Activated carbon to comply with new NOM-001-SEMARNAT-2021 limits

0 0 0 26 June, 2024

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New limits allowed by NOM-001-SEMARNAT-2021 and how activated carbon can comply with it.

NOM-001-SEMARNAT establishes the permissible limits for pollutants in wastewater discharges into receiving water bodies owned by the nation. The limits established by the modification made in 2000 to the 1996 version of this standard (NOM-001-SEMARNAT-1996) applied until March 10, 2023.

On March 11, 2022, a new version of the standard was published: NOM-001-SEMARNAT-2021, which establishes the permissible limits indicated in the following two tables:

TABLE 1 – PERMISSIBLE LIMITS

Parameters (*) (milligrams per liter, except when specify)	Rivers, streams, canals, drains			Reservoirs, lakes and lagoons			Mexican marine areas				Soil
	Rivers, streams, canals, drains			Reservoirs, lakes and lagoons			Mexican marine areas				Irrigation of green areas			Infiltration and other irrigation			Karst
	P.M	P.S.	V.I.	P.M	P.S.	V.I.	P.M	P.S.	V.I.		P.M	P.S.	V.I.	P.M	P.S.	V.I.	P.M	P.S.	V.I.
Temperature (°C)	35	35	35	35	35	35	35	35	35		35	35	35	35	35	35	35	35	35
Fats and Oils	15	18	21	15	18	21	15	18	21		15	18	21	15	18	21	15	18	21
Total Suspended Solids	60	72	84	20	24	28	20	24	28		30	36	42	100	120	140	20	24	28
Chemical Oxygen Demand	150	180	210	100	120	140	85	100	120		60	72	84	150	180	210	60	72	84
Total Organic Carbon	38	45	53	25	30	35	21	25	30		15	18	21	38	45	53	15	18	21
Total Nitrogen	25	30	35	15	25	30	25	30	35		NA	NA	NA	NA	NA	NA	15	25	30
Total Phosphorus	15	18	21	5	10	15	15	18	21		NA	NA	NA	NA	NA	NA	5	10	15
Eggs of Helminths (eggs/liter)	NA	NA	NA	NA	NA	NA	NA	NA	NA		1
Escherichia coli, (MPN/100 ml)	250	500	600	250	500	600	250	500	600		250	500	600	250	500	600	50	100	200
Enterococci fecal* (MPN/100 ml)	250	400	500	250	400	500	250	400	500		250	400	500	250	400	500	50	100	200
pH (UpH)	6-9
True color	Wavelength									Maximum spectral absorption coefficient
	436 nm									7,0 m -1
	525 nm									5,0 m -1
	620 nm									3,0 m -1
Acute toxicity (UT)	2 after 15 minutes of exposure
N.A: Not Applicable P.M: Monthly Average P.S: Daily Average V.I: Instantaneous Value MPN: Most probable number UpH: pH units TU: Toxicity Units * If Chlorides is less than 1000 mg/L, COD is analyzed and reported. * If Chlorides is greater than or equal to 1000 mg/L, TOC is analyzed and reported. * If the electrical conductivity is less than 3500 µS/cm, E. coli is analyzed and reported. * If the electrical conductivity is greater than or equal to 3500 µS/cm, fecal Enterococci are analyzed and reported. Electrical conductivity and chloride determinations do not require the accreditation and approval of the corresponding entity.

