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Tuesday, August 9, 2016

Adsorption (Activated Carbon)

Compiled by:
Félicien Mazille (Aquasis, cewas international centre for water management services) , Dorothee Spuhler (seecon international gmbh)
Activated carbon filtration is a commonly used technology based on the adsorption of contaminants onto the surface of a filter. This method is effective in removing certain organics (such as unwanted taste and odours, micropollutants), chlorine, fluorine or radon from drinking water or wastewater. However, it is not effective for microbial contaminants, metals, nitrates and other inorganic contaminants. The adsorption efficiency depends on the nature of activated carbon used, the water composition, and operating parameters. There are many types of activated carbon filters that can be designed for household, community and industry requirements. Activated carbon filters are relatively easy to install but require energy and skilled labour and can have high costs due to regular replacement of the filter material.
In Out
Freshwater,Non-biodegradable Wastewater, Treated Water Drinking Water, Treated Water

Introduction

The use of carbon in the form of charcoal has been used since antiquity for many applications. In Hindu documents dating from 450 BC charcoal filters are mentioned for the treatment of water. Charred wood, bones and coconut charcoals were used during the 18th and 19th century by the sugar industry for decolourising solutions (CECEN 2011). Activated carbon is a material prepared in such a way that it exhibits a high degree of porosity and an extended surface area.

A typical carbon particle has numerous pores that provide a large surface area for water treatment. Source: LEMLEY et al. (1995)
During water filtration through activated carbon, contaminants adhere to the surface of these carbon granules or become trapped in the small pores of the activated carbon (AMIRAULT et al. 2003). This process is called adsorption. Activated carbon filters are efficient to remove certain organics (such as unwanted taste and odours, micropollutants), chlorine, fluorine or radon, from drinking water or wastewater. However, it is not effective for microbial contaminants, metals, nitrates and other inorganic contaminants.Activated carbon filtration is commonly used in centralised treatment plants and at household level, to produce drinking water and in industries to treat effluents. It is also an upcoming treatment applied for the removal of micropollutants both in drinking water production and for the purification of treated wastewater before disposal (see also surface disposal or surface and subsurface groundwater recharge).

Treatment Principles

(Adapted from LEMLEY et al. 1995)

Activated carbon filters for water treatment. Source: FOCUS TECHNOLOGY CO LTD (2011)
There are two basic types of water filters: particulate filters and adsorptive/reactive filters. Particulate filters exclude particles by size, and adsorptive/reactive filters contain a material (medium) that either adsorbs or reacts with a contaminant in water. The principles of adsorptive activated carbon filtration are the same as those of any other adsorption material. The contaminant is attracted to and held (adsorbed) on the surface of the carbon particles. The characteristics of the carbon material (particle and pore size, surface area, surface chemistry, etc.) influence the efficiency of adsorption.
The characteristics of the chemical contaminant are also important. Compounds that are less water-soluble are more likely to be adsorbed to a solid. A second characteristic is the affinity that a given contaminant has with the carbon surface. This affinity depends on the charge and is higher for molecules possessing less charge. If several compounds are present in the water, strong adsorbers will attach to the carbon in greater quantity than those with weak adsorbing ability.

Preparation of Activated Carbon

(Adapted from DROVAC and SKIPTON 2008)

Wood based powder activated carbon for drinking water treatment. Source: GCHFF (2011)
The medium for an activated carbon filter is typically petroleum coke, bituminous coal, lignite, wood products, coconut shell or peanut shell. The carbon medium is “activated” by subjecting it to stream (a gas like water, argon or nitrogen) and high temperature (800-1000°C) usually without oxygen. In some cases, the carbon may also undergo an acidic wash or be coated with a compound to enhance the removal of specific contaminants. The activation produces carbon with many pores and a high specific surface area. It is then crushed to produce a granular or pulverised carbon product.

Use of Activated Carbon Units

(Adapted from DROVAC and SKIPTON 2008)

 Types of activated carbon units. Source: AMIRAULT et al. (2003)
Activated carbon units are commonly used to remove organics (odours, micropollutants) from drinking water at centralised and decentralised level. At centralised level, they are generally part of one of the last steps, before the water is fed into the water distribution network. At decentralised level, activated carbon filtration units can either be point-of-use (POU) or point-of-entry (POE) treatment. A POE device is recommended for the treatment of radon and volatile organic compounds because these contaminants can easily vaporise from water in showers or washing machines and expose users to health hazards. POU devices are useful for the removal of lead and chlorine. The structure of POU devices can either be in-line, line-bypass faucet mounted (see also advanced filters) or pour-through (similar to the design of ceramic candles, colloidal silver or biosand filters).
Activated carbon filters can also be used as a tertiary treatment in wastewater treatment plants to remove micropollutants from municipal effluents or recalcitrant contaminants from industrial effluents.

