What Do the Main Biological Treatment Processes for Landfill Leachate Include?

Biological treatment is widely used to degrade the organic pollutants in leachate because it is highly efficient, low in cost and produces no chemical sludge. As a mature and widely adopted technology, it is implemented mainly as aerobic treatment, anaerobic treatment and combined processes that integrate the two.

1. Aerobic biological treatment

Aerobic biological treatment relies on the activity of microorganisms in an oxygen-rich environment. The microorganisms use the organic matter in the leachate as a nutrient source and complete their metabolism through aerobic decomposition, thereby purifying the water. The technology can remove a wide range of pollutants and reacts quickly. Its mainstream processes include the activated sludge process, aerated oxidation ponds and aerobic biofilm processes.

• Sequencing batch reactor (SBR)

The SBR process is an efficient wastewater treatment method developed by deepening and improving the conventional activated sludge process. It features simple structures, easy operation and management, high sludge activity, fast biochemical reaction, thorough solid-liquid separation and little risk of sludge bulking. Spagni et al. treated leachate with a real-time controlled SBR and reported average removal rates of 98% for ammonia nitrogen and 95% for total nitrogen; removal of chemical oxygen demand (CODcr), however, was limited to only 20%-30%. Domestic research now focuses largely on further improving SBR performance, and new directions such as SBR combined with other processes and bioaugmented SBR have emerged. In a field pilot test, Sun Tiegang's team combined bioaugmentation with an SBR-AO process to treat landfill leachate and confirmed that the bioaugmented system performed significantly better than the conventional SBR process.

• Membrane bioreactor (MBR)

MBR is one of the most actively researched membrane processes for leachate treatment in China, integrating a membrane separation unit with a bioreactor. Wang Jinhui et al. studied treatment with a submerged polypropylene MBR and found high removal of COD and ammonia nitrogen, with suspended solids (SS) in the effluent close to non-detectable; at the same time, membrane flux declined rapidly and stayed at a low level. Ouyang Ke et al. treated leachate with a submerged MBR and systematically assessed removal of COD, ammonia nitrogen (NH4+-N) and total nitrogen (TN). The data showed that leachate COD could be reduced to 650-1500 mg/L. The study also found that membrane fouling progressed quickly and showed a two-stage pattern, and that combined cleaning with alkali and an oxidant effectively restored membrane performance and lowered the filtration pressure.

2. Anaerobic biological treatment

Anaerobic treatment uses the metabolic function of anaerobic microorganisms in the absence of oxygen and can effectively break down the organic pollutants in landfill leachate. Compared with aerobic technology, anaerobic processes require a long hydraulic retention time and a large reactor volume, but they produce little sludge, are simple to operate and have low capital and operating costs; they are particularly suited to leachate with poor biodegradability and high organic concentration. In recent years the technology has been applied ever more widely in leachate treatment.

Because of its excellent solid-liquid separation capability, the upflow anaerobic sludge blanket (UASB) reactor is the most widely applied anaerobic process today. Wang Wei et al. studied an improved externally circulating UASB reactor and showed that, when treating high-strength leachate, the reactor achieved a high biogas yield under high loading. Although COD removal was good, the effluent was still difficult to bring reliably within limits and required further advanced treatment.

3. Combined anaerobic-aerobic treatment

Anaerobic processes excel at high-strength organic wastewater but are slow, while aerobic processes need only a short hydraulic retention time. Used alone, neither can usually bring landfill leachate to the national discharge standard. Combining the two into an economically sound integrated process therefore allows several pollutants to be removed together. Studies show that combined processes perform significantly better than either process alone.

Jin Yongxiang et al. applied a composite A/O system to mature leachate and achieved effluent ammonia nitrogen (NH4+-N) below 20 mg/L, meeting the requirements of the Standard for Pollution Control on the Landfill Site of Municipal Solid Waste. To remove total nitrogen (TN) and ammonia nitrogen (NH4+-N) simultaneously and efficiently, Wu Lina et al. developed a UASB1 + A/O + UASB2 combined process in which the downstream UASB stage makes full use of the carbon source in the upstream effluent, achieving cost-effective nitrogen removal. With this combined process, NH4+-N and TN removal reached 97% and 95% respectively.

