Treatment Of Old Landfill Leachate Containing High Ammonia Concentration And Refractory Organics

It is well known that landfill leachate is a complicated and non-biodegradable wastewater, and it always contains high strength pollutants which have an adverse effect on the environment. It has been generally known as a high-strength wastewater that is most difficult to deal with. This is due primarily to its large variability of organic, inorganic and heavy metal contents, strongly depending on the age and type of solid wastes of a landfill site. The monitored sample taken from the Chang Sheng bridge landfill site in Chongqing city in China, has its concentrations of COD, BOD₅, and NH₃-N about 1650, 75 and 1100 mg/l, respectively. According to some reference literatures of the same filed for high-ammonia and low-carbon leachate in Changsheng landfill which they concluded that it is difficult to remove ammonia and COD at the same process; therefore, biological and physical-chemical processes were used to remove COD and the ammonia. Firstly, The SBBR reactor and MAP precipitating process were used separately to remove ammonia from the leachate, their efficiency and their influencing factors were investigated during this treatment, and comparison between the two processes were also studied. Second, Physical-chemical treatment was adopted to remove COD; four processes like coagulation-flocculation, Fenton oxidation, combination of coagulation-flocculation and Fenton oxidation, and combination of Fenton oxidation and coagulation-flocculation were adopted to remove COD, each of their efficiency was investigated to make a comparison among them. Third, to improve the biodegradability of the leachate, research has been done to use ultrasonic method to improve the biodegradability of leachate and followed by SBBR was set to remove both COD and ammonia. The author hopes that leachate in this proposal thesis work can be discharged within standards accepted values. The results are shown as follows:Each cycle of the SBBR was divided into three periods; they are fill, react and draw. During the setup period of SBBR reactor, the temperature was between 30-33℃, and Na₂CO₃ was added twice every cycle to keep the inside pH-value more than 7, meanwhile, the dissolved oxygen was also kept more than 4.0mg/L, because the leachate used in this study contain high ammonia concentration was about 1100mg/L, it should be dilute with water or treated wastewater so that ammonia concentration of overall liquid inside the SBBR reactor do not exceed 200mg/L. When the packing density was 30% and cycle time was 24h, SBBR attained stable after two months'operation.During the domestication period, influencing factors such as pH-value, temperature, packing density and nitrogen loading rate were examined to study their effects on ammonia removal. Results showed that, first when nitrogen loading was 0.035 kgN/(m³.d), and HRT was 6 days, pH decreased to less than 3.7 at the end of each cycle time, therefore an alkaline solution (Na₂CO₃ was used) should be added twice every cycle to raise pH in order to keep microorganism activity. Second, when nitrogen loading rate was 0.048 kgN/(m³.d), and HRT was 3.33 days, two temperatures, 20℃and 31℃were choose to see their effect on ammonia removal, the change of nitrogen compounds with time shows that higher temperature as 30℃(usually 30-33℃) is good for nitrification. Third, study on packing density showed that, under the same temperature and volume, when packing density was 30% and influent ammonia concentration inside the SBBR reactor was 125mg/L, the HRT became 2.75 days, effluent ammonia concentration became 14mg/L, and nitrogen loading rate reached to 0.076 kgN/(m³.d); while when packing density was 42% and influent ammonia concentration inside the SBBR reactor was 121.275mg/L, the HRT became 2 days, effluent ammonia concentration became 14.7mg/L, and nitrogen loading reached to 0.105 kgN/(m³.d). Therefore, higher packing density improved biodegradation. Fourth, effect of nitrogen loading rate on ammonia removal was also studied, it was showed that when nitrogen loading rate between 0.01 and 0.02 kgN/(m³.d), the removal efficiency of ammonia showed that there was no significant decline between these values, ammonia removal efficiency reached to 99.3% and effluent ammonia concentration was 5mg/L at nitrogen loading was 0.01 kgN/(m³.d), while ammonia removal efficiency attained 98.2% and effluent ammonia concentration was 14mg/L at nitrogen loading was 0.02 kgN/(m³.d). They all comply with the 1st degree of the Chinese standards of landfill leachate. However, when nitrogen loading rate increased to 0.035 kgN/(m³.d), effluent ammonia concentration was 45mg/L which is beyond the standards; when nitrogen loading rate was increasing to 0.08 kgN/(m³.d), ammonia removal efficiency dropped dramatically to 74.2% and effluent ammonia concentration was 295mg/L.From the above experiment, the optimum conditions for SBBR reactor were: the HRT was 2 days, packing density was 42% and the reaction temperature was between 30 and 33℃. Research has been done under