By: ALVARO OROZCO JARAMILLO CONSULTANT ENGINEER Professor (retired) Universidad de los Andes aorozcos1@une.net.co
CONTENIDO: 1. INTRODUCTION 2. GLOBAL WARMING AND GREENHOUSE GASES (GHG) 3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 4. IMPACT OF WWTP ON GLOBAL WARMING 5. CONCLUSIONS 6. BIBLIOGRAPHY
1. INTRODUCTION Global warming is the increase in the average temperature of the Earth's nearsurface air and oceans since the mid 20th century and its projected continuation. Global warming is very likely due to the observed increase in anthropogenic GHG concentrations (mostly CO 2 ) generated by fossil fuels burning (carbon, oil, natural gas, etc.). Other GHG (CH4, N2O, SF 6 ) emissions also contributes to global warming. Water vapor is also a GHG but its concentration depends on Earth s temperature and is considered in mathematically based Global Climate Models (GCM) as the water vapor feedback. The major non gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the greenhouse gases. Earth's most abundant greenhouse gases are: (i) water vapor, which contributes 36 70%; (ii) carbon dioxide, which contributes 9 26%; (iii) methane, which contributes 4 9%; and (iv) ozone, which contributes 3 7%.
1. INTRODUCTION The Intergovernmental Panel on Climate Change (IPCC) is a scientific intergovernmental body tasked to evaluate the risk of climate change caused by human activity. The panel was established in 1988 by the World Meteorological Organization(WMO) and the United Nations Environmental Programme (UNEP), two organizations of the United Nations. The IPCC shared the 2007 Nobel Peace Prize with Al Gore. The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change (UNFCCC or FCCC) produced at the United Nations Conference on Environment and Development (UNCED), (also known as the Earth Summit), held in Rio de Janeiro (Brazil) in June 1992. The treaty is intended to achieve "stabilization of green house gas (GHG) concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system
1. INTRODUCTION Wastewater Treatment Plant (WWTP) practice only considers as GHG the CH 4 (produced by sludge and wastewater anaerobic treatment) and N 2 O (produced by nitrification, denitrification and, in a minor scale, activated sludge treatment). The 100 years global warming potential (GWP) of CH4 is 23 times higher than that of CO 2 (1 t CH4 = 23t equivalent CO 2 or CO 2 e) and is the GHG more important in WWTP. The 100 years global warming potential (GWP) of N 2 O is 320 times higher than that of CO 2 (1 t N 2 O = 296 t CO 2 e). Nitrous oxide (N 2 O) may be generated by nitrification and denitrification processes during biological treatment of the wastewater. Direct discharges of wastewater into water masses will produce N2O anyway and should be account for as indirect emissions. CO 2 emission in wastewater treatment processes was found to be neutral in terms of global warming due to its biogenic origin, and is not considered by IPCC as a global warming contributor by WWTP. WWTP accounts for less than 0.5% of the total GHG emissions worldwide. In any case, WWTP practices should incorporate methods to control GHG.
2. GLOBAL WARMING Sun is the Earth s main source of energy. There are others (e.g. geothermic sources) but they are minute in comparison with solar radiation. Earth s temperature is the resultant of: (i) incoming solar radiation (S) mostly irradiated in the range of visible light of the electromagnetic spectrum, minus the fraction (α) which is reflected by polar caps and clouds. This fraction is called the terrestrial albedo; (ii) the emission of terrestrial radiation, which depend on mean Earth s temperature. GHG are transparent to short wave radiation (99% emitted by the Sun) and opaque to long wave radiation (99% emitted by Earth)
2. GLOBAL WARMING How much is the solar incoming radiation?: On the average the solar incoming radiation is 1370 W/m 2. This radiation coming through the Earth s projected circle is distributed on the whole area of Earth s sphere which is four times greater. In other words, the average radiation arriving to Earth surface is 1370/4 = 342 W/m 2. However, of the total solar incoming radiation 30% is reflected by the polar caps, clouds, etc. This percentage in decimal form (0.3) is the terrestrial albedo (α).
2. GLOBAL WARMING How much heat is re radiated by Earth?: Assuming that Earth is in thermal equilibrium, then the incoming Sun radiation (heat) should be equal to Earth s radiation as a black body (Stefan Boltzmann s Law). Solving the equilibrium equation for a terrestrial albedo α = 0.3 obtain an Earth s average temperature of 18 o C. It seems a bit cold! (real Average surface temperature is 15 C). Is the physics wrong? No, it s just incomplete. To obtain the right answer it is necessary to have into account the Greenhouse Effect.
2. GLOBAL WARMING What happens then? Because of its temperature, the Earth s surface emits radiation in the 4.0 to 100.0 μm region. Most of this is absorbed by greenhouse gases (GHG). But the atmosphere is at a similar temperature, so these gases will re emit much of this radiation, some to space, but more back to the surface, making the surface warmer. This is known as the Greenhouse Effect. Taking into account the Greenhouse Effect, the GHG reflection from the atmosphere back to Earth is 155 W/m 2 and solving again the thermal balance equation we found that the mean temperature is 15ºC, in agreement with reality. The Greenhouse Effect is a natural phenomenon and without it life on Earth would be inexistent! The GHG have existed for millions years.
