The properties of landfill leachate change with the operating age of the landfill, which is primarily determined by the stabilization process of the waste within the landfill. The stabilization process of a landfill is generally divided into five stages: the initial adjustment phase, the transition phase, the acid phase, the methane fermentation phase, and the maturation phase.

In addition, this technology has been widely applied and practiced in related fields.

3. Acid phase: When hydrogen (H2) is continuously generated in the landfill, it indicates that landfill stabilization has entered the acid phase. At this stage, the microorganisms primarily responsible for waste degradation are facultative and obligate anaerobic bacteria. The main component of landfill gas is carbon dioxide (CO2). The concentrations of leachate COD, VFA and metal ions continue to rise, reaching a maximum in the middle of this phase and then gradually declining; pH continues to fall to its lowest value and then gradually rises.

At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.

The treatment of leachate from municipal solid waste landfills has always been a highly challenging issue in landfill design, operation and management. Leachate is the product of liquid flowing through the landfill under gravity, originating mainly from precipitation and the intrinsic moisture of the waste itself. Because many factors may affect the properties of the leachate as the liquid percolates - including physical, chemical and biological factors - leachate characteristics vary over a considerable range. In general, its pH ranges from 4 to 9, COD from 2,000 to 62,000 mg/L, BOD5 from 60 to 45,000 mg/L, while heavy metal concentrations are broadly consistent with those in municipal sewage. Municipal landfill leachate is a high-strength organic wastewater with a complex composition. If discharged directly into the environment without treatment, it will cause serious environmental pollution. For the purpose of environmental protection, treatment of leachate is indispensable.

Treatment methods for landfill leachate include physicochemical and biological processes. Physicochemical methods mainly include activated carbon adsorption, chemical precipitation, density separation, chemical oxidation, chemical reduction, ion exchange, membrane dialysis, air stripping and wet oxidation. At COD levels of 2,000-4,000 mg/L, physicochemical methods can achieve COD removal rates of 50%-87%. Compared with biological treatment, physicochemical treatment is not affected by fluctuations in water quality or flow rate, and the effluent quality is relatively stable. It is particularly effective for leachate with a low BOD5/COD ratio (0.07-0.20) that is difficult to treat biologically. However, physicochemical methods have higher treatment costs and are not suitable for large-volume leachate treatment, so biological methods remain the mainstream approach for landfill leachate.

At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.

Many researchers have also found that activated sludge can remove 99% of the BOD5 in leachate, and more than 80% of the organic carbon can be removed by activated sludge. Even when influent organic carbon reaches 1,000 mg/L, the sludge biota can quickly adapt and perform degradation. An activated sludge system operating at low loading can remove 80%-90% of the COD in leachate, with effluent BOD5 below 20 mg/L. For leachate with COD of 4,000-13,000 mg/L, BOD5 of 1,600-11,000 mg/L and NH3-N of 87-590 mg/L, the completely mixed aerobic activated sludge process can maintain COD removal above 90%. Numerous full-scale leachate treatment systems in operation demonstrate that the activated sludge process performs better than chemical oxidation and other alternatives.

Modified activated sludge processes such as the low-oxygen/aerobic activated sludge process and the SBR process are more effective than conventional activated sludge because they can sustain higher operating loads and require less time. Xu Dimin et al. of Tongji University treated landfill leachate using the low-oxygen/aerobic activated sludge process, and the tests demonstrated excellent results under controlled operating conditions. The average effluent CODCr, BOD5 and SS were reduced from 6,466 mg/L, 3,502 mg/L and 239.6 mg/L to CODCr < 300 mg/L, BOD5 < 50 mg/L (average 13.3 mg/L) and SS < 100 mg/L (average 27.8 mg/L) respectively, with overall removal rates of 96.4% for CODCr, 99.6% for BOD5 and 83.4% for SS.

The two-stage process also achieves better nitrogen and phosphorus removal from leachate than conventional biological methods, with average removal rates of 90.5% for phosphorus and 67.5% for nitrogen. In addition, this process compensates for the shortcomings of the anaerobic-aerobic two-stage biological treatment, in which excessive NH3-N formed in the first stage hinders the second stage and the two aerobic treatment steps take too long.

In addition, this technology has been widely applied and practiced in related fields.

For example, at Bryn Posteg Landfill in the UK, GBP 60,000 was invested to build a 1,000 m3 aerated lagoon equipped with two surface aerators, with a minimum hydraulic retention time of 10 days. After settling, the lagoon effluent flows through a 3 km pipeline into the municipal sewer. The system began operation in 1983. Maximum leachate CODCr was 24,000 mg/L and maximum BOD5 was 10,000 mg/L, with F/M = 0.05-0.3 kgCOD/(kgMLSS·d) and flow varying from 0 to 150 m3/d. Average effluent BOD5 was 24 mg/L, occasionally exceeding 50 mg/L, and COD removal reached 97%. Phosphorus dosing was required during operation. Taking into account daily operating costs, capital repayment and interest, the system saves GBP 750 per year compared with discharging leachate directly to the municipal network.

