New Water Treatment Index System toward Zero Liquid Discharge for

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New water treatment index system toward zero liquid discharge for sustainable coal chemical processes Peizhe Cui, Yu Qian, and Siyu Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03737 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Sustainable Chemistry & Engineering

New water treatment index system toward zero liquid discharge for sustainable coal chemical processes

Peizhe Cui, Yu Qian, Siyu Yang∗

School of Chemistry and Chemical Engineering, South China University of Technology, NO.381, Wushan road, Guangzhou 510640, P.R. China

*Corresponding author: Professor Siyu Yang Ph.D. School of Chemical Engineering South China University of Technology Guangzhou, 510640, PR China. Phone: +86-20-87112056, +86-18588887467 Email: [email protected]

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Abstract: Shortage of water resource is the major bottleneck for coal chemical industry development. It is imperative to approach zero liquid discharge for coal chemical processes. Varieties of wastewater treatment processes have been developed, but none of them have yet achieved zero discharge in practice. The water quality of reuse does not meet the requirements, resulting in inactivation of microorganisms in biochemical system and accelerated corrosion and fouling of heat exchangers. These problems can even threatening production and operation. Current water treatment units are pieced simply together leading to unsatisfactory water quality for reuse. Therefore, a new water treatment index system is proposed for ensuring water quality for reuse and zero liquid discharge. In order to establish the water treatment index system, this paper focuses on study of the whole water treatment process. For better illustration, a water treatment process for zero liquid discharge is established. A mass balance is calculated depending on several practical industrial cases. Analysis shows that the content of polyphenols, PAHs, long-chain alkanes, and ammonia nitrogen is the key for water reuse quality. Therefore, concentration limits for these components in the indexes are carefully studied and determined. The indexes at each joint point between units are determined based on data from current existing treatment processes and technical demands of specific treatment units. Depending on the index system, appropriate treating technologies are selected and optimized. We finally give out suggestions on improvement and optimization of water treatment processes for zero liquid discharge. .

Keywords: coal chemical industry; water sustainable; treatment index; water reuse.

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Introduction As one of the three major sources of energy, coal account for 28.1% in the world primary energy consumption. Pursuit of creating a greener environment has directly boosted clean and efficient utilization of coal energy. In emerging coal clean utilization industries, coal is gasified, liquefied to further produce a large number of chemicals and fuel besides for energy production as shown in the Figure 1.

Figure 1. Main production chain for coal based energy and chemicals The emerging coal clean utilization industries located in all over the world. Such as Sasol indirect coal liquefaction project in South Africa with annual consumption of coal is 33Mt. The world largest coal based integrated ammonia and urea plant is established in Australian in 2013 with capacity of 3500 t/d ammonia and 6200 t/d urea. The Beaumont project in Texas can produce 225 million gallons of methanol and 255,000 tons of ammonia per year taking coal as raw materials. China is the largest energy consumer in the world. Coal is the main fuel1 in China accounting for 65% as shown in Figure 2. . Coal dominant characteristics will be in energy and chemical industries for a long time. 3



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Coal

Oil

Natural gas

Nuclear

Hydro power

Renewable resources

2% 1%

8%

6%

18% 65%

Figure 2. Energy consumption structure of China in 2015

To realize the sustainable development of coal chemical industry has become an imperative to the utilization of coal resource.3,4 In China, coal chemical projects are mostly located in the northwest with rich coal resources. However, the regions face with the severe shortage of water resource, inadequate capacity of water environment and even lack of pollution-holding water body.5 Water resource has become the focus of sustainable development of coal chemical industry.6 Thus, “Water Pollution Prevention and Control Action Plan” is issued by the State Council in 2015 and the “Environmental Access Conditions for Modern Coal Chemical Construction Projects (Trial)” are meanwhile issued by the Ministry of Environmental Protection. These two government documents have given strict requirements for coal chemical water pollution prevention and control. Zero liquid discharge (ZLD) for coal chemical processes is the target necessary for sustainable productions.7 With differences in feed coal (brown coal, bituminous coal and anthracitic coal) and in coal chemical process (fixed bed, fluidized bed and entrained bed), the compositions of coal gasification wastewater have a tremendous difference.8 The crude gas generated by fixed bed gasification is high in the hydrogen and methane. 4


