China is a country lacking of water resources. The per capita share of water resources is only 1/4 of the world's per capita share of water resources. With the development of society and economy, the demand of water is increasing constantly, and the contradiction between water shortage and social and economic development is more prominent.
Printing and dyeing industry is the major water consumption and wastewater discharge in China. According to incomplete statistics, China's effluent from printing and dyeing is about 3 * 106~4 * 106m3/d, accounting for about 35% of the total industrial effluent discharge, but the recycling rate is less than 10% . It is of great practical and economic significance to treat printing and dyeing wastewater in depth and improve the reuse rate of waste water.
Status of treatment and reuse of printing and dyeing wastewater in China
There have been many researches on the reuse of printing and dyeing wastewater in China. From the current research and application, the following characteristics are mainly shown:
(1) the recycling technology is mostly in the experimental research stage, mostly in the pilot and pilot tests. There are few practical engineering applications, and the water recycling rate is low, generally no more than 50%. It is mainly used in the pre-process with low water quality requirements.
(2) reuse treatment is mainly to further treat the printing and dyeing wastewater on the basis of standard treatment, so as to meet the water quality standards for reuse. The treatment process mainly USES coagulation, adsorption, filtration and oxidation technologies, among which the key technologies to remove salinity and hardness are less studied.
(3) due to the limitations of current technical level, the large-scale reuse of printing and dyeing wastewater will bring a series of problems to the production and wastewater treatment system, including the accumulation of organic pollutants and inorganic salts. At present, little research has been done on the water quality problem of long-term reuse of wastewater and the impact on water treatment system, especially the accumulation of inorganic salt.
Technology and process of deep treatment of printing and dyeing wastewater
The deep treatment of printing and dyeing wastewater mainly deals with the effluent of the conventional secondary treatment system. The pollutants removed are mainly chroma, COD and salinity (electrical conductivity), etc., so that the effluent quality can meet the requirements of the production process. Printing and dyeing process and product quality requirements are different, and the water quality requirements for reuse are different. Therefore, there is no uniform water quality standard for printing and dyeing wastewater. According to industry experience, water quality indicators must be controlled within the water indicators. Therefore, the requirements of textile printing and dyeing industry for reuse water quality are much higher than that of urban household water quality.
2.1 deep processing unit technology
2.1.1 adsorption treatment technology
By passing the waste water through a filter bed composed of absorbent, the pollutant is absorbed on the surface of the porous material or is filtered out. Activated carbon is the most commonly used adsorbent in the deep treatment of printing and dyeing wastewater. It has many micropores and the specific surface area can be up to 500 ~ 600 m2/g. It has strong adsorption decolorization performance and is especially suitable for decolorization of water-soluble dyes with relative molecular weight less than 400. However, activated carbon has a poor adsorption effect on hydrophobic dyes, and its regeneration is complicated and expensive, which limits the application of adsorption method in the deep treatment of printing and dyeing wastewater. Natural minerals such as kaolinite, diatomite, active clay and pulverized coal also have high adsorption properties and are also used in the deep treatment of printing and dyeing wastewater. In addition, li mengying et al.  studied the adsorption treatment of printing and dyeing wastewater with penicillium, and found that the chromaticity of black and red dyebath wastewater had a better treatment effect, with the removal rate reaching 98.0% and 74.5%, which provided a new choice for the development of adsorption method. Although the adsorption method has a quick effect, it is difficult to regenerate the adsorbent after use. Therefore, the research and development of new adsorbent with high efficiency and easy regeneration is the development direction of current adsorption methods.
2.1.2 membrane separation technology
Membrane has different permeability to different materials. Membrane separation technology is a method to separate the mixture under certain driving force of mass transfer by using this characteristic of membrane. The membrane separation techniques used in the deep treatment of printing and dyeing wastewater mainly include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). MF and UF are often pretreated as NF and RO. UF can separate macromolecular organics, colloid and suspended solids. NF can achieve desalination and concentration at the same time; RO can remove soluble metal salts, organics, colloids, and other ions. Ruan huimin et al.  treated the effluent of a certain printing and dyeing factory in zhejiang after biochemical treatment with UF+RO process. The COD in the film system was 100~350 mg/L, with a color degree of 180 times and a conductivity of 800~ 1,350 molar S/cm. The outlet COD of the film system after treatment was < 10 mg/L, the chroma was 1-2 times, and the conductivity was < 30 year-old S/cm. Xujie Lu et al.  adopted the method of biofilter combined with membrane separation. When the inflow COD was 150~450 mg/L, the effluent COD dropped below 50 mg/L, and the removal rate was as high as 91%.
