Methodology for the quantification of greenhouse gas emissions during land application of sewage sludge

Abstract

Land application of sewage sludge improves soil physical, chemical and biological properties, minimizes the mineral fertilizer application and reduces maintenance of soil. In this work, various methods to estimate greenhouse gas (GHG) emissions due to land application of sewage sludge described in the literature were summarized and their limitations were presented. Moreover, an improved methodology to evaluate GHG emissions due to the land application of sewage sludge was proposed. Further, based on the proposed methodology, GHG emissions due to land application of sewage sludge at lower, higher and average concentrations of nutrients (carbon, nitrogen and phosphorus) were assessed. The results revealed that GHG emissions substantially decreased when higher nutrients containing sewage sludge were applied to land. For methodologies presented in the literature, GHG emissions or reductions due to land application did not change with nutrients concentrations of sewage sludge. However, based on the proposed methodology GHG emissions or reductions varied according to sludge nutrients concentrations.

Keywords:

• sewage sludge
• greenhouse gas emissions
• land application
• anaerobic digestion
• digestate
• nutrient concentration

1. Introduction

The reduction in greenhouse gas (GHG) emissions is the prime focus and goal of several researchers, scientists, industrialists and municipalities. The radiatively active trace gases in the Earth's atmosphere are called GHGs, which principally include water vapour, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃) (Gohar & Shine, 2007). CO₂ is used as the basis to calculate the level of relative contribution to global warming potential of each gas. Per molecule, the global warming potential of CH₄ and N₂O for 100 years is about 28 and 265 times, respectively, more powerful than CO₂ (Stocker et al., 2013). According to the Intergovernmental Panel on Climate Change (IPCC), population growth, deforestation, factory farming, biomass degradation (from the wastewater treatment plants (WWTPs) and during sludge management) and the widespread use of fossil fuels create excess GHG emissions. These GHGs while in the atmosphere absorb radiation emitted to ground and thus contribute to the global warming and climate change. Alarming levels of global warming and climate change arising from unprecedented release of GHGs have made it essential to quantify GHG emissions from every source.

Sewage sludge is an unavoidable by-product collected from different stages of the wastewater treatment process. In general, specific sludge production varies widely from 35 to 85 g dry solids per population equivalent per day during the wastewater treatment (Ginestet & Camacho, 2007). From the existing WWTPs the annual sludge production in USA and Canada is 6 515 × 10³ and 550 × 10³ Mg dry solids per year, respectively (LeBlanc, Matthews, & Richard, 2009). In Europe, during the last decade sewage sludge production has increased significantly (Fytili & Zabaniotou, 2008; Heras, Mañas, & Labrador, 2005). In China from 2005 to 2010 sludge production steadily rose from 11 × 10⁶ to 21  × 10⁶ Mg per year (sludge with 80.0% water content) (Liu, Wei, Zhang, & Bi, 2013). In Taiwan, the prevalence of the public sewage system could reach to 36.0% of the population by 2014, and sewage sludge production could reach up to 1040 Mg/day (Wang, Shih, Chiueh, & Huang, 2013). Sewage sludge production has increased by 45.0% in France (i.e. 58 × 10⁴ Mg dry solids per year in 2001 to 13 × 10⁵ Mg dry solids per year in 2005) (Pradel & Reverdy, 2012). Moreover, sewage sludge production is expected to increase in the future due to industrialization, population increase, greater volume of water used, higher levels of wastewater treatment and increased stringency of laws for wastewater treatment and disposal.

In general, the common sludge treatment methods are aerobic digestion, anaerobic digestion followed by dewatering and composting, and the common disposal methods are incineration, landfill or land application (Figure 1). Sludge disposed by different methods in different countries is given in Table 1. Sludge treatment and disposal contribute to GHG emissions (Pradel & Reverdy, 2012). During sludge treatment, disposal and/or reuse (as fertilizer, biotransformation), sewage sludge is metabolized to soluble or gaseous end-products mainly consisting of CO₂, CH₄ and N₂O (depending upon the environmental conditions). These three gases are considered as the principal GHGs emitted during sludge treatment and disposal routes. Sludge management (treatment, disposal and/or reuse) may account for approximately 40.0% of the total GHG emissions from a WWTP (Brown, Beecher, & Carpenter, 2010). GHG emissions from sludge treatment in wetlands are 17×10⁻³ Mg CO₂ equivalent/population equivalent and for the untreated sludge are 0.162 Mg CO₂ equivalent/population equivalent (Uggetti, Garcia, Lind, Martikainen, & Ferrer, 2012). Landfilling of sewage sludge will contribute GHG emissions of 0.297 Mg CO₂ equivalent/Mg of sludge in Taiwan (Wang et al., 2013). Among the various disposal methods land application of sludge is the most common method in practice around the world (Table 1).

Figure 1. Sludge management with the most common treatment process ‘E’ is the energy required for the process (generated from fossil fuels).

Table 1. Percentage of wastewater sludge by various disposal methods.

Region	Year	Land application (%)	Incineration (%)	Land-filling(%)	Compost ^a(%)	Others (%)	Ref.^a
USA	–	52.0	22.0	17.0	9.0	–	[1]
Canada		43.0	47.0	4.0		6.0	[2]
Western Canada	–	66.0	–	4.0	27.0	–	[1]
Japan	–	–	80.0	–	–	–	[1]
China	2010	48.5	3.5	34.5	–	13.5	[3]
Australia	2011	55.0	15	30.0	15.0	–	[4]
France	2008	60.0	–	–	–	–	[5]
UK	2005	69.0	14.0	5.0	1	9.0	[6]
Denmark	2007	59.0	16.0	6.0	–	19.0	[5]
Sweden	2009	24.0	1.0	24.0	33.0	18.0	[5]
Germany	2008	28.0	51.0	–	17.5	3.5	[5]
Italy	2005	23.0	3.0	42.0	21.0	11.0	[5]
Belgium	2008	14.0	53.5	–	–	32.5	[5]
Netherlands	2006	–	67.0	4.5	13.0	15.5	[5]
Greece	2008	1.0	46.0	38.0	–	15.0	[5]

^a[1] = LeBlanc et al. (2008); [2] = Roy, Dutta, Corscadden, Havard, and Dickie (2011); [3] = Chen, Yan, Ye, Meng, and Zhu (2012); [4] = http://www.biosolids.com.au/benefits-land-application.php; [5] = Kelessidis and Stasinakis (2012) and [6] = Maisonnave, Montrejaud-Vignoles, Bonni, and Ensct, (2002); Compost is mainly applied to agriculture land or used for horticulture. Other applications: used in cement industry, bricks, etc.

