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China has seen a significant expansion in the number of wastewater treatment plants constructed over recent years, and the first sludge drying plant in Beijing is currently under construction. As more operators worldwide consider options for sludge treatment, a leading company in the field examines the benefits of sludge drying.
by Klaus Stanke, Juergen Geyer and Yan Xu
An important consideration for operators of wastewater treatment plants (WWTPs) is how to handle the disposal of the residual sludge in a reliable, sustainable, legal and economical way. This by-product of wastewater treatment contains abundant organic material, including many kinds of bacteria. It also contains heavy metals and its composition is generally unknown.
Although reliability and flexibility are important factors in the operation of a WWTP, the choice of the optimal solution for sludge disposal has to take account of a number of factors such as legislation, government guidelines, the cost and possible applications for the dried sludge. This means that there is no universal solution to the problem of sludge disposal for all plants.
The benefits of drying sludge can be seen in two main treatment options:
- use of the dewatered sludge as a fertilizer or in fertilizer blends
- incineration with energy recovery.
Use as a fertilizer
This option takes advantage of the high organic content (40%-70%) of the dewatered sludge and its high levels of phosphorus and other nutrients. However, there are a number of concerns about this route including:
- the chemical composition of the sludge (e.g. heavy metals, hormones and other pharmaceutical residues)
- pathogen risk (e.g. salmonella, Escherichia coli, prionic proteins, etc.)
- potential accumulation of heavy metals and other chemicals in the soil.
To prevent a build-up of heavy metals and other contaminants in the soil, there are restrictions on the amount that can be applied per area. In addition, fertilizer use is not necessary on a particular field every season but sludge is produced daily from WWTPs receiving a continuous flow of domestic sewage and other wastewaters.
Sludge can be applied as a fertilizer in three forms:
- liquid sludge
- wet cake blended into compost
- dried granules.
Thermally dried sludge in granular form reduces the pathogen risk and provides flexibility as it can be stored until needed.
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Energy recovery
This option takes advantage of the energy available in the sludge’s organic content. Drying the sludge reduces its water content, thus increasing its calorific value and making it easier to combust. It also reduces odours and improves handling, with lower transport and storage costs.
Sludge from WWTPs is typically combusted in:
- cement kilns
- coal-fired power plants
- mono-incinerators, i.e. plants burning refuse-derived fuel or a single waste stream
- mixed waste incinerators, e.g. municipal waste incinerator.
Different types of plants show different flexibility in terms of accepting sludge as an input material. In addition, the thermal balance of an existing plant may further restrict what may be acceptable in addition to the existing load. Table 1 summarizes how different forms of sludge are used in different energy recovery plants.
The recognized advantages of energy recovery from sludge include:
- the high calorific value (similar to lignite) of dewatered sludge
- the use of dewatered sludge as a carbon dioxide (CO2) neutral substitute for primary fuels such as oil, gas and coal
- the use of dewatered sludge is a ‘sink’ for pollutants such as heavy metals, toxic organic compounds and pharmaceutical residues, thus offering a potential disposal route for these substances provided the combustion plant has adequate flue gas cleaning
- the potential, under certain circumstances, to utilize the inorganic residue from sludge incineration (incinerator ash), such as in cement or gravel. As a result, this can offer a complete solution to the problem of disposing of the WWTP sludge.
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But there are also concerns. Incineration of mechanically dewatered sludge cake will produce zero or only a small amount of excess energy. Indeed, additional energy is often needed to maintain combustion of such sludge. While thermally dried sludge will have a calorific value comparable to that of lignite or light coal, the drying process also requires energy; thus the overall energy balance is not always clear and positive. Much depends on the heat source and the drying process applied. Use of waste heat from the incineration plant to dry the sludge is, however, beneficial.
Where does this leave us?
It is evident that the drying of sludge plays an important role in both its utilization as a fertilizer and for the recovery of energy. Fully dried sludge offers a flexible product that can be channelled into either route (see Figure 1). Its application depends to some extent on its dried solids (DS) content.
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The demands placed on the drying system are therefore critical and include:
- high process stability
- high mechanical reliability
- high safety standards under all operating conditions
- compliance with environmental legislation, such as emission limits
- a product with properties suitable for a wide range of uses.
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Such demands are not easy to meet.
The Andritz fluidized bed approach
The sludge drying system developed by Andritz Fliessbett Systeme GmbH has been successfully applied for more than 15 years in a growing number of installations. The system includes a fluidized bed dryer, the direct feeding system, the gas recycling system, cooling loop and product handling. The main elements are outlined below.
