**3.3 Carbon footprint**

Carbon footprint is relatively a new measure of sustainability in WWTPs to determine its overall impact on climate change and as a result WWTPs performance has recently been evaluated based on carbon minimization [55, 56]. To address sustainability, carbon footprint minimization has become an important environmental indicator [57]. For carbon footprint, assessment, all relevant forms of the energy demand in WWTPs, sludge production and common GHGs emissions are accounted. This review aims to investigate previously unexplored relationships between carbon footprint and sustainability in the context of UASB+DHS system, focusing particularly on the impact of energy minimization measures.

#### *3.3.1 GHGs emission*

Global Warming Potential (GWP) is generally used as a metric for weighting the climatic impact of emissions of different greenhouse gases [58]. Among GHGs stated by Kyoto Protocol, the most common GHGs emitted during operation and on-site anthropogenic activities in WWTPs are carbon dioxide (CO2), methane

### *Downflow Hanging Sponge System: A Self-Sustaining Option for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.94287*

(CH4), nitrous oxide (N2O) [49]. According to USEPA, WWTPs are the 7th largest contributors of CH4 and nitrous N2O emissions in the atmosphere [59]. Particularly, WWTPs produce GHGs during the biological wastewater treatment processes. For calculation, all GHGs emission can be expressed as CO2 equivalents (CO2e) with respect to their GWP. CH4 and N2O have 28 and 265 times greater GWP compared to CO2 in a 100-year time horizon [60]. Therefore, more stringent regulatory efforts, mandatory reports and measurements on GHGs emissions from WWTPs are being enforced to control GHGs emissions.

Since, UASB+DHS system is also an anaerobic and aerobic biological treatment process, this information would be vital for the wastewater specialists. For almost all WWTPs, CO2 production is attributed to two main factors: biological treatment process and electricity consumption. In UASB+DHS system, CO2 is emitted during the production of the energy required for the plant operation. Emission of N2O is generated by nitrification and denitrification processes used to remove nitrogenous compounds from wastewater. Similar to the mainstream WWTPs, the organic carbon of wastewater is either incorporated into biomass or oxidized to CO2. During anaerobic digestion in UASB, it is mainly converted to CO2 and CH4. It is assumed that all the CH4 produced is oxidized to CO2 during biogas combustion. Estimation of (CO2e) is attained using units and equations summarized in **Table 1**. For the calculation of GHGs, considering the total treatment process is important. Therefore, GHGs emissions of both the system were estimated based on CO2 emission from COD oxidation, CH4 combustion and N2O emission as presented in **Table 3**. The data for GHGs calculation for UASB+DHS system and UASB+TF were taken from these studies [27, 60, 62]. The value for GWP by UASB+DHS system was 0.59 kg CO2 equivalent m�<sup>3</sup> d�<sup>1</sup> and that for UASB+TF was 0.50 kg CO2 equivalent m�<sup>3</sup> d�<sup>1</sup> . It is to be noted that for the calculation of GWP of UASB+TF, N2O emissions value was not available as there were no literatures reporting its values. Nevertheless, other studies associated with GHGs emission of TF + ASP system and TF+ Lagoon system showed GWP values of 1.232 kg CO2 equivalent m�<sup>3</sup> d�<sup>1</sup> and 0.898 kg CO2 equivalent m�<sup>3</sup> d�<sup>1</sup> respectively. Therefore, it could be assumed that the UASB+TF system might show fairly higher values compared to UASB+DHS system. The another reason behind assuming the lower GWP values by UASB+DHS system could be justified by its higher solid retention time (SRT) values of almost 92–101 days [17] compared to 2–4 days of TF [65]. Higher values of SRT supports endogenous respiration of biomass which increases the amount of COD oxidized to CO2 thus decreasing the overall sludge production [17]. This decrease of sludge production reduces the methane production and therefore, a decrease in CO2 emissions is associated with its combustion. Similarly, higher SRT capacity of DHS system helps to maintain low ammonia and nitrite concentrations in the media which leads to minimum N2O emissions to the atmosphere. Despite the accuracy of estimated GWP value is not exact, conclusive potential of operating UASB+DHS system at low GHGs emission levels has been assured. Hence, the analysis of GWP revealed the potential of UASB+DHS system to become a sustainable option in the future of wastewater treatment.


