more than its solubility plot, the film will surface in precipitation, or scaling; industrial or municipal Wastewater treatment system there are a lot organics, inorganic, viruses and microorganisms such as bacteria and algae, such constraints is not just its physical and chemical factors, microbial index, a film with a pro-reaction and the organic carbon or biological degradable Dissolved organic carbon. Brackish water in a very wide scope, in order to put a better process design, the need for full analysis of water quality.
Figure 1：Water quality analysis of the entire log sheet
|Register table of water quality analysis|
|Item||Unit||Testing value||Item||Unit||Testing value|
|Na+ (Sodium)||ppm||CO3 2-(Carbonate ion)||ppm|
|K+ (Potassium)||ppm||HCO3 -(Bicarbonate ion)||ppm|
|Ca2+ (Calcium)||ppm||SO4 2-(Sulfate)||ppm|
|Total Fe(Iron)||ppm||NO3 -(Nitrate)||ppm|
|Item||Unit||Testing value||Item||Unit||Testing value|
|Total plate count||CO2(Carbon dioxide)|
|The integrated evaluation of water quality:||Common seal
Remove suspended solids and colloid
Reverse osmosis water turbidity of the requirements
Reverse osmosis water control devices solid particles in one of the indicators is turbidity, the general roll components require water turbidity is less than 1 NTU, the best to 0.2 NTU. Pretreatment system in the design, consideration should be given not to let more than 5 μ m particles into the high-pressure pumps and reverse osmosis devices, high-pressure pump to avoid scratches leaves, to prevent these particles accelerated by the high-pressure pump breakdown after the film components. General roll module requirements SDI <4.
Depth commonly used method of removing suspended
In order to meet the installation of reverse osmosis water turbidity and SDI value of the request, often in the pretreatment system set up multi-media filter, fine sand filters, micro-filter and the filter, such as ultra-depth filtration devices. Multi-media filter also known as multi-media filter, quartz sand and anthracite used by the composition of the two-tier media filter; fine sand filters generally used for 0.3-0.5 mm diameter quartz sand, high layer for 800 -- 1000mm; filter also known as micro-porous filters, safety filter, aperture range 0.01-350 μ m; ultra-filter filtration accuracy by withholding molecular weight, its value in the general 500-500000 Dalton, similar to the corresponding aperture (20-1000 ) × 10-10m.
The traditional reverse osmosis system design, common aperture 5 μ m micro-filter (commonly known as 5 μ filter), as a pretreatment system in the final steps in the reverse osmosis unit from the security role, it also called the security filter. With the rise of ultra-filtration technology, the pretreatment system in the last processes tends to UF. Ultra filtration water SDI <2, the water quality is better than micro-filter.
Reverse osmosis water treatment sector has yet to micro-organisms in water for specific provisions. Generally believe that, SDI includes micro-particle pollution with comprehensive features, often focused on the pretreatment on SDI. However, at present, China's reverse osmosis membrane of the microorganisms increasingly serious pollution problems, SDI qualified reverse osmosis system is not operational security, the same will of scale arising from the non-fault. These failures, the problem of micro-organisms most serious.
All raw water microorganisms such as bacteria, alga, spore, viruses, and higher organisms. The typical size of bacteria is about 1～3μm. Microorganisms can be regard as colloidal matter and removed during pretreatment as discussed in Colloidal and Particulate Fouling Prevention. The difference between microorganisms and non-living particles, however, is the ability of microorganisms to reproduce and form a biofilm under favorable conditions.
Microorganisms entering a RO system find a large membrane surface where dissolved nutrients from the water are enriched due to concentration polarization, thus creating an ideal environment for the formation of a biological fouling of the membranes may seriously pressure from feed to concentrate, finally leading to telescoping and mechanical damage of in membrane element, and a decline in membrane flux. Sometimes, biogouling develops even on the permeate side, thus contaminating the product water.
