Reduction of Excess Sludge in Sewage Treatment by Ozone Application

A new process which attains both excess sludge reduction and phosphorus recovery has been developed. In this paper, characteristics of sludge solubilization by ozone were investigated in batch experiments with sludge taken from an A/O reactor supplied with synthetic sewage and several real wastewater treatment plants. Then, performance of the new developed process was investigated in a laboratory-scale plant. The process eliminated withdrawn excess sludge by 100% with recovering more than 80% of phosphorus in influent. Water quality (S-BOD, SS and phosphorus) of effluent was maintained at the same level as that from conventional process. Introduction Excess sludge is an inevitable product of wastewater treatment process with activated sludge. However, treatment of the excess sludge costs much, requires much energy, and has been a serious issue because of lack in landfill space. Hence, reduction of excess sludge has been gathering attention in wastewater treatment sector in Japan. Therefore, several kinds of technologies for excess sludge reduction were developed so far. One of the methods is sludge ozonation. Some researches showed that ozone can reduce excess sludge [1][2], so that ozone can be a possible alternative. However, there is a drawback of installing the excess sludge reduction technology with ozone in some areas of Japan. The wastewater treatment process with the technology cannot remove phosphorus because phosphorus is to be removed as the content of withdrawn excess sludge. Thus, in the area which has an enclosed water body, the installation of the technology raises the concern of eutrophication in the water body. In order to attain the both targets, excess sludge reduction and phosphorus removal, a new advanced wastewater treatment process has been developed [3][4]. The new process consists of conventional anaerobic/oxic (A/O) phosphorus removal process, sludge ozonation process for excess sludge reduction and crystallization process for phosphorus removal and recovery. The schematic diagram of the new process is shown in Figure 1. In this process, a part of the returned sludge is treated by ozone in ozonation tank resulting in increase of biodegradable organic substance through sludge solubilization. The solubilized substrate generated in ozonation tank can serve as energy source for phosphorus release when ozonated sludge is returned into the anaerobic tank in A/O process. Phosphorus accumulating organisms (PAOs) in A/O tank releases phosphorus under anaerobic condition and then phosphorus rich condition is made in bulk water. The P-rich liquor is then introduced to crystallization tank, where phosphorus is recovered as a crystalline product with Mg or Ca ion. Wasser Berlin 2006 35 International Conference Ozone and UV, April 3 2006 As previous study showed important parameters for designing this process were solubilization degree in sludge ozonation, activity change of microbes, and biodegradability of solubilized sludge [3], sludge ozonation experiment was conducted to obtain basic information to determine process operation condition using several kinds of sludge. Based on the result, the process was continuously operated in lab scale plant to investigate the performance. effluent Oxic influent Anaerobic A/O tank Crystallization tank Ozonation tank excess sludge P crystal Figure 1 Schematic diagram of advanced wastewater treatment process with excess sludge reduction and phosphorus recovery Sludge Ozonation Experiment 1. Materials and method 1.1 Sludge ozonation apparatus Semi-batch system (liquid; batch, gas; continuous flow), which consisted of an ozone generator, ozone contactor, ozone meter and exhausted ozone decomposer, was utilized to investigate effect of ozone on sludge solubilization, biodegradability and activity. Sludge was ozonated in a cylindrical glass contactor with 2L effective volume (Φ=120 mm, H=200 mm). Ozone gas, generated from pure oxygen with an ozone generator (Mitsubishi OS-1N), was continuously bubbled into the contactor through a diffuser equipped at the bottom of the contactor. The ozone concentrations in the gas phase at inlet and outlet of the reactor were determined by ozone meter. Ozone consumption and solubilization degree of sludge were defined in the following equations: × 3 3 [amount of inf. O amount of eff. O ] (mgO ) Ozone consumption = (Initial SS) (mg/L) Sludge volume (L) 3