Activated Carbon: Domestic Supply
The last in a five part series on the applications of activated carbon, focus shifts to the domestic setting. Ensuring both clean water ingress for domestic/municipal supplies and effective outflow/sewerage treatment are crucial for localities everywhere. Activated carbon is one of many tools used to enable high quality domestic supplies - whilst also minimising the dumping of untreated wastewater.
Introduction
Ensuring satisfactorily clean waste water from homes is as important as providing clean drinking water to them. Municipal sewage systems over the years have changed little and rely primarily on chemical treatment (for example with chlorine), crude filtration (such as through a metal mesh) and gravity filtration (where a pool of water is left undisturbed until its impurities ‘drop’ out). This is all well and good but it fails to acknowledge the wealth of compounds finding their way into contemporary domestic wastewater.
Inbound treatment
Water pretreatment is largely covered elsewhere in this series. But there are a few examples where granular activated carbon has had specific impacts on the production of clean and safe water for drinking, cooking and washing.
1,1,1-trichloroethane at concentrations up to 40,000 ppb were detected in a plume in an aquifer in the state of New Jersey which supplies drinking water to thousands. The cause of such a plume was attributed to a nearby plant. The plant had water treatment in place but it was clearly insufficient and so a granular activated carbon in line filtration system was brought in and in just a few months, the plume was significantly smaller and the risk of entering drinking water was reduced(1). This system remains in place to this day. Tetrachloroethane is a class 2A carcinogen and is a popular choice in dry cleaning solvents.
Similar to the previous example, dichloroethylene, methylene chloride and benzene contaminants were found in a town just outside Boston. Activated carbon treatment there was able to restore the water supply to safe levels in a relatively short amount of time(2). Granular activated carbon was combined with an aeration technique to ensure near complete removal.
Radiation hazards are not uncommon for water supplies. Radon levels - particularly in areas with a lot of granite - may be elevated in water. Removal using GAC is an effective course of action, with one study suggesting that equilibrium can be reached in as little as six hours(3) in areas of medium level radon. Radon removal by GAC is typically used only at the lower radon concentration range, up to around 185 Bq L-1(4).
Removal Of Pharmaceuticals From Wastewater
With a greater development in the pharmaceutical industry, more medicines than ever are being prescribed. Some of these medications pass through the body, either because of higher dosing than needed or as a breakdown product. Naturally, these end up in the municipal sewage system. Traditional sewage treatment may not be sufficient to remove these compounds to acceptable levels. Enter granular activated carbon.
The adsorption of pharmaceuticals from water by GAC has been reported as effective up to 477.1 mg g-1 for the tetracycline group of antibiotics(5). This group of compounds is particularly bad for wastewater streams because of their intended activity as antibacterials, the possibility for production of antibiotic resistant bacteria downstream is raised - potentially affecting aquatic and mammalian life. Endocrine disruptors are becoming more prevalent in domestic wastewater, which is problematic because they are toxic to aquatic life at trace concentrations. Combination membrane and GAC filtration has been shown as effective at complete endocrine removal(6).
Hormonal compounds, such as antiestrogens, antiandrogens and other estrogenics, are becoming more prevalent in wastewater. A demonstration plant has opened in the South West of England which uses granular activated carbon filtration specifically to remove such compounds. The impact on the outflow of this plant (i.e. a local river) was overwhelmingly positive. Steroidal estrogens were reduced by between 43 and 64%, with some related pharmaceutical compounds being near-completely removed (99% of mebeverine) but others largely unchanged (carbamazepine and propranolol 17 to 23%)(7). The removal of other antiestrogens and antiandrogens via a combined ozone and granular activated carbon treatment has been shown as effective, with removal efficiency of estrogenic and androgenic compounds in the range of 67 to 75%, as confirmed by biological assay and fluorescence spectroscopy(8). The latter is especially welcome as a technique, owing to its ability to provide a live readout of contaminants present.
Antibiotics may be removed from wastewater by coagulation followed by GAC filtration. This process relies on both size exclusion and adsorption. Polyaluminium chlorides were added as coagulants - with these metal species being easily adsorbed by the GAC. The balance between acidic and aqueous water is manipulated to ensure optimal filtration(9).
