Collectivities wastes management has become an issue of growing global concern as urban populations continue to increase and thus their consumption patterns are changing. Nowadays, more than half of the world’s population is living in urban centers.
When talking about those types of wastes, it traditionally refers first to municipal solid waste which consists of everyday items that people use and then throw away, such as product packaging, furniture, clothing, bottles, food scraps, newspapers, paint, batteries etc… and secondly to sewage sludge’s, which are generated primarily by municipal wastewater treatment plants. The quantities have increased substantially in recent years because of an increase in the number and size of urban communities and as well as in the amount of wastewater discharged by industrial processes.
In many developing countries, the final disposal for those wastes is either the landfill or worse in the case of sludge’s sometime used as fertilizer in the agriculture: their content in pollutants may be toxic to plants and humans. Dried/dewatered sewage sludge that is landfilled contains heavy metals and a significant amount of carbon that can produce methane, a gas with a global warming potential 21 times that of CO2.
Co-processing of those waste streams in cement kilns could be part of the solution to this problem.
The co-processing concept has been adopted by numerous countries and clearly defined by the Basel Convention (2011) as “the use of waste materials in manufacturing processes for the purpose of energy and/or resource recovery”
Due to its specific process capabilities (temperatures, residence time, self-gas cleaning etc…) since more than 20 years, Co-Processing in Cement kilns is under development worldwide. In fact, more than 20’000’000 tons of wastes are co-processed yearly by the Cement sector.
For optimizing the recovery operations, both (MSW and Sewage sludge’s) need to be pre-processed prior their acceptance at the cement plant gate.
The MSW has to go through a mechanical treatment process. During this procedure, raw MSW is screened and treat first to recover recyclable materials such as metals, PET plastics, aluminum cans, secondly to separate the compost and heavy part as demolitions debris, and finally, the remaining part, representing about 25% by weigh of the original MSW can be co-processed in Cement kilns. Size reduction through shredding is an essential operation in mechanical treatment process of MSW. Shredding of MSW to about 200-250 mm is common in many waste treatment facilities. Additional shredding steps might be required to produce RDF to be fed in the main burner and thus need to be smaller than 50 mm (2D) and 20 mm (3D). The RDF (Refuse-derived fuel) which is the outcome of the preprocessing of MSW may replace 15 to 40 % of primary fossil fuels used in cement plants – more for fully optimized combined processes (pre & co-processing). We may consider that this waste has an average of 3’500 kcal/kg and can vary from one country to another. The important characteristics of RDF as a fuel are the calorific value, water content, ash content, sulphur and chlorine content. The Chlorine issue usually comes from the presence of kitchen salt in the waste and should be minimized during pre-processing operations..
When looking at sewage sludge’s, after the current treatment, they often contains approximately 80 % water. This high water content will result in negative heat gain when the sludge is directly used in cement kilns, So sewage sludge’s must undergo a dewatering process. There are different types of dewatering processes from centrifuges to simple belt or filter press. Normally, filter press represent the best way to maximize the water reduction via a mechanical way. The “sewage cakes” will contain around 50 % dry materials. Those cakes are non-sticky, easy to transport and their dry part has as well a significant calorific value for the organic fraction and represent a significant source of alternative mineral to substituting virgin raw materials.
Even if those above solutions are in favor of the environment protection and resource preservation, we still face an important issue. The costs of RDF production and sewage sludge pre-processing and co-processing are usually higher than existing landfill fees. So lobbying with the authorities to implement the “polluters pay” principle becomes the priority number 1.
Last but not least, both MSW and sewage sludge has significantly lower carbon emission factors than coal. Therefore, replacing coal with MSW and sewage sludge will significantly reduce CO2 emissions.
Landfill is not an environmentally friendly solution and incineration is not only extremely costly but it neither allows for CO2 reduction nor it helps reducing the use of non-renewable combustible by the cement industry. Co-processing in the cement kilns is by far the best solution for the environment.
Further progress in this direction needs a close collaboration between the cement industry and the local authorities under the umbrella of Environmental Ministry to define a clear road map based on win-win economic conditions.