2. GLOBAL WARMING
2. GLOBAL WARMING In the long range, solar radiation is also affected by Milankovitch cycles, due to position changes of the planet respect to the Sun.
2. GLOBAL WARMING
2. GLOBAL WARMING There are two ways of measuring the greenhouse effect: (i) The first is the 33 C difference between the effective temp ( 18 C) and the actual (average) surface temp (15 C). (ii) The second is the difference between the 390 Wm 2 surface emission and the 237 Wm 2 emission to space: 153 Wm 2 (iii) about 50, Of this 153 Wm 2, H 2 O vapor accounts for about 95, CO 2 for and N 2 O, CH 4, O 3 and CFCs about 2 each. The interesting question which now confronts us is: How are these numbers changing, as a result of our actions?
2. GLOBAL WARMING
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS Wastewater is a source of methane (CH 4 ) when treated or disposed of anaerobically. It can also be a source of nitrous oxide (N 2 O) emissions. Carbon dioxide (CO 2 ) emissions from wastewater are not considered in the IPCC Guidelines because these are of biogenic origin and should not be included in inventories of emissions. Wastewater originates from a variety of domestic, commercial and industrial sources and may be treated: (i) on site (uncollected), (ii) sewered to a centralized plant (collected) or (iii) 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. The most common wastewater treatment methods in developed countries are centralized aerobic wastewater treatment plants and lagoons for both domestic and industrial wastewater. The degree of wastewater treatment varies in most developing countries. Domestic wastewater is treated in centralized plants, pit latrines, septic systems or disposed of in unmanaged lagoons or waterways, via open or closed sewers. Presently, anaerobic treatment in warm climate is increasingly used.
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 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 microorganisms. These may include aerobic stabilization 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
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 3.1 N 2 O EMISSION FACTOR (EF) Nitrous oxide (N 2 O) is associated with the degradation of nitrogen components in the wastewater, e.g., urea, nitrate and protein. Direct emissions of N 2 O may be generated during both nitrification and denitrification of the nitrogen present. Both processes can occur in the plant and in the effluent receiving water body. Nitrification is an aerobic process converting ammonia and other nitrogen compounds into nitrate (NO 3 ) Denitrification occurs under anoxic conditions (without free oxygen), and involves the biological conversion of nitrate into nitrogen gas (N 2 ). Nitrous oxide can be an intermediate product of both processes, but is more often associated with denitrification. N 2 O emissions should be converted into CO 2 equivalent. (1 t N 2 O = 296 t CO 2 e)
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 3.1.2 Emissions from advanced centralized wastewater treatment plants are typically much smaller than those from effluent and may only be of interest for centralized wastewater treatment plants with controlled nitrification and denitrification steps. The overall emission factor (EF) to estimate N 2 O emissions from such plants is 3.2 g N 2 O/person/year. If there are commercial and industrial discharges a default factor (F ind/com ) of 1,25, should be applied: (kg N 2 O /yr) WWTP =P x 3,2 kg N 2 O /person.yr x F ind/com = P x 4 kg N 2 O /person.yr where P is the population equivalent treated at the WWTP
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 3.2 CH 4 EMISSION FACTOR (EF) Wastewater as well as its sludge components can produce CH 4 if it degrades anaerobically. The extent of CH 4 production depends primarily on the quantity of degradable organic material (DOM) in the wastewater. The temperature, and the type of treatment system. With increases in temperature, the rate of CH 4 production increases. This is especially important in uncontrolled systems and in warm climates. Below 15 C, significant CH 4 production is unlikely because methanogens are not active and the lagoon will serve principally as a sedimentation tank. 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. CH 4 production should be converted into CO 2 e (1 t CH 4 = 23 t CO 2 e)
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 3.2.1 The IPCC general equation to estimate CH 4 emissions from domestic wastewater is as follows: CH 4 Emissions =[ (U i T i,j EF j )] (TOW S ) R CH 4 Emissions = CH 4 emissions in inventory year, kg CH 4 /yr TOW= total organics in wastewater in inventory year, kg BOD/yr S = organic component removed as sludge in inventory year, kg BOD/yr U i = fraction of population in income group i in inventory year. degree of utilization of treatment/discharge pathway or system, j, for each income group T i,j = fraction i in inventory year. i= income group: rural, urban high income and urban low income j= each treatment/discharge pathway or system R = amount of CH 4 recovered in inventory year, kg CH4/yr EF j = emission factor, kg CH 4 / kg BOD