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Furthermore, fixed bed gasification is adapted to inferior coal such as brown coal. Thus, fixed bed gasification is broadly applied in China. However, the wastewater generated by the fixed bed gasification process is large in amount and complex in composition. ZLD for coal gasification wastewater is difficult and long-term and key researches focus. In order to implement ZLD, attentions in the industrial and academic circles have been paid on treating high-concentrated organic wastewater and reusing saline wastewater. For insoluble pollutants such as tar and other oils in wastewater, Li9 proposed nitrogen flotation degreasing for pretreatment of wastewater. Qian et al.10 proposed a phenol and ammonia recovery process. With this technology, the total phenols content in wastewater is decreased efficiently and the treated water can goes directly to biochemical treatment unit. As one of the improved anaerobic treatment process, a two-phase anaerobic treatment process shows favorable effects in treating the fixed bed coal gasification waste water.11 55% to 60% of COD components can be removed by this technology. Wang et al.12 reported in their research that anaerobic co-metabolism can significantly improve biodegradability of coal gasification wastewater. With 500 mg/L methanol, the removal rate of COD and phenol in the anaerobic system can increase to 71% and 75%. According to experimental analysis, 4g/L powdered activated carbon (PAC) is stuffed on a membrane bioreactor (MBR) to improve COD removal. This can increase the removal rates of COD, total phenol and ammonia nitrogen upto 93%, 99% and 63%, respectively.13 The wastewater after biochemical treatment is mixed with other saline wastewater. This waste water need to be treated by membrane before reuse back to production operation. Jin et al.14 adopted the ultra-filtration and reserve osmosis (UF/RO) dual-membrane process to deeply treat the waste water. This research finds out that the RO system runs well with the steady desalinization ratio. In terms of industrial applications, the dual-membrane process has been widely applied for water reuse. The corresponding successful applications can be found in the Yili Xintian Coal-to-SNG Project, Tuk Chemical Fertilizer Project, and State Power Investment 5



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Yinan Coal to SNG Project in China.15 The concentrated brine separated by membrane treatment need to be treated further through a vaporization and crystallization unit to further recovering water resource. These techniques include multi-effect evaporation (MEE), mechanical vapor recompression (MVR) and natural evaporation.16 The treatment efficiency of individual wastewater treatment technologies has been tested by industrial practices. However, when these technologies work together for implementing ZLD, the whole process is difficult to achieve satisfying effects. Few research attentions are paid on synthesis of the whole water treatment processes. Very few scholars study on how to synthesize different treatment technologies to implement ZLD. For the above problem, the typical fixed-bed coal gasification wastewater treatment process is taken as the study case in this paper. The case is established referring to several practical industrial processes. The shortcomings of the existing water treatment system are analyzed and explored depending on the mass balance. A new index system, including the index of different units, is built for zero liquid discharge. We give out suggestions on improvement and optimization of the treatment process.

A treatment process for zero liquid discharge For ZLD study, we firstly built a water treatment case process for fixed bed coal gasification wastewater treatment. This treatment case process refers to existing running processes, i.e. Datang coal to SNG project with the scale of 4 billion m3 SNG in Inner Mongolia, Qinghua coal to SNG project with the scale of 5.5 billion m3 SNG in Sinkiang and some others shown in supporting information. We built the treatment case by six basic units. They are gas liquor separation, phenol & ammonia recovery process, biochemical treatment, advanced treatment, water reuse unit and multi-effect evaporation & crystallization. The main purpose of the gas liquor separation unit is to separate high insoluble oil of wastewater generated by fixed bed gasifier. In the unit, insoluble oil is separated from wastewater through gravitational sedimentation. The conventional oil separation process is adopted for this unit in this paper.18 High 6


