Sludge is a great problem for wastewater treatment plants. This problem grows every day since the amount of sludge increases continuously and the land required for the storage of the sludge. Moreover, it has some other problems like odor when it is kept in open areas.
Sludge originates from the process of treatment of waste water. Due to the physical-chemical processes involved in the treatment, the sludge tends to concentrate heavy metals and poorly biodegradable trace organic compounds as well as potentially pathogenic organisms (viruses, bacteria etc) present in waste waters. Wastewater characteristics and specific sludge production vary from country to country and from region to region. There are several sludge characteristics that have a great influence on the costs of sludge treatment. The water content is very important as it determines the sludge volume and therefore feasibility and costs of transportation and disposal. The solids and water content of sludge depends on the type of sludge (e.g. primary sludge, waste activated sludge, chemical sludge) and the type and quality of its treatment (e.g. sludge digestion) and on the method of sludge thickening and dewatering.
Moreover, for the land application of sludge there exist some limiting factors such as heavy metal content. In the case of land application, harmful effects of the sludge will be presented on soil, vegetation, animals and humans.
As a result of the existing methods used in regular wastewater treatment plants, water content of the sludge can be reduced approximately to 75%. This means that sludge still contains 75% water and only 25% solid matter. Therefore, total volume and weight of the sludge still remains as a great problem. Landfilling is not a solution for sludge disposal. On the contrary, it creates other problems and it has a great cost for the transportation and Landfilling. Furthermore, EU regulations states that wastes are to reduce by 2010 the amount of biodegradable municipal waste sent to landfill to 75 per cent of that arising in 1995, with further reductions to 50 percent by 2013 and 35 percent by 2020. Therefore, Landfilling seems not to be a solution for sludge handling.
At this point, Santes offers a system which decreases the total volume of the sludge to about 5% of its original volume with the name of Sludge Fuelisation System. Within this system, Sludge first fed to the drying unit and dried sludge is directed to the incineration unit. With the incineration of dried sludge necessary energy for the drying process is provided. Santes Sludge Fuelisation System consists of 2 processes which are;
- Sludge Drying System and
- Waste Incineration System
Sludge Drying System
The exchange of mass and heat between dried sludge and air (material and factor) is of essential significance for the drying process. The heat exchange is achieved through radiation, convection and conduction. Moisture mass goes from the area of higher concentration to the area of smaller concentration as a result of diffusion.
Drying process is based on granulated sludge with granulated equipment. This equipment provides aerateable layer with a large surface is formed which is essential for an efficient drying process.
The product is dried down to the desired moisture content. Afterwards it is transferred to a collecting conveyor belt.
At the end of this process, sludge has a 95% solid matter and only 5% of water. This process both decreases the amount of sludge to be incinerated and increases the calorific value of the sludge. The calorific value of the sludge increases about 300% of its original value. Therefore, this dried product can be fed to following incineration unit as an energy source.
The main goals of thermal drying of sludge are:
- To eliminate water from sludge and diminish volume of sludge (approx. 4-5 times) in order to make the transportation cost lower and the sludge storage easier,
- To increase sludge calorific value, so that sludge could be easily incinerated with the existence of limited auxiliary fuel,
- To make sludge hygienic (without pathogenic organisms),
- To stabilize sludge (what is achieved by drying sludge to the sludge dry mass about 95% of DS).
Waste Incineration System
Waste incineration system is for combustion and destruction of dried sludge. With this incineration system, sludge is turned into ash and can be safely disposed into waste deposition sites or can be used as packing material. The system includes;
- Automatic Loading Unit,
- Main Combustion Chamber,
- Heat Exchanger,
- Flue Gas Treatment System,
- ID Fan,
- Main Stack,
- Automatic Control Unit,
Dried sludge is fed to the system with the help of Automatic Loading Unit which is fully automated and controlled by Program Logic Controller (PLC). Loading unit is designed as an air-tight structure and wastes are loaded to the loading unit via screw (helical) feeder.
Main combustion unit is the part of the whole incineration system in which the solid phase is turned into ash and combustible gases at 1000ºC. Approximately 5% by volume of the waste remains as incombustible ash and accumulates at the ash removal unit. It is removed at the end of the operating cycle automatically.
In the Post combustion chamber, gases originated during the incineration of wastes in the main combustion chamber are re-burnt in excess air at 1100 -1200 °C during 2 seconds.
Heat exchanger is included in the system for both decreasing the temperature of the flue gas coming from the combustion chamber and heating the air necessary for the drying process. Therefore, at this stage auxiliary fuel or energy is excluded necessary for the drying process. This energy simply provided from the sludge itself by incinerating it.
After cooling the flue gas it is directed to the flue gas treatment system in order to be fully in compliance with the desired emission standards. Whole system is controlled by the automatic control unit.
Advantages of the System
- Destruction of organic matter
- Destruction of living organism, therefore elimination of health problems that may be occurred due to untreated sludge,
- Less investment and operation cost,
- Safe disposal and independence for operators.
- Minimum area needed,
- Minimized transportation costs,
- Minimized insertion of pollutants in the environment,
- Use of heat for sludge drying so that optimized energy usage,
- Outstanding energy efficiency,
- Full compliance with the desired standards such as European Directive on Waste - 2000/76/EC, EPA or any local standards.