Pay more attention to the public account: environmental protection water treatment
Landfill leachate has always been the focus and difficulty in the field of sewage treatment in mainland China due to its large amount of organic matter and ammonia nitrogen. The water quality characteristics of leachate were summarized, and then the advantages and disadvantages of different landfill leachate treatment processes were discussed in combination with the research on landfill leachate treatment at home and abroad. At present, leachate treatment technologies mainly include physicochemical method and biochemical method, and the use of biochemical method to achieve deep denitrification of landfill leachate and reduce the treatment cost of landfill leachate is the development direction of landfill leachate treatment technology in the future.
With the continuous development of urbanization in the mainland, the daily output of municipal solid waste has increased sharply, and in 2013, the total output of domestic waste in the mainland has reached 173×105t, of which more than 80% of the waste is disposed of by landfill.
The landfill leachate produced by the landfill is a kind of high-concentration wastewater with high organic pollutant content, complex nature and difficult to treat, and the leachate contains a large number of refractory organic matter, heavy metal ions, high ammonia nitrogen and a variety of toxic and harmful pollutants, which will have long-term potential harm to the environment, animals, plants and humans. GB16889-2008 "Domestic Waste Landfill Control Standards" has stricter discharge standards for landfill leachate, and seeking and developing a leachate collection and efficient treatment process has become an urgent water treatment problem in mainland China.
1. Source and characteristics of landfill leachate
1.1 Landfill leachate sources
Landfill leachate refers to the secondary pollution caused by fermentation and leaching and erosion of rainwater and the soaking of surface water and groundwater in the process of stacking and landfilling.
The leachate is derived from natural rainfall, snowfall, runoff (the main source), water content of the waste itself, groundwater infiltration, and anaerobic decomposition of microbial water.
1.2 Characteristics of landfill leachate
Water quality characteristics of landfill leachate: 1) The water quality changes greatly, and the leachate composition is different with different landfill plant years (as shown in Table 1); 2) The concentration of organic matter is high and there are many components, mostly light, dark brown or black, and there is a very heavy garbage rotten odor; 3) High content of heavy metals; 4) The concentration of ammonia nitrogen can reach more than 3000mg/L; 5) the proportion of nutritional factors was adjusted, and P was deficient; 6) Biochemical treatment will produce a lot of foam.
From Table 1, it can be seen that the water quality of the early leachate is characterized by high organic matter content and strong biodegradability, but the ammonia nitrogen concentration is relatively low. The water quality of late-stage leachate is characterized by high ammonia nitrogen content, poor biodegradability and a significant decrease in C/N, and the water quality of medium-stage leachate is between early and late leachate.
Characteristics of leachate at different times of landfill
2. Chemical treatment technology
Physicochemical treatment mainly includes adsorption method, coagulation and precipitation method, advanced oxidation method, membrane separation technology, ammonia blowing method, etc.
2.1 Adsorption method
In 1995, granular activated carbon was used for the first time in Germany to treat advanced landfill leachate, and the COD removal rate reached 91%; Modin studied the comparison of the removal efficiency of heavy metals in landfill leachate by granular activated carbon, bone meal and iron powder, and found that the removal rate of heavy metals such as Co and Cr by activated carbon was more than 90%. The adsorption method is simple and convenient to operate, and the treatment effect is stable, but the treatment cost is high and the regeneration of adsorbent is difficult.
2.2 Coagulation and sedimentation method
Amokrane et al. treated the leachate with ρ(COD) and ρ(NH3-N) of 4100 and 5690 mg/L respectively by coagulation method, and found that Fe salt had a higher organic matter removal rate than Al salt. The precipitation method can select different coagulants according to the different removal of ammonia nitrogen and heavy metal ions. Ye Biao et al. treated the leachate with influent ρ(NH3-N) of 800-1100mg/L by magnesium ammonium phosphate method, and the optimal removal rate of NH3-N could reach 90%.
Coagulation and sedimentation is widely used as a pretreatment before biological or reverse osmosis processes or as an advanced treatment for the removal of refractory biodegradable organic matter, but this process has the following disadvantages: large sludge production, possible presence of aluminum or iron in the liquid phase, high consumption of precipitant, sensitive pH value, and further treatment of sludge.
2.3 Advanced oxidation technology
2.3.1 Advanced oxidation single-stage technology
According to the different ways of generating free radicals and reaction conditions, advanced oxidation technologies can be divided into Fenton technology, ozone oxidation technology, electrochemical oxidation method, etc.
Introduction to Advanced Oxidation Technology
As can be seen from Table 2, the removal rates of COD by Fenton method, ozone oxidation method and electrochemical oxidation method are all higher than 50%. The treatment effect of the Fenton method is higher than that of other methods, but the actual treatment process requires a variety of structures and produces secondary pollution. Ozone oxidation and electrochemical oxidation have no secondary pollution, but the equipment cost is high, the consumption is large, and the maintenance work is complicated.
