Sludge acclimatization in coking wastewater treatment plant

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Sludge acclimatization in coking wastewater treatment plants. Industry: Water treatment. Keywords: Coking wastewater, Wastewater treatment, Activated sludge. According to Polaris Environmental Protection Network, the coking production process generates large amounts of high-concentration phenol-containing wastewater. This type of wastewater is characterized by a complex composition, high concentrations, and high toxicity, making it one of the most difficult industrial wastewaters to treat. Currently, the activated sludge process is mostly used for the treatment of coking wastewater. http://img01.bjx.com.cn/news/UploadFile/201701/2017010309114752.jpg The aerobic biological treatment method features high efficiency, fast speed, and low cost; it is the primary method used for treating wastewater from coking and printing and dyeing industries. However, during aerobic biological treatment, sludge is highly sensitive to changes in the quality of wastewater; sudden changes in any of the parameters such as the pH value of the wastewater, toxic substances, dissolved oxygen, nutrients, and water temperature can inhibit the growth and reproduction of microorganisms or even cause their death. Therefore, the cultivation and acclimatization of activated sludge is a very important task. Taking the cultivation and acclimatization of activated sludge in the aerobic biological treatment system for coking wastewater as an example, this paper introduces the process of cultivation and acclimatization for treating coking wastewater using dry sludge from domestic sewage treatment plants as the microbial strain. Process of culturing and acclimating sludge 1. Preparation work 1) Clean the pipelines involved in the entire biological phenol removal process to ensure smooth flow; inspect and test all equipment such as tanks, fans, pumps, and motors to confirm they are in good condition with no deficiencies, and ensure that backup equipment is ready for use. 2) Test the quality of the wastewater used for acclimatization; the test parameters include: pH value, water temperature, COD, BOD5, phenols, ammonia nitrogen, **, oil, and volatile ammonia. 3) Ensure a stable stream of ammonia-containing wastewater, and prepare wastewater with a phenol concentration of <10 mg/L in the mixing tank. 4) Prepare an adequate amount of trisodium phosphate and glucose. 5) When using the inoculation culture method, it is also necessary to have an adequate supply of dry (or concentrated) sludge from other wastewater treatment plants (stations) with similar wastewater characteristics, to be used as a strain for culturing activated sludge microorganisms. 6) For this acclimation process, a set of aeration tanks from the biological defoaming treatment system was used; the total volume of these aeration tanks is approximately 450 m3, and they are divided into an adsorption section and a regeneration section, with equal volumes in each section. First, inject approximately 300 m3 of tap water into the aeration tank and aerate it for more than 2 days (to remove residual chlorine from the tap water). 2. The selection of seeding sludge and the methods for culturing activated sludge are divided into natural cultivation and inoculated cultivation. The incubation time for the inoculation culture method is short; it is a commonly used method for culturing activated sludge and is suitable for most industrial wastewater treatment plants. There are two common methods for inoculation culture: 1) Inoculation culture using concentrated sludge. The concentrated sludge is approximately paste-like but still retains fluidity. Using concentrated sludge is not only inconvenient for transportation but also results in high transportation costs; on the other hand, a large amount of concentrated sludge cannot be obtained in a short period of time. Therefore, when the aerobic tank is large, the amount of sludge used is substantial, making it unsuitable to use concentrated sludge for inoculation and culture. 2) Inoculation and cultivation using dry sludge. ““Dry sludge” generally refers to the sludge cake obtained after dehydration using a dehydrator, with a moisture content of 70% to 80%. This law applies to remote areas and situations where the distance for transporting the seed sludge is long. The amount of dry sludge added is generally 2% to 5% of the tank volume. If dry sludge contains a certain concentration of chemical agents (used for sludge conditioning), and if the concentration of these agents is too high and their toxicity is significant, it is not suitable to be used as seed sludge for culturing bacteria. To determine whether sludge can be used for inoculation, a small amount of the sludge can be crushed and placed in a small container such as a beaker or plastic bucket; water is added and aeration is provided. If the color of the sludge changes to yellow after some time, it can then be used for inoculation. For this domestication process, dry sludge from municipal wastewater treatment plants was used as the microbial strain. Considering that when the dry sludge is directly added to the aeration tank, it does not dissolve completely; the deposition of sludge can lead to blockages in the aeration equipment, and moreover, the efficiency of using the microbial strain is also reduced. First, the dry sludge is placed in an iron tank with a volume of about 1 m3. Water is added while the sludge is introduced, and the mixture is stirred to turn the sludge into a slurry. This slurry is then pumped into the aeration tank using a slurry pump. Sludge was added starting from October 20, 2008, and the addition was completed by October 22 (continuous aeration was carried out during sludge addition); at this time, the sludge settling volume ratio (SV) of the mixture in the aeration tank was 35%. Approximately 24 m3 of dry sludge was used as the culture medium for this domestication process; about 60% of the total amount of microorganisms was added to the adsorption stage, while about 40% was added to the regeneration stage. 