The coagulation process is one of the key links in modern municipal water supply and industrial wastewater treatment processes. It can not only remove the raw water turbidity and color and other sensory indicators, but also remove a variety of toxic and harmful pollutants can be self-contained independent treatment system, but also can be used in combination with other units of the process for pre-treatment, intermediate treatment and final treatment process [1-2].
Currently, there are many types of coagulants, which can be broadly divided into inorganic, polar polymer type and composite coagulants. Inorganic coagulants can be divided into low molecular type and polymer type, the application of more iron sulfate, ferric chloride, polymerized aluminum chloride (PAC), etc.; organic polymer coagulants can be divided into natural polymer type, such as starch derivatives, chitin; synthetic polymer type, such as polyacrylamide (PAM), as well as microbial flocculants; composite coagulants are mainly some iron salts, aluminum salts, and silicic acid and other polymers. The coagulation process is the process of aggregation of colloidal particles in water, and also the process of growth of colloidal particles, and this process is carried out under the hydrolysis of coagulants, so the coagulation mechanism is related to the following three factors: one is the nature of the colloidal particles; second is the hydrolysis products of different coagulants under different conditions; third is the interaction between the colloidal particles and hydrolysis products of the coagulant [3].
The application of coagulants in coking wastewater in practical studies will be described below.
1 Fenton coagulation and precipitation method
This method is to oxidize the coking wastewater with Fenton reagent first, and then mix the coagulation.
Wu Keming et al[4] conducted a systematic study on the Fenton oxidation method of treating high-concentration complex coking wastewater with H2O2 as the oxidizing agent and FeSO4-7H2O as the catalyst, and ferric chloride was used as the coagulant after oxidation; the results showed that: when the pH was controlled to be around 3, the reaction time was 30 min, and the temperature of the reaction was 80 ℃, COD, NH3-N, turbidity and coloration of the coking wastewater could be reduced by the Fenton method. -N, turbidity and color removal reached 93.1 %, 96.2 %, 90.8 % and 90.2 %, respectively.
Wang Chunmin et al [5] used this method to determine the reaction conditions of Fenton oxidation stage as follows: H2O2 dosage of mmol/L, [Fe2+]/[H2O2]=1︰10, pH 3, time 30 min, coagulation stage Fe2(SO4) dosage of 600 mg/L, pH adjusted to 6.5, the COD removal rate of treated effluent reached 97.5 %, and meet the national level of COD removal. 97.5 %, and in line with the national discharge standards.
Peng Xianyu et al[6] used Fenton-coagulation and precipitation method to treat coking wastewater with ferric chloride and polyacrylamide (PAM) as coagulants, and the effluent COD, COD and NH3-N removal rates were 84.3%, 92.9% and 96.2%, respectively, which were up to the national emission standards.
When treating coking wastewater by Fenton-coagulation method, the choice of coagulant, the amount of coagulant added and the pH of adaptation in the subsequent coagulation stage will affect the effect of the effluent, especially the choice of coagulant has a bearing on the cost of treating wastewater, therefore, it should be careful.
2 Composite coagulant
Composite polymer coagulant has the effect of its components of the various agents, can take the strengths and weaknesses of the treatment of wastewater, often to achieve unexpected results, and therefore the attention.
Wang Juan et al [7] used BC method + composite overconsideration technology process of coking wastewater biochemical effluent depth treatment experiments, the results show that: in the use of iron salts, aluminum salts, magnesium salts and oxidation of the nucleus and the ratio of 3 ︰ 4 ︰ 1 ︰ 2 of the SE coagulant dosing of 30 mg/L, the retention time of the BC pool of 1.5 h, composite filters hydraulic loading of 2.4 m3/ (m2-h) conditions, when the depth of treatment of influent water, the coagulant is used in the treatment of wastewater, and the coagulant is used to treat wastewater. Under the condition of 2.4 m3/(m2-h), when the influent water quality of deep treatment was COD=196.1 mg/L, chromaticity=120 times, NH3-N=35mg/L, the removal rate was 74.4%, 86.7%, 69.7%, and the effluent water could reach the requirement of reuse water.
Li Yanguang, Guo Jinhua [8] with a quantitative polymerization of aluminum, polymerization of iron and cationic quaternary ammonium salts and other substances at a certain temperature and pressure composite into a JY-202 composite coagulant, through experiments to determine the optimal conditions: coagulation pH 7.1, dosing quality concentration of 250mg / L, the temperature of 15 ~ 40 ℃, when the precipitation time of 12 min, COD, SS and chromaticity removal rate of 12 times, respectively. When the precipitation time was 12 min, the removal rates of COD, SS and color were 51.2%, 92.7% and 85.7% respectively, and the indicators of the effluent reached the national secondary discharge standard.
