8.1 Sector overview
In 2006, the waste sector accounted for 1,857.8 Gg carbon dioxide equivalent (CO2-e) (2.4 per cent) of total emissions. Emissions from the waste sector are now 647.8 Gg CO2-e (25.9 per cent) below the 1990 baseline value of 2,505.7 Gg CO2-e (Figure 8.1.1). The reduction has occurred in the “solid waste disposal on land category” as a result of initiatives to improve solid waste management practices and increase landfill gas capture rates 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 is a key category (level and trend assessment).
Solid waste disposal on land
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 1,475.4 Gg CO2-e (79.4 per cent) of emissions from the waste sector in 2006. This is a decrease of 646.1 Gg CO2-e (30.5 per cent) from the 1990 level of 2,121.5 Gg CO2-e.
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 tonnes 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 2002 Landfill Review and Audit reported that there were 115 legally operating landfills in New Zealand, a reduction of 65 per cent from 1995.
Recently, New Zealand’s focus has been towards waste minimisation and resource recovery. In March 2002, the Government announced its New Zealand Waste Strategy (Ministry for the Environment, 2002a). The strategy sets targets for a range of waste streams as well as 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.
8.2.2 Methodological issues
New Zealand has used the IPCC Tier 2 First Order Decay approach (IPCC, 2000) to report emissions from solid waste in the inventory. New Zealand uses country-specific values for the degradable organic carbon factor (0.15 Gg C/Gg waste), methane generation potential (Lo) (0.05 Gg C/Gg waste), and a methane generation rate constant (k) (0.06) based on waste composition and conditions at New Zealand landfills. The IPCC default oxidation correction factor of 0.1 is used (IPCC, 2000). The calculations for estimating emissions from the waste sector are included in the excel workbooks available for download with this report from the Ministry for the Environment’s website.
The degradable organic carbon factor varies according to the composition of waste disposed to landfill. Linear extrapolations are used between years where no new data was available. Consistent and comparable data is only available for two years, 1995 and 2004, and reported in the National Waste Data Report (Ministry for the Environment, 1997 and the report on Waste Composition and Construction Waste Data (Waste Not Consulting, 2006). The estimate of DOC in 1995 (and 1990) was 0.14 Gg C/Gg waste, and in 2004 (and 2006) 0.15 Gg C/Gg waste.
Calculation of the methane generation potential is based on the same data contained in the reports in the paragraph above, and adjusted for changes in the management of landfilled waste through the methane correction factor. New Zealand SWAP baseline results as reported in the report on Waste Composition and Construction Waste Data (Waste Not Consulting, 2006). The methane generation potential was 0.04 Gg CH4/Gg waste in 1995 (and 1990), and 0.05 Gg CH4/Gg waste in 2006.
A methane generation rate constant of 0.06 is used for New Zealand’s landfills. International measurements support a methane generation rate constant in the range of 0.03 to 0.2 (IPCC, 2000). The 0.03 represents a slow decay rate in dry sites and slowly degradable waste, whereas the 0.2 value represents high moisture conditions and highly degradable waste. The IPCC recommended value is 0.05 (IPCC, 2000). The relatively wet conditions in most regions of New Zealand mean that the methane generation rate constant is likely to be slightly above the 0.05 default value. This was confirmed by a comparison of CH4 generation and recovery estimates to actual recovery rates at a limited number of solid waste disposal sites in New Zealand (SCS Wetherill Environmental, 2002).
There has been no new solid waste compositional data for 2005 and 2006, hence degradable organic content per Gg waste remained constant from 2004 values. However, the methane correction factor has been increasing due to 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)3. The IPCC (1996) default values are used for the carbon content of the various components of the solid waste stream.
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 2.08 kg per person per day. This shows a reduction in waste generation from 2.35 kg per person per day in 1995.
The fraction of degradable organic carbon that actually degrades (0.5) and the methane oxidation factor (0.1) are drawn from the Topical Workshop on Carbon Conversion and Methane Oxidation in Solid Waste Disposal Sites, held by the IPCC Phase II Expert Group on Waste on 25 October 1996. The workshop was attended by 20 international experts with knowledge of the fraction of degradable organic carbon that is converted to CH4 and/or the oxidation of CH4 by microbes in the soil cover.
The recovered CH4 rate per year was 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.
