Characteristics of leachate from garbage
Leachate from garbage refers to the wastewater generated during the stacking and landfill process due to fermentation, precipitation leaching, surface water and groundwater infiltration. The composition of leachate from garbage is influenced by factors such as garbage composition, landfill time, landfill technology, and climatic conditions, among which landfill time is the most important influencing factor. If classified according to the age of the landfill site, generally those with a landfill time of less than 1 year are considered young leachate, those with a landfill time of 1-5 years are considered middle-aged leachate, and those with a landfill time of more than 5 years are considered old leachate [1]. Table 1 shows the characteristics of different types of leachate from garbage [2].
The water quality of garbage generally has the following characteristics: (1) complex composition, containing various organic pollutants, metals, and plant nutrients; (2) The concentration of organic pollutants is high, with COD and BOD reaching tens of thousands of mg/L; (3) There are many types of metals, including more than 10 types of metal ions; (4) High ammonia nitrogen and wide range of variation; (5) The composition and concentration will undergo seasonal changes [2]
At present, the treatment methods for leachate from garbage mainly rely on biological methods. Among them, young leachate has a higher content of easily biodegradable organic matter, a higher B/C ratio, and lower ammonia nitrogen, making it suitable to use biological methods for treatment. However, as the age of the landfill site increases, the biodegradability of the leachate will decrease and the ammonia nitrogen will increase significantly, which will inhibit the effectiveness of biological treatment. Therefore, it is not suitable to directly use biological treatment for middle-aged and elderly leachate. Moreover, biological methods are sensitive to changes in temperature, water quality, and water quantity, and cannot treat difficult to biodegrade organic matter. The physicochemical method has a good removal effect on garbage leachate with poor biodegradability and high ammonia nitrogen content, and is not affected by changes in water quality and quantity. The effluent water quality is relatively stable, and it is widely used for pre-treatment and deep treatment of garbage leachate. On the basis of existing physical and chemical treatment technologies, the author reviewed the research progress of adsorption method, blow off method, coagulation precipitation method, chemical precipitation method, chemical oxidation method, electrochemical method, photocatalytic oxidation method, reverse osmosis and nanofiltration method, in order to provide some reference for practical work
2 Physical and Chemical Processing Technologies
2.1 Adsorption
Adsorption method is to use the adsorption effect of porous solid substances to remove toxic and harmful substances such as organic matter and metal ions in leachate from garbage. Currently, research on activated carbon adsorption is the most extensive. J. Rodr í guez et al. [4] studied the adsorption of anaerobic treated leachate using activated carbon, resin XAD-8, and resin XAD-4. The results showed that activated carbon had the strongest adsorption capacity and could reduce the COD of influent from 1500 mg/L to 191 mg/L. N. Aghamohammadi et al. [5] added powdered activated carbon when using activated sludge method to treat leachate from garbage. The results showed that the removal rates of COD and chromaticity were almost twice as high as those without activated carbon, and the removal rate of ammonia nitrogen was also improved. Zhang Futao et al. [6] studied the adsorption behavior of activated carbon on formaldehyde, phenol, and aniline in landfill leachate, and the results showed that the adsorption isotherm of activated carbon conforms to the Freundlich empirical formula. In addition, adsorbents other than activated carbon have also been studied to some extent. M. Heavey et al. [7] conducted coal slag adsorption experiments using leachate from the Kyletalesha landfill in Ireland. The results showed that after coal slag adsorption treatment, the leachate with an average COD of 625 mg/L, an average BOD of 190 mg/L, and an average ammonia nitrogen of 218 mg/L had a COD removal rate of 69%, a BOD removal rate of 96.6%, and an ammonia nitrogen removal rate of 95.5%. Due to the abundant and renewable coal slag resources, without secondary pollution, it has good development prospects. The main problem faced by activated carbon adsorption treatment is that activated carbon is expensive and lacks simple and effective regeneration methods, which limits its promotion and application. At present, the adsorption method for treating leachate from garbage is mostly laboratory scale and requires further research before it can be applied in practice.
