Application of OMRON PLC in the water return of Ningbo urban drainage

1. Introduction

Ningbo is located in the Jiangnan water town, but seasonal water shortages and uneven spatial distribution of water resources still exist. Since the early 1990s, a series of figures provided by an evaluation team composed of experts from relevant ministries of the State Council have shown that Ningbo has been listed as one of the water-scarce cities in the country. With the rapid development of Ningbo's industrial economy and the continuous improvement of urbanization, the city's demand for water has increased year by year, and Ningbo urgently needs new water sources to alleviate the water shortage. In order to alleviate the contradiction between water supply and demand, it is necessary to adhere to the combination of "increasing water" and "saving water", and vigorously promote the recycling of sewage resources (recycled water).

2. Use of Recycled Water

Recycled water refers to non-drinking miscellaneous water that can be reused within a certain range after being treated to meet certain water quality standards. Since 2002, the state has successively promulgated technical standards such as "Classification Standards for Urban Sewage Recycling", "Water Quality Standards for Urban Miscellaneous Water for Urban Sewage Recycling", "Water Quality Standards for Landscape and Environmental Water for Urban Sewage Recycling", "Sewage Recycling Engineering Design Specifications", and "Building Recycled Water Design Specifications", which standardize the design of sewage recycling and provide a basis for the design of urban sewage recycling projects. Recycled water reuse means that people use recycled water that has been treated by a certain process for agricultural and animal husbandry irrigation, municipal landscaping, vehicle washing, building internal toilet flushing, landscape water and industrial water, etc., which do not require high water quality. According to the actual situation in Ningbo, recycled water is mainly used for three categories: environmental water, industrial water and urban miscellaneous water.

3. Current status and treatment technology of recycled water in the three districts of the city

Compared with the degree of reclaimed water application in northern cities such as Beijing, Dalian and Qingdao, the reclaimed water reuse in our city is still in its infancy and on a relatively small scale. The Municipal Urban Drainage Co., Ltd. has carried out a series of reclaimed water development and operation work by taking advantage of the favorable condition that the secondary tail water of the sewage treatment plant is reclaimed water after deep treatment.

In 2006, OMRON CS1 series was applied to the reclaimed water project (20,000 m3/day) of Jiangdong North District Wastewater Treatment Plant. Due to the limitation of land use, the reclaimed water project adopted the "coagulation-filtration-ultraviolet disinfection" process. The designed reclaimed water quality is close to the GB18918-2002 Class B standard. PAC coagulant is added after the water pump is lifted to flocculate the inlet water and produce flocs. The water flows into the characteristic structure of this project - D-type filter tank. The D-type filter tank is a practical, new and efficient filter tank. It adopts a real-time multi-task distributed control system composed of a programmable controller and an industrial computer (PLC+IPC) to control the filtration, backwashing and constant water level of the filter tank.

(1) Filtration control

The author installed water level sensors and head loss sensors at the corresponding parts of the filter tank. The filtration of the filter tank is to measure the water level and head loss of the filter tank through them, and send the water level value and the opening degree of the valve after filtration to a special module installed in each PLC cabinet. The adjustment module can adjust the opening degree of the valve to make the filter tank reach the balance of inlet and outlet water, thereby realizing automatic filtration with constant water level and constant filtration speed.

