Process’s Fundamental Requirements
Given the characteristics of leachate, the design and process selection for leachate treatment in landfills need to meet the following conditions:
● Meeting the requirements of water quantity changes.
The processing capacity of any chosen scale of water treatment process has an upper limit on water treatment. As a result, the design process should have a high anti-water pressure shock load capacity to accommodate large fluctuations in water quantity.
● High resistance to shock load due to water quality.
Because of the wide variation in leachate water quality, the treatment process must be shock-load resistant. Considering the significant decrease in leachate biodegradability and the imbalance of carbon and nitrogen ratio as landfill age increases is especially important.
● High capacity for COD and BOD removal.
The COD concentration in landfill leachate can range between 4000 and 20000 mg/l, and national environmental protection policies demand increasingly stringent water quality standards for leachate treatment effluent. As a result, the treatment process must have a very high organic pollutant removal capacity.
● Excellent denitrification efficiency.
The ammonia nitrogen concentration of landfill leachate typically ranges from hundreds to thousands of mg/L, which is tens to hundreds of times higher than that of municipal sewage. Furthermore, as a result of the project’s use of the GB16889-2008 standard, the discharge requirements for ammonia nitrogen and total nitrogen in the effluent are extremely stringent. Therefore, for ammonia nitrogen removal, the treatment process necessitates a denitrification rate greater than 99%.
● The treatment facility is stable, and the operation and management are straightforward.
● The treatment method is safe and environmentally friendly.
DTRO (Disc Tube Reverse Osmosis) Technology Overview
DTRO (Disc Tube Reverse Osmosis) is a cutting-edge reverse osmosis membrane technology. The component is built differently than traditional spiral-wound membranes. The disc tube membrane module’s feed channel features a patented flow channel design with an open channel. The feed solution enters the pressure vessel through the inlet and flows to the other end of the module through the channel between the guide plate and the housing.
The feed solution enters the guide plate through eight channels at the opposite end of the flange (as shown in Figure 1). The treated liquid flows through the filtration membrane as quickly as possible, then reverses 180o to the other membrane surface and flows into the following guide plate through the slot in the center of the guide plate (as shown in Figure 2).
On the membrane surface, this creates a double “S”-the shaped path from the guide plate’s circular edge to the center, back to the edge, and back to the center. Finally, the concentrated liquid emerges from the feed end flange. The DTRO module’s two guide plates are separated by 3mm, and the guide plate surface has protrusions arranged in a specific pattern.
This unique hydraulic design causes turbulence in the treated liquid as it passes through the membrane surface and collides with the protrusions under pressure, increasing the permeation rate and self-cleaning function, effectively avoiding membrane fouling and concentration polarization, and successfully extending the service life of the membrane module. Cleaning the module is also simple, making the disc tube membrane module suitable for harsh feed water conditions.
This is a technical explanation of the benefits of using a disc-tube reverse osmosis membrane to treat leachate. Here are the main points:
● Reliable effluent quality that meets discharge standards, unaffected by leachate biodegradability.
Because the disc-tube reverse osmosis technology rejects pollutants at a high rate, the effluent can meet discharge standards in the early, middle, and late stages of leachate treatment without being affected by the leachate’s biodegradability or carbon-nitrogen ratio. This technology is especially useful for treating leachate from old landfills in the middle and late stages.
● Low investment and operating costs.
When compared to other processes, disc-tube reverse osmosis technology has the shortest process flow and the lowest energy consumption while meeting high-level discharge standards, resulting in low investment and operating costs. The operating costs of the two-stage DTRO are much lower than those of other treatment processes that can meet the new standards.
● Extended membrane life.
The DTRO components have a 3mm open wide channel and a unique flow guide disc with protrusions, allowing the fluid in the element to form a turbulent flow state, minimizing membrane fouling, pollution, and concentration polarization to the greatest extent possible.
The DTRO’s distinctive shape and hydraulic design make it simple to clean and restore flux after cleaning, extending membrane life. Practical engineering demonstrates that the lifespan of a first-stage DTRO membrane pack in treating original leachate can reach up to 3 years or even longer, and the lifespan of a second-stage DTRO can reach more than 5 years, which is difficult to achieve for available reverse osmosis treatment systems.
● Membrane components are simple to maintain.