A biofilm is difficult to remove because it protects its microorganisms against in action of shear forces and biocides chemicals. In addition, if not completely removed, remaining parts of a biofilm lead to rapid regrowth. Biological fouling prevention is therefore a major objective of the pretreatment process. The control of microbiological activity is also part of system design in the system operation, in the sanitization of system and in the preservation of system.
Microbial control methods
Methods to prevent and control biological fouling are: coagulation, activated carbon adsorption, sterilization, ultra filtration, ultraviolet disinfection, electronic sterilization, disinfected regularly.
- Coagulation. Microorganisms can be seen as colloidal, negatively charged, using conventional methods such as the pretreatment coagulation, to clarify and filter can remove most, but must be removed, is very difficult.
- Activated carbon adsorption. Organic matter in water with activated carbon adsorption, reducing microbial source of nutrition. However, the activated carbon adsorption organic nutritional adequacy has become an ideal breeding place of microorganisms.
- Sterilization. Microbial control with fungicides very effective, it is the most commonly used method, but also control of anti-microbial systems the last barriers. Fungicides commonly used as a capacity of the compounds, such as Cl2, NaClO, H2O2, O3, and so on and KmnO4. Fungicide-point increase as much as possible in the higher processes, so that the contact time is sufficient so that the water entering the film installations completed before the disinfection process.
- Ultra filtration (UF). UF can remove microorganisms and especially algae that are sometimes very difficult to remove by standard techniques. The UF membranes should be made from a chlorine-resistant material to withstand periodic treatment with biocides. UF membranes, however, do not remove the low molecular weight fractions of organic matter and other compounds that are nutrients for microorganisms. Pretreatment with UF membranes helps to retard and to control the onset of bio-fouling, but it is no safeguard by itself.
- UV Irradiation. Ultraviolet (UV) Irradiation at 254nm is known to have a germicidal effect. Its application has come into use especially for small-scale plant. No chemicals are added, and the equipment needs little attention other than periodic cleanings or replacement of the mercury vapor lamps. UV treatment is limited; however, to relatively clan waters because colloids and organics matter reduce the penetration of the radiation.
- Electronic sterilization. E-Sterilization is actually a low-pressure cell, the treated water through the cell, the negatively charged cathode to attract bacteria, the bacterial cell and electrode between the electron transfer reactions in reducing the oxygen activity. Because the ultraviolet light disinfection and sterilization of electronic sterilization n no residual capacity, reverse osmosis system in general rarely used.
- Regular sterilization. Even with these means of sterilization, a small amount of bacteria will survive. Microbial breeding fast, so these bacteria survive in those who do not have the ability to sterilization large number of breeding places again, there is a need for these places disinfected regularly. For example, the box regularly to the Anti-Water chlorination.
Remove organic matter
Organic food is not only the micro-organisms, but also when its concentration to a certain extent, the film can be dissolved organic material, the film performance degradation. A wide range of organic matter in water, reverse osmosis membranes of different organic compounds the harm is not, therefore reverse osmosis pretreatment system design, it is difficult to give a quantitative indicators, but if the water total organic carbon (TOC) of the level over the 2 mg / l, , Should be cause enough attention.
Remove organic methods
Remove the organic methods to increase oxidants, such as Cl2, NaClO, H2O2, O3 and KmnO4 of organic matter, organic matter or activated carbon adsorption.
Can be activated carbon adsorption many impurities in the water, with the exception of residual chlorine, there are suspended solids, organic matter, such as oil.
Prevent the deposition of iron and manganese
Fe and Mn transition metals, such as reverse osmosis system on the harm are: 1, blocking ormation of sewage. Colloidal iron-manganese compounds (such as hydrogen manganese oxide and iron oxide) from surface water channel and blocking the sewage. 2, 2000 for iron bacteria feed, breeding of mud stick. 3 hours, sometimes as oxidation catalyst, they exist; will speed up the oxidation and aging of the film. 4, 2000 inhibitors reduce efficacy.
