System for treating landfill leachate by coupling forward osmosis and membrane distillation

Abstract

The utility model relates to a system for treating landfill leachate by coupling forward osmosis and membrane distillation, which mainly solves the following problems in landfill leachate concentrated water treatment: high comprehensive energy consumption, membrane pollution risk and difficulty in regeneration of drawn liquid. The device comprises a pretreatment unit, a forward osmosis module, a membrane distillation module and a draw solution storage tank, a water inlet of the pretreatment unit is connected with a landfill leachate concentrated water source, and a discharge port of the pretreatment unit is connected to a leachate side inlet of the forward osmosis module through a first delivery pump; a draw liquid side outlet of the forward osmosis module is connected to a liquid inlet of the membrane distillation module through a membrane distillation heater and a second delivery pump; a concentrated liquid outlet of the membrane distillation module and a draw liquid side inlet of the forward osmosis module form a closed loop through a draw liquid storage tank; an on-line conductivity monitor is arranged in the draw solution storage tank, and a salt supplementing valve is arranged at a feeding port of the draw solution storage tank. And a reflux pump is arranged between the draw solution storage tank and the forward osmosis module. The method is used for treating the leachate concentrated water of the domestic waste landfill.

Description

Technical Field

[0001] This utility model relates to a landfill leachate treatment system, and in particular to a system for treating landfill leachate by coupling forward osmosis and membrane distillation, which is mainly used for treating concentrated leachate from municipal solid waste landfills. Background Technology

[0002] Landfill leachate concentrate is characterized by high COD (>4000mg / L), high ammonia nitrogen (>1000mg / L), high salinity (TDS>30g / L), and complex pollutants (heavy metals, organic matter). Traditional technologies such as reverse osmosis (RO) treatment face problems such as rapid membrane fouling, high energy consumption, and low recovery rate. Evaporation crystallization treatment is expensive and has low efficiency.

[0003] Forward osmosis (FO) technology can treat high-concentration, high-salt landfill leachate, but the degree of concentration is limited by the concentration of the draw solution. Currently, draw solution regeneration relies on multi-effect evaporation or secondary membrane separation (RO), which is costly and has limited draw solution regeneration concentration, thus failing to fully utilize the capabilities of FO technology.

[0004] Membrane distillation (MD) technology can achieve high brine concentration using low-grade heat sources (60-85℃), but it is susceptible to organic contamination when directly treating landfill leachate. However, it can be used in drawdown regeneration to obtain drawdown with higher concentrations.

[0005] Coupling FO concentrated leachate with MD concentrated extract to form a closed loop can reduce system energy consumption, extend membrane life, and improve resource recovery rate. Utility Model Content

[0006] The purpose of this invention is to provide a system for treating landfill leachate by coupling forward osmosis and membrane distillation, which mainly solves the following problems in the treatment of landfill leachate concentrate: reducing the overall energy consumption of the system, reducing the risk of membrane fouling, and achieving efficient regeneration of the extract.

[0007] This utility model provides a system for coupled forward osmosis and membrane distillation to treat landfill leachate, comprising: a pretreatment unit, a forward osmosis module, a membrane distillation module, and a draw liquid storage tank. The inlet of the pretreatment unit is connected to a concentrated leachate source, and the outlet of the pretreatment unit is connected to the leachate-side inlet of the forward osmosis module via a first transfer pump. The draw liquid-side outlet of the forward osmosis module is connected to the inlet of the membrane distillation module via a membrane distillation heater and a second transfer pump. The concentrated liquid outlet of the membrane distillation module forms a closed loop with the draw liquid inlet of the forward osmosis module through the draw liquid storage tank. The draw liquid storage tank has a built-in online conductivity monitor, and a salt replenishment valve is provided at the feed port. A reflux pump, which is a variable frequency diaphragm pump, is provided between the draw liquid storage tank and the forward osmosis module.

[0008] The pretreatment unit includes a security filter (retaining particles with a diameter >10μm), an ultrafiltration module (retaining a molecular weight cutoff of 5000Da), and a feed tank (which can be filled with HCl solution to adjust the pH to 6-6.5) connected in sequence by pipelines.

[0009] The forward osmosis module is a hollow fiber forward osmosis membrane module. The membrane distillation module is a plate-type hydrophobic membrane module.

[0010] Condensation design: A spiral tube condenser and a connected vacuum pump are installed on the freshwater side of the membrane distillation module.

