The Role of Diatomite Particles in the Activated Sludge System for Treating Coal Gasification Wastewater*

ZHANG Wenqi (張文啟)**, RAO Pinhua (饒品華), ZHANG Hui (張輝) and XU Jingli (徐菁利)

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The Role of Diatomite Particles in the Activated Sludge System for Treating Coal Gasification Wastewater*

ZHANG Wenqi (張文啟)**, RAO Pinhua (饒品華), ZHANG Hui (張輝) and XU Jingli (徐菁利)

School of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China

Diatomite is a kind of natural low-cost mineral material. It has a number of unique physical properties and has been widely used as an adsorbent in wastewater treatment. This study was conducted to investigate the aerobic biodegradation of coal gasification wastewater with and without diatomite addition. Experimental results indicated that diatomite added in the activated sludge system could promote the biomass and also enhance the performance of the sludge settling. The average mixed-liquor volatile suspended solids (MLVSS) is increased from 4055 mg·L－1to 4518 mg·L－1and the average settling volume (SV) are changed only from 45.9% to 47.1%. Diatomite additive could enhance the efficiency of chemical oxygen demand (COD) and total phenols removal from the wastewater. The COD removal increased from 73.3% to near 80% and the total phenols removal increased from 81.4% to 85.8%. The mechanisms of the increase of biomass and pollutants removal may correlates to the improvement of bioavailability and sludge settlement characteristics by diatomite added. Micrograph of the sludge in the diatomite-activated sludge system indicated that the diatomite added could be the carrier of the microbe and also affect the biomass and pollutant removal.

diatomite, phenols, coal gasification wastewater, activated sludge

1 Introduction

Coal gasification wastewater is generated in coal gas purification. The composition of this kind of wastewater is very complex and varies from one factory to another. Phenols are primarily and constitute 60% to 80% of the organic content of the wastewater. Other compounds of concern in these wastewaters are ammonia, cyanide, and thiocyanate [1].

Powdered activated carbon treatment (PACT) for activated sludge processes has found special use in coal gasification, petrochemical and other chemical wastewater treatment for nearly 30 years. Advantages of the system, as opposed to adsorption on activated carbon, has been addressed by several authors, although the mechanism through which powdered activated carbon (PAC) enhances the activated sludge process remains to be fully elucidated [5, 6]. Application of this system for wastewater treatment has usually been focused on slowly biodegradable and adsorbable compounds in view of the mechanism suggested to explain pollutant removal [7]. This mechanism consists of “stimulation of biological activity” and “bio-regeneration” of activated carbon by microorganisms. Although the PAC-activated sludge system is effective for wastewater treatment, the PAC added may be lost with the effluent sludge and is difficult to recover in the process of the operation. Therefore the actual application of this process is restricted by the high cost of PAC.

Most researches have focused on adsorption of dissolved or emulsified organic contaminants from wastewater by some natural low-cost minerals [8-10]. Several literatures have researched the effects of some clay minerals on the behaviour of microorganisms and bio-films [11, 12]. In general, clay particles, such as kaolin,have the ability to enhance the bacterial activity [13].

Diatomite has a unique combination of physical and chemical properties, which make it applicable as an adsorbent for the pollutants and as flocculants used in wastewater treatment [14-18].

Its high permeability, high porosity, low thermal conductivity, and chemical inertness, make it a cheap alternative to activated carbon [19]. According to the patent by Pollock, modified diatomite can be used to remove the polyaromatic hydrocarbons (PAHs), cyanides, and phenol from coal tar effectively [20].

Diatomite particles are also used in the biological reactors as carriers for microorganisms, which are called bio-diatomite. The microbial colonies on diatomite can form zoogloeas through microbial capsules and surface mucus [21, 22]. However, few literatures mention bacterial activity enhancement by natural diatomite in toxic wastewater treatment.

The purpose of this study is to research the role of diatomite particles in the performance of activated sludge system for coal gasification wastewater treatment.

2 Materials and Methods

The experimental setup for coal gasification wastewater treatment is shown in Fig. 1. The wastewater quantity is 500 L·h－1and controlled by flow meter. The volumes of the reactor and settler are 12 m3and 0.69 m3, and the hydraulic retention times (HRT) are 24 h and 1.38 h respectively. The solid retention time (SRT) is 25 d. The sludge in the settler is returned continuously and the return ratio is 1︰1. Dissolved oxygen concentration was maintained in the range of 4-6 mg·L－1by aeration. The reactor is placed in a room with a central heating system and controlled at (20±2)°C. Phosphoric acid is added in amount of 1% of the total organic carbon (TOC) of wastewater for microbe growing.

Figure 1 Schematic diagram of diatomite-activated sludge process

In the first stage of the experiment, wastewater is treated by the common activated sludge system. After 50 days, the diatomite is added into the activated sludge reactor to form diatomite-activated sludge. The concentration of diatomite added is 1000 mg·L－1in the aeration tank and the dose of diatomite added in the process of the system operating is about 480 g·d－1, corresponding to the diatomite discharged.

The microstructures of the sludge in the diatomite- activated system are examined by optic microscope with camera and some samples are dyed by purple dyestuff.

