Emerging pollutants

Emerging pollutants in treatment plants

The occurrence of pharmaceutical and personal care products (PPCP) in water and wastewater have been extensively reported after the early findings of Ternes in 1998 (Water Res., 32, 3245-3260). PPCP constitute a reason for concern because they are used and continuously released in large quantities, many of them are specifically designed to exert effect on living organisms and their physical and chemical properties contribute to their widespread distribution into the environment. In fact, many of these substances escape to conventional activated sludge wastewater treatments and reach surface water streams. Many other get included in the sludge and endangers its potential uses such as land amendment. The fate of PPCP in an Sewage Treatment Plant (STP) depends on their hydrophobicity expressed as its apparent octanol-water distribution coefficient, Dow, that considers both the dissociation constant of acidic of basic solutes, pKa, and the current pH of wastewater. The figure plots the removal efficiency for forty PPCP related to Dow.

[Removal efficiency during conventional activated sludge treatment: (1) paraxanthine, (2) caffeine, (3) acetaminophen, (4) nicotine, (5) ibuprofen, (6) ketorolac, (7) clofibric acid, (8) furosemide, (9) ciprofloxacin, (10) fluoxethine, (11) ofloxacin, (12) naproxen, (13) hydrochlorothiazide, (14) 4-amino-antipyrine, (15) metronidazole, (16) N-acetyl-4-amino-antipiryne, (17) codeine, (18) N-formyl-4-amino-antipiryne, (19) 4-methylaminoantipyrine, (20) ranitidine, (21) antipyrine, (22) gemfibrozil, (23) benzophenone-3, (24) triclosan, (25) tonalide, (26) galaxolide, (27) atenolol, (28) sulfamethoxazole, (29) fenofibric acid, (30) metoprolol, (31) bezafibrate, (32) ketoprofen, (33) trimethoprim, (34) Diclofenac, (35) indomethacine, (36) propanolol, (37) mefenamic acid, (38) omeprazole, (39) carbamazepine, (40) erythromycin]. From Water Res. 44, 578, 2010.

Non-polar compounds are efficiently removed in conventional activated sludge STP, but their presence in untreated wastewater still results in high concentrations in treated effluents. In particular, the widespread use of synthetic musks leads to their release to the environment in high amounts. Synthetic musks are persistent compounds and their presence have been assessed in a number of environmental samples, even including lipid-based fish tissue from remote Alpine lakes (where they entered by atmospheric precipitation). The following figure plots a GC×GC contour plot for the analysys of a wastewater sample carried out by stir-bar sorptive extraction (SBSE) followed by comprehensive two-dimensional gas chromatography (GC×GC-TOF-MS). The GC×GC contour plot shows a high number of compounds represented as dots in a bidimensional plot in which the first (x-axis) and second (y-axis) dimensions represent retention times in a non-polar (Rtx-5) column and a polar (Rxi-17) column respectively. Two of the most used synthetic musks, Galaxolide and Tonalide, are marked together with their mass spectra. From Water Res. 46, 4435, 2012.

Removal of emerging pollutants

Non-polar pollutants can be efficiently removed by ozone treatment with a global efficiency >95% for a set of compounds including UV filters, synthetic musks, herbicides, insecticides, antiseptics and polyaromatic hydrocarbons. The ozone dosage required was about 200 mmol/L. The irradiation with germicidal 254 nm UV reduced the total concentration of the tracked pollutants by an average of 63% with the highest removal for the synthetic musks 7-acetyl-1,1,3,4,4,6-hexamethyltetrahydronaphthalene and musk ketone and most herbicides. Visible light Ce-TiO2 photocatalysis reached 70% overall removal after 15 min with the best results obtained for synthetic musks. In terms of energy usage, ozonation was by far the most efficient treatment, one order of magnitude ahead of visible light Ce-TiO2 photocatalysis. Expressed in terms of energy per order, ozonation was again preferred, with 0.18 kWh m-3 order-1, while UV irradiation at 254 nm consumed 2.56 kWh m-3 order-1. Visible light irradiation photocatalysis led to collector areas per order of 0.16 kWh m-3 order-1. From Water Res. 47, 5546, 2013

The depletion of micropollutants with ozone has been studied with different ozone dosages. Beyond certain energy intensity, expressed in kWh/m3, further increases in pollutant removal are accompanied by a sharp decrease in ozone usage efficiency. For the best ozonation scenarios, the energy input was in the 0.03–0.26 kWh/m3 range with ozone efficiencies in the 90–100% range and joint pollutant depletion in the 67–98% range for a complete set of wastewater pollutants for ozone doses transferred to wastewater in the 5.5–8.5 mg/L range, which represented 100–200 mol of ozone transferred per mole of pollutant removed. From Sci. Total Environ. 437, 68, 2012.

