Coke plant wastewater (CPW) is an intractable chemical wastewater, and it contains many toxic pollutants. This article presents the results of research on a semi-industrial adsorption method of coking wastewater treatment. As a sorbent, the coking coal (CC) was a dozen times less expensive than active carbon. The treatment was conducted within two scenarios, as follows: (1) adsorption after biological treatment of CPW with CC at 40 g L−1; the chemical oxygen demand (COD) removal was 75.66%, and the concentration was reduced from 178.99 to 43.56 mg L−1; (2) given an adsorption by CC of 250 g L−1 prior to the biological treatment of CPW, the eliminations of COD and phenol were 58.08% and 67.12%, respectively. The CC that adsorbed organic pollution and was returned to the coking system might have no effect on both coke oven gas and coke.
Coke plant wastewater (CPW) is generated in the coal coking, coal gas purification and by-product recovery processes of coke plants in the iron and steel industries (Kim et al. 2008). Thus, the chemical composition of wastewater is very complex. The wastewater contains the following components, among others: ammonia, cyanides, thiocyanates, phenols, sulphide and other organic compounds, such as polycyclic aromatic hydrocarbons (PAHs) (Burmistrz & Burmistrz 2013; Burmistrz et al. 2014). Phenolics are the primary organic constituents, accounting for approximately 80% of the total chemical oxygen demand (COD) (Zhang et al. 1998), and the concentrations of heavy metals and phosphorus are very low. The concentration of the primary pollutants in CPW depends on the quality of the raw coal, the carbonation temperature and the method used for by-product recovery (Zhang et al. 1998).
CPWs have been considered among the most toxic emissions that are discharged into the environment. Biological treatment is the primary action of the CPW treatment system, and 94% of pollutants as assessed by COD means are primarily removed during the biological treatment process (Burmistrz et al. 2014). The anoxic–oxic and anaerobic–anoxic–oxic methods are common biological treatments in China (Yu et al. 1996, 1997; Lee & Park 1998). Despite complex treatments, CPWs can have adverse impacts on ecosystems and human health (Zhang et al. 2013).
Adsorption is a simple method that is widely used in the field of wastewater treatment. Many studies have confirmed the removal of residual phenols (Vazquez et al. 2007), PAHs and other organic compounds (Zhang et al. 2013; Tong et al. 2014; Apul & Karanfil 2015) from industrial wastewaters after the use of activated carbon, coke dust and carbon nanotubes.
The selection of the adsorbent is the key part of CPW treatment by adsorption. However, the high price and cost of regeneration is the biggest barrier to the implementation of commercial sorbents. The most common thermal regeneration of coal consumes a large amount of energy and results in a 5–10% mass loss from regenerated coal (Asghar et al. 2013). An attempt to overcome this barrier was made by using materials with fairly good sorptive properties at the lowest possible price and without the need for regeneration.
Wang et al. described a sustainable wastewater treatment technology (Wang et al. 2014). The technology was based on the following two key elements: a sorbent with good sorptive properties along with a low price and a sorbent that could go back into the coking system after adsorption instead of entering a regeneration process. The aim of this study was to explore the sorptive properties of coking coal (CC) for CPW and to investigate the toxic pollutants on the sorbent after adsorption to find whether it can be converted into a harmless substance during the coking process.
MATERIALS AND METHODS
Characterization of CC
CC was taken from a coke plant in Mongolia, China. It was the raw material of coke with a particle size of −3 mm, and it was ground up to obtain the 80% mass of −0.074 mm fine coal.
The average particle size of adsorbent material was determined by using a laser particle size analyser. The surface area and pore volume were analysed by the Brunauer–Emmett–Teller (BET) method by using N2 adsorption.
Adsorption of CPW after biological treatment by CC
CPW was treated by physico-chemical and biological treatments through the anaerobic–anoxic–oxic method; it was introduced to a mixing tank with a volume of 30 L, the pH value was adjusted to 4, and then different quantities of CC were added. The average time the wastewater remained in the mixing tank was 5 min. The effluent wastewater with the CC was pumped into a filter, from which water that meets the discharge standards of China can be obtained. The schematic diagram of the technological system for research on adsorptive CPW treatment is shown in Figure 1(a).
During that period, wastewater samples were taken from points A1 and A2.
Adsorption of CPW before physico-chemical and biological treatment by CC
The operation of the raw coke wastewater plant was similar to that of wastewater by biological treatment. The wastewater was introduced to a mixing tank with a volume of 30 L, the pH value was adjusted to 4, and then CC was added. The adsorption time was also 5 min.
The scheme for this process is presented in Figure 1(b). During the test, wastewater samples were taken at points B1 and B2.
Analytic method for testing CPW
All CPW samples were analysed as soon as possible after filtration for their COD, ammonia, and phenol content. These parameters were determined by potassium dichromate oxidation, salicylic acid spectrophotometry, and the 4-AAP spectrophotometric method (National Environment Bureau Water & Wastewater Monitoring Analysis Committee 2002).
RESULTS AND DISCUSSION
Characteristics of CC
The CC particle size was −0.125 mm and the content of −0.074 mm particles was approximately 81%. The special surface area was 11.447 m2g−1. The total pore volume and average pore diameter were 0.016 cm3g−1 and 5.703 nm as measured by the N2 adsorption isotherm with BET surface area equipment (BELSORP-max ver. 2.1, Japan), respectively. In contrast to activated carbon, the CC had a smaller specific surface area and pore volume (Zhang et al. 2010). However, it had a greater share of mesopores, which was beneficial for adsorbing large molecule pollutants from aqueous solution (Li et al. 2002).
Figure 2 shows a scanning electron microscope (SEM) photograph of CC. This image illustrates that the CC surface was rough and uneven, and its porosity was helpful for adsorbing organic compounds.
