Factores de emission de CH4 para plantas de tratamiento de aguas residuals municipales:

lodos activados

Adalberto NoyolaInstituto de Ingeniería

Universidad Nacional Autónoma de México

• Wastewater treatment may produce methane depending on the chosentechnology and how it is operated.

•Second most abundant GHG after CO2 with a GWP of 28 (now 34 as IPCC, 2013)

•Methane produced from wastewater management accounts for between 8 to11% of overall anthropogenic CH4 emissions*

• Wastewater treatment facilities may be intensive in energy use, depending onchosen technology.

• Electricity requirements for wastewater treatment have an impact on CO2

production at the generation facility (indirect emissions from fossil fuels).

• Energy efficiency, less energy demanding treatment processes or energy fromwaste (co-generation) schemes are the options for reducing indirect CO2

emissions in WWT facilities.

CH4 and CO2 in WWTP

* (Abdulla & Al-Ghazzawi, 2000)

Mexican GHG Inventory, 2015

INECC (2018)https://www.gob.mx/inecc/documentos/investigaciones-2018-2013-en-materia-de-mitigacion-del-cambio-climatico

Waste (6.7%), (30% on CH4)

Wastewater treatment and discharge (3.3%), (14% on CH4), (2.6% on CH4 from municipal sewage)

National commitments on GHG emissions reduction

• National Strategy on Climate Change, 10-20-40 years (SEMARNAT, 2013)• To reduce GHG emissions up to 30% with respect to the business-as-usual

(BAU) scenario by 2020

• To reduce GHG emissions up to 50% with respect to the 2000 emissions levels

Up-date:

• 2016 NDC: to reduce 25% of GHG and SLCP emissions (below BAU) for year 2030 (reduction of 22% of GHG and 51% of Black Carbon)

• Clean electricity generation (clean energies)

202425%

CONAGUA (2016)

2477 Municipal WWT facilities

Collected flow: 212 m3/s

Treated flow: 120.9 m3/s

43%57%

0 0Municipal wastewater treatment in Mexico

treated untreated

Industrial wastewaterTotal: 214.6 m3/sTreated: 70.5 m3/s (33%)2832 WWT facilities

Treated flow:55% aerobic processes (Act. Sludge)11% Facultative ponds

CONAGUA, 2016

5021

42

101

28

9968

32

752 746

10 19

63

140

28 21

7452

14

118

0

100

200

300

400

500

600

700

800

No

. de

PTA

R

Tipos de tecnologías

Las tres principales tecnologías (en número):

- Lagunas de Estabilización

- Lodos Activados

- Reactor Anaerobio de Flujo Ascendente (Upflow Anaerobic Sludge Blanket)

Total: 1729 (70 %)

Tres tecnologías:

- Lodos Activados

- Lagunas de Estabilización

- Dual

Representan el 79% del caudal tratado en México

Municipal WWT facilities with anaerobic digesters

1. Acapatzingo

2. Agua Prieta*

3. Aguascalientes

4. Atotonilco*

5. Cd. Juárez Sur*

6. Chihuahua Norte

7. Chihuahua Sur

8. Colima

9. Cuautitlán

10. Culiacán

11. Dulces Nombres

12. El Ahogado*

13. FIRIOB Veracruz (UASB)

14. Hermosillo*

15. La Paz

16. León*

17. Morelos Tam.

18. Monterrey Norte

19. Paso Limón

20. Reynosa II

21. Saltillo*

22. San Fco del Rincón*

23. San Pedro Mártir*

24. Tanque Tenorio

25. Tierra Negra (Tampico)

26. Toluca Norte

27. Xalapa

28. Pto. Vallarta

29. Puebla Atoyac Sur

30. Puebla Alseseca Sur

3

2

1

67

4

279

10

8

5

15

14

13

12

2524

21

19

1720

11

18

22

2316

26

Adapted from GIZ (2017) and IMTA (2017)

28

2930

In red: WWTP with sludge anaerobic digestersIn black: Candidates to adopt sludge anaerobic digestión* Biogas recovery for energy production (9)

Comparison of five mitigation scenarios for

municipal WWTP in Mexico

centro

molinamariocentro

molinamariocentro

molinamariomolinamario

7,000

9,000

11,000

13,000

15,000

1990 1995 2000 2005 2010 2015 2020 2025 2030

Escenario Base Escenario A2030 Escenario B1 Escenario B2 Escenario B3 B4 EBBase scenario Mexican Water Agenda (WA) 2030

WA 1 WA 2 WA 2z WA 2e

34%

23%

14%

10%

Base

6%

Year

Gg

of

CO

2eq

Noyola et al. (2016)

Most attractive scenario (WA 2e)

• Scenario considerations• 100% of collected municipal wastewater is treated• All WWT facilities comply with the NOM-001 (discharge standards)• New WWTP based on combined processes: Anaerobic reactor (UASB)

followed by:• Activated sludge• Aerated ponds• Trickling filters• Biological rotating contactors

• Methane is burned in flares (76% of produced methane; 20% is dissolved in effluent)

• 50% of dissolved methane in the anaerobic effluent is collected• Biogas is used for electricity production in WWT facilities larger than 500 L/s.

• Results• 34% lower CO2e emissions if compared to BAU scenario• 25% lower CO2e emissions if compared to the 1990 level.