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concentrated phenols and ammonia in the wastewater are removed and recycled through the phenol and ammonia recovery process. In the phenol and ammonia recovery process, ammonia-removal is first conducted by stripping. Phenols are removed by solvent extraction. Treated water leaves the unit and goes into the biological treatment unit.19 The purpose of biochemical treatment is to remove organic components and ammonia nitrogen from wastewater. It is mainly made up of aerobic process and anaerobic process.20 The effluent of anaerobic process will be further processed by aerobic process to eliminate organics. Under the effect of nitrification, ammonia nitrogen in wastewater is transformed into nitrate and nitrite. However, there are many different organic pollutants hard to decompose in the wastewater. Advanced treatment is required to remove or decompose these organics. The current advanced treatment is mainly to remove ammonia nitrogen and organics hard to decompose in the effluent after biochemical treatment. The effluent is usually treated by physical and chemical methods, such as coagulating sedimentation, absorption of active carbon (active coke) and advanced oxidation, or combination biological techniques, such as membrane bioreactor (MBR) and biological aerated filter (BAF).21 Advanced treatment in this case is suggested to be the combination of coagulating sedimentation, advanced oxidation and BAF. For zero liquid discharge, recycling and reuse of water resources have to be included. In this benchmark case, the water reuse unit consists of two membrane treatment systems. The water is treated by ultra-filtration (UF)/reverse osmosis (RO) dual-membrane treatments. As a pre-treatment process of the RO system, UF is mainly to remove suspended solids, colloids and organics to decrease turbidity and sludge density index (SDI) of the wastewater.22 The UF/RO dual-membrane separation system can concentrate the wastewater by three to ten times. The water reuse rate can approach up to 60%. The recycled water obtained through UF/RO can be used as cooling water, while the concentrated brine separated is sent for continue processing. In the concentrated brine wastewater, the concentration of Ca2+, Mg2+ and SiO2 is high. These ions can easily form scale on the membrane surface. It is necessary to adopt additional method to 7



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slow down fouling and scaling of membrane. The high efficiency reverse osmosis (HERO) is developed and included in the benchmark case. In order to realize zero liquid discharge of coal gasification wastewater, the concentrated brine is treated by the evaporation and crystallization. The multi-effect evaporation (MEE) is chosen in this study since it is widely used in industrial applications.23 The benchmark case of wastewater treatment for fixed bed coal gasification processes is shown in Figure 3.

Figure 3. Flow sheet of the water treatment process

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Analysis of the water treatment process Analysis of feed wastewater quality For zero liquid discharge, waste water analysis of coal gasification processes is of the most importance. Specific pollutants in the wastewater are divided into eight groups, i.e. insoluble oils, sour gases, ammonia nitrogen, phenols, heterocyclic compounds, monocyclic and polycyclic aromatic hydrocarbons (MAHs and PAHs), long-chain alkanes and fatty acids. The water analysis is collated and collected from different reports and literatures, as shown in Table 1.

Table 1 Water quality of a fixed bed coal gasification wastewater Types Insoluble oils24 Sour gases25 Ammonia nitrogen10

Phenols19

Heterocyclic compounds26

MAHs and PAHs27

Long-chain alkanes28

Pollutants Conctrn.(mg/L) Middle distillate 12000 Tar 15000 CO2 4500 H2 S 300 Free ammonia 5600 Fixed ammonia 2400 Phenol 2000 Methylphenol 1000 Dimethylphenol 200 Trimethylphenol 150 Ethylphenol 150 O-dihydroxybenzene 600 Resorcin 500 Hydroquinone 1500 Methyl diphenol 150 Polyphenols 300 Indoline 150 Pyridine 100 Quinoline 1100 Imidazole 100 Benzene 300 Methylbenzene 200 Ethylbenzene 250 Biphenyl 50 Naphthalene 20 450 9



