By comparing the characteristics of waste leachate from waste incineration plants and landfill plants, we determined the UASB reactor-CASS reactor composite process to treat waste incineration plant leachate and determined its optimal treatment parameters. The results show that after treatment by this system, the total CODcr removal rate reaches 98.1, and the total NH4-N removal rate reaches 96.3, achieving better organic matter removal and denitrification effects.
Keywords: landfill leachate UASB reactor CASS reactor
1. Introduction
With the development of economy and technology and the acceleration of urbanization, traditional The landfill treatment of urban domestic waste is subject to more and more restrictions. According to the basic principles of harmlessness, reduction and resource utilization of urban domestic waste treatment, waste incineration power generation has become a new direction to solve the problem of urban domestic waste in recent years. . At present, most domestic research on landfill leachate treatment technology remains at the stage of leachate treatment in landfill plants. Due to the different characteristics of leachate from waste incineration power plants and leachate from landfill plants, they cannot be simply applied.
2. Characteristics and comparison of waste leachate from waste incineration power plants and leachate from landfill plants
Ningbo Fenglin Green Energy Development Co., Ltd. (Ningbo waste incineration power plant) waste The water quality characteristics of leachate and leachate from a landfill plant in Ningbo are shown in Table 1.
2.1CODcr and BOD5
The average concentration of CODcr in landfill leachate is mostly around 2500~5000mg/L, and the average concentration of BOD5 is mostly around 1450~2000mg/L. /CODcr is about 0.50, and its biodegradability is average. Since landfills are generally located in the open air, their pollutant concentrations are greatly affected by rainwater and vary greatly. Generally speaking, CODcr, BOD5, and BOD5/CODcr decrease with the age of the landfill plant, while the alkalinity content increases.
The average concentration of CODcr in the landfill leachate from the incineration plant is as high as 10,000~20,000mg/L, and the average concentration of BOD5 is as high as 3,800~5,000mg/L. The concentrations are quite high. The landfill leachate from the incineration plant is primary leachate, and most of it is produced on the same day. The landfill leachate has not undergone anaerobic fermentation, hydrolysis, and acidification processes, and contains heterocyclic aromatic hydrocarbon compounds such as benzene, naphthalene, and phenanthrene, polycyclic aromatic hydrocarbons, phenols, alcohol compounds, aniline compounds, and other refractory organic compounds. It is less affected by rainwater than landfill leachate. BOD5/CODcr is about 0.38, which is worse than the biodegradability of landfill leachate.
2.2 The ammonia nitrogen content is high and the heavy metal content is high
The ammonia nitrogen content in the garbage leachate from the incineration plant is high and the biodegradability is poor, which often brings certain difficulties to the biochemical treatment. After oxygen treatment, some refractory organic matter in the leachate is acidified and hydrolyzed into small molecular compounds that are easy to biochemically. The ammonia nitrogen content will increase to a certain extent with the decomposition of aniline compounds, etc. The concentrations of iron, lead, zinc, and calcium in landfill leachate are all high, so using a suitable process is crucial to the treatment effect.
3. Treatment process
The existing urban waste landfill plants in my country mostly use anaerobic and aerobic biological treatment processes for leachate. According to surveys, existing leachate sewage treatment plants generally have poor operating results. There are two reasons for this: 1. The leachate has gone through a long period of anaerobic fermentation process before entering the sewage treatment plant, and it is no longer suitable to use anaerobic hydrolysis and acidification processes. 2. The ammonia nitrogen content in the leachate is high. If the general activated sludge treatment process is used, not only will the ammonia nitrogen degradation effect be poor, but the sludge will not be cultivated or the cultivated sludge will be difficult to maintain.
Based on the characteristics of the landfill leachate treatment process in my country and the characteristics of the landfill leachate in incineration plants, we use the following process to conduct research.
3.1 Process Flow
The process flow is shown in Figure 1
3.2 Process Description
After the landfill leachate passes through the fine grid, the leachate is removed The suspended solids and floating solids in the filtrate enter the regulating tank and are pumped to the UASB upflow anaerobic reactor for anaerobic fermentation. The generated biogas is connected to the garbage incinerator for combustion support. The sludge is dehydrated and then landfilled or incinerated. The effluent is added After adjusting the alkalinity, CaO flows into the CASS reactor. CASS is a cyclic intermittent treatment process with good nitrogen and phosphorus removal functions. The entire system goes through five stages: water inlet period, reaction period, precipitation period, drainage period and standby period. The CASS reactor is divided into three Zone: The first zone is the biological selector, the second zone is the facultative zone, and the third zone is the aerobic zone. The effluent flows through the biological selector area, which can not only improve the stability of the system, prevent sludge swelling, but also produce significant denitrification. The effluent enters the facultative and aerobic zones from the biological selector. This zone mainly completes the degradation of organic matter and nitrification/denitrification processes. It is then excreted after a precipitation period.
4. Test part
4.1 Test method
Use the process flow shown in Figure 1 for a small test in the laboratory. The UASB reactor is modified from a polyvinyl chloride column with a three-phase separator with a volume of 5L. The CASS reactor uses a rectangular polyvinyl chloride pool with baffles inside and a volume of 5L.
