Cities around the world produce huge quantity of municipal wastewater (or sewage) which represents a serious problem due to its high treatment costs and risk to environment, human health and marine life. Sewage generation is bound to increase at rapid rates due to increase in number and size of urban habitats and growing industrialization.
An attractive disposal method for sewage sludge is to use it as alternative fuel source in cement industry. The resultant ash is incorporated in the cement matrix. Infact, several European countries, like Germany and Switzerland, have already started adopting this practice for sewage sludge management. Sewage sludge has relatively high net calorific value of 10-20 MJ/kg as well as lower carbon dioxide emissions factor compared to coal when treated in a cement kiln.
Use of sludge in cement kilns can also tackle the problem of safe and eco-friendly disposal of sewage sludge. The cement industry accounts for almost 5 percent of anthropogenic CO2 emissions worldwide. Treating municipal wastes in cement kilns can reduce industry’s reliance on fossil fuels and decrease greenhouse gas emissions.
The use of sewage sludge as alternative fuel in clinker production is one of the most sustainable option for sludge waste management. Due to the high temperature in the kiln the organic content of the sewage sludge will be completely destroyed. The sludge minerals will be bound in the clinker after the burning process. The calorific value of sewage sludge depends on the organic content and on the moisture content of the sludge. Dried sewage sludge with high organic content possesses a high calorific value. Waste coming out of sewage sludge treatment processes has a minor role as raw material substitute, due to their chemical composition.
The dried municipal sewage sludge has organic material content (ca. 40 – 45 wt %), therefore the use of this alternative fuel in clinker production will save fossil CO2 emissions. According to IPCC default of solid biomass fuel, the dried sewage sludge CO2 emission factor is 110 kg CO2/GJ without consideration of biogenic content. The usage of municipal sewage sludge as fuel supports the saving of fossil fuel emission.
Sludge is usually treated before disposal to reduce water content, fermentation propensity and pathogens by making use of treatment processes like thickening, dewatering, stabilisation, disinfection and thermal drying. The sludge may undergo one or several treatments resulting in a dry solid alternative fuel of a low to medium energy content that can be used in cement industry.
The use of sewage sludge as alternative fuel is a common practice in cement plants around the world, Europe in particular. It could be an attractive business proposition for wastewater treatment plant operators and cement industry to work together to tackle the problem of sewage sludge disposal, and high energy requirements and GHGs emissions from the cement industry.
Use of (dried) sewage sludge as an alternative fuel can be an interesting treatment method, but several issues have to be taken into account when considering this option. First of all drying of the sewage sludge can cost just as much energy as it will deliver when burned. Therefore drying should take place with waste heat to have a positive nett energy balance. Secondly sewage sludge can contain significant amounts of mercury that wil than be emitted by the cement kiln. Appropriate flue gas treatment is therefore to be considered. Finally sewage sludge contains a large amount of phosphate and this valuable resource will then be dilluted in the cement. In addition the phosphate will degrade the binding properties of the cement and will limit the amount that can be co-incinerated. The importance of phosphate as a resource has already lead to legislation in Switzerland that does not allow the phosphate in waste streams to be diluted, See also: http://www.aiforo.com/development/
Dear Leon,
thanks for your comments. The use of sewage sludge as alternative Fuel in the cement industry is, as you stated, only one solution for a safe and environmental friendly usage. First of all, the use as alternative Fuel is limited by the relevant environemntal emission regulations., So Sludges, which contain mercury will not be used. There are strict regulations and guidleines for the use of alternative fuels which have to be meet. The Cement Sustainability Initiative describes Alternative Fuels as follows: “Selected waste and by-products with recoverable calorific value can be used as fuels in
a cement kiln, replacing a portion of conventional fossil fuels, like coal, if they meet strict
specifications. Sometimes they can only be used after pre-processing to provide ‘tailor-made’
fuels for the cement process“
These Guidelines also stating, that wastes, which can be used as alternative Fuels have to be reviewed under the “end of pipe” proces; means, if wastes can be recycled, there first should be recycled and not used as fuel.
To reclaim the phosphate from sewage sludge, various technologies are tested- but still under development and not yet described as Best available technology. It will take some years time, before phosphate recycling will be adapted in a large scale. Till now, e.g. in germany, around 53% of all sewage sludge ( around 1,2 mio tons dry substance) is used as fuel- in coal fired power plants or in cement Plants. This saves around 500.000 tons of coal ( coal CV 6200 kcal, dreied sewage sludge 3500 kcal).
Drying- you are absolutely right- should be done and is currently done with available waste heat from the plants. However, we are always observing new technologies and evaluating, if these technologies can be implemented in our projects around the world.
The effect of using sewage sludge with a phosphate content has a effecgt on the cement Quality. Investigations show that phosphate participates actively in the reactions during the clinker burning process. It displaces the limits of stability of individual phases.
Phosphate reacts to form a C2S-C3P mixed crystal that exhibits continuous mixed crystal formation with ?-C2S. At P2O5 levels in the clinker of up to 1.0% the early strengths decrease due to the reduced content of alite and aluminate, only slightly at first but more sharply at higher levels of P2O5. On the other hand, the 28-day strengths increase somewhat at P2O5 levels of up to 1.0% – so no negative impact on the cement Quality.
When we look at recycling we always have to see the development of recycling and reycling technologies with different views. In high developed countries like in the Netherlands or Germany, were up to 50€/ ton of sewage sludge is paid for a safe disposal or recycling, you can implement expensive and highly developed technologies. But- unfortunately- in most of the countries in the world, either there is no environmental friendly solution- means the sludge is even not seperated and the waste water is polluting rivers and the sea- or it is just landfilled or dumped somewhere. Nobody is paying – and able- to pay for sophisticated recycling.
So we always need to have a different view- and if cement plants- which are available in almost all countries of the world- using sewage sludge in a safe and environmental friendly manner this is the best solution…
Bst regards
dirk
Hello Dirk,
I think we agree on the possibilities and limitations of using sewage sludge in cement kilns. I also fully agree that you should take into account the local situation. There is one solution for every situation. With respect to mercury: most sludge will contain 1-3 ppm of mercury and that is the reason that co-firing of sewage sludge in power plants is not allowed in The Netherlands. Also special mercury limits are in force for the (only) cement kiln in The Netherlands. The problem is that the mercury is so diluted in the huge gasvolumes that it is difficult to detect. Also there is a problem of a level playing field as dedicated sewage sludge or MSW incinerators have to meet lower mercury limits (as well as NOx, SOx and dioxin-limits).
Recovery of phosphate is still evolving but it’s application will increase in the future and can be of special importance for developping countries as nutrient deficiency is especially an issue in these countries. Some interesting developments for phosphate removal of sewage sludge before incineration are being developped at Wetsus (www.wetsus.nl), where I lead a research theme on phosphate recovery.
Sorry, I meant: there is NOT one solution for every situation
i agree with Leon Korving, Aiforo