Table 1 Characteristics of granularity distribution of the experimental diatomite (%)

Table 2 Characteristics of the chemical composition of diatomite (%)

Table 3 Characteristics of wastewater used in the experiment

3 Results and Discussion

3.1 Characteristics of the activated sludge

In the first stage, the reactor was operating without diatomite. The average mixed-liquor volatile suspended solids (MLVSS) was 4055 mg·L－1and the average settling volume (SV) was 45.9% (see Fig. 2). After 50 days, 12 kg diatomite was added in the reactor and operated for more than two months. In this stage, the average MLVSS was 4518 mg·L－1and the average SV was 47.1% (see Fig. 3).

Figure 2 Characteristics of the MLVSS and SV of the activated sludge system without diatomite

○?MLVSS; - SV

Figure 3 Characteristics of the MLVSS and SV of the activated sludge system with diatomite

○?MLVSS; - SV

The results indicated that more biomass was gained in the diatomite-activated sludge system with a similar influent and had near SV values corresponding to the system without diatomite.

The biomass usually correlates to the bioavailability of pollutant, sludge settlement characteristics and the configuration of the reactor. It can be deduced from the experiment that the added diatomite added can improve the settlement characteristics of the activated sludge. This may be one of the mechanisms of the biomass increase.

Diatomite particles may also be used in the biological reactors as carriers. Micrograph of the sludge in the diatomite-activated sludge system indicated that the microbe grew adhering to the diatomite and some microbes formed little granules [see Figs. 4 (a) and (b)]. The phenomenon implies that the added diatomite may be a carrier of the microbe like bio-film process and may be core of sludge. This also affects the biomass.

Figure 4 Micrograph of the sludge in the diatomite-activated sludge system

There are reports showing that sorption of organic contaminants on the solid particles can enhance the biodegradation of sorbed compounds [23-25]. Diatomite added may affect the bioavailability of refractory organics in the wastewater.

3.2 Effect of diatomite additives on COD removal

The COD removal of the wastewater in the common activated sludge system is shown in Fig. 5. It can be seen that the average COD of the influent is 1956 mg·L－1, effluent is 510 mg·L－1and the ratio of COD removal is about 73.3%. On the other hand, in the diatomite-activated sludge system (operated after 50 days), the average COD of the influent is 1951 mg·L－1, but the value of the effluent decreases to 389 mg·L－1and COD removal increases to near 80% ( see Fig. 6). Therefore, diatomite added to the activated sludge can enhance the efficiency of COD removal from the wastewater.

Figure 5 Characteristics of COD removal of the wastewater in activated sludge system

□?influent;◆?effluent; - removal

Figure 6 Characteristics of COD removal of the wastewater in diatomite-activated sludge system

□?influent;◆?effluent; - removal

3.3 Effect of diatomite additives on the total phenols removal

Some phenols are refractory and are the main reasons that induce the COD of the effluent to increase. Total phenols removal from the wastewater in the common activated sludge system and the diatomite- activated system are shown in Fig. 7 and Fig. 8, respectively.

Figure 7 Characteristics of total phenols removal from the wastewater in the activated sludge system

□?influent;◆?effluent; - removal

Figure 8 Characteristics of total phenols removal in the diatomite-activated sludge system

□?influent;◆?effluent; - removal

It can be seen from Fig. 7 that the average total phenols of the influent is 395.2 mg·L－1, effluent is 67.2 mg·L－1and the ratio of total phenols removal is about 81.4%. On the other hand, in the diatomite- activated sludge system , the average total phenols of the influent is 388.5 mg·L－1, but the value of the effluent decreases to 55 mg·L－1and the removal increases to nearly 85.8%. The results indicate that the added diatomite can improve total phenols removal in the activated sludge system.

Diatomite can be used to remove the contamination by its adsorption. However, in this study, the dose of diatomite added was only 40 mg·L－1. Previous experiments indicated that the adsorption capacity of the diatomite was inadequate to affect the experimental results. The adsorption capacities of the diatomite for COD and total phenols were 12.4 mg·g－1and 30 mg·g－1respectively in the coal gasification wastewater with 1542 mg·L－1COD and 188.9 mg·L－1total phenols [26]. In addition, compared to the results of PAC, the adsorption capacity of diatomite would be decreased in the activated sludge reactor [27]. Hence, diatomite enhanced the COD and phenols removal by promoting biodegradation. The bioavailability of refractory organics of wastewater in the activated sludge system was improved by adding diatomite. Other experiment showed that the added diatomite could obviously enhance the microbe respiratory activity in the wastewater with total phenols concentrations of 188.9 mg·L－1- 312.4mg·L－1. This may be the mechanism of diatomite promoting biodegradation.

4 Conclusions

From the above results of this study, the following conclusions can be derived:

(1) Diatomite added in the activated sludge system can promote the biomass in the phenols wastewater treatment system and also can enhance the performance of sludge settling.

(2) Diatomite additive can enhance the efficiency of COD and total phenols removal from the wastewater.

(3) Micrograph of the sludge in the diatomite- activated sludge system indicated that the added diatomite may be a carrier of the microbe.

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2008-04-16,

2008-10-09.

the Shanghai Committee of Education (07ZZ158).

** To whom correspondence should be addressed. E-mail: zhangwenqi@sues.edu.cn