Exploration of leachate treatment technologies for WtE plants
In recent years, with the advancement of ecological civilization, the number of waste-to-energy projects has grown rapidly, drawing increasing public attention to the treatment of leachate. This paper focuses on the current leachate-treatment technologies adopted by domestic waste incineration plants.At present, incineration is a primary means of treating municipal solid waste. Before burning, the waste is usually tipped into a storage pit and left for 3–5 days to allow thorough fermentation and maturation, thereby draining moisture and raising its calorific value so that the subsequent incineration process can run smoothly; this inevitably generates leachate. Leachate contains high concentrations of organic matter and ammonia-nitrogen pollutants, along with certain toxic substances. If it is not promptly and effectively treated, it can contaminate the groundwater, surface water and soil around the plant. It is therefore essential to treat such leachate to meet discharge standards and prevent "secondary pollution". For this reason, research into and application of leachate-treatment technologies must be given high priority.
1 Recirculation (Re-spray) Treatment Technology
The re-spray process is suitable for incinerators that generate small volumes of leachate and are fed with high-calorific waste. It is not appropriate for plants receiving low-calorific, highly wet refuse with large leachate flows, because spraying large quantities of low-temperature liquid can depress furnace temperature and impair combustion efficiency. Literature indicates that this technology is widely used in developed countries, where municipal solid waste contains little food residue, has a high overall heating value, and produces limited leachate; under these conditions the leachate is simply re-injected into the furnace for high-temperature oxidation. For example, a 1500t/d waste-to-energy plant in New York City generates a maximum of 4t/d of leachate. The liquid is collected in a storage tank and, when the waste calorific value is high, is pressurised by a high-pressure pump, filtered, and sprayed into the furnace through a recirculation nozzle; injection is stopped when the calorific value is low. For waste with a calorific value of about 5115kJ/kg and 48% moisture, the theoretical maximum recirculation rate is 3.20% of the total waste feed. The technique is generally unsuitable for China, where municipal refuse usually has high moisture content, produces large amounts of leachate, and exhibits a low overall heating value.
2 Biochemical Treatment Technologies
(1) UASB Anaerobic Treatment
Leachate from renewable waste contains high concentrations of organic pollutants, most of which are readily biodegradable volatile fatty acids (VFAs). The up-flow anaerobic sludge blanket (UASB) technology achieves good removal of these organics, with literature reporting COD removal efficiencies above 70%. The process can be operated at volumetric loading rates up to 10kg/m3・d and does not require energy-intensive aeration, thus substantially reducing both the footprint and the operating costs of the treatment unit.
(2) SBR Aerobic Treatment
The sequencing batch reactor (SBR) process is a time-controlled, cyclic technology in which influent filling, mixing, aeration, sedimentation and decantation are completed sequentially in a single tank. It offers high resistance to shock loads and allows operating parameters to be adjusted flexibly to match the complex and variable characteristics of leachate. Usually combined with an anaerobic stage, SBR significantly enhances nitrogen and phosphorus removal efficiency and reliability.
(3) Ammonia Stripping Treatment
Municipal solid waste leachate is characterized by extremely high ammonia-nitrogen concentrations, often ranging from tens to several thousand mg/L. Such elevated NH₄⁺-N levels strongly inhibit biological processes, upset the ρ(C)/ρ(N) ratio and make biological nitrogen removal difficult, so that the treated leachate cannot meet discharge standards. Consequently, leachates with high ammonia-nitrogen concentrations are usually subjected to ammonia stripping before biological treatment.
At present, ammonia stripping in China is mainly carried out in aeration tanks or packed stripping towers. The aeration-tank option gives low removal efficiency because of the limited gas–liquid contact area and is therefore unsuitable for leachates with very high NH₄⁺-N levels. Towers can achieve higher removal rates, but capital and operating costs are high and the off-gas from stripping is difficult to treat. For example, at a Shenzhen waste-to-energy plant the investment for ammonia-stripping equipment and infrastructure accounted for nearly 30% of the total project cost, while the daily operating expense of the stripping stage represented about 70% of the total leachate-treatment cost. The main reasons are: (i) the leachate pH must be raised to 10–12 for stripping and then readjusted to neutrality before biological treatment, requiring large amounts of acid and alkali; (ii) high-power blowers must run continuously to provide the large air flow needed to increase the gas–liquid contact area. All these factors make the process expensive.
3 Photocatalytic Treatment Technology
Photocatalysis is an emerging advanced wastewater process. When a semiconductor is irradiated by UV light, valence-band electrons are promoted to the conduction band, generating highly reactive electron–hole pairs. Upon reaching the surface, these species initiate redox reactions with oxidants, thereby mineralising pollutants. Chinese researchers have employed a ZnO/TiO₂ composite catalyst for the tertiary treatment of incineration-plant leachate and achieved effluent quality that meets discharge standards. Recently, heterogeneous photocatalysis has been widely adopted for wastewater remediation; its use as a polishing step for MSWI leachate can markedly upgrade final water quality.
4 MBR Treatment Technology
In recent years, advances in science and engineering have allowed a variety of innovative processes to be introduced for leachate treatment at waste-to-energy plants, leading to significant overall progress. Among these, membrane technology—primarily ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO)—is the most mature and widely adopted. By combining micro- or ultrafiltration with aerobic biological treatment, the membrane bioreactor (MBR) has emerged as a stable and highly efficient wastewater treatment system. In an MBR, a membrane (usually UF) replaces the conventional secondary clarifier, eliminating the dependence on sludge settleability. Compared with the traditional activated-sludge process, MBRs achieve markedly higher organic removal efficiencies: microbial concentrations can be maintained at 10–25g/L, values well above those typical of conventional systems, and superior effluent quality is obtained in shorter hydraulic retention times. Consequently, MBR technology offers substantial advantages in both treatment efficiency and final water quality.
In conclusion, each leachate-treatment technology has its own scope of application, advantages, and limitations. Selection must be based on the specific operational conditions of the incineration plant, the characteristics of the leachate, and the socio-economic context of the locality; on-site pilot trials are essential to identify the most suitable option. When formulating a treatment scheme and selecting equipment, project-specific economic, safety, and efficiency considerations should be fully integrated to ensure an optimal, holistic solution.
Source:https://mp.weixin.qq.com/s/NkLJjiTa6GhKqABUmexB0g?scene=1&click_id=15