1.1. Land application of sewage sludge

Sewage sludge contains a substantial amount of organic matter rich in nutrients such as carbon (C), nitrogen (N) and phosphorous (P), and also provides macronutrients such as potassium and sulphur, and micronutrients such as copper and zinc (Bettiol & Ghini, 2011; Singh & Agrawal, 2008; Wang, Shammas, & Hung, 2008a). The N and P contents in sewage sludge reported by various researchers are summarized in Table 2. The concentrations of N and P present in sewage sludge vary depending upon the original effluent composition and the treatment method (Quilbe et al., 2005). An extensive study on sewage sludge composition (nutrients composition) was conducted by Basta (1995). It was reported that sewage sludge consists mainly of partially decomposed organic matter (30.0–60.0%) and other significant amount of nutrients required by plants that include N (0.5–10.0%), P (1.0–6.0%), sulphur (0.5–1.5%), calcium (1.0–20.0%), magnesium (0.3–2.0%) and micronutrients such as iron (0.1–5.0%), copper, manganese, and zinc (<0.2%), nickel, boron, cobalt and molybdenum (<0.05%) (Basta, 1995). The cost-effectiveness of the land application is evaluated based on the fertilizing value (nutrient content) of sludge. Therefore, land application of sewage sludge is considered to be the most economical (Champagne, 2007; Laboy-Nieves, Schaffner, Abdelhadi, & Goosen, 2008; Lu, He, & Stoffella, 2012; Singh & Agrawal, 2008).

Table 2. Nitrogen and phosphorus contents of sewage sludge.

Type of sludge	Nitrogen content	Phosphorus content	Reference
Sewage sludge	NA	1.2–9.5%	Furrer, Gupta, and Stauffer (1984)
Secondary sludge	3.8%	2.2%	Smith (1996)
Fermented sludge	3.9% (average)	2.5% (average)	Sommers (1977)
Anaerobic sludge	32–96 g N/kg dry mass	24–47 g P2O5/kg dry matter	Johannesson (1999)
Anaerobic sludge	15 g N/kg dry matter	78 g P2O5/kg dry matter	Quilbé et al.(2005)
Thermally stabilized sewage sludge	19 g N/kg dry matter	22 g P2O5/kg dry matter
Aerobic sludge	48 g N/kg dry matter	25 g P/kg dry matter	Sigua, Adjei, and Rechcigl (2005)
anaerobic sludge	40 g N/kg dry matter (pH 11; calcium hydroxide treated)	22 g P/kg dry matter
Cake biosolids	39 g N/kg dry matter	33 g P/kg dry matter
Secondary sludge	21–50 g N/kg dry matter	16–31 g P/kg dry matter	Fernandes, Bettiol, and Cerri (2005a)
Sewage sludge	2.3%	1.1%	Wang et al. (2008b)
Sewage sludge	7 mg TN/kg dry matter	17 g TP/kg dry matter	Wang, Chen, Ge, and Jia (2008c)
Sewage sludge	2.8–5.1%	0.8–3.0%	Chan (2009)
Sewage sludge	30 g TN/kg dry matter	10 g P/kg dry matter	Franco et al. (2010)
Sewage sludge	2.3%	0.4%	Keskin, Bozkurt, and Akdeniz (2010)
Sewage sludge	47 g N/kg dry matter	<1 g P2O5/kg dry matter	Lakhdar et al. (2010)
Sewage sludge	1.4%	0.8%	Kabir, Kamal, Jahan, Faizullah, and Ullah (2012)

Note: NA, not applicable.

The organic carbon content has been reported in the range of 17.5–25.3% w/w (average 21.4% w/w) of the dry mass for the aerobically digested sludge and in the range of 6.8– 28.5% w/w (average of 17.6%) of dry mass for anaerobically digested sludge (Parker & Sommers, 1983). Depending on the sludge solids content, the organic form of the N available is around 50.0–90.0% of the total N of sludge (average total nitrogen is 3.9% on a dry basis) (Sommers, 1977). The N components of sludge (identified as proteinaceous, amino acids and hexosamines) are predominantly organic in nature (Epstein, 2002). The P content of sludge has often been closer to half of the N content (Johannesson, 1999). Moreover, the crop's requirement of P is just 1/10–1/5 of the N requirement (Johannesson, 1999).

The land application of sewage sludge refers to spreading sludge on or just below the soil surface with a tilling depth of 15–30 cm (Delgado Arroyo, Miralles de Imperial Hornedo, & Martín Sánchez, 2011; Epstein, 2002; Lu et al., 2012; Wang et al., 2008b). Moreover, the land application of sewage sludge has been practised in many countries for centuries (Jiménez et al., 2010; Singh & Agrawal, 2008). In general, research on land application of sewage sludge has focused on its effects on plant development (Singh & Agrawal, 2008; Wang et al., 2008b) and also on soil contamination due to heavy metals (Bettiol & Ghini, 2011; Haynes, Murtaza, & Naidu, 2009; Wang et al., 2008b). However, very few studies pertaining to GHG emissions due to land application of sewage sludge are available.

2. Advantages and disadvantages of land application of sewage sludge

2.1. Advantages

The land application of sewage sludge reduces the need for chemical fertilizers and pesticides, increases the crop production and improves soil structure (i.e. enhancement of workability and lower resistance to ploughing and tilling) (Aggelides & Londra, 2000; Haynes et al., 2009; Singh & Agrawal, 2008; Torri & Lavado, 2008; Wang et al., 2008b). Therefore, fossil fuel (energy) usage in the production of chemical fertilizers and pesticides will be reduced along with corresponding GHG emissions. The fossil fuel (energy) required by the tractors for ploughing, tilling and other activities during irrigation will also be reduced as well as the corresponding GHG emissions. The physical properties of soil such as soil aggregate stability, water holding capacity, porosity and humus content are increased with sewage sludge amendment (Singh & Agrawal, 2008). On the other hand, land application of sewage sludge decreases bulk density and erosion of the soil. With sewage sludge amendments the soil organic carbon, electrical conductance, cation exchange capacity and N and P are increased (Singh & Agrawal, 2008). The land application of sewage sludge has the potential to increase soil carbon stores and gain credits for carbon sequestration (Singh & Agrawal, 2008; Sylvis, 2009). The use of compost on soils results in 8.0% of the compost carbon (this equates to 0.13 Mg CO₂eq/dry Mg wastewater solids) being sequestered for 100 years (Recycled Organics Unit, 2006). A larger mass of carbon sequestration (0.25 Mg CO₂eq/dry Mg wastewater solids) was estimated at biosolids land reclamation sites in British Columbia (Sylvis, 2009). Sewage sludge also modifies the soil's biological properties (Singh &Agrawal, 2008; Torri & Lavado, 2008; Wang et al., 2008b). Biological properties of soil basically govern the dynamics of the microbiotia, which control the soil ecosystem and its functioning, that is, humus formation, organic matter decomposition, nutrient cycling, physical and other functions.

