A case of leachate treatment of landfill in a city

2022-08-24

　　Project overview

　　The city is a subtropical monsoon humid climate area, with an average annual rainfall of 998.4mm, evaporation 1000mm, wind speed 2.8m/s, population 1.35 million, daily waste treatment 1200t, now produces about 600m3/d landfill leachate, leachate water quality of about 10000mg/L CODCr, 1500mg/L ammonia nitrogen, BOD5 about 7000mg/L,

　　The quality and quantity of landfill leachate vary greatly, especially the range of BOD/COD。The landfill leachate treatment plant has been built for 3 years，In practice, the biodegradability of leachate wastewater was found to be good in general，The concentration of ammonia nitrogen increased year by year，处理后排放水质标准为：CODCr<300mg/L，BOD5<150mg/L，NH3—N<25mg/L，SS<200mg/L。The main control indexes of effluent are CODCr and NH3-N。

　　In order to meet the treatment requirements, the leachate treatment process adopts a multi-stage treatment scheme combining biochemical treatment and physicochemical treatment. The process flow is as follows: regulation tank - UASB - FEO reactor - ammonia blowoff - CASS reaction tank - flocculation float - disinfection - oxidation pond - effluent。

　　Process flow and related processing units

　　The main treatment facilities and equipment of this project include: leachate regulation tank lifting pump house，Regulating pool (2 jointly built，分2格)，UASB reactor (2 units jointly built，分2格)，Primary sedimentation tank (1 unit)，Ammonia stripping tower (2)，Absorption tower (2)，CASS reaction tank (2 jointly built，分4格)，FEO Reactor (3 sets)，Condensate float tank (2 sets)，Sludge thickening tank，Lime dosing room，Air room (1 block)，Pump house (combined with reaction sedimentation tank，1座)，Accident emptying pool，Monitoring building。

　　UASB reactor

　　The UASB reactor works well in practice，Can fully meet the design requirements，It has the following characteristics: ① the sludge concentration in the reactor is high，The treatment effect is stable and efficient，Impact load resistance，Especially suitable for the treatment of high concentration organic wastewater;② The yield of residual sludge is low，No need to drain mud frequently，Simple operation and management，At the same time, it has a long sludge age;③ Can be processed at room temperature，No special heating equipment is required，Especially when the reactor is built underground or semi-underground, the treatment effect is better;Generally has a high volume load and gas production rate。

　　FEO reactor

　　FEO is an electrochemical treatment technology that uses many local microcells formed between iron and other metal crystal structures and carbon in electrolyte solution to treat organic wastewater，It is in the condition of no external electric energy，Use the potential difference between metal - metal, non-metal - non-metal to produce the role of countless tiny batteries，The pollutants in wastewater can be purified by electrochemical oxidation-reduction reaction, condensation, air flotation and sedimentation。

　　Landfill leachate and FEO reactor packing (mainly composed of Fe, Al, C, Mn, Zn and other 20 kinds of substances evenly mixed in a certain proportion) after contact with the corresponding time，A catalytic REDOX reaction occurs，The molecular structure of refractory and non-biochemical substances in wastewater was changed，The heterocyclic and heterocyclic chains are opened，To form small molecules that are biodegradable，At the same time, Fe2+ and other ions are produced during the reaction，The interaction has strong adsorption and flocculation activity，It can absorb a large number of tiny ions and polymer substances dispersed in wastewater，The flocculant can be removed by precipitation or air flotation。The FEO reactor has a certain removal effect on CODCr and NH3-N, and can degrade a considerable amount of non-biodegradable substances。

　　Ammonia stripping absorber

　　Ammonia blowing method is adopted for physical and chemical nitrogen removal, and its parameter setting is relatively mature. The pH of leachate is adjusted to 10 ~ 10 by hydrated lime in the reaction tank first.5, the sewage is pumped to the packing tower, and the centrifugal fan supplies air from the bottom of the tower to maintain the water-air ratio (1500 ~ 2000) : 1;Countercurrent contact, the NH3 free in water to the air, the higher the pH, the higher the blowing rate of NH3, the blowing effect can be adjusted by circulating water volume and pH。It is found that the efficiency of the ammonia stripping tower is very low in the actual operation, and often can not reach the designed removal rate。The pH of the effluent is adjusted from sulfuric acid to the requirements of the next stage of aerobic biochemical treatment, and the NH3 discharged must be equipped with an absorption device, and the absorption solution can be absorbed by 20% ~ 25% dilute sulfuric acid。

