Keywords

1 Introduction

Most direct emissions resulting from wastewater treatment plants (WWTPs) based on biological processes are greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), while other indirect emissions are released by on site energy generation from biogas combustion (De Haas and Foley 2009; Campos et al. 2016). The CO2 emitted in relation to energy demand can be directly reduced enhancing the energy efficiency of WWTPs (Libralato et al. 2012). In this way, both the reduction of environmental impacts and the decrease of treatment costs, increasing energy savings, can be accomplished simultaneously. In terms of costs, the main efficient way to reduce GHG emissions is to modify the operational conditions of WWTP units even if this is could not be always possible due to the operational limitations of the installed units (Panepinto et al. 2016).

In this study, we monitored for one year the energy consumption in all treatment units (pre-treatment and pumping stations; primary treatment rainwater and aerated storage; secondary treatment; tertiary treatment; sludge treatment; return liquor treatment; and odour treatment) of Medio Sarno WWTP (Nocera Superiore, Campania, Southern Italy) (300.000 p.e.) managed by Consorzio Nocera Ambiente. Moreover, GHG emissions were evaluated in order to support their minimization.

2 Materials and Methods

To estimate the overall electric energy consumption of Medio Sarno WWTP, the calculated power values (P) were multiplied for the operating time of each device. During the survey of the devices operating in the WTTP, the electro-mechanic equipment was later grouped and classified in homogeneous categories according to ENERWATER methodology.

3 Results and Discussion

Results were summarised in Table 1 and showed that the phase requiring the highest amount of electricity was the biological oxidation (>50%) followed by pre-treatment and pumping stations.

Table 1. Electric energy consumption for each stage according to ENERWATER methodology
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The values of key performance indicators (KPIs) on the base of the estimated energy consumption were reported in Table 2. The comparison of KPIs of Medio Sarno WWTP with other WWTPs outlined a general equivalency in their values. The main deviations involved the indexes related to total nitrogen removal, while the values of the index connected to wastewater volume and COD removal were more similar (Panepinto et al. 2016).

Table 2. Critical evaluation of electric energy demand of Medio Sarno WWTP; EEC = electric energy consumption
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In order to estimate GHG emissions, we referred to the “Methodology Guide for Evaluating Greenhouse gas emissions by water and sanitation services” (2013) prepared by ASTEE and based on IPCC Guidelines for National Greenhouse Gas Inventories (2006) and the GHG Protocol prepared by WBSCD and WRI. WWTP operational data were summarized in Table 3.

Table 3. Operational data for GHG emissions calculations
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GHG emissions (Table 4), as required by the GHG Protocol, were quantified in the following order: Scope (1) direct emissions from the sewage process and discharge into surface water; Scope (2) indirect emissions associated with the consumption of electricity, steam or gas; Scope (3) other indirect emissions related to production and transport of chemicals, transport and treatment of sludge and by-products.

Table 4. GHG estimates (ton CO2-eq/yr), with breakdown according to scope (IPCC, 2007)
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As shown in Fig. 1, energy consumption provided the greatest contribution to carbon footprint (39%) followed by sewage process (31%), biosolids, screening and grits (24%), effluent (5%) and others (1%).

Fig. 1.
figure 1

Carbon Footprint (%) breakdown at Medio Sarno WWTP

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4 Conclusions

This study evidenced that:

  • The phase requiring the highest fraction of the electricity consumption is the biological oxidation (> 50%) followed by pre-treatment and pumping stations;

  • The energy consumption associated to the oxidation tank and pumping stations can be greatly reduced thought optimization;

  • Energy consumption provided the greatest contribution to the carbon footprint (39%) followed by sewage process (31%), biosolids, screening and grits (24%), effluent (5%) and others (1%).

Our next goal will be to set the best operational condition to keep the WWTP efficient as well as to minimize its carbon footprint.