1. Introduction
At present, a complete desalination process system includes the pretreatment of raw water, the pre-desalination of reverse osmosis device, ion exchange or secondary desalination of EDI.
Among them, the reverse osmosis system has its specific requirements for the pretreatment of raw water, and the water source is roughly divided into groundwater, surface water, municipal tap water and urban water, etc. These water sources are affected by various factors, different geographical conditions, different seasonal climates lead to the characteristics of the water source and the impurities it contains, so the relative process will be different;
Among them, for municipal tap water, due to the pure water quality can not be set up without a clarification tank, directly into the mechanical filter after running, and no need to sterilize treatment; and for other water sources, due to the complex composition of suspended solids, impurities and salts contained in the water, in view of these situations and the requirements of the main process design parameters such as the recovery rate of the reverse osmosis system, choose the appropriate pretreatment process system to reduce the fouling and scaling of the reverse osmosis membrane, prevent the desalination rate of the reverse osmosis membrane, and the reduction of the water yield, especially for the current lack of water sources. Water quality deteriorates day by day, and choosing the right pretreatment system will directly affect the function of the entire water treatment system.
As we all know, reverse osmosis systems do not work well, and in most cases, they are caused by imperfect pretreatment system functions. In order to ensure the normal progress of the reverse osmosis process, the raw water must be strictly pretreated.
2. Introduction to the pretreatment process
At present, the mainstream pretreatment process is basically composed of coagulation treatment equipment, mechanical filters and ultrafiltration equipment. The coagulation treatment equipment is specifically composed of sedimentation tank (clarification tank), scrubbing filter, etc.;
At present, mechanical filters mainly include activated carbon, multi-media filters, etc.; ultrafiltration is used as the last pair of barriers, so as to ensure that the pretreated water quality meets the operation of reverse osmosis equipment.
2.1 Sedimentation tank (clarification tank):
Here temporarily take the mechanical stirring clarification pool as an example, the type of clarification tank is to combine the mixing chamber and the reaction chamber into one, that is, the raw water directly into the first reaction chamber, here due to the stirring and lifting of the impeller of the mixer, so that the inlet water, the agent and a large number of reflux mud are quickly contacted and mixed, the mechanical reaction is completed in the first reaction chamber, and the original mud slag in the reflux sludge collides with the adsorption, forms a large flocculent particle, and then is lifted by the impeller to the second reaction chamber, and then separated by the diversion to the clarification area, and the clean water rises from the collection tank. The mud and slag are returned to the first reaction chamber in the lower part of the clarification area, scraped by the sludge scraper to the sludge thickening chamber, and the discharge is controlled by the mud discharge valve at the bottom of the pool, so as to achieve the effect of clarifying and separating the raw water, and the normal operation ensures that the turbidity of the effluent is less than 10mg/L.
2.2 Gravity sand filter:
Here temporarily take the gravity air scrub filter as an example, this type of filter is the mechanical stirring of the clarification pool condensed water treated, through the inlet pipe evenly into the filter pool diaphragm water tank, through the filter area sand layer, water cap to the bottom of the porous plate water collection chamber, from top to bottom gravity filtration (quartz sand filter layer from bottom to top for backwashing, backwashing water source for the upper tank of the filter pool, while using a Roots fan to air scrub the quartz sand filter layer, the sand filter layer intercepted suspended solids and other impurities for cleaning, so as to ensure the quality of the effluent water) After the filtered water is stored in the upper tank of the water barrier through the connecting pipe, after the water tank is filled, the clean water overflows from the overflow tank of the upper water tank, and the outlet pipe is sent into the chemical pool as ultrafiltration reverse osmosis desalination water, and a part of it can also be sent into the domestic fire fighting pool as a living and fire fighting water, and the normal operation ensures that its effluent turbidity is less than 2mg/L.
