8.1 Sector overview
The waste sector totalled 1,847.1 Gg CO2-e in 2005 and this represented 2.4 per cent of national greenhouse gas emissions. Emissions in 2005 are now 645.6 Gg CO2-e (25.9 per cent) below the 1990 baseline value of 2,492.8 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.
Gg CO2 equivalent (thousands)
Emissions from the waste sector are calculated from solid waste disposal on land, wastewater handling (Figure 8.1.2) and waste incineration (not shown in Figure 8.1.2 as emissions are negligible). Methane from solid waste disposal was identified as a key category for New Zealand in 2005 (tables 1.5.2 and 1.5.3).
Gg CO2 equivalent
Percent of total
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 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 most important gas is the CH4 produced as a by-product of anaerobic decomposition.
8.2 Solid waste disposal on land (CRF 6A)
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. Carbon dioxide from aerobic decomposition is not reported in the inventory. 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 them in landfills. In 1995, a National Landfill Census showed there were 327 legally operating landfills or solid waste disposal site in New Zealand that accepted approximately 3,180,000 tonnes of solid waste (MfE, 1997). Since that time 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 national focus has been towards waste minimisation and resource recovery. In March 2002, the Government announced its New Zealand Waste Strategy (MfE, 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 (MfE, 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 both the IPCC Tier 1 and Tier 2 approaches to calculate emissions from solid waste. The data reported in the inventory follow the IPCC Tier 2, first order decay approach (IPCC, 2000). New Zealand uses country-specific values for the degradable organic carbon factor, methane generation potential (Lo), and a methane generation rate constant (k) based on conditions at New Zealand landfills. The IPCC default oxidation correction factor of 0.1 is used (IPCC, 2000). Worksheets showing the waste sector calculations are included in Annex 8.
Data on municipal solid waste generation rates, waste composition, the fraction of degradable organic carbon (DOC) and the percentage of municipal solid waste disposed to solid waste disposal sites are obtained from the National Waste Data Report (MfE, 1997), the Landfill Review and Audit (MfE, 2002a), a report on Waste Composition and Construction Waste Data (MfE, 2006b), and the Solid Waste Analysis Protocol (SWAP) baseline results (MfE, 2003); surveys for 1995, 2002 and 2003. The proportion of waste for each type of solid waste disposal site is obtained from the 2003 solid waste disposal sites baseline results. It is estimated that in 1995, 90 per cent of New Zealand’s waste is disposed to managed solid waste disposal sites and 10 per cent to uncategorised sites (MfE, 1997)1. The IPCC (1996) default values are used for the carbon content of the various components. Calculation of the methane generation potential is also based on the New Zealand SWAP baseline results.
Based on the 2002 Landfill Review and Audit, the 2006 report on Waste Composition and Construction Waste Data and using the SWAP classification system, it is estimated that the quantity of solid waste going to landfills in New Zealand in 2005 was equivalent to 2.14 kg per person per day. This shows a reduction in waste generation from 2.35 kg per person per day in 1995.
There has been no new solid waste compositional data for 2004 and 2005, hence degradable organic content per Gg waste has remained constant. However, the methane correction factor has been increasing due to closure of unmanaged landfills and increasing volumes being disposed to larger modern landfills. These inputs have resulted in some increases to the methane generation potential of solid waste to landfills.
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).
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 (WMNZ, 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 2004 inventory, but an improvement on prior submissions. The improvement was due to the sampling and survey guidelines from the Solid Waste Analysis Protocol and the 2002 Landfill Audit and Review. 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 are primarily from uncertainty in waste statistics based on the 1997 National Waste Data Report (total solid waste disposed to landfills and the recovered methane rate).
The New Zealand waste composition categories from the Solid Waste Analysis Protocol 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 Solid Waste Analysis Protocol baseline report. 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).