these conditions, and results showed that during the reaction time, ammonia concentration decreased gradually until reached to 11mg/L, and ammonia removal efficiency attained 99%, the effluent comply with 1st degree of the Chinese standard of landfill leachate. During the whole cycle of the SBBR reactor, it was observed that there was accumulation of nitrite significantly at the first 3 hours, nitrite concentration inside SBBR reactor increased with time, and this point represents the maximum nitrite concentration, after that the nitrite was concerted through reaction and its concentration decreased with time. It was suggested that the reason for nitrite accumulation was the presence of a lot of free ammonia inside the SBBR reactor that inhibited the activity of nitrifier bacteria.The optimum treatment conditions of MAP were determined through examination of pH-value and molar ratio. Firstly, adjusted different addition of MgCl₂6H₂O and Na₂HPO₄·12H₂O to form 13 groups of different molar ratio among Mg²⁺:NH₄⁺:PO₄^3-, through this experiment , it was clear that when the molar ratio changed from 1:1:0 to 1:1:1, ammonia removal rate increased from 4.2% to 97.5%; then, when the molar ratio kept increasing to 1:1:1.5, ammonia removal efficiency showed that there was no rise at its value; while when the molar ratio was 0:1:1, ammonia removal efficiency decreased dramatically to 14.6%, therefore, the optimum molar ratio among Mg²⁺:NH₄⁺:PO₄^3- was 1:1:1, under such conditions, when the molar ratio was 1:1:1, pH value was examined to find out the best interval, and the selected pH value was between 5 to 11. When pH-value increased from 5 to 8.5, ammonia removal efficiency increased greatly from 9.1% to 97%, as it continued to increase, ammonia removal efficiency changed slowly when pH-value increased from 10 to 11, and there was no significant decline in ammonia removal efficiency, so the optimum pH-value should be between 8.5 and 9.Study of ammonia removal by MAP precipitating process has been done under the condition of pH-value 8.5-9 and the moral ratio among Mg²⁺:NH₄⁺:PO₄^3- to be 1:1:1, results showed, when the ammonia concentration of the original leachate was 1100mg/L, and the pH-value was about 9, ammonia concentration was reduced to 28mg/L within 15min and the removal efficiency was 97.5%.Comparison between SBBR reactor and MAP precipitating process showed that, when it was used to remove ammonia, influent of SBBR should be diluted, while in MAP precipitating process do not need dilution; SBBR reactor produced less sludge compared with MAP precipitating process, but sludge from MAP was easy to remove; SBBR reactor needed long time to remove ammonia, while for MAP precipitating process just 15min; since MAP need chemicals, its cost is so much than SBBR reactor; the removal efficiency for SBBR reactor and MAP precipitating process were 99% and 97.5%,respectively. The reality should be considered to choose from the two processes when it comes to the real project.Coagulation-flocculation, Fenton oxidation and their combination were employed to remove COD and color. The study can be divided into four parts:â‘ Research of COD removal by coagulation-flocculation. The results showed that, when pH-value was 6, which was represent the optimum of pH-value, the highest COD removal efficiency was 19%. COD removal efficiency increased with increasing of PAM addition, and then dropped at certain amount, it was reached to 16% at the optimum addition 4mg/L of PAM dosage. Under the conditions mentioned above, PAC was added. The COD removal efficiency increased with increasing addition until reached 3000mg/L, after that COD removal efficiency decreased slowly as more PAC addition. All of these showed that pH-value and coagulant dosage has great effect on COD removal, and COD removal efficiency attained 41% under the optimum conditions. The effluent did not meet with the Chinese standards.â‘¡Research of COD removal by Fenton oxidation. In this part influencing factors such as pH-value, reaction time, Fe²⁺ and H₂O₂ addition, as well as temperature on COD removal were studied. Firstly, different pH-values were selected under the same reaction time 1h and the same Fe²⁺/ H₂O₂ ratio 1:1, since it was known that lower pH-value is preferred in Fenton oxidation process for three reasons: [Fe(H₂O)]²⁺ and H₂O₂ react slowly to produce less OH- under lower pH-value; also, lower pH-value can make H⁺ and OH^- react easily; furthermore, lower pH-value inhibited reaction between Fe3+ and H₂O₂, so the optimum pH-value was 3. Second, results of reaction time showed that, as reaction time increased from 10min to 90min, effluent COD concentration decreased, however, as reaction time climbed from 90min to 180min, effluent COD concentration increased, so the optimum reaction time should be 90min. Third, to obtain the optimum Fe²⁺/ H₂O₂ ratio, Fe²⁺ and H₂O₂ was studied separately because H₂O₂ dosage depends heavily on initial COD, the results showed that, as Fe²⁺ dosage reached to 800mg/L, effluent COD concentration began to increase, so the optimum Fe²⁺ dosage was 800mg/L, under such