Chinese J. Chem. Eng., 14(3) 398-401 (2006) RESEARCH NOTES

Pretreatment of Coal Gasification Wastewater by Acidification Demulsion

ZHANG Wenqi(%%g), MA Jun(4 %)*, YANG Shidong(t%g%), ZHANG Tao(%@) and LI Yongfeng(+%q) School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China

Abstract General pretreatment processes of ammonia stripping and phenols solvent extraction can reduce the concentration of toxic compounds of the coal gasification wastewater for the following biological treatment. How- ever, some emulsified coal tar still exists in the influent and many substances in coal tar are refractory and toxic to microorganisms. This study is mainly on the removal of emulsified coal tar by acidification demulsion. The ex- perimental results show that the acidification process of the wastewater by pure hydrochloric acid can reduce the chemical oxygen demand (COD), total organic carbon (TOC), total phenolics and oil about 3.1%-11.3%, 6%- 10.8%. 5.3%-8.6% and 25.2%-57.4% respectively with pH value in the range of 4 to 7. The analysis of molecu- lar weight distribution indicates that compounds removed from the wastewater by this process are large molecular substances. The experiment also shows that the efficiency of COD removal in the demulsion process by different acids is different and the phosphoric acid is prominent. The preserved time of the wastewater also affects the effi- ciency of demulsion. Small amount low-cost solid additives including kaolin and diatomite can improve the rate of coal tar sedimentation and enhance the removal efficiency of organics in the phosphoric acidification process. Keywords coal gasification wastewater, acidification, demulsion, pretreatment, coal tar

1 INTRODUCTION Coal gas is a kind of clean fuel and coal gasifica-

tion is important to the environment of China in which coal is the main energy resource. However, a large amount of wastewater is also produced in the process of coal gasification. Pollution of wastewater contami- nated with phenols, poly-aromatic hydrocarbons (PAHs), ammonia, and cyanide has been a severe problem for decades in China"].

Biodegradation is the main method for coal gasi- fication wastewater treatment. As the wastewater is of high strength and toxicity to microorganism, pre- treatment processes of ammonia stripping and solvent extraction are generally needed for the reduction of ammonia and phenols prior to the biological treat- ment12]. However, these pretreatment processes cannot effectively remove the emulsified coal tar existing in the wastewater.

Coal tar, a major byproduct of gas production process, is dense and non-aqueous phase liquid primar- ily composed of PAH compounds. Many substances in coal tar are refractory and cannot be removed by con- ventional biological system^[^-^'. Therefore, most of the emulsified compounds in the wastewater will be discharged to environment after the biological treat-

Received 2005-03- 17, accepted 2005-09-08.

ment and result in long-term contamination. Thus it is necessary to combine biological treatment with a physical andor chemical pretreatment or post-treatment for coal gasification wastewater treatment to achieve minimal impact on the receiving ecosystems. In the process of coal gasification wastewater biological treatment, we found that a lot of sticky coal tar com- pounds existed in the tube and on the wall of reactor. These compounds can also boost up the foam during aeration, and flow out with bacteria from the reactor. Microscopic characteristics showed that coal tar could adhere with the bacteria, forming lighter and bigger agglomerates. This would not only limit the activity of bacteria but also result in the loss of biomass from the reactor.

Few researches on post-treatment of carboniza- tion wastewater treated by activated sludge process have been reportedf6]. This article mainly focuses on the pretreatment process of coal gasification waste- water by demulsion.

2 MATERIALS AND METHODS Pure acids including HC1, HzS04 and HjP04

were used as the main chemicals for acidification de- mulsion jar tests. Low-cost solid additives including

* To whom correspondence should be addressed. E-mail: majun@hit.cn

Pretreatment of Coal Gasification Wastewater by Acidification Demulsion 399

kaolin and diatomite were used as the secondary chemicals at a dosage of 100mg.L-', added before the main chemicals in the test process. Mechanical stirring was need. Quantity of the main chemicals added was determined by pH of the wastewater. The coal gasifi- cation wastewater was from Harbin Coal Gasification Plant where it had been pretreated by ammonia strip- ping and solvent extraction. Main characteristics of the wastewater are showed in Table 1. The soluble chemical oxygen demand (COD), NH3-N, oil, and phenols were analyzed according to the standard meth- ods for water and wastewater examination (APHA, 1980). The total organic carbon (TOC) was measured with TOC analyzer (Shimazu TOC-VCPN, Japan). Molecular weight distribution was analyzed with HPLC-GPC (Shimazu LC-IOA, Japan) with detector and p. Bondage1 E-125 column (Water Co. USA).