Compound coagulant is suitable for deep treatment of coking wastewater, not suitable as a separate treatment system. See more related technical documents.
3 The combination of coagulants
Because of the traditional coagulants used in a larger amount of input, and the traditional aluminum coagulant used too much residual aluminum in the effluent water is very difficult to control, too much residual aluminum discharged into the water body will be harmful to the human body; and traditional iron coagulant used too much residual iron in the wastewater will affect the chromaticity of the effluent. In addition polymer coagulants such as polymerized aluminum chloride (PAC), polymeric ferric sulfate (PFS), polyacrylamide (PAM) and other combinations between the use of the visible.
Zhou Jijun et al [9] to coking wastewater treatment process examples to confirm the polymerization of aluminum chloride (PAC) plus polyacrylamide (PAM) combination of excellent oil removal performance; test effluent petroleum-based substances, turbidity, cyanide, volatile phenols, and the removal rate of COD were 93%, 86%, 19 %, 27%, 17%, the water body becomes clear, basically no oil traces, irritating The odor was reduced and the appearance was greatly improved. If this method is used for the pretreatment of coking wastewater, it will provide a good environment for the subsequent treatment process.
Cheng Shengyu [10] through the study of different coagulants on the treatment of coking wastewater, comparisons were made: when the PAC and PAM dosage of 4.0 mg / L, 0.3 mg / L, respectively, the effluent chromaticity, COD removal rate of up to 80%, respectively, 55.63%; and the experimental formula was applied to the actual coking wastewater treatment of the wastewater treatment station of the Dandong Vantone Company Limited, the effluent reached the GB8978-1996 standards. Ltd., the effluent reached GB8978-1996 comprehensive sewage discharge standard (Grade I), which can be discharged in compliance with the standard, and can also be directly reused for coke quenching.
Zheng Yi et al [11] selected several coagulants for the depth treatment of coking wastewater after biological treatment, and concluded that the use of PFS + PAM combination of coagulants, under the condition of pH = 5, the dosage of (40 + 6) mg / L, this time, the effluent chromaticity, COD removal rate of 73.08%, respectively, 62.22%, effluent chromaticity and COD are able to meet the "Comprehensive Emission Standards for Wastewater" (GB8978-1996) (Class I), and can also be directly reused for coke quenching. Comprehensive Emission Standards for Sewage (GB8979-1996) in the requirements of the second level, is more suitable for the depth of coking wastewater treatment method.
The combination of coagulants should also pay attention to some principles. Such as the addition of a small amount of PAM to the role of bridging flocculation and network flocculation, not only can improve the removal rate of COD, but also can make the floc particles to increase the settling speed; but the amount of injection is too large, PAM can also produce colloidal protective effect of colloids just destabilized and turned to the suspension in the resulting decline in the removal rate of COD [12]. In the application of organic coagulants and inorganic coagulants can be used in conjunction with the organic polymer coagulant to give full play to the organic polymer coagulant adsorption bridging properties and inorganic coagulants and the ability of electric neutralization, so as to ensure that the composite coagulant high efficiency, so that the treatment of the effluent to achieve a better result [13].
4 Application of fly ash
Fly ash is a waste residue collected from the waste discharged from the boiler of a coal-fired power plant, and most of the fly ash is spherical, with a loose and porous surface, a large specific surface area, and certain active groups and adsorption properties; its composition contains Fe3+, Al3+, Mg2+, Ca2+, and so on.
Online treatment of coking wastewater from biochemical treatment process with fly ash as adsorbent can achieve better pollutant removal [14]. Zhang Changming, Yu Changshun [15] studied in the treatment of 100 t/h of water, fly ash dosage of 1.747 t/h (17.47 kg/m3), COD, phenol, cyanide, sulfide, oil, ammonia nitrogen, BOD, chromaticity of the average removal rate of 57.41%; treated water in addition to ammonia nitrogen is slightly higher, the rest of the pollutants have reached the China's first-class coking new plant discharge standards; treated water 60 % was reused. 60 % of the treated water was reused, and the used fly ash could be used as building materials. Zhang Changming, Li Aiying [16-17] through experimental research, in the fly ash additive amount of 15 g / L and retention time of 20 ~ 25 min conditions, the treated wastewater in addition to ammonia nitrogen, the other items can also meet the standards for external discharge, in similar conditions to get the average removal rate of 48.85%.
Fly ash as an adsorbent or coagulant in wastewater treatment has the advantage of low price; however, due to the low adsorption capacity of fly ash, the efficiency of removing pollutants is relatively low, and much of the research coming in centers around the modification of fly ash and the search for optimal control conditions [18].