8.1 Sector overview
In 2007, the waste sector accounted for 1,821.8 Gg carbon dioxide equivalent (CO2-e) (2.4 per cent) of total emissions. Emissions from the waste sector were 616.4 Gg CO2-e (25.3 per cent) below the 1990 baseline value of 2,438.2 Gg CO2-e (Figure 8.1.1). This reduction occurred in the solid waste disposal on land subcategory as a result of initiatives to improve solid waste management practices and increase the rate of landfill gas capture in New Zealand.
Emissions from the waste sector are calculated from solid waste disposal on land, wastewater handling and waste incineration (Figure 8.1.2). Methane from solid waste disposal was a key category (level and trend assessment) in 2007.
|Solid waste disposal on land||2,063.2||1,438.0|
Disposal and treatment of industrial and municipal waste can produce emissions of CO2 and CH4. The CO2 is produced from the aerobic decomposition of organic material. These emissions are not included as a net emission because the CO2 is considered to be reabsorbed in the following year. The CH4 is produced as a by-product of anaerobic decomposition.
8.2 Solid waste disposal on land (CRF 6A)
Solid waste disposal on land contributed 1,438.0 Gg CO2-e (78.9 per cent) of emissions from the waste sector in 2007. This was a decrease of 625.3 Gg CO2-e (30.3 per cent) from the 1990 level of 2,063.2 Gg CO2-e. Methane emissions from solid waste were identified as a key category (level and trend) for 2007.
Organic waste in solid waste disposal sites is broken down by bacterial action in a series of stages that result in the formation of CO2 and CH4. The CO2 from aerobic decomposition is not reported in the inventory and assumed to be reabsorbed in the following year. The amount of CH4 gas produced depends on a number of factors including the waste disposal practices (managed versus unmanaged landfills), the composition of the waste, and physical factors such as the moisture content and temperature of the solid waste disposal sites. The CH4 produced can go directly into the atmosphere via venting or leakage, or it may be flared off and converted to CO2.
In New Zealand, managing solid wastes has traditionally meant disposing of solid waste in landfills. In 1995, a National Landfill Census showed there were 327 legally operating landfills or solid waste disposal sites in New Zealand that accepted approximately 3,180,000 t of solid waste (Ministry for the Environment, 1997). Since 1995, there have been a number of initiatives to improve solid waste management practices in New Zealand. These have included preparing guidelines for the development and operation of landfills, closure and management of landfill sites, and consent conditions for landfills under New Zealand’s Resource Management Act (1991). As a result of these initiatives, a number of poorly located and substandard landfills have been closed and communities rely increasingly on modern regional disposal facilities for disposal of their solid waste. The 2006 Landfill Census reported there were 60 legally operating municipal landfills in New Zealand, a reduction of 82 per cent from 1995 (Ministry for the Environment, 2007). The same census reported that 3,156,000 t of solid waste was deposited in 2006.
New Zealand’s focus regarding waste is towards waste minimisation and resource recovery. In March 2002, the Government announced the New Zealand Waste Strategy (Ministry for the Environment, 2002a). The Strategy sets targets for a range of waste streams as well as for improving landfill practices by the year 2010. As part of the implementation and monitoring of the Strategy, the Government developed the Solid Waste Analysis Protocol (Ministry for the Environment, 2002b) that provided a classification system, sampling regimes and survey procedures to measure the composition of solid waste streams. In 2008, the Government passed the Waste Minimisation Act that imposes a levy of $10 per tonne of municipal solid waste from 1 July 2009, extends product stewardship regimes, and enables regulations to require landfill operators and others to report on various waste targets and measures. Reporting under this Act will significantly improve New Zealand’s knowledge of solid waste volumes and composition.
8.2.2 Methodological issues
New Zealand has used a first order decay approach with the Intergovernmental Panel on Climate Change (IPCC) model contained in the IPCC 2006 guidelines to report emissions from solid waste in the inventory. New Zealand uses default values for starting year, delay time, degradable organic carbon content of specific waste streams, and the default “wet temperate” values for methane generation rate constants (k) for each compositional type. New Zealand-specific activity data on current and historic waste disposal and waste composition is used. An oxidation correction factor of 0.1 is used as landfills in New Zealand are capped and are categorised as well managed (IPCC, 2006).
