Control scheme and instrument selection for anaero

Key points: introduce the main control schemes of anaerobic treatment in the 30000m3 sewage treatment plant of Nanzhi daily treatment plant, including: liquid level control of pre acidification pool, pH control of sewage in key lifting circulating tank, feed flow control of IC reactor, gas level control of biogas storage tank, etc. The selection of main instruments such as ultrasonic level meter, microwave level meter and pH analyzer is also discussed

key words: anaerobic treatment of sewage; Control scheme; Ultrasonic level meter; Microwave (radar) level meter; PH analyzer

Wu Jianqi, engineer of Fujian Nanzhi Co., Ltd

1 overview of the whole sewage treatment process

the daily treatment of 30000m3 sewage treatment plant of Nanzhi paper adopts the "Anaerobic + aerobic" process, in which: anaerobic is used to treat high concentration sewage including TMP (wood chip mill pulp) and dip (waste paper deinked pulp); Aerobic is used to treat the effluent after anaerobic reaction and low concentration sewage such as BKP (chemical pulp), GP (mechanical pulp), sand pipe wrapping paper, etc. The key process technology and equipment of the sewage treatment project are from Paques water systems bv Company introduction

the main process flow includes two parts: high concentration sewage treatment process and low concentration sewage treatment process

main high concentration sewage treatment: water inlet with grille → primary sedimentation tank → cooling tower → pre acidification tank → circulating tank → IC reactor → gas-liquid separator → biogas tank → flare system

after anaerobic treatment, the sewage is transferred from the circulating tank to the distribution tank and enters the aerobic treatment process

main low concentration sewage treatment: water inlet well with grille → switching tank → regulating tank (or accident tank) → distribution tank → screening tank → gas tank of Donghua University → secondary sedimentation tank → water outlet well (up to standard discharge)

mainly because the high concentration sewage of TMP and dip accounts for more than 80% of the COD pollution load of the whole sewage treatment system, it can be seen that anaerobic treatment plays an important role in the whole sewage treatment process. In terms of the control system of the whole sewage treatment plant, the process parameters and process control of anaerobic treatment are the most complex, focusing on the middle and high-end, striving for a new breakthrough in the green and energy-saving plastic processing industry, and striving for the most complex 3 The oil collector of the horizontal tension oil pump is not pressed on the pump body

2 full anaerobic treatment process control

when formulating the sewage treatment process control scheme compared with bromide flame retardant polypropylene, it must be considered that all process equipment can be operated manually or automatically. The control of anaerobic treatment is relatively special. Here are several typical control schemes in the process of high concentration sewage treatment, mainly including the control schemes for the process processes such as the liquid level of pre acidification pool, the pH value of sewage in circulating tank, the feed flow of IC reactor and the gas level of Biogas gas storage tank, and briefly introduce the main functions of the control system of sewage treatment plant

2.1 pre acidification pool level control scheme

for pre acidification pool level monitoring, it is not only necessary to control the pre acidification pool level by interlocking with the operation of the pre acidification pool outlet pump, but also to interlock with the submersible agitator in the pre acidification pool to prevent solid precipitation

considering safety and other factors comprehensively, the control idea is as follows: in the automatic operation state, consider the two pre acidification tank outlet pumps as mutually standby pumps, and automatically rotate at a fixed time; The manual operation status is divided into local manual operation and control room manual operation. The control room cannot be operated manually during local manual operation, that is, the principle of local priority, to ensure the absolute safety of on-site operation. The control scheme of liquid level interlocking control submersible agitator is as follows: when the liquid level is higher than the set position required by the process, in order to avoid affecting the service life of the submersible agitator due to frequent startup and shutdown at the set liquid level, it is specially considered that the submersible agitator will not start running until the liquid level is higher than the set position and lasts for a certain time; In addition, from the perspective of safety, in case of leakage (detected by the humidity sensor) of the submersible mixer, the working power supply of the submersible mixer will be automatically cut off and an alarm will be given. See Figure 1 for the liquid level control logic diagram of pre acidification pool

Figure 1 liquid level control logic diagram of pre acidification pool

2.2 pH control scheme of sewage in circulating tank

it is very important to control the pH value of sewage for the whole sewage treatment plant. In order to treat high concentration sewage anaerobic effectively, the pH value of sewage in the circulating tank must be well controlled

first of all, we consider that NaOH or HCl storage tank should have normal liquid level to ensure use (otherwise, alarm and take measures); The second is to ensure that the measured pH value of the circulating tank is true. Therefore, it is necessary to consider that the circulating measurement pump used to measure the pH value is in operation, and the pH value of the circulating tank can be judged to be true only when the flow switch for circulating measurement of pH value detects that there is flow passing. Only when the pH value of sewage in the circulating tank is true and meets the process requirements, it is allowed to enter the IC reactor

for the pre control of pH value, it naturally occurred to me that the pH value of the circulating tank is used to interlock and control whether to start the NaOH outlet metering pump or the HCl outlet metering pump

the implementation of the above control scheme can fully ensure that the pH value of the sewage in the circulating tank meets the process requirements and enters the IC reactor for anaerobic treatment

see Figure 2 for the logic diagram of pH control of sewage in the pre circulation tank