Measures for discharge of waste water by coal chemical enterprises

The waste water discharged by coal chemical enterprises is mainly concentrated gas washing waste water, which contains a lot of toxic and harmful substances such as phenol, cyanide, oil and ammonia nitrogen. In the comprehensive wastewater, CODCr is generally about 5000mg/ L and ammonia nitrogen is about 200-500mg/L. The organic pollutants contained in the wastewater include phenols, polycyclic aromatic compounds and heterocyclic compounds containing nitrogen, oxygen and sulfur, etc. It is a typical industrial wastewater containing refractory organic compounds. Phenolic compounds and benzene compounds are the main easily degradable organic compounds in wastewater. Pisha, naphthalene, furan and squinazole belong to degradable organic compounds. The refractory organic compounds mainly include pyridine, carbazole, biphenyl, triphenyl, etc.

At present, the domestic technology for the treatment of coal chemical wastewater mainly uses the biochemical method, which has a good removal effect on phenols and benzene substances in wastewater, but the treatment effect on quinoline, indole, pyridine, carbazole and other refractory organic compounds is poor, making it difficult for the coal chemical industry drainage CODCr to reach the first level standard.

At the same time, after biochemical treatment, coal chemical wastewater has the characteristics of high chromaticity and turbidity (due to the organic matter containing a variety of chromophore and chromophore, Such as 3-methyl-1,3,6 heptatriene, 5-norbornene-2-carboxylic acid, 2-chloro-2-norbornene, 2-hydroxy-benzofuran, phenol, 1-methylsulfonyl-4-methylbenzene, 3-methylbenzothiophene, naphthalene-1, 8-diamine, etc.).

Therefore, CODCr, ammonia nitrogen, chroma, turbidity and other indexes should be further reduced in order to meet the reuse or discharge standards after the treatment of such coal gasification wastewater.

4. Difficulty in the treatment of coal chemical wastewater:

In recent years, new methods and technologies have been developed to treat coal chemical wastewater, but each has its own advantages and disadvantages.

The pure biological oxidation method contains a certain amount of refractory organic matter, COD value is high, can not fully meet the discharge standard.

Although the adsorption method can remove CODCr well, it has the problems of adsorbent regeneration and secondary pollution.

Although catalytic oxidation can degrade the organic matter which is difficult to biodegrade, it has the problem of high operating cost in practical industrial application.

Anaerobic and aerobic combined treatment of coal chemical wastewater can obtain ideal treatment effect, operation management and cost is relatively low, this process is the main selection of coal chemical wastewater process.

However, when the inflow concentration is high and contains more refractory organic matter, the effluent is difficult to reach the standard stably, so it needs to be combined with catalytic oxidation, coagulation and sedimentation and other processes.

Combined treatment of coal chemical wastewater by various methods is the developing direction of coal chemical industrial wastewater treatment technology.

Treatment of coal chemical wastewater

The treatment process route of coal chemical wastewater basically follows the "physicochemical pretreatment +A/O biochemical treatment + physicochemical advanced treatment", which is briefly introduced below.

1. Physical and chemical pretreatment

Common pretreatment methods: oil separation, air flotation, etc.

Because too much oil will affect the effect of subsequent biochemical treatment, the role of gas flotation coal chemical wastewater pretreatment is to remove the oil and recycle it, in addition to play the role of pre-aeration.

2. Biochemical treatment

For the pretreated coal chemical wastewater, anoxic and aerobic biological method is generally used at home and abroad to treat it (A/O process). However, due to the polycyclic and heterocyclic compounds in the coal chemical wastewater, the COD index in the effluent treated by aerobic biological method is difficult to reach the standard steadily.

In order to solve these problems, some new treatment methods, such as PACT, carrier flow bed biofilm (CBR), anaerobic biological method and anaerobic - aerobic biological method, have been developed in recent years.

1) Anaerobic biological method

A technology called upflow anaerobic sludge bed (UASB) is used to treat coal chemical wastewater. The reactor used in this method was successfully developed by G.Lettinga of the Netherlands in 1977. Wastewater flows from bottom to top through a reactor with a sludge layer at the bottom, where most of the organic matter is converted by microorganisms into CH4 and CO2 at the upper part of the reactor. Equipped with three-phase separator to complete the separation of gas, liquid and solid three phases.

In addition, activated carbon anaerobic expansion bed technology has also been used in the treatment of coal chemical wastewater, which can effectively remove phenols and heterocyclic compounds in wastewater.

2) Improved aerobic biological method

PACT method is to add the activated carbon powder to the activated sludge aeration tank, and make use of the adsorption of the activated carbon powder to the organic matter and dissolved oxygen to provide food for the growth of microorganisms, so as to accelerate the ability of oxidation and decomposition of organic matter. Activated carbon was regenerated by wet air oxidation.

