Summary

Carbon footprint refers to the total set of greenhouse gas (GHG) emissions in a region due to anthropogenic activities. Major sources of GHG are forests (due to human induced land cover changes leading to deforestation), power generation (burning of fossil fuels), agriculture (livestock, farming, rice cultivation and burning of crop residues), water bodies (wetlands), industry and urban activities (building, construction, transport, solid and liquid waste). Higher concentrations of GHG in the atmosphere have contributed to global warming and changes in the climate. Climate change refers to a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Carbon dioxide, nitrous oxide, methane, chlorofluorocarbon, etc. are major greenhouse gases (GHG). Carbon dioxide (CO2) is one of the more abundant greenhouse gases and a primary agent of global warming. It constitutes 72% of the total anthropogenic greenhouse gases. IPCC (2007) reported that the amount of carbon dioxide in the atmosphere has increased from 280 ppm (1750) to 394 ppm in 2012. Similarly, methane (CH4) and nitrous oxide (NOx) concentrations have risen substantially from pre-industrial levels (from 715 ppb to 1730 ppb, and 270 ppb to 319 ppb respectively). For these gases, most of the concentration increases have occurred during the last 100 years.  Forests mitigate global warming through sequestration of carbon in the environment due to anthropogenic activities. Atmospheric carbon dioxide is taken up by trees, grasses, and other plants through photosynthesis and stored as carbon in biomass (trunks, branches, foliage, and roots) and soils. The sink of carbon sequestration in forests and wood products helps to offset anthropogenic sources of carbon dioxide to the atmosphere. Emission and sequestration of carbon needs to be in balance in the Earth system to maintain the environmental conditions.

First section of this chapter introduces carbon foot print concept with a scientific definition based on accounting principles and modelling approaches. Subsequent sub-section addresses questions such as system boundaries, etc.
The second section of this chapter estimates carbon footprint in the wastewater treatment plants. This shows that the major C foot print through electricity usage i.e. ~38.4 ktonnes CO2e (scope 2 emissions by) of the  total emission of ~41.53 ktonnes CO2e. The average biomass productivity found in the pond systems ranged from 10-20 g/m2/d and considering the surface BOD loading 400-900 kg/ha/d. The CO2 sequestration potential in the native L. ovum species is 1.23-1.48 tonnes CO2e/ha/d providing an annual C sequestration of ~490 tonnes CO2e/ha.  C sequestered accounts to about 10 tonnes CO2e/ha/yr, which gets transformed to lipids that can be used effectively as a biofuel. These unique way of C sequestration and transformation into valorisable algal biomass with ~20% lipids paves path for new avenues for energy sustainability with adequate reduction of wastewater C footprint.

Wastewaters generated from domestic households are a source of nutrients to growth and development of algae and consequent lipid extraction. Algae, helps in the mitigation of GHG emission through sequestration. CO2  sequestration in algae is accomplished by a) direct  sequestration,  which comprises  the  capture  of  CO2  from  the  wastewaters before  its emission to atmosphere, and its subsequent storage and  ii) indirect sequestration, based on the capture of  CO2  that  is  already  in  the  atmosphere,  through photosynthesis. C present in wastewater can be either inorganic C (carbonic acid/bicarbonates/carbonates) or organic C (dissolved organic acids etc.). Many wastewater algae are capable of consuming both C forms and are called mixotrophic algae and are extremely beneficial in rapid C fixation and reduction of wastewater C footprint.  Often bulk of the wastewater C fixed in the algal biomass is stored in the form of lipids (neutral) that can be potentially used as feedstock for biofuel as biodiesel and gasoline. Sequestration aspect of microalgae is discussed in Section 3.

Section 4 presents the possibility of mitigating GHG emissions from wastewater sector through integrated wetlands ecosystem. Integration with wetlands (consisting of typhabeds and algal pond) would help in the complete removal of nutrients in the cost effective way. However, this requires regular maintenance of harvesting macrophytes and algae (from algal ponds). Harvested algae would have energy value, which could be used for biofuel production. The joint activity of algae and macrophytes in the wetland systems helps in the removal 77 % COD, ~90 % BOD, ~33 % NO3-N and ~75 % PO43—P. Implementation of intgrated wetlands system help in treating the water and more importantly in the mitigation of GHG emissions from wastewater sector.