2.2. Disadvantages

The land application of sewage sludge requires relatively less capital, which is generally lower than other sludge management technologies, however, the process can be labour intensive (Shammas & Wang, 2009). The land application of sewage sludge is limited during rainy and snowing weather conditions. Public opposition and odour problems also limit the land application of sewage sludge. The presence of contaminants (metals, etc.) in sludge may also limit the land application. Moreover, the land application of sewage sludge should not affect the groundwater, surface watercourses, wells and other environmentally sensitive features. Contrary to the positive effect of land application of sewage sludge on the microbial activity, there are reports that find a reduction in microbial activity (Baggs, Watson, & Rees, 2000; Fernandes et al., 2005a; Fernandes, Bettiol, Cerri, & Camargo, 2005b; Lu et al., 2012). The land application of sewage sludge and its effect on the microbial activity were investigated by researchers by measuring the flux of gases (CO₂, CH₄ and N₂O) at the soil–atmosphere interface (Baggs et al., 2000; De Urzedo et al., 2013; Fernandes et al., 2005a, 2005b; Lu et al., 2012; Ruíz-Valdiviezo et al., 2013). The land application of sewage sludge will provide organic matter and nutrients to the soil, but on the other side the biological processes undergo altercations, and as a consequence, the flux of the gases at the soil–atmosphere interface is also changed.

Sludge land applications increase GHG emissions with increased flux of gases at the soil–atmosphere interface (Baggs et al., 2000; Fernandes et al., 2005a, 2005b; Lu et al., 2012). Emerging contaminants (endocrine-disrupting compounds), antibiotics and heavy metals present in sewage sludge will affect the soil microbial flora, and as a consequence will affect the flux of gases at the soil–atmosphere interface.

3. Methods in literature to quantify GHG emissions due to land application of sewage sludge

The estimation methods used by government bodies and researchers to evaluate GHG emissions during the land application of sewage sludge are summarized in Table 3. The methane or nitrous oxide emissions will vary vastly depending upon the types of sludge, prevailing moisture content, the ambient environmental conditions and also on the diurnal temperature variations. These factors are not considered in the IPCC estimation methodology. CO₂ emissions due to land application are considered as biogenic emissions (IPCC, 2007), (since they are generated from biological pathways) and they were not taken into account, due to the fact that they are considered (by convention) ‘carbon neutral’ (global warming potential equal to zero). Indirect CO₂ emissions due to fossil fuel utilization during land application are considered. RTI International has estimated the CO₂ emissions from the land application of sewage sludge by assuming a constant biomass population (dying and decaying biomass equalling new biomass growth). The CO₂ generation rate for the land application will be directly proportional to the carbon application rate to the land treatment unit. This methodology gives an overestimation of CO₂ emission, as it presumes the complete conversion of carbon content of sewage sludge to CO₂.

Table 3. Description of methods used to estimate GHG emissions due to sludge land application.

Method	Gases included	Parameters used	Limitations of the methods
IPCC	CO2 emissions from vehicles used for transport and during land application of sewage sludge		This method does not account for any benefits of land application of sewage sludge (IPCC, fourth Assessment Report by Bonger et al., 2009). Increase in the flux of gases (CO2, CH4 and N2O; Fernandes et al., 2005b) following the land application are not considered. Emissions reduction resulting from offsetting fertilizer use is not considered
	CH4 and N2O emissions following sludge land application	10.0% of CH4 potential or 5 kg CH4/Mg raw dry solids
	Indirect N2O emissions	1.0% of total nitrogen applied
		Indirect N2O emissions = application rate (kg N applied) × FracGASM × EF4
		Where FracGASM is the fraction of the added N that will volatilize (default value is 0.20) and EF4 is a conversion factor (default value is 0.01) for the fraction of N that volatilizes what will convert to N2O
Research triangular institute (RTI)	CO2 emissions following land application of sewage sludge. The emissions will be directly proportional to the carbon application rate to the land	GHGs emissions = Mg CO2/year, The MW = annual mass of waste applied to the land treatment unit (Mg/yr, wet basis), TSW = total solids content of waste material applied to the land (kg dry solids/kg wet solids), CCw = carbon content of sewage sludge applied to the land (kg C/kg dry solids), 44 = molecular weight of CO2 (kg/kg-mol), 12 = molecular weight of carbon (kg/kg-mol)	This process gives an overestimation of CO2 emissions, as it presumes the complete conversion of carbon content of sewage sludge to CO2. Following the land application, carbon will be converted into CO2 and CH4 due to the presence of anaerobic pockets within the land. Increased in the flux of gases (CO2, CH4 and N2O; Fernandes et al., 2005b) following the land application of sewage sludge are not considered. Emissions reduction resulting from offsetting fertilizer use was not considered
BEAM (Sylvis, 2009)	CO2 emissions from vehicles used for transport and during land application of sewage sludge		CO2 emissions from land due to biosolids application are considered as biogenic sources and they are not included in GHG estimation. CH4 emissions are considered negligible (Ball, Ball, McTaggart, & Scott, 2004; Jones et al., 2006). Increased in the flux of gases (CO2, CH4 and N2O; Fernandes et al., 2005b) following the land application are not considered
	CO2 emissions reductions from carbon sequestration in soil (credit) are considered	CO2eq sequestered (–Mg/day) = dry sludge mass (Mg/day) * estimated CO2 equivalent sequestered (Mg CO2eq/dry Mg sludge; default value = 0.25).
	CO2 emissions reduction resulting from the offsetting fertilizer use.	Replaces commercial nitrogen (N) fertilizer: CO2 equivalent reduction (–Mg/day) = dry biosolids mass (Mg/day) *% total N (default = 4.0%) * N fertilizer credit (–Mg CO2eq/Mg N; default value = 4.00) (Recycled Organics Unit, 2006)
		Replaces commercial phosphorus (P) fertilizer: CO2 equivalent reduction (–Mg/day) = dry sludge mass (Mg/day) *% total P (default = 1.5%) * P fertilizer credit (–Mg CO2eq/Mg P; default = 2.00) (Recycled Organics Unit, 2006)
	N2O emissions following land application of sludge.	N2O emissions (Mg/day) = biosolids applied (dry Mg/day) *% N in biosolids *% of biosolids applied to fine textured soils *% of N that goes to N2O
		% of N that goes to N2O
		For fine textured soils = 2.3%
		For coarse textured soils = 0.05%
		For fine and coarse-textured soils (factor for N conversion to N2O)
		C/N > 30, N conversion to N2O factor is 0
		C/N < 30, N conversion to N2O factor is 1.57
		C/N in the biosolids
Lundin et al. (2004)	Positive benefits of nitrogen and phosphorous (18 kg of phosphrous/Mg of dry solids) are considered. N2O and NO3 are likely to be released when the organically bound nitrogen in sludge is mineralized	GHG emission factors (kg/Mg of dry solids)  CO2  CH4 N2O  25.00 −0.01 −0.19	CH4 and CO2 emissions due to sludge land application are not described. An increase in the flux of gases (CO2, CH4 and N2O; Fernandes et al., 2005b) following the land application are not considered. The negative value was not defined by the authors
Hospido, Moreira, Martín, Rigola, and Feijoo (2005)	Benefits of nitrogen 17.87 kg N/Mg of dry solids and phosphrous 14.32 kg P/Mg of dry solids) due to sludge application was considered	CO2  CH4  N2O  NA  3.18  NA	There was no explanation for the values utilized for evaluating the GHG emissions. The authors did not consider (avoided) CO2 and N2O emissions from the land application of sludge
Hong, Hong, Otaki, and Jolliet (2009)	Positive benefits of nitrogen are considered. The other factors are taken from inventory data without proper explanation	CO2  CH4  N2O 17.20 −21.30 −0.19	The authors have not considered carbon sequestration due to land application of sludge. Moreover, authors have not included increased flux of gases at the soil–atmosphere interface due to the land application of sludge. Compare to Lundin et al. (2004) and Hospido et al. (2005) the methane emission factor was different. The representation of negative sign was not mentioned
Niu, Huang, Dai, and Zhao (2013)	Positive benefits of the nitrogen content present in sludge was considered. Further explanation of the factors was detailed	CO2  CH4  N2O 17.2  3.18  NA	Authors have considered CO2 emissions values from Hospido et al. (2005) and CH4 factor from Hong et al. (2009) but they have not included N2O emissions. There was no explanation behind the assumption considered in computing GHG emissions
This study	• CO2, CH4 & N2O emissions and reductions are considered	Details of these parameters used are explained in Section 4. The parameter (or factors) considered in evaluating GHG emissions due to land application of sludge are as follows	Since there are gaps in the knowledge associated with land application of sewage sludge. Few of the parameters are considered from sewage sludge compost. Moreover, we have not included the GHG emissions during sludge generation, dewatering and during the transportation of dewatered sludge to the land application site
		(i) Increased GHG emissions due to increased flux of gases at the soil–atmosphere interface
		(ii) Replacement of the synthetic fertilizers
		(iii) Carbon sequestration in the soil
		(iv) Additional water holding capacity of the soil
		(v) Improved tillage and workability of the soil
		(vi) Photosynthetic absorption of carbon by crops