Other Wastewater Examples
Overall, in excess of 40 studies have probed the use of granular activated carbon in the removal of organic micropollutant contaminants from domestic water and wastewater flows. There seems to be broad agreement that GACs are highly effective and robust choices for the removal of such compounds - although issues like residence time and flow rates seem to be wide(10). Authors in that meta analysis conclude that bed volume ratio should be in excess of 20,000. The bed volume ratio is described as the ratio between volume of water (cubic metres) and the volume of GAC (also in cubic metres). Authors suggest that such ratios are viable starting points for filtration at scale which will be tolerant of high flow rates, pressures and tolerant to backwashing without risking a breakthrough.
It has also been shown that in combined membrane and granular activated carbon filtration systems, the performance enhancement conferred by the GAC is applicable to the overall system, rather than the instantaneous single process when applied to micropollutants. GAC filtration can, in this type of filtration, be compared to the use of anthracite as a filter in advance of a reverse osmosis filtration membrane as used in water desalination(11).
In the removal of orthophosphate contamination from wastewater - orthophosphates can lead to eutrophication - limestone has often been used as a filter. In “optimal conditions”, limestone is able to remove around 90% of orthophosphate in laboratory experiments. Sadly though, optimal conditions are hard to come by. With the addition of GAC, the impressive 90% removal value can be achieved in a greater diversity of conditions, particularly with regards to pH, to which limestone is acutely sensitive(12).
Water treatment is not limited to municipal systems - rural wastewater is an important consideration. Point-of-use granular activated carbon filters have been successfully deployed to improve local bacteriological quality in wastewater. The addition of powdered granular activated carbon is effective at removal of 12 bacterial strains - although there are profound seasonal effects ostensibly related to bacterial concentration as a function of temperature(13). Greywater is a feature of many treatment/waste systems downstream from domestic washing machines and dishwashers, yet it can contain contaminants such as cyclooctadiene amongst other organic compounds. Simple filtration with GAC in a fixed bed type arrangement has been shown to effectively remove such compounds with low residence times(14).
Using 595 to 2380 μm mesh granular activated carbon in a relatively crude bucket-like set up to filter agricultural run off. Compounds such as didecyldimethylammonium chloride, imidacloprid, paclobutrazol and glyphosate are popular choices in modern agriculture yet are highly problematic if released into water supplies in any appreciable concentration. With as little as 64 seconds of exposure to the GAC, the didecyldimethylammonium chloride and paclobutrazol were removed to such a level that they were undetected, whilst imidacloprid and glyphosate were removed at rates of 85.3 and 99.0% respectively(15).
Summary
- Wastewater treatment as as important to the environment as supplying clean potable water to homes is
- Granular activated carbon is an effective and popular choice for municipalities to ensure superior water and wastewater treatment
- Relying on GAC’s excellent porosity and surface chemistry profiles, particularly stubborn contaminants such as tetrachloroethane and other chloroalkanes are readily removed - as shown by case studies in the United States
- Radiation levels in groundwater can be modulated, with examples showing GAC is able to effectively immobilise radon
- Pharmaceuticals are becoming more common in wastewater, which could have devastating effects on marine life. GAC can be used to remove these to acceptable levels
- Finally, GAC may be used for treating runoff and stagnant water in rural and agricultural communities, providing a robust and cheap method for water treatment
References
1 W. F. Althoff et al., Groundwater, 1981, 19, 495
2 J. E. MacLeod et al., Amer. City and Country, 1983, 1, 32
3 H. Maghrawy et al., J. Radioanaly. Nucl. Chem., 2011, 287, 77
4 S. Rydell et al., J. New England Water Works Assoc., 1989, 103, 1
5 A. N. Alshirifi et al., J. Pharma. Sci. Res, 2018, 10, 2252
6 S. A. Snyder et al., Desalination, 2007, 202, 156
7 J. L. Zhou et al., J. Hazard. Mater., 2011, 185, 1005
8 L. Chen et al., Chemosphere, 2016, 153, 346
9 K.-J. Choi et al., J. Hazard. Mater., 2008, 151, 38
10 F. Benstoerm et al., Chemosphere, 2017, 185, 105
11 H.-S. Kim et al., Desalination, 2009, 243, 132
12 H. A. Aziz et al., Desalination, 2011, 271, 265
13 J. W. Snyder Jr. et al., Appl. Environ. Microbiol., 1995, 61, 4291
14 A. Sharaf, Granular Activated Carbon Biofilters for Greywater Treatment, Graduate Thesis, University of Alberta, 2020
15 P. R. Fisher et al., Water, Air and Soil Pollution, 2019, 230, 4056
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