8.2.3 Uncertainties and time-series consistency
The overall estimated level of uncertainty is estimated at ± 20 per cent, which is the same uncertainty as the 2007 inventory submission, but an improvement on prior submissions. The improvement was due to the sampling and survey guidelines from the Solid Waste Analysis Protocol, the 2002 Landfill Audit and Review, and as 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. Uncertainty in the data is primarily from uncertainty in total solid waste disposed to landfills and the recovered methane rate based on the 1997 National Waste Data Report (Ministry for the Environment).
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 managed by the use of IPCC default carbon contents which are similar for the non-food (17 per cent carbon content) and food categories (15 per cent carbon content). New Zealand has chosen to use the 15 per cent carbon content figure. While using the 15 per cent figure may potentially underestimate annual emissions, it provides the best estimate for emission reductions between 1990 and 2006. This approach was recommended by the international review team (UNFCCC, 2007).
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 preparation for this inventory, the data for this category underwent Tier 1 quality checks.
A technical review of New Zealand’s inventory recommended that gross CH4 estimates from solid waste emissions should be compared with the IPCC Tier 1 and Tier 2 approaches (UNFCCC, 2001c). For the 2006 year, the Tier 2 value of gross annual CH4 generation is 138.1 Gg CH4 and the Tier 1 value is 158.0 Gg CH4. The assumptions used to calculate net CH4 emissions from gross CH4 are the same for both tiers.
8.2.5 Source-specific recalculations
Municipal solid waste composition values for 2004 and 2005 were updated after discussion with the authors of the Waste Composition and Construction Data report (Waste Not Consulting, 2006). It was decided that the composition analysis in that report, which was based on SWAP baseline data for 2003, made appropriate adjustments for sampling errors including seasonal and geographical location issues. Use of that composition analysis for the 2004 year meant linear extrapolation back to 1995 for the 2003 year, and changes to the constant value used in 2005. This new data has lead to increased values of degradable organic carbon in MSW in 2003, 2004 and 2005.
The waste generation rate per capita increased for 2004 due to applying improved data which was published in a report on Waste Composition and Construction Waste Data (Waste Not Consulting, 2006).
The waste generation rate per capita decreased for 2005 due to extrapolating from recent national data that was published in the 2006/07 National Landfill Census (Ministry for the Environment, 2007).
The waste generation rate per capita increased for 1996, 2000 and 2004 due to changes in the formula used to adjust for calendar leap years. A similar adjustment decreased the DOC modelled as in landfills prior to 1990, thereby decreasing 1990 gross emissions onwards.
Recalculations were performed back to 1990 and have resulted in a decrease of 1.4 Gg CO2-e in 1990 and an increase of 60.1 Gg CO2-e in 2005.
8.2.6 Source-specific planned improvements
There are no specific improvements planned for this category.
8.3 Wastewater handling (CRF 6B)
In 2006, wastewater handling produced 377.3 Gg CO2-e (20.3 per cent) of emissions from the waste sector. This is an increase of 7.6 Gg CO2-e (2.1 per cent) from the 1990 level of 369.7 Gg CO2-e.
Wastewater from virtually every town in New Zealand with a population over 1,000 people 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 methodology (IPCC, 1996). A population using each municipal treatment plant in New Zealand has been assessed (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 methane producing capacity factor (Bo). Where no data are 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 the 2008 inventory submission.
Methane emissions from industrial wastewater treatment
The IPCC default methodology is also used to calculate emissions from industrial wastewater treatment. For each industry, an estimate is made of the total industrial output in tonnes per year, the average chemical oxygen demand load going to the treatment plant and the proportion of waste degraded anaerobically. Methane is only emitted from wastewater being treated by anaerobic processes. Industrial wastewater that is discharged into a sewer with no anaerobic pre-treatment is included in the domestic wastewater section of the inventory.
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. Any CH4 emissions from landfilled sludge are reported under the solid waste disposal sites category. Other sources of emissions from sludge are discussed below.
In large treatment plants in New Zealand, sludge is handled anaerobically and the CH4 is almost always flared or used.4 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 desludged 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 a modification of the IPCC methodology (IPCC, 1996). The IPCC method calculates nitrogen production based on the average per capita protein intake; however in New Zealand, raw sewage nitrogen data are available for many treatment plants. The raw sewage nitrogen data are used to calculate a per capita domestic nitrogen production of 13 g/day and a per capita wastewater nitrogen value of 4.75 kg/person/year. The IPCC default method uses an emissions factor (EF6) to calculate the proportion of raw sewage nitrogen converted to N2O. New Zealand uses the IPCC default value of 0.01 kg N2O–N /kg sewage N.