The calculations for estimating emissions from the waste sector, including the IPCC 2006 workbook tables, are included in the MS Excel worksheets available with this report from the Ministry for the Environment’s website (here). Some modifications were made to the standard tables, including the addition of an assumptions worksheet that documents the sources of data and judgements made for the method described above. Another additional worksheet provides an estimate of emissions using the IPCC 1996 Tier 1 calculations. These estimates were used for a quality assurance check as described in section 8.2.4.
New Zealand does not have sufficient data to be able to categorise solid waste as either municipal solid waste or industrial waste, as many municipal landfills accept industrial waste. All national data is therefore contained in the municipal solid waste class.
Activity data on solid waste composition was documented for 1995 and 2004 (Ministry for the Environment, 1997; Waste Not Consulting, 2006). Linear extrapolations and interpolations were used between years where no new data was available. The estimate of degradable organic carbon in 1995 (and 1990) was 0.15 Gg C/Gg waste, and increased over time to 0.17 Gg C/Gg waste in 2004 (and 2007) mainly through increases in the proportion of wood waste going to landfill.
Calculation of the CH4 generation potential is based on the same data contained in Ministry for the Environment (1997) and Waste Not Consulting (2006) reports, and adjusted for changes in the management of landfilled waste through the CH4 correction factor. In 1990, 1995 and 2007, the CH4 generation potential was 0.05 Gg CH4/Gg waste.
There is no New Zealand-specific composition data on the specific half-lives of solid waste. Consequently, New Zealand uses the IPCC default CH4 generation rate for a wet temperate climate. This climate type is considered the best fit for New Zealand’s complex climate systems and geography.4
There has been no new data on solid waste composition since 2004. Consequently, the value for degradable organic content per Gg of waste has remained constant since 2004. However, the CH4 correction factor has been increasing due to the closure of unmanaged landfills and increasing volumes being disposed to larger, modern landfills. It is estimated that, in 1995, 90 per cent of New Zealand’s waste was disposed to managed solid waste disposal sites and 10 per cent to uncategorised sites (Ministry for the Environment, 1997)5. The IPCC (2006) default values are used for the carbon content of the various components of the solid waste stream.
Total waste to landfill has been estimated for the years 1995, 1998, 2002, 2003, 2004 and 2006. Based on the 2006/07 National Landfill Census, the 2002 Landfill Review and Audit, and the 2006 report on Waste Composition and Construction Waste Data, it is estimated that the quantity of solid waste going to landfills in New Zealand in 2006 was equivalent to 749.4 kilograms per person per year. This is a reduction of 12.6 per cent in waste generation from 858.0 kg per person per year in 1995. The 2006 data on kilograms of solid waste per person per day was extrapolated for 2007 using estimated national population data. The reduction in solid waste per person per day since 1995 is due to waste minimisation initiatives from central and local government and increased recycling.
New Zealand uses the IPCC 2006 default value for the fraction of degradable organic carbon that actually degrades (0.5).
The rate of recovered CH4 per year is estimated based on information from a 2005 survey of solid waste disposal sites that serve populations of over 20,000 in New Zealand (Waste Management New Zealand, 2005). There was no landfill gas collected in 1990 and 1991, with the first flaring system installed in 1992. The consultants surveyed 18 landfills known to have installed, or that were planning to install, landfill gas systems. The method involved initial modelling of the major landfills that had good operational data, to establish benchmarks for the CH4 generation potential (Lo), the CH4 generation rate constant (k), and system recovery efficiency. This information was then used as a starting point for preparing estimates for other sites, with adjustments made based on knowledge of site conditions, system design, and assessed operating performance. The landfill CH4 recovery data was then compiled for all of the sites. The consultants used the IPCC (1996) first order decay model. The quality assurance and quality control worksheet was used to check the data.
