Small reverse osmosis water preparation system process details

A kind of technically mature water treatment technology. According to the different water quality selection of reverse osmosis membrane, so that the reverse osmosis water to meet the “drinking water quality standards” (CJ94-2005) standards.
At present, reverse osmosis technology has been widely used in water treatment systems, this paper introduces the 0.5t/h small reverse osmosis water preparation system, the system can be applied to office building drinking water, experimental pure water preparation, pharmaceutical, food, electronics, biology and chemical fiber and other different industrial scenarios.

Process Flow Design

1、Pre-treatment process
The use of “pretreatment + reverse osmosis” water purification process, the pretreatment system includes multi-media filters and ultrafiltration filters, the pretreatment process can effectively extend the service life of the reverse osmosis membrane, and reverse osmosis system of water production depends largely on the pretreatment.
After the tap water through the tap water valve into the raw water tank, which began to carry out the pretreatment process; pump extracted raw water into the multi-media filter, which contains quartz sand and activated carbon, quartz sand to remove suspended particles in the water, activated carbon to remove the water of the foreign chemical heavy metal substances, and absorption of residual chlorine, etc., multi-media filters need to carry out backwashing, backwashing water into the concentrated water treatment system;
Multi-media filter treated water into the ultrafiltration filter, can effectively remove particles in water, colloids, bacteria, heat and organic matter, etc., and the size of the solute molecules than the membrane aperture is larger than the membrane retention, ultrafiltration filters also need to be backwashed, backwash water and concentrate into the concentrated water treatment system, the pretreatment process is completed.
High-pressure pump to extract the water after pretreatment into the reverse osmosis system, retention of bacteria, viruses, heavy metal ions and other pollutants; after the reverse osmosis system after the water shunt part of the cold pure water tank cooled as room temperature water, the other part of the hot water after electric heating, stored in the hot pure water tank, reverse osmosis water production after all the water in contact with the material all for sanitary level.
From the theoretical level, reverse osmosis membrane can remove microorganisms in tap water completely, but because of the actual operation process once the system stops running, the system is very likely to produce bacteria, in order to ensure that the cold, hot water tank water quality, the internal need to arrange the ultraviolet lamp for sterilization and disinfection, to ensure that the drinking water is sterile and non-toxic.

2、Backwashing system
In the normal operation of the reverse osmosis system, feed water, concentrated water flow along the surface of the reverse osmosis membrane to a certain flow rate, the original water pollutants are difficult to be deposited, but if the reverse osmosis system stops running, these pollutants will be immediately deposited on the surface of the membrane, the membrane element caused by pollution. Therefore, it is necessary to set up a backwash system in the reverse osmosis system, using the reverse osmosis membrane produced water on the surface of the membrane element to stop the operation of rinsing, to prevent secondary pollution.
If a new water production is not carried out within four hours for timed automatic flushing, the PLC brain control system will open the switching valve, automatic flushing valve, relying on the pressure of tap water, the reverse osmosis membrane automatic flushing for 30s.


3、Treatment of concentrated water
Concentrated water to be treated includes the concentrated water of the reverse osmosis system as well as the multi-media filter, ultrafiltration filter and the backwash water of the reverse osmosis system. It will be introduced into the concentrated water multi-media filter, add flocculant, the specific dosage needs to be based on the water quality, the pre-test to determine.

In this paper, the design of the system is used to add flocculants to treat concentrated water, and the treated water is returned to the original water tank to increase the recovery rate.
In order to realize the full use of water, the treated concentrated water can also be reused as production water, can be used as filtering device backwash water, dust removal water, cooling water; for the water quality requirements are not high can be directly reused, such as ash flushing, slag flushing; can also be used as water miscellaneous, such as road sprinkling to reduce dust, landscaping water, flushing toilets, vehicle flushing, workshop floor flushing, construction; in line with the relevant requirements and municipal and urban If it meets the relevant requirements and the municipal and urban sewage treatment plant agrees to receive it, it can be discharged into the municipal sewage network and sent to the urban sewage treatment plant for treatment.

4, control system
When the water level meter in the cold pure water tank and hot pure water tank is at low water level, the PLC control system starts the water inlet switching valve and each delivery water pump, so the tap water is pretreated by the multi-media filter and ultrafiltration filter, and the pretreated water is extracted by the high-pressure pump and enters into the reverse osmosis membrane pipe system. This control system uses PLC as the controller to control the water level by means of feedback control, as shown in Figure 3.
PLC connects the sensor input module and pump output module, where the input module is the data acquisition and processing module. The sensor input module has been collecting water level data information from the cold and hot water tanks, and the water level sensor is used to collect the data.
When the water level meter in the cold and hot water tank reaches the low water level, the input module triggers the control signal of PLC to control the water pump to start pumping to raise the water level; when the water level in the cold and hot water tank exceeds the specified high water level, the input module triggers the control signal of PLC to stop the pump pumping work to stabilize the water level, so as to realize the water level stabilization in the specified range.