2.2 Blow off method
Blow off method is to introduce gas (carrier gas) into water, and after sufficient contact, the volatile soluble substances in the water are transferred to the gas phase through the gas-liquid interface, thereby achieving the purpose of removing pollutants. Air is commonly used as the carrier gas. The ammonia nitrogen content in the leachate of middle-aged and elderly garbage is relatively high, and the blowing off method can effectively remove the ammonia nitrogen from it. S. K. Marttinen et al. [8] used the blow off method to treat ammonia nitrogen in leachate from garbage. Under the conditions of pH=11, 20 ° C, and hydraulic retention time of 24 hours, the ammonia nitrogen decreased from 150 mg/L to 16 mg/L. Liao Linlin et al. [9] studied the factors affecting the efficiency of liquid ammonia stripping in garbage infiltration, and found that pH, water temperature, and gas-liquid ratio had a significant impact on the stripping efficiency. The denitrification effect was improved when pH was between 10.5 and 11; The higher the water temperature, the better the denitrification effect; When the gas-liquid ratio is 3000~3500 m3/m3, the denitrification effect is as shown in Jay Chou's new song; The concentration of ammonia nitrogen has little effect on the blowing efficiency. Wang Zongping et al. [10] used three methods, namely jet aeration, blast aeration, and surface aeration, to pretreat leachate with ammonia stripping. The results showed that jet aeration was effective at the same power. According to foreign data, the removal rate of ammonia nitrogen in leachate treated with gas extraction combined with other methods can reach as high as 99.5%. However, the operating cost of this method is relatively high, and the generated NH3 needs to be removed by adding acid in the blow off tower, otherwise it will cause air pollution. In addition, carbonate scaling will also occur in the blow off tower.
2.3 Coagulation precipitation method
The coagulation sedimentation method is a method of adding coagulants to the leachate of garbage, causing suspended solids and colloids in the leachate to aggregate and form flocs, and then separating them. Aluminum sulfate, ferrous sulfate, ferric chloride, and other inorganic flocculants are commonly used. Studies have shown that using iron based flocculants alone to treat leachate from garbage can achieve a COD removal rate of 50%, which is better than using aluminum based flocculants alone. A. A. Tatsi et al. [11] pretreated the leachate with aluminum sulfate and ferric chloride. For young leachate, the highest COD removal rate was 38% when the influent COD was 70 900 mg/L; For middle-aged and elderly landfill leachate, the COD removal rate can reach 75% when the influent COD is 5350 mg/L. When the pH is 10 and the coagulant reaches 2 g/L, the COD removal rate can reach as high as 80%. In recent years, bioflocculants have become a new research direction. A. I. Zouboulis et al. [12] studied the treatment effect of bioflocculants on landfill leachate and found that only 20 mg/L of bioflocculants were needed to remove 85% of humic acid from landfill leachate. Coagulation precipitation method is a key technology for the treatment of leachate from garbage. It can be used as a pre-treatment technology to reduce the burden of post-treatment processes, and as a deep treatment technology to become the guarantee of the entire treatment process [3]. But its main problem is the low removal rate of ammonia nitrogen, the generation of a large amount of chemical sludge, and the addition of metal salt coagulants may cause new pollution. Therefore, developing safe, efficient, and low-cost coagulants is the foundation for improving the treatment efficiency of coagulation sedimentation methods.
2.4 Chemical precipitation method
The chemical precipitation method is to add a certain chemical substance to the leachate of garbage, generate a precipitate through chemical reaction, and then separate it to achieve the purpose of treatment. According to data, hydroxide ions of alkaline substances such as calcium hydroxide can precipitate with metal ions, which can remove 90% to 99% of heavy metals in leachate and 20% to 40% of COD. The bird guano stone precipitation method is widely used in chemical precipitation methods. The bird guano stone precipitation method, also known as the ammonium magnesium phosphate precipitation method, involves adding Mg2+, PO43-, and alkaline agents to the leachate of garbage to react with certain substances and form a precipitate. X. Z. Li et al. [13] added MgCl2 · 6H2O and Na2HPO4 · 12H2O to the leachate from garbage. When the ratio of Mg2+to NH4+to PO43- was 1:1:1 and the pH was 8.45-9, the ammonia nitrogen in the original leachate decreased from 5600 mg/L to 110 mg/L within 15 minutes. I. Ozturk et al. [14] used this method to treat leachate from anaerobic digestion. When the influent COD was 4024 mg/L and ammonia nitrogen was 2240 mg/L, the effluent removal rates reached 50% and 85%, respectively. B. Calli et al. [15] also achieved a 98% removal rate of ammonia nitrogen using this method. The chemical precipitation method is simple to operate, and the generated precipitate contains fertilizer components such as N, P, Mg, and organic matter. However, the precipitate may contain toxic and harmful substances, which have potential environmental hazards.
