Methane emissions from stabilization ponds for municipal wastewater treatment in Mexico

Figures & data

Table 1. Parameters used to calculate theoretical CH₄ emissions from sewage treatment in Mexico (Year 2010).

Parameter	Value	Units
Population^a	112,336,538	Person
Wastewater generated	235	m^3/s
Collected wastewater^b	208	m^3/s
Treated wastewater	94	m^3/s
% of treated flow^c	45.1	%
Generated BOD^d	2,460,170,182	kg BOD/year
BOD that enters WWTP^e	709,998,981	kg BOD/year
BOD removed by WWTP	606,001,912	kg BOD/year
Removal efficiency	85.35	%

^a CONAPO 2007. http://www.conapo.gob.mx/

^b Per-capita wastewater generation 180 L/inhab/day (CONAGUA 2010).

^c Percentage of collected wastewater in municipal sewage systems being treated.

^d Per-capita generation BOD of 60 g/inhab/day (CONAGUA 2010).

^e [BODin] 242 mg/L obtained from a representative sample of 124 municipal WWTP.

Figure 1. Geographical location of the evaluated stabilization ponds.

Figure 2. Configuration schemes of the evaluated stabilization ponds: (1) Torreón (TOR), (2) Los Mochis (MOC), (3) Comitán (COM), (4) Coatzacoalcos (COA) and (5) Irapuato (IRA). Anaerobic units are in gray, facultative units in white. Sampling points are represented by asterisks. Arrows indicate the flow direction.

Table 2. General information of evaluated stabilization ponds with wastewater quality parameters (average values).

	TOR		MOC		IRA		COA		COM
Location	25°30′43.40″N 103°20′9.78″O		25°43′30.79″N 109° 4′24.87″O		20°38′42.24″N 101°21′17.12″O		18° 6′55.77″N 94°29′0.61″O		16°14′26.42″N 92° 5′46.43″O
State	Coahuila1900		Sinaloa1100		Guanajuato700		Veracruz 340		Chiapas210
Installed capacity (L/s)
Treated flow (L/s)	1357 ± 128		1036 ± 191		775 ± 65		220 ± 60		160 ± 14
Years in operation	10		14		22		12		10
Final disposal/reclamation	Discharge into water bodies		River discharge		Agricultural irrigation		River discharge		Agricultural irrigation
Parameter	Influent	Effluent	Influent	Effluent	Influent	Effluent	Influent	Effluent	Influent	Effluent
BOD5 (mg/L)	344 ± 36	88 ± 7	61 ± 15	36 ± 17.5	241 ± 45	108 ± 15	61 ± 15	41 ± 15	278 ± 35	140 ± 39
COD (mg/L)	644 ± 55	270 ± 20	195 ± 48	145 ± 40	686 ± 230	273 ± 150	NA	NA	NA	NA
TSS (mg/L)	285 ± 63	89 ± 12	162 ± 92	70 ± 17	198 ± 72	75 ± 24	NA	168 ± 79	537 ± 40	40 ± 1.5
N Total (mg/L)	42 ± 2	39 ± 4	22 ± 4	17 ± 4.6	41 ± 15	31 ± 13	NA	NA	18 ± 3	NA
P Total (mg/L)	10 ± 3	8 ± 0.8	4.2 ± 0.8	4.3 ± 0.7	9 ± 4	7.7 ± 2.7	NA	NA	4.1 ± 0.5	NA
Temperature (°C)	32.4 ± 1.7	29 ± 3	27 ± 3	27.6 ± 3	30 ± 2	30 ± 2	31 ± 2	31 ± 2	24 ± 2	24 ± 2
pH	7.26 ± 0.05	8.3 ± 0.12	7.4 ± 0.2	8.1 ± 0.3	7.54 ± 0.16	7.34 ± 0.27	NA	7.0 ± 0.3	7.1 ± 0.0	7.7 ± 0.3
BOD removal (%)	75	40	55	33	50

NA: Not available

Table 3. Dimensions of the evaluated stabilization ponds.