According to test the conductivity of the final water to test the degree of purification of pure water production. According to the conductivity of the final water production, control the valves, pumps frequency converter, so that the filters and reverse osmosis system is in the best operating state; according to the conductivity of the final water production, to determine whether it is necessary to replace the membrane.

Main equipment selection
1、 Reverse osmosis membrane
Reverse osmosis membrane selection is crucial, according to the comprehensive balance of various factors, it is generally believed that the water production capacity of 5t / h above the reverse osmosis system using 8040 membrane element is more appropriate, 3t / h below the reverse osmosis system using 4040 membrane element is more appropriate, 3 ~ 5t / h reverse osmosis system using 8040 membrane element or 4040 membrane element can be.
Selection of membrane based on raw water quality and user design requirements, reverse osmosis membrane desalination rate, reverse osmosis membrane water production and reverse osmosis membrane recovery rate are three reference basis, which has been determined in the reverse osmosis membrane element manufacturing molding.
According to the balance of the above factors, the choice of reverse osmosis membrane is produced by the U.S. DuPont 2 BW30-LCLE 4040, with an inner diameter of 101mm membrane group container.

2, high pressure pump
The actual use pressure of membrane element is affected by various factors such as ambient temperature, average water flux, salt content of feed water, system recovery rate, type of membrane element and so on, and the use pressure of membrane element changes with the working condition.
The working pressure of the reverse osmosis membrane is selected as 0.6MPa, but because the water flux of the reverse osmosis membrane group will decay every year, in order to ensure that the working pressure after three years, a slightly larger head high-pressure pump is selected to ensure that the pump pressure is rich.
As can be seen from Table 1, take the surface water flux of the maximum attenuation value of 9.9% per year, the working pressure of 0.6MPa, within a certain range of working pressure and water flux is positively correlated, according to the attenuation value of 10% per year, in order to ensure that after three years to achieve the design of the water production pump, the selection of high-pressure pumps with a head of not less than 78m high-pressure pumps.

3, multi-media filter
Multi-media filter can reduce suspended solids and colloidal solids in reverse osmosis raw water, which is very important for the selection of filter media, filter media types, and different functions.
Combined with the raw water quality selection of double media filtration, quartz sand and activated carbon combination of sets of multi-media filter. The upper layer adopts quartz sand with large particle size and small relative density to increase the dirt holding capacity, and the head loss increases slowly; the lower layer adopts activated carbon with small particle size and large relative density to increase the adsorption effect.

4、 Ultra-filtration filter
Ultrafilter has better effect on removing particles, colloids, bacteria, heat and various organic substances in solution, and it is a necessary equipment in the field of water treatment system engineering and material separation.

Calculation of process selection

1, reverse osmosis membrane selection
Reverse osmosis membrane model BW30-LCLE 4040, water production is 0.25 t / h, that is, two membranes can meet the requirements of water production. Table 4 shows that the recovery rate of the system is 29%, and the recovery rate of a single membrane is 16%.
Tandem - a two-stage system calculation of the actual recovery rate of 29.4%. In order to ensure that 0.5 t / h of water production, the membrane group in parallel with the water intake of 3.125 t / h, the membrane group in series with the water intake of 1.7 t / h, after comparing the choice of reverse osmosis membrane group in series arrangement. Therefore, when the design water production requirements are relatively small, through the tandem arrangement of reverse osmosis modules can improve the recovery rate.

2, reverse osmosis piping system resistance calculation
In principle, in order to determine the optimal pipe diameter, different flow rates can be selected as the program calculation, from which the economic and reasonable optimal flow rate or pipe diameter can be found. For the pipeline inside the workshop, the flow rate can be selected empirically according to the economic flow rate range, and then the pipe diameter can be calculated by the mass conservation formula, and then rounded according to the pipeline standard.
The resistance to flow consists of two main components: one is the straight pipe and the other is the various fittings such as elbows and valves. The mechanical energy loss caused by the straight pipe is called flow resistance loss; the mechanical energy loss caused by the pipe fittings is called local resistance loss. The flow resistance loss is negligible compared to the local resistance loss.
In this design system, the water production is 0.5 t/h, the inlet flow of reverse osmosis system is 1.7 t/h, and the inlet flow of water treatment system is 2.0 t/h. The working condition is 25 ℃ at room temperature, and under a standard atmospheric pressure, the viscosity of the water is 0.894 9 mPa-s, and the density is 997.13 kg/m3.

In summary, the head loss caused by the sum of straight pipe resistance loss and local resistance loss is Hf = 6.761m, that is, the head of the pump selected in the whole design system must be greater than the head loss, select the pump model 25GP-30, head of 30m, that is, to test and verify the correctness of the pump selection.