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Fatty acids29

Acetic acid Propionic acid Butyric acid Valeric acid Hexanoic acid

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600 200 200 200 200

Insoluble tar and oil is mixed with crude gas. They are washed off into the washing water. Due to low gasification temperature, contention of tar and oil in the waste water is above 10000 ppm. Zhang30 reported that the tar concentration is 0.8%~1 w% and the oil concentration is 0.12%~0.5 w%. The specific concentrations can be found in Table S1, referring to the waste water from several fixed bed coal gasification processes. Sour gases in the wastewater are CO2 and H2S. The contention of CO2 takes a larger proportion with 4000-11000 mg/L.10 The concentrations from different processes are given in Table S2. Ammonia nitrogen in the wastewater has two forms, free ammonia and fixed ammonia, with the concentrations of 3000-9000 mg/L and 1500-4000 mg/L.31 Phenols in the waste water takes the highest proportion, with the concentration between 5000 and 10000 mg/L accounting for 40%~60% of the waste water COD.15The concentration of phenols in several processes can be found in Table S3.19 Phenols in the waste water mainly contains phenol, methylphenol and hydroquinone. Besides phenols, the wastewater contains many heterocyclic compounds, MAHs and PAHs, long-chain alkanes, and fatty acids. Data collected by Engelbart, a Germany company, from South Africa Sasol coal to oil project, showed that the content of quinoline is above 1,000 mg/L.28 Other heterocyclic pollutants, such as indoles, pyridines and imidazoles, are low in content. In the wastewater, aromatic hydrocarbons include

benzene,

methylbenzene, ethylbenzene,

biphenyl and

naphthalene. Amongst them, the proportion of MAHs is relative high, with the concentration around 300mg/L.32 The alkanes in wastewater are mainly long-chain alkanes of C20 - C30.26 Wu et al. used the gas chromatographic method to test the concentration of fatty acids in coal chemical wastewater. They found out that 10


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short-term fatty acids of C2 - C6 are in the majority. The contention of acetic acid is the highest about 583 mg/L.29 Insoluble oils have a high viscosity and might cause blocking of the pipeline and equipment. Thus, it is considered to be removed at first of the ZLD process. Gravitational sedimentation can be adopted to separate these oils.33 In order to guarantee the separation effects of insoluble oils, flotation can be added after gravitational sedimentation. Conventional flotation adopts air as the medium. Corresponding research shows that air flotation can reduce biodegradability of wastewater. Some efforts considers nitrogen flotation as a favorable choice and the corresponding successful industrial practices can be found.9 Sour gases and ammonia are soluble gases. The most commonly-adopted separation method is steam stripping. Sour gas and ammonia can be separated by a dual-tower steam stripping or by a single stripper with side-draw.34 Phenols are major organic pollutants in the wastewater. There are many processes which can be used to remove phenols from wastewater, such as distillation, extraction, absorption, membrane evaporation and liquid membrane extraction.35 Dephenolization using solvent extraction is widely adopted to separate phenols from the wastewater. Solvent extraction can recycle crude phenols to obtain certain amount of economic benefits.25 The total phenols and COD in wastewater can be reduced to about 500 mg/L and 4,000 mg/L. Fatty acids in the wastewater can be easily removed through biochemical treatment. Wang et al. found that fatty acids in wastewater are fully eliminated by anaerobic biochemical treatment.26 Apart from phenols, heterocyclic compounds, MAHs, PAHs and long-chain alkanes belong to the typical organics. They are hardly decomposed. Treatment process such as anaerobic biochemical reaction and aerobic biochemical reaction should be combined to realize degradation of these pollutants. The anaerobic treatment process can effectively decompose phenols (e.g. resorcinol and hydroquinone), long-chain alkanes (e.g.

