Methodological issues 6311 Choice of method

Nitrous oxide (N2O) emissions can occur as direct emissions from treatment plants or from indirect emissions from wastewater after disposal of effluent into waterways, lakes or the sea. Direct emissions from nitrification and denitrification at wastewater treatment plants may be considered as a minor source and guidance is offered in Box 6.1 to estimate these emissions. Typically, these emissions are much smaller than those from effluent and may only be of interest to countries that predominantly have advanced centralized wastewater treatment plants with nitrification and denitrification steps.

No higher tiers are given, so it is Good practice to estimate N2O from domestic wastewater effluent using the method given here, No decision tree is provided. Direct emissions need to be estimated only for countries that have predominantly advanced centralized wastewater treatment plants with nitrification and denitrification steps.

Accordingly, this section addresses indirect N2O emissions from wastewater treatment effluent that is discharged into aquatic environments. The methodology for emissions from effluent is similar to that of indirect N2O emissions explained in Volume 4, Section 11.2.2, in Chapter 11, N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application. The simplified general equation is as follows:

Where:

N2O emissions = N2O emissions in inventory year, kg N2O/yr N EFFLUENT = nitrogen in the effluent discharged to aquatic environments, kg N/yr EFeffluent = emission factor for N2O emissions from discharged to wastewater, kg N2O-N/kg N The factor 44/28 is the conversion of kg N2O-N into kg N2O.

6.3.1.2 Choice of emission factors

The default IPCC emission factor for N2O emissions from domestic wastewater nitrogen effluent is 0.005 (0.0005 - 0.25) kg N2O-N/kg N. This emission factor is based on limited field data and on specific assumptions regarding the occurrence of nitrification and denitrification in rivers and in estuaries. The first assumption is that all nitrogen is discharged with the effluent. The second assumption is that N2O production in rivers and estuaries is directly related to nitrification and denitrification and, thus, to the nitrogen that is discharged into the river. (See Volume 4, Table 11.3 of Section 11.2.2 in Chapter 11, N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application.)

6.3.1.3 Choice of activity data

The activity data that are needed for estimating N2O emissions are nitrogen content in the wastewater effluent, country population and average annual per capita protein generation (kg/person/yr). Per capita protein generation consists of intake (consumption) which is available from the Food and Agriculture Organization (FAO, 2004), multiplied by factors to account for additional 'non-consumed' protein and for industrial protein discharged into the sewer system. Food (waste) that is not consumed may be washed down the drain (e.g., as result of the use of garbage disposals in some developed countries) and also, bath and laundry water can be expected to contribute to nitrogen loadings. For developed countries using garbage disposals, the default for non-consumed protein discharged to wastewater pathways is 1.4, while for developing countries this fraction is 1.1. Wastewater from industrial or commercial sources that is discharged into the sewer may contain protein (e.g., from grocery stores and butchers). The default for this fraction is 1.25. The total nitrogen in the effluent is estimated as follows:

Equation 6.8 Total nitrogen in the effluent

NEFFLUENT =(P • Protein • FNPR • FNON-CON • FIND-COM ) - NSLUDGE

Where:

Neffluent = total annual amount of nitrogen in the wastewater effluent, kg N/yr

P = human population

Protein = annual per capita protein consumption, kg/person/yr

Fkpr = fraction of nitrogen in protein, default = 0.16, kg N/kg protein

FNON-CON = factor for non-consumed protein added to the wastewater

Find.com = factor for industrial and commercial co-discharged protein into the sewer system

NSLUDGE = nitrogen removed with sludge (default = zero), kg N/yr

Box 6.1

Subcategory - Emissions from advanced centralised wastewater treatment plants

Emissions from advanced centralised wastewater treatment plants are typically much smaller than those from effluent and may only be of interest for countries that have predominantly advanced centralized wastewater treatment plants with controlled nitrification and denitrification steps. The overall emission factor to estimate N2O emissions from such plants is 3.2 g N2O/person/year. This emission factor was determined during field testing at a domestic wastewater treatment plant in the Northern United States (Czepiel et al, 1995). The emission data were obtained at a plant that received only domestic wastewater. This wastewater already included non-consumption protein, but did not include any co-discharged industrial and commercial wastewater. No other country-specific emission factors are available. The emissions from N2O from centralized wastewater treatment processes are calculated as follows:

Equation 6.9 N2O emision from centralized wastewater treatment processes

Where:

N2OpLANTS

TPLANT

find-comm EFpLANT

= total N2O emissions from plants in inventory year, kg N2O/yr = human population

= degree of utilization of modern, centralized WWT plants, % = fraction of industrial and commercial co-discharged protein (default = 1.25, based on data in Metcalf & Eddy (2003) and expert judgment) = emission factor, 3.2 g N2O/person/year

Note: When a country chooses to include N2O emissions from plants, the amount of nitrogen associated with these emissions (NWWT) must be back calculated and subtracted from the Neffluent. The Nwwt can be calculated by multiplying N2Oplants by 28/44, using the molecular weights.

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