Addition of CC after biological treatment
The indicators showed that the post-biological treatment wastewater met the discharge standards of China, except for the COD. Thus, COD was the indicator that was detected after adsorption during the experiment.
The semi-industrial test results for the adsorptive CPW treatment confirmed the high efficiency of the process. More than 70% of the COD was removed from the wastewater (the COD concentration was reduced from 178.99 to 43.56 mg L−1). Figure 3 reveals the effects of different CC doses on the COD removal efficiency. This finding illustrated that the COD removal rate was 72.57% when the CC dose was only 20 g L−1. The COD removal rate increased gradually from 72.57 to 75.66% when the CC dose increased from 20 to 40 g L−1. This increase was related to the number of active sites and the increased surface areas with the CC dose increase, which was helpful for the removal of COD. CC addition resulted in the efficient removal of organic pollutants. An average of 6.49 g COD kg−1 CC was removed. Figure 4 provides a view of wastewater after biological treatment before and after adsorption with different CC doses. The colour of the wastewater after adsorption was crystal clear.
Addition of CC prior to biological treatment
Preliminary work indicated that there were numerous organic compounds in the wastewater, especially PAHs and oily and tar-like substances. These materials had strong toxicity on micro-organisms. Burmistrz et al. showed that oily and tar-like substances were primarily removed during physico-chemical processes (Burmistrz et al. 2014), and thus it was important to add a pre-treatment before the biological treatment.
Adsorption by CC resulted in a greater elimination of oil, tar substances and PAHs during physico-chemical treatment, and these results were assessed by using the COD. When the CC dose was increased from 100 to 250 g L−1, the degree of COD elimination increased from 29.33 to 58.08%, the ammonia went from 7.80 to 12.62%, and the phenol increased from 19.14 to 67.12%. These changes caused a significant reduction in the load of compounds that could inhibit biological processes, and better conditions for micro-organisms were thus established in the activated sludge, causing an intensification of biodegradation, nitrification, denitrification and thio-oxidation processes (Burmistrz et al. 2014).
As shown in Figure 5, the CC had high removal efficiency for organic matter, for example phenol and COD, and its ammonia removal was low. This finding may be explained by the fact that the CC is primarily based on organic matter, and the surface contains a plurality of functional groups, carbon and oxygen atoms included. However, the ammonia was an ionic contaminant, and it was also strongly hydrophilic. Therefore, the removal efficiency of ammonia was far less than that of COD. Cai et al. reached the same conclusion (Cai et al. 2010). Figure 6 provides a view of the wastewater prior to the biological treatment before and after adsorption with different quantities of CC. As shown in Figure 6, the CC had an efficient removal rate for chromaticity.
The adsorption of phenolic compounds on CC implies the formation of electron donor–acceptor complexes in which the basic surface oxygen groups act as acceptors. Furthermore, the dispersive forces between the π-electrons in the carbons have a significant impact (Ahmaruzzaman & Sharma 2005).
The simulation of the coking system
To study the impact on coke oven gas and coke during the coking process after adsorption, tests were performed to simulate a coke oven by using a tube furnace at a heating rate of 5 °C/min to raise the temperature to 900 °C, and 1.5 g of CC was added to the furnace cavity. The composition and content of the coke oven gas were then analysed on-line by gas chromatography, with argon as a carrier gas.
As shown in Table 1, the composition and content of the coke oven gas produced by CC before and after adsorption were similar. This similarity may be related to the fact that CC adsorbed a smaller amount of organic compounds compared with the volatile components of the coal, and the organic matter may be changed to CO or short-chain alkanes at a temperature of 900°C. Thus, the CC after adsorption that was returned to the coking system may have no impact on the coke oven gas.
|Content||CC (%)||CC after adsorption (%)|
|Content||CC (%)||CC after adsorption (%)|
The surface area and pore size of semi-coke have a significant impact on the reaction rates, gasification and combustion process (Zhou et al. 2013), and the pore is the primary part of the physical structure of semi-coke. BET equipment could therefore be used to analyse the surface area and pore volume of semi-coke. The surface areas of semi-coke produced by CC before and after adsorption were 113.4 m2 g−1 and 113.6 m2 g−1, the pore volumes were 0.15 mL g−1 and 0.14 mL g−1, and the average pore diameters were 6.385 nm and 6.392 nm, respectively.
The CC after adsorption had no effect on the surface areas and pores of semi-coke. This lack of effect may be related to the character of adsorption, which was primarily determined by surface properties, and the quality of semi-coke is determined by the composition of raw coal (Zhou et al. 2013).
CC, which is the raw material of coking, has special and beneficial properties for use in the adsorptive treatment of CW, which contains large molecules of organic pollution.
Adsorptive treatments of CPW after biological treatment, in which 40 g L−1 of CC was obtained from the raw material of the coking process, were able to remove 75.66% of the COD, and an average of 6.49 g COD kg−1 CC was removed.
Adding CC before the biological treatment of wastewater reduced the concentration of biodegradation inhibitors. The removal of these inhibitors established better conditions for micro-organisms in the activated sludge and intensified the biodegradation, nitrification and denitrification processes.
Because CC is primarily based on organic matter, and the surface contains many functional groups that contain carbon and oxygen atoms, the CC had high removal efficiency for organic matter. Ammonia is an ionic contaminant, and thus the CC led to lower ammonia removal.
Through the detection of coke oven gas and semi-coke, which was produced by the CC before and after adsorption, the findings indicated that the raw CC after adsorption may have no effect on the by-products.
This research was supported by the Fundamental Research Funds for the Central Universities Grant (no. 2014XT05), for which the authors express their appreciation.