(2) Backwash control

The backwashing of a group of filters is controlled by a common PLC. When the filtration reaches the filtration cycle or the filter pressure difference (water head) setting value, the filter makes a backwashing request, and the PLC forms a request backwashing queue according to the priority order of the filter. Once responding to the request of a certain filter, the PLC implements the entire backwashing process. In a group of filter plates, two filters are not allowed to be backwashed at the same time. When a filter is backwashing, the signals requesting backwashing from other filters are stored in the common PLC, and then the filters are backwashed in turn according to the storage order. When the filter tank is backwashing, the control process of the public PLC is: ① Close the water inlet valve to be filtered. When the water level of the filter tank drops to
the top of the sand washing drainage tank, close the filter water control valve and open the backwash drainage valve; ② Start the blower. After 5 seconds, open the filter tank backwashing air valve to pre-wash the filter tank for 1 minute; ③ Open the backwashing water valve, start the backwashing water pump, and backwash the filter tank with air and water for 7 minutes; ④ Close the backwashing air valve. After 5 seconds, stop the blower, open the air diaphragm valve to exhaust, backwash and rinse with clean water for 5 minutes, and then stop the backwashing water pump. After 5 seconds, close the water backwashing valve, then close the backwashing drainage valve, open the water inlet valve to be filtered, and the filter tank resumes filtering. The whole backwashing process takes about 25 minutes. In addition, the PLC can also control the number of filter tanks to be opened. It determines the number of filter tanks to be opened according to the water flow rate of the filter tank, and determines the opening and stopping of a filter tank according to the principle of first stop first open and first open first stop.

4. System application effect

It has the advantages of traditional rapid filter tanks, and the design filtration rate can generally reach 20-40m/h, which is 3-4 times that of general V-type filter tanks. Considering that no sedimentation is set, the design flow rate is 16m/h, which indirectly plays a partial sedimentation role. Using De'an's patented product DA863 fiber filter material, floccules are intercepted by the filter material, and clean water flows into the ultraviolet disinfection tank. After ultraviolet disinfection, the effluent meets the design standard and is used for river landscape water. Considering that this process is to remove suspended solids (SS) in sewage and remove some organic pollutants it carries, it is a physical removal process. This conventional treatment process has a high removal efficiency for raw water turbidity and SS, and a very low removal efficiency for TN and ammonia nitrogen. Online COD and NH3-N meters are installed at the effluent of the sewage plant and connected to the nearest PLC station. When the value exceeds the design value, the PLC program will respond and control the valve installed on the pipeline to add bleaching powder into the pipeline to make the effluent meet the standard. Then, in 2007, OMRON PLC CS1 series was applied in the water reuse project of the South District Sewage Treatment Plant, with a treatment scale of 10,000 m3/day. The tail water of the South District Sewage Treatment Plant is used as the inlet of the water reuse project, and the water quality of the inlet is designed to be GB18918-2002 Class B standard. The inlet water quality standard is relatively high, and the water reuse project adopts coagulation-filtration-chlorination disinfection. PAC is added at the inlet end, and flocs are generated under the stirring of four agitators with different linear speeds from high to low controlled by the PLC program. PAM is added at the front end of the water inlet pump, and the flocs are further formed into flocs through hydraulic agitation. The water flow is pumped from bottom to top through the quartz layer of the high-efficiency quicksand filter, and the flocs are intercepted by the quartz sand, and the effluent flows out through the outlet pipe. The intercepted material is transported to the sand collector with the sand through the rotation of the sand separator, and discharged from the top with the compressed air. The suspended matter flows out through the adjustable weir, and the sand flows into the sand washer and is washed several times in a special way. It is transported to the top of the filter by the sand pump and returns to the filtration process. The high-efficiency quicksand filter has a turbidity and SS removal rate of up to 90%, and a COD and BOD removal rate of about 50%. The effluent enters the clean water pool with chlorine for disinfection, and the effluent will be used for landscape water, industrial water and urban miscellaneous water.

5. Conclusion

In general, the reuse of reclaimed water in Ningbo is still in its infancy, with a small scale and a scope of utilization to be expanded. Although the reclaimed water projects of the North District Sewage Treatment Plant and the South District Sewage Treatment Plant have been completed, the utilization rate is not high. Although it is increasingly receiving attention and concern from the government and the public, the overall progress is slow. There are still many constraints. In the next step, we need to do a good job in the regulations on the reuse of reclaimed water, the planning of reclaimed water, etc., introduce corresponding preferential policies, increase water prices, and increase publicity on the recycling of water resources to promote the vigorous development of Ningbo's reclaimed water industry.