The coking wastewater is a kind of toxic organic wastewater with high concentration, which is difficult to be degraded. The COD, SS, Cl, chroma and hardness in the wastewater are still difficult to reach the standard after the biological coagulation treatment. The wastewater can only be reused after the advanced treatment
Experiment of coking wastewater treatment by ultrafiltration membrane and nanofiltration membrane process
Source and characteristics of coking wastewater
Coking wastewater mainly comes from coal coking, gas purification process and chemical product refining process
It belongs to high concentration organic wastewater, with high concentration of harmful substances, various pollutants and complex components. Among them, inorganic compounds are mainly large amount of ammonia salt, thiocyanate, sulfide, cyanide, etc. organic compounds include phenols, single ring and multi ring aromatic compounds, nitrogen, sulfur, oxygen heterocyclic compounds, etc
Experimental water quality
In the experiment, the effluent from the secondary sedimentation tank of a coking plant in Tangshan was treated by the combined process of ultrafiltration and nanofiltration.
Refer to the picture on the right for wastewater, secondary sedimentation tank effluent (experimental water inlet) and discharge standard.
Raw water tank: 1m × 1.5m
Ultrafiltration water tank: 0.8m × 1m
Nanofiltration water tank: 0.2 × 0.8 × 1.2m3
Wedge wire screen pretreatment filter
Ultrafiltration experimental device
Nanofiltration experimental device
Ultrafiltration membrane: produced by Saehan company, model uf4040, material PVDF, filtering aperture 0.1 μ m, water yield 1000L / h, working pressure 0.1mp, transmembrane pressure difference 0.1mp, water yield recovery 90%.
Nanofiltration membrane: produced by Saehan company, model nf4040, material PA, filtration aperture 1nm, water yield 80L / h, working pressure 0.6mp, transmembrane pressure difference 0.04mp, water yield recovery 90%.
In the combined process of ultrafiltration and nanofiltration, coking wastewater is first filtered by cross flow of ultrafiltration membrane.
The water from the ultrafiltration membrane is concentrated water and produced water.
The concentrated water contains a large number of macromolecular substances such as suspended solids, colloids, proteins and microorganisms.
The produced water only contains inorganic salts and small fraction substances.
Ultrafiltration water is used as the influent of nanofiltration membrane, and the concentrated ultrafiltration water is directly returned to the anaerobic tank for further biochemical treatment
The ultrafiltration water is delivered to the nanofiltration membrane by high pressure pump. After the separation of the nanofiltration membrane, it is also divided into concentrated water and produced water. The concentrated water is returned to the anaerobic tank for biochemical treatment or incineration.
Nanofiltration membrane can intercept small molecules with molecular weight of 200-1000 and divalent anions of 99%. Therefore, nanofiltration water only contains a small amount of small molecular organics and a small amount of inorganic salts, which can meet the quality standard of reclaimed water as supplementary water for circulating cooling system in code for design of wastewater recycling engineering (gb50335-2002).
The experimental process of coking wastewater is shown in the figure.
The experiment adopts the way of continuous operation, sampling and analysis at regular time every day. The collected samples are influent water and produced water. The analysis contents are cod, NH3-N, hardness, alkalinity, Cl -, etc.
In addition, the pressure difference and water yield of the equipment shall be recorded regularly every day. The equipment shall be flushed and flushed regularly (once a day for ultrafiltration and nanofiltration membrane flushing and once every 10d for dosing flushing). The equipment shall be operated continuously for 20d.
Test results and discussion
Water quality and removal efficiency
During the experiment, the change of COD concentration in the inlet and outlet water is shown in Figure 2. It can be seen from Figure 2 that the influent water quality fluctuates greatly, cod is in the range of 180-240mg / L, ultrafiltration water quality changes with the change of influent water, while nanofiltration water quality is stable, cod is maintained in the range of 30-50Mg / L, and COD removal rate is 80% – 90%.
The hardness of influent water is 150-180mg / L, the hardness of ultrafiltration water is 100-150mg / L, and the hardness of nanofiltration water is about 40-50mg / L. the removal rate of hardness is 70% – 75%.
The influent Cl concentration is 340-450mg / L, the ultrafiltration Cl concentration is 250-360mg / L, the nanofiltration Cl concentration is 170-250mg / L, and the CL removal rate is 40% – 50%.
The TDS in the influent water is 1900-2700mg / L, the total dissolved solid (TDS) in the ultrafiltration water is 1600-2400 mg / L, the TDS in the nanofiltration water is 650-1000mg / L, and the removal rate of TDS is 57% – 68%. In addition, in the experimental water, the suspended solids and chroma are not detected, that is, the removal rate is 100%.
In general, the removal rate of COD, hardness, suspended matter, chroma, TDS and other pollution factors in Coking Wastewater by the combined process of ultrafiltration and nanofiltration is very high, while the removal rate of chloride ion is relatively low.
Among them, the COD concentration in the produced water is stable below 50mg / L, the removal rate is 80% ~ 90%, the hardness is stable at 40 ~ 50mg / L, the removal rate is 70% ~ 75%, the TDS content is 650 ~ 1000mg / L, the removal rate is 57% ~ 68%, the removal rate of suspended solids and chroma is 100%, and the CL removal rate is 40% ~ 50%.
Stability of equipment operation
During the experiment, the pressure of ultrafiltration membrane was maintained at 0.9kg/cm2, and the pressure difference was kept at about 0.1kg/cm2.
