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Chapter 8: Waste
8.1 Sector overview
The waste sector totalled 2394.18 Gg CO2 equivalent in 2002 and represented 3.2% of all greenhouse gas emissions. Emissions in 2002 are now 17.7% below the 1990 baseline value of 2,908.46 Gg CO2 equivalent (Figure 8.1.1). The reduction has occurred in the solid waste disposal on land category, which has decreased by 20.7% as a result of initiatives to improve solid waste management practices in New Zealand.
Emissions from the waste sector are calculated in three components (Figure 8.1.2): solid waste disposal on land, wastewater handling and waste incineration (negligible emissions). CH4 from solid waste disposal was identified as a key source category for New Zealand in 2002 (Tables 1.5.2 and 1.5.3).
Disposal and treatment of industrial and municipal waste can produce emissions of CO2, CH4, and NMVOC. The CO2 is produced from the decomposition of organic material, however these emissions are not included as a net emission as 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)
8.2.1 Description
Organic waste in solid waste disposal sites (SWDS) is broken down by bacterial action in a series of stages that result in the formation of CO2 and CH4. The amount of gas produced depends on a number of factors including the waste disposal practices (managed vs. unmanaged landfills), the composition of the waste, and physical factors such as the moisture content and temperature of the SWDS. 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 SWDS 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 the Resource Management Act. As a result of these initiatives, a number of poorly located and substandard landfills have been closed and communities increasingly rely on modern regional disposal facilities for disposal of their solid waste.
Recently, the national focus has been towards waste minimization and resource recovery. In March 2002, the Government announced its New Zealand Waste Strategy (MfE, 2002). The strategy sets targets for a range of waste streams as well as improving landfill practices by the year 2010.
8.2.2 Methodological issues
New Zealand has used both the IPCC Tier 1 and Tier 2 methods to calculate emissions from solid waste (SCS Wetherill Environmental, 2002). The data reported in the CRF follow the IPCC Tier 2, first order decay methodology (IPCC, 2000). New Zealand uses country specific values for the degradable organic carbon factor (DOC), 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).
Data on municipal solid waste (MSW) generation rates, waste composition and the percentage of MSW disposed to SWDS are obtained from the National Waste Data Report (MfE, 1997) and the Waste Analysis Protocol (WAP; MfE, 1992) surveys for the period 1993 to 1995. Based on the 1995 data, every person in New Zealand sends approximately 1.08 kg of residential waste per day to SWDS. Industrial waste that is landfilled averages approximately 1.31 kg per person per day. As a result, it is estimated that the total quantity of solid waste that is landfilled in New Zealand is 2.39 kg per person per day [ It has been noted that the results for New Zealand are higher than for other OECD nations. The residential result for New Zealand includes 'bulky waste' associated with garden and home renovations. In addition, the industrial result for New Zealand includes construction and demolition waste. These two categories are counted separately by most other OECD nations. ] . This value is used in the calculations for all years.
The proportion of waste for each type of SWDS is obtained from the 1995 National Landfill Census. It is estimated that 90% of New Zealand's waste is disposed to managed SWDS and 10% to uncategorised sites (MfE, 1997). The fraction of DOC is calculated using waste composition data from the National Waste Data Report (MfE, 1997) and the Waste Analysis Protocol (WAP; MfE, 1992) surveys for the period 1993 to 1995. The IPCC (1996) default values are used for the carbon content of the various components. The DOC estimate also includes an estimate of the quantity of wood waste in the construction and demolition waste stream. Calculation of the methane generation potential is also based on the New Zealand waste statistics.
A methane generation rate constant of 0.06 was selected 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 SWDS in New Zealand (SCS Wetherill Environmental, 2002).
The fraction of DOC 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. These figures are consistent with good practice (IPCC, 2000).
The recovered CH4 rate per year was estimated based on information from a previous survey of SWDS that serve populations of over 20,000 in New Zealand. The survey information was updated based on local knowledge and experience (SCS Wetherill Environmental, 2002).
8.2.3 Uncertainties and time-series consistency
The overall estimated level of uncertainty is estimated at the ±35 percent reported in previous inventories (SCS Wetherill Environmental, 2002). 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 determine uncertainty levels. Uncertainty in data are primarily from uncertainty in waste statistics based on the National Waste Data Report dated 1997, waste composition and how changes in waste management practices can alter the composition of waste to SWDS, the methane generation constant and the recovered methane rate.
