Introduction

Wastewater can be a source of methane (CH4) when treated or disposed anaerobically. It can also be a source of nitrous oxide (N2O) emissions. Carbon dioxide (CO2) emissions from wastewater are not considered in the IPCC Guidelines because these are of biogenic origin and should not be included in national total emissions. Wastewater originates from a variety of domestic, commercial and industrial sources and may be treated on site (uncollected), sewered to a centralized plant (collected) or disposed untreated nearby or via an outfall. Domestic wastewater is defined as wastewater from household water use, while industrial wastewater is from industrial practices only.1 Treatment and discharge systems can sharply differ between countries. Also, treatment and discharge systems can differ for rural and urban users, and for urban high income and urban low-income users.

Sewers may be open or closed. In urban areas in developing countries and some developed countries, sewer systems may consist of networks of open canals, gutters, and ditches, which are referred to as open sewers. In most developed countries and in high-income urban areas in other countries, sewers are usually closed and underground. Wastewater in closed underground sewers is not believed to be a significant source of CH4. The situation is different for wastewater in open sewers, because it is subject to heating from the sun and the sewers may be stagnant allowing for anaerobic conditions to emit CH4. (Doorn et al, 1997).

The most common wastewater treatment methods in developed countries are centralized aerobic wastewater treatment plants and lagoons for both domestic and industrial wastewater. To avoid high discharge fees or to meet regulatory standards, many large industrial facilities pre-treat their wastewater before releasing it into the sewage system. Domestic wastewater may also be treated in on-site septic systems. These are advanced systems that may treat wastewater from one or several households. They consist of an anaerobic underground tank and a drainage field for the treatment of effluent from the tank. Some developed countries continue to dispose of untreated domestic wastewater via an outfall or pipeline into a water body, such as the ocean.

The degree of wastewater treatment varies in most developing countries. In some cases industrial wastewater is discharged directly into bodies of water, while major industrial facilities may have comprehensive in-plant treatment. Domestic wastewater is treated in centralized plants, pit latrines, septic systems or disposed of in unmanaged lagoons or waterways, via open or closed sewers. In some coastal cities domestic wastewater is discharged directly into the ocean. Pit latrines are lined or unlined holes of up to several meters deep, which may be fitted with a toilet for convenience. Figure 6.1 shows different pathways for wastewater treatment and discharge.

Centralized wastewater treatment methods can be classified as primary, secondary, and tertiary treatment. In primary treatment, physical barriers remove larger solids from the wastewater. Remaining particulates are then allowed to settle. Secondary treatment consists of a combination of biological processes that promote biodegradation by micro-organisms. These may include aerobic stabilisation ponds, trickling filters, and activated sludge processes, as well as anaerobic reactors and lagoons. Tertiary treatment processes are used to further purify the wastewater of pathogens, contaminants, and remaining nutrients such as nitrogen and phosphorus compounds. This is achieved using one or a combination of processes that can include maturation/polishing ponds, biological processes, advanced filtration, carbon adsorption, ion exchange, and disinfection.

Sludge is produced in all of the primary, secondary and tertiary stages of treatment. Sludge that is produced in primary treatment consists of solids that are removed from the wastewater and is not accounted for in this category. Sludge produced in secondary and tertiary treatment results from biological growth in the biomass, as well as the collection of small particles. This sludge must be treated further before it can be safely disposed of. Methods of sludge treatment include aerobic and anaerobic stabilisation (digestion), conditioning, centrifugation, composting, and drying. Land disposal, composting, and incineration of sludge is considered in Volume 5, Section 2.3.2 in Chapter 2, Waste Generation, Composition, and Management Data, Section 3.2 in Chapter 3, Solid Waste Disposal, Section 4.1 in Chapter 4, Biological Treatment and Disposal, and Chapter 5, Incineration and Open Burning of Waste, respectively. Some sludge is incinerated before land disposal. N2O emissions from sludge and wastewater spread on agricultural land are considered in Section 11.2, N2O emissions from managed

1 Because the methodology is on a per person basis, emissions from commercial wastewater are estimated as part of domestic wastewater. To avoid confusion, the term municipal wastewater is not used in this text. Municipal wastewater is a mix of household, commercial and non-hazardous industrial wastewater, treated at wastewater treatment plants.

soils, in Chapter 11, N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application, in Volume 4 of the Agriculture, Forestry, and Other Land Use (AFOLU) Sector.

Figure 6.1 Wastewater treatment systems and discharge pathways

Figure 6.1 Wastewater treatment systems and discharge pathways

Note: Emissions from boxes with bold frames are accounted for in this chapter.

