Effects of operational conditions on sludge degradation and organic acids formation in low-critical wet air oxidation

https://doi.org/10.1016/j.jhazmat.2008.05.038Get rights and content

Abstract

Wet air oxidation processes are to treat highly concentrated organic compounds including refractory materials, sludge, and night soil, and usually operated at supercritical water conditions of high temperature and pressure. In this study, the effects of operational conditions including temperature, pressure, and oxidant dose on sludge degradation and conversion into subsequent intermediates such as organic acids were investigated at low critical wet oxidation conditions. The reaction time and temperature in the wet air oxidation process was shown an important factor affecting the liquefaction of volatile solids, with more significant effect on the thermal hydrolysis reaction rather than the oxidation reaction. The degradation efficiency of sludge and the formation of organic acids were improved with longer reaction time and higher reaction temperature. For the sludge reduction and the organic acids formation under the wet air oxidation, the optimal conditions for reaction temperature, time, pressure, and oxidant dose were shown approximately 240 °C, 30 min, 60 atm, and 2.0 L/min, respectively.

Introduction

The subject in sludge treatment recently is the organic sludge containing more than 40% of organic matter and typically formed in sewer and wastewater discharge facility systems. This sludge possesses high moisture content, in addition to high organic content, due to the difficulty of excluding moisture efficiently at dewatering stage, and may cause secondary environmental pollution. Therefore, it should be treated by an appropriate method as soon as it is formed. Considering the wastewater treatment systems increasing every year, the amount of wasting sludge will continuously increase, and the countermeasures to handle it should be prepared quickly. By the year 2000, about 1.5 million tons of sewage sludge was being generated annually in Korea and the amount to be generated in 2005 was expected to exceed 3.5 million tons. Then, it would cost approximately 46 million U.S. dollars for the sludge disposal. Despite the increasing trend of sludge generation, it still shows no sign of less reliance on landfill and ocean dumping for the disposal. Ocean dumping still accounts for 74% whereas landfill has been decreased to as low as 12% by stricter regulations in Korea. As the law regulating the waste management gets more stringent, landfill is banned if the water content of sludge is >75%, and ocean dumping is being restricted by the international treaties. Subsequently, a technology to replace the current sludge treating process is a prerequisite, and the thermal oxidation (TO) was applied in this study to develop an alternative sludge reduction technology. Wet air oxidation (WAO) among TO methods is considered suitable for highly concentrated organic matters including difficult-to-decompose ones, excretions, or sludge containing small amount of organic matters when incinerated and biologically processed [1], [2], [3], [4], [5], [6]. The WAO accomplishes oxidation at elevated temperatures (150–325 °C) and oxygen pressures (10–200 atm). High temperature accelerates the dissolution of sludge, and high pressure, higher than the vapor pressure, increases the solubility of oxidant and represses pollutants that are transferred to the air [7]. The oxidation products may be inorganic salts, simpler forms of biodegradable compounds, or carbon dioxide and water through the complete oxidation. Water which makes up the bulk aqueous phase serves to modify the oxidation reactions to proceed at relatively low temperatures as well as to moderate them to remove excess heat by evaporation. In addition, water is an excellent heat transfer medium which enables the efficient heat transfer. The oxygen required by the WAO reactions is provided by air bubbles through the liquid phase in a reactor used to contain the process [2], [8], [9]. One advantage of WAO is that it causes no secondary environmental pollution as the reaction proceeds under the liquid condition, capable of minimizing the discharge of air pollutants [10], [11], [12]. The other is that organic acids can be obtained as byproducts through the WAO process, at relatively low temperature and pressure, subsequently capable of reducing high initial investment and energy cost needed for maintaining high temperature and pressure. The aim of this study was to investigate whether the wasting sludge generated from the wastewater treatment system could be reduced and simultaneously produce such useful matters as organic acids under the sub-critical WAO condition of <250 °C and 20–60 atm, a more relaxed condition than the general operational condition of WAO.

Section snippets

Lab-scale WAO system

The laboratory-scale experimental setup is shown in Fig. 1. The WAO system was composed of sludge inflow pump, TO reactor, and oxidizer supplying device, and the whole process was designed automatically controlled and operated by the computer program. The thermal reactors were made up of completely stirred tank reactor (CSTR; 1 L capacity) and plug-flow reactor (PFR; 1 L capacity). A feeding tank (50 L capacity) was located before CSTR (Reactor 1) with impeller to keep the sludge concentration

Effects of reaction temperature on degradation of organic compounds

In order to investigate the effects of reaction temperature and time on the biodegradation of organic matter as total chemical oxygen demand (TCOD), the initial COD concentration used for RT1 (180 °C), RT2 (200 °C), RT3 (220 °C), and RT4 (240 °C) under the corresponding reaction temperature (RT) was 7610, 7230, 7740, and 7530 mg/L, respectively. As shown in Fig. 2(a), the higher the reaction temperature was, the faster the TCOD concentration decreased. During the reaction time of 10 min, the

Conclusions

In order to investigate operational factors in a sewage sludge reduction technology applying the sub-critical wet oxidation process, experiments were performed using the thickened sludge taken from a municipal wastewater treatment facility. The influence of such factors as reaction temperature and time, reaction pressure, and oxidant dose on the sludge reduction was considered. The reaction temperature directly affected the thermal hydrolysis reaction rather than the oxidation reaction, and the

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