The gravel layer is a lower part of the conventional system of bioretention that contains fine, medium, or coarse-grade gravel. This layer is the more porous layer of the bioretention system. The primary function of the gravel layer is to collect and transport the treated water to the outlet pipe or surrounding soil, as well as to prevent the washout of engineered soil [20,102].

The inclusion of a saturated zone (SZ) in the bioretention system has a positive effect on the reduction of nitrogen (N), especially nitrate (NO₃) and nitrite (NO₂). Among the research community, this is a well-known principle. The SZ is used mainly to provide anaerobic conditions as well as plant survival between events in the dry season [102,103]. The main reason for inadequate N removal in the traditional bioretention system is the lack of a denitrification process and anaerobic condition [45,67]. An anaerobic condition is essential for NO₃ removal to complete the deoxidation or denitrification process. The denitrification process is the process of releasing gaseous nitrogen in the forms of N₂O, NO, and N₂ [102]. Several published studies describe the link between the removal of N and SZ via the provision of an anoxic zone to improve microbial activity [21,63,76,104]. The denitrification process is unstable; in some cases, it has been found that there was no significant effect of SZ on TN removal [12,105]. This may be due to the presence of a carbon source in the soil media that has been converted into NH₄. Furthermore, some studies have found that SZ does not affect NO_X removal (amendment media and SZ) [106,107]. The presence of carbon sources has the main role in SZ enhancement [26,108,109]. To improve the denitrification process, different forms of carbon sources have been used, including woodchips, newspapers, sawdust, and sulfur [109,110,111]. Adding a carbon source within SZ enhanced TN [26,108]. The presence of SZ can also enhance TN and NO₃ removal, whereas NH₄ reduction is not dependent on the presence of a saturated zone (SZ) [51]. On the other hand, NH₄ removal in a bioretention system without a saturated zone is more efficient. Increasing the depth of the saturated zone has a negative effect on NH₄ removal [108]. Up to 95.42% of NH₄ is retained in the soil media [112]. Different saturated zone depths have been suggested, ranging from 150 to 600 mm [11,23,111,113]. In terms of cost, Xu and Zhang [113] recommended that the best SZ depth for TN removal was 450 mm, and including SZ would mean more excavation work and higher costs. Table 3 shows a list of studies with different SZ depths and removal efficiency.

Recently, it has been suggested that additives can be used to enhance filter media because they are known to be effective in eliminating nitrogen (N). Waste products are mainly used to improve bioretention efficiency because they are cost-effective, require less effort, and can solve environmental issues. Several types of additives are used as a layer or mixed with soil media, including newspapers, woodchips, sawdust, wheat straw, Skye sand, shredded hardwood bark mulch, and water treatment residuals (WTR). In addition, the bilayer media concept is also used to enhance the bioretention system; it involves different layers of modifier media with various mechanical and chemical properties. The wide range of layer properties including porosity, permeability, particle size, water holding capacity, moisture content, bulk density, CEC, and pH would provide adsorption, nitrification, and denitrification conditions [7,12,39,75,118,119]. The bilayer of bioretention forms an anaerobic condition and increases nitrogen removal by applying a low-porosity layer in the lower portion of the media, which results in best nitrogen removal [39,105]. The less-permeable layer in the bottom of the bioretention media decreases water flow, thereby impeding the diffusion of oxygen and forming an anoxic zone [105].

Furthermore, the available carbon source in this layer promotes the denitrification process [27,39,120]. The denitrification process could be provided by the inclusion of a low-porosity layer at the bottom of the soil media [39]. Providing denitrification conditions in soil media is encouraged, especially in wet climates [28]. The inclusion of a saturated zone (SZ) in the bioretention system is not necessary for tropical countries with rainfall depth of over 2000 mm [28,76,103]. Overall, amendment materials improve nitrogen removal and offer a promising approach for bioretention enhancement [106]. The common additives that have been used as absorptive, nitrifier, and denitrifier materials are shown in Table 4. However, most studies on this topic do not study the removal of nitrate and nitrite and focus only on the reduction of TN.

The combination of modified filter media (as a mixture or as a layer) and the saturated zone (SZ) is the latest development in nitrogen removal enhancement [11]. This configuration is considered the best since the development has improved the conditions of nitrification and denitrification [127]. Nitrification occurs through enhanced soil media, especially in the dry season, and denitrification via SZ. In this approach, the process of nitrogen removal occurred in steps through soil media, where ammonium (NH₄) was adsorbed in the upper part of the filter media and transferred by the nitrification process to nitrate (NO₃) [128]. The addition of additives containing a carbon source increases NH₄ adsorption. Furthermore, these additives improve the microbial activity of soil media, thereby enhancing the removal of NO₃ by microorganism assimilation and dissimilation [106]. Microbial activity plays a critical role in minimizing NO₃ compared with soil adsorption [129]. The combination of the saturated zone and modified media in the bioretention system promotes the nitrogen cycle [11,27,120]. Emma V. Lopez-Ponnada et al. [26] conducted a field study and compared the modified system (combined woodchips layer with SZ) and the traditional bioretention system without any modification. The findings have shown that removals of NH₄ and NO_X (NO₂, NO₃) in the modified system were 83% and 81%, respectively, and for the traditional system 74% and 29%, respectively. Some of the studies applied modifiers as a mixture with the media [130]. Xiong et al. [106] compared the performance of the traditional system and retrofitted media with biochar or iron-coated biochar (ICB) and rice husk (RHB). The results have shown that with the inclusion of the SZ, the enhancement media with ICB and RHB give a better performance than the traditional system. The efficiency of the amendment material depends on the CEC and surface area [106]. Another study was undertaken using various materials for waste modification, including flyash, shells, ceramsite, pyrite, quartz, grinding slag, bottom ash, electric arc furnace slag (EAFS), and basic oxygen furnace slag. The results showed that the retrofitted media with bottom ash yielded the best performance with TN removal, indicating an improvement from 58% to 70% [11]. At present, limited research has been conducted to examine the feasibility of this strategy.