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 3.2.2 The IPCC proposes the following equation for calculating the CH 4 Emission Factor of anaerobic WWTP: kg CH 4 = P * DOM * (1 % DOM disposed of as sludge) * (CH 4 ) max * MCF kg (CH 4 ) burned To calculate the EF from sludge treatment, the IPCC proposed the following equation: kg CH 4 = P * DOM* % DOM disposed of as sludge * (CH 4 ) max * MCF kg (CH 4 ) burned In both equations, DOM is the degradable organic matter per capita (as BOD). A default value of DOM= 0.05 kg DBO5/person.d, is suggested unless the country real value is known. The good practice IPCC guide proposes a maximum production of 0.6 kg CH4/kgDBO. The correction factor for methane production depends on treatment type and affect the proposed maximum production (see table below)
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS
3. WASTEWATER TREATMENT PLANT EMISSION FACTORS 3.3 INDIRECT EMISSIONS For the fully aerobic process, up to 1.4 kg CO 2 /kg COD removed originates from power generation needed for aeration. This means that considerably more CO 2 is produced in power generation needed for aeration than in the actual treatment process, and all of this is typically from fossil fuels, whereas the energy from the wastewater pollutants comes primarily from renewable energy sources (food, i.e. biogenic origin ). A change from anaerobic to aerobic sludge treatment processes (for the same aerobic main process) has a massive impact on the CO 2 production from fossil fuels: an additional 0.8 kg CO 2 /kg COD removed is produced by changing to aerobic sludge digestion. In terms of GHG production, the total output (in CO 2 equivalents) can be reduced from 2.4 kg CO 2 /kg COD removed for fully aerobic treatment to 1.0 kg CO 2 /kg COD removed for primarily anaerobic processes. Major advantages can be gained by using primarily anaerobic processes as it is possible to largely eliminate any net energy input to the process, and therefore the production of GHG from fossil fuels.
4. IMPACT OF WWTP ON GLOBAL WARMING
4. IMPACT OF WWTP ON GLOBAL WARMING
4. IMPACT OF WWTP ON GLOBAL WARMING
4. CONCLUSIONS Wastewater Treatment Plants (WWTP) contributes 21% of total CO 2 e from sector Waste, which is the 2.3 % of total CO 2 e USA GHG emissions. This account for less than 0.5% of USA greenhouse gas emissions. Wastewater as well as its sludge components can produce CH 4 if it degrades anaerobically. Nitrous oxide (N 2 O) is associated with the degradation of nitrogen components in the wastewater, e.g., urea, nitrate and protein. Direct emissions of N 2 O 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. Carbon dioxide (CO 2 ) emissions from wastewater are not considered in the IPCC Guidelines because these are of biogenic origin and should not be included in inventories of emissions.
4. CONCLUSIONS More CO 2 is produced in power generation needed for aeration than in the actual treatment process, and all of this is typically from fossil fuels, whereas the energy from the wastewater pollutants comes primarily from renewable energy sources (food, i.e. biogenic origin ). Major advantages can be gained by using primarily anaerobic processes as it is possible to largely eliminate any net energy input to the process, and therefore the production of GHG from fossil fuels. Burning produced CH 4 in WWTP additionally reduces de CO 2 e emissions due that the Potential Global Warming of 1 t CH 4 = 23 t CO 2 e.
7. BIBLIOGRAFÍA Orozco, A. (2007) La lucha contra el calentamiento global: un conflicto de intereses, Corporación Otraparte, Medellín. Orozco, Álvaro (2005), Bioingeniería de aguas residuales: teoría y diseño, ACODAL, Bogotá, Colombia. METCALF & EDDY (2003), Wastewater engineering: treatment and reuse, 4th Edition, McGraw Hill Co. http://unfccc.int/files/meetings/cop_13/application/pdf/cp_bali_action.pdf http://themes.eea.europa.eu/sectors_and_activities/energy/indicators/en01,2007.04/fig1.gif/view Yochanan Kushnir, The Green House Effect and the two dimensional pattern of Earth s Radiation Budget, Greehouse pdf http://en.wikipedia.org/wiki/milankovich_effect http://www.abelard.org/briefings/global_warming_atmospheric_chemistry_physics.php. Thomsen Marianne y Erik Lyck (2005). Emission of CH 4 and N 2 O from Wastewater Treatment Plants (6B). IPCC, WMO, UNEP (2006), 2006 IPCC guidelines for national greenhouse gas inventories, Volume 5. Dautremont Smith, J (2002); en Guidelines for College Level Greenhouse Gas Emissions Inventories UNFCCC/CCNUCC (1996), Approved baseline and monitoring methodology AM0039: Methane emissions reduction from organic waste water and bioorganic solid waste using composting. FAO (2006), Food Security Statistics Colombia Keller J. and Hartley K. (2002), Greenhouse gas production in wastewater treatment: process selection is the major factor, Water science and technology ISSN 0273 1223 CODEN WSTED4, London, UK. EPA (2008), Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 2006, USA. Daiger, G, R. Peterson, J. Whiterspoon and E. Allen (2000), Impact of global warming concerns on wastewater treatment plant design and operation, ASCE, USA.