The advantage of membrane separation technology is that it can not only remove residual organic matter in water and reduce chroma, but also remove inorganic salts and prevent accumulation of inorganic salts in the system. It is a promising technology in the deep treatment of printing and dyeing wastewater. However, the cost of membrane treatment process is high, and the membrane module is easy to be polluted, which shortens its service life. The membrane separation technology will be more widely used in the deep treatment of printing and dyeing wastewater only by controlling and reducing membrane pollution to extend the membrane life and reduce the cost.
2.1.3 advanced oxidation depth treatment technology
(1) chemical oxidation technology. O3 and Fenton reagents are commonly used as oxidants in the deep treatment of printing and dyeing wastewater. O3 has strong decolorization effect. Although it has little effect on COD removal, it can change the B/C of waste water, thus improving the biodegradability of waste water. Lu ningchuan et al.  used O3 oxidation to treat printing and dyeing wastewater. The results showed that the removal rate of COD was 72%, while the color was reduced by 94%. Guo zhaohai et al.  studied the effect of O3 on chromaticity removal and B/C, and found that when the amount of ozone is about 15 mg/L, the chromaticity removal rate can reach 70%, and B/C has more than doubled. The main advantages of O3 oxidation are simple and compact equipment, small footprint and easy automation control. The main disadvantage is that the treatment cost is high, not suitable for the treatment of large discharge of waste water.
Fenton reagent is a compound oxidant made by H2O2 and Fe2+. ·OH produced under acidic conditions has a strong oxidation effect and is especially suitable for the treatment of dye wastewater with complex components. Jiang xinghua et al.  used Fenton reagent to deeply treat the printing and dyeing wastewater. The results showed that: pH 2~3, H2O2 dosage 3.2ml /L, ferro-carbon volume ratio 1:1, reaction time 90 min, COD of effluent was removed more than 90%, chromaticity was reduced by 99%, salinity was reduced by 64%, and the reuse water quality indicators all met the recycling requirements. Shi hongxiang et al.  also studied the treatment of printing and dyeing wastewater with Fenton reagent and obtained similar results. Fenton oxidation has a strong ability to remove COD and chroma, but the presence of iron ions may affect the color of water, and the reaction pH is low, which may affect other treatment processes.
(2) photocatalytic oxidation technology. ·OH generated by strong oxidants under UV radiation can be used to treat wastewater. It has the advantages of low energy consumption, no secondary pollution and thorough oxidation. The most commonly used are UV/Fenton, UV/O3, UV/H2O2 and so on. There are many researches on photocatalysis, among which TiO2 photocatalyst is the most widely used and has the best treatment effect. Under the light radiation, the valence band of TiO2 produces electron holes (h+) pairs, and the organic matter adsorbed on the surface of TiO2 is activated and oxidized by h+ with strong oxidation. Li-na feng etc.  adopted TiO2 / carbon load system of treatment, secondary treatment of dyeing effluent water COD in about 300 mg/L, the optimum reaction conditions, the effluent COD to 50 mg/L, reduced to 2 times, chromaticity, research has shown that the use of activated carbon adsorption, help to solve the problem of the loss of TiO2, separation and recycling, improve the effect of the processing of photocatalyst. However, the light transmittance and utilization ratio of wastewater restrict the application of photocatalytic technology in wastewater treatment industry.
(3) electrochemical oxidation technology. Under the action of external electric field, a large number of free radicals are generated through a certain chemical reaction, electrochemical process or physical process in a specific reactor, and the process of degradation of pollutants in waste water is conducted with the strong oxidization of free radicals. Electrochemical technology is easy to control, pollution-free or less pollution, highly flexible and other characteristics.
M. Kennedy  pointed out that the electrochemical method is very effective for decolorization of printing and dyeing wastewater. When the mass concentration of Fe2+ in the mainstream wastewater in the electrochemical reactor is 200~500 mg/L, the chromaticity removal rate reaches 90%~98%, and the removal rate of COD and BOD reaches 50% and 70% respectively. However, the consumption of the soluble electrode oxidation method is too large, so the development of the new electrode becomes one of the research focuses. Jajinping  used the composite electrode of activated carbon fiber and iron to degrade various simulated printing and dyeing wastewater, and achieved good results. Lei yangming et al.  treated simulated printing and dyeing wastewater with PbO2/Ti as anode, and the removal rate of chroma and COD was up to 99.5% and 78.6%.
2.1.4 efficient biological treatment technology
The secondary effluent pollutants of printing and dyeing wastewater are not highly biodegradable, so it is difficult to biodegrade. The emphasis of biological method is to develop new bioreactor with enhanced biological technology to further remove COD and chroma.