　　CASS tank

　　The selection of aerobic biochemical treatment process should focus on the strong nitrogen removal effect。目前，In practical application, the treatment effect of activated sludge method is quite stable，CASS technology is used in this system，A biological selection area is provided at the inlet of the reactor，And about 20% of the sludge reflux was carried out，It is beneficial to improve the activity of sludge，The soluble and easily degradable matrix can be quickly removed，Further effectively inhibit the proliferation of filamentous bacteria，The amount of denitrification also increased。The main reaction zone is the aerobic zone, where the organic matter is fully oxidized and decomposed。The CASS process is highly resistant to impact load and can be flexibly adjusted to cope with different situations. The aeration reaction precipitation process is carried out in one tank to prevent sludge swelling and improve the nitrogen removal effect。And when the treatment effect of single tank can not meet the requirements, the multi-stage CASS tank series system can be considered。

　　Coagulation sedimentation tank

　　The effluent after biochemical treatment needs a physical and chemical treatment device to remove the refractory and non-biodegradable organic matter. At present, taking into account the reduction of operating costs, a relatively simple coagulation and precipitation method can be used for treatment。It is estimated that the residual sludge produced in the leachate treatment process is about 30m3/d, which is concentrated by sludge concentration tank and transported to landfill, and the yield of residual sludge in actual operation is much lower than the estimated value。After the leachate treatment, the effluent is discharged to the naturally formed and artificially modified oxidation pond。

　　The actual operation parameters of the project

　　Hefei Longquanshan domestic waste leachate treatment plant was built and put into operation in early 2005。At the initial stage of operation, the water quality and water quantity did not reach the design load, and the treatment effect of each unit was satisfactory, and the effluent basically met the design requirements。With the extension of operation time, the water quality and water quantity reached and exceeded the design load, and the related problems of each treatment unit were gradually exposed, so in the middle of 2008, the treatment effect of each treatment unit during the actual operation of Longquanshan domestic garbage leachate treatment plant was monitored throughout the whole process。

　　The main problems in operation

　　During the operation of the project, the treatment efficiency of the ammonia blow-off absorber is too low, and the ammonia removal rate is only about 20%, which is far from the designed 80%。This also reflects the present situation of physical and chemical nitrogen removal in our country, that is, the construction and operation costs are high, and the operation and maintenance are difficult。This has been proved many times in the treatment of industrial wastewater with high ammonia nitrogen such as chemical industry and fertilizer，Even for the relatively mature ammonia blow-off absorption tower process, the nitrogen removal efficiency will be extremely low due to the reasons of the equipment itself, such as the efficiency and improper operation of the centrifugal fan，In particular, the amount of circulating water is insufficient and the amount of lime is mainly insufficient，Making the inlet pH of the ammonia stripping tower too low (about 9)，Lower than the design value (10 ~ 10.5), which seriously affects the removal rate of ammonia nitrogen, making the subsequent aerobic treatment unit run in a long-term environment with high ammonia nitrogen (500-1000mg /L)。However, it is precisely for this reason that biological nitrification provides a rare research opportunity for the treatment of high ammonia nitrogen wastewater。In the past, it was generally believed that the biological nitrification method was not suitable for the treatment of high ammonia nitrogen wastewater, but the actual operation of this project proved that the biological nitrification method was feasible for the treatment of high ammonia nitrogen wastewater, and the specific removal effect was shown in Table 3。

　　Study on the treatment of wastewater with high ammonia nitrogen by biological nitrification

　　(1) In the nitrification process of CASS pond，Ammonia is converted to nitrite and nitrate by autotrophic microorganisms，These aerobic autotrophs have very high environmental requirements，The conversion process is determined by the growth conditions in the treatment unit，The main influencing factors are dissolved oxygen, temperature, carbonate content and pH, SRT, activated sludge concentration and performance, toxic and inhibiting substance content (heavy metals, NH3-N, NO-X-N, etc.).。There is a relationship between heterotrophic microorganisms and autotrophic microorganisms growing in CASS pond, and appropriate growth conditions should be selected to regulate the removal rates of CODCr and NH3-N。Since the removal of organic matter in CASS tank is an inevitable factor, it is only possible to remove organic matter without removing ammonia nitrogen, and it is impossible to remove only ammonia nitrogen without removing organic matter。In addition, the removal rate of NH3-N in this treatment unit is the control factor, and priority should be given to meeting the conditions suitable for biological nitrification。

