Chapter 14: Treatment processes, filtration and adsorption

14.7 Adsorption processes

14.7.1 Activated alumina

Activated alumina based products have been used historically for the reduction of fluoride, arsenic and selenium. Point-of-use (POU) products using such media have been developed since the USEPA indicated acceptability of POU treatment as an alternative for small communities. Several types of media including unmodified activated alumina, manganese modified or iron modified alumina, and iron based granules are being investigated for use in a manner similar to granular activated carbon (GAC), packed into columns and inserted into housings. Some of these products are being tested for arsenic (V) reduction from 0.05 mg/L per the protocols recently incorporated into NSF/ANSI Standard 53 – Drinking water treatment units – Health effects.
A recent report (NSF 2005) describes the reduction of arsenic (V) from 0.025 mg/L to less than 0.002 mg/L using an activated alumina point-of-use treatment system. The unit shut down automatically after processing 800 gallons. The study found that the cost of POU treatment was less than the cost of operating a central treatment plant. The spent cartridges from the trial were tested for disposal safety according to the California Waste Extraction Test (WET) and EPA Toxicity Characteristics Leaching Procedure (TCLP) test. They passed both tests, indicating that disposal in the household refuse would be acceptable. Currently, there are 20 different products shown as certified for this capability on the NSF web site.

While activated alumina primarily removes fluoride and As V (arsenate) and performs better at a lower pH (best between 5.6 and 6), iron-based media generally are more effective at removing both arsenic (III) (arsenite) and arsenic (V), although oxidation of arsenite to arsenate prior to filtration can increase its removal efficiency, depending on pH. In addition, activated alumina is more likely to experience interference affecting arsenic removal from competing ions such as silica, fluoride, phosphate, and sulphate than iron-based media. As with most column treatment systems, the granules can support the growth of micro-organisms. It has been reported that the bacteria can reduce arsenate to arsenite, thereby reducing the efficacy of the activated alumina.

Note that RO devices are certified under NSF/ANSI Standard 58 only for arsenate removal (USEPA 2006).
In New Zealand, geothermal or hydrothermal waters are the most likely to contain excessive fluoride levels; these waters usually contain high silica concentrations too. Arsenic V and silica are preferentially adsorbed by activated alumina media, so fluoride removal may not be very efficient. Fluoride removal is covered in USEPA (1984).
See Chapter 19: Small, Individual and Roof Water Supplies, section 19.3.4, for further discussion on point-of-use and point-of-entry treatment systems.
See USEPA (2003c) for a full discussion on arsenic removal technologies.

14.7.2 Solid block activated carbon (SBAC) filters

All types of carbon filters effect the removal of organic substances by adsorption on to the carbon surface. The filter in this device consists of extremely small particles of activated carbon that are fused together into a solid block with uniform pore size. If the carbon block configuration is constructed properly, the pore size may be uniformly 0.5 micrometer (μm or microns), which would be effective at removing asbestos fibres, protozoal (oo)cysts, and some bacteria. SBAC filters are less prone than GAC filters to channelling and can also be effective at removing organic contaminants such as some pesticides and chlorinated solvents. In addition, some SBAC devices are certified by NSF International for removal of methyl tert-butyl ether and selected disinfection byproducts (DBPs) such as total trihalogenated methanes. They can also remove chlorine and can be formulated to remove metals such as mercury and lead.
With regard to limitations, SBAC filters typically will not remove most heavy metals, viruses, small bacteria, arsenic, fluoride, iron, or nitrate. These filters also tend to harbour bacteria that grow on trapped organic matter, and the bacteria can migrate from the filter to the water at a later time.
Most manufacturers recommend that the filters be replaced about every six months, even though the adsorptive capacity may not yet be totally exhausted. However, replacement may be required sooner depending on the quality of the incoming water and the amount of usage. USEPA (2006).

14.7.3 Granular activated carbon (GAC) filters

GAC is extremely porous and can have a surface area of about 1000 square metres per gram. Many organic compounds, such as chlorinated and nonchlorinated solvents, naturally occurring organic matter, some gasoline components, and trihalomethanes, can be adsorbed on to the GAC surface. However, for some pesticides, such as atrazine and alachlor, GAC has a very low adsorptive ability.
This material is also effective for removal of chlorine and moderately effective for removal of some heavy metals and metals that are bound to organic molecules. Activated carbon processes show promise for removal of biotoxins and other potential organic contaminants of concern.
Regardless of the design, GAC filters are subject to clogging and, like all types of activated carbon filters, provide an environment for bacterial growth (see BAC below) which may present problems. Backwashing can improve long-term effectiveness for removal of organic compounds and provide some control of bacterial growth, but it does not improve radon removal efficiency. ANSI/AWWA (2012) has a Standard (B604-12) for GAC, and B605-13 covers its reactivation.
GAC is not effective at removing fluoride, chloride, nitrate, hardness, or most metal ions, and is not recommended at the point-of-use for removal of radon or VOCs. GAC is not as effective as SBAC, especially with regard to removal of chlorine, taste-causing substances, or halogenated organic compounds. USEPA (2006).
See Chapter 19: Small, Individual and Roof Water Supplies, section 19.3.4, and Chapter 18: Aesthetic Considerations, section 18.3 (under the heading of Taste and Odour), for further discussion on activated carbon treatment systems.

14.7.4 Biologically active filters (BAC)

These are normally granular active carbon filters (GAC) where the bacteria are actually encouraged to develop on the granules. BAC filters develop layers of microbes, along with their associated exopolymers, on the surface of or within the granular medium matrix. This biologically active layer, called the schmutzdecke in conventional slow sand filters, retains microbes and often leads to their inactivation and biodegradation. These microbes can also degrade natural organic matter and industrial organic chemicals. Ozone can be effective in partially oxidising organics in the water to biodegradable compounds that can be removed more readily by biological filtration. This increase in the biodegradable fraction of organic carbon occurs as a result of moderate to high levels of ozonation. These ozone levels are typical of the doses commonly applied for disinfection.
Adsorption processes are discussed in Chapters 9 and 13 of AWWA (1990). ANSI/AWWA B13013 covers membrane bioreactor systems. Chapter 3 of USEPA (2004) is the Environmental Technology Verification (ETV) Technology Specific Test Plan for evaluation of drinking water treatment equipment utilising fixed bed adsorptive media for volatile organic chemical (VOC) removal.