Noyola et al. (2016)

All scenarios

Anaerobic sludgedigestion with biogas use

(cogeneration)

Covered anaerobic ponds

CH4

CH4 mitigation approaches for

WWT facilities

Optimize energy use in existing facilities (indirect CO2 emissions)

Adopt good operationpractices

Capture of dissolvedmethane

Paredes M.G. (2017)

Paredes et al. (submitted)

Methane emission factors obtained for the anaerobic digesters in the six WWTP evaluated (at normal conditions: 273 K and 1 atm)

Emission factor Units CHI MTY GDL QRO SLP XAL

CH4 emission factor (*) kg CH4/kg DBOrem 0.66 (1.38) 0.49 0.68 (1.05) 0.35

CH4 emission factor per VS removed

kg CH4/kg VSrem 0.39 ± 0.02 0.81 ± 0.10 0.29 ± 0.01 0.40 ± 0.04 0.62 ± 0.04 0.21 ± 0.04

CH4 emission factor per VS removed

m3 CH4/kg VSrem 0.54 ± 0.03 1.13 ± 0.14 0.40 ± 0.02 0.56 ± 0.05 0.86 ± 0.05 0.29 ± 0.05

CH4 emission factor per VS fed m3 CH4/kg VSfed 0.13 ± 0.03 0.35 ± 0.05 0.21 ± 0.01 0.23 ± 0.02 0.34 ± 0.05 0.07 ± 0.02

(*) Emission factor based on the corresponding kg CH4/kg VSrem value after converting kg VS to kg BOD (see section 2.2)

Paredes et al. (submitted)

Cerro de la Estrella, Cd de México Dulces Nombres, Monterrey NL

• The theoretical CH4 emissions should be zero (this is a fully aerobicsystem)

• Experimental measurements showed that actual CH4 emissions from theWWTP were 0.464 Gg CH4 /year.

• Emission factor of 6.4 g CH4 per m3 treated for this specific facility,corresponding to a 1.7% of the influent COD or 3.6% as BOD (4.2%BODrem).

• There are poor operating practices, related with deficient primary settleroperation (sludge withdrawal).

• In addition, methane dissolved in the influent sewage should not beneglected (5 – 6 mgCH4/L).

Noyola et al. 2018

Noyola et al., 2018

Las PTAR bien operadas pueden emitir CH4, debido a metano disuelto presente

en el influente.

La eliminación de nutrientes (nitrógeno) también producirá este gas en la etapa

anaerobia (o anóxica).

Ninguna de estas fuentes se considera en las directrices actuales del IPCC.

No hay PTAR libre de emisiones de CH4. El drenaje es una fuente exógena

de este gas

Por lo tanto, los factores de corrección de metano (MCF) en la metodología de

nivel 1 del IPCC deben ser revisados.

Noyola et al., 2018

- Se propone 0.06 MCF para una PTAR aerobia

centralizada bien operada (regiones

intertropicales). (IPCC actual: 0.0)

- La eliminación de nutrientes debe agregarse a las

directrices del IPCC con un 0.08 MCF. (IPCC

actual: no considerado)

- Se debe agregar una PTAR aerobia + un digestor

anaerobio como un proceso integrado (MCF

0,32). (IPCC actual: 0.8 digestor anaerobio)

Paredes et al., 2018 (submitted)

CH4 emissions from anaerobic sludge digestion using the IPCC Tier 1 method presented an overestimation concerning the values obtained on-site at six full-scale treatment facilities in Mexico. In contrast, the results obtained with the Tier 1 method, but applying an alternate MCF (0.26 and 0.32), were much closer to the actual values determined on-site.

The CH4 emission factors determined in the GDL and QRO facilities (0.49 and 0.68 kg CH4/kg DBOrem, respectively) may be considered as representative of anaerobic sludge digestion associated with activated sludge processes with good operational practices in Mexico.

Based on the stoichiometry of methane oxidation, 0.49 kg CH4/kg DBOrem may be recommended as country-specific emission factor for Mexico.

The on-site CH4 emission factor here obtained allows the IPCC Tier 2 application for the calculation of national GHG inventories for activated sludge processes associated with anaerobic sludge digesters in Mexico.

Doing so, uncertainty may be reduced in the corresponding inventories, and improved technical assessments may be attained for identifying more effective mitigation actions in the water sector.

centro

molinamariocentro

molinamariocentro

molinamariomolinamario

100% of collected municipal wastewater is treated

All these facilities comply with the NOM-001 (discharge standards)

The same conditions for WA

New WWTP based on aerobic processes:

•Activated sludge•Aerated ponds•Trickling filters•Biological rotating contactors

WA1WA Mexican Water Agenda for 2030

Five mitigation scenarios for municipal WWTP in Mexico

centro

molinamariocentro

molinamariocentro

molinamariomolinamario

The same conditions for WA New WWTP based on combined processes: Anaerobic reactor (UASB) followed by:

Activated sludgeAerated pondsTrickling filtersBiological rotating contactors

Methane is burned in flares (76% of produced methane; 20% is lost dissolved in effluent)

The same conditions for WA2

100% of dissolved methane in the anaerobic effluent is collected and burned

Burning efficiency 95%.

WA2(anaerobic + aerobic)

•The same conditions for WA2

• 50% of dissolved methane in the anaerobic effluent is collected

•Biogas is used for electricity supply in the WWTP (facilities larger than 500 L/s)

WA2e(biogas for electricity generation)

WA2z(“zero” methane emissions)

Five mitigation scenarios for municipal WWTP in Mexico