Nitrous oxide emissions from industrial wastewater treatment
The IPCC does not offer a methodology for estimating N2O emissions from industrial wastewater handling. 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 was 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 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 –40 per cent to +60 per cent (SCS Wetherill Environmental, 2002; Beca, 2007).
Methane from industrial wastewater
The method used in estimating CH4 emissions from industrial wastewater treatment limits the ability to undertake a statistical analysis of uncertainty.
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 +1,200 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 domestic and commercial waste water treatment has been updated following the development of a national treatment facility database in 2006. New total organic product data for 2006 was used to recalculate CH4 emissions from 2002–2005, resulting in a decrease of estimated emissions of 0.8 Gg CH4 for 2005 (Beca, 2007).
Improvements to the accuracy of calculations for emissions of CH4 from industrial waste water 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 2005.
8.3.6 Source-specific planned improvements
No improvements are planned for this category.
8.4 Waste incineration (CRF 6C)
In 2006, waste incinerations accounted for 5.1 Gg CO2-e (0.3 per cent) of waste emissions. This is a decrease of 9.3 Gg CO2-e (64.5 per cent) from the 1990 level of 14.5 Gg CO2-e.
New Zealand has previously not estimated emissions from waste incineration as they were considered to be negligible. 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, and these practices have been declining since the early 1990’s due to introduced environmental regulations and alternative technologies, primarily sterilisation techniques. Resource consents under New Zealand’s Resource Management Act control non-greenhouse gas emissions from these incinerators.
In 2004, New Zealand introduced national environmental standards for air quality. The standards effectively require all existing low temperature waste incinerators in schools and hospitals to obtain resource consent by 2006, irrespective of existing planning rules. Incinerators without consents will be prohibited.
The IPCC methodology (2006) IPCC Guidelines for National Greenhouse Gas Inventories, Volume 2, Chapter 2: Stationary Combustion, Volume 1, Chapter 3: Uncertainties and Volume 5, Chapter 5: Incineration and Open Burning of Waste was used to estimate direct greenhouse gas emissions of CO2, CH4 and N2O. The 2006 IPCC guidelines (IPCC, 2006) were used to calculate emissions from the incineration of waste as the revised 1996 IPCC guidelines (IPCC, 1996) did not contain methodologies 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 from paper, cardboard and food is excluded in accordance with the 2006 IPCC guidelines (IPCC, 2006). Also excluded were emissions from waste to energy incineration facilities, which are reported within the energy sector of the national greenhouse gas inventory.
Default compositional values from Volume 5 Waste: Chapter 2: Table 2.6 of IPCC 2006 were used to estimate the fossil fuel derived carbon. These were 27.5% for hazardous waste (being the mean of the recommended range) and 25% for clinical waste.
Many incinerators were quarantine waste incinerators. The 2006 IPCC guidelines (IPCC, 2006) do not have a default category for quarantine incinerators. Only three default classifications were available: clinical waste, hazardous waste or sewage sludge. None of these categories appropriately represents quarantine waste, which 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 were made using the default Tier 1 emission calculations (IPCC, 2006). Default factors for a Tier 1 assessment were used in the calculation of CO2 and N2O. Default emission factors for the calculation of CH4 were taken from the 2006 IPCC guidelines (Volume 2, Chapter 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 a annul basis. Estimates based on consented limits are likely to be over estimates 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 2006.
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 been 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 was 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, which meant that activity data had to be interpolated and extrapolated. This could have lead to inaccuracies in the quantification of the total waste incinerated annually. There was generally no detailed information about the actual composition of the waste incinerated; only the consented types of waste allowed.
8.4.5 Source-specific planned improvements
No improvements are planned for this category.
3 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. The inventory assumes that by 2010 all solid waste will be disposed to “managed” solid waste disposal sites, which has lead to a linearly increasing Methane Correction Factor in Lo calculations.
4 An exception is the Christchurch sewage treatment plant that uses anaerobic lagoons for sludge treatment. Based on volatile solids reduction measurements in the lagoons they estimate CH4 production of 0.46 Gg/year plus an additional 0.16 Gg/year from unburned CH4 from the digester-gas fuelled engines.