2.3 Mechanical filter:
Here for the time being, the activated carbon filter as an example, this type of filter can adsorb the residual chlorine that cannot be removed in the pretreatment to prevent the reverse osmosis membrane from being oxidized by it and irreversible damage, while also adsorbing small molecule organic matter and other polluting substances leaked from the previous pretreatment, which has a more obvious adsorption and removal effect on the odor, colloid and pigment, heavy metal ions, etc. in the water, and also has the effect of reducing COD
2.4 Ultrafiltration equipment:
Ultrafiltration (UF) device is a kind of physical filtration device with a transition accuracy of 0.01μm, which is driven by the use of external pressure to intercept colloids, particles and substances with relatively high molecular weight in the water; the control of ultrafiltration water SDI is not more than 5, which is sufficient to ensure that the water production of the ultrafiltration device is pure and meets the requirements of the reverse osmosis equipment for the quality of the inlet water.
2.5 Precision Security Filters
Its main purpose is to ensure that the reverse osmosis inlet water does not damage the membrane module, according to the operation mode can be divided into backwashing type and non-backwashing type; non-backwashable filter element is disposable, the operating cost is higher, but the effect is better; a small number of power plants use backwashable security filters, which are more complex in operation and have lower operating costs. Security filters have become the main hidden danger of bacterial growth and organic matter deposition in the system.
3. Problems and adjustments faced by operation
As mentioned earlier, the normal operation of pretreatment will directly affect the operation of the entire reverse osmosis equipment, and the problems faced by most power plants are roughly divided into: improper coagulant dosing, improper control of fungicides and other issues.
3.1 Control of the dosage of coagulant dosing
At present, most of the power plant coagulants use polyaluminum chloride, the alumina concentration of crystalline polyaluminum chloride is 95%, and it is necessary to prepare a low concentration of polyaluminum chloride solution, and the concentration of alumina in the liquid is about 10%, and the drug is added by continuous dosing method of diaphragm metering pump;
By adding to the sedimentation tank (clarification tank), the flocculation effect of the clarification tank is controlled, so as to ensure that the effluent quality of the clarification tank meets the standard, that is, less than 10mg/L; in actual operation, due to the influence of many aspects, it may lead to the phenomenon of overdosing, if the excessive dosing will lead to an increase in the residual aluminum molecules in the water, etc., and the colloidal substance it generates is easy to deposit on the surface of the membrane, so it produces colloidal pollution, and these colloidal pollutants are difficult to be treated because of the same charge. Thus forming an aluminum salt (if polyferroic, it may be an iron salt), which will cause the flux of the membrane to decrease, resulting in a rise in the membrane permeable pressure difference;
Similarly, if the amount of addition is insufficient, it will affect the operation of the clarification tank, which is not enough to ensure the effluent quality of the clarification tank, which will cause the suspended solids in the effluent of the clarification tank to rise. It can be seen from Table 1 that the total aluminum concentration of the No. 3 clarification pool of a certain plant is ultra-high, and the amount of PAC is seriously excessive. As can be seen from Table 2, after adjustment, the total aluminum concentration and PAC dosing amount of No. 3 clarification pool of a certain plant returned to normal.
3.2 Control of Microorganisms
At present, most of the water intake of power plants is mainly surface water, and there are more organic matter and microbial pollution in surface water, and these pollutants will cause irreversible damage to the operation of reverse osmosis equipment, so the raw water must be sterilized to control the breeding of microorganisms in the water and reduce the impact on reverse osmosis equipment.
A power plant uses prometic CDVa8A chlorine dioxide generator, due to the large amount of raw water flux, high water temperature, increase in microorganisms and bacteria in the water in the spring and summer, and need to be regularly added to the mechanical stirring clarification tank for sterilization and algae treatment to control the breeding of microorganisms and bacteria in the water;
In autumn and winter, due to the low water temperature of the raw water, microorganisms and bacteria in the water are not easy to survive, so the sterilization and algae treatment can be stopped. At the same time, it is also necessary to control the residual chlorine of the mechanical stirring clarification tank at 0.1 ~ 0.3mg /l, and the excessive amount of residual chlorine in the water will cause the residual chlorine content in the water to be too high, and if the subsequent reducing agent is insufficient, it will lead to the oxidation of the reverse osmosis membrane, resulting in irreversible damage to the reverse osmosis membrane.
