Questions about activated carbon

Questions about activated carbon processes

Unfortunately, there is no simple answer to this question. Besides the pure adsorption capacity, other factors also play a role, such as density, settleability, particle size distribution, hardness (abrasion) and price. Activated carbon manufacturers and suppliers usually have a great deal of know-how about the adsorption capacities of their carbons. Environmentally friendly production, if possible from renewable raw materials, is to be strived for.

PAC in itself is not a hazardous substance. However, inhalation of PAC dust can cause respiratory problems. PAC can also cause mild irritation if it comes into contact with the skin or eyes. What’s more, PAC can cause dust explosions when stirred up in the air with an ignition source present. This factsheet (in German, French and Italian) describes safety aspects in handling PAC.

The dosing quantities of activated carbon are relatively low; the tests conducted so far have shown that sufficient removal (> 80%) can be achieved with 10–20 mg of activated carbon per litre of wastewater. In the effluent of WWTPs, 3–15 mg of filterable substances per litre are usually measured. It is clear that after a PAC stage, a filter is needed to retain the fine fraction of the carbon. In one of the platform projects, the loss of PAC was measured for different processes and the results were compiled in a factsheet (in German and French).

So far, regenerating PAC is not economical. Presently, it is planned to treat the PAC with the primary and activated sludge in the existing sludge treatment. However, no detailed information is available yet.

In the Ulmer process, PAC, precipitants and flocculants can be dosed into the contact reactor in this order. At the WWTP Bachwis in Herisau, a three-point dosing for the precipitants was examined (biology, contact reactor inflow and directly in front of the filter), which proved to be more efficient. The flocculants are added at the end of the contact reactor.

In the process where PAC is dosed in front of the spatial filter, the precipitant is dosed with the PAC into the contact reactor. In this case, no flocculant is dosed.

When dosing PAC into the biology, the addition of precipitation and flocculation aids must be assessed on a case-specific basis.

At WWTP Bülach-Furt (GAC filtration), the pilot tests are being carried out with fresh GAC (see final report, in German). At WWTP Penthaz (GAC in fluidized bed), the 1st phase of the pilot test was carried out with fresh GAC and the 2nd phase with reactivated GAC (see interim report in German and French). At WWTP Glarnerland, three different GAC have been tested, including a reactivated one (see final report in German). After each reactivation, only about 10 to 20% fresh GAC is needed to restore the adsorption capacity. In this way, CO2 emissions can be significantly reduced in comparison to PAC. Currently, there is no possibility to reactivate GAC in Switzerland. However, the activated carbon can be reactivated in kilns in northern Italy.

Questions about the loss of activated carbon

There is no threshold value for activated carbon loss. However, it must be ensured that the activated carbon used is retained to a large extent to protect the receiving water (Art. 63 WPO, Proper Water Protection). As part of a broad-based measurement campaign, various effluents from WWTPs with an activated carbon stage were examined for activated carbon loss. The results have shown that with most processes, the activated carbon concentrations in the effluent are within the detection limit (0.1 mg/L) and an activated carbon retention of over 95% is technically feasible. More information can be found in this report (in German) and this factsheet (in German and French).

In the project “Activated Carbon Loss from Treatment Stages for the Removal of Micropollutants”, two methods were compared: thermogravimetric analysis (TGA) and the gradient total organic carbon (TOC) method. Both methods determine the activated carbon content relative to the amount of total solids. The detection limit is 0.1 mg AC/L. It has been shown that the two methods are comparable and suitable for this application. It is therefore recommended to periodically measure the activated carbon content in the WWTP effluent using one of the two methods. This report (in German) and this factsheet (in German and French) give more information. The current data does not show a clear correlation between the activated carbon concentration in the effluent and common sum parameters such as total suspended solids (TSS) or turbidity. When these values are low, however, it can generally be assumed that the activated carbon loss is also low.

After dosing PAC, it must be separated as well as possible from the treated wastewater. Various processes have been tested for this purpose: flocculation filtration, membrane filtration, or sedimentation followed by sand or cloth filtration. A final filtration stage can ensure that the TSS concentration in the effluent is reliably below 5 mg/L. However, the quantitative composition of these TSS is often unclear, as is the proportion of PAC (other components are essentially activated sludge and precipitant flocs).

New analytical methods (e.g. thermogravimetry) can be used to determine the proportion of PAC in the TSS. Studies to date show that the above-mentioned separation methods can retain over 99% of the dosed PAC (cf. report, article in German). This provides an up-to-date assessment of activated carbon retention and can assist in determining the number of provisions necessary for the activated carbon concentration.

The risk of re-dissolution of micropollutants from the PAC into the water bodies is currently estimated to be very low (cf. studies by the Competence Centre for Micropollutants Baden-Württemberg, Meckes et al., 2014). From long-term operations of numerous PAC plants in Germany, no negative consequences of PAC loss on water bodies are known. Nevertheless, it is legally stipulated (Art. 63 WPO, Appropriate Water Protection) that the input of PAC into water bodies must be minimized.

No, efficient PAC retention can also be ensured by using processes other than membranes. The important thing is to ensure that the PAC is retained as completely as possible.

There are no studies on this. The re-dissolution of micropollutants in the water is not to be expected on a large scale. In laboratory experiments, where attempts were made to dissolve the micropollutants back from the PAC, this was hardly possible.

No, the micropollutants are measured in the dissolved phase – both in the inflow and in the outflow.

Questions about activated carbon quality

There are various parameters that can be used to assess the quality of activated carbon. The specific requirements should be defined in advance and contractually regulated with the supplier.

A key parameter is the effective abatement performance achieved in comparison to the sample carbon based on which the product was selected. The abatement performance is usually determined by means of stirring tests in the laboratory (instructions here in German). Such tests require a few days and can therefore only be carried out after the activated carbon has been filled into the silo.

It is also advisable to determine the water content (according to the DIN EN 12902 standard at 150°C), which should be below 10%. The ash content and the ash residue can be used to determine the proportion of fresh carbon and reactivated carbon; the ash content of fresh carbon is around 5–10% and that of reactivated carbon is below 15% (Rössler et al., 2019). Currently, there is no general procedure for GAC, as it is more difficult to test in the laboratory due to its coarse grain size and because a GAC filtration system takes several months to operate.

In principle, information on sustainability and social/ethical aspects is limited and difficult to verify. One way of taking social aspects into account in a tender is to formulate compliance with the core conventions of the International Labour Organization (ILO) as a condition of participation.

By choosing an activated carbon product made from a renewable raw material (e.g. wood) or with a high proportion of reactivates (i.e. activated carbon that has already been used in other applications and regenerated), the environmental impacts (in particular greenhouse gas emissions and primary energy consumption) can be greatly reduced. To assess the environmental impact in tenders, it is possible to evaluate the information on the raw material and the proportion of reactivates in the corresponding activated carbon product, as long as they are comparable. According to current knowledge, sustainable products are not worse than less sustainable products in terms of abatement performance, but tend to be more expensive.

So far, there is no label or equivalent for activated carbon products for WWTPs. However, such a label would be welcomed by many stakeholders, including manufacturers. In other areas, activated carbon labels or certificates already exist, for example for barbecue charcoal or for biochar, which is used, for example, as a soil conditioner.

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