Note: NA, not applicable.

The biosolids emissions assessment model (BEAM) was developed by SYLVIS environmental for the Canadian Council of Ministers of the Environment to allow municipalities to estimate GHG emissions from biosolids management. Anthropogenic CO₂ emissions are considered for the land application of biosolids, that is, fuel burned for hauling and transporting biosolids to farm fields and during the land application process (e.g. using tractors and spreaders). The fuel used during the land application of biosolids as 3.2 l/Mg dry biosolids applied. CO₂ emissions from land due to biosolids are considered as biogenic sources and they are not included in GHG estimation. In BEAM, CH₄ emissions from land application of biosolids are considered negligible based on the studies by Ball, Ball, McTaggart, and Scott (2004) and Jones, Rees, Kosmas, Ball, and Skiba (2006). Methane emissions from storage of biosolids before land application are considered in this methodology. The N₂O emissions from the land application of the biosolids are evaluated as given in Table 3. Reducing the soil maintenance and increasing the water holding capacity will minimize machinery usage and fossil fuels and corresponding GHG emissions. These factors are not considered in estimating CO₂ emissions. Increased microbial activity in air pockets formed in the soil will increase the CH₄ emissions. N to N₂O conversion factors are used in estimating N₂O, which will not present the exact emissions due to the land application of sewage sludge. There will be increased N₂O flux between atmosphere and soil interface due to the land application of sewage sludge, which need to be considered in estimating N₂O emissions.

Several other researchers have evaluated GHG emissions during the life cycle analysis of sewage sludge management (considering land application or agriculture application as sludge disposal option) (Table 3). Lundin, Olofsson, Pettersson, and Zetterlund (2004) have considered P and N presented in sludge replaces mineral fertilizer. Energy savings due to the replacement of mineral N (by sewage sludge) was less than the energy required for the pasteurization, transportation and spreading of sewage sludge. These resulted in higher CO₂ and N₂O emissions due to the land application of sewage sludge. However, it was assumed that CH₄ emissions due to the land application were −0.012 kg/Mg of dry solids without explaining the details. Fernandes et al. (2005a) concluded that there will be increased flux of CO₂ and N₂O gases from the land applied of sewage sludge. On the other hand, some of the N and P present in sewage sludge will be utilized by plants as a source for their development. It is unclear if the land application of sewage sludge corresponds to either GHG emissions or GHG reductions. Moreover, the sewage sludge application will change the physical properties of the soil, which will affect GHG emissions. Therefore, in this study a methodology was developed (by considering possible factors responsible for GHG reductions and GHG emissions) to evaluate precise net GHG emissions due to the land application of sewage sludge.

4. Proposed methodology for the quantification of GHG emissions due to land application of sewage sludge

There are many limitations in GHG estimation methods (Table 3). In this proposed methodology, net GHG emissions (GHG reductions-GHG emissions) due to the land application are considered. The land application of sewage sludge reduces GHG emissions due to the following reasons: (i) replacing the synthetic fertilizers, (ii) carbon sequestration in soil, (iii) additional water holding capacity of soil, (iv) improved tillage and workability of soils and (v) photosynthetic absorption of CO₂ by crops. On the other hand, the land application of sewage sludge increases the flux of gases at the soil–atmosphere interface. Further, the energy required during the land application of sewage sludge will contribute to GHG emissions. Therefore, the net GHG emissions due to the land application of sewage sludge are evaluated using Equation (1).(1)

