Linking Sanitation, Climate Change & Renewable Energies
Sustainable sanitation projects can contribute to both climate change mitigation (through energy or nutrient recovery) and to climate change adaptation (through innovative sanitation systems and wastewater management). Measures of renewable energy production consist basically of either biogas production from waste water or biomass production through the use of waste water to grow short rotation plantations for firewood. Biogas can also be used for heat generation while heat exchangers can recover heat energy from wastewater in sewers. Measures of nutrient recovery are primarily based on nitrogen reuse. Adaptation measures in the area of sanitation aim at coping with increasing water scarcity or flooding.
Contents
1. Introduction
1.1 Overview
The United Nations Framework Convention on Climate Change (UNFCCC) defines ‘Climate change’ as a
"change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods."
Some of the major climate change effects that have been predicted are the significant rise in temperature due to greenhouse gases, rising sea level and shifts in precipitation and evapotranspiration patterns (IPCC, 2007a).
Sustainable sanitation has a strong link to climate change and renewable energy production. For example, 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. Sanitation systems may also serve to help people adapt to climate change by reusing energy, nutrients and treated wastewater and thus substituting the use of primary resources.
Another example is dry toilets such as Urine Diversion Dehydrating Toilets (UDDT) with a raised platform and safe containment of excreta and which use no water for flushing (suitable for areas with increasing water scarcity) or which can still function during flooding events. UDDTs are potentially resilient to all expected negative climate change impacts while water born systems (flush toilets and sewers) are more vulnerable to different climate change scenarios (WHO, DFID, 2009).
1.2 Greenhouse effect and relevant greenhouse gases
The greenhouse effect is the phenomenon where the presence of so-called greenhouse gases (GHG) cause warming of the earth's surface: GHG allow solar radiation to enter the earth's atmosphere but prevent heat from escaping back out to space. They absorb infrared radiation and reflect it back to the earth's surface leading to its warming. In the field of sanitation, the following GHG are climate relevant:
- Carbon dioxide (CO2) is produced as a result of combustion of any fossil or biomass fuel. However, CO2 from biomass combustion does not contribute to global warming as it originates from the atmosphere; it is a step in the organic carbon cycle. In sanitation, CO2 emissions occur whenever fossil energy is used, as fossil fuelbased electricity. The treatment of wastewater for removal of organic matter and nutrients in wastewater treatment plants requires energy. The same holds true for the production of mineral fertilisers which is a very energy intensive process.
- Methane (CH4) is a potent greenhouse gas with a global warming potential 25 times higher than that of CO2 in a 100 year perspective (IPCC/TEAP, 2005). In anaerobic processes, organic matter contained in domestic waste and wastewater is decomposed and biogas is formed which contains 60-70% methane. In soak pits, anaerobic ponds, septic tanks and other anaerobic treatment systems or even at the discharge of untreated wastewater into water bodies, anaerobic processes take place to different extents and methane is released to the atmosphere.
- Nitrous oxide (N2O) is a strong greenhouse gas with a global warming potential 298 times higher than that of CO2 in a 100 year perspective (IPCC, 2005). Nitrous oxide emissions occur during the denitrification process in wastewater treatment, at the disposal of nitrogenous wastewater into aquatic systems and also during mineral nitrogen fertiliser production. For climate protection, nitrogen in excreta or wastewater can be recovered and reused as a fertiliser to save energy.
2. Climate change mitigation and adaption potential of sanitation
2.1 Mitigation measures (energy and nutrient recovery)
Sanitation systems can be designed and operated to produce renewable energy in the forms of either biogas or biomass and thus reduce primary energy consumption (see Section 3 for details). Small scale biogas systems can generate enough biogas to cook main family meals and thus replace part of the traditional used cooking fuels. It should, however, be kept in mind that particularly in small systems the organic load from human excreta alone is in most cases not high enough for the economical usage of biogas for cooking, lighting or heating but still beneficial. Much more biogas is produced if animal excreta, organic solid waste (e.g. from kitchens and/or markets), or agricultural waste is co-digested as well.
The macronutrients nitrogen (N), phosphorus (P) and potassium (K) contained in human and animal excreta can be locally recovered and safely used as fertiliser in agriculture. Hence, a substitution to the manufactured mineral fertilisers with their associated energy intensive production and transport over long distances (for information on the safe use of excreta in agriculture see references (WHO, 2006) and (Gensch et al., 2012). A life cycle analysis study comparing the energy demands for nutrient removal and mineral fertiliser production versus nutrient recovery identified a considerable energy saving potential with urine diversion nutrient recovery (Maurer, Schwegler, Larsen, 2003).
2.2 Adaptation measures
In order to adapt sanitation systems both to water scarcity and increasing amounts and periods of rainfall and flooding, the measures that can be taken include for example:
- In cases where potable water is used for irrigation, the use of treated wastewater would substitute the extraction, processing and distribution of potable water and thus may lead to energy savings (see guidelines of the WHO (2006). The nutrient content of the wastewater also reduces the need for mineral fertiliser input. Other possible reuse options are the irrigation of parks, lawns and other public spaces and groundwater recharge.
- Dry toilets as an alternative to conventional flush toilets and subsurface drip irrigation instead of conventional irrigation reduce the daily water footprint.
- An effective measure to adapt sanitation systems to flooding is building them on slightly elevated concrete structures.
3. Renewable energy production from sanitation
3.1 Biogas
The biogas generation by anaerobic bacterial decomposition of organic matter uses organic sources such as fresh faecal sludge from public toilets and septic tanks and pit latrines or animal manure. The production of both energy and fertilizer makes the anaerobic degradation an economically viable wastewater treatment (in China for example the construction of 5 million small-scale biogas plants was governmentally supported (Balasubramaniyam et al., 2008). For the production of biogas for the cooking needs of one person the excreta of about 10 people, 1 pig or 1/2 cow is needed (indicative values) (Balasubramaniyam et al., 2008). Hence, the available energy potential in human excreta should not be overestimated.
The biogas can either be burnt in a gas stove or used within a combined heat and power unit (CHP) for electricity generation. The CHP is equipped with a gas engine for producing electricity and heat. The efficiency is 30% for electricity generation and 60% for heat production which may sum up to a total energy efficiency of 90% in case the excess heat is used on-site. This high efficiency represents the main advantage of a CHP compared to a biogas plant.
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.
3.2 Biomass
Biomass is a non-fossil energy source which can substitute fossil fuels. According to the UNFCCC definition, renewable biomass is understood as (UNFCCC, 2006):
- wood (provided that wood harvest does not exceed wood growth),
- other wooden biomass (provided that the cultivated area remains constant),
- animal or human manure or
- solid organic waste (domestic or industrial).
While food production is often in the focus when talking about nutrient cycles, another interesting aspect is the cultivation of energy crops. In a new approach, the so-called Short-Rotation-Plantations (SRP), domestic wastewater is reused to irrigate and fertilise fast growing energy crops. The term SRP refers to plant species which are harvested after short periods, usually between 2-8 years, but also annually in the case of herbaceous plants or grasses. Drawbacks that have to be considered are a possible pollution of groundwater and an increase in soil salinity.
With a 10 hectare SRP, the wastewater of approximately 6,500 people with a daily discharge of 100 L/person may be treated, corresponding to an area of 15 m2/person.
4. Acknowledgements
SuSanA factsheet: Links between sanitation, climate change and renewable energies. April 2012. susana.org