2.3.2 Advanced oxidation combination technology
Asaithambi et al. used the "ozone + ultrasound + Fenton" process to treat the leachate, and the COD removal rate could reach 95%. Yan et al. [17] used ultrasonic-enhanced TiO2 photocatalytic technology to treat the influent water (COD) and ρ (NH3-N) of 2646 and 1330 mg/L, respectively, and the optimal removal rates of COD and NH3-N were 50.1% and 75%, respectively.
Advanced oxidation technology has the advantages of fast reaction speed, thorough degradation of organic matter, pollution-free, and wide application range of water quality, which can greatly improve the biodegradability of filtrate effluent and reduce leachate toxicity. However, advanced oxidation technology is not only expensive to invest in, but also has a high demand for electrical energy, so the treatment cost is high.
2.4 Membrane separation technology
2.4.1 Membrane separation technology method for the treatment of landfill leachate
Chaudhari et al. used nanofiltration membranes to remove metal ions such as Cr3+ and Ni2+, and the removal rate was more than 90%. Trebouet et al. used nanofiltration membranes to treat leachate, and the removal rate of COD could reach more than 74%. RO membrane has a good removal effect on heavy metals, suspended colloidal substances and dissolved solids in leachate. Linde et al. used RO membrane to treat the leachate, and the removal rate was more than 98%.
The combined treatment of leachate by reverse osmosis and biological treatment ensures optimal treatment results. Ahn et al. used RO membrane to further treat the bioprocess effluent, and the COD removal rate was about 97%. The biggest advantage of membrane separation technology is the stable quality of the effluent, but the disadvantages are membrane fouling, high investment and operating costs, and concentrate treatment problems. At present, the membrane process is usually used for advanced treatment to remove macromolecular refractory organic matter and total nitrogen in the leachate to ensure the quality of the effluent.
2.4.2 Treatment of membrane technology concentrates
Treatment of leachate with NF and RO results in a concentrate with very high concentrations of contaminants. At present, the concentrate is mainly treated by advanced oxidation method. Zheng et al. used ozone to treat the concentrated solution with ρ (COD) of the influent water ρ (COD), and the removal rate reached 67.6%. Yang Zhenning compared the treatment effects of UV-Fenton, Fenton and ozone methods on the concentrate with 4114mg/L influent ρ(COD), and the COD removal rates of UV-Fenton, Fenton and O3 processes were 72%, 60% and 68%, respectively.
2.5 Ammonia blowing technology
Wu Fangtong used packing tower to blow off the leachate with ammonia nitrogen content of 1500-2500mg/L, and the optimal blowing rate of ammonia nitrogen was more than 95%. As a pretreatment process, ammonia blow-off can reduce the inhibition of ammonia nitrogen on the biological treatment process and improve the biodegradability of leachate, but the disadvantages are high pH value, tail gas treatment, scaling of the blowing tower and foaming.
3. Biological treatment technology
3.1 Anaerobic biological treatment technology
Anaerobic biological treatment technologies mainly include UASB, ASBR, etc. UASB has a high processing efficiency and a short hydraulic retention time, and can exhibit better performance when reaching high volumetric organic loading rate values. Agdag et al. used UASB to treat the leachate, the HRT was 1.25d, the influent ρ (COD) increased from 5400mg/L to 20000mg/L, and the removal rate of COD could reach 96%-98%.
In addition to the typical characteristics of SBR, ASBR also has the advantages of less influence by temperature, wide adaptation range, good sludge sedimentation performance and high activity, and is more suitable for the change of leachate water quality and quantity. Gao Feng et al. used ASBR as an anaerobic digestion reactor, and the influent ρ (COD) was 6000-8000mg/L, and the COD removal rate of ASBR was maintained at about 41.2%.
Wang et al. treated the early leachate with ASBR, and the removal rate of COD could reach more than 80%. Anaerobic biotechnology has the advantages of low energy consumption, simple operation, low investment and operation costs, less sludge production and required nutrients, but its biggest disadvantage is that it cannot remove ammonia nitrogen and the effluent COD is high, and the effluent cannot be discharged up to the standard, so it is generally used as a pretreatment process for aerobic biological treatment.
3.2 Aerobic biological treatment
Aerobic biotechnology is the main body of leachate treatment at present, with SBR, MBR and other technologies. Wei et al. studied the difference between alternating intermittent aeration and traditional continuous aeration and SBR on the removal of leachate organic matter, and the COD removal rate was more than 80% under both operation modes. Zaloum et al. used SBR to remove COD from the anaerobic reaction of leachate up to 91%.
