The process route is MBR + DTRO. This route combines biological and physical treatment, fully utilizing the advantages of each while overcoming their respective disadvantages. This ensures stable and reliable treatment that meets discharge standards, while saving land, investment, and operating costs, and facilitating operation and management.
The biological system includes a homogenization and equalization tank, a homogenization tank, a denitrification tank, a nitrification tank, a sludge treatment system, and auxiliary systems (feed pumps, chemical storage systems, cooling systems, and all individual equipment not listed in other systems).
Leachate from the equalization tank is first lifted into a bag filter installed above the homogenization tank. The bag filter can intercept particulate matter larger than 0.5mm. The filter bags need to be replaced when the pressure loss reaches a certain value. The filter bags can be reused after cleaning.
The filtered leachate flows by gravity into the homogenization tank. A mixer inside the tank agitates and mixes leachate from different sources and at different times, achieving homogenization and stabilizing the influent to subsequent treatment facilities. The biological system mainly consists of a denitrification tank, a nitrification tank, a defoaming system, and a cooling system. Auxiliary equipment from other systems is also included in the biological system. There is one set of denitrification tanks and one set of nitrification tanks. The effluent from the equalization tank enters the biological system production line. The biological system is an AO-type biological reactor. Aerobic microorganisms in the reactor decompose and utilize organic matter in the water, synthesizing cell tissues and releasing water and carbon dioxide. A portion of the ammonia nitrogen in the water is used for cell synthesis in the carbon removal reaction, and the remainder is utilized by nitrifying bacteria to produce nitrates and nitrites. The nitrates and nitrites are returned to the denitrification tank with the nitrification liquid, where denitrification occurs in an anaerobic environment. The nitrates and nitrites are reduced, generating nitrogen gas which escapes, thus achieving nitrogen removal.
The aeration system consists of jet aerators, jet pumps, and blowers. The jet pumps provide high-flow-rate pressurized water, while the blowers provide compressed air. These two components mix and release through the venturi tubes of the jet aerators, forming uniform microbubbles that diffuse evenly throughout the water, achieving an oxygenation efficiency exceeding 25%. This simultaneously achieves overall water volume agitation. The biological treatment tank is a completely mixed reactor; high-concentration leachate entering the system is immediately diluted and diffused, preventing damage to microorganisms.
Due to the high sludge concentration in the biological treatment tank, it is recommended to use high-oxygenation-efficiency European-imported jet aerators to ensure smooth biological reactions.
Due to the special characteristics of landfill leachate, a large amount of foam may be generated during the biological cultivation stage and operation. This system incorporates both chemical defoaming and hydraulic defoaming. Chemical defoaming suppresses foam formation by adding defoaming agents, while hydraulic defoaming eliminates foam by spraying water at key locations such as manholes and installation holes.
The biochemical process generates a significant amount of heat, raising the reactor temperature and hindering system operation. Therefore, a cooling system is implemented, using a cooling tower to supply cooling water to the biochemical tanks via heat exchangers. This single cooling system cools both nitrification tanks. The biochemical system's automatic control system primarily consists of various sensors, input/output modules, and a PLC. The influent water monitoring system mainly monitors flow rate, conductivity, and pH. The biochemical tanks primarily monitor pH, dissolved oxygen, temperature, and liquid level. Analysis of these parameters controls the aeration rate, sludge discharge rate, and ultrafiltration (UF) operation time to create a suitable environment for microorganisms.
The UF system takes water from the nitrification tanks for sludge-water separation. The concentrated sludge-water mixture is then returned to the denitrification section as internal reflux.
The biochemical system discharges approximately 12 tons of sludge daily, with a designed moisture content of 98.5%. Due to the low sludge concentration, a sludge thickening tank is included in the system for gravity thickening of the discharged sludge. The thickened sludge is then pumped to a centrifugal dewatering machine. After centrifugation, the dewatered filtrate is pumped back to the biochemical system for further treatment. The sludge cake, with a moisture content below 80%, is sent to a landfill for disposal.
This ultrafiltration system uses an external tubular ultrafiltration system. Water is drawn from the nitrification tank. The UF inlet pump distributes the mixed liquor from the biochemical tank to the UF loop. The maximum ultrafiltration pressure is 6 bar. The sludge-water mixture is pumped into the tubular membrane module via a circulating pump. Under the pressure difference across the membrane, the liquid phase permeates through the membrane, forming permeate, achieving sludge-water separation. The sludge is returned to the biochemical tank to increase the sludge concentration there, while simultaneously achieving internal recirculation of the nitrification liquor. A portion of the sludge is discharged as surplus sludge into the sludge thickening tank. The permeate is discharged into the ultrafiltration clear water tank and enters the next treatment process.
Compared to traditional biological treatment processes, microbial cells are separated from the effluent through a high-efficiency ultrafiltration system, ensuring that particulate matter larger than 0.02 µm, microorganisms, and COD-related suspended solids are safely retained within the system. This achieves true separation of hydraulic retention time and sludge retention time. By controlling the sludge age, the goal of cultivating a large number of nitrifying bacteria is achieved, thereby significantly improving the ammonia nitrogen removal rate. The ultrafiltration effluent is sterile, free of solids, and free of biologically active substances. Sludge concentration is maintained through continuous reflux in cross-flow ultrafiltration.
