## 2 Data, Methods and Techniques  
### Filter Supplemented with PAC  
The filter bed designed for adsorption on powdered activated carbon (PAC) may consist of two or three-layers. PAC, as an aqueous suspension, is dosed by a peristaltic pump into the top layer of a two-layer bed or the top/middle layer of a three-layer bed. PAC is introduced into the filter after backwashing while this layer is still in a fluidized state; the dosing is continued until traces of carbon start to appear above the layer. Once the appropriate dose of PAC suspension has been introduced, the backwash flow is turned off and PAC stays deposited inside the layer. The lower one/two layers of the filter protect the installation against carbon particle wash-out. Once the sorbent adsorption capacity is exhausted, PAC is removed from the filter during backwashing.  
In the study, a column with an internal diameter of 3 cm was used (Figure 1). It comprised four segments, each 0.5 m long, joined together with flanges and screws. The filter was a dual media filter (total height of 1.1 m) with an upper anthracite layer 0.5 m in depth (Figure 2) and a lower supporting layer of sand and gravel 0.6 m in depth (Figure 3); the depth of each layer was measured at the end of each experiment. A tube with an external diameter of 0.6 cm was permanently inserted into the bed (Figure 1) to supply the PAC suspension into the fluidized anthracite layer. Similar experiments have been reported using a filter filled with pumice particles [28; 29]. The authors used PAC collected at the Dlubnia Water Treatment Plant in Krakow (Poland), a product of AquaSorb C, Jacobi Carboons GmbH, widely used as a sorbent in both industrial and municipal applications.  
Figure 1: Filter column. P1—feed pump. P2—pump dosing PAC suspension. PPAC—tube supplying PAC suspension into an anthracite layer.  
Figure 3: Sand layer and supporting (gravel) layer.  
Figure 2: Anthracite layer.  
The PAC's main parameters are as follows:  
* Methylene number: min. 20 mL,
* Iodine number: min. 850 mg/g
* Moisture: max. 8-10%
* Surface area: min. 800 m\({}^{2}\)/g  
In the filter, the sorbent stays in contact with organic substances at concentrations comparable to the feed concentrations for a long time (several or several dozen hours, depending on the filter cycle). In homogeneous flow units, i.e., units with a plug or dispersed flow (e.g., flocculators), concentrations of the adsorbate in contact with PAC are lower than in the filter. Therefore, the adsorption rate in the drive modules for the filter will be higher than that for the other units [14; 15; 16; 17]. A higher adsorption rate in drive modules guarantees better utilization of the PAC's adsorption capacity. Higher sorbent doses to flocculators would be required to observe filter adsorption effects similar to those of PAC [14; 15; 16; 17].  
In additional tests on a semi-technical scale, two columns with a diameter of \(d=0.150\) m were used. The columns were filled from the bottom with a layer of gravel 0.15 m in depth. The next layer was sand with a diameter of 0.3-1.02 mm, diameter \(d_{10}=0.61\) mm, with a coefficient of grain unevenness \(d_{60}/d_{10}=1.3\) and a depth of 0.5 m. Above the sand, there was a layer of anthracite with a particle diameter of 0.75-1.5 mm, a diameter of \(d_{10}=0.73\) mm, with a coefficient of grain unevenness \(d_{60}/d_{10}=1.37\) and a depth of 0.5 m. The filter bed used in the laboratory tests also had the same technical parameters. Water from the settling tank at the Dlubnia Water Treatment Plant in Krakow (Poland) was pumped into the filter beds. Water from the Dlubnia River was initially purified in the coagulation process using the PAX\(-\)16 coagulant, a product of Kemipol, Police, Poland.