The benchmarks for the CH4 generation potential (Lo) and the CH4 generation rate constant (k) for landfills with gas recovery systems developed by Waste Management New Zealand are not the same as the values used to estimate New Zealand’s gross CH4 emissions in the IPCC 2006 worksheets. Waste Management New Zealand assumed higher Lo and k values because it argued waste would be managed to biodegrade and emit CH4 faster at landfills with gas recovery systems. This methodological inconsistency probably results in an underestimation of gross and net national CH4 emissions from solid waste disposal. It will be remedied for the 2010 inventory submission.
8.2.3 Uncertainties and time-series consistency
The overall estimated level of uncertainty is estimated at ±20 per cent. This level of uncertainty is the same as the 2008 inventory submission, but an improvement on prior submissions. The improvement was due to the utilisation of the IPCC 2006 spreadsheet model, the 2002 Landfill Audit and Review, and an assessment of comparability between data sources as performed in Waste Not Consulting (2006). Due to the unknown level of uncertainty associated with the accuracy of some of the input data, it has not been possible to perform a statistical analysis to precisely determine uncertainty levels. The incomplete dataset makes statistical analysis impractical. Uncertainty in the data is primarily from uncertainty in changes to solid waste composition since 1990 and actual recovered CH4, based on the 1997 National Waste Data Report (Ministry for the Environment, 1997), the Waste Composition and Construction Waste Data (Waste Not Consulting, 2006) and the Landfill Methane Recovery Estimate Report (Waste Management New Zealand, 2005).
The New Zealand waste composition categories from the Waste Not Consulting (2006) report do not exactly match the categories required for the IPCC degradable organic carbon calculation. The major difference is that in New Zealand’s degradable organic carbon calculation, the putrescibles category includes food waste as well as garden waste. A separation into the IPCC categories was not feasible given the available data in the report by Waste Not Consulting (2006). The effect of this difference is zero, as the IPCC 2006 default carbon contents are identical for non-food (15 per cent carbon content) and food categories (15 per cent carbon content).
8.2.4 Source-specific QA/QC and verification
Methane from solid waste disposal was identified as a key category (level and trend assessment) in 2007. In preparation for this inventory submission, the data for this category underwent Tier 1 quality checks.
The data and associated text for the solid waste subcategory was peer reviewed by the Caledonian Environment Centre as part of the QA/QC procedures implemented before this inventory was submitted.
A centralised review of New Zealand’s inventory (UNFCCC, 2001c) recommended that gross CH4 estimates from solid waste emissions should be compared with the IPCC Tier 1 and Tier 2 approaches. For the 2007 year, the IPCC (2006) Tier 2 value of gross annual CH4 generation was 131.9 Gg CH4 and the IPCC (1996) Tier 1 value was 179.3 Gg CH4. The assumptions used to calculate net CH4 emissions from available activity data were the same for both Tier approaches.
8.2.5 Source-specific recalculations
Municipal solid waste composition values for all years prior to 2004 were updated to remove nappies from the paper classification. The National Waste Data Report (Ministry for the Environment, 1997) included nappies within “paper”, whereas the Solid Waste Analysis Protocol as reported in the Waste Composition Data report (Waste Not Consulting, 2006) separated these categories. The proportion of nappies that made up solid waste disposal in landfills prior to 2004 is assumed to be the same as the 2004 value. This is due to no existing national data before 2004. This recalculation improved the accuracy of national waste composition estimates for all years prior to 2004. It also reduced the estimates of degradable organic carbon in solid waste for those years, because nappies are assumed to have lower degradable carbon content than paper.
The use of the IPCC 2006 spreadsheet model has resulted in many recalculations in this inventory submission. Firstly, this submission has used the default six-month delay in the anaerobic decomposition process, whereas earlier submissions from New Zealand did not account for this. Secondly, this submission has used default IPCC 2006 degradable organic carbon (DOC) values for decomposable waste. These default values differ for textiles and wood from the values published in IPCC 1996 guidelines and used in earlier submissions. Thirdly, this submission has applied individual half life (k) values to separate categories of waste, whereas earlier submissions used a weighted average for mixed municipal waste. Finally, the starting year for the model was changed from 1940 in earlier submissions, to the default IPCC 2006 spreadsheet model of 1950 for this submission.