tetracosane, heptacosane and

hexacosane), and some PAHs and heterocyclic compounds.36 Under the anaerobic 11



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conditions, these pollutants can be converted into intermediates. They are easier to be degradated.37 Intermediates and pollutants hard to be decomposed in anaerobic process can be further treated through the aerobic biochemical process. Research shows that pyridine, quinolone and benzene can be fully removed through a three-stage rotating biological contactor (RBC).38 Experimental study showed that after a series of anaerobic and aerobic reaction, hazardous pollutants in fixed bed coal gasification wastewater decrease both in quantity and variety. The final COD concentration can be reduced to about 80 mg/L. The BOD5/COD ratio is around 0.06. This means biodegradability of the wastewater is low. Residues in the wastewater include heterocyclic compounds and long-chain alkanes.39 In order to further eliminate these pollutants, advanced physical and chemical methods should be adopted, such as heterogeneous catalytic ozonization, TiO2 photocatalytic oxidization and catalytic ultrasonic oxidization.

Mass balance of the water treatment process Zero liquid discharge system of fixed bed coal gasification wastewater consists of the water use unit and the wastewater treatment unit. The topology diagram of the system is shown in Figure 4. The water reuse unit includes a desalination unit, cooling water unit, and gasification and conversion unit. The desalination plant provides desalted water for the coal gasification process and the brine produced enters the water reuse unit. The wastewater of circulating water system also enters the water reuse unit. The gasification and conversion unit are major production units of the fixed-bed coal gasification wastewater. Based on practical operation data, the mass balance of the fixed-bed coal gasification wastewater ZLD system is analyzed and shown in Table 2. These data refers to on-line running data or quarterly operation reports from Datang coal to SNG project, Qinghua coal to SNG project and some other project shown in supporting information. The wastewater is around 1800 t/h. In this study, the water involved in the reaction is not taken into account for mass balance.

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Figure 4. The coal gasification wastewater treatment process

Steam (Stream 14) and coal moisture (Stream 15) are the water input for gasification and conversion unit. After reaction, the wastewater produced by gasification and conversion unit (Stream 1) is 1800 t/h. Content of pollutants in the waste water are presented in Table 1. After the gas liquor separation unit, the dissolved crude gas and insoluble oils are removed. The content of insoluble oils lowers down to around 300 mg/L. The effluent flow of the unit is divided into two sub-streams. One is Stream 2 with the flow rate of 700 t/h. It is fed back to the gasification and conversion unit to clean and cool crude gas. The other stream is Stream 3, with the flow rate around 1100 t/h. This stream is fed into the phenol and ammonia recovery unit. In the phenol and ammonia recovery unit, the steam stripping is firstly used to remove sour gas and ammonia. After that, the concentration of acid gases can be reduced to 50 mg/L, while that of ammonia nitrogen to 200 mg/L. Dephenolization using solvent extraction is adopted to reduce the total phenols to 450 mg/L.19 The extraction solvent also removes part of other organics at the same time. Monocyclic and polycyclic aromatic hydrocarbons and heterocyclic compounds have the similar molecular structure with phenols, and are easy to be removed. Their concentrations are around 120 mg/L and 650 mg/L. The concentration of fatty acids and long-chain alkanes is reduced to 860 mg/L and 230 mg/L. The COD after the recovery is reduced to the value between 4,000 and 5,000 mg/L.27 13