[0011] The beneficial effects of this invention are as follows: 1. Improved treatment efficiency: the system recovery rate after forward osmosis coupled membrane distillation is >80%. 2. Reduced energy consumption and cost: utilizing waste heat from waste incineration to drive membrane distillation reduces overall energy consumption by 60% compared to RO + evaporation crystallization. 3. Extended membrane lifespan: low fouling on the forward osmosis side and no phase change operation on the membrane distillation side extend the membrane replacement cycle to 12-18 months. 4. Resource utilization: the regenerated high-concentration NaCl solution (TDS 22-25%) can be directly reused or crystallized to produce salt, achieving zero liquid discharge (ZLD). Attached Figure Description

[0012] Figure 1 This is a structural block diagram of the present utility model.

[0013] In the diagram: 100-Pretreatment unit, 110-Security filter, 120-Ultrafiltration module, 130-Feed tank; 200-Forward osmosis module, 201-Leachate side inlet, 202-Draw liquid side outlet, 203-Draw liquid side inlet, 210-First transfer pump, 300-Membrane distillation module, 310-Membrane distillation heater, 320-Draw liquid storage tank, 321-Brine replenishment valve, 322-Online conductivity monitor, 330-Second transfer pump, 340-Condenser, 350-Vacuum pump, 360-Reflux pump. Detailed Implementation

[0014] Reference Figure 1 A municipal solid waste landfill leachate concentrate treatment project:

[0015] Influent water quality: COD 5000 mg / L, NH3-N 1200 mg / L, conductivity 35 mS / cm.

[0016] A system for treating landfill leachate by coupling forward osmosis and membrane distillation includes: a pretreatment unit, a forward osmosis module, a membrane distillation module, and a drawup liquid storage tank;

[0017] The inlet of the pretreatment unit 100 is connected to a concentrated leachate source, and the outlet of the pretreatment unit is connected to the leachate-side inlet 201 of the forward osmosis module 200 via a first transfer pump 210. The draw liquid-side outlet 202 of the forward osmosis module 200 is connected to the inlet of the membrane distillation module 300 via a membrane distillation heater 310 and a second transfer pump 330. The concentrate outlet of the membrane distillation module 300 forms a closed loop with the draw liquid-side inlet 203 of the forward osmosis module 200 via a draw liquid storage tank 320. The draw liquid storage tank 320 has a built-in online conductivity monitor 322, and the feed port of the draw liquid storage tank 320 is equipped with a salt replenishment valve 321. A reflux pump 360, which is a variable frequency diaphragm pump, is provided between the draw liquid storage tank 320 and the forward osmosis module 200.

[0018] The pretreatment unit 100 includes a security filter 110 (retaining particles with a diameter >10μm), an ultrafiltration module 120 (retaining a molecular weight cutoff of 5000Da), and a feed tank 130 (which can add HCl solution and adjust the pH to 6-6.5) connected in sequence through pipelines.

[0019] Function: Removes suspended solids from leachate concentrate (TDS 3-5%, COD>5000mg / L) and adjusts pH value.

[0020] The forward osmosis module 200 is a hollow fiber forward osmosis membrane assembly (using Danish AQP hollow fiber forward osmosis membrane, model HFFO02).

[0021] Parameter configuration:

[0022] Membrane area: 248.8 m² (adjusted according to processing capacity);

[0023] Operating conditions: Flow rate on the leachate side: 0.5-1.2 m / s; Flow rate on the draw solution side (1.5-2.5 mol / L NaCl solution): 0.1-0.4 m / s.

[0024] Function: By utilizing the osmotic pressure difference across the forward osmosis membrane, water molecules migrate from the permeate to the driving liquid side, thereby concentrating the permeate (TDS ≥ 8% after concentration).

[0025] The membrane distillation module 300 is a plate-type hydrophobic membrane assembly (using the second-generation plate membrane distillation assembly from EvCon, Germany, model VMEMD01).

[0026] Extraction fluid regeneration process:

[0027] The diluted draw solution (TDS 5%-8%) is heated to 50-70℃ by the membrane distillation heater 310 and then enters the membrane distillation module 300;

[0028] A negative pressure (-80 kPa to -95 kPa) is applied to the hot side (salt water side) of the membrane, and water vapor passes through the hydrophobic membrane to the cold side and condenses.

[0029] After regeneration, the TDS of the driving fluid increases to >20%, and it is returned to the forward osmosis module via a reflux pump for recycling.