Recalculations were performed back to 1990 and have resulted in a decrease of 58.3 Gg CO2-e in 1990 and a decrease of 35.6 Gg CO2-e in 2006.
8.2.6 Source-specific planned improvements
New estimates of landfill gas collected will be developed for the Next inventory submission.
8.3 Wastewater handling (CRF 6B)
In 2007, wastewater handling produced 381.7 Gg CO2-e (21.0 per cent) of emissions from the waste sector. This was an increase of 21.3 Gg CO2-e (5.9 per cent) from the 1990 level of 360.4 Gg CO2-e.
Wastewater from almost every town in New Zealand with a population over 1,000 is collected and treated in community wastewater treatment plants. There are approximately 317 municipal wastewater treatment plants in New Zealand and approximately 50 government or privately-owned treatment plants serving more than 100 people.
Although most of the treatment processes are aerobic, and therefore produce no CH4, there are a significant number of plants that use partially anaerobic processes such as oxidation ponds or septic tanks. Small communities and individual rural dwellings are generally served by simple septic tanks followed by ground soakage trenches.
Large quantities of industrial wastewater are produced by New Zealand’s primary industries. Most of the treatment is aerobic and any CH4 from anaerobic treatment is flared. There are a number of anaerobic ponds that do not have CH4 collection, particularly serving the meat-processing industry. These are the major sources of industrial wastewater CH4 in New Zealand.
8.3.2 Methodological issues
Methane emissions from domestic wastewater treatment
Methane emissions from domestic wastewater handling have been calculated using a refinement of the IPCC method (IPCC, 1996). The population using each municipal treatment plant in New Zealand has been determined (SCS Wetherill Environmental, 2002; Beca, 2007). Where industrial wastewater flows to a municipal wastewater treatment plant, an equivalent population for that industry has been calculated based on a biological oxygen demand (BOD) loading of 70 g per person per day.
Populations not served by municipal wastewater treatment plants have been estimated and their type of wastewater treatment assessed (SCS Wetherill Environmental, 2002; Beca, 2007). The plants have been assigned to one of nine typical treatment processes. A characteristic emissions factor for each treatment is calculated from the proportion of biological oxygen demand to the plant that is anaerobically degraded, multiplied by the CH4 conversion factor (SCS Wetherill Environmental, 2002; Beca, 2007).
It is good practice to use country-specific data for the maximum CH4 producing capacity factor (Bo). Where no data is available, the revised 1996 IPCC guidelines (IPCC, 1996) recommend using Bo of 0.25 CH4/kg COD (chemical oxygen demand) or 0.6 kg CH4/kg BOD. The IPCC biological oxygen demand value is based on a 2.5 scaling factor of chemical oxygen demand (IPCC, 2000). New Zealand has used these IPCC default factors in this inventory submission.
New Zealand uses a value of 0.026 kg BOD/1000 person/yr, as it is equivalent to the IPCC high-range default value for the Oceania region of 70g/person/day.
Methane removal via flaring or energy use is known to occur at eight plants in New Zealand. They all use anaerobic digesters as a component of the treatment. However, because these plants are categorised as “centralised aerobic treatment plant, well managed” according to the 2006 IPCC guidelines, the CH4 emission factor is zero. The CH4 generated in those plants is an abnormality by that classification, as all the CH4 generated is flared or used for energy production The net result is no CH4 emission and no CH4 flared volumes are included in the equation.
Methane emissions from industrial wastewater treatment
The IPCC 2006 default method is also used to calculate emissions from industrial wastewater treatment. Three industries were identified as having organic-rich wastewaters that are treated anaerobically. These are (in order of significance): meat processing, pulp and paper, and dairy processing. The meat industry is divided into kills and rendering, because the emissions from kills are calculated based on a pro-rata of previous inventories, and actual carcass numbers, whereas emissions from rendering are calculated based on wastewater volume. The dairy industry predominantly uses aerobic treatment. There is only one remaining factory that uses anaerobic treatment. The wastewater is covered and the majority of the captured biogas (55 per cent CH4) is used to operate the boilers. The remainder is flared.