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In the proposed benchmark process, the biochemical process contains anaerobic treatment and aerobic treatment. The biochemical treatment techniques pay more attention on degradation of phenols. Thus, the total phenols in biochemical treatment can be lowered down to around 2-8 mg/L.40 Fatty acids are easy to be decomposed, and none fatty acids is found after the treatment. The content of MAPs and HAPs is also low, with the total concentration of about 10 mg/L. Organic pollutants in the biochemical effluent can mainly be divided into two groups - heterocyclic compounds and long-chain alkanes, with the concentrations of around 80 mg/L and 40 mg/L.39 Heterocyclic compounds inhibit activity of biological species. Removal and degradation of these pollutants is difficult by biochemical treatment method. Meanwhile, metabolism of long-chain alkanes is time-consuming. The designed duration time for biochemical treatment is often not long enough and make removal of these pollutants is also inefficient. Due to the above problems, COD of the effluent is still about 350 mg/L, while the concentration of ammonia nitrogen is around 50 mg/L. Some running treatment processes include advanced treatment units for further treatment of organics. After the treatment, the concentration of total phenols can be reduced down to less than 1 mg/L. The concentrations of heterocyclic compounds, aromatic hydrocarbons and long-chain alkanes can be reduced down to about 15 mg/L, 2 mg/L, and 10 mg/L, respectively. The total COD can decrease down to 80 mg/L. As shown in the figure, the wastewater after advanced treatment (Stream 6) is mixed together with the discharge from the desalination unit (Stream 7) and cooling water unit (Stream 8). The mixture then goes into the reuse unit, and the overall flow rate is 2100 t/h. The mixed water is with low COD content and a high salt content. After the dual-membrane treatment, the flow of the recycled water (Stream 10) is 1960 t/h. The content of organic pollutants and soluble solids is low. The membrane system suffers from a low interception effect of ammonia nitrogen. The content of ammonia nitrogen in the effluent is still as high as 12 mg/L. The membrane treatment can separate the concentrated brine with a flow of 140 t/h from the reused water. The 14


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content of total dissolved solids (TDS) is high, up to around 29300 mg/L. The concentrated brine then enters to the evaporation and crystallization unit. The condensate (Stream 11) generated by evaporation reused as cooling water with a flow rate of 112 t/h. The reuse water (Stream 12) produced by the system is the combination of Stream 10 and Stream 11. The total reuse water is 2072 t/h, with the content of ammonia nitrogen is about 12 mg/L and that of COD is around 50 mg/L.

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Table 2 Mass balance of the water treatment process

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Flow (t/h) 1800 700 1100 1100 1100 1100 600 400 140 1960 112 2072 2800 1044 56 2828 28

Insoluble oils 27000 300 300 50 -

Acid gas 4800 4800 4800 50 -

Ammonia nitrogen 8000 8000 8000 200 50 20 5 5 25 12 15 12 -

Total phenols 6550 6550 6550 450 5 1 5 0.2 0.1 0.2 -

Heterocyclic compounds 1450 1450 1450 650 80 15 20 7 7 -

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MAPs & HAPs 820 820 820 120 10 2 1.7 1 1 -

Long chain alkanes 450 450 450 230 40 10 8.6 5 5 -

Fatty acids 1400 1400 1400 860 15 20 51 5 24 6 5 -

TDS 2000 2000 2000 2000 2000 2000 2500 2000 29300 205 112 200 200 -

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The total water input of the system is 2856 t/h. One is fresh water (Stream 13) with a flow of 2,800 t/h and the other is the water in coal (Stream 15) with 56 t/h. As for water loss, the ZLD system has the one in the crystalized salt (Stream 17) about 28 t/h and that in evaporation of the cooling tower (Stream 16) about 2,828 t/h. None liquid is directly emitted from the system. However, in practice, the reuse water usually does not meet the quality requirement. This might cause a series of problems such as heat exchanger sever fouling and corrosion.

Shortcomings of the treatment process The standard indexes for water reuse are not clear yet for coal chemical process. Thus, water reuse standard indexes for other industrial fields are referred as shown in Table 3. Please see Table S4 for more details. These indexes refer to those in petrochemical processes. Table 3 Several water reuse standard indexes in industries Pollutants Index 1 Index 2 Index 3 Index 4 pH 7.0-8.5 6.5-8.5 6.0-9.0 6.5-8.5 Turbidity 5 3 10 5 Suspended solids 10 0.5 20 30 CODCr 30 40 80 60 Oils 5 1 0.5 Volatile phenol 0.5 0.5 5 5 10 10 Ammonia nitrogen Total hardness 250 250 350 450 Total alkalinity 200 300 350 Chloridion 250 200 500 Total phosphorus 1 1 1 1 TDS 1000 800 1000 1000

