Sewage treatment DCS

Process flow:

The entire sewage treatment process consists of the following five processes:

(1) Pretreatment process

The industrial sewage from each production workshop is discharged into the regulating tank. Due to the high water temperature, it is not conducive to the growth of fungi that decompose organic matter in the subsequent process, and it can be discharged after natural cooling. A temperature node is set in the tank. When the water temperature drops to the set value, the valve is opened to allow the sewage to flow into the flocculation sedimentation tank. Install an electromagnetic flowmeter in the wastewater pipeline to complete the function of sewage measurement and settlement.

Turn on the lime pump and valve regularly in the sedimentation tank, and add a slaked lime solution that is controlled according to the ratio to enhance the dewatering capacity of the sludge and improve the pH value of the sewage. The wastewater in the pond is collected offline for a certain period of time and its pH is measured. The scraper at the bottom of the sedimentation tank works regularly to scrape the precipitated sludge into the flocculation sedimentation tank, and then regularly control the daily discharge of a quantitative concentrated sludge into the sludge dewatering process. The supernatant liquid in the sedimentation tank is diluted by adding water and discharged into the homogenization tank, and discharged into the next process at regular intervals.

(2) Hydrolysis and acidification process

The main equipment for hydrolysis and acidification is four anaerobic desulfurization towers, which accept the raw sewage in the four homogenization tanks, and each adds an electromagnetic flowmeter for cost settlement. The anaerobic bacteria cultivated in the desulfurization tower can remove part of the BOD and COD of the sewage raw liquid to achieve the purpose of sewage desulfurization. According to the process requirements, the temperature in the tower is the most critical for the growth of anaerobic bacteria, and the temperature at the top of the tower and the bottom of the tower are used as reference quantities. Since the opening of the steam valve controls the temperature to be constant, and the steam flow interference has the greatest impact on the temperature, the design takes the temperature in the tower as the main control point, and the steam flow control is a temperature-flow cascade control system in the secondary loop to ensure the tower The internal temperature is stable. The desulfurized sewage is discharged into the SBR tank.

(3) SBR process

SBR is the abbreviation of Sequncing Batch Reactor AcTIvated Sludge Process, which is an activated sludge wastewater treatment technology that operates by intermittent aeration. The SBR reaction tank is the core of the entire sewage treatment plant, where the macromolecular organic matter in the sewage is decomposed into small molecular organic matter, and then decomposed into carbon dioxide and water. Therefore, the automatic control of the SBR reaction tank is very important to the entire sewage treatment system.

There is a large amount of activated sludge at the bottom of the SBR tank, and its main component is aerobic bacteria. Aerobic bacteria have a strong degrading effect on organic pollutants in sewage, and their population is directly related to the sewage treatment capacity of the SBR tank. From the equation of the nitration reaction that has been going on in the reaction tank: NH4++2O2→NO3-+2H++H2O+energy Only with enough oxygen molecules can the nitrification reaction proceed normally and the aerobic bacteria that degrade organic matter can survive; however, too much oxygen will cause a large amount of foam to be produced, the sludge sedimentation performance will decrease, and the energy consumption will increase. Therefore, the ventilation of the SBR tank Control of the most critical. The ventilation volume is controlled by timing. When the deviation of the concentration indication of the online dissolved oxygen meter is too large, the setting value is changed to adjust the intake volume duration. Sewage feed valve and sludge discharge valve are also controlled by timing. The SBR tank is equipped with a drop-in level gauge control valve to prevent sewage overflow, an ultrasonic flowmeter is installed to measure the sewage, and a temperature node is set to heat the water to achieve a constant temperature in the reaction tank.

(4) Contact oxidation process

The primary and secondary contact oxidation tanks in this process can be regarded as the continuation of the aeration tank. After sufficient oxidation, they are discharged into the corresponding sedimentation tank at regular intervals. The principle of gravity sedimentation is used to remove suspended solids such as sludge. Drive the sludge suction machine to scrape the sludge into the sludge thickening tank. The treated water in the secondary sedimentation tank has been filtered and disinfected to meet the national discharge standards and can be directly discharged or enter the return water system for reuse.

(5) Sludge dewatering process

In the above process, the sludge is discharged into the thickening tank through the sludge scraper and dehydrated by the dehydrator to form a sludge filter cake with lower water content, which is sent to the rotary kiln for combustion, and finally discharged in the form of harmless carbon dioxide and water. The ash is buried deeply.

Control System:

(1) Hardware configuration

The system is composed of an engineer station, an operator station, an I/O station and a process control network, and is connected to the management computer of the workshop director via Ethernet to realize remote query and remote control. The engineer station is equipped with a configuration software platform to complete the tasks of modifying parameters and system maintenance; the operator station is a platform for operators to complete the monitoring and management of the production process; the I/O station completes the implementation and monitoring of the entire industrial process, and the main control card and data In order to ensure the reliability of the system, the main control card and data forwarding card are redundantly configured.

Equipped with 2 SP243X main control cards; SP313 4 analog signal input cards 30; SP322 4 analog output cards 3; SP363 7 contact type switch input cards 8; SP364 7 relay output cards 10 One. There are totally 108 AI signals; 8 AO signals; 40 DI signals; 50 DO signals.

 (2) Network structure 

Figure 2 Network structure of sewage treatment system

(3) Software configuration 

1. HMI

Display data overview, historical curve of each inspection point, process flow chart of each process, and state of each valve on site, and set up different permissions to ensure the safety of the system.

2. Report

Shift report at 8:30, 16:30, 0:30, in the form of: display the whole point data of current shift temperature and flow, flow accumulation, valve opening and closing time record, equipment operation status; monthly report , The format is: taking each day as the recording unit, display the daily average value of the flow (sum of the daily punctual records/24), monthly cumulative value, and the daily average value of the temperature (the sum of daily punctual records/24), and input at the same time Edit other data.

3. SBR reaction tank valve control: as shown in Figure

Figure 3 Flow chart of valve control algorithm

4. SBR reaction tank valve status alarm:

If the valve in the aeration tank fails and is not discovered in time, the sewage will be discharged into the next process without full degradation, and it will not be able to meet the national discharge standards for sewage purification, causing greater losses. So with the help of smart valves The valve alarm program is designed with the feedback signal, so as to deal with the failure in time and reduce the loss.

The alarm is defined as: after the valve is opened/closed, there is no feedback signal within 12 seconds and it is regarded as a valve failure. The sound and light alarm method is used to remind the maintenance personnel to go to troubleshoot and resume production.