For each industry, an estimate is made of the total industrial output in tonnes per year. The IPCC 2006 default values for wastewater generated and chemical oxygen demand are used. The exception is for the pulp and paper industry where the chemical oxygen demand (COD)/t product is determined from industry figures of biochemical oxygen demand (BOD)/t product, using a conversion factor of COD = 2.2 × BOD.
For meat processing (rendering), total organic wastewater is a function of the IPCC 2006 default COD value (4.1kg COD/m3) and site-specific estimates of wastewater treatment activity. For dairy processing, the IPCC 2006 default method is followed.
Methane emissions from sludge
The organic solids produced from wastewater treatment are known as sludge. In New Zealand, the sludge from wastewater treatment plants is typically sent to landfills.
In large treatment plants in New Zealand, sludge is handled anaerobically and the CH4 is almost always flared or used.6 Smaller plants generally use aerobic handling processes such as aerobic consolidation tanks, filter presses and drying beds.
Oxidation ponds accumulate sludge on the pond floor. In New Zealand, these are typically only de-sludged every 20 years. The sludge produced is well stabilised with an average age of approximately 10 years. It has a low, biodegradable organic content and is considered unlikely to be a significant source of CH4 (SCS Wetherill Environmental, 2002; Beca, 2007).
Sludge from septic tank clean-out, known as “septage”, is often removed to the nearest municipal treatment plant. In those instances, it is included in the CH4 emissions from domestic wastewater treatment. There are a small number of treatment lagoons specifically treating septage. These lagoons are likely to produce a small amount of CH4 and their effect is included in the calculations.
Nitrous oxide emissions from domestic wastewater treatment
New Zealand’s calculation uses the IPCC 2006 method (IPCC, 2006). The IPCC method calculates nitrogen production based on the average per capita protein intake. A value of 36.135 kg N/person/year is assumed for 1990 to 2007. This is the maximum value as reported to the Food and Agriculture Organisation of the United Nations by New Zealand, and was used as there was no discernable trend between 1990 and 2007. Default IPCC 2006 values are used for the fraction of nitrogen in protein, fraction of non-consumption protein, fraction of industrial and commercial co-discharged protein, and nitrogen removed with sludge. The IPCC default emission factor of 0.005 kg N2O-N/kg N is used.
Nitrous oxide emissions from industrial wastewater treatment
The 2006 IPCC guidelines state that, compared with domestic wastewater, the N2O emissions from industrial wastewater are insignificant and can therefore be ignored. However, this statement does not take into account the significance of the meat industry in New Zealand in relation to nitrogenous-rich wastewaters. Due to the prevalence of anaerobic treatment plants within the meat industry, New Zealand has chosen to report N2O emissions from this source.
The 2006 IPCC guidelines do not have a method for calculating N2O emissions from industrial wastewater. Emissions are calculated using an emissions factor (kg N2O-N/kg wastewater N) to give the proportion of total nitrogen in the wastewater converted to N2O. The total nitrogen is calculated by adopting the chemical oxygen demand load from the CH4 emission calculations and using a ratio of chemical oxygen demand to nitrogen in the wastewater for each industry.
8.3.3 Uncertainties and time-series consistency
Methane from domestic wastewater
It is not possible to perform rigorous statistical analyses to determine uncertainty levels because of biases in the data collection methods (SCS Wetherill Environmental, 2002). The uncertainty reported for wastewater values is based on an assessment of the reliability of the data and the potential for important sources to have been missed from the data. It is estimated that domestic wastewater CH4 emissions have an accuracy of –25 per cent to +40 per cent (SCS Wetherill Environmental, 2002; Beca, 2007). This is less uncertainty than reported in previous submissions, due to the added confidence in activity data provided by the new national wastewater treatment database.
Methane from industrial wastewater
Total CH4 production from industrial wastewater has an estimated accuracy of ±40 per cent based on assessed levels of uncertainty in the input data (SCS Wetherill Environmental, 2002, Beca 2007).
Nitrous oxide from wastewater
There are very large uncertainties associated with N2O emissions from wastewater treatment and no attempt has been made to quantify this uncertainty. The IPCC default emissions factor, EF6, has an uncertainty of –80 per cent to +1200 per cent (IPCC, 1996) meaning that the estimates have only order of magnitude accuracy.