The dryer
Figure 2 shows the operation of the fluidized bed dryer. It is essential that the fully dried, granular sludge particles are kept in suspension against gravity by a constant gas flow. Blowing the fluidizing gas uniformly across the entire area of the dryer generates a fluidized layer of dry, granular sludge particles that look much like a boiling liquid.
The direct (sludge) feed system
Unlike most other sludge drying systems, the Andritz system does not need to recycle dry granules in order to generate wet granules in a mixer, which would then subsequently be fed to a dryer.
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The direct feed system disintegrates the wet dewatered sludge cake pumped into the dryer into small particles. This method of granulation (or disintegration) makes it independent of the nature of the incoming sludge and/or cake moisture content. It is applied, for example, in regional sludge drying centres where sludge is accepted from up to 40 different WWTPs without blending or any other kind of homogenization.
Gas recycling system
The fluidized bed dryer operates in a closed inert gas loop (Figure 3). The fluidizing gas leaving the dryer carries fines and evaporated water from the fluidized bed dryer. The fines are separated in a cyclone and the evaporated water is condensed from the gas stream in a scrubber-condenser using a direct water spray. The gas is recycled to the dryer using a blower.
The system generates small quantities of inert gas which are enough to keep the entire dryer loop permanently under a low-oxygen atmosphere. Oxygen concentrations of 1%-3% measured in the dried sample gas are typical.
The excess inert gas from the dryer loop flows through the entire downstream sludge-handling system including the dry granules storage, thus keeping the entire system under low-oxygen atmosphere and keeping it safe from dust explosions, fires and smouldering.
Final product and downstream handling
The quantity of dust recovered in the cyclone is accumulated in a hopper and fed back in batches to the dryer after rehumidification with a small quantity of sludge cake. This assures a final product that is completely granular and virtually dust-free. The size distribution is typically 1-5 mm. Bulk densities of 500-750 kg/m3 are achieved, depending on the sludge quality.
The dried product from the fluid bed dryer is cooled to < 40°C using a fluidized bed cooler under inert gas conditions as described above. From there, the granules are conveyed to storage silos.
Control
Control of the fluid bed process is simple, reliable and allows for fully automatic operation without permanent operator supervision. A number of such plants even run without any operating personnel present during the night or at weekends.
Measurement and adjustment of the dry solids content in the final product is not necessary. The intensive energy and mass transfer process in the fluidized bed ensures that the DS content of the final product is > 90% at a temperature of 85°C. To keep this temperature constant, the sludge feed rate into the dryer is adjusted by automatically reducing or increasing the speed of the dryer feed pumps.
Application of the technology
Andritz is active in different markets worldwide. A growing market for this technology is China, where the expanding economy is accompanied by growing environmental problems. A huge number of wastewater treatment plants have been built in China during recent decades and more are planned. WWTPs in China currently produce about 1.3 million tonnes of dewatered sludge each year, with an average annual growth rate of about 10%. The box above gives details of the evolving picture in China.
Klaus Stanke is Managing Director, Juergen Geyer is Product Manager Biosolids Drying, and Yan Xu is Sales Manager Asia at Andritz Fliessbett Systeme GmbH in Ravensburg, Germany.
e-mail: Yan.Xu@andritz.com
Practical experiences in China
Until recently, disposal of WWTP sludge received relatively little attention in China. Particularly in more rural areas, sludge was used directly as a fertilizer or just landfilled. But the situation is changing. An increasing number of studies and projects are looking for alternative sludge handling technologies in China, focusing initially on the bigger coastal cities.
Andritz was one of the first companies active in China for sludge drying, delivering its first sludge drying plant in 1996. This plant handled sludge from the WWTP of a chemical production site in Shanghai, using a fluidized bed system with direct feed system (basically as described in this article).
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In 2004, the first drying plant for sludge from municipal sources (sewage) in China started operation at the Shanghai Shidongkou Wastewater Treatment Plant. A direct-feed fluidized-bed sludge-drying system was chosen. The drying plant is combined with a sludge incinerator, which delivers the thermal energy for the drying step.
As a consequence of the successful operation of the Shanghai plant, a fluidized bed with direct-feed system was chosen by Beijing Drainage Group in March 2006 for the Qinghe WWTP in Beijing. As the first sludge drying plant in the capital city, it will dry about 400 tonnes per day of dewatered sludge cake. The drying plant consists of two fluidized bed dryers. According to the schedule, the project must be completed before the end of 2007 - a deadline given to most infrastructure projects in Beijing ahead of the Olympic Games in 2008.
Design data for the three different plants are given in Table A. Local partners have built or will build standard or non-process items at all the plants, with Andritz supervising the projects and supplying the sludge drying technology.