　　During the operation study, DO in CASS pool was generally kept above 2mg/L, the temperature was greater than 20℃, SRT was more than 60 days, and the heavy metal content in the influent water was very low, which had little effect on microorganisms. All these factors met the conditions for biological nitrification。

　　However, the effects of activated sludge concentration and performance, carbonate content, pH, NH3-N, NO-X-N concentration on nitrification process are complicated, so it is necessary to conduct comprehensive analysis and monitoring of these influencing factors。

　　An important feature of CASS system in the treatment of low and medium concentration ammonia nitrogen wastewater is that nitrification and denitrification are carried out at the same time in the aeration stage. Controlling the concentration of dissolved oxygen during operation can make the activated sludge external aerobic and internal anaerobic, and the external aerobic nitrification of the floc internal denitrification to achieve complete nitrogen removal。However, in the treatment of high ammonia nitrogen wastewater to meet the treatment requirements, it is necessary to consider the use of multi-tank series operation，First, the ammonia nitrogen in the wastewater was reduced to less than 200mg/L, and then the subsequent denitrification treatment was carried out，Based on the principles of biological reaction dynamics，Multistage CASS pool is run in series，The flow of wastewater in the pool presents a whole push flow and a completely mixed complex flow state in different areas，Maintain a stable treatment effect，Increase capacity utilization。In the first CASS tank, emphasis should be placed on biological nitrification to increase ammonia nitrogen conversion as much as possible, which requires a long hydraulic retention time and biosolid retention time to ensure a high number of nitrifying bacteria in the tank。According to the actual situation, the load rate of N/MLSS should be less than 0 as far as possible.3. If the nitrogen removal effect is higher than this value, the MLSS should be increased as much as possible when the influent ammonia nitrogen is certain, so that it cannot be lower than 3000mg/L。在CASS tank实际运行中可采取极高的MLSS(>10000mg/L)进行生化处理运行依旧平稳，氨氮去除率达80%左右，CASS tank内活性污泥的监测情况见表4。

　　(2) The increase of activated sludge concentration is achieved by improving the sludge settling performance，SVI should be less than 50mL/g，The SVI of the processing unit of this system is generally only about 25mL/g，Such good settling performance is due to the iron salt produced in the FEO reactor and the addition of coagulants PAC, PAM, etc., which improve the flocculation effect of the sludge。

　　(3) In addition to the high activated sludge concentration, the amount of carbonate and pH also have a great impact on biological nitrification nitrogen removal。The amount of carbonate required for the complete oxidation of ammonia nitrogen is about 7.1mg carbonate /mg ammonia nitrogen, and there must be sufficient alkalinity to prevent acidity in the reaction process, the nitrification process is difficult to proceed smoothly when there is insufficient carbonate, unless there is sufficient inorganic carbon (unlikely to be achieved in operation), the actual operation of the system found that the amount of lime is insufficient (less than 4.2t/d) will directly affect the removal rate of ammonia nitrogen。

　　(4) The effect of pH on nitrification The existing opinions are somewhat different, and to control the pH of most of the treatment areas in CASS pond within this range, the influent pH of CASS pond needs to be controlled at 8.4～8.9. Because more strong acid (HNO3) is generated in the process of treating wastewater with high ammonia nitrogen, the pH value of CASS pond decreases greatly. In general, the influent pH of CASS pond is 8.Above 5, the effluent pH will be below 7, which can ensure the treatment effect of most areas in the CASS tank。This system monitors the pH of each processing unit, and the influent pH is 7.5～7.At 0900 hours, the UASB is flooded with water pH7.5～7.9，出水7.6～7.9, ammonia blow-off absorption tower water pH9 ~ 10.5，出水8.6～9.5. Water inlet pH8 in CASS pool.4～9，出水6.5～7.1。This reflects that after more than three years of operation of the landfill, the pH of the leachate produced by the landfill has changed to a weak alkaline。At present, the pH of leachate shows a relatively stable law, and the pH of leachate gradually changes from weakly acidic to weakly alkaline with the increase of time。

　　In actual operation, it is found that the nitrogen removal efficiency of ammonia blow-off absorber is lower than that of pH 10 when the influent pH is 9.Therefore, the insufficient amount of lime is also an important reason for the low nitrogen removal efficiency of the ammonia blow-off absorption tower。The influent pH of CASS pool is 8.At 6, the effluent pH is generally about 6.7. In this state, the removal rate of NH3-N can reach about 80%, and the effluent NH3-N can be basically controlled below 200mg/L, and it only needs to be subjected to corresponding subsequent biochemical treatment。