*3.2. Hydraulic Properties of Modifiers*

Excellent hydraulic performance, including high water retention capacity and permeability, are fundamental criteria for choosing filter additives in bioretention systems, which are also shortcomings of biochar at present [19], when compared to biological waste and mineral materials. Basic experiments were conducted to inspect the improvement of PCB and the results are illustrated in Table 1. After being modified by polyurethane, the material became lighter, with a bulk density of 0.165 g/cm3, compared to its former bulk density of 0.378 g/cm3. Such lightweight polyurethane gave PCB a sizable performance boost in water retention capacity [40], whose saturated water content was improved from 195.65% to 383.5%. If added to the filter layer with the same mass ratio (4% of additive materials in traditional bioretention facilities [41]), PCB can reduce 42–63 mm of stormwater within the unit area according to the following water absorption formula:

$$\mathcal{W}water = \rho filter layer \times h \times n \times \omega sat,\tag{5}$$

where *Wwater* is the stormwater retention volume of the filter additive within unit area; *ρfilterlayer* is the density of the filter layer in bioretention facilities, ranging from 0.8 to 1.2 g/cm3; *h* is the filling height, usually calculated as 70 cm; *n* is the mass ratio of the filter additive; *ωsat* is the saturated moisture content of the filter additive.


**Table 1.** Physicochemical properties of PCB and HB.

<sup>1</sup> *ρ* is natural bulk density. <sup>2</sup> *e* is pore ratio. <sup>3</sup> *ωsat* is saturated moisture content. <sup>4</sup> *K* is permeability coefficient. <sup>5</sup> TP is total phosphorus content.

This improvement is of great significance and will achieve a high storage volume in order to reduce peak flow and enhance the removal of many contaminants from stormwater [42]. The Permeability of HB was also improved by the polyurethane polymerization, the infiltration coefficient of which changed from 6.57 × <sup>10</sup>−<sup>4</sup> to 8.56 × <sup>10</sup>−2, with an improvement of more than two orders of magnitude. Apart from the bigger particle size, as an influencing factor on permeability, the internal throats formed in the foaming process also took effect, where stormwater flowed inside the network through the throats, increasing the number of available flowing paths [43].