**Table 3.**

*Carbon footprint assessment of UASB+DHS system and UASB+TF.*

#### *3.3.2 Sludge production*

For most of the WWTPs, one of the biggest challenges is its sludge production, its post treatment and disposal. Being an aerobic system, DHS system has advantage over other biological treatment system for sludge management [61]. Any sludge accumulated in the clarifier of DHS is called as excess sludge. The sludge production in DHS reactor is calculated by taking the sum of SS volumes in the DHS effluent and the settled excess sludge in the clarifier and relative to the COD or BOD removed by the system. For bench scale experiment, the excess sludge produced by UASB+DHS system was 0.02 kg SS/kg COD removed which is basically 2.5% of the total COD removed or 7% of the total SS load removed [17]. Further, excess sludge from UASB usually varied from 0.03 to 0. 2 kg SS/kg COD removed [8]. Therefore, a total sludge from UASB+DHS system was 0.06 kg SS/kg COD removed. While, excess sludge production from UASB+TF system was 0.38 kgSS/kgCOD removed [63]. The sludge production from UASB+TF system was almost 6 times higher than UASB+DHS system. The DHS sponges are designed with the high void ratio and reticulated structure which cater as a favorable site for the attachment, adsorption and growth of active biomass [25, 61]. Further, the profiling data from same researchers stated that the majority of organic removal especially SS occurred at highest part of the reactor, however after attaining stable state, uniform distribution of sludge was observed along its height. In real scale DHS, for every liter of wastewater treated, about 0.04 kg-COD was wasted as excess sludge which is quite negligible as compared to the other treatment systems. The basic mechanism for the sludge removal in DHS is the physical entrapment of the sludge inside and outside of the sponge which lengthens the solid retention time and provide ample time for self-degradation of sludge minimizing the excess sludge production [61, 66].

#### *3.3.3 Energy consumption*

Nowadays, for developing countries energy efficiency has become the first priorities in the WWTPs hierarchy [67]. Minimizing net energy consumption for WWTPs has become mandatory [68]. Generally, for aerobic treatment processes, the aeration is the highest energy consuming process of the wastewater treatment technology which can account upto 50–60% of all electricity consumption followed by 15–25% of energy by sludge treatment and 15% by secondary sedimentation including recirculation pumps [69].

The energy consumed in UASB+DHS system is through electricity required for pumping [27]. The pumps are used for supplying UASB effluent to the top of DHS system. It is usually estimated on the basis of treatment performance and electricity utilized by pumps. Comparison of energy consumption of UASB+DHS system [27] with UASB+TF [64] is summarized in **Table 3**. From the data, it is evident that the energy consumption of UASB+TF is approximately 5 times higher than that of UASB+DHS system. For UASB+DHS system, 0.05 kWhm�<sup>3</sup> of energy was consumed by main pumping from UASB unit and 0.07 kWhm-3 for the pump of the DHS system, which sums up the total energy consumption for the system of 0.12 kWh per m<sup>3</sup> of wastewater treated. It is noteworthy that the energy consumption for both these systems was solely by pumping. The UASB+DHS system has likelihood of becoming energy sufficient system. The energy sufficiency of UASB+DHS system can be explained by its minimized energy consumption. In addition, when constructing a UASB+DHS system, the energy sufficiency or neutrality can be achieved if UASB is designed in such a way where the outlet is positioned above the DHS distributor or maintained through gravity.

*Downflow Hanging Sponge System: A Self-Sustaining Option for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.94287*

Considering the overall arguments, environmental indicators suggest that the UASB+DHS system is considerably superior in terms of high treatability, less land requirement and reduced carbon footprint. This information could assist the planners and stakeholders in developing nations for good decision making while selecting WWTPs in future.