Method to remove iron-manganese
Should try to reduce the reverse osmosis water installations in the concentration of iron and in manganese. Rail to allow the PH value and the concentration of dissolved oxygen content vary, usually 0.05-0.1 mg / l.
For surface water, the chlorination, clarification, filtration, the water of iron and manganese content in general is qualified; for groundwater, particularly of iron and manganese-rich groundwater, should take measures to remove the iron-manganese. For example: aerated water, iron generated Fe (OH) 3 precipitation, and then using contacts to get rid of filtration; increase Na2SO3 remove dissolved oxygen, to prevent the oxidation of iron and manganese to maintain the dissolved state.
Scaling of RO/NF membranes may occur when sparingly soluble salts are concentrated within the element beyond their solubility limit. For example, if a reverse osmosis plant is operated at 50% recovery, the concentration in the concentrate stream will be almost double the concentration in the feed stream. As the recovery of a plant is increased, so is the risk of scaling.
Due to water scarcity and environmental concern, adding a brine (RO concentrate) recovery system to increase recovery has become more popular. To minimize precipitation and scaling, it is important to establish well-designed scale control measures and avoid exceeding the solubility limits of sparingly solute salts. In an RO/NF system, the most common sparingly soluble salts encountered are CaSO4, CaCO3, and silica. Other salts creasing a potential scaling problem are CaF2, BaSO4, SrSO4, and Ca3 (PO4)2. Solubility products of sparingly soluble inorganic compounds are listed in Table 5-1.
Table 5-1 Solubility products of sparingly soluble inorganic compounds.
|Substance||Formula||Temp °C||Solubility Product KSP||- Log Ksp|
|Aluminum Hydroxide||Al(OH)3||25||3 x 10 – 34||33.2|
|Aluminum phosphate||AlPO4||25||9.84 x 10 - 21||20|
|Barium carbonate||BaCO3||25||2.58 x 10 – 9||8.6|
|Barium sulfate||BaSO4||25||1.1 x 10 – 10||10|
|Calcium carbonate||CaCO3||25||3.36 x 10 – 9||8.5|
|Calcium Fluoride||CaF2||25||3.45 x 10 – 11||10.5|
|Calcium phosphate||Ca3(PO4)2||25||2.07 x 10 – 33||32.7|
|Calcium sulfate||CaSO4||25||4.93 x 10 – 5||4.3|
|Iron (II) hydroxide||Fe(OH)2||25||4.87 x 10 – 17||16.3|
|Iron(II) sulfide||FeS||25||8 x 10 - 19||18.1|
|Iron(III) hydroxide||Fe(OH)3||25||2.79 x 10 – 39||38.6|
|Iron (III) phosphate hydrate||FePO4·2H2O||25||9.91 x 10–16||15|
|Lead carbonate||PbCO3||25||7.4 x 10–14||13.1|
|Lead fluoride||PbF2||25||3.3 x 10–8||7.5|
|Lead sulfate||PbSO4||25||2.53 x 10–8||7.6|
|Magnesium ammonium phosphate||MgNH4PO4||25||2.5 x 10–13||12.6|
|2.6 x 10–5
|7.1 x 10–9
5.16 x 10–11
|1.2 x 10–11
5.61 x 10–12
|Magnesium phosphate||Mg3(PO4)2||25||1.04 x 10–24||24|
|4.0 x 10–14
2 x 10–13
|Strontium carbonate||SrCO3||25||5.6 x 10–10||9.25|
|Strontium sulfate||SrSO4||17.4||3.8 x 10–7||6.42|
|Zinc carbonate||ZnCO3||25||1.46 x 10–10||9.84|
In order to prevent the membrane surface in inorganic salt scaling, should adopt the following measures:
220.127.116.11 Acid add
Most natural surface and ground waters are almost saturated with CaCO3. The solubility of CaCO3 depends on the pH, as can be seen from the following equation:
Ca2+ + HCO3- - H+ + CaCO32-
By adding H+ as acid, the equilibrium can be shifted to the left side to keep calcium carbonate dissolved. Use food –grade quality acid.
