Commissioning techniques for the A/O activated sludge process in textile dyeing wastewater treatment

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This post was last edited by fangqiong on 2010-10-28 at 21:31. The tuning techniques for treating dyeing and printing wastewater involve the A/O activated sludge process. The pollutants in textile dyeing and printing wastewater include impurities on textile fibers such as cotton and wool, salts, oils, and fats, as well as various pastes, dyes, surfactants, additives, and alkalis used during the processing stage. The main process adopted is: “grid + equalization tank + anaerobic tank + aerobic tank + sedimentation tank + disinfection tank”. Tuning is carried out by simultaneously performing inoculation, cultivation, and acclimatization; good activated sludge can be successfully cultivated within 1 month, and the system can meet the required standards after 3 months. ⑴ The entire debugging process is divided into two phases: Phase 1: The composition of printing and dyeing wastewater is very complex; there are no two wastewater samples that are exactly identical. Although we inoculate the activated sludge from wastewater treatment plants with similar characteristics, the various enzyme systems within the microbial cells need time to adapt to the new wastewater. After an adaptation period, the microbial cells begin to divide, and the microorganisms start to proliferate; their numbers increase at a geometric rate. Once bacterial growth becomes vigorous, they multiply in large numbers, consuming large amounts of nutrients present in the wastewater. These nutrients then gradually become a limiting factor for further bacterial growth. Active sludge can be properly formed when the residual organic pollutants (BOD5) in the aeration tank are low, and the ratio of organic matter to bacteria (F/M) is low. Therefore, in the first stage of debugging, intermittent operation is adopted: activated sludge from a printing and dyeing wastewater plant, accounting for 15% of the tank volume, is added. After 1 day of anoxic aeration, with the water temperature in the control tank kept below 42°C and the pH between 6 and 10, water inflow and aeration are carried out intermittently. The daily water inflow amount is 40% of the designed total amount, while the aeration volume is 25% of that during normal operation. The biodegradability of printing and dyeing wastewater is low, as the nutrients in the wastewater are insufficient to support the reproduction and growth of activated sludge microorganisms. A carbon source is added to the anaerobic and aerobic tanks every day (the dosage is such that it increases the BOD5 level in these tanks by 200 mg/L). Dosage of nitrogen and phosphorus: In the anaerobic tank, they are added at a ratio of BOD5:N:P = 300:5:1, while in the aerobic tank, the ratio is BOD5:N:P = 100:5:1. During intermittent operation, the amount of sludge in the sedimentation tank is low, and all of it is returned to the aerobic tank. After 20 days of intermittent operation, well-aggregated activated sludge flocs with good sedimentability appeared in the aerobic tank. The sludge concentration reaches 1000 mg/L. Stage 2: In the activated sludge treatment system, the process of removing organic pollutants from wastewater essentially involves these organic pollutants being taken up by the microorganisms in the activated sludge as nutrients, and then metabolized and utilized by them. It is the so-called “activated sludge reaction” process. The result of this process is that the wastewater is purified, microorganisms obtain energy to synthesize new cells, thereby causing an increase in the activated sludge. After intermittent operation, the formation of activated sludge flocs with good sedimentation properties and an increase in the microbial load of the activated sludge created the conditions for continuous operation of the biochemical system. Initially, water was supplied continuously at 50% of the total daily treatment capacity; during continuous operation, the growth of the sludge was primarily influenced by the sludge load (F/M). If F/M was too low, the activated sludge microorganisms would disintegrate and age due to a lack of nutrients, which hindered their growth. An excessively high F/M ratio causes the flocs to break apart into free bacteria, which is also unfavorable for the growth of activated sludge. Therefore, it is crucial to control the F/M ratio in the aerobic tank; we maintain this ratio at 400 mg/LBOD5/mgMLSS·day, and use an inverter to keep the DO level of the water leaving the aerobic tank at 3 mg/L. The sludge in the anaerobic tank grows very slowly; to increase its concentration, 5% of the aerobic activated sludge, corresponding to 5% of the tank’s capacity, is returned to the anaerobic tank on a daily basis. The BOD5 level in the anaerobic tank is kept between 300 and 400 mg/L. Apart from a small amount that is returned to the anaerobic tank, all the activated sludge from the sedimentation tank is sent back to the aerobic tank. The amount of sludge to be returned is determined using the formula Q1 = Q·SV30/(1-SV30), where Q1 represents the volume of sludge returned and Q represents the volume of water entering the system. This formula should be used flexibly; as the concentration of activated sludge increases, the water inflow can be gradually increased, provided that the sludge load (F/M) remains within acceptable limits. After 3 months of continuous operation, the daily wastewater treatment capacity reached the designed level; the sludge concentration in the anaerobic tank was as high as 10 Kg/m3, the color removal rate was as high as 70%, and the removal rates for COD and BOD exceeded 30%. The pH value ranged from 6.8 to 7.5. The sludge concentration in the aerobic tank reaches 3.5 Kg/m3, with an SVI of 200–300; the removal rates for COD and BOD exceed 85%. COD of the effluent from the sedimentation tank