The New Zealand waste composition categories from the National Waste Data Report do not exactly match the categories required for the IPCC DOC calculation. The major difference is that in New Zealand's DOC calculation, the garden waste category also includes food waste. A separation into the IPCC categories was not feasible given the available data in the National Waste Data Report. This effect of this difference is mediated by the use of IPCC default carbon contents which are similar for the non-food (17% carbon content) and food categories (15% 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. For the 2002 inventory, the Tier 2 value of gross annual methane generation is 160.6 Gg CH4 and the Tier 1 value is 171.4 Gg CH4. The assumptions used to calculate net CH4 emissions from gross CH4 are the same for Tier 1 and 2.
CH4 from solid waste disposal was identified as a key source category for New Zealand in 2001. In preparation of the 2002 inventory, the data underwent Tier 1 QC procedures (refer Annex 6 for examples).
8.2.5. Source-specific recalculations
There have been no changes introduced to the methodology for estimating greenhouse gas emissions in the solid waste disposal on land sector since the IPCC Tier 2 approach was introduced in the 2000 inventory. The emissions for 1990-2001 are recalculated based on improved population data from Statistics New Zealand.
8.2.6 Source-specific planned improvements
New Zealand will consider reviewing the data and assumptions used for calculations in the solid waste disposal on land category in 2004. Updated information will be included in New Zealand's next submission.
8.3 Wastewater handling (CRF 6B)
8.3.1 Description
Wastewater from virtually all towns 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 around 50 government or privately owned treatment plants serving more than 100 people.
While 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.
Very large quantities of high-strength industrial wastewater are produced by New Zealand's primary industries. Most of the treatment uses aerobic treatment and any CH4 from anaerobic treatment is flared. There are however 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.
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 has been assessed for each municipal treatment plant in New Zealand. Where industrial wastewater flows to a municipal wastewater treatment plant, an equivalent population for that industry has been calculated based on a BOD (Biological Oxygen Demand) 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 BOD to the plant that is anaerobically degraded multiplied by the CH4 conversion factor. The emission factors are shown in the worksheets accompanying this chapter.
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 BOD value is based on a 2.5 scaling factor on COD (IPCC, 2001). New Zealand uses a Bo of 0.25 CH4 / kg COD but calculates a country-specific value of 0.375 kg CH4 / kg BOD, based on a scaling-up factor of 1.5*COD. The New Zealand scaling factor is based on information from New Zealand waste sector experts (SCS Wetherill Environmental, 2002) and research (Metcalf and Eddy, 1991).
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 COD load going to the treatment plant and the proportion of waste degraded anaerobically (refer to the accompanying worksheets). 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 landfilled. Any CH4 emissions from landfilled sludge are reported under the SWDS 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 [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 a year 2001 CH4production of 0.46 Gg/year plus an additional 0.16 Gg/year from unburned CH4from the digester-gas fuelled engines.] . 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 twenty 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 cases it has been included in the inventory for that plant. Where sludge is landfilled, the CH4 production has been included under solid waste disposal. 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 is 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 COD load from the CH4 emission calculations and using a ratio of COD to nitrogen in the wastewater for each industry. Refer to the accompanying worksheets in this chapter for details.
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. 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% to +60% (SCS Wetherill Environmental, 2002).
Methane from industrial wastewater
The method used in estimating CH4 emissions negates a statistical analysis of uncertainty.
Total CH4 production from industrial wastewater has an estimated accuracy of ± 40% 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 and no attempt has been made to quantify this uncertainty. The IPCC default emissions factor, EF6, has an uncertainty of -80% to +1,200% (IPCC, 1996) meaning that the estimates have only an order of magnitude accuracy.
8.3.4 Source-specific QA/QC and verification
There was no specific QA/QC performed on the wastewater handling category in the 2002 inventory.
8.3.5 Source-specific recalculations
There have been no recalculations in the 2002 inventory. The major change was in the 2000 inventory, where the calculations for CH4 emissions from industrial wastewater handling were changed to COD from BOD. This change was because COD is the preferred IPCC measure (IPCC, 1996) and reliable COD measurements were available from the meat industry - the largest source of industrial wastewater CH4 emissions.
8.4 Waste incineration (CRF 6C)
8.4.1 Description
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 waste (five incinerators), veterinary and laboratory waste (6 incinerators), quarantine waste (three incinerators) and hazardous waste (two incinerators). Most regional and territorial councils have banned the open burning of waste.
In 2003, New Zealand's MfE proposed national environmental standards for the discharge of potentially harmful contaminants into the air. Once the proposed standards come into force, all existing low temperature waste incinerators in schools and hospitals will require a resource consent (authorisation to operate) by 2008, irrespective of existing planning rules. Such incinerators without consents will be prohibited. The MfE will work with the Ministries of Education and Health to encourage other forms of waste disposal in schools and hospitals.
Worksheets accompanying the waste sector
The worksheets for the waste sector document the underpinning data (population, solid waste disposed to landfills, system of wastewater treatment etc), the emission factors, variables and calculations used to collate emissions from the waste sector.