For aged leachate with poor biodegradability, microbiological treatment alone is often ineffective and other processes are needed as an adjunct. Dosing a composite microbial agent is one strategy for improving performance: bioengineering techniques are used to acclimatise and culture highly efficient degrading strains.

Li Hong et al. dosed leachate with a BM composite microbial agent and studied the treatment performance and mechanism, confirming that the dosed test group performed markedly better than the group without dosing. New process development has also attracted attention, such as the JS-BC process derived from the conventional activated sludge process and the rotating biological contactor. By effectively proliferating and activating soil bacteria (Bacillus), this process removes BOD, COD, NH4+-N, TN and TP efficiently while simultaneously breaking down the odour generated by the system.

4. Advanced oxidation processes

Advanced oxidation processes (AOPs) are a family of technologies that has emerged in water treatment in recent years. Under conditions involving electricity, light radiation, catalysts or oxidants they generate hydroxyl radicals with extremely strong oxidising power, which can oxidise refractory organics in water into low-toxicity or non-toxic small molecules, or even mineralise them completely into carbon dioxide and water. Depending on how the radicals are generated, AOPs can be divided into the Fenton process, ozonation, electrochemical oxidation and others.

• Fenton process

The Fenton process uses homogeneous catalysis by Fe2+ to decompose hydrogen peroxide (H2O2) into hydroxyl radicals, which then degrade the organics in leachate. Chen Di et al. applied the Fenton process to circulating semi-aerobic landfill leachate and, under optimum operating conditions, achieved removal of 84.77% for CODcr and 60% for colour. The conventional Fenton process, however, suffers from difficulties in recovering the catalyst, a requirement for low reaction pH, the generation of large amounts of iron-bearing sludge, high Fe2+ in the effluent that can cause secondary pollution, long treatment times and high reagent consumption. Improved Fenton-like processes have therefore been developed, such as sono-Fenton, photo-Fenton and electro-Fenton. Mohajeri et al. used the electro-Fenton process on leachate and achieved maximum removal of 94.07% for COD and 95.83% for colour.

• Ozonation

Ozone has extremely strong oxidising power, with a redox potential second only to fluorine. It can oxidise organics directly and can also decompose to generate hydroxyl radicals that indirectly oxidise target pollutants. Ozone performs well in deodorisation, disinfection, decolourisation and organic removal in wastewater and produces no secondary pollution. Used alone on landfill leachate, however, it suffers from low utilisation efficiency, limited oxidising capacity, high cost and modest degradation. Researchers have therefore explored various catalytic routes combined with ozone, forming enhanced ozonation AOPs intended to raise ozone utilisation, oxidation rate and treatment capacity. Liu Weihua et al. compared catalytic ozonation with plain ozonation for wastewater rich in refractory organics and found that catalytic ozonation markedly improved TOC and COD removal.

• Electrochemical oxidation

Compared with other AOPs, electrochemical oxidation can be carried out at ambient temperature and pressure and, because it is efficient and easy to operate, is regarded as one of the most promising advanced treatment methods for wastewater. Lei et al. applied electrochemical oxidation to the advanced treatment of biologically treated leachate and achieved removal of 98.5% for COD, 99.9% for ammonia nitrogen and 99.9% for BOD, demonstrating outstanding pollutant removal. The principle is as follows: reduction at the cathode releases electrons and can remove most heavy metals, while oxidation at the anode absorbs electrons and mineralises and degrades organics, with oxidation also occurring in the bulk solution. Because this takes place directly in the water without added catalysts and so avoids secondary pollution, the technology is described as environmentally friendly and is one of those with the greatest potential for industrial application.

Besides the methods above, plasma oxidation, supercritical water oxidation and photochemical oxidation are also applied to landfill leachate treatment. Although AOPs give strong results, they generally face high energy consumption and are not yet fully mature. From an economic point of view, they can be used in combination with other technologies, or effort can go into developing low-energy AOPs, or even into using the material resources of the leachate and the landfill itself to power the treatment process - all directions worth developing further.