With growth of human population, a sewage sludge production is now becoming an important issue through the world. Thereby, in European Union, the future forecast for annual wastewater sludge volume is reached up to 13 million dry tons at 2020. The process of sludge handling encounters challenges, such as dewatering, solids biodegradation, cake transportation and consequent application. Moreover, costs required to cover all of these arrangements can achieve up to 50% of plant maintenance. Therefore, the application of cost-effective treatment methods is being developed in sludge management.
To date, there are variety methods applying for sewage sludge disposal such as landfill, drying-incineration, agriculture usage, recycling as building material and anaerobic digestion. The anaerobic digestion has been widely applied as a method for biogas production reducing greenhouse gas emission (GHG), refunding input in the form of partly energetic independence of plant and profit earning from a renewable energy market.
Except biogas production in the form of methane, the anaerobic digestion possesses important benefits, such as sludge stabilization, pathogens and odor removal. To improve biodegradability of organic matter, increase methane yield and improve final dewatering step, thermal hydrolysis has been developed and proven as a successful approach making sludge more amenable to anaerobic digestion and increasing dewaterability of digested solids significantly .
However, the reject water containing hard-degradable organic compounds, recirculated to the head of the process, can disturb biological activity in the main treatment process.
Figure 1 – Sludge treatment line with Thermal Hydrolysis and anaerobic digestion 
Several methods can be applied to destroy hard-degradable organics: bioaugmentation, UV-disinfection and ozonation. Ozonation tends to have some advantages such as strong oxidation power, capability in wide pH, rapidly reaction with viruses and bacteria. To evaluate biodegradability of organic matter in reject water, the Zahn-Wellens test is applied modelling biological treatment in laboratory scale . The Figure 2 represents example of ozone influence on hard-degradable organic compounds. In order to be relevant with sludge ages in real conditions, biodegradation has been calculated for 14 and 21 days.
Figure 2 – Ozone influence on hard-degradable organic compounds
Ozonation of reject water improves biodegradability of recalcitrant organic compounds by 8 times at 14 days of sludge age and 2.5 times at 21 days. This potentially prevents negative influences of recycled hard-degradable organics on activated sludge in the main biological treatment process.
1. Anaerobic digestion is recognized as the best method of sludge treatment to reduce the amount of sludge, GHG emissions and at the same time producing biogas.
2. Pretreatment step increases methane yield and sludge parameters such as biodegradability, hygienization, stabilization, odor, dewaterability.
The biodegradation of the reject water can be improved by applying ozone.