4.1. Reduction in GHG emissions due to replacement of synthetic fertilizers

The type of sludge and the N and P contents on the dry basis were presented in Table 2. The US Department of Agriculture and University of Minnesota have conducted extensive research on the use of sewage sludge from crop production and revealed that land-applied sewage sludge produces high crop yields and high-quality crops similar to commercial fertilizer. Fossil fuels are mainly used during chemical fertilizer production, which is accountable for GHG emissions. During fertilizer application to soil, various fossil fuel-consuming machinery is also used, further contributing to GHG emissions. The land application of sewage sludge will minimize the use of chemical fertilizers and machinery, thereby offseting fossil fuels and thus reducing GHG emissions. The production of 1 kg N synthetic fertilizers produce a total of 5645 g of CO₂ equivalents (Flessa et al., 2002) and 1 kg of P synthetic fertilizer generates 165 g CO₂ equivalents (Wood & Cowie, 2004). The N present in organic fertilizers normally exists in two forms: (i) ammonium-N, which is readily available for plant uptake and (ii) organic-N compounds, which are less available to plants. Therefore, the amount of ammonium-N relative to the total N content will determine the N availability in the organic fertilizer. The N availability also depends upon the moisture content of sludge. In general, the N-nutrient value of organic fertilizers varies in the range of 20.0–50.0% of the available total N content (http://www.northwaymushrooms.com/organic-fertilizers-a-valuable-commodity/). Therefore, N available for the plants was considered as a function of the N mineralization rate, which was 30.0% (Fernandes et al., 2005a). In the case of P, organic fertilizers can generally be considered as 100.0% available (i.e. each kg of P applied in organic fertilizer can replace 1 kg P from chemical fertilizer) (http://www.northwaymushrooms.com/organic-fertilizers-a-valuable-commodity/). The GHG reductions due to synthetic fertilizer replacement by sewage sludge are evaluated as per Equation (2).

GHG reductions due to N replacement + reductions due to P replacement(2)

where ‘’ Mg/yr of dry sewage sludge applied to the land, %N and %P represent the amount contained in sewage sludge on dry weight basis.

4.2. GHGs reduction due to carbon sequestration in soil

The average organic carbon content of aerobically digested sludge is 21.4% w/w and that for anaerobically digested is 17.6% w/w (Parker & Sommers, 1983). When sludge is applied to the land, the organic matter immediately begins to decompose by the soil microbes as well as by those in sludge, provided that the temperature is above freezing and there is sufficient moisture. Determination of carbon sequestration from sewage sludge to soil is difficult, because of the many factors that affect the longevity of the sequestration, that is, tilling, precipitation, air, runoff, soil temperature, pH, etc. Most of the carbon of sewage sludge applied to the soil will eventually be converted to CO₂ and CH₄ depending upon the aerobic and anaerobic pockets, but some carbon remains in the organic matter. To the best of our knowledge, there are no reports available to date on this value (i.e. carbon remained in the organic form in the soil). When sewage sludge compost is applied to soils, it is assumed that 8.2% carbon of the compost remains in the soil for 100 years or more (NEBRA, 2008). Even though sewage sludge and sewage sludge compost are entirely two different things, however, the application of sewage sludge in the soil will eventually undergo aerobic, facultative and anaerobic degradation process by the soil and sludge microbes, similar to a composting operation. Therefore, in the absence of the real data of carbon sequestration from sewage sludge, the same percentage (as that of compost) of carbon can be assumed as carbon sequestration credit for direct sewage sludge application to soil. This GHG reductions can be calculated as per Equation (3).

GHG reductions due to carbon sequestration(3)

where, ‘’ Mg/yr of dry sewage sludge applied on land and organic carbon content in sewage sludge (g/g).

4.3. GHG reductions due to additional water holding capacity of the soils

Land application of sewage sludge improves the soil structure. The organic matter present in sewage sludge will improve water holding capacity of the soil. Depending upon the region, soil type, soil texture and compost application rate, the compost can save more than 30.0% of irrigation water and thereby reduces GHG emissions related to the electrical energy requirement for supplying that extra water through irrigation systems (Compost Australia). The application of compost can save between 0.13 and 0.95 mL of water per hectare per year, depending on crop and soil types (Compost Australia, http://www.wmaa.asn.au/lib/pdf/05_d/ca/dca_1212_capabilitystatement.pdf).

A mixture of sewage sludge and fly ash applied to the soil increased the water holding capacity from 53.0% to 58.5% (Veeresh et al., 2003). In the absence of practical study of the extra water holding capacity of the soil due to sewage sludge application to agricultural land, the same degree of water holding capacity as that of the compost can be assumed for the computation. Irrigation water requirements will significantly reduce due to the compost application. When sewage sludge compost with a thickness of 53.40 mm is applied over the soil, it will save 0.251 m³ of water/m³ of sludge compost (NEBRA, 2008). Moreover, it is reported that 9.25 kWh electricity is required per cubic metre of water for agricultural use (E2, 2007). If the water content of sludge before land application is X%, sludge bulk density is Y kg/m³, then GHGs reduction due to irrigation water saving due to application of Mg/yr of dry sewage sludge can be calculated as per Equation (4).

GHG reductions due to water holding capacity of soil.(4)

where ‘κ’ is the national emission intensity coefficient 0.53 × 10⁻³ Mg CO₂/kWh (developed by an Environment Canada study based on a Resources for the Future (RFF) model).

4.4. GHGs reductions due to improved tillage and workability of soils

Due to the availability of nutrients, sewage sludge will restore the health of the soil by building organic matter and tilth, and it will also improve the soil's physical and chemical properties (Torri & Lavado, 2008). The change in physical and chemical properties will reduce the management and maintenance required due to the fact that the machinery will pass through the soil (after sewage sludge application) more easily. This will result in the reduction in the fossil fuel consumption from machinery and therefore will result in lower GHG emissions. There are no available quantitative data on this aspect, but it is assumed that the improvement of tillage and workability will reduce the tractor fuel consumption by 5.0%. This assumption of 5.0% reduction in fuel consumption is based upon the difference in fuel consumption between hard soil and soft soil (Moitzi, Weingartmann, & Boxberger, 2006). Due to the sewage sludge application the hard soil will behave like a soft soil. The average fuel consumption of hard soil is 12.80 L/ha (Moitzi et al., 2006) and emission factor for fuel is 2.73 kg CO₂/L of diesel (Gassara et al., 2011). Therefore, GHG reductions when sludge is applied with an average thickness of 53.40 mm can be calculated as per Equation (5).

GHG reductions due to improved tillage and workability(5)

where ‘’ Mg/yr of dry sewage sludge applied to land, water content of sludge before land application is X% and Y is sewage sludge bulk density in kg/m³.