condition mentioned above, H₂O₂ was added to find the best ratio, and it was 2.5:1. Forth, study of temperature showed that, effluent COD concentration decreased as temperature increased from 10℃to 23℃, while it turned to higher concentration as temperature continued to rise, therefore, the optimum temperature was 23℃. When all of the parameters were determined, COD removal efficiency can attain 78.2%, and it did not meet with Chinese standards of landfill leachate for discharge. â‘¢Research of COD and color removal by combination of coagulation-flocculation and Fenton oxidation process. In this process, coagulation-flocculation was the primary treatment process, and according to results mentioned above inâ‘ , at a pH value of 6, PAC and PAM dosage of 2400 mg/L and 6mg/L respectively; effluent COD and color were at their lowest values. The COD concentration and color of raw leachate were 1650 mg/L and 1000 respectively. After treatment by coagulation-flocculation the COD concentration and color value decreased to 390 mg/L and 200 respectively. Then the effluent obtained from coagulation-flocculation method, was as influent in Fenton oxidation method. In this method, the COD concentration and color became 108 mg/L and 5 respectively at H₂O₂/Fe²⁺ molar ratio of 2000 mg/L / 800 mg/L (2.5:1). The removal efficiencies of COD and color increased to 93.5% and 99.5% respectively, which are close to 1st degree of Chinese standard for leachate treatment.â‘£Research of COD and color removal by combination of Fenton oxidation and coagulation-flocculation process. In this process, Fenton oxidation was the primary treatment process, after which the effluent COD concentration and color became 345 mg/L and 50 respectively. The effluent obtained from Fenton oxidation was used as influent in coagulation-flocculation method thereby giving removal efficiencies for COD and color of 84% and 97.5% respectively. The corresponding concentration of COD and color value were decreased to 265 mg/L and 25 respectively. The final effluent complies with 2nd degree of Chinese standard for leachate treatment.From the results obtained for COD and color removal, combination of coagulation-flocculation and Fenton oxidation might be the optimal (novel) method.Since the physical-chemical process need more chemicals, which are usually expensive. Ultrasonic method was used as alternative to improve the biodegradability of low-carbon and high ammonia of landfill leachate. The effluent obtained from the ultrasonic method was treated by SBBR reactor. Reaction time, ultrasonic power and temperature were studied to find the best operational conditions. Results showed that, at ultrasonic power of 400W, reaction temperature of 35℃and ultrasonic time in a range between 10min to 360min. While BOD₅ increased from 64mg/L to 138mg/L, when ultrasonic time increased to180min. However, from an ultrasonic time of 180 min, the BOD₅ increased slowly throughout. Therefore, the optimum ultrasonic time should be 180 min. Second, when the ultrasonic time was 3h, ultrasonic powers of 220,330,440,550 and 660W were selected. Results showed that, BOD₅ increased from 65mg/L to 261mg/L as the ultrasonic power increases. However, due to the power rating of the equipment used, optimum ultrasonic power should be 660W. Third, 9 values of temperature were employed to see the best reaction, they were 30, 35, 40, 45, 50, 60, 70, 80, and 90℃. Results showed that when temperature increased from 30℃to 40℃, effluent BOD₅ concentration increased from 230mg/L to 245mg/L, and then dropped with higher temperature. Therefore, the optimum reaction temperature was 40℃.Finally the optimum operational conditions were : ultrasonic power was 660W, ultrasonic time was 180min, and temperature was 40℃. Under such conditions, the effluent BOD₅ increased from 65mg/L to 245mg/L, and the BOD₅/COD ratio also increased from 0.038 to 0.140.The average effluent concentration of COD and ammonia were 1755mg/L and 735mg/L from ultrasonic process, which was the influent of SBBR reactor. Study has been done to examine its biodegradability. The results showed that effluent COD and ammonia were 1000 mg/L and 125 mg/L respectively, and their corresponding removal efficiencies were 43% and 81.6% respectively. This means that the ultrasonic method may effectively increase the biochemical nature of the leachate of high ammonia and low carbon.From the experimental results, for ammonia removal, MAP precipitation method even though expensive, is much more suitable than SBBR method. In fact, MAP method provides leachate treatment without dilution which is needed in SBBR method. It is also worth mentioning that MAP method is far faster than SBBR.Still, from the experimental results, it is has been found out that, for COD and color removal, the best method is combined treatment of coagulation-Fenton.Both MAP and combined treatment of coagulation– Fenton methods comply with Chinese standards for landfill leachate treatment and are easy for implementation. Thus, an efficient method could be made by combining these two methods, which is in fact a good recommendation.