Niu Ruisheng et al. used the MBR reaction as the core process to treat leachate, and when the MBR inlet ρ(COD) and ρ(NH3-N) were 4700 and 600 mg/L, respectively, the effluent ρ(COD) and ρ(NH3-N) were 380 and 28 mg/L, respectively. Aerobic biological method is the core process of leachate treatment, which has the characteristics of low energy consumption, low secondary pollution and recyclability, although the method has about 80% and 90% removal rates of leachate COD and ammonia nitrogen, but the effluent still needs further advanced treatment.
3.3 Anaerobic-aerobic biological treatment
Chen et al. studied that the final effluent ρ(NH+4-N) was less than 10mg/L, and the removal rates of COD and NH+4-N in the system were >97% and 92%-95%, respectively. Wang Shuying et al. treated the leachate by ASBR and SBR processes, and the removal rate was more than 83% when the ASBR influent ρ(COD) was 7338-10445mg/L. When the SBR influent ρ(NH+4-N) is about 912.0 mg/L, the total nitrogen removal rate is more than 90%, and the total nitrogen in the effluent is less than 40 mg/L.
Anaerobic biological methods are effective for the treatment of high-concentration organic wastewater, but the effluent COD and removal rate are determined by the quality of the leachate. In general, the effluent COD by anaerobic biological method alone cannot achieve the standard discharge, and further treatment is required. The aerobic biological method can remove ammonia nitrogen in the leachate, but the energy consumption is higher than that of the anaerobic biological method, so the combined anaerobic-aerobic treatment process can not only reduce the organic matter and nitrogen-containing substances in the leachate at the same time, but also save energy costs.
3.4 New landfill leachate biological denitrification technology
3.4.1 Short-range nitrification and denitrification
Short-range nitrification and denitrification is an effective way to control nitrification in the nitrite nitrogen generation stage, which can reduce energy consumption and save carbon sources, and is an effective way to improve the efficiency of leachate biological treatment. When the NH+4-N load of the system is less than 0.45kg/(m3 ̇d), the removal rate of NH+4-N is greater than 98%, and the short-range nitrification rate of 90%-99% can be obtained, and the effluent ρ(NH+4-N) <15mg/L, and the TN removal rate is 70%-80%, which realizes stable short-range nitrification denitrification.
At the end of nitrification, the effluent ρ(NO-3-N) of the A/O reactor was stable at about 57mg/L, ρ(NO-2-N) was about 162mg/L, and the nitrous nitrogen accumulation rate was 74%, which achieved a relatively obvious short-range nitrification reaction.
3.4.2 Endogenous denitrification
Endogenous denitrification is a reaction in which denitrifying bacteria convert the organic matter in the leachate into a stored internal carbon source such as PHA, and use the stored internal carbon source to denitrify under anaerobic conditions. Endogenous denitrification technology can improve the nitrogen removal effect of the process, reduce energy consumption and achieve sludge emission reduction.
Wang et al. used improved SBR to treat leachate, and under the condition of not adding any organic carbon source, the effluent ρ(TN) of the system was < 40mg/L, and the denitrification rate reached more than 95%, and the effect of stored internal carbon source on the endogenous denitrification rate was explored, and it was found that the organic matter adsorption time, aeration amount and aeration time could determine the endogenous denitrification rate.
3.4.3 Anaerobic ammonia oxidation
Anaerobic ammonia oxidation is a new denitrification technology discovered by Delft University in the Netherlands, which is characterized by low energy consumption and no additional carbon source. Zhang Fangzhai et al. used the CANON process to treat the late leachate with influent ρ(NH+4-N) and ρ(TN) of 1625±75 and 352mg/L ±2005, respectively, and the TN removal rate reached 98.76%. Miao et al. used a three-stage SBR process of carbon removal, short-path nitrification and anaerobic ammonia oxidation to treat the late-stage leachate with an influent ρ (ammonia nitrogen) of 2000mg/L, and the TN removal rate could reach more than 90%.
Short-range nitrification denitrification, endogenous denitrification and anaerobic ammonia oxidation are all explorations of landfill leachate treatment, and the rational use of this process is bound to greatly improve the total nitrogen removal efficiency of leachate.
4 Conclusion
Landfill leachate treatment is the focus of the water treatment field, and the three key challenges of landfill leachate treatment are cost, efficiency and quality. Membrane technology can effectively ensure the quality of effluent, but the investment and operation costs are high, and the concentrate produced by membrane separation technology needs to be further treated. The biochemical method is the main body of landfill leachate at present, but the removal rate of total nitrogen in the leachate is not high due to the limitation of the process.
Improving the denitrification efficiency of biochemical methods through new denitrification technology and reducing the grade of subsequent membrane processes to reduce the cost of leachate treatment are the next development directions of leachate treatment.