8.3.4 Source-specific QA/QC and verification
No specific quality checks were carried out for this category.
8.3.5 Source-specific recalculations
The inventory of emissions from industrial wastewater treatment has been updated with adjustments made to activity data for both the pulp and paper processing and meat-processing categories. This new data adjusted total organic product estimates for all years from 1990 and resulted in a decrease of 15.2 Gg CO2-e in 1990 and a decrease of 10.7 Gg CO2-e in 2006.
The estimates of total organics in wastewater (TOW) before 1997 from domestic and commercial sources were adjusted to reflect population growth. In earlier submissions, the constant total organic waste (TOW) value, first established in 1997, was used back to 1990. This improvement to TOW estimates resulted in recalculations in the years 1990 to 1996, with a decrease of 15.5 Gg CO2-e in 1990 and no change to the 2006 estimate.
Improvements to the accuracy of calculations for emissions of CH4 from industrial wastewater treatment resulted in recalculations for emissions estimates for all years from 1990. The recalculations resulted in a decrease of 0.2 Gg CO2-e in 1990 and a decrease of 10.7 Gg CO2-e in 2006.
8.3.6 Source-specific planned improvements
No improvements are planned for this category.
8.4 Waste incineration (CRF 6C)
In 2007, waste incineration accounted for 2.2 Gg CO2-e (0.1 per cent) of waste emissions. This was a decrease of 12.4 Gg CO2-e (85.1 per cent) from the 1990 level of 14.6 Gg CO2-e.
There is no incineration of municipal waste in New Zealand. The only incineration is for small specific waste streams including medical, quarantine and hazardous wastes. The practice of incinerating these waste streams has declined since the early 1990s due to environmental regulations and alternative technologies, primarily improved sterilisation techniques. Consents under New Zealand’s Resource Management Act control non-greenhouse gas emissions from these incinerators.
In 2004, New Zealand introduced a national environmental standard for air quality. The standard effectively requires all existing, low-temperature waste incinerators in schools and hospitals to obtain a resource consent by 2006, irrespective of existing planning rules. Incinerators without consents will be prohibited.
The 2006 IPCC guidelines (IPCC, 2006) are used to calculate emissions from the incineration of waste as the revised 1996 IPCC guidelines (IPCC, 1996) do not contain methods for estimating emissions from waste incineration. New Zealand considers the 2006 IPCC guidelines (IPCC, 2006) contain the most appropriate and current methodologies for estimating emissions from waste incineration.
Incineration devices that do not control combustion air to maintain adequate temperature, and do not provide sufficient residence time for complete combustion, are considered as open burning systems (IPCC 2006). This excluded many small facilities that may have burned plastics and other mixed waste, such as at schools.
Only CO2 resulting from burning of carbon in waste that is fossil in origin is included under the IPCC methodology, such as in plastics, synthetic textiles, rubber, liquid, solvents and waste oil. Biogenic CO2, such as that from paper, cardboard and food, is excluded in accordance with the 2006 IPCC guidelines (IPCC, 2006). Also excluded are emissions from waste to energy incineration facilities, as they are reported within the energy sector of the inventory.
Default compositional values from the IPCC 2006 guidelines are used to estimate the fossil fuel-derived carbon. These values are 27.5 per cent for hazardous waste (being the mean of the recommended range) and 25 per cent for clinical waste.
Many incinerators are quarantine waste incinerators. The 2006 IPCC guidelines (IPCC, 2006) do not have a default category for quarantine incinerators. Only three default classifications are available: clinical waste, hazardous waste, or sewage sludge. None of these categories appropriately represent New Zealand quarantine waste that contains paper, plastics, food and dunnage. However, for the purposes of the calculations, the composition of quarantine was assumed to be more closely aligned with clinical waste than with the other categories. This is because clinical waste may also contain paper, plastics and biological matter (SKM, 2007).