Sulfuric acid is easier to handle and in many countries more readily available than hydrochloric acid, however, additional sulfate is added to the feed stream, potentially causing sulfate scaling.
CaCO3 tends to dissolve in the concentrate stream rather than precipitate. This tendency can be expressed by the Langelier Saturation Index (LSI) for brackish waters ad the Siff & Davis Stability Index(S&DSI) for sea waters. At the PH of saturation (PH5), the water is in equilibrium with CaCO3.
The definitions of LSI and S&DSI are:
LSI = pH – pHs (TDS ≤ 10,000 mg/L)
S&DSI = pH – pHs (TDS > 10,000 mg/L)
To control calcium carbonate scaling by acid addition lone, the LSI or S&DSI in the concentrate stream must be negative. Acid addition is useful to control carbonate scale only.
18.104.22.168 Anti scalelants
Scale inhibitors (antiscalants can be used to control carbonate scaling, sulfate scaling sulfate scaling, and calcium fluorde scaling. There are generally three different types of scale inhibitors: sodium hexametaphosphate (SHMP), organ phonates and polyacrylates.
SHMP is inexpensive but unstable compared to polymeric organic scale inhibitors. Minor amount adsorb to the surface of microcrystal, preventing further growth and precipitation of the crystals. Food-grade quality SHMP should be used. Care must be taken to avoid hydrolysis of SHMP in the dosing feed tank. Hydrolysis will not only decrease the case inhibition efficiency, but also create a calcium phosphate scaling risk. Therefore, SHMP is generally not recommended.
Organophosphonates are more effective and stable than SHMP. They act as antifoulants for insoluble aluminum and iron, keeping them is solution.
22.214.171.124 Strong acid-softening resins
Can use the Na ion exchange to remove the water and fouling cation such as Ca2+, Ba2+ and Sr2+. After the exchange of saturated ion-exchange resins used NaCl regeneration, a process known as the softening of raw water treatment. In this process, water pH will not change. Therefore, do not need to take degassing operation, but the raw water can be dissolved CO2 gas products through the membrane into the side, caused the increase in conductivity, the operator can still soften the water by adding a certain amount NaOH (until pH8.2) to Will be residual CO2 into the water re-carbonate, to be re-carbonate film by the removal, reverse osmosis water conductivity lower middle class.
If timely regeneration, the use of strong acid cation exchange for softening the tree that is very effective and insurance scale, but mainly for small and medium-sized bitter and salty water system. The main shortcoming of this process is very high NaCl consumption, the existence of environmental problems, not economic.
126.96.36.199 Resin from weak acid-alkalinity
A weak acid cation exchange resin from alkalinity is large brackish water treatment system, it can achieve some softening of regeneration to achieve the purpose of conservation. In this process, and only re-carbonate hardness of the same amount of time being in the Ca2 +, Ba2 + and Sr2 + H +, etc. have been replaced by a removed, so the pH value of raw water will be reduced to 4 to 5. As the acid resin for the carboxyl group, when the pH reached 4.2, no longer dissociation, ion exchange process would stop. Therefore, only to achieve some softening, and re-integration of the carbonate scaling cation can be removed. Therefore this process to re-carbonate high content of the water better, re-carbonate can also be converted into CO2
H2O + CO2 ↔ HCO3-+ H+ ↔ CO32+ + 2 H+
In most cases, there does not want to yield water CO2, then can the original water or water-degassing to achieve, but when the existence of biological contamination when the suspect (surface water, high or high TOC colony total), water from the middle Gas more appropriate. Membrane system in the higher CO2 concentration can inhibit the growth of bacteria, when the hope that the system is running at a high rate of desalination, water use degassing more appropriate, the removal of CO2 will cause the pH increased influent pH> 6, the membrane system Removal of water than the rate of pH <5 should be high.
