Activated carbon use in Ontario drinking water treatment plants

The Treatment Committee of the Ontario Water Works Association (OWWA) performed two surveys of activated carbon usage at drinking water treatment plants in the province. A 1995 survey was to determine the extent and application of activated carbon use. A follow-up survey was undertaken in 1998, to determine changes in carbon usage since the first survey. The perception was that taste and odour events in Ontario had increased and activated carbon was increasingly being used in drinking water treatment.

The surveys were conducted by mailing questionnaires to water treatment plants throughout the province, using the OWWA utility database. In the 1995 survey, 93 completed questionnaires were received and in the 1998 survey, 43 completed questionnaires were received.

One of the major findings of the survey was that activated carbon usage has increased substantially in the past three years. In the 1995 survey, 27% of the plants used activated carbon and in the 1998 survey this percentage had increased to 49%. (Figure 1)

Typically, activated carbon is applied either in powdered form as Powdered Activated Carbon (PAC) or in granular form as Granular Activated Carbon (GAC).

In the powdered form, activated carbon is added as either a dry powder or a slurry to the raw water, similar to the addition of any other chemical. The compounds in the water which are adsorbable are adsorbed on the carbon and then the carbon is removed from the water by the downstream sedimentation and filtration processes.

In the granular form, the water to be treated flows through a bed of activated carbon and the compounds in the water are adsorbed onto the carbon. When the adsorptive capacity of the GAC is exhausted, it is removed, replaced or regenerated.

The type of carbon used was split almost equally between Powdered Activated Carbon and Granular Activated Carbon. Of the plants using activated carbon that responded to the 1998 survey, 40% used PAC, 45% used GAC and 15% used a combination of PAC and GAC. (Figure 2)

It was also shown that activated carbon usage was more prevalent for some raw water sources than for others (Table 1).

Powdered Activated Carbon

One of the interesting findings from the surveys was that from 1995 to 1998, PAC dosages have increased. The median PAC dose from the 1995 survey respondents ranged from 0 to 5 mg/L. By the 1998 survey, this median PAC dose had increased to 5 to 10 mg/L. The survey found that the majority of the plants used PAC during the summer and Fall periods. In the 1995 survey, however, 30% of the plants used PAC continuously. By 1998, the plants using PAC continuously had decreased to less than 10%. This is likely a result of operations staff optimizing PAC use.

Approximately 50% of the plants using PAC used it for 60 to 180 days of the year. The length of time PAC was used during the year, increased from the 1995 survey to the 1998 survey. In 1995 the median period of use was 30 to 60 days/year and by 1998 this had increased to 60 to 90 days/year.

Granular Activated Carbon

Most of the plants using GAC apply it in the filter adsorber mode; only one plant responding to the survey used dedicated GAC contactors. Filter adsorbers are typically constructed from conventional dual media sand anthracite filters in which the anthracite has been replaced with granular activated carbon.

The contact time for GAC contactor affects the efficiency of the contactor in that the more contact time available, the more efficient the adsorption will be. The median Empty Bed Contact Time based on the survey responses was 5 to 10 minutes.

The life expectancy of the GAC, based on the responses from the survey, averaged approximately three years. Approximately half of the plants using GAC leased the GAC and the other half had purchased it.

The OWWA intends to update this survey on a regular basis to track changes in carbon usage. A future survey will compare raw water temperature with taste and odour episodes.