Since activated carbon plays an adsorption role, mainly adsorbing small molecules of organic matter in water, in addition, the removal rate of COD in activated carbon for water is generally 40% to 98%; so it is necessary to add activated carbon filters before ultrafiltration to meet the water intake needs of ultrafiltration equipment.
Because there will still be a small number of microorganisms and bacteria in the water, and microorganisms and bacteria will be directly attached to the filter element of the precision security filter, thus affecting the operation of the security filter, so the fungicide dosing system is also set up before the ultrafiltration equipment, if there is sterilization treatment in the early stage, the amount of fungicide can be reduced here, and the amount of residual chlorine imported by the ultrafiltration is controlled at 0.1 ~ 0.2mg/L;
If the water source of a small number of small thermal power plants is municipal tap water, because the municipal tap water itself has been sterilized and the water quality is better, it can be considered to stop adding fungicides; considering that the effluent of the ultrafiltration equipment has residual chlorine, it is necessary to add a reducing agent (sodium bisulfite) at the entrance of reverse osmosis to offset the impact of residual chlorine on reverse osmosis operation.
3.3 Contamination of organic matter
Reducing agent: that is, sodium bisulfite, the solid is a white monoclinic crystal powder, there is a sulfur dioxide odor, the concentration is 99%, the relative density is 1.49, it is easily soluble in water, easy to decompose when heated, slightly soluble in ethanol, the aqueous solution is acidic, the reducibility is strong, it is easy to be oxidized or lose sulfur dioxide in the air, and it can be stored for a longer time in a closed, dark and dry environment;
In the aqueous solution of sodium bisulfite, to avoid the exposure of the sun to the solution measurement box, under the action of sunlight, it will react with oxygen in the air at any time, and the shelf life of different concentrations of sodium bisulfite solution is also different, see Table 3 for details.
The size of the reducing agent dosing amount needs to be monitored with a redox potential (ORP) to control the size of residual chlorine in the water. However, excessive dosing will lead to the growth of anaerobic bacteria on the surface of the membrane, the formation of new organic pollution, long-term operation will cause pollution to the reverse osmosis membrane, resulting in a decline in water production and the rise of differential pressure in operation, and the filter element of the security filter is also replaced frequently;
Usually control its dosing amount of 3 ~ 5ppm, control reverse osmosis inlet residual chlorine between 0.05 ~ 0.1mg / l, reverse osmosis inlet of the redox potential (ORP) between 200 ~ 300mV; reducing agent overdosing is currently a common phenomenon, but also one of the main reasons for the reverse osmosis membrane fouling, many power plant operators simply look at the ORP meter for addition, which is not taken for granted, ORP meter is actually a potentiometer,
The measurement data is lagging behind, susceptible to factors such as solution temperature, pH and reversibility of chemical reactions; in addition, the redox potential is related to the partial pressure of oxygen, and is also affected by pH (that is, the redox potential is high when the pH is low, and the redox potential is low when the pH is high), in actual operation, it is necessary to pay attention to the linear relationship between the ORP redox potential, residual chlorine, conductivity or pH;
In addition, it should be noted that the flow rate required for the redox potentiometer is 8-10L/h, due to the presence of a small number of microorganisms in the water, it will gradually adsorb on the measurement electrode, and the long-term non-cleaning will cause measurement errors, so after 30 days of continuous operation (if the pretreatment water quality is better, it can be controlled for 60 days) it is necessary to clean the measurement electrode of the ORP meter, thereby ensuring the accuracy of the measurement results of the ORP table.
4. Conclusion
In summary, the normality of reverse osmosis operation is actually to see the dosage of each link, as long as any one of the dosing amounts is abnormal, it will cause the reverse osmosis system to run abnormally, and these are a chronic process, we only have to do regular testing of the water quality of each link and through the data analysis of each monitoring table, from the source to control, in order to get a good prevention; under normal circumstances, continuous operation of reverse osmosis cleaning 1 to 2 times a year is normal.