3 RESULTS AND DISCUSSION 3.1 Effect of demulsion by pure acid on the char- acteristics of wastewater

The characteristics of the wastewater in the process of demulsion at different pH conditions are listed in Table 2. It shows that the COD, TOC, total phenols and oil concentration of the wastewater are gradually decreased as the acidification level increases. The maximum COD, TOC, total phenols and oil re- moval are 11.3%, 10.8%, 8.6% and 57.4% respec- tively. The mono-phenols removal is only 3.2%. The substances removed in the demulsion process are mainly oil and poly-phenols that are refractory to the bio-treatment. Considering the effectiveness of fol- lowing biological treatment and the cost of adjusting

pH, the acidification level should be controlled at pH value of 6. Then 7.6%, 8.2%, 6.0% and 36.6% of COD, TOC, total phenols and oil removal could be achieved through pure hydrochloric acid demulsion.

Table 3 shows the results of demulsion with three different kinds of acids applied on two wastewater samples with different preserved time. At pH 6, the effectiveness of demulsion by these acids was differ- ent. H3P04 is the most effective to the COD removal among the three chemicals. Besides, with respect to the selection of acids, the effect of constitution of ac- ids on following biological systems should also be considered. C1- and SO:- are harmful to biological systems, and C1- is more harmful to activated sludge system than SO:- in coal gasification wastewater treatment"'. Although P is a necessary nutrition ele- ment, and absent in common coal gasification waste- water, it also can result in some environmental prob- lems beyond the limited concentration.

On the other hand, the preserved time of the same wastewater sample affected the demulsion effective- ness. The wastewater preserved for a long time (5 days) needed more phosphoric acid to adjust the pH value to 6 for demulsion and the COD removal can increase from 10.9% to about 18.2%. This also oc- curred to the other acid dose (Table 3).

Previously published composition analyses of coal tar concluded that it is a mixture of al- kyl-aromatic and poly-cyclic aromatics. Typical sur- factant compounds containing the functional groups, such as phenolic, carbonyl, or sulfonyl, are not present in appreciable quantities in the bulk coal tar. However, much bound water was found in the coal tar-water

Table 1 Characteristics of wastewater of Harbin Coal Gasification Plant CODc,, NH-N, Mono-phenols, Poly-phenols, Oil, CN-,

PH mg.L-' mg.L-' mg .L-' mg.L-' mg.L-' mg.L-' Sample

wastewater 8 - 9 . 2 5000-8000 3-400 150-400 800-1200 40-100 0.1

Table 2 Effect of acidification by hydrochloric acid on the characteristics of wastewater

Oil removal,

TOC Mono- Mono Total Total phenols Oil, COD

removal, mg.L-' % Phenols,

% %l

removal, mg.L-~ removal, phenols,

% % mg.L-'

COD, pH mg.L-1 removal, mg.L-~

8.8 7068.4 2346.5 298.1 1320.1 88.7

7 6847.2 3.1 2205.2 6 298.1 0 1250.5 5.3 66.3 25.2

6 6528.4 7.6 2124.3 8.2 288.0 3.4 1240.2 6.0 56.3 36.6

5 6379.1 9.8 2065.3 9.4 304.4 1211.3 8.2 45.9 48.3

4 627 1.6 11.3 2013.0 10.8 288.6 3.2 1206.6 8.6 37.8 57.4

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Chinese J. Ch. E. 14(3) 398 (2006)

400 Chinese J. Ch. E. (Vol. 14, No3)

Table 3 Effect of acid types and preserved time of the wastewater on COD removal Samples Preserved time, d pH Acid type COD, m g C ' COD removal, %

A 1 8.8 7068.4 6 hydrochloric acid 6528.4 7.6 6 sulfuric acid 6772.3 4.2 6 phosphoric acid 6300.5 10.9

8.9 6948.7 6 hydrochloric acid 6358.1 8.5 6 sulfuric acid 6267.7 9.8 6 phosphoric acid 5684.0 18.2

B 5

emulsion[81. Therefore, it is the bound water, not sur- factants, that should be the main reason of emulsion of the coal tar-water.

It has been speculated for several years that aro- matic rings may act as hydrogen-bond acceptors. Subsequently, compelling but indirect evidence for the hydrogen-bonding character of aromatic rings to water has been found in a number of experiments. Suzuki et al. demonstrated that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the x cloud[". Thus we can conclude that the intermolecular interac- tion between water and coal tar components are the main mechanism of emulsion.