A weak acid alkalinity from the advantages is as follows:
- Regeneration of the acid required by not more than 105 percent of the acid consumption theory, this would reduce operating costs and the impact on the environment;
- Through the removal of heavy carbonate, the water to reduce the TDS, this middle and lower water TDS;
The shortcomings of this law are:
- Residual hardness
If you need completely softened, and can be created strong acid-sodium exchange resin process, or even weak acid resin can be placed on the same exchange column, so the consumption of renewable still than the separate use of strong acid resin at the low, but a higher initial investment, a Only when the combination of large capacity when meaningful.
Another way to overcome this shortcoming in the alkalinity of the water from Canada inhibitors, although so far, people used on their own weak acid-base from that tree, also unprecedented in scale, but we still strongly recommend You calculate the solubility of salt residue insoluble, and take corresponding measures.
The water treatment process will change in pH
Due to the saturation level of resin in the run-time changes, the weak acid from alkali treatment of effluents from its pH value will be 3.5 to 6.5 within the framework of changes, such cyclical changes in pH, the rate of the desalination plant becomes very difficult to control. When the pH <4.2, inorganic acid will be covered by the film, may increase the production of water TDS, therefore, we recommend that users of a parallel increase in weak acid to soften, control at different times for regeneration, in order to deal with the water evenly weak acid pH, to prevent other very The low pH of the water is removal of CO2 by adding NaOH or weak acid to soften after adjusting the pH of the water.
188.8.131.52 Preventive cleansing
In some cases, the film can carry out preventive cleansing to control scaling problems, this system can and does not require chemicals or softening scale. Such systems usually run the recovery rate is very low, about 25 percent, and 1 to 2 years to consider the replacement of membrane components. These systems usually tap water or sea water for water, drinking water production unit is not important pieces of small systems, the simplest way is to open clean water valve for strong low-pressure washing, cleaning set intervals shorter than the model of a long pattern Effective, such as the common run every 30 minutes of low pressure washing 30 seconds. Can also be used in wastewater treatment similar to the approved mode of operation, that is, in each operation after cleaning a membrane component. Cleaning steps, cleaning chemicals and cleaning frequency of the need for case management and optimization. Special attention should be paid to take measures to prevent the scale of the operation of the extension of further aggravated.
184.108.40.206 Adjusted operating parameters
When other control measures scaling does not work, we must adjust the system operating parameters, to prevent a scaling problem, because the guarantee that the water insoluble salt concentration below the solubility product, it will not appear precipitation, which need to reduce system recovery To reduce the rate of the dense concentration of the water.
Solubility also depend on temperature and pH, the water containing silicon, raising the temperature and pH can increase its solubility, silica is often the only consideration for the need to adjust these operating parameters to prevent scaling reasons, because the regulation of these parameters, there are some Shortcomings, such as energy consumption or other high-scaling of risk (such as high pH of precipitation-prone CaCO3).
For small systems, the low recovery rate and choice of preventive cleaning mode of operation is to control the scaling one of the most convenient means.
5.2.7 Prevent scaling
220.127.116.11 The prevention of calcium carbonate scaling
The concentration of water salinity TDS ≤ 10000 mg / L of the bitter and salty water, Langgelier index (LSIC) as CaCO3 said the possibility of scaling indicators
pHC as dense as water pH value pHS CaCO3 saturated when the pH value
When LSIC ≥ 0, there will be CaCO3 scale.
Most natural water without treatment, LSIC will be positive, in order to prevent CaCO3 scale, unless the water system in the film adding inhibitors on the front or take preventive measures to clean, or else the need to ensure LSIC was negative.
CaCO3 control of the conditions for scaling:
LSIC < 0, do not need to increase inhibitors;
LSIC ≤ 1.8 ~ 2.0, adding a separate scale use of chemical agents or completely softened
LSIC > 1.8 ~ 2.0, and acid to LSIC of 1.8 to 2.0,
And then adding inhibitors; full use of chemical or softening.