[0030] Condensation design: The membrane distillation module 300 is equipped with a spiral tube condenser 340 and a connected vacuum pump 350 on the fresh water side. The cooling water temperature is ≤20℃ and the fresh water recovery rate is ≥85%.

[0031] The flow path of the draw solution is as follows: forward osmosis dilute draw solution outlet, membrane distillation heater, membrane distillation module, draw solution storage tank, and forward osmosis draw solution inlet. Unless otherwise specified, all components involved in this invention are commercially available conventional products.

[0032] Detailed operating procedures and steps

[0033] Step 1: System Startup

[0034] Inject leachate concentrate into the forward osmosis module 200 and start the first transfer pump 210; the forward osmosis module 200 has a flux of 18 LMH, an operating pressure of 0.3 bar, and operates continuously for 24 hours.

[0035] Inject a 15% NaCl solution to the draw liquid storage tank 320 to a volume of 2 m³, start the reflux pump 360, and match the flow rate to the feed liquid side; preheat the membrane distillation module 300: turn on the membrane distillation heater 310 to raise the temperature of the hot side solution to 70°C, start the vacuum pump 350 to evacuate the vacuum to -80 kPa, and the flux is 10 LMH.

[0036] Step 2: Continuous operation

[0037] The permeate from the forward osmosis module 200 enters the permeate tank, and the diluted NaCl solution enters the membrane distillation module 300 via the membrane distillation heater 310 and the second transfer pump 330.

[0038] The membrane distillation module produces 300 tons of condensate, which meets emission standards, and the concentrate is recycled.

[0039] Automatic crystal replenishment: Based on the data from the online conductivity monitor 322, add 2-3 kg of NaCl to the extract liquid storage tank 320 for every 1 m³ of leachate treated.

[0040] Step 3: Maintenance and Cleaning

[0041] Chemical cleaning: Backwash the forward osmosis membrane weekly with a 0.1% NaOH + 0.05% EDTA solution (pH=11) for 30 min.

[0042] Physical cleaning: The membrane distillation membrane is flushed with 60℃ pure water in the forward direction every 48 hours (10 min, flow rate 1 m / s).

[0043] Troubleshooting: If the membrane distillation module flux decreases by more than 20%, start by soaking in 1% citric acid (by mass percentage) for 2 hours to remove inorganic scale.

[0044] Treatment results: The conductivity of the permeate is <100 μS / cm (meeting the standard of Table 2 of GB16889-2008); the conductivity of the concentrate is 210 mS / cm (80% reduction due to outsourced incineration); the TDS of the regenerated extract is 23.5%, saving 120,000 yuan in reagent costs annually.

Claims

1. A system for treating landfill leachate by coupling forward osmosis with membrane distillation, characterized in that, The application relates to a landfill leachate treatment system, which comprises a pretreatment unit, a forward osmosis module, a membrane distillation module and a draw solution tank; a water inlet of the pretreatment unit is connected with a landfill leachate concentrated water source; a discharge port of the pretreatment unit is connected with a leachate side inlet of the forward osmosis module through a first conveying pump; a draw solution side outlet of the forward osmosis module is connected with a liquid inlet of the membrane distillation module through a membrane distillation heater and a second conveying pump; a concentrated liquid outlet of the membrane distillation module forms a closed loop with the draw solution side inlet of the forward osmosis module through the draw solution tank; an online conductivity monitor is arranged in the draw solution tank; a salt supplement valve is arranged at a feeding port of the draw solution tank; and a reflux pump is arranged between the draw solution tank and the forward osmosis module. The pretreatment unit comprises a safety filter, an ultrafiltration module and a feeding tank which are sequentially connected through pipelines.

2. The system for treating landfill leachate by coupling forward osmosis with membrane distillation according to claim 1, characterized in that, The forward osmosis module is a hollow fiber forward osmosis membrane assembly. 3.The system for treating landfill leachate by coupling forward osmosis with membrane distillation according to claim 1, characterized in that, The membrane distillation module is a plate type hydrophobic membrane assembly.

4. The system for treating landfill leachate by coupling forward osmosis with membrane distillation according to claim 1, characterized in that, The reflux pump is a variable frequency diaphragm pump.

5. The system for treating landfill leachate by coupling forward osmosis with membrane distillation according to claim 1, characterized in that, A spiral pipe type condenser and a connected vacuum pump are arranged at a fresh water side of the membrane distillation module.

6. The system for treating landfill leachate by coupling forward osmosis with membrane distillation according to claim 1, characterized in that, ​

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