WWTP	Anaerobic ponds (AP)					Facultative ponds (FP)
	Nr of ponds	Retention time (day)	Depth (m)	Area (m^2)	Volume (m^3)	Nr of ponds	Retention time (day)	Depth (m)	Area (m^2)	Volume (m^3)
TOR	4	6	3	55,640	166,380	4	12	2	136,000	272,000
	2	6	3	38,080	114,240	2	12	2	104,000	208,000
MOC	8	5	2	79,500	159,000	12	12	1.5	53,530	80,295
IRA	1	4	2	123,930	247,860	–		–	–	–
COA	2	3	3.5	16,400	57,400	2	6	1.9	20,000	38,000
COM	3	4	4.5	8,625	38,813	3	7	1.5	52,900	79,350

Table 4. Values used to estimate theoretical methane emissions from the evaluated stabilization ponds.

Parameter	Unit	TOR	MOC	IRA	COA	COM
Treated flow	L/s	1550	900	700	250	160
Influent BOD^a	kg BOD /year	11,731,392	6,811,776	5,298,048	1,892,160	1,210,982
Removed BOD ^b	kg BOD /year	10,264,968	5,960,304	4,635,792	1,655,640	1,059,610
Theoretical removal	%	87.5	87.5	87.5	87.5	87.5
CH4 correction factor (MCF)^.^c	–	0.8	0.8	0.5	0.5	0.8

^a [BODin] 242 mg/L obtained from a representative sample of 124 municipal WWTP.

^b [BODeff] 30 mg/L according to Mexican discharge standards in NOM-001-SEMARNAT-1996 (SEMARNAT 1997).

^c The methane correction factors are the recommended default values in Tier 1 method (IPCC 2006). An MCF of 0.5 was chosen based on expert judgement.

Figure 3. A view of the static chamber used in this study (five similar units).

Table 5. CH₄ fluxes from the five stabilization pond systems.

WWTP	Anaerobic ponds		Facultative ponds
	Nr. of ponds	CH4 flux	Nr. of ponds	CH4 flux
		(mg CH4/m^2 h)		(mg CH4/m^2 h)
TOR	4	1,157 ± 430	4	127 ± 21
	2	2,440 ± 1,018	2	156 ± 29
MOC	8	235 ± 62	12	139 ± 19
IRA	1	366 ± 104	–	–
COA	2	348 ± 113	2	124 ± 30
COM	3	878 ± 304	3	192 ± 27

Table 6. Data from other studies determining CH₄ flux from municipal stabilization ponds.

Systems	Location	Flow treated (L/s)	COD (mg/L)	Temperature (°C)	CH4 flux (mg CH4/m^2 h)	Reference
Stabilization ponds	Mexico	160–1,357	195–686	24–32	124–2,440	This study
Stabilization ponds	Bolivia	317	1,336	13–14	5–152	Parra et al. (2010)
Stabilization ponds	Portugal	15.5	699	20	541–1,450	Toprak (1995)
Anaerobic ponds	England	46.3	–	10–20	175–842	Czepiel et al. (1993)
Anaerobic ponds	France	10.2	685	12–24	55–2,035	Paing et al. (2000)
Anaerobic ponds	China	3,472	200	12−24	89–143	Wang et al. (2011)
Anaerobic ponds	France	Population served 13,800	589	18	358–2,970	Picot et al. (2003)

Figure 4. Methane emissions in CO2 eq. for the five stabilization ponds evaluated, accordingly to IPCC methodology and on-site data.

Table 7. Methane emission factors based on on-site measurements.

WWTP	kg CH4/kg BOD rem	kg CH4/m^3 treated water
TOR	0.431	0.110
MOC	2.561	0.064
IRA	0.122	0.016
COA	1.044	0.021
COM	0.669	0.092

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