3. Cultivation and acclimatization of sludge 1) Control of inlet and outlet parameters: Wastewater is fed in according to the principle of increasing phenol content, and once the removal rate of phenol reaches over 90%, wastewater with a slightly higher phenol concentration is introduced. The inlet and outlet parameters are shown in Table 1. http://img01.bjx.com.cn/news/UploadFile/201701/2017010309122937.jpg 2) Inlet flow control: After the addition of sludge on October 22, aeration was carried out for 1 day, and the sludge turned brownish-yellow. Stop aeration, allow sedimentation for 2 hours, then use a submersible pump to remove about 30% of the supernatant from the tank. At 10:00 the next morning, start feeding phenol-water with a prepared phenol concentration of <10 mg/L into the aeration tank using a sewage pump. Operation is carried out with intermittent water feeding and continuous aeration to ensure uniform water supply. The inlet flow rate control is shown in Table 2. http://img01.bjx.com.cn/news/UploadFile/201701/2017010309131169.jpg 3) Changes in the sludge: In the first few days, there were no significant changes in the sludge; its color remained brownish-yellow. After 7 days, motile paramecia were detected under the microscope; after 10 days, colonial flagellates were observed. After 23 days, more than 20 colonies of flagellates were found, at which point the sludge took on a slightly black color. By 25 days, the sludge had turned tea-brown. After 33 days, a significant increase in the number of stalked ciliates was observed, along with motile pseudopodiate protozoa. Some microscopic photographs are shown in Figure 1. http://img01.bjx.com.cn/news/UploadFile/201701/2017010309135351.jpg4. Adjustment of process parameters – Detection indicators: pH value, phenol, cyanide, CODcr, oil, volatile ammonia, BOD5, ammonia nitrogen ； Aeration tank, dissolved oxygen, pH value, phosphorus, MLSS, SV, water temperature, SVI ； Secondary sedimentation tank: CODcr, ammonia nitrogen, sulfides, phenols, cyanide, BOD5, oil. 1) The temperature for thermal biochemical treatment is 10–40°C, with the optimal temperature being 25–35°C. When the temperature is too low, microbial metabolism slows down and their activity is suppressed ； When it is too high, the protoplasmic colloid of microbial cells coagulates, stopping enzyme activity and leading to the death of the microorganisms. During acclimation, the water temperature in the aeration tank was between 17 and 30°C. In November, the low temperature of the incoming water caused the water temperature in the aeration tank to drop to 17°C. Immediately, the steam pipes in the equalization tank were activated to raise the temperature of the incoming water, keeping it between 35 and 40°C; as a result, the water temperature in the aeration tank could be maintained at around 27°C. 2) pH value: The optimal pH range for most bacteria and protozoa is 6.5–8, as it is under these conditions that they grow and reproduce best. In the aeration system for treating wastewater using activated sludge, bacterial flocs, which constitute the main component of the activated sludge, can produce large amounts of viscous substances at pH levels between 6.5 and 8.5, thereby forming good flocs. During operation, the pH value should not change drastically, otherwise it will inhibit the growth of microorganisms or cause them to die. During the domestication period, the pH value in the aeration tank remained between 6 and 7. However, starting from November 14, the pH value in the aeration tank dropped to 5.41. A low pH level is extremely detrimental to the growth and reproduction of sludge. Analysis showed that this was due to a malfunction in the water supply pump on November 14, which resulted in a reduced water supply volume. At this point, the amount of glucose added was increased appropriately, and by November 19, after the pump failure was resolved and the water inflow was increased, the pH value rose to 6.68 and remained relatively stable; microscopic examination showed that the sludge was developing well. 3) The microorganisms in nutrient-containing wastewater must continuously absorb nutrients; through catabolic metabolism (disintegration), they break down complex polymers or high-energy compounds into simple low-molecular-weight substances or low-energy compounds, releasing energy in the process ； By anabolic (assimilative) processes, it utilizes the energy and substances provided by catabolism to convert them into its own cellular materials ； At the same time, the resulting metabolites are excreted from the body. Water, carbon source, nitrogen source, phosphorus, inorganic salts, and the growth environment are the conditions for microbial growth. Nutrients should be added to the wastewater in a ratio of carbon:nitrogen:phosphorus = 100:5:1 to create favorable conditions for the growth of activated sludge. Sodium triphosphate dosage (kg/d) based on a phosphorus content of 3 mg/L in the mixture: 3 × biochemical water inflow volume (t/d) × 0.00529. Care should be taken to ensure that the chemical is evenly added to the aeration tank