4.2 Test water
The effluent was taken from the garbage leachate tank of Ningbo garbage incineration plant. The effluent water quality is shown in Table 2. It can be seen from Table 2 that the wastewater BOD5/CODcr=0.335 has poor biodegradability.
4.3 Screening and domestication of bacterial strains
The sludge in the UASB reactor and CASS reactor were taken from the anaerobic tank and aerobic tank sludge of Ningbo Wastewater Treatment Plant respectively. During domestication, firstly, the landfill leachate and domestic sewage are gradually prepared into mixed water in the ratio of 1:10, 1:6, 1:3, 1:1, 2:1, and 4:1 for stepwise domestication of sludge until the The water is all landfill leachate and is put into normal tests. Before the start of the test, we conducted a 3-month cultivation and acclimation period for the activated sludge in the CASS reactor to acclimate, screen and cultivate efficient denitrification bacteria in the activated sludge, which is the key to this process. As a result of long-term acclimation, the CASS reactor can tolerate high ammonia nitrogen concentration in the water above 1000mg/L, and can tolerate the toxicity caused by heavy metals. 4.4 Analysis items and methods
CODcr, BOD5, NH4-N and sludge concentration are carried out according to "Water and Wastewater Monitoring and Analysis Methods (Third Edition)".
5. Test results and discussion
5.1 UASB anaerobic reactor test results
The results show that when the sludge concentration is 7.5g/L, the residence time When it is 48H, the CODcr removal rate can reach up to 75.5, the BOD5 removal rate is 56.5, and the NH4-N concentration increases due to the decomposition of aniline compounds. When the volumetric load Nv reaches 5.0g/L.d, the gas production increases significantly. Due to the increase in gas production, bubble vibration and mixing phenomena cause the sludge to be in a good dynamic mixing state. Due to acidification and hydrolysis in the UASB reactor, the BOD5/CODcr value is significantly improved, which is beneficial to subsequent biochemical treatment.
The effluent from the UASB anaerobic reactor is shown in Table 3
5.2 CASS reactor test results
We measured CODcr and NH4-N based on various factors in the CASS reactor To determine the influence of removal rate, sedimentation time, drainage and sludge discharge time, standby time and pH during reaction are changed, and the reaction time and sludge concentration are changed to determine the best removal effect of CODcr and NH4-N.
5.2.1 Determination of pH value
The nitrification reaction is an alkaline process. On average, 1 mg of NH4-N for nitrification requires 7.07 mg of alkalinity (calculated as CaCO3). The optimal pH for the nitrification reaction is =7.5~8.5. Therefore, no further research was conducted in this experiment. CaO was added to the wastewater to adjust the pH, and the pH range in the CASS reactor was controlled between 7.5 and 8.5.
5.2.2 Effect of reaction time on CODcr and NH4-N removal rates
Among the various influencing factors, reaction time is the main operating parameter, and the increase in reaction time is beneficial to CODcr and NH4-N removal rates. For the removal of NH4-N, according to Cheng Jiehong et al.’s research on the SBR method for treating landfill leachate, in this test, when the sludge concentration was tentatively set at 5g/L, the reaction time was changed to test the removal of CODcr and NH4-N. rate, the results are shown in Table 4
The results in Table 4 show that under the conditions of sludge concentration of 5g/L, idle time of 6h, and pH=8.0, the optimal reaction time is 36h and the CODcr removal rate is 89.5 , NH4-N removal rate is 95.2.
5.2.3 Effect of sludge concentration on CODcr and NH4-N removal rate
According to the test results in Table 4, it is determined that the reaction time is 36h, the idle time is 6h, and the pH=8.5 Under the conditions, the sludge concentration was changed to observe the removal rate of CODcr and NH4-N. The sludge concentrations were selected as 3.5g/L, 5.0g/L, 6.5g/L and 8.0g/L as test parameters. The results are shown in Table 5.
As can be seen from Table 5, when the sludge concentration is 8.0g/L, the CODcr removal rate is the highest, and when the sludge concentration is 6.5g/L, the NH4-N removal rate is the highest, which shows that the sludge Although the increase in concentration can improve the CODcr removal rate, the demand for dissolved oxygen increases, and the increase in sludge volume makes the mass transfer of oxygen difficult, which cannot meet the needs of normal growth and metabolism of activated sludge, and the treatment effect will not be achieved. improve.
6 Conclusions
(1) The following operating parameters were obtained through testing using the UASB anaerobic reactor-CASS reactor process:
UASB anaerobic reactor; . The sludge concentration is 7.5g/L, and the residence time is 48H.
CASS reactor: reaction time 36h, idle time 6h, PH=8.0, sludge concentration 6.5g/L.
(2) The data of landfill leachate treated by the above process are shown in Table 6. Under optimal operating conditions, the CODcr and NH4-N of the original landfill leachate dropped from 10000mg/L and 510mg/L to 191.1mg/L and 18.88mg/L respectively. The total CODcr removal rate was 98.1, and the total NH4-N removal rate was 98.1. The rate is 96.3. It shows that this process can better treat landfill leachate from incineration plants.