4.5. GHGs reductions due to photosynthetic absorption of CO₂ by crops

It was concluded that the land application of sewage sludge produce high crop yields and high-quality crops similar to commercial fertilizers(based on three to seven years of field study by the US Department of Agriculture and University of Minnesota). Clapp, Stark, Clay, and Larson (1986) reported increased yield for several crops with sewage sludge land application as fertilizer compared to commercial NPK (nitrogen, phosphorus and potassium) fertilizers, (38.0% increase for corn fodder, 51.0% for grain and 14.0% of reed canary grass). Similarly, Togun, Akanbi, and Dris (2003) reported an increase of 28.0% yield of tomato due to sewage sludge land application (tomato yield increased from a typical average of 14.40 Mg/ha yield to 18.50 Mg/ha). The effect of sewage sludge amendment on growth, yield and heavy metal accumulation in plants is reported by various researchers and is summarized by Singh and Agrawal (2008). An increase in crop yield signifies higher carbon sequestration (or GHG minimization) due to photosynthetic CO₂ absorption by plants and crops. The land application of sewage sludge was studied by Kharub (2012) and reported a positive effect on the emergence and growth of Abelmoschus esculentus. Thus, an increase in the yield of plants growth and crops due to sewage sludge application will reduce GHG emissions compared to the control (without sewage sludge application). The extra carbon sequestration will be due to additional growth of plants (leaves, stem, branches, etc.) and for additional yield of crops (fruits, grains, etc.). If sewage sludge is applied on dry basis ( in Mg/yr) with an average thickness of 53.40 mm, GHGs reduction due to additional photosynthetic absorption of CO₂ can be calculated as given by Equation (6).

GHGs reduction due to additional photosynthetic absorption of CO₂ by surplus growth of plants (leaves, stem, branches, etc.) and crops (i.e. fruits, grains, etc.)(6)

where ‘’ Mg/yr of dry sewage sludge applied on land, ‘¢₁’ is the carbon content in the plant (kg/kg weight of plants without crops) and ‘¢₂’ is the carbon content in the crops (kg/kg weight of crops), ‘a₁’ is a yield of plant (kg/ha) and ‘a₂’ is the yield of crops (kg/ha) after the land application of sewage, ‘b₁’ is yield of plants without crops (kg/ha) and ‘b₂’ is the yield of the crops (kg/ha) before (or without) the land application of sewage sludge.

4.6. Increased GHG emissions due to land application of sewage sludge

The function of soil ecosystems is governed by the dynamics of its microbiota. The land application of sewage sludge will change the dynamics of soil microbiota. The microbiota have a very essential role in organic matter degradation. Reduction or increase in microbiota due to sewage sludge land application will affect the nutrient cycling (Giller, Witter, & McGrath, 1998). Few studies are available on the flux of gases in sewage sludge applied to tropical soils (Alvarez, Alconada, & Lavado, 1999; Blechschmidt, Schaaf, & Hüttl, 1999; Flessa & Beese, 2000). Soil respiration, metabolic quotient and soil enzyme activity are the important parameters or indicators to evaluate the effect of sewage sludge application on soil microbial activity (Bettiol & Ghini, 2011). The application of sewage sludge has been reported to cause an increased flux of CO₂, N₂O and CH₄ to the atmosphere (Bettiol & Ghini, 2011; Chiaradia et al., 2009; Fernandes et al., 2005a, 2005b). The flux of CO₂, N₂O and CH₄ with sludge land application are increased by 224.0%, 316.0% and 162.0%, respectively, compared with the control soil without any fertilizers (Fernandes et al., 2005a, 2005b). Similarly, when soils are amended with chemical fertilizers the flux of CO₂, N₂O and CH₄ to the atmosphere is increased by 85.0%, 45.0% and 106.0%, respectively, compared with the control soil without any fertilizers. Fernandes et al. (2005a) observed the flux of CO₂, N₂O and CH₄ to the atmosphere with sewage sludge application as 115.10 mg C.m⁻² h⁻¹, 8.60 mg N.m⁻² h⁻¹ and 0.05 μg C.m⁻² h⁻¹, respectively. Therefore, GHG emissions into the atmosphere due to land application of sewage sludge with an average thickness of 53.40 mm can be calculated as per Equation (7). Further, it was also considered the energy required during sewage sludge land application as 351.68 kWh/Mg of dry sludge solids (Wang et al., 2008a).

GHG emissions due to microbial mineralization(7)

where A is the net increase in the CO₂ flux (mg C.m⁻² h⁻¹) with respect to control (i.e. without sewage sludge or without fertilizer application), B is the net increase in the N₂O flux (mg N.m⁻² h⁻¹) with respect to the control (i.e. without sewage sludge or fertilizer application), C is the net increase in the methane flux (μg C.m⁻² h⁻¹) with respect to the control (i.e. without sewage sludge or fertilizer application) due to the application of fertilizers or sewage sludge; ‘’ is Mg of dry sewage sludge applied to land.

Therefore, the net GHG emissions due to the land application of sewage sludge are evaluated as per Equation (8).(8)

The negative value of Equation (8) will indicate a reduction in GHGs, whereas a positive value will indicate GHG emissions due to the land application of sewage sludge.

5. Hypothetical case study for estimating GHG emissions from land application of sewage sludge

In order to evaluate whether the land application of sewage sludge would result in either GHG emissions or reductions, a hypothetical case study has been presented based on the literature values. In this case, it was considered that the WWTP had a treatment capacity of 20 million gallons of wastewater per day (75708 m³/day). The sludge production rate is considered as 0.225×10⁻³ Mg of dry solids/m³ of treated wastewater (Ghazy, Dockhorn, & Dichtl, 2011). Considering 45.0% (w/v) total volatile solids degraded during anaerobic digestion (Metcalf, Eddy, & Tchobanoglous, 2004), the weight of the digestate required for land application was calculated. Chemical composition of anaerobic digested sludge (ranging from lower to higher concentration) was adopted from Sommers (1977). Further, three possibilities were considered to evaluate the GHGs emissions for land application of the digested solids. Case 1: with lower percentage of carbon, N and P. Case 2: with higher concentration of organic carbon, N and P. Case 3: average concentration of organic carbon, N and P (Table 4). The nutrients concentrations in sewage sludge defined by various researchers (Table 2) fall within the range of cases 1, 2 and 3. Further, it also evaluated GHG emissions based on the available methodologies in the literature and the results are compared with the present study (Table 6).

Table 4. Parameters considered for evaluation of GHGs during the land application of sludge digestate.