Luthy et al. suggested that the coal tar-water in- terface undergoes a visible change as coal tar is aged in wate+'". And the coal tar-water interface is stabilized by the formation of an organized, semi-gelatinous film or thin emulsion-like layer. The film is very similar in organic composition to the bulk coal tar without sig- nificant enrichment, depletion, or polymerization of tar components. The notable difference in the composition of the film material was the presence of water bound to coal tar constituents by weak, reversible bonds.

The mechanisms of demulsion by acids are still not very clear. It have been reported that the rigidity of the film may depend on the pH value of the aque- ous solution"]. Addition of acids provides many H+ that can occupy the hydrogen bond site of x cloud and destroy the thin emulsion layer. For the coal gasifica- tion wastewater, some surfactant compounds can in- tensify the emulsification. Addition of H+ can also decrease the effect of these surfactants.

3.2 Effect of demulsion on the variation of mo- lecular weight distribution of wastewater

Effect of demulsion at pH of 6 by phosphoric acid on molecular weight distribution of the wastewa-

ter is shown in Fig.1. A great deal of large molecular weight substances appear in the early retention time in the wastewater and is reduced after demulsion treat- ment. The removed substances should be some poly- meric compounds with molecular weights larger than ten thousands or more. On the other hand, the compo- sition of smaller molecular weight appeared in the retention time of 10-1 lmin increases (Volts in- creases). The phenomena show that the demulsion is important to the following biological treatment sys- tems because the polymeric organics in the wastewa- ter are refractory, and also color causing compounds.

A 0.100

I 2 3 4 5 6 7 8 9 1 0 1 1 t , min

Figure 1 Effect of coagulation on molecular distribution of wastewater

3.3 Effect of solid additives on the efficiency of demulsion

Demulsification is one of the critical steps in emul- sion liquid membrane processes. Generally, the two processes involving in disrupting the emulsion are: co- agulation in which the dispersed phase in emulsion gath- ers together in a reversible mode, and the irreversible coalescence process in which the coagulated droplets begin to adhere to one another to form larger droplets["].

Our experiments showed that the flocs produced in the demulsion process by pure acid were fragile and flighty, and resulted in slow sedimentation rate and poor sedimentary effectiveness. Thus it will require longer sedimentation time and higher cost in actual

June, 2006

Pretreatment of Coal Gasification Wastewater by Acidification Demulsion 401

Table 4 Effect of kaolin and diatomite additives on the acidification demulsion of wastewater

COD TOC, TOC Oil, Oil PH COD’ mg.L-’ removal, % mg.L-1 removal, % mg .L-’ removal, % Additives

8.8 7068.4 2403.5 88.7

phosphoric acid 6 6300.5 10.9 2132.7 11.3 52.3 41.3

kaolin + H3P04 6 6082.8 13.9 1952.7 18.8 34.8 60.8

diatomite + H3P04 6 6013.5 14.9 1934.9 19.5 40.8 54.0

wastewater treatment process. Fig.2 illustrates that both kaolin (about 1 p ) and diatomite (3-50~) ad- ditives can change the structure of flocs and increase the sedimentation rate at a dosage of lOomg.L-’. The sedimentation ratio is defined as the ratio of the dec- rement to the initial bulk of the sediment. In fact, the effectiveness of this coagulation process is similar to the flocculation in drinking water treatment cited by Desjardins ef a1.“21.

100 I I

5 10 15 20 25 30 sedimentation time, min

Figure 2 Characteristics of sedimentation ratio in the acidification process

I3 phosphoric acid; kaolin; 0 diatomite

It has been demonstrated that approximately 17% of the coal tar is classified as ‘polar’ components that are adsorbed finnly to clays[*’. Many literatures re- ported the effectiveness of kaolin and diatomite on enhancing coagulation and color and organic con- taminants r e m ~ v a l “ ~ ” ~ ] .

Table 4 shows the COD, TOC and oil removal in the process of demulsion with or without solid additives.

It could be seen that the kaolin and diatomite added in the process of demulsion by pure phosphoric acid could also enhance COD, TOC and oil removal. This suggests that kaolin and diatomite have high ad- sorption capability to the organic matter in the waste- water and can cohere to the agglomerates produced and sedimentate together.

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Chinese J. Ch. E. 14(3) 398 (2006)