In the initial operation, the water yield of ultrafiltration membrane is 920l / h. After 10 days of continuous operation of the experimental device, the water yield decreases to 909l / h, and the water yield attenuation is about 1%.
After chemical cleaning, the water yield can be restored to the initial value.
The water yield of the ultrafiltration membrane decreased from 920l / h to 909l / h when the experimental device continued to operate to the 20th day. After the second chemical cleaning, the water yield can still be restored to the initial value.
Through the analysis of the discharged water after ultrafiltration cleaning, it is concluded that the main pollutants of ultrafiltration membrane are the suspended solids and organic colloids in coking wastewater, which can be easily removed by cleaning.
It can be seen that during the operation of the experimental equipment, the pressure difference of the ultrafiltration membrane is very stable, and the change of water yield is small, even if the membrane surface is polluted, it can be easily removed by cleaning
During the experiment, the pressure of nanofiltration membrane was kept at 6.0kg/cm2, and the pressure difference was kept at about 1.8kg/cm2.
During the operation of nanofiltration membrane, the initial water yield was 180L / h, and after 10 days of continuous operation, the water yield decreased to 165l / h, the attenuation range was 8%.
After chemical cleaning, the water yield can be restored to the initial value. When the experimental device continues to operate to the 20th day, the water yield of the nanofiltration membrane decreases from the original 180L / h to 163L / h for the second chemical cleaning
After chemical cleaning, the water yield can still be restored to the initial value
Through the analysis of the discharged water after nanofiltration cleaning, it is concluded that the main pollutants of nanofiltration membrane are organic colloid and inorganic salt.
It can be seen that during the operation of nanofiltration membrane, the change of water yield is small, which indicates that the surface pollution of membrane is less, the anti pollution ability of membrane is stronger, and the pollutants on the surface of membrane are easy to be removed by washing, the operation of nanofiltration membrane is stable, that is to say, the selected process is feasible.
Cleaning and treatment of cleaning water
During the experiment, the cleaning of membrane is divided into daily cleaning and chemical cleaning.
The production water of nanofiltration is used as the cleaning water of membrane for daily cleaning.
The ultrafiltration cleaning includes positive washing and back washing.
The nanofiltration only has positive washing. The cleaning time is 30min each time, and the cleaning water is recycled. After calculation, the daily cleaning water volume is very small, accounting for about 2% of the water volume, and the content of pollutants is small.
It can be returned to the biological wastewater treatment regulating tank to regulate the water quality of the anaerobic tank.
The cleaning agent is NaOH and citric acid. In the cleaning process, NaOH is added to the raw water tank filled with cleaning water, the pH is adjusted to 12, and the centrifugal pump is started to clean the ultrafiltration membrane and nanofiltration membrane respectively for 30min, soak for 20min, and drain water.
Then adjust the pH of cleaning water to 2 with citric acid, start centrifugal pump to clean ultrafiltration membrane and nanofiltration membrane respectively for 30min, soak for 20min, and drain water.
After chemical cleaning, determine the water yield of ultrafiltration and nanofiltration, and the water yield can be restored to the initial value.
Due to the long interval of chemical cleaning and the small amount of cleaning water, it can be directly returned to the anaerobic tank to regulate the water quality.
Cleaning agents are cheap and can reduce operating costs.
Direction of concentrated water
The concentrated water produced by the combined process of ultrafiltration and nanofiltration accounts for about 10% of the volume fraction, and the COD concentration is 1600-1800mg / L, which is mainly the polycyclic organic matter that can not be degraded by biology.
The common treatment methods of this kind of wastewater are advanced oxidation, wet oxidation and incineration, which are difficult to degrade and complex in composition. However, it is not cost-effective to treat this kind of organic wastewater with high concentration by advanced oxidation. Because the consumption of oxidant increases with the increase of pollutant concentration, the COD content of wastewater suitable for treatment by wet oxidation technology is 20-150mg / L, and the incineration method is suitable for treatment The content of COD in wastewater is more than 100mg / L.
After summary, the concentrated water produced by the combined process of ultrafiltration and nanofiltration is treated by incineration.
The method is to spray the waste water into the high temperature combustion furnace in the form of mist, make the water mist completely vaporized, oxidize the organic matter in the waste water in the furnace, and decompose it into the complete combustion product CO2, H2O and a little inorganic ash.
There are a lot of organic matter and a small amount of NH3-N in the concentrated water. Whether the ammonia nitrogen generates no in the combustion process and causes secondary pollution is a sensitive problem in the incineration process of coking wastewater treatment.
It is found that the main product of ammonia nitrogen under the non catalytic oxidation condition is N2, which will not cause no secondary pollution. Therefore, it is feasible to treat the concentrated water produced by the combined process of ultrafiltration and nanofiltration by incineration.
a. The advanced treatment of coking wastewater by membrane combined process is as follows: COD of effluent is less than 50mg / L, hardness is less than 50mg / L, Cl concentration is less than 250mg / L, TDS is less than 1000mg / L, suspended solids and chroma are 0, which conforms to the quality standard of reclaimed water as supplementary water for circulating cooling system in code for design of wastewater recycling engineering (gb50335-2002).
b. During the experiment, the water yield and membrane pressure difference of ultrafiltration and nanofiltration are all in the normal range, and the membrane water yield and membrane pressure difference can be recovered by flushing and backwashing the membrane regularly. Therefore, it is feasible to treat coking wastewater with ultrafiltration nanofiltration process.
c. The concentrated water produced by the combined process of ultrafiltration and nanofiltration can be treated by incineration.