The UK Water Research Centre also conducted a pilot study of aerated stabilization ponds on leachate with CODCr > 15,000 mg/L at New Park Landfill in southeast England. At loadings of 0.28-0.32 kgCOD/(kgMLSS·d) (or 0.04-0.64 kgCOD/(kgMLSS·d)) and a sludge age of 10 days, COD and BOD5 removal rates reached above 98% and 91% respectively. Phosphoric acid dosing was also required during operation.

Compared with the activated sludge process, the biofilm process offers better resistance to shock loads in both flow and water quality, and the biofilm can support microorganisms with longer generation times, such as nitrifying bacteria. C. Peddie and J. Atwater of the University of British Columbia in Canada used a 0.9 m diameter rotating biological contactor to treat weak leachate with CODCr < 1,000 mg/L and NH3-N < 50 mg/L, achieving effluent BOD5 < 25 mg/L; as the temperature rose again, the nitrification capacity of the microorganisms recovered. It should be noted, however, that the properties of this leachate are close to those of municipal sewage, and whether the method is applicable to stronger leachate requires further study.

The purposeful application of anaerobic biological treatment has a history of nearly a century. However, only over the past two decades, with the development of microbiology and biochemistry and the accumulation of engineering practice, have new anaerobic processes been continuously developed that overcome the long hydraulic retention times and low organic loading of traditional processes. This has brought major progress in both theory and practice, achieving good results in treating high-strength (BOD5 >= 2,000 mg/L) organic wastewater.

Anaerobic biological treatment has many advantages, the most important being low energy consumption and simple operation, resulting in low capital and operating costs. Because less excess sludge is produced, fewer nutrients are required - for instance, a BOD5/P ratio of only 4,000:1 is needed. Although the phosphorus content of leachate is usually less than 1 mg/L, this still satisfies the microbial phosphorus requirement. With conventional anaerobic digestion at 35 degrees C, a loading of 1 kgCOD/(m3·d) and a retention time of 10 days, COD removal from leachate can reach 90%.

On this basis, industry experts have also carried out extensive research and improvements.

Anaerobic filters are suitable for treating soluble organics. Leachate at the Halifax Highway 101 landfill in Canada had an average COD of 12,850 mg/L, a BOD5/COD ratio of 0.7 and a pH of 5.6. The leachate was first adjusted to pH 7.8 with lime water and settled for 1 hour before entering the anaerobic filter (this step also removes heavy metals such as Zn). At a loading of 4 kgCOD/(m3·d), COD removal exceeded 92%; when the loading was further increased, removal efficiency dropped sharply.

J. G. Henry et al. of the University of Toronto in Canada also successfully used anaerobic filters at room temperature to treat leachate from landfills aged 1.5 years and 8 years, with COD of 14,000 mg/L and 4,000 mg/L and BOD5/COD ratios of 0.7 and 0.5 respectively. At loadings of 1.26-1.45 kgCOD/(m3·d) and hydraulic retention times of 24-96 hours, COD removal exceeded 90% in both cases. Again, removal dropped sharply when loading was further increased. This shows that although anaerobic filters can handle loadings of 5-20 kgCOD/(m3·d) when treating high-strength organic wastewater, loadings must be kept low for leachate in order to achieve satisfactory treatment.

The UK Water Research Centre reported using an upflow anaerobic sludge blanket (UASB) reactor to treat leachate with COD > 10,000 mg/L. At loadings of 3.6-19.7 kgCOD/(m3·d), an average sludge age of 1.0-4.3 days and a temperature of 30 degrees C, COD and BOD5 removal rates were 82% and 85% respectively - loadings far higher than those of anaerobic filters.

Beyond this, many additional factors must be considered in practical engineering applications.

Although practice has demonstrated the effectiveness of anaerobic biological processes for high-strength organic wastewater, it is rare for anaerobic treatment alone to be used for leachate. For high-strength landfill leachate, a combined anaerobic-aerobic process is both economical and highly efficient, achieving COD and BOD removal rates of 86.8% and 97.2% respectively.

The Department of Biology at Southwest Normal University applied an anaerobic-aerobic biochemical process to leachate with pH 8.0-8.6, COD 16,124 mg/L, BOD5 214-406 mg/L and NH3-N 475 mg/L, achieving excellent results with effluent pH of 7.1-7.9, COD of 170.33-314.8 mg/L, BOD5 of 91.4 mg/L and NH3-N of 29.1 mg/L.