2.6 Electrochemical method
Electrochemical method is a process in which pollutants in leachate from garbage are directly subjected to electrochemical reactions on electrodes under the action of an electric field, or undergo redox reactions using · OH and ClO - generated on the electrode surface. Currently, electrolytic oxidation is commonly used. P. B. Moraes et al. [19] used a continuous electrolytic reactor to treat leachate from garbage. When the influent flow rate was 2000 L/h, the current density was 0.116 A/cm2, the reaction time was 180 min, the influent COD was 1855 mg/L, TOC was 1270 mg/L, and ammonia nitrogen was 1060 mg/L, the effluent removal rates reached 73%, 57%, and 49%, respectively. N. N. Rao et al. [20] used a three-dimensional carbon electrode reactor to treat leachate with high COD (17-18400 mg/L) and high ammonia nitrogen (1200-1320 mg/L). After 6 hours of reaction, the COD removal rate was 76% -80%, and the ammonia nitrogen removal rate could reach up to 97%. E. Turro et al. [21] studied the factors affecting the electrolytic oxidation treatment of landfill leachate, using Ti/IrO2-RuO2 as the electrode and HClO4 as the electrolyte. The results showed that reaction time, reaction temperature, current density, and pH were the main factors affecting the treatment effect. Under the conditions of temperature of 80 ℃, current density of 0.032 A/cm2, and pH=3, the reaction time was 4 hours, and COD decreased from 2960 mg/L to 294 mg/L, TOC decreased from 1150 mg/L to 402 mg/L, and the color removal rate could reach 100%. The electrochemical method has a simple process, strong controllability, small footprint, and does not generate secondary pollution during the treatment process. The disadvantage is that it consumes electricity and has high treatment costs. Currently, most of them are in laboratory research scale.
2.7 Photocatalytic oxidation
Photocatalytic oxidation is a new type of water treatment technology that is better at treating certain special pollutants than other methods, and therefore has good application prospects in the deep treatment of leachate from garbage. The principle of this method is to add a certain amount of catalyst to the wastewater, generate free radicals under the irradiation of light, and use the strong oxidizing property of free radicals to achieve the treatment goal. The catalysts used in photocatalytic oxidation mainly include titanium dioxide, zinc oxide, and iron oxide, among which titanium dioxide is widely used. D. E. Meeroff et al. [22] conducted experiments on photocatalytic oxidation of leachate using TiO2 as a catalyst. After 4 hours of ultraviolet photocatalytic oxidation, the COD removal rate of leachate reached 86%, the B/C ratio increased from 0.09 to 0.14, the ammonia nitrogen removal rate was 71%, and the chromaticity removal rate was 90%; After the reaction is completed, 85% of TiO2 can be recovered. R. Poblete et al. [23] used by-products from the titanium dioxide industry (mainly composed of TiO2 and Fe) as catalysts and compared them with commercial TiO2 in terms of catalyst type, removal rate of recalcitrant organic matter, catalyst loading, and reaction time. The results showed that the by-product had higher activity and better treatment effect, and could be used as a catalyst for photocatalytic oxidation. A study has found that the content of inorganic salts can affect the effectiveness of photocatalytic oxidation in treating leachate from garbage. J. Wiszniowski et al. [24] studied the effect of inorganic salts on the photocatalytic oxidation of humic acid in leachate using suspended TiO2 as a catalyst. When only Cl - (4500 mg/L) and SO42- (7750 mg/L) are present in the leachate of garbage, it does not affect the photocatalytic oxidation efficiency of humic acid, but the presence of HCO3- greatly reduces the photocatalytic oxidation efficiency. Photocatalytic oxidation has the advantages of simple operation, low energy consumption, load resistance, and no pollution. However, in order to put it into practical operation, it is necessary to study the type and design of the reactor, the efficiency and lifespan of the catalyst, and the utilization rate of light energy.
