Heat Recovery from Sludge

The costs for sewage sludge treatment often represent more than 50% of total wastewater treatment costs. Looking into the future, sludge management will consist in innovative treatment methods focussing on three aspects:

• Recovery and reuse of valuable products from sludge (nutrients or heat).
• Complete solution of the sludge problem (final disposal), especially regarding toxins.
• Acceptable costs.

In this respect, it can be expected that the recovery of sustainable energy from sewage sludge will become more and more of interest (RULKENS 2007). In this factsheet, each one of the following techniques are introduced down below:

• Anaerobic digestion of sewage sludge
• Production of biofuels from sewage sludge
• Direct production of electricity from sewage sludge in microbial fuel cells,
• Incineration of sewage sludge with energy recovery
• Co-incineration of sewage sludge in coal-fired power plants
• Gasification and pyrolysis of sewage sludge
• Use of sludge as an energy and raw material source in the production of Portland cement and building materials,
• Supercritical wet oxidation of sewage sludge
• Hydrothermal treatment of sewage sludge

Large-scale anaerobic biogas digesters are reactors used for the conversion of the organic fraction of large volumes of slurries and sludge into biogas by anaerobic digestion. Biogas is recovered and used either directly for heating the reactors or transformed into combined power and heat and fed into the grid. Read more about anaerobic digestion in general or large scale anaerobic digestion.

An overview of the various conservation processes (either liquid or gaseous energy carriers from biomass) are given by CLAASSEN et al. (1999). What type of energy carrier is produced strongly depends upon the types of microorganisms and the applied process conditions. Depending upon the type of microorganisms, energy carriers, such as methane, ethanol, acetone, butanol, or hydrogen, can be produced. For sewage sludge, by far, the most information is available on the production of biogas (methane, see anaerobic digestion). There are not many studies about the production of ethanol, butanol, or acetone from sewage sludge. One reason could be that the process to separate the components is very complex and therefore less attractive (RILKENS 2007). There are a few studies about the production of hydrogen from sewage sludge, e.g. TING and LEE (2006). Another approach is to produce biodiesel from waste sludge. Nowadays, sources for biodiesel are vegetable oils or animal fats, which are quite expensive (pure vegetable or seed oils constitute between 70% and 85% of the overall biodiesel production cost). On the other hand, biodiesel production from sludge could be very profitable in the long run (KARGBO 2010).

Microbial fuel cells can be used to directly produce electricity from a wastewater stream, such as municipal wastewater containing biodegradable organic compounds. In the figure to the left, a schematic presentation of a microbial fuel cell is given. Basically, a microbial fuel cell consists of an anode chamber and a cathode chamber separated from each other by a cation exchange membrane.In the anode chamber, organic compounds present in the wastewater, such as carbohydrates, are oxidised by specific microorganisms (RULKENS 2007). You can find more information about it in AELTERMAN et al. (2006) or LIU et al. (2004).

Adapted from RULKENS (2007)

Incineration (either small scale or large scale) of sewage sludge is aimed at a complete oxidation at high temperature of the organic sludge compounds, also including the toxic organic compounds. The process can either be applied to mechanically dewatered sludge or dried sludge (e.g. drying beds). Potential environmental problems related to sludge incineration are the emissions of pollutants with the exhaust gases to the atmosphere as well as with the quality of the ashes.

The energy produced in the incineration process can be used for the drying of the mechanically dewatered sludge cake prior to the incineration process or can be used for the production of electricity. Currently, sludge incineration processes are increasingly focused on the recovery of energy from the sludge in the form of heat (steam) or electricity. Incineration of sludge is currently applied worldwide more and more in combination with energy recovery, mainly on a large scale.

Adapted from RULKENS (2007)

To avoid the high costs of a stand-alone incineration plant for sludge and to improve the energy recovery efficiency, possibilities have been investigated to incinerate dried sludge in a coal-fired power plant. In this case, beneficial use can be made from existing coal combustion installations and existing exhaust gas treatment systems. Because the amount of incinerated sludge is small compared to the amount of coal, the effect of the incineration of the sludge on the air and ash qualities can be neglected. The energy recovery from the incineration of the sludge can often be strongly increased by improvement of the dewatering and drying processes of the sludge and probably also by the use of the low-caloric waste heat from the exhaust gases of the power plant. Also, the use of the polluted drying gas in the incineration process can contribute to a better total process performance. Apart from co-incineration of sewage sludge in coal-fired power plants, other co-incineration processes (such as in combination with municipal waste), are also currently practised.

Adapted from RULKENS (2007)

Pyrolysis is a thermal treatment process in which the sludge (or biomass) is heated under pressure to a temperature of 350–500°C in the absence of oxygen. In this process, the sludge is converted into char, ash, pyrolysis oils, water vapour, and combustible gases. Part of the solid and/or gaseous products of the pyrolysis process are incinerated and used as heating energy in the pyrolysis process.

Adapted from RULKENS (2007)

In sewage sludge, both the organic carbon-containing compounds and the inorganic compounds represent valuable materials. There are several possibilities to use these compounds simultaneously in a beneficial way. Especially in Japan, a lot of effort has been put into the manufacturing of valuable products by thermal solidification of the inorganic sludge compounds. The starting point in this production process is either incinerator ash, obtained after combustion of sludge, or dried sludge. The solidification process occurs at high temperatures, up to 1000 °C, which is high enough to destroy toxic organic compounds. Waste heat is available for the drying process (e.g. mechanical dewatering). Depending on the needs, various types of products can be made, such as artificial lightweight aggregates (IMBIGS n.y.), glass-ceramic (SUZUKI et al. 1997), slags, and bricks (HEGAZY et al. 2012 or WENG et al. 2003). In general, the energy efficiency in the production process is not very high and it is expensive. Currently, the practical application is very limited.

Another beneficial way to use both the inorganic and organic compounds of the sludge is the use of sludge in the production of Portland cement. In this process, which is also applied in practice, either ash or dried sludge can be used. Toxic organic pollutants in the sludge are completely oxidised, and because of the high process temperature, heavy metals are immobilised in the cement.

Adapted from XU & LANCASTER (n.y.)

Supercritical Water Oxidation (SCWO), also referred to as hydrothermal oxidation, is a process that oxidises organic solutes in an aqueous medium using oxygen/air or hydrogen peroxide as oxidants, at temperatures and pressures above the critical point of water (374°C and 22 MPa). After reaction, the effluent is cooled and energy is recovered. According to Svanstrom et al. (2004), about half of the heating value of the sludge can be recovered in the studied process. The solid and liquid products are separated and the wet inorganic solids can be sent to a landfill or spread on dedicated land while the water can be redirected to a wastewater treatment plant.

Adapted from RULKENS (2007)

Hydrothermal treatment (or thermal hydrolysis) is a process in which the sludge is heated as an aqueous phase to temperatures (normally) varying between 120 and about 400°C. The hydrothermal treatment process aims to disintegrate the sludge and results in a formation and accumulation of dissolved products. This makes it possible to recover and recycle useful resources from the sludge, such as volatile fatty acids, phosphorous compounds, organic compounds for enhanced anaerobic biogas production, and coagulants. The potential of hydrothermal treatment for the recovery of energy from sewage sludge strongly depends upon the process performance.

The described methods and processes are highly engineered and therefore expensive and require expert knowledge and professional operators.