The following analysis is based on the Likeng Landfill in Guangzhou. The wastewater treatment plant at Likeng Landfill was designed for a flow of 300 m3/d, with influent BOD5 of 2,500 mg/L, CODCr of 4,000 mg/L, NH3-N of 1,000 mg/L, SS of 600 mg/L and colour of 1,000 times; effluent BOD5 of 30 mg/L, CODCr of 80 mg/L, NH3-N of 10 mg/L, SS of 70 mg/L and colour of 40 times. The selected process train is: anaerobic oxidation ditch - facultative pond - flocculation and sedimentation. When influent quality is good and the facultative pond effluent meets standards, the pond water can be discharged directly; when influent quality is poor and the facultative pond effluent fails to meet discharge standards, the coagulation-sedimentation system is activated and the clarifier supernatant is then discharged.

Beyond this, many additional factors must be considered in practical engineering applications.

This process is used for leachate treatment at the Datianshan Sanitary Landfill. Based on monitoring data and simulation tests on similar landfill leachate conducted by the Guangzhou Institute of Environmental Sanitation, combined with site-specific conditions, the design parameters for leachate treatment were determined. Influent quality: CODCr 8,000 mg/L, BOD5 5,000 mg/L, SS 700 mg/L, pH 7.5; effluent quality: CODCr 100 mg/L, BOD5 60 mg/L, SS 500 mg/L, pH 6.5-7.5. Given that the site is far from the urban area, a combined anaerobic-aerobic process was selected after comparison in order to simplify management and save energy. The anaerobic stage uses an upflow anaerobic sludge blanket reactor and the aerobic stage uses biological contact oxidation, supplemented by chemical coagulation-sedimentation and a biological oxidation pond, with discharge after treatment to standard. Excess sludge is thickened and returned to the landfill for disposal.

At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.

In 1995, Fuzhou completed the largest modern integrated municipal solid waste treatment facility in China - the Fuzhou Hongmiaoling Sanitary Landfill. The leachate treatment capacity is 1,000 m3/d; influent leachate quality is CODCr 8,000 mg/L and BOD5 5,500 mg/L; the required treatment performance is 95% CODCr removal and 97% BOD5 removal.

The wastewater undergoes aerobic biochemical treatment in an Orbal oxidation ditch. The Orbal ditch adopts a three-channel A/O process with advanced nitrogen removal performance. A key advantage of this process is that the first channel can both nitrify ammonia nitrogen and denitrify nitrate using BOD as the carbon source, achieving total nitrogen removal of up to 80%. Because the BOD in the wastewater is used as the carbon source, BOD5 is simultaneously removed and the oxygen demand of the wastewater is reduced. To further improve nitrogen removal, effluent from the third channel is pumped back to the first channel by submersible pumps for internal recirculation and denitrification.

In addition, this technology has been widely applied and practiced in related fields.

"Older" landfills are typically in the methane fermentation phase, and their leachate has a relatively high ammonia nitrogen content, usually 100-1,000 mg/L. There are two main methods for ammonia removal: nitrification and denitrification; or raising the pH above 9 followed by air stripping. Robinson and Maris aerated leachate from a 20-year-old landfill at 10 degrees C with a sludge age of 60 days (conditions similar to lagoon operation) and achieved complete nitrification. Other aerobic methods such as rotating biological contactors have also been successful, so it is generally accepted that nitrification of leachate poses no difficulty.

The nitrification/denitrification process is a biochemical treatment method targeting ammonia nitrogen removal. Through the combined action of nitrification and denitrification stages, both COD and ammonia nitrogen are thoroughly removed, and the effluent finally meets national discharge standards through MBR sludge-water separation and deep ion rejection by RO.

In addition, this technology has been widely applied and practiced in related fields.

For the reasons above, anaerobic treatment (followed by aerobic treatment) is recommended for high-strength landfill leachate with COD above 50,000 mg/L, while aerobic biological treatment is recommended for leachate with COD below 5,000 mg/L. For leachate with COD between 5,000 and 50,000 mg/L, either aerobic or anaerobic methods may be used, and process selection should be based mainly on other factors.

At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.

1.2 High CODcr and BOD5 concentrations. CODcr and BOD5 in leachate can reach up to 90,000 mg/L and 38,000 mg/L respectively, or even higher.

1.4 Large variation in water quality. According to landfill age, leachate is divided into two categories: young leachate from landfills less than 5 years old, characterized by high CODcr and BOD5 concentrations and good biodegradability; and old leachate from landfills more than 5 years old, in which fresh waste has gradually become stabilized waste, the pH is close to neutral, CODcr and BOD5 concentrations are reduced, the BOD5/CODcr ratio decreases and the ammonia nitrogen concentration increases.

It is worth noting that technologies and standards in this field continue to evolve and improve.

Fly ash is mainly treated by solidification and stabilization to meet acceptance criteria before being disposed of in municipal solid waste landfills. In the landfill, fly ash is washed by leachate and releases large amounts of fine particles, accelerating the occurrence of physical clogging. [1]