호기·호열성 소화공정을 이용한 정유공장 슬러지의 처리

김 영기, 배 진혜, 엄 용석, 최 정우, 이 원홍

서강대학교 화학공학과

Thermophilic Aerobic Digestion Process

for Waste Activated Sludge of Oil Refinery plant

Young-Kee Kim, Jin-Hye Bae, Yong-suk Eom, Jeong-Woo Choi, Won Hong Lee

Department of Chemical Engineering, Sogang University

Introduction

Conventional waste sludge treatment technology has involved mesophilic- anaerobic digestion, but such technology is no longer appropriate for  satisfactory either toxic chemicals or nonbiodegradable compounds. Additional   disadvantages of conventional mesophilic-anaerobic digestion processes include extraordinary long residence times, frequently exceeding 20 days, for treatment. Thermophilic aerobic digestion (TAD) has been used at a few sewage-treatment works for a number of years and considered as a suitable method of treating sludge.[1][2] Thermophilic aerobic digestion processes operated in high temperature ranges could expect to offer advantages of pathogenic organism elimination and reduced nonbiodegradeable elements. Also it offers significant reducing residence times. Although thermophilic aerobic digestion process has a number of advantages, it has been hardly applied to industrial wastewater sludge digestion because thermophilic aerobic digestion shows quite slow degradation rate in waste activated sludge digestion processes.

The object of our study is the enhancement of sludge degradation rate and developement of industrial waste activated sludge digestion process by adding aerobic, thermophilic bacteria. Expecially, the present paper deals with problems regarding of waste activated sludges in oil refinery plant which contain aromatic and long chain hydrocarbon compounds. The thermophilic aerobic digestion of waste activated sludge in oil refinery plant was performed using three strains of aerobic thermophilic bacteria.

Materials and Methods

Materials

three strains of aerobic thermophilic bacteria were purchased from ATCC. There were one strain of Thermus aquatics and two strains of Thermus sp. (ATCC 25104, 27737, 43814). Bacteria in the late exponential phase were used as a seed for digestion experiments. The waste activated sludge was obtained from wastewater treatment plant of Hanwha oil refinery plant.

Methods

Digestion experiments were performed using 3L Bio Reactor (BIOFLO Ⅲ, New Brunswick Scientific Co., U. S. A) with the following conditions; working volume 2.5L, temperature 55℃, and impeller speed 300rpm. Batch digestion processes were carried out during 100 hours. Sludge digestion rate was measured with total suspended Solid (TSS), total organic carbon (TOC), and dissolved organic carbon (DOC) concentrations. TSS concentration was measured by Standard Method 2540D[3]. DOC and TOC concentrations were measured by TOC analyzer (TOC-5050A, Shimadzu Co., Tokyo, Japan). Experiments for investigation of growth behavior of Thermus bacteria were carried out in flask, and dry cell weight was measured with the UV/VIS spectrophotometer (Model V-550, JASCO) at 660nm wavelength except Thermus aquatics (ATCC 25104) at 640nm.

Results and Discussion

Growth behavior of aerobic, thermophilic bacteria.

The growth patterns of Thermus aquatics, Thermus sp.(ATCC 25104, 27737, 43814) were shown in Fig. 1. After 3 hrs lag phase, bacteria were grown rapidly (exponential phase) until 10~15 hrs, and then stationary phase was occurred. Maximum dry cell weights were obtained as 1.0~1.5g/L. Maximum specific growth rates of ATCC 25104, 27737 and 43814 were determined as 0.245, 0.333 and 0.1917 (1/hr) respectively.

Fig. 1. The growth pattern of three strains of thermophilic bacteria

--□... Thermus aquatics(ATCC 25104)

_□... Thermus sp.(ATCC 27737)

_□... Thermus sp.(ATCC 43814)

Sludge digestion (batch) experiment

TSS concentration was decreased in all digestion experiments as illustrated in Fig 2. Without Thermus bacteria addition, TSS concentration was decreased only about 15% during digestion process. This degradation of TSS concentration might be due to the fact that solubilization of solid particles was done by thermal effect. On the other hand, the experiments with addition of each Thermus bacteria represented TSS reduction of 25~40%. It can be considered that decomposition of solid particles was accomplished not only by thermal effects but also by enzymes secreted from thermophilic bacteria. Thermus aquatics (ATCC 25104) was superior to other thermus sp. in terms of TSS concentration reduction rate. The maximum reduction of TSS concentration was obtained by Thermus aquatics, as about 40%.

Fig. 2. Time course behavior of TSS concentration in aerobic thermophilic digestion

_□-_ Without bacteria addition

_□_ Thermus aquatics(ATCC 25104)

_□_ Thermus sp.(ATCC 27737)

_□-_ Thermus sp.(ATCC 43814)

In the early stage of digestion experiments, TSS concentrations were sharply decreased until about 10 hrs, and then gradually decreased until the end of the experiment. In the contrast to TSS concentrations, DOC concentrations were drastically increased at the beginning of experiments as shown in Fig. 3. It is considered that these phenomena were occured at this phase because of hydrolysis and solubilization of compounds. After the early stage, DOC concentrations were slowly reduced by activity of thermophilic bacteria. TOC concentrations were decreased continuously as shown in Fig 4. But, TOC concentrations were not decreased without bacteria addition. Thermus aquatics (ATCC 25104) demonstrated higher TOC decrease than others. TOC concentration was decreased about 36% comparison with the initial concentration (Fig. 4).

Consequently, Thermus aquatics (ATCC 25104) showed best results for degradation  of TSS, DOC, and TOC concentrations. From this study, although waste activated sludge produced from oil refinery plant contains a large amount of hydrocarbon  compounds, it was revealed that the reduction of activated sludge is possible by thermophilic aerobic digestion (TAD) process with Thermus bacteria addition.

Fig 3. Time course behavior of DOC concentration in aerobic thermophilic digestion

Fig 4. Time course behavior of TOC concentration in aerobic thermophlic digestion

_□-_ Without bacteria addition

_□_ Thermus aquatics(ATCC 25104)

_□_ Thermus sp.(ATCC 27737)

_□-_ Thermus sp.(ATCC 43814)

References

[1] Mason C. A., Wat. Sci. Tech., 25(1), 113-118, (1992).

[2] Hamer G., Mason C. A., Wat. Res., 28(4), 1863-869, (1994).

[3] Standard Methods for the Examination of Water and Wastewater., 20th edn,

APHA/AWWA/WEF, Washington DC, U.S.A. (1998).