2.8 Reverse osmosis (RO)
RO membrane has selectivity towards solvents, using the pressure difference on both sides of the membrane as a driving force to overcome the osmotic pressure of solvents, thereby separating various substances in leachate from garbage. Fangyue Li et al. [25] used a spiral RO membrane to treat the leachate from the Kolenfeld landfill in Germany. COD decreased from 3100 mg/L to 15 mg/L, chloride decreased from 2850 mg/L to 23.2 mg/L, and ammonia nitrogen decreased from 1000 mg/L to 11.3 mg/L; The removal rates of metal ions such as Al3+, Fe2+, Pb2+, Zn2+, Cu2+, etc. all exceed 99.5%. Research has shown that pH has an impact on the removal efficiency of ammonia nitrogen. L. D. Palma et al. [26] first distilled the leachate from the garbage and then treated it with an RO membrane, reducing the influent COD from 19000 mg/L to 30.5 mg/L; The removal rate of ammonia nitrogen is highest at pH 6.4, decreasing from 217.6 mg/L to 0.71 mg/L. M. R et al. [27] conducted a pilot experiment on purifying leachate from garbage using two-stage continuous RO membranes and found that the removal rate of ammonia nitrogen was highest when the pH reached 5, decreasing from 142 mg/L to 8.54 mg/L. The reverse osmosis method has high efficiency, mature management, and is easy to automatically control, and is increasingly being applied in the treatment of leachate from garbage. However, the membrane cost is relatively high, and pre-treatment of the leachate before use is required to reduce the membrane load, otherwise the membrane is prone to contamination and blockage, resulting in a sharp decrease in treatment efficiency.
2.9 Nanofiltration (NF)
NF membrane has two significant characteristics: it has a microporous structure of about 1 nm, which can intercept molecules with a molecular weight of 200-2000 u; The NF membrane itself is charged and has a certain retention rate for inorganic electrolytes. H. K. Jakopovic et al. [28] compared the removal of organic matter in landfill leachate using three technologies: NF, UF, and ozone. The results showed that under laboratory conditions, different UF membranes could achieve a COD removal rate of 23% for Jay Chou's new song; The removal rate of COD by ozone can reach 56%; The removal rate of Jay Chou's new songs on COD by NF can reach 91%. NF also has a relatively ideal removal effect on ions in leachate. L. B. Chaudhari et al. [29] used NF-300 to treat electrolytes in aged leachate from the Gujarat landfill in India. The sulfate levels in the two experimental waters were 932 and 886 mg/L, respectively, and the chloride ions were 2268 and 5426 mg/L, respectively. The experimental results showed that the removal rates of sulfate were 83% and 85%, respectively, and the removal rates of chloride ions were 62% and 65%, respectively. The study also found that the removal rates of Cr3+, Ni2+, Cu2+, and Cd2+by NF membrane reached 99%, 97%, 97%, and 96%, respectively. NF combined with other processes has better post-treatment effects. T. Robinson [30] used the MBR+NF combined process to treat the leachate from Beacon Hill, UK. COD decreased from 5000 mg/L to below 100 mg/L, ammonia nitrogen decreased from 2000 mg/L to below 1 mg/L, and SS decreased from 250 mg/L to below 25 mg/L. NF technology has low energy consumption, high recovery rate, and great potential. But the biggest problem is that the membrane will scale after long-term use, which will affect its performance such as membrane flux and retention rate. Further research is needed to apply it to engineering practice.
3 Conclusion
The above-mentioned physical and chemical treatment technologies can achieve certain results, but there are also many problems, such as the regeneration of adsorbents, the recovery of photocatalytic oxidation catalysts, the high energy consumption of electrochemical methods, and membrane fouling. Therefore, it is difficult for leachate from garbage to meet the national emission standards through a single physical and chemical treatment, and its treatment process should be a combination of multiple treatment technologies. The complete treatment process of general garbage leachate should include three parts: pretreatment, main treatment, and deep treatment. Pre treatment methods such as blow off, coagulation precipitation, and chemical precipitation are commonly used to remove heavy metal ions, ammonia nitrogen, chromaticity, or improve the biodegradability of leachate from garbage. The main treatment should adopt low-cost and high-efficiency processes, such as biological methods, chemical oxidation and other combined processes, with the aim of removing most of the organic matter and further reducing the content of pollutants such as ammonia nitrogen. After the first two stages of treatment, certain pollutants may still exist, so deep treatment is necessary, which can be achieved through methods such as photocatalytic oxidation, adsorption, membrane separation, etc.
Due to the complex composition of leachate and its variability over time and location, in practical engineering, it is necessary to first measure the composition and analyze its characteristics in detail before treating leachate, and select appropriate treatment techniques. At present, the treatment technologies for leachate from garbage have their own advantages and disadvantages. Therefore, upgrading and transforming existing technologies, developing new and efficient treatment technologies, and strengthening the integration research and development between different technologies (such as the integration of photocatalytic oxidation technology and biochemical treatment technology, the integration of precipitation method and membrane treatment), in order to improve the overall treatment efficiency of leachate and reduce investment and operating costs, will be the focus of future research on leachate from garbage.