Most of the high salinity of natural water without treatment, S & DSIC is generally positive, in order to prevent CaCO3 scale, it is necessary to increase the S & DSIC acid into a negative, if by adding inhibitors to prevent precipitation of CaCO3, S & DSIC value for Positive, the maximum allowed S & DSIC value and scale of the required dosage, please refer inhibitors manufacturer's technological information.
18.104.22.168 The prevention of calcium sulfate scaling
Regarding CaCO3, we can achieve a higher recovery rate of reverse osmosis by one of the following measures:
- Strong acid-softening resins or weak acid-base resin in addition, can be calcium removal of all or part;
- Lime or lime - soda to soften and reduce the concentration of water Ca2 +;
- Inhibitors providers under the provisions of the water in the vote plus inhibitors,
- Reduce system recovery.
22.214.171.124 The prevention of barium sulfate scaling
Barium sulfate is all in the alkaline earth metal salts dissolved the lowest, when the water in the barium sulfate, will cause a lot of precipitation phenomenon, it is because of its calcium sulfate and strontium sulfate scale promote the formation.
Most natural water, the barium content will lead to concentration of barium sulfate in water sedimentation, the limit concentration of barium in brackish water is less than 5μg/L, when adding the current face of sulphuric acid, it should be less than 2μg/L. Preventive measures and preventive measures calcium sulfate the same.
126.96.36.199 SrSO4 scalen prevention
The preventive measures are as same as CaCO3
188.8.131.52 CaF2 scale prevention
When the concentration of calcium is very higher, as long as fluoride contains in the water reach 0.1 μ g / L, it may form the CaF2 sediment.
The Prevention measures are as the same as CaSO4’s
184.108.40.206 Silicon scale prevention
Most water’s solubility of silicon dioxide (SiO2) is in the range of 1 ~ 100 mg / L. Supersaturating of silicon dioxide (SiO2)can automatically polymerize into insoluble colloidal silicon or silicon jelly, an cause the contamination of membrane. The maximum concentration of SiO2 in the water depends on the SiO2 solubility degrees.
The process of silicon scale formation in the Concentrated water is different from the feed water, because the pH Value of the concentrated water are changing as the increased concentration of SiO2, so the calculation of SiO2 scale should be according to the feed water quality analysis and reverse osmosis operating parameters.
If a certain amount of metal appears, such as Al3+, formation of metal silicate. The existence of aluminum or iron in the water is the main reason of silicon Scale formation. Therefore, if the existence of silicon, it should ensure that there is no aluminum or iron, and it’s better to use the 1 μ m security filter, meanwhile take the preventive measures such as acid washing.
In order to increase the recovery rate, when taking the lime - Soda softening pretreatment, some amount of aluminum or magnesium should be added into the water to reduce the solubility of SiO2, meanwhile, the effective operation of the softening process is very important to prevent the appearance of insoluble metal silicate in the reverse osmosis system;
Because the pH value lower than 7the or higher than 7.8 can increase the solubility of silicon, regarding to the prevention of silicon scale formation, add the acid or alkaline in the water can increase the recycling rate, but in the high pH conditions, it should avoid the formation of CaCO3 sediment; regarding to the silica scale, when use a heat exchanger to increase water temperature, it can significantly increase the water recycling rate, but the maximum temperature of the membrane system allowable is 45 ℃; polymer resin of polypropylene acid inhibitors can be used to increase the solubility of silica.
220.127.116.11 The summary of pretreatment methods
Chart5-1 the treatment list of membrane contamination factors
|Ion exchange From base||2||2||2||2||2|
|Adjustment of recovery||1||1||1||1||1||1||1||1||1||1||1||1|
|Oxidation - filter||1||2|
Notes: 1. Likely to be effective, 2. Very effective, 3. Combination of several methods can be used together, blank is void.