Estimates of direct emissions are made using the default Tier 1 methodology (IPCC, 2006). Default emission factors for CO2, CH4 and N2O are taken from the 2006 IPCC guidelines. New Zealand uses the mid point where these emission factors are presented as a range.
The default emission factor for industrial waste is used for hazardous waste, and the default emission factor for municipal/industrial waste is used for clinical waste. As the CH4 factors are presented as kg/TJ, the calorific value for the relevant waste is needed to convert the figures to Gg/yr. The calorific value was sourced from chapter 11 of the New Zealand Energy Information Handbook (Baines, 1993). Only the gross calorific value was available from the energy handbook, so this value was used, although it is noted this is inconsistent with the IPCC approach that uses net values.
The Japanese emission factor is used for sewage sludge. The IPCC 2006 guidelines note that the most detailed observations of CH4 emissions from waste incineration have been made in Japan (Volume 5: section 5.4.2).
8.4.3 Uncertainties and time-series consistency
The measurement of uncertainty in the data collected from each individual site was difficult to quantify. For most sites, tonnes per year of waste incinerated was obtained from file information or this was calculated from a mass burn rate (kg per hour) and assumed operating hours on an annual basis. Estimates based on consented limits are likely to be overestimates of the actual waste burnt.
The annual rates were projected for the corresponding number of years of operation. This provided an estimated total amount of wet waste incinerated from 1990 to 2007.
As per the recommendation for uncertainties relating to activity data (IPCC 2006 Volume 5, section 5.7.2), the conservative estimated uncertainty for the amount of wet waste incinerated is around ±10 per cent. The estimated value in the 2006 IPCC guidelines is ±5 per cent. This uncertainty has increased to ±10 per cent due to the lack of detailed data. The uncertainty for the data is likely to be greater than this, particularly where projections are based on a mass burn rate and assumed operating hours (SKM, 2007).
The data collected for the composition of waste is not detailed. Therefore, as per the recommendation for uncertainties relating to emission factors (IPCC 2006 Volume 5, section 5.7.1), the estimated uncertainty for default CO2 factors is ±40 per cent. Default factors used in the calculation of CH4 and N2O emissions have a much higher uncertainty (IPCC 2006 Volume 5, section 5.7.1); hence, the default estimated uncertainty for default CH4 and N2O factors is ±100 per cent (SKM, 2007).
8.4.4 Source-specific QA/QC and verification
All data collected was from reliable sources and all default emission factors for emissions calculations were extracted from the 2006 IPCC guidelines. All calculations were externally and internally reviewed. Hand calculations were used to check calculations. Limited information was provided by some individual sites. This meant activity data had to be interpolated and extrapolated from the available data. This could have led to inaccuracies in the quantification of the total waste incinerated annually. There is generally no detailed information about the actual composition of the waste incinerated; only the consented types of waste allowed.
8.4.5 Source-specific recalculations
Recalculations have been performed for all years from 1990 to 2006 after adjusting for several modelling errors. These recalculations resulted in a 0.1 Gg CO2-e increase in estimated emissions in 1990 and a 3.0 Gg CO2-e decrease in estimated emissions in 2006.
8.4.6 Source-specific planned improvements
No improvements are planned for this category.
4 Mean average temperatures vary from 10 degrees Celsius in the south to 16 degrees in the north (NIWA). Mean annual precipitation ranges from 600 to 1600 mm (NIWA). Mean annual potential evapo-transpiration ranges from 200 mm to 1100 mm (Tait A, and Woods R, 2007).
5 The 10 per cent of solid waste not disposed to “managed” solid waste disposal sites, went to sites that fell outside the definition of “managed”, yet insufficient information is held about the sites to classify them as deep or shallow, unmanaged solid waste disposal sites, hence the “unclassified” status. This submission, assumed that, by 2010, all solid waste would be disposed to “managed” solid waste disposal sites. This has lead to a linearly increasing CH4 correction factor.
6 An exception is the Christchurch sewage treatment plant that uses anaerobic lagoons for sludge treatment. Based on volatile solids reduction measurements in the lagoons, the plant estimates CH4 production of 0.46 Gg/year plus an additional 0.16 Gg/year from unburned CH4 from the digester-gas fuelled engines.