Granular Activated Carbon (GAC) Adsorbers

PAC is costly and is discarded after one use whereas GAC, although about two to three times the cost of PAC, can be reactivated after exhaustion and reused. Since the introduction of more rigorous standards for drinking water quality the use of GAC has become the predominant process for the removal of organic matter including micropollutants. GAC adsorbers are commonly installed downstream of rapid gravity filters used for turbidity removal (Section 9.9).

GAC adsorbers are of conventional rapid gravity or pressure filter design (Section 9.9) and the basic design parameters are the EBCT and bed depth or hydraulic loading (m³/h.m²). Bed depths up to 2.5 m for rapid gravity filters and 3 m for pressure filters are used (Section 9.9).

GAC characteristics vary according to the base material used. For example, the adsorptive capacity for the pesticide atrazine varies in the order wood>coconut shell>peat>coal (Paillard, 1990a). However, coal-based GAC finds wide use for most water treatment applications as it has a distribution of both mesopores (2–50 nm diameter) and micropores (up to 2 nm diameter), a structure suitable for medium to large (colour, taste and odour) and small organic molecules (micropollutants), respectively. Pilot plant work or Rapid Small Scale Column tests (RSSCT) should be used to optimize the GAC type and other design parameters such as adsorption capacity (by Freundlich adsorption isotherm) and to determine the life of carbon between reactivation (by RSSCT). EBCT varies for different micropollutants and is usually in the range 5–30 minutes; for pesticides it is 15–30 minutes and for DBPs and VOCs it is about 10 minutes.

Although GAC removes most micropollutants efficiently, the adsorption capacity towards some is low, so that frequent reactivation may be necessary which makes the adsorption process costly. For example, using an EBCT of 10–30 minutes most pesticides or DBPs may show breakthrough in 6–12 months and VOCs in 12–18 months. If only taste and odour removal is required, breakthrough normally occurs in 2–3 years when using an EBCT of about 10 minutes. Breakthrough of TOC generally occurs in about 3 months. In one UK works it has been shown that the TOC removal efficiency reduced from 90% to 10% in 14 weeks, but this did not have any adverse effect on the final water THM concentration which was significantly less than that before the installation of GAC (Smith, 1996). For TOC removal a rule-of-thumb used for estimating GAC life is 50 m³ water treated per kg of GAC (Langlais, 1991).

Backwash requirements are similar to those used for GAC as a filter medium (Section 9.9). The frequency of backwashing of GAC adsorbers at groundwater sites can vary between once every 2 and once every 8 weeks depending on the raw water quality; wash is normally by water only. Frequent air scour tends to break down GAC and where air scour is necessary it should be limited to every 5–10 washes.

At surface water sites, in order to maintain low bacterial counts in the filtered water, the backwashing should include sequential application of air scour and water wash, with the wash frequency being every 2–3 days. This can also help to control the growth of microanimals (zooplanktons such as nematodes, and chironomid midge larvae) because the wash frequency is shorter than their reproductive cycle. The problem of microanimals can also be overcome by chlorinating the backwash water or taking a filter out of service for a period sufficient to produce anaerobic conditions in the filter, to kill the microanimals (Weeks, 2003). This should be carefully controlled so as not to lose biological activity in the filter and to prevent the formation of ammonia and nitrite in the filter. Alternatively, microanimals in the filtered water could be removed by the use of microstrainers (which would also remove any carbon fines). A combination of ultrasound and sand filtration was found to be successful in a demonstration plant (Matsumoto, 2002).

GAC should be reactivated as and when it is exhausted with respect to organic compounds. Reactivation can be achieved (either on or off-site) by heating to 800°C in steam or CO₂, or chemically. Up to 25% of the carbon may be lost in these processes. The loss in adsorption capacity after 1, 4 and 7 reactivations is about 5%, 10% and 20%, respectively (Marc, 1998). The addition of virgin carbon to make up for carbon lost after each reactivation helps restore the GAC to almost its original capacity. Typically the iodine number should not be allowed to fall below about 60% of its initial value (Table 11.3), as reactivation recovers only about 300 points.

When reactivation of the carbon is required it is usually removed from the adsorber by means of a water operated eductor (5 volumes of water to 1 volume of GAC) or by recessed impeller centrifugal pumps (3 volumes of water to 1 volume of GAC) operating at less than 1000 rpm. The removal is only about 90–95%. In some designs, adsorbers are provided with a sloping floor or recessed drain in the floor discharging to a collector system, which helps to improve GAC removal efficiency. All pipework, in particular bends, should be in stainless steel. Straight lengths could be in ABS or PVC-U. Bend radii should be 5–10 pipe diameters. The pipeline velocities should be maintained between 1.5 and 2.0 m/s. The same equipment and design parameters should be used for carbon placement in adsorbers.

Virgin GAC (coal-based, as used in water treatment) contains contaminants such as aluminium (0.65%), iron (0.35%), copper (0.0025%) as well as traces of manganese and arsenic, and reactivated GAC contains additional chemicals adsorbed in the process and not completely removed in the reactivation. Materials that could leach into the filtrate when GAC is placed in adsorbers include sulphides, sulphites and bisulphites (causing chlorine demand and odours), alkali (resulting in high pH), phosphates (if phosphoric acid is used in the activation process) and metals such as aluminium, iron, manganese and copper (Lambert, 2002). Repeated backwashing with water followed by running to waste of the filtrate should be carried out until tests confirm that water is of acceptable quality for supply. For GAC activated by phosphoric acid the phosphate content after reactivation should not exceed 1% w/w.

The impact of reactivated GAC on water quality can be minimized by pre-acid and post acid wash in the reactivation process. Pre-acid wash is useful for manganese, aluminium hydroxide and calcium carbonate and post acid wash is useful for manganese.