The ultrafiltration system has one loop, with four membrane tubes in each loop. A separate circulation pump provides a required flow rate along the inner wall of the membrane tubes, creating turbulence, generating a large filtration flux, and preventing clogging.
The membrane tubes also require flushing and cleaning after a period of operation, which is accomplished by a cleaning pump using a cleaning tank containing clear water or clear liquid. Each loop can be flushed, cleaned, or maintained while other loops are operating. An automatic compressed air control valve can simultaneously cut off the feed, and sludge remaining in the pipes is carried to the biological treatment tank by the flushing water.
CIP is an even-frequency process. During the later stages of cleaning, valves open according to a program, allowing cleaning water to circulate in the membrane loops and return to the cleaning tank until thorough cleaning is achieved. If necessary, a small amount of membrane cleaning agent can be added to the cleaning tank during the later stages of cleaning.
While most organic matter and nitrogenous pollutants in the MBR effluent have been removed, the levels are still far from meeting discharge standards. The water still contains a large amount of organic matter, scaling ions, and other pollutants, posing a significant risk to the stable operation of subsequent membrane separation equipment. Due to stringent discharge standards, and the new standards' requirements for total nitrogen, the membrane system must have a high rejection rate for nitrates, etc. This design employs a DTRO (Disc Tube Reverse Osmosis Membrane System) with strong anti-fouling capabilities and high recovery rates. Landfill leachate has a complex composition, containing various sparingly soluble salts such as calcium, magnesium, barium, and silicon. These sparingly soluble inorganic salts are highly concentrated after entering the reverse osmosis system. When their concentration exceeds their solubility under certain conditions, scaling will occur on the membrane surface. Adjusting the pH of the raw water effectively prevents scaling by carbonate inorganic salts; therefore, the pH of the raw water must be adjusted before entering the reverse osmosis system.
MBR effluent is pumped into the DTRO membrane column via the DTRO feed pump. Simultaneously, acid is added from the acid storage tank to adjust the pH and is refluxed for mixing, achieving pH equilibrium. The DTRO system feed line is equipped with a pH sensor. The PLC determines the raw water pH and automatically adjusts the metering pump frequency to regulate the acid addition, ultimately ensuring the pH of the raw solution before entering the reverse osmosis system reaches 6.1–6.5. The pH-adjusted raw water is pumped into the DTRO membrane column. The DTRO system consists of one set, each equipped with two security bag filters. Pressure sensors at both the inlet and outlet of each security bag filter automatically detect the differential pressure. When the differential pressure exceeds a certain threshold, the system prompts for filter bag replacement. The two security bag filters operate in parallel, allowing for online filter bag replacement. The security filter has a filtration accuracy of 50μm, providing the final protective barrier for the membrane. The raw water passing through the security bag filter directly enters the high-pressure plunger pump. The effluent, pressurized by the high-pressure pump, enters the DTRO membrane column. The first membrane stage is directly supplied with water by the high-pressure pump. Each membrane stage is equipped with an online circulation pump, which returns a portion of the concentrate from the membrane outlet to the online pump inlet to ensure sufficient flow rate and velocity at the membrane surface, preventing membrane fouling.
The membrane effluent is divided into concentrate and permeate. A pressure regulating valve at the concentrate end controls the pressure within the membrane module to achieve the necessary purified water recovery rate. The concentrate is discharged into a concentrate tank for further treatment. The permeate enters a clear water storage tank to meet its own cleaning needs. Excess water is discharged into an effluent equalization tank and then pumped into the receiving water body. The system recovery rate is 80%.
The reverse osmosis system is a medium-pressure reverse osmosis system using disc tube reverse osmosis membranes. The average operating pressure of the disc tube reverse osmosis membrane is 30–70 bar, with a maximum pressure limit of 70 bar for medium-pressure reverse osmosis. Since the feed water is ultrafiltration clarified liquid, the clarified liquid yield can reach 80%. The DTRO system has one unit, with a total of 70 disc tube reverse osmosis membrane columns and a total membrane area of 658.3 m².
⑴ CIP (Clean-In-Place) system: The CIP system is used for flushing, water cleaning, and chemical cleaning of the reverse osmosis unit.
⑵ Acid dosing system: To prevent inorganic scaling during reverse osmosis operation, an acid dosing system is installed to adjust the pH value of the reverse osmosis system feed water.
⑶ Antiscalant dosing system: The antiscalant dosing system is also used to prevent the formation of inorganic scaling during reverse osmosis operation.
Because the leachate is pretreated using a fully biological denitrification membrane bioreactor, the ultrafiltration effluent is free of suspended solids and biodegradable organic matter. This largely avoids the formation of inorganic and organic fouling on the reverse osmosis membrane, thereby reducing the frequency of reverse osmosis membrane cleaning and allowing the reverse osmosis system to operate under relatively low pressure, extending the service life of the reverse osmosis membrane.