(2) the method of moving bed biofilm carrier (CBR) CBR is, in fact, a kind of biological fluidized bed technology based on the special structure packing, the technology in the same biological treatment unit will be an organic combination of biofilm and activated sludge method, through the carrier and special packing in the activated sludge pool make attached microbes grow in suspended filler surface, form a certain thickness of microbial film layer. The attached growth of microorganisms can achieve a high biomass, so the concentration of organisms in the reaction tank is 2-4 times that of the suspended growth activated sludge process, up to 8-12g/L, and the degradation efficiency is also doubled.

The uniquely designed filler floats with the flow in the reaction tank under the disturbance of air blowing and aeration, which drives the biological flora attached and growing to fully contact with the pollutants and oxygen in the water. The pollutants enter into the biofilm through adsorption and diffusion and are degraded by microorganisms. The degradation efficiency of the whole system is high.

By microorganisms for attached growth way (different from active sludge suspended growth), flow bed long sludge age of microorganisms on the surface of the carrier (20-40 days), very beneficial to slow growth of denitrifying bacteria such as autotrophic microbe breeding, packing surface has a large number of nitrifying bacteria breeding, so the system has a very strong nitrification ability of removing ammonia nitrogen.

At the same time, the adhesion growth method is conducive to the natural selection of other special bacteria, and these special bacteria can effectively degrade the characteristic pollutants in coal gasification wastewater, especially some pollutants that are difficult to degrade, so as to obtain a lower effluent COD concentration.

CBR technology can be applied to the treatment of high concentration coal chemical wastewater, and can also be applied to the subsequent advanced treatment and reuse unit.

3) Anaerobic and aerobic combined biological method

The treatment of coal chemical wastewater by aerobic or anaerobic technology alone can not achieve satisfactory results. The combined biological treatment of anaerobic and aerobic has been paid more and more attention by researchers.

After the anaerobic acidification treatment of coal chemical wastewater, the biodegradation performance of organic compounds in the wastewater is significantly improved, and the CODCr removal rate in the subsequent aerobic biological treatment is more than 90%. The removal rates of naphthalene, quinoline and pyridine, which are more difficult to degrade, were 67%, 55% and 70% respectively, while the removal rates of these organics were less than 20% under normal aerobic treatment.

The treatment of coal chemical wastewater by anaerobic fixed membrane and aerobic biological process has also achieved satisfactory results.

3. Deep processing

After the biochemical treatment of coal chemical wastewater, although the concentration of CODCr and ammonia nitrogen in the effluent has been greatly reduced, the COD and chroma of the effluent have not reached the discharge standard due to the existence of refractory organic matter. Therefore, the effluent after biochemical treatment still needs further treatment. The advanced treatment methods mainly include coagulation sedimentation, immobilized biological technology, adsorption catalytic oxidation method and reverse osmosis membrane treatment technology.

1), advanced oxidation technology

Due to the complexity and diversity of organic matter in coal chemical wastewater, phenols, polycyclic aromatic hydrocarbons, nitrogen-containing organic matter and other refractory organic matter accounted for the majority, the existence of these refractory organic matter seriously affected the effect of subsequent biochemical treatment.

Advanced oxidation technology is to produce a large number of HO. free radicals in wastewater HO. free radicals can non-selectively degrade organic pollutants in wastewater into carbon dioxide and water. Advanced oxidation technology can be divided into homogeneous catalytic oxidation method, photocatalytic oxidation method, heterogeneous wet catalytic oxidation method and other catalytic oxidation method.

Catalytic oxidation method can be applied in the first stage of coal chemical wastewater treatment process to remove part of COD and enhance the biodegradability of wastewater, but there are problems such as large consumption and uneconomic operation, so the application of this technology in the subsequent advanced treatment unit can obtain better economic and degradation effect.

2), adsorption method

Because the solid surface has the ability to adsorb solutes and colloid in water, when the wastewater passes through the solid particles with large specific surface area, the pollutants in the water are adsorbed to the solid particles (adsorbent), thereby removing the contaminants. This method can achieve good results, but there are some problems such as large amount of adsorbent, high cost and secondary pollution, so it is generally suitable for small-scale sewage treatment application.

3) Coagulation and precipitation

Sedimentation method is a process of solid-liquid separation by using the settling property of suspended solids in water to sink under the action of gravity. The aim is to remove suspended organic matter to reduce the organic load of subsequent biological treatment.

In production, coagulants such as aluminum salt, ferric salt, polyaluminum, polyiron and polyacrylamide are usually added to strengthen the precipitation effect. The influencing factors of this method are the pH of wastewater, the type and dosage of coagulant and so on.

4) Immobilized biotechnology

Immobilized biotechnology is a new technology developed in recent years, which can selectively immobilize dominant strains and treat wastewater containing refractory organic poisonants.

The degradation ability of quinoline, isoquinoline and pyridine of the domesticated dominant strains is 2-5 times higher than that of the ordinary sludge, and the degradation efficiency of the dominant strains is higher. After 8h of treatment, quinoline, isoquinoline and pyridine can be degraded by more than 90%.