The comparison between water quality of Stream 12 and the data in Table 3 shows that the COD of this stream meet the requirements of reuse. However, the content of ammonia nitrogen exceeds the standard. This is the main cause for heat exchange corrosion. Furthermore, we found that ammonia is accumulated in the whole process (shown in Figure 5). Some ammonia in Stream 6 passes through the membrane without treatment to the reuse water (Stream 10) and the other ammonia 17



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enters the multiple-effect evaporation and crystallization unit (Stream 9). Most of the ammonia is evaporated during the evaporation process (Stream 11). The Streams 8 12 forms a closed loop and has no ammonia nitrogen emission and removal unit. Meanwhile, there is an input of new ammonia nitrogen (Stream 6). This will lead to continuous increase of ammonia content in the reuse water. Stream Content (mg/L) 6 20 8 5 9 25 10 12 11 15 12 12

Figure 5. Ammonia close loop in the ZLD process

Ammonia degradation effects by current biochemical treatments are far from satisfaction. Microbial metabolisms of pollutants are interacted with each other. The interactional relationship of those metabolisms is illustrated in Figure 6. Those of Phenols, heterocyclic compounds and aromatic hydrocarbons inhibit that of nitration. They restrain nitrification and denitrification of the ammonia. Furthermore, the degradation of heterocyclic compounds produces ammonia nitrogen. Thus, for better efficiency of the ZLD system, it is better to systematically reduce these pollutants content to appropriate levels.

Figure 6 Interactional relationship of metabolism of pollutants

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New water treatment index system for ZLD To implement ZLD, it is of the most importance to ensure the water quality for reuse back in production processes. However, there has been no reuse water standard index for coal chemical process. In practice, coal chemical plants are referred to the experiences from other industrial production fields. As a result, it is difficult to find a unified standard for water reuse. Thus, this paper attempts to give our suggestion on a unified standard index of reused water. The index is proposed by referring the standards form other industries as shown in Table 3. Based on the above indexes 1-4 and the practical wastewater treatment system operation of certain coal chemical projects, this paper proposes a set of reuse standard indexes as shown in Table 4.

Table 4 The new reuse water quality indexes Items Control indexes pH 6.5-8.5 COD ≤40 Ammonia nitrogen ≤5 Total phenols ≤0.2 Heterocyclic compounds ≤5 Long-chain alkanes ≤5 Fatty acids ≤5 TDS ≤400 Turbidity ≤3 Suspended solids ≤10 Total hardness (Calculated by CaCO3) ≤300 Total alkalinity (Calculated by CaCO3) ≤250 Chloridion ≤100 Total phosphorus ≤1

Unit mg/L mg/L mg/L mg/L mg/L mg/L mg/L NTU mg/L mg/L mg/L mg/L mg/L

According to the above discussions, major quality indexes for water reuse are analyzed as following: (1) The COD of water reuse in current coal chemical process is around 50~80 mg/L, referring to several running processes.15,20 We propose that it is 19



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possible to fix the standard to a further low level of 40 mg/L. This is an achievable goal by selection of suitable process technologies and further optimizes the treatment process. (2) The concentration of ammonia nitrogen for water reuse is currently selected to 10 mg/L in the ZLD system. This is unreasonable for the circulating water system, since it might accelerate growth of microorganisms, and cause corrosion of heat exchangers and pipelines. The content of ammonia nitrogen in recycled water should refer to the index shown in Table 3, less than 5 mg/L; (3) Organic pollutants including phenols, heterocyclic compounds, long-chain alkanes and fatty acids. The interception by the membrane is inefficient. Those pollutants are kept in the circulating water system, with the concentrations of 0.2 mg/L, 7 mg/L, 5 mg/L and 5 mg/L, respectively. In order to avoiding for accumulation of these pollutants, the concentration of these pollutants should lower than the above values. (4) After dual-membrane desalting and evaporation desalination, the content of TDS in recycled water is significantly decreased. Thus, TDS index for coal chemical processes should be lower than the current index for petrochemical processes. The wastewater can reach the reuse standards after a series of treatments. In order to help reused water quality reach standards, it is of the most importance to control the water indexes at specific joint point between units. These indexes proposed in this paper are shown in Figure. 7. Referring to the technical demands for several membrane systems (Referring to Table S5), the waste water enters into the membrane system (Stream 6) should has the COD lower than 50 mg/L. At this situation, the fouling of the membrane system is reduced to the accepted value, and the lifetime of the membrane system is therefore increased. Research suggests that the interception rate of low molecular weight organics by the RO system is around 60%-90%.41 For this, the concentrations of total phenols, heterocyclic compounds, 20