The DTRO membrane components are designed to be disassembled and maintained easily. Opening the DTRO component allows for simple inspection and maintenance of any filter membrane pack and other parts. When the number of parts is insufficient, the component can allow for the installation of fewer membrane packs and flow guides without affecting the use of the DTRO membrane component, which other membrane components cannot do.
● Low cost of replacing filter membranes.
Each component of the DTRO can be replaced individually. The filter is made up of several filter membrane packs and flow guides. When a filter membrane pack needs to be replaced, it can be done individually, and good-performing membrane packs can still be used to keep membrane replacement costs to a minimum. Other membrane components, such as roll membranes and hollow fiber membranes, cannot achieve this. When a rolling membrane develops quality issues, such as patches or local leaks, or when new membranes are required, the entire membrane component must be replaced.
As a membrane separation technology, the DTRO membrane system offers the following advantages over traditional biochemical processes:
- Operational adaptability
The DTRO membrane system is a highly flexible physical separation device that can be used continuously or intermittently. The system’s series-parallel configuration can also be adjusted to meet water quality and quantity requirements.
- Short construction time, as well as quick debugging and startup
Mechanical processing is used to build the DTRO membrane system, with supporting facilities such as factory buildings and water pools. The system is built on a small scale and can be completed quickly. Installation and debugging can be completed in about two weeks after the equipment is delivered to the site.
- High level of automation and ease of use
The DTRO membrane system is fully automatic and features an advanced monitoring and control system. To protect the system, the PLC can automatically adjust based on sensor parameters and issue alarm signals promptly. To troubleshoot error codes, operators only need to consult the operating manual, which does not require extensive experience.
- Compact size
The DTRO membrane system is installed as an integrated unit, with small structures and facilities that require a small footprint.
The adjustment tank’s filtration liquid is pumped to the filtration raw water storage tank for pH adjustment, simple pretreatments such as sand filtration and security filtration, and then into the first stage DTRO. In the first stage, DTRO concentrate is discharged to the concentrate storage tank for recycling. DTRO permeate enters the degassing tower for additional treatment before being discharged to meet the standard.
The following figure illustrates the complete process flow of DTRO treatment for filtration liquid:
Description of the Process
- The pH of the permeate varies with plant age, changes in various environmental conditions, and the complex composition of the feed water, which contains a variety of insoluble salts such as calcium, magnesium, barium, and silica. Scaling occurs on the membrane surface when the concentration of these insoluble inorganic salts exceeds the solubility under these conditions.
- The pH value of the feed water must be adjusted before entering the reverse osmosis system to effectively prevent the scaling of carbonate inorganic salts. Simultaneously, suspended solids in the feed water must be pretreated to reduce pollution from suspended solids in the permeate to the membrane.
- Before the raw water is transported from the regulating pool to the permeate raw water tank by the booster pump, it is first passed through a basket filter to remove any particulate matter that may have been introduced into the feed water. The basket filter has a filtration aperture of 1.0mm. The raw water enters the permeated raw water tank after being filtered. At the same time that the permeate enters the raw water tank, acid from the acid storage tank is added to adjust the pH value.
- At the same time, the acid mixing pump begins to operate for reflux mixing to achieve the desired pH value. The raw liquid storage tank reflux pipeline in the system is equipped with a pH sensor, and the PLC judges the pH value of the raw water and automatically adjusts the frequency of the dosing pump to adjust the amount of acid added so that the pH value of the raw liquid before entering the reverse osmosis is eventually in the range of 6.1-6.5. There is no need to adjust the acid if the pH of the raw waterfalls within this range.
- The booster pump sand filter pumps raw water into the sand filter. The sand filter has a single unit and a filtration accuracy of 50um. Pressure gauges are located at the sand filter’s inlet and outlet. Backwashing must be performed when the pressure difference exceeds 1.5 bar. The frequency of sand filter backwashing is determined by the number of suspended solids in the influent.
2. First-stage DTRO
Two stages of reverse osmosis comprise the membrane system. After the sand filter, the first-stage reverse osmosis must be fed, and the second-stage reverse osmosis treats the permeate from the first stage.
The sand filter effluent is fed into the first-stage DTRO equipment, which provides water. It first passes through the core filter, where suspended solids are removed from the permeate. The equipment is outfitted with a single set of core filters, and pressure sensors on both the inlet and outlet ends detect pressure differences automatically. When the pressure difference exceeds 2.0 bar, the system indicates that the filter element should be replaced—the accuracy of the core’s filtration.