Thermal dried sludge meets EU standards
Efficient sludge management is facing the twin challenges of more stringent land disposal regulations and an increase in the quanity of wastewater. Doris Thamer, senior sales manager, and Werner Jenewein, senior process manager, of Austrian sludge specialist Andritz claim that drying technologies can meet both these demands.
Volume reduction has long been the focus of sludge treatment; drying evaporates most of intracellular and chemically-bound water, providing dry granules which are easy to store, transport and dispose of. Ideally volume reduction should lead to odour reduction and biological stabilisation of the organic matter in order to prevent degradation of the biomass.
In order to convert sludge to a product, enhanced thermal treatment is necessary. Sludge contains several nutrients (nitrogen, phosphorus, potassium, micronutrients) and organic compounds and can be used for fertiliser as long as the product is fully pasteurised, sanitised and fulfils requirements on heavy metal concentrations. Another beneficial use for dried sludge stems from its heating value.
The calorific value of fully dried sludge (>90% DS) is roughly equivalent to that of brown-coal, at approximately 11.000kJ/kg. It can therefore be used as secondary fuel for coal-fired power plants, waste incinerators or as an additive in cement and brick production, replacing primary fuel.
EU legislation has become more severe since the European BSE crisis and amid growing awareness of the risks posed by the heavy metals and hormones carried in sludge. Sludge reuse as fuel comprises an growing alternative use.
Energy recycling is also an attractive option and Andritz has developed the Eco-Dry process by which dewatered sludge is dried without using primary energy. This dried sludge is further used as fuel for the cyclone furnace producing the required heat for the drying process.
Eco-Dry utilises the calorific value of the sludge for its disposal without the need for any other fuel. The ash produced from this process is ready for use in road construction work or for landfill, meeting even the most stringent requirements.
Andritz drying technologies
Drum drying system (DDS)
Mechanically dewatered sludge is fed into the drying plant where water is evaporated in a triple-pass drum-dryer consisting of three concentric cylinders, and one joint-axis. To ensure the formation of granules, the feed to the drying drum is kept at about 65% dry solids (DS) by mixing the mechanically dewatered sludge with a certain portion of already dried granulate.
A feed of lower solids content has to be prevented because of the extremely sticky properties of the sludge, which may cause it to adhere to the drum surface or create lumps. As the drum turns slowly, the sludge moves forward in the air stream, from the inner cylinder to the middle one, and finally, into the outer cylinder.
During this process the granulate will conduct a rolling motion, which contributes to the formation of stable, globular granulate. Water evaporates from the pre-formed granulate in the rotating triple-pass drum. The air stream and the drum rotation cause the material to move across the drum until it is sufficiently dry (and light) for pneumatic discharge.
The smaller, lighter particles are discharged more quickly than larger, heavier particles which ensures the same drying result for particles of different size und therefore provide a very homogeneous dried granulate. After passing through the drum, the granulate has a dry solids content ¡Ý90% and is carried by the circulating air stream to the pre-separator and polycyclone.
In the pre-separator, the granulate is separated from the air and finally discharged via a cooling device. Separated fines and dust from the polycyclone are fed back into the drying circuit.
After separation of the dried sludge particles from the air stream, the warm and wet air enters the condenser where the water evaporated during the drying process is separated. The air is routed back to the furnace to be reheated to process temperature.
After the condenser, some of the air is separated from the circulating air stream and led to an exhaust air treatment. Possible energy sources for sludge drying with the drum dryer are primary energy (natural gas, biogas, light oil) and waste energy (exhaust gases from gas turbines, gas engines, off gas from incineration processes).
About 90 DDSs are in operation around the world. The first one has been in successful operation since 1974.
The world¡¯s largest DDS is currently being installed in Changi, Singapore, and will be ready in 2007. This plant comprises five lines of DDS, with a capacity of 11 tons/h water evaporation with more than 60 tons/h of dewatered sludge converted into product.
Fluidised-bed drying system (FDS)
A fluidised bed is characterized by the movement of granules, achieved by a gas stream passing through the bulk of product. Fluidisation gas is uniformly blown over the entire cross-section of the dryer generating a fluidised bed consisting of dry granulates.
The granules become free-floating and at the same time mixed thoroughly. The fluid bed dryer itself consists of three main sections:
- Wind box with a gas distribution plate
- Middle section, holding the heat-exchanger immersed in the fluidised bed. This heat exchanger transfers all the required energy for evaporating the water coming along with the wet sludge. Steam or thermal oil is the ideal heat transfer medium.
- Dryer hood, first separation step for particles lifted from the layer with the fluidisation gas. The gas leaves the dryer carrying the evaporated water and fines.
The dryer operates in a closed inert gas loop. The recycled gas leaving the dryer hood, carries evaporated water and fines. The fines are separated in the cyclone and the evaporated water is condensed out of the gas stream in a scrubber-condenser.
The dust is fed to a mixer and structured together with the original dewatered sludge. The dry, dust-free granules leaving the fluidised bed unit through the discharge opening are cooled by a vibrated fluid-bed cooler to a temperature below 40¡ãC and are ready for further utilisation.
More than 40 Andritz FDS systems are installed in the municipal field. The largest FDS installation (in one single line) is located in the Netherlands, converting 11 tons/h of dewatered sludge into a valuable, dried product.
Belt drying system (BDS)
The peripheral equipment of the BDS is similar to the DDS and FDS. The dryer itself is designed as a belt conveyor in a casing where the drying gases flow through the granulate which is evenly distributed on top of the belt. The belt is slow-moving which enables the drying gases to heat up the wet granulate and evaporate the water, whereas the granulate bed acts as a filter media to prevent dust generation inside the dryer.
The low operating temperature (around 110¡ãC-150¡ãC) and low dust levels ensure safe operation even without inertia inside the dryer, which is a requirement of the DDS and FDS process. The belt dryer is the ideal unit for integration of all forms of primary and especially secondary energy sources, which makes it highly attractive from a commercial point of view.
Three alternatives are possible:
- Direct heating with burner (heat is generated by burning fossil fuel or biomass (natural gas, biogas, fuel extra-light)
- Indirect heating with burner and heat exchanger
- Indirect heating with heat exchanger (heat is generated with saturated steam, thermal oil, hot off gases, hot water etc)
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