폐활성슬러지의 열-화학적 전처리를 통한 혐기소화 공정의 효율 향상

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

박철환1,2, 김정식1,2, 김탁현1, 이명구1,2, 김승욱2, 이진원3, 김상용1

한국생산기술연구원 신에너지환경팀1, 고려대학교 화학공학과2, 광운대학교 화학공학과3

Thermo-chemical Pretreatment For Improving Anaerobic Digestion Process Of Waste Activated Sludge

Chulhwan Park1,2, Jeongsik Kim1,2, Tak-Hyun Kim1, Myung-Gu Lee1,2, Seungwook Kim2, Jinwon Lee3, Sangyong Kim1

Advanced Energy & Environment Research Team, KITECH1

Department of Chemical Engineering, Korea University2

Department of Chemical Engineering, Kwangwoon University3

Introduction

The activated sludge has been the most widely used biological process for the industrial wastewater treatment. It allows the transformation of dissolved organic pollutants from wastewater into biomass, and these are finally converted into CO2 and H2O by microorganisms. The main by-product of biological wastewater treatment is the waste activated sludge (WAS), which mainly consists of 70% organic matter. WAS has been also increased as the result of expansion of wastewater treatment and many methods have been used to solve these problems such as pasteurization, mesophilic anaerobic digestion, thermophilic aerobic digestion, composting and lime stabilization/conditioning/dewatering.

Additionally, anaerobic digestion is the most applied technique for the WAS stabilization resulting in the reduction of volatile solids (VS) and the production of biogas such as methane. There are technical difficulties in rapidly treating the sludge by adding the WAS directly to the recently developed anaerobic bioreactor that maintains a high microorganism concentration.

To improve the digestion efficiency, various methods of pretreatments have been studied because the anaerobic stabilization is a very slow process. In the case of anaerobic digestion, the biological hydrolysis has been identified as the rate-limiting step and the effects of such pretreatments are to improve the hydrolysis of a particulate organic matter and to promote effective solubilization of organic substances and to split complex polymers into smaller molecules. This produces an organic substance that is easily converted to methane gas via volatile organic acids. Improvement WAS digestion efficiency using a pretreatment method that combined thermal treatment with chemical treatment, ultrasonic treatment and mechanical treatment has also been researched.

This study investigated the operating conditions of thermal, chemical, ultrasonic, thermo-chemical, thermo-ultrasonic pretreatments to enhance soluble chemical oxygen demand (SCOD) and methane production and evaluated five pretreatment methods for the improvement  of subsequent anaerobic digestion of WAS.

Materials and Methods

WAS characterization and Pretreatment

Table 1 shows characteristics of WAS.

The experiment of thermal pretreatment is as follows: Samples were thermally treated with an autoclave (JeioTech, Korea) at a temperature of 121℃ and a pressure of 1.5 atm for 30 min. The treated WAS was cooled to ambient temperature and cooling time was 3 h.

That of chemical pretreatment is as follows: Various alkaline agents were used such as sodium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide. Alkali agents were added to 1,000 ml of WAS to reach amounts ranging from 0 to 21 g alkali agent.

Samples of WAS were placed in beakers and pretreated with the ultrasonicator (Branson, USA, 42 kHz) for the ultrasonic pretreatment. Samples were stirred for the experiments to increase mixing effect and contact surface area.

Table 1. Characteristics of WAS used in experiments

Item	Values
Total COD	31,000 mg/l
SCOD	2,160 mg/l
TS (total solids)	38.0 mg/l
VS (volatile solids)	26.0 mg/l
pH	6.4

Analysis

Standard methods were used for the estimation of COD, SCOD, TS, VS and pH. TS was measured gravimetrically using glass microfiber filter (4.7 cm diameter, Whatman, GF/C), COD and SCOD using a colorimetric method after digestion of the samples in the COD reactor, Model 45600 (HACH Co. USA). Additionally, SOCD was measured after centrifugation at 4,000 rpm for 40 min.

The values of COD solubilization were calculated by below equation:

[eqn: COD~Solubilization~(%)~=~{Soluble~COD~measured~after~pretreatment} over {Total~COD~measured~after~pretreatment}~×~100]

Results and Discussion

Chemical pretreatment was performed with various alkaline agents such as sodium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide. Among them, sodium hydroxide showed the highest COD solubilization. And then the sodium hydroxide addition of different concentration was tested. As a result, Fig. 1 shows the influence of sodium hydroxide addition on COD solubilization. And COD solubilization of thermal pretreated sample was higher than that non-pretreated sample. COD solubilization increased as the dose of sodium hydroxide increased, COD solubilization reached 86.5 percent when 9 g of sodium hydroxide/l were added. A slower COD solubilization increase was observed from 9 g of sodium hydroxide/l to 21 g of sodium hydroxide/l.

Fig. 2 shows the result of ultrasonic pretreatment. The change of COD solubilization was not significant in the case of thermal pretreatment, but that increased in the case of non-pretreatment. From these results, we adopted thermo-chemical method as a proper pretreatment.

Fig. 1. The influence of sodium hydroxied

addition on COD solubilization

Fig. 2. The influence of ultrasonic

pretreatment on COD solubilization

Fig. 3 shows the change of WAS in a graduated cylinder after gravity settlement for 24 h. The supernatant after pretreatment was more turbid than that before pretreatment, it says that colloidal and dissolved solids in WAS increase after pretreatment and that they remain in supernatant, as we know that they are not removed by settling.

Alkali pretreatment allowed an enhanced solubilization which was even more pronounced when heating was applied. And the thermo-chemical pretreatment of WAS allowed a decrease in the anaerobic digester sludge retention time (SRT), without major effects on the process performance and on effluent quality.

(a) Non-pretreatment    (b) Thermo-chemical pretreatment

Fig. 3. WAS images in a graduated cylinder after 24 h gravity settlement.

Acknowledgement

This research was supported by KITECH (The project of international cooperation). The authors deeply appreciate their financial support.

References

1. APHA, Standard Methods for the Examination of Water and Wastewater, 19th Ed (1995)

2. S. Tanaka, T. Kobayashi, K. Kamiyama, and M.L.S. Bildan, Effects of thermochemical pretreatment on the anaerobic digestion of waste activated sludge, Water Science and Technology, 35, 209-215 (1997)

3. V. Pennaud, J.P. Delgenes, and R. Moletta, Thermo-chemical pretreatment of a microbial biomass: influence of sodium hydroxide addition on solubilization and anaerobic biodegradability, Enzyme and Microbial Technology, 25, 258-263 (1999)