Parameter	Quantity	Unit	Reference
Volume of wastewater	75708.0	m^3/day	Assumed
Sludge production rate	0.225	kg of dry solids/ m^3 of wastewater treated	Ghazy, Dockhorn, and Dichtl (2009); Ghazy et al. (2011)
Sludge produced during wastewater treatment	17034.3	kg dry solids/day	Calculated
Volatile solids content in sludge	65.0	%	Metcalf et al. (2004)
Total dry solids degraded during AD	40.0	%	Metcalf et al. (2004)
Digestate for land application	12051.8	Kg dry solids/day	Calculated
Case 1. low percentage of
	(w/w)
Organic carbon	18.0	(%)
Nitrogen	0.5	(%)	McFarland (2001); Sommers (1977)
Phosphrous	0.5	(%)
Case 2. high percentage of
Organic carbon	39.0	(%)
Nitrogen	17.6	(%)
Phosphrous	14.3	(%)	McFarland (2001); Sommers (1977)
Average nutrient concentration
Organic carbon
Nitrogen	28.5	(%)	McFarland (2001); Sommers (1977)
Phosphrous	9.1	(%)
	7.4	(%)
Digestate applied on the land mm
	mm
Thickness	53.4		NEBRA (2008)
Water content of sludge before land application	70.0	%	McFarland (2001)
Bulk density of sludge	1020.0	kg/m^3	McFarland (2001)

The carbon content in the plant and carbon content in the crops (as required in Equation 6) are not available for sewage sludge application. Therefore, for the evaluation of GHGs reduction due to the photosynthetic absorption of CO₂ by crops, the data increase in the yield of tomato after compost application were considered from Togun et al. (2003). The typical carbon content of tomato is 24.80 g C/kg of tomato or 90.90 kg CO₂/Mg of tomato (Ministry of Agriculture and Forestry, http://www.mpi.govt.nz/mafnet/rural-nz). In case 2, due to non-availability of suitable data, a rational figure of 10.0% increase in the crop yield (compared to case 1) was assumed. An increase in the flux of gases in the soil–atmosphere interface at different nutrients concentrations is not available. Therefore, it was considered that there was a similar effect on gas fluxes at the soil–atmosphere interface by different nutrients concentrations.

5.1. GHG emissions and reductions at different nutrients concentrations

The computations of net GHG emissions (GHG reductions-GHG emissions) at different nutrients concentrations are summarized in Table 5. The GHG reduction, GHG emission and net GHG emissions for cases 1, 2 and 3 are presented in Figure 2. In case 1 at lower nutrients concentrations, GHG reductions due to the replacement of fertilizers represent only −40.88 Mg CO₂/yr, since the reductions are directly proportional to the nutrients concentrations (N and P) in the sludge. In cases 2 and 3, the GHG reductions corresponding to the fertilizer replacement were −1414.97 and −53.74 Mg CO₂/yr, respectively, that is, the maximum GHG reduction was observed for sludge with high nutrients concentrations (case 2). GHG reductions due to the carbon sequestration were proportional to the amount of carbon content in sewage sludge, therefore at high nutrients concentrations (case 2) the carbon sequestration was high (−515.81 Mg CO₂/yr). In cases 1 and 3, the carbon sequestration was relatively low compared with the case 2 (Table 5). Due to non-availability of the data at different nutrients concentrations and their effect on water holding capacity, improved tillage and photosynthetic absorption of CO₂ by crops, similar factors were assumed (for cases 1, 2 and 3) which resulted in similar GHG savings. However, further studies are required to establish the data. GHG emissions corresponding to the energy utilization were directly proportional to the volume of the digestate applied to the soil. Therefore, GHG emissions corresponding to the energy utilization were similar to cases 1, 2 and 3 (Table 5). GHG emissions corresponding to the increase flux of gases were similar at different nutrients concentrations (since a similar flux factor was assumed).

Figure 2. Net GHG emissions due to the land application of sewage sludge with different nutrients concentrations.

Table 5. Net GHG emissions due to sewage sludge land application at different nutrient concentrations.

		Case 1	Case 2	Case 3
		GHG emission (Mg CO2/yr) at
	Description	Low nutrient concentration	High nutrient concentration	Average nutrient concentration
I.	GHG reductions due to replacement of fertilizers	−40.88	−1414.97	−53.74
II.	GHG reductions due to carbon sequestration	−2.38	−515.81	−259.10
III.	GHG reductions due to additional water holding capacity	−17.71	−17.71	−17.71
IV.	GHG reductions due to improved tillage	−0.05	−0.05	−0.05
V.	GHG reductions due to photosynthetic absorption of CO2 by crops	−10.03	−10.03	−10.03
VI.	Sum of the reductions (I+II+III+IV+V)	−71.05	−1958.58	−340.63
VII.	GHG emissions due to increased flux	439.23	439.23	439.23
VIII.	GHG emissions due to energy utilization	819.91	819.91	819.91
IX.	Sum of the GHG emissions (VII+VIII)	1259.15	1259.15	1259.15
	Net GHG emissions due to land application of digestate	1188.10	−699.43	918.51
	The reduction in net GHG emissions compared to case 1	–	158.0%	22.0%

Note: Negative sign indicates a reduction in GHG emissions; positive sign indicates increased GHG emissions.

At lower nutrients concentrations (case 1) GHG emissions corresponding to the energy required during land application represent 62.0% of the net GHG emissions (Table 5). Moreover, GHG emissions corresponding to the increased flux of gases in the soil–atmosphere interface are 33.0% of the net GHG emissions. Thus GHG reductions due to digestate application represents only 5.0% of the net GHG emissions. In case 1, the GHG reductions due the replacement of fertilizers were only −40.88 Mg CO₂/yr, because of the lower nutrients concentrations in the digestate, but increased flux has resulted in GHG emissions of 493.23 Mg CO₂/yr. Thus, the overall balance represents increased GHG emissions for case 1. With increased nutrients concentrations (case 2), GHG emissions corresponding to the energy required during land application represent only 25.0% of the net GHG emissions, whereas GHG emissions corresponding to increased flux represent 14.0% of the net GHG emissions. At higher nutrients concentrations in sludge, the GHG reductions corresponding to the replacement of synthetic fertilizer are 44.0% of the net GHG emissions. GHG reductions corresponding to carbon sequestration represent 16.0% of the net GHG emissions. The sum of GHG emissions and reductions due to the land application of the digestate with higher nutrients concentrations was greater than the sum of GHG emissions. Therefore, the overall balance of higher nutrients concentrations result in GHG reductions. The land application of sewage sludge having average nutrients concentrations (case 3) will result in increased GHG emissions (Table 5). At average nutrients (case 3) concentrations the major factor for GHG reductions was due to carbon sequestration −259.10 Mg CO₂/yr. The sum of GHG reductions and emissions are −340.63 Mg CO₂/yr and 1259.14 Mg CO₂/yr, respectively. The net GHG emissions for case 3 were 918.51 Mg CO₂/yr. Therefore, the land application of digestate possessing lower nutrients concentrations results in net GHG emissions. The replacement of synthetic fertilizers and carbon sequestration represents the highest percentage of GHGs savings compared with other parameters (i.e. GHGs reduction due to additional water holding capacity, improved tillage and photosynthetic absorption of CO₂ by crops) (Table 5).