(1) biological aerated filter (BAF). After secondary biochemical treatment, the amount of COD and BOD in the water is relatively low, and the nutrient-poor microorganisms such as pseudomonas and bacillus, which grow on the aerated biological filter filler, are relatively large in surface area, and have a strong affinity to the organics in the waste water. Zhou feng  studied the secondary effluent of printing and dyeing wastewater treated by BAF, and increased BAF depth treatment process after hydrolysis acidification + aerobic process. When inflow COD < 200 mg/L, hydraulic load 1.0 ~ 2.0 m3/ (m2·h), gas-water ratio was (2 ~ 3) : 1, the removal rate of effluent COD was above 50%, reaching the first-level discharge standard. The biological concentration and organic load in aerated biological filter are high, the treatment effect is stable, and the effluent quality is good. The smaller the size of the filter material in the filter tank, the better the treatment effect, but small particle size will make the working cycle shorter, the filter material is not easy to clean, the corresponding backwash water will increase. Therefore, the selection of suitable particle size is the key to give full play to the function of biological aeration filter.
(2) mobile bed biofilm reactor (MBBR). MBBR is a new type of biological membrane reactor. Microorganisms are enriched on the filler in the reactor, and the filler is suspended in the reactor and flows with the mixed liquid. Therefore, the three elements of gas, water and filler can fully contact in the reactor. The oxygen utilization rate and organic pollutant mass transfer efficiency are high. MBBR also has the advantages of no need of backwashing, strong impact load and stable effluent quality .
At present, there are few researches on treating printing and dyeing wastewater with MBBR process. Huo tao mei  found that MBBR can remove COD and ammonia nitrogen in the deep treatment of printing and dyeing wastewater. The COD of water inflow decreased from about 200 mg/L to less than 50 mg/L, and the ammonia nitrogen decreased from 10 mg/L to less than 2 mg/L, but the chroma removal rate was only 25%.
There are many kinds of organic pollutants in the printing and dyeing wastewater, and the multi-bacteria system on the biological filler has great degradation capacity. Therefore, MBBR as a deep treatment process has great advantages for the secondary biochemical treatment of effluent with low concentration of organics. In the future, the research and application of MBBR in the deep treatment of printing and dyeing wastewater can be taken as a development direction.
(3) membrane bioreactor (MBR). Membrane bioreactor integrates membrane separation and biodegradation, and can remove most residual COD, chroma and all SS in wastewater. After further treatment through the NF (RO) process, most of the salinity is removed, and the effluent quality can generally meet the reuse requirements. Dai shu et al.  treated the printing and dyeing wastewater by using the external MBR combined with the ultrafiltration membrane composed of aerobic reactor and ultrafiltration membrane, and the results showed that the removal rate of COD, chroma and turbidity of the system was 99% and the electrical conductivity was 97%. P.Schoeberl et al.  first treated the printing and dyeing wastewater with the combination of MBR and NF, and the effluent quality all met the reuse water index. However, considering the technical difficulty and high economic cost, they replaced NF with UF and achieved good results. The advantages of MBR lie in its short process, small area and stable water quality. The disadvantages are similar to membrane separation technology, mainly due to short membrane life, high cost and high power consumption caused by membrane pollution.
2.2 integrated process of deep treatment and reuse of printing and dyeing wastewater
2.2.1 traditional technology combination process
Due to the complex water quality of printing and dyeing wastewater, the reuse of wastewater is difficult to be realized only by a single technology. Therefore, it is necessary to combine all kinds of methods organically and adopt combined process for comprehensive treatment. Xiaojun Wang et al.  used ozone combined biological method to treat printing and dyeing wastewater. After ozone oxidation, wastewater B/C increased from 0.18 to 0.36. Huang ruimin et al.  treated the dyeing wastewater of knitted cotton with coagulation decolorization -- aeration biological filter -- ion exchange process, and the effluent chromaticity was reduced to less than 10 times, with COD < 20 mg/L, SS < 2 mg/L and turbidity < 3 NTU. Guo zhaohai et al.  studied the effect of the combined process of O3 oxidation and biological filter on the treatment of printing and dyeing wastewater, and found that the combined technology of O3 biological filter plays a very good role in the synergism of chemical oxidation, adsorption and biodegradation, and has the advantages of low operating cost, no concentration and little residual sludge. Single technology is difficult to solve the problems of decolorization, COD reduction and salt removal at the same time when applied in deep treatment. The organic combination of various single technologies can achieve better treatment effect and guarantee the full play of the advantages of each technology to improve the removal rate of pollutants.
2.2.2 integration of membrane technology and traditional technology
The composition of printing and dyeing wastewater is complex. For example, if the printing and dyeing wastewater is treated with membrane technology, appropriate pretreatment technology must be selected to prevent the contamination of the membrane by colloid, organic matter and suspended matter in the wastewater. A. bes-pia et al.  used the combination of O3 and NF to treat and reuse the dyeing and printing wastewater after biochemical treatment and caused by O3 oxidation