Methane( CH4 )

Wastewater as well as its sludge components can produce CH4 if it degrades anaerobically. The extent of CH4 production depends primarily on the quantity of degradable organic material in the wastewater, the temperature, and the type of treatment system. With increases in temperature, the rate of CH4 production increases. This is especially important in uncontrolled systems and in warm climates. Below 15°C, significant CH4 production is unlikely because methanogens are not active and the lagoon will serve principally as a sedimentation tank. However, when the temperature rises above 15°C, CH4 production is likely to resume.

The principal factor in determining the CH4 generation potential of wastewater is the amount of degradable organic material in the wastewater. Common parameters used to measure the organic component of the wastewater are the Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). Under the same conditions, wastewater with higher COD, or BOD concentrations will generally yield more CH4 than wastewater with lower COD (or BOD) concentrations.

The BOD concentration indicates only the amount of carbon that is aerobically biodegradable. The standard measurement for BOD is a 5-day test, denoted as BOD5. The term 'BOD' in this chapter refers to BOD5. The COD measures the total material available for chemical oxidation (both biodegradable and non-biodegradable). 2 Since the BOD is an aerobic parameter, it may be less appropriate for determining the organic components in anaerobic environments. Also, both the type of wastewater and the type of bacteria present in the wastewater influence the BOD concentration of the wastewater. Usually, BOD is more frequently reported for domestic wastewater, while COD is predominantly used for industrial wastewater.

2 In these guidelines, COD refers to chemical oxygen demand measured using the dichromate method. (American Public Health Association, American Water Works Association and Water Environment Federation, 1998)

Nitrous Oxide (N2O)

Nitrous oxide (N2O) is associated with the degradation of nitrogen components in the wastewater, e.g., urea, nitrate and protein. Domestic wastewater includes human sewage mixed with other household wastewater, which can include effluent from shower drains, sink drains, washing machines, etc. Centralized wastewater treatment systems may include a variety of processes, ranging from lagooning to advanced tertiary treatment technology for removing nitrogen compounds. After being processed, treated effluent is typically discharged to a receiving water environment (e.g., river, lake, estuary, etc.). Direct emissions of N2O may be generated during both nitrification and denitrification of the nitrogen present. Both processes can occur in the plant and in the water body that is receiving the effluent. Nitrification is an aerobic process converting ammonia and other nitrogen compounds into nitrate (NO3-), while denitrification occurs under anoxic conditions (without free oxygen), and involves the biological conversion of nitrate into dinitrogen gas (N2). Nitrous oxide can be an intermediate product of both processes, but is more often associated with denitrification.

Treatment and Discharge Systems and CH4 and N2O Generation Potential

Treatment systems or discharge pathways that provide anaerobic environments will generally produce CH4 whereas systems that provide aerobic environments will normally produce little or no CH4. For example, for lagoons without mixing or aeration, their depth is a critical factor in CH4 production. Shallow lagoons, less than 1 metre in depth, generally provide aerobic conditions and little or no CH4 is likely to be produced. Lagoons deeper than about 2-3 metres will generally provide anaerobic environments and significant CH4 production can be expected.

Table 6.1 presents the main wastewater treatment and discharge systems in developed and developing countries, and their potentials to emit CH4 and N2O.

Table 6.1

CH4 and N2O emission potentials for wastewater and sludge treatment and discharge systems

Types of treatment and disposal

CH4 and N2O emission potentials

Collected

Untreated

River discharge

Stagnant, oxygen-deficient rivers and lakes may allow for anaerobic decomposition to produce CH4.

Rivers, lakes and estuaries are likely sources of N2O.

Sewers (closed and under ground)

Not a source of CH4/N2O.

Sewers (open)

Stagnant, overloaded open collection sewers or ditches/canals are likely significant sources of CH4.

Treated

Aerobic treatment

Centralized aerobic wastewater treatment plants

May produce limited CH4 from anaerobic pockets.

Poorly designed or managed aerobic treatment systems produce CH4.

Advanced plants with nutrient removal (nitrification and denitrification) are small but distinct sources of N2O.

Sludge anaerobic treatment in centralized aerobic wastewater treatment plant

Sludge may be a significant source of CH4 if emitted CH4 is not recovered and flared.

Aerobic shallow ponds

Unlikely source of CH4/N2O.

Poorly designed or managed aerobic systems produce CH4.

Anaerobic treatment

Anaerobic lagoons

Likely source of CH4. Not a source of N2O.

Anaerobic reactors

May be a significant source of CH4 if emitted CH4 is not recovered and flared.

Uncollected

Septic tanks

Frequent solids removal reduces CH4 production.

Open pits/Latrines

Pits/latrines are likely to produce CH4 when temperature and retention time are favourable.

River discharge

See above.

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