TABLE 2 – PERMISSIBLE LIMITS FOR METALS AND CYANIDES

Parameters (milligrams per liter)	Rivers, streams, canals, drains			Reservoirs, lakes and lagoons			Marine areas mexican			Soil
	Rivers, streams, canals, drains			Reservoirs, lakes and lagoons			Marine areas mexican			Irrigation of green areas			Infiltration and other irrigation			Karst
	P.M.	P.S.	V.I.	P.M.	P.S.	V.I.	P.M.	P.S.	V.I.	P.M.	P.S.	V.I.	P.M.	P.S.	V.I.	P.M	P.S.	V.I.
Arsenic	0,2	0,3	0,4	0,1	0,15	0,2	0,2	0,3	0,4	0,2	0,3	0,4	0,1	0,15	0,2	0,1	0,15	0,2
Cadmium	0,2	0,3	0,4	0,1	0,15	0,2	0,2	0,3	0,4	0,05	0,075	0,1	0,1	0,15	0,2	0,05	0,075	0,1
Cyanide	1	2	3	1	1,5	2	2	2,50	3	2	2,5	3	1	1,50	2	1	1,5	2
Copper	4	5	6	4	5	6	4	5	6	4	5	6	4	5	6	4	5	6
Chrome	1	1,25	1,5	0,5	0,75	1	1	1,25	1,5	0,5	0,75	1	0,5	0,75	1	0,5	0,75	1
Mercury	0,01	0,015	0,02	0,005	0,008	0,01	0,01	0,015	0,02	0,005	0,008	0 01	0,005	0,008	0,01	0,005	0,008	0,01
Nickel	2	3	4	2	3	4	2	3	4	2	3	4	2	3	4	2	3	4
Lead	0,2	0,3	0,4	0,2	0,3	0,4	0,5	0,75	1	0,5	0,75	1	0,2	0,3	0,4	0,2	0,3	0,4
Zinc	10	15	20	10	15	20	10	15	20	10	15	20	10	15	20	10	15	20
Parameters measured in full way	P.M: Monthly Average P.S: Daily Average V.I: Instantaneous Value

Of the parameters listed in the tables above, all but two, Acute Toxicity and True Color, began to apply as of April 3, 2023. The two aforementioned will apply as of March 12, 2026.
The main changes with respect to the previously applicable limits are as follows:

The permissible limit in Biochemical Oxygen Demand (BOD5) changed by Chemical Oxygen Demand (COD) or Total Organic Carbon (TOC), in case the chloride concentration is higher than 1000 mg/L (the COD test does not apply to waters with high salinity). The reason for this change is that BOD5 does not detect organic compounds that are not biodegradable; in many cases, these represent a significant percentage of the organic load and are toxic to living organisms.

The limit as to True Color was not previously established. The color, in addition to its unpleasant aesthetic effect, usually corresponds to compounds that are also toxic.

The acute toxicity limit was also not established.

PARAMETERS REGULATED BY NOM-001-SEMARNAT-2021 THAT CAN BE MET WITH ACTIVATED CARBON.

Activated carbon is an adsorbent for organic compounds.

Fats and oils, chemical oxygen demand and total organic carbon are parameters that correspond entirely to organic compounds.

True Color and Acute Toxicity parameters do not always correspond to organic compounds, but do in most cases.

In the table shown above, these five parameters are marked in yellow.

From an economic point of view, activated carbon is a competitive method to reduce the presence of organic compounds present in water, when the following two conditions are met:

a) The total concentration of these is less than about 30 mg/L (corresponding to a COD of about 100 mg/L or a TOC of about 25 mg/L); and

b) It is required to reach very low concentrations.

Regarding condition (a), activated carbon, either in granular or powdered form, is applied as tertiary treatment of wastewater containing organic compounds. This means that, in the vast majority of cases, such waters will have been previously required:

Primary treatment (which may include prefiltration, pH adjustment, coagulation, flocculation, clarification); and
Secondary treatment (this is biological treatment and may include anaerobic and/or aerobic).

Condition (b) is present in the case of the application of activated carbon as tertiary treatment, since the limits established by the new standard are low.

After tertiary treatment with activated carbon, the water requires disinfection.

SOLUTION WITH BIOLOGICAL ACTIVATED CARBON

In tertiary wastewater treatment where biodegradable pollutants are present, granular activated carbon can act as “biological activated carbon”. This method is feasible when the water does not contain a significant concentration of toxic or difficult to degrade compounds.

The job of an activated carbon is to adsorb and concentrate organic molecules. Within a short time, bacteria begin to form and attach themselves to the carbon surface. Bacteria convert organic compounds into methane, carbon dioxide, water and more bacteria. The conversion of these compounds releases sites where the carbon can continue to adsorb more of them.

Eventually, the excess bacteria form a sludgy biomass that clogs the charcoal bed. This excess biomass is removed by backwashing. Proper backwashing (in which the carbon bed is expanded, and carried out as often as required) can make the carbon last much longer. There are cases where this dynamic can do more than a standard activated carbon can do (thus, the carbon is changed not because it is depleted, but because it is eroded and lost during backwashing).

It is important to mention that the water treated by this dynamic does not reach the typically low levels achieved by an activated carbon, but the reduction in the concentration of total organic carbon is significant.