8.2.4 Source-specific QA/QC and verification
The Tier 1 and Tier 2 approaches have been used for solid waste emission estimates and the gross CH4 results compared, as recommended from the technical review of New Zealand’s greenhouse gas inventory conducted in May 2001 (UNFCCC, 2001c). For the 2005 inventory, the Tier 2 value of gross annual methane generation is 127.6 Gg CH4 and the Tier 1 value is 148.23 Gg CH4. The assumptions used to calculate net CH4 emissions from gross CH4 are the same for both tiers.
CH4 from solid waste disposal was identified as a key category for New Zealand in 1990 and 2005 (level and trend assessment). In preparation for this inventory, the data for this category underwent Tier 1 quality checks.
8.2.5 Source-specific recalculations
Municipal solid waste values for 2003 and 2004 were updated after obtaining new data on solid waste disposed to landfill in Auckland (New Zealand’s largest urban area). The 2003 and 2004 inventory submissions assumed a constant fraction of municipal solid waste disposed at solid waste disposal sites in year x (MSWx), when the new data indicated that in the landfills measured, solid waste to landfill had increased faster than the population size. This new data has lead to increased total MSW tonnage figures in 2003 and 2004.
Textiles were added to the calculation of degradable organic carbon. Data on the proportion of solid waste volumes made up of textile waste was only available for 2003, and has been linearly applied for other years.
The waste generation rate increased for 2004 due to more accurate data becoming available for New Zealand’s largest region which was published in a report on Waste Composition and Construction Waste Data (MfE, 2006).
Food and garden waste are combined in New Zealand’s compositional analysis. This inventory submission revised the degradable organic carbon value associated with this combined category from 0.17 to 0.15GgC/Gg waste. This change was made following the recommendation of the UNFCCC expert review team to be conservative about reductions achieved relative to the 1990 base year.
Recalculations were performed back to 1990 and have resulted in a reduction of 2.7 Gg CH4 in 1990 and an increase of 0.6 Gg CH4 in 2004.
8.2.6 Source-specific planned improvements
There are no specific improvements planned for this category.
8.3 Wastewater handling (CRF 6B)
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
CH4 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. 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. 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. The emissions calculations are shown in Annex 8.
It is good practice to use country-specific data for the maximum methane producing capacity factor (Bo). Where no data are available, the 1996 IPCC methodology recommends 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.
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 (refer to Annex 8). CH4 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 used2. 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).
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 figure 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 all wastewater figures 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).
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).
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 scaling factor to convert chemical oxygen demand to biological oxygen demand was changed to 2.5 (from 1.5). This change was made following the recommendation from the UNFCCC expert review team. Wastewater volume data for 1997 and 2001 were used to extrapolate the data for the entire time-series. These changes resulted in recalculations in estimates of methane emissions from domestic and commercial wastewater treatment back to 1990. Emissions from domestic and commercial wastewater treatment have changed from 4.0 Gg CH4 to 7.1 Gg CH4 in 1990, and 4.0 Gg CH4 to 6.6 Gg CH4 in 2004.
8.3.6 Source-specific planned improvements
A comprehensive database of industrial/commercial and municipal wastewater treatment plants in use in New Zealand was developed during 2006. A subsequent project to update estimates of emissions from wastewater treatment will be performed through 2007. This project will include improvements such as using year-specific average per capita protein intake data and using the same industrial activity data, such as agricultural production, as used in the inventory.
8.4 Waste incineration (CRF 6C)
New Zealand has not estimated emissions from waste incineration as they are 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. Resource consents under New Zealand’s Resource Management Act control non-greenhouse gas emissions from these incinerators. As the quantity of material being disposed through these incinerators is not required to be measured under resource consents, it is not possible to estimate the quantity of greenhouse gas emissions being released.
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.
8.4.2 Source-specific planned improvements
During 2007, the Ministry for the Environment will be contracting a project to estimate emissions from solid waste incineration. Data will be available for the 2008 inventory submission.
1 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.
2 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.