Toxicity of emerging pollutants

The presence of small concentration of PPCP has been associated to chronic toxicity, endocrine disruption and the development of pathogen resistance. The large quantities of compounds discharged by wastewater treatment plants keeps concentrations in the nanogram per liter to microgram per liter range for most effluents and receiving bodies. The ecological impact of PPCP is not well understood. The lack of experimental approaches for the identification of pollutant effects in realistic settings (low doses, complex mixtures, and variable environmental conditions) supports the widespread perception that these effects are often unpredictable. Understanding the effects of exposure to chemical mixtures has been addressed to by both pharmacology and ecotoxicology. In risk assessment-oriented ecotoxicology, the prediction of the effect of chemical mixtures typically relies on the information from individual chemicals and uses the standard of Loewe additivity - Concentration Addition model. Despite additive models demonstrating a reasonnably good predictive power in the assessment of chemical mixture risk, concerns still exist due to the occurrence of unpredictable synergism or antagonism in certain experimental situations.

[Polygonograms showing pair-interactions among five antibiotics for three selected effect levels, fa (0.1, 0.5, and 0.9) in the bioluminescent cyanobacterium Anabaena CPB4337. AMO: Amoxicillin, NOR: Norfloxacin, LEV: Levofloxacin, ERY: Erythromycin, TET: Tetracycline. Green lines denote synergism and red lines antagonism. The thickness of the lines indicates the strength of the interactions.] From Water Res. 47, 2050, 2013..

Moreover, deviations from additivity have been reported even at very low concentrations at which no toxic effect at all would be expected. The use of Combination Index methodology allows the identification and quantification of synergism and antagonism. Using a novel screening method that couples global sensitivity analysis and high-throughput screening techniques. In a recent paper (Sci. Adv. 2, e1601272, 2016) a case study was presented identifying the biological effect of a set of mixtures of the main pharmaceutical pollutants at low-doses. The experiments analyzed nearly 2700 observations from an array of 180 low-dose, which is the most complex experimental mixture effect assessment to date. Ecological scaling-up experiments confirmed that a subset of pollutants also affects typical freshwater microbial community assemblages. Contrary to the established scientific opinion the bioactivity of the mixtures was not predicted by conventinal models.The results suggest that current effect assessment methods overlook a substantial number of ecologically dangerous chemical pollutants.

Further reading

Click to download

Hidden drivers of low-dose pharmaceutical pollutant mixtures revealed by the novel GSA-QHTS screening method, Sci. Adv. 2, e1601272, 2016

Toxicity of five antibiotics and their mixtures towards photosynthetic aquatic organisms: Implications for environmental risk assessment, Water Res. 47(6), 2050-2064 (2013)

Transformation products and reaction kinetics in simulated solar light photocatalytic degradation of propranolol using Ce-doped TiO2, Appl. Catal. B Environ. 129(1), 13-29 (2013)

Energy efficiency for the removal of non-polar pollutants during ultraviolet irradiation, visible light photocatalysis and ozonation of a wastewater effluent, Water Res. 47(15), 5546-5556 (2013)

Oxidative and photochemical processes for the removal of galaxolide and tonalide from wastewater, Water Res. 46(14) 4435-4447 (2012)

Environmental optimization of continuous flow ozonation for urban wastewater reclamation, Sci. Total Environ. 437, 68–75 (2012)

Oxidation by-products and ecotoxicity assessment during the photodegradation of fenofibric acid in aqueous solution with UV and UV/H2O2, J. Hazard. Mater. 194(1) 30-41 (2011)

Chemical and toxicological evolution of the antibiotic sulfamethoxazole under ozone treatment in water solution, J. Hazard. Mater. 192(1) 18-25 (2011)

Occurrence of emerging pollutants in urban wastewater and their removal through biological treatment followed by ozonation, Water Res. 44(2) 578-588 (2010)

Ecotoxicological assessment of surfactants in the aquatic environment: combined toxicity of docusate sodium with chlorinated pollutants, Chemosphere 81(2) 288-293 (2010)