2.5 Energy

The importance of energy recovery from sanitation (wastewater and faecal sludge) is evident from the fact that waste minimization and alternative energy generation can help in resource optimization. The advantages include associated energy cost savings, a reduced environmental impact (i.e. pollution) and compliance with strengthening regulations. The recovery of renewable energy also reduces greenhouse gas emissions and provides an option of gaining carbon credits. Sludge streams have high calorific values and are rich in energy sources that can be potentially recovered. Energy recovery can be an incentive for complete sanitation value chain and services.

Sanitation has a strong linkage to renewable energy production and climate change. Sanitation systems can be designed in a way to produce renewable energy sources (biogas or biomass) which in turn may mitigate climate change by reducing greenhouse gas emissions.

There are different pathways for generating energy from sanitation (waste water and faecal sludge) as briefly described below –

• Anaerobic digestion producing biogas – biological conversion method which is widely used due to lower costs and ability to utilize organic waste with high moisture content without reducing the high calorific value of the produced gas (combination of methane and carbon dioxide).
• Thermochemical conversion producing biomass/fuel/gas – combustion (or mono-incineration), pyrolysis and gasification. These processes require lower moisture level in sludge because energy efficiency of the process is reduced due to energy consumed for drying the sludge.
  • Incineration is one of the most prominent technology which was not traditionally meant for energy recovery but rather to reduce the volume of waste and destroy harmful contaminants. The process of heat recovery coverts the traditional incinerator into a combustor where heat is harnessed from flue gas and is utilized for heating fluid which can be used directly by heating or for generating electricity via a steam turbine.
  • In pyrolysis, combustion occurs in an inert atmosphere to produce pyrolytic oil, biochar and non-condensable gases. Biochar, non-condensable gases and bio-oil can be used as solid, gaseous and liquid fuels for electricity and heat generation via combustion. Bio-oil can also be reformed to synthesis gas for energy recovery while biochar can be utilized as soil conditioner.
  • Gasification involves conversion of organic compounds via partial oxidation at high temperatures for production of synthesis gas which can be used in combined heat and power (CHP) for heat and electricity generation.
• Co-incineration and Co-processing – Co-incineration involves combusting sludge (municipal) in municipal solid waste incinerators. In co-processing sewage sludge serves as an alternative fuel in cement kilns and coal fired power plants. This requires additional fuel which has higher calorific value than sewage sludge, but the process substitutes 15-20 percent of the conventional fossil fuels.

Biogas is a renewable energy that can be used for cooking, lighting, heating and for generating electrical power. Biogas production depends on the amount of organic matter removed by anaerobic digestion. Biogas can either be burnt in a gas stove or used within a combined heat and power unit (CHP) for electricity generation. For use in a CHP, biogas must be filtered to remove aggressive sulphur compounds. If the biogas cannot be used, then it should at least be flared (this converts methane to carbon dioxide which has 25 times lower GHG potential than methane). After the generation of biogas, the residue of anaerobic digestion (called “slurry or digestate“) still contains all the nutrients and some organic matter. This residue is therefore suitable for application in agriculture as a fertiliser and soil conditioner. The macronutrients (N, P and K) which are contained in the substrates remain in the digestate and are easily available to plants. Due to the two benefits of energy production and fertiliser production, anaerobic digestion is receiving interest as an option in sustainable sanitation concepts.

Biomass is a non-fossil energy source which can substitute fossil fuels. For example, biochar is a solid material obtained from pyrolysis, the thermochemical conversion of biomass in an oxygen-limited environment. Biochar derived from pyrolysis of sludge, faeces and/or organic waste may be applied to soils in order to improve soil properties and crop yields or as a feedstock for energy recovery as well as acting as a carbon sink to reduce climate change impacts. It is typically called “biochar” when it is used as a soil conditioner and “char” when it is used as a fuel. Biochar has other applications such as using as an adsorption material for filters, especially for water purification purposes.