within 24 hours. During the acclimation period, the phosphorus content in the aeration tank and in its effluent should be monitored daily; it is advisable for the phosphorus concentration in the effluent to be greater than 1 mg/L. If the phosphorus content in the effluent is too high or too low, it is necessary to adjust the dosage of trisodium phosphate promptly. The amount of glucose to be added (starch can also be used) is calculated as follows: W = ((P – P_out) × 100 – LCOD × B/C) × Q ÷ 1000 × 2.5. Where: W is the amount of glucose added, kg/d ； P is the phosphorus content in the aeration tank, in mg/L ； Pout represents the phosphorus content in the effluent from the aeration tank, in mg/L ； Q is the biochemical influent volume, t/d ； LCOD is the COD content in the biochemical influent, in mg/L ； B/C is BOD5/CODcr. As can be seen from the inlet water parameters, the CODcr level in the water since November 23, 2008 has remained around 1000 mg/L, indicating that there is no shortage of carbon source; therefore, it is no longer necessary to add glucose. 4) The dissolved oxygen (DO) in the aerobic tank should be around 2 mg/L; it should not be too high or too low, just enough to allow the mixture in the aeration tank to be agitated. During the acclimation period, the DO level in the aeration tank was found to fluctuate within the range of 3–7 mg/L. On November 20, the DO level in the aeration tank suddenly rose to 16 mg/L. In the following days, it reached a peak value of 27 mg/L. These values are significantly higher than the saturation level of DO in water. Analysis suggests that the reason is that the iodometric method is used for measuring dissolved oxygen; the presence of nitrites, iron ions, and free chlorine in water can interfere with this measurement. In such cases, a modified version of the iodometric method should be employed. However, when wastewater contains substances such as sulfides, humic acid, and lignin, even the modified iodometric method cannot mask their effects, and it is then appropriate to use electrochemical probes for measurement. Later, a dissolved oxygen meter was used for measurement, and the DO in the aeration tank fluctuated within the range of 2–6 mg/L. Therefore, when measuring the DO of the mixture in the aeration tank, selecting an appropriate method is crucial to ensuring accurate results. 5) The return principle is to ensure that all the sludge discharged into the secondary sedimentation tank is promptly returned to the aeration tank. Since the amount of sludge increase during the acclimatization period is very small, sludge loss should be avoided. During the acclimatization period, the SV30 of the mixture in the aeration tank remained around 40%. However, starting on November 13, the SV of this mixture in the aeration tank dropped sharply; by November 14 it had fallen to 19%, with a continued downward trend. At the same time, a large amount of sludge was floating on the secondary sedimentation tank. The reason for this was a malfunction in the sludge scraper inside the secondary sedimentation tank, which led to excessive accumulation of sludge in that tank. Most of the sludge could not be returned to the aeration tank, and the sludge deposited at the bottom of the secondary sedimentation tank floated to the surface due to lack of oxygen, resulting in sludge loss. Solution: Lower the sludge pump to the lower part of the secondary sedimentation tank, and use this pump to return as much of the sludge in the secondary sedimentation tank as possible back to the aeration tank. Starting from the evening of November 14, a sludge pump was used to create backflow into the secondary sedimentation tank; by the morning of November 15, the SV value of the mixture in the aeration tank had risen to 35%. 6) Maintaining stable and qualified influent water quality: Sludge is highly sensitive to changes in wastewater quality; therefore, maintaining stability in the influent water quality is key to the proper operation of the biochemical system. The wastewater entering the aeration tank should have an oil content of <50 mg/L; if the oil content is too high, it can cause the sludge to float to the surface, leading to sludge loss. During the acclimation process, the oil content in the mixing tank remained at around 200–400 mg/L. To reduce the oil content in the incoming water, an iron grate was installed at the end of the mixing tank; this grate was covered with cotton cloth of high density, so that the wastewater passed through the cotton cloth before being sent to the aeration tank. The oil content in the wastewater after filtration is approximately 50 mg/L. Conclusion: In this study, the dry sludge from a domestic sewage treatment plant was used as the strain for acclimatization, and the method of continuous aeration with intermittent water feeding was employed for the acclimatization process. The advantage of this method is that the process flow is easy to control during acclimatization, unaffected by the size of the tank, while the disadvantage is that it requires stable water quality in the incoming water. The measure taken was to use two mixing tanks to prepare the phenolic water in advance, ensuring a stable influent water quality. By adjusting the process parameters, the concentration of phenol in the influent water and the volume of water fed into the system were gradually increased, thereby enabling the domestic sludge to regain its activity and adapt to the environment of coking wastewater within 32 days. As a result, the removal rate of phenol from the wastewater exceeded 99.8%, while the removal rate of CODcr exceeded 60%.