Combination of Activated Carbon With Other Processes

Activated carbon is often used as pre-treatment to protect other water treatment units such as reverse osmosis membranes and ion exchange resins from possible damage due to oxidation or organic fouling. The combination of ozonation with activated carbon is a very efficient technique for eliminating organic matter including micropollutants. Besides, the lifetime of activated carbon filters is extended drastically when used in combination with ozone, deceasing operation costs substantially (AEPPLI and DYER-SMITH 1996).

Cost Considerations

(Adapted from AMIRAULT et al. 2003)
Installation costs are moderate but additional technical equipment is required. Operating costs are usually limited to filter replacement. Depending on the type and concentration of the contaminant being removed, some carbon filters may require special hazardous waste handling and disposal, which can be costly.

Operation and Maintenance

(Adapted from LEMLEY et al. 1995)
Carbon filters are relatively easy to install and maintain but skilled labour is required at least occasionally for monitoring the removal performance over time of both POU and POE equipment. Activated carbon filters have a limited lifetime. After long-term use, their surfaces are saturated with adsorbed pollutants and no further purification occurs. The filter material therefore has to be replaced at regular intervals, according to manufacturer's instructions. Replacement intervals should be calculated based on the average daily water use through the filter and the amount of contaminant being removed. Cartridge disposal depends on usage. A carbon cartridge can be backwashed and then reused or discarded if non-toxics have been adsorbed.

At a Glance

Working PrincipleThe pollutants are removed from water through adsorption on the surface of the activated carbon. Use at the POE or POU (e.g. advanced filters).
Capacity/AdequacySimple technique using abundant raw material (e.g. petroleum coke, bituminous coal, lignite, wood products, coconut shell or peanut shell). Skilled labour required at least occasionally.
PerformanceEfficient for pollutant having high affinity with activated carbon surface (non-polar compounds).
CostsRelatively low operation costs.
Self-help CompatibilityInitial analysis of water is required to choose proper adsorbent (type of activated carbon).
O&MRegular replacement or regeneration of carbon cartridge.
ReliabilityReliable if the water composition is taken into account when choosing the type of activated carbon used as filter material.
Main strengthActivated carbon can be produced relatively easily everywhere in the world.
Main weaknessFilter has to be replaced on a regular basis.

Applicability

Activated carbon filters are widely used to produce drinking water at household and community level (to remove certain organics, chlorine or radon from drinking water) and to treat industrial or municipal wastewaters. It is not efficient for disinfection and nitrates removal. Adsorption on activated carbon is a simple technology based on materials such as fossil fuels (petroleum coke, lignite...) and even agricultural waste (e.g. coconut shell, wood, etc.).
To choose the most applicable type of activated carbon for a given application it is important to analyse the composition of the influent water previously. The carbon filter has to be replaced or regenerated regularly to remain efficient. Activated carbon can also be used as a pre-treatment to protect other water treatment units.

Advantages

  • Easy to install and maintain
  • Can be used at the point-of-entry (semi-centralised drinking water treatment plants, wastewater treatment plants) or at the point-of-use (household/community filters)
  • Efficient to remove certain organics, chlorine, radon
  • Based on materials available everywhere

Disadvantages

  • Filter has to be replaced regularly
  • Skilled labour required, at least occasionally
  • Water analysis is required to choose the most adapted type of activated carbon
  • Contaminants are separated from water but not destroyed

References

AEPPLI, J.; DYER-SMITH, P. (1996): Ozonation and Granular Activated Carbon Filtration the Solution to Many Problems. In: Proceedings of the First Australian Conference of the International Ozone Association. URL [Accessed: 04.10.2011].
AMIRAULT, R.; CHOBANIAN, G.; MCCANTS, D.; MCCANN, A.; BURDETT, H.; NEPTIN,B. (2003): Activated Carbon Treatment of Drinking Water Supplies. In: Healthy Drinking Waters for Rhode Islanders. URL [Accessed: 04.10.2011]. PDF
CECEN, F. (2011): Water and Wastewater Treatment: Historical Perspective of Activated Carbon Adsorption and its Integration with Biological Processes. In: WILEY-VCH Verlag GmbH & Co. KGaA. URL [Accessed: 04.10.2011].
DROVAC, B. I.; SKIPTON, S.O. (2008): Drinking Water treatment: Activated Carbon. In: University of Nebraska-Lincoln Extension . URL [Accessed: 04.10.2011].
FOCUS TECHNOLOGY CO LTD (Editor) (2011): Water Treatment System (Active Carbon Filter). Zhangjiagang Beyond Machinery Co. Ltd.. URL [Accessed: 10.11.2011].
GCHFF (Editor) (2011): Wood based Powder Activated Carbon. Henan: Gongyi City Hongda Filter Factory (GCHFF) . URL [Accessed: 10.11.2011].
LEMLEY, A.; WAGENET, L.; KNEEN, B. (1995): Activated Carbon Treatment of Drinking Water. In: Water Treatment Notes Cornell Cooperative Extension. URL [Accessed: 04.10.2011]. PDF
For further readings, case studies, awareness raising material, training material, important weblinks or the related powerpoint presentation, see www.sswm.info/taxonomy/term/