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long-chain alkanes, MAHs and HAPs should be controlled to lower than 0.5 mg/L, 10 mg/L, 10 mg/L and 1 mg/L, respectively. After the biochemical treatment and the advanced treatment, these specific pollutants are mostly removed. As for ammonia nitrogen, the interception by the membrane system is not efficient.42 To meet the quality standards of recycled water, the content of ammonia nitrogen should be reduced to less than 5 mg/L before fed into the membrane system.

Figure 7 The new indexes at joint points

After the biochemical treatment and the advanced treatment processes, the removal rate of COD and ammonia nitrogen in wastewater is found to be about 95~98% and 90~95%.20,43

The content of COD and ammonia nitrogen in Stream 4 in Figure

7 should be under 3,000 mg/L and 50 mg/L. This is to ensure to meet the quality standards of Stream 6. As for phenols, past work from our group found that it is possible to be further lower down in the phenol and ammonia recovery unit. In lab scale the concentration of phenols can reduce to 130 mg/L by new solvent and new extraction conditions.44 Studies show that the removal rate of heterocyclic compounds is 80% by biochemical treatment45 and 90% by advanced treatment.46 In order to achieve the concentration of heterocyclic compounds lower than 10 mg/L in Stream 6, the heterocyclic compounds content of Stream 4 should be lower than 400 mg/L. Stream 3 contains high content of oils and dust, resulting in blocking of heat exchangers and towers in the phenol and ammonia recovery process. The current gravity sedimentation is inefficient, causing frequent maintenance hold every 9 to 12 months.47 Thus, it is important to control the content of insoluble oils less than 200 21



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mg/L and that of ash to be lower than 50 mg/L, depending on our industrial experiences obtained from Harbin gasification plant and Tuk Chemical Fertilizer Project.

Conclusion It is recognized that zero liquid discharge is of the most importance for sustainability of coal based chemical processes, due to severe shortage of water resource at the regions of these plants. At present, water treatment systems for the coal chemical projects are not running efficiently and far from zero liquid discharge. This is because different techniques are simply combined without systematically considering their interrelationship. Thus, this paper proposed a water treatment index system to combine each unit of the whole process. The study of wastewater quity and the mass balance of the ZLD system show that special attention need to be paied on the concentration limits of polyphenols, PAHs, long-chain alkanes, and ammonia nitrogen in the water treatment index system.. This index system is proposed depending on references of those in other industrial field and some running experience from existing projects. The water quality of water reuse is determined first. Then the indexes at each joint point between units are calculated based on the removal efficiency of each point. Suggestions on how to implement this ZLD system is also given in this paper in the last part. The authors believe this paper gives reasonable suggestions for sustainable development of coal based chemical processes in China.

Acknowledgements We would like to express our appreciation to National key research and development program (NO. 2016YFB0600501) for their great funding and support of this study.

Supporting Information Supporting information is supplied. Table S1-S3 is the data of wastewater water 22


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quality we collected. Table S4-S5 is the water index of some other industry.

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Zero liquid discharge of coal chemical wastewater - key point for sustainable coal chemical industry

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New Water Treatment Index System toward Zero Liquid Discharge for

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