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A review of methods for the thermal utilization of sewage sludge

A review of methods for the thermal utilization of sewage sludge

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Introduction

In the course of sewage treatment, municipal sewage sludge can be generated at wastewater treatment plants. The waste stream from sewage sludge rapidly grows, creating wastes that require management by the law. According to the Ministry of the Environment of Poland sewage sludge is in group No. 19 includes the wastewaters from treatment installations and equipment for waste management, as well as wastewater from wastewater treatment plants. Poland is home to 38.2 million people. It has an average population density of 122 per square kilometer. The country covers 322,577km2, of which 311,904 km2 is land. In 2008, more than 1100 000 tons of sewage were produced in Polish sewage-treatment facilities (both municipal and industry). Produced a lot of municipal and industrial wastewater sludge. Only 63% of the country's wastewater treatment plants were able to serve the needs of its residents in 2008 (87% in urban areas and 26% rural areas where 39% of the population resides). This compares with Western Europe, which has more than 78%. 

 

Only 456 of Poland's 559 villages and towns had modern wastewater treatment plants with enhanced nitrogen and/or phosphorus removal. These plants treated 918 hm 3 waste, which represents 73% of all waste that is discharged from urban and rural water systems. In most countries, this ratio fluctuates between 70% and 70%. Polish wastewater treatment plants produce 0.25 kg of sewage sludge per m 3 of treated wastewater. This structure of sewage-sludge usage in Poland is extremely unfavorable, according to the EU Directives. Problems include a high level of sewage sludge stored and insufficient thermal utilization installations. The conversion of large amounts of sewage sludge into useful energy can be done using thermal processes. Thermal utilization of sludge is possible at existing facilities (e.g. power plants, cement plants, or heating plants), or in new buildings. Dehydration and drying should precede any thermal methods for sewage-sludge usage. 

 

Polish sewage-treatment plants employ 98% of the biological treatment. 36% use enhanced biogenic removal. There are 50% of sewage-treatment facilities in the EU that use anaerobic digestion; 18% have aerobic digestion and 4% lime stabilizer, while 24% do not. The same trends can be observed in Poland.

 

There are regulations and options for sewage-sludge usage methods

Digestive methods can reduce the amount of sludge by a large margin. There is also a tendency in Europe for sewage-sludge management priorities to be avoided, minimized, recycled, and thermal methods with energy recuperation and landfilling. There are many options for municipal sewage-sludge usage. However, this decision is subject to serious legal restrictions. These first results are a result of the EU Directive 91/271/EEC Urban Wastewater Treatment Directive and its amended Directive 98/15/EC. Article 14 of the directive, which states that "sludge resulting from wastewater treatment shall re-use whenever necessary", is the most important. This sentence clearly explains the directive's purpose in directing the treatment of sewage sludge. This document implies that new sewerage systems must be built and upgraded to treat sewage. This would increase the volume of sewage sludge produced.

 

The Sewage Sludge Directive 86/278/EEC is the second act. It places restrictions on the use of sewage sludge for agriculture. It is primarily concerned with the high levels of heavy metals found in sewage sludge and limits its natural and agricultural use. The directive was further supplemented with entries in subsequent acts of the European Community, the so-called Waste Directive.

 

This paper reviews the current state of the art in thermal technology for sewage-sludge utilization, taking into consideration the Polish requirements and existing facilities.

The use of thermal processes allows for significant weight and volume reductions in transformed sewage sludge. It is difficult to use sewage sludge for agricultural purposes due to its high heavy metal content. Thermal methods are also required because there is not enough land available for agriculture near urban sewage-treatment facilities. Thermal methods apply to sewage-sludge disposal based on their properties, e.g. heat of combustion (calorific values) and composition (including humidity content). The heating value for raw sludge is approximately 17 MJ/kg. For activated sludge, it's about 15 MJ/kg and for stabilized (digested anaerobically, aerobically, or lime stabilization) it's around 11 MJ/kg.

 

Review of the thermal methods for sewage-sludge usage

For the thermal treatment of sewage sludge, there are many thermal technologies on the market or in development. There are many ways to classify these technologies. They can be classified in many ways. Pyrolysis is the basis of oil-from-sludge. This process involves submitting sewage sludge containing 95% dried matter to pyrolysis at more than 450 degrees Celsius for 30 minutes at atmospheric pressure. Solid hydrocarbons and carbonization products, such as pyrolytic coke, are created. You can also generate liquid hydrocarbons. This product can be used as a raw material in many industries, such as petrochemical. The Carver-Greenfield technology, (C-R), is a crucial step in the pyrolysis of sewage sludge. It produces a refuse-derived fuel. This allows for simultaneous drying of the sewage sludge before its combustion or gasification. This process involves a mixture of raw sewage and oil waste (e.g. used motor oil). To remove any water, the prepared mixture is put through an evaporating unit. The sludge is dried and then passed to a centrifuge to separate the liquid from the particles. The liquid phase is then returned to the facility for fuel.

 

Gasification refers to the process of turning a solid fuel into gas. It involves using oxygen, steam, and air to convert the fuel. Marrero and colleagues. The gasification of sewage waste sludge produces a high-quality, flammable gas that can be used to generate electricity or for supporting processes such as drying sewage sludge. A gas composition that results from gasification of sewage waste. The main combustible elements are the ones that are included in this composition. Other components, depending on the gasification medium used, may include carbon dioxide, nitrogen, and oxygen. Different experimental protocols were used to analyze the emission of nitrogen oxides as well as ash comminution. The turbulence in the bed causes granular sewage to rapidly mix in bubbling fluidized-bed combustors. The ash agglomerates that form during combustion are broken down by the mechanical action of grains. The rapid equalization of temperature and high heat-transfer coefficient results in a vigorous and even combustion process that produces low NOx emissions. Shimizu and Toyono presented a study on a process that allows for the combustion and co-combustion of sewage sludge in a circulating fluidized bed. This type of bed has a much higher velocity than the bubbling fluidized one. Thus, uniform combustion and mixing are possible.

 

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