5.2. Comparison of different methodologies to quantify GHG emissions due to the land application of sewage sludge

Computation of GHG emissions due to the land application of sewage sludge by employing different methodologies exhibited variable GHG emissions (Table 6). IPCC and RTI methodologies revealed that GHG emissions decreased with a decrease in the concentrations of nutrients in sewage sludge (Table 6). In IPCC methodology (Eggleston, Buendia, Miwa, Ngara, & Tanabe, 2006), estimation of GHG emissions is linked directly to nutrients (C, N) concentration with default factors (Table 3). RTI's methodology predicted the highest GHG emissions (Table 6); because it is assumed that CO₂ emissions are directly related to the carbon content of sewage sludge (Table 3). Therefore, GHG emissions are lower at a lower carbon (nutrient) concentration and higher at a higher carbon concentration of sewage sludge. The IPCC and RTI methodologies do not take into account the replacement of the synthetic fertilizer, carbon sequestration and improved characteristic of soil (increased water holding capacity, improved tillage and movement of vehicles), which are the factors responsible for the reduction in GHG emissions.

Table 6. Comparison of GHG estimation methodologies due to land application of sewage sludge.

Description			GHG reductions	GHG emissions	Net GHG emissions	Reference
Case 1 :
Organic carbon 18.0%, nitrogen 0.5% and phosphorus 0.5%			NA	686	686	IPCC
			NA	2903	2903	RTI
			−1496	1030	−465	BEAM
			−71	1259	1188	This study
Case 3:			NA	1882	1882	IPCC
Organic carbon 28.5%, nitrogen 9.1% and phosphorus 7.4%			NA	3159	3159	RTI
			−8085	4629	−3456	BEAM
			−340	1259	919	This study
Case 2:			NA	3078	3078	IPCC
Organic carbon 39.0%, nitrogen 17.6% and phosphorus 14.3%			NA	6290	6290	RTI
			−14745	8228	−6516	BEAM
			−1988	1259	−699	This study
Default values of 4.0% N and 1.5% P			−4046	2503	−1543	BEAM

GHG emissions (kg/Mg of dry solids)
CO2	CH4	N2O
25.00	−0.01	−0.19
			−223	110	−112	Lundin et al. (2004)
NA	3.18	NA	NA	392	392	Hospido et al. (2005)
17.2	−21.30	−0.19	−224	75	−148	Hong et al. (2009)
17.2	3.18	NA	NA	467	467	Niu et al. (2013)

Note: NA means not applicable; IPCC is an intergovernmental panel on climate change; RTI is research triangular institute; BEAM is biosolids emissions assessment model; negative sign indicates a reduction in GHG emissions; positive sign indicates increased GHG emissions.

The BEAM methodology revealed higher GHG reductions (Table 6). The lowest GHG emissions are due to various assumptions, that is, (i) carbon sequestration is 25.0% of the dry sewage sludge applied, (ii) fertilizer credit for N is assumed as 4 and (iii) fertilizer credit for phosphorus as 2. Moreover, GHG emissions were also lower when the BEAM methodology was used by employing default values (i.e. 4.0% N and 1.5% P content present in sewage sludge) (Table 6). Moreover, the methodology developed by BEAM did not consider the increased water holding capacity of the soil, improved tillage and movement of vehicles and increased crop production due to the land application of sewage sludge. GHG emissions studies conducted by Hospido et al. (2005) and Niu et al. (2013) indicated an increase in GHG emissions due to sewage sludge land application, whereas GHG emissions studies conducted by Lundin et al. (2004) and Hong et al. (2009) revealed a decrease in GHG emissions due to sewage sludge land application. It was considered that CH₄ and N₂O emissions are reduced in these studies (Hong et al., 2009; Lundin et al., 2004). These authors have assumed a value of −0.012 kg CH₄/Mg of dry solids and −0.190 kg N₂O/Mg of dry solids (Table 3) for estimating GHG emissions without explaining the details.

In the proposed methodology, the estimated GHG emissions decreased with increased nutrients concentrations. In the proposed methodology the factors responsible (defined in Section 4) for GHG emissions are considered. Therefore, GHG emissions are representative in accordance with nutrients concentrations of sewage sludge applied to the land. Computations of GHG emissions performed according to the proposed methodology revealed that the estimation of GHG emissions was more effective and representative.

6. Limitations and future perspective

It is imperative to note that the nutrient content of sewage sludge is unpredictable. The release of nutrients in available form that plants can use may not occur at the right plant growth stage. Moreover, sewage sludge nutrient content, their solubility and nutrient release rates, in general, are lower than inorganic fertilizers. On the whole, the nutrients in sewage sludge are both more diluted and much less readily available to plants. In case of a shorter time frame, nutrients of sewage sludge will not be available to the plants and therefore will not contribute to GHG savings. However, for a longer time frame, all the nutrient content of sewage sludge will eventually be available to plants and thus the calculation of GHGs saving would yield more or less realistic values.

Organic fertilizers from compost and other sources (such as sewage sludge) can be quite variable from one batch to the next. Therefore, without testing each batch, the amount of the applied nutrients cannot be accurately known. Therefore, the consideration of compost derived data for calculating additional water holding capacity of soils due to land application of sewage sludge may not give accurate results. The type of sludge (primary, secondary or mixed) applied to agriculture land will also affect GHG emissions. Moreover, the type of sludge treatment process (aerobic or anaerobic) and treatment duration (solids retention time) will also affect GHG emissions. The type of sludge and the type of treatment mainly influence the background microbial community of the soil, which affects the degradation of sludge solids or organic matter after sludge is land applied.

The availability of N and P of sewage sludge for plants needs to be established. The carbon percentage sequestrated in the soil, increased water holding capacity of soil and increase in the flux of gases due to land application of sewage sludge vary with type of soil, moisture content of the soil and atmospheric conditions. Therefore, further study of these components is required to estimate either GHG emissions or reductions. The carbon content of the plants and the crop, before and after sewage sludge application is required to estimate the GHG reductions. Further, the antibiotics and other emerging contaminants, which are present in sewage sludge at the nano-concentrations will impact the soil microbiota and also the flux of gases at the soil–atmosphere interface. Thus, data need to be generated on the impacts of emerging contaminants on the gas flux at the soil–atmosphere interface.

7. Conclusion

Different methodologies to evaluate GHG emissions due to the land application of sewage sludge presented in the literature are summarized. A progressed methodology to evaluate GHG emissions was proposed. Based upon the proposed methodology, it was clear that increased nutrients concentrations in sewage sludge substantially sequestrated GHG emissions. The land application of sewage sludge proved to be beneficial in reducing GHG emissions only at high nutrients concentrations (39.0% organic carbon, 17.6% N and 14.3% P) with sequestration in GHG emissions (−699 kg CO₂/yr). A comparison of different methodologies for GHG emissions showed that the methodology proposed in this study was more effective and reliable, that is, GHG emissions or reductions vary according to the nutrients concentrations of the sludge.