3.1. Performance of Organic Matter and Settleable Solids Removal
The average BOD influents and effluents were 234 ± 79, 142 ± 52, 119 ± 31, 104 ± 26, 120 ± 32 mg L−1
, for BTFLV
respectively. The average efficiencies for BOD were 39, 49, 56, and 49% for BTFLV
, respectively (Figure 2
A). The values in general are lower than indicated in literature considering low rate BTFs. According to Ali et al. [1
], BTFs achieve usually 85–90% BOD and 80–85% COD removal efficiencies after the second clarifier. The reasons that led to the obtained average values for BOD may be related to the high solid input contribution in the BTFs (Figure 2
C) in which the system had not retained enough and due to the detachment of biofilm.
However, for filtered BOD, the average efficiencies were 77, 82, 78, and 80% for BTFLV, BTFL, BTFGV, and BTFG, respectively. These satisfactory values regarding filtered BOD indicate a high capacity of removing dissolved BOD as most of the remaining BOD was in the particulate form. Part of this material can be removed in the secondary clarifiers, providing a lower level of suspended solids in the final effluent and consequently a lower concentration of organic matter in the receiving water body.
shows results from the factorial statistical analysis, showing the effects caused by each factor (media and vegetation) and the effect caused by the interaction between them.
We can observe in Table 3
that the units with gravel as support media differed significantly. The solids retention in the gravel might create a favorable environment for BOD removal, probably because this media worked as a barrier preventing a portion of the suspended solids to be routed out of the BTFs. Kishimoto et al. [27
] evaluated the performance of trickling filters using two types of plastic media of the same material and concluded that the rougher surface retained twice more biomass of microorganisms than the smoother, contributing to greater COD removal. Furthermore, according to the same authors, the roughness of the filter media may prolong the hydraulic retention time. Therefore, despite its smaller void ratio and specific surface area compared to LECA, the rougher surface of the gravel media maybe has enabled better adherence and retention of the bacteria, besides prolonging retention time in comparison to LECA.
The higher settleable solids values in BTFLV
effluent in comparison to the other BTFs (Figure 2
C) can be explained by the use of media with different specific surface and structure which, according to Pérez et al. [28
], could produce changes in the transfer and diffusion of nutrients to the packing media, causing different degrees of development and detachment of biofilms in BTFs. The uneven sloughing of slime layer is able to led high settleable solids in the effluent [1
]. This discontinuity might be increased by the penetration of the roots while it grows. Furthermore, it is known that clay has a high void ratio, which is positive for providing a greater airflow into the system, but enables higher solids carryover out of the filter. These prerogatives might explain the higher levels of settleable solids in BTFLV
of 1.5 mg L−1
against 0.5, 0.8 e 0.2 mg L−1
, and BTFG.
For this parameter, BTFG
has the lowest level, reaffirming the solids’ retentions by this material. According to the results showed in Figure 2
A, a significant part of BOD not removed from BTFLV
may be settleable in a second clarifier.
A large part of this BOD in the BTFLV
effluent (Figure 2
A) comes from the organic matter contained in the biofilm. It is indicated by the difference between the results of filtered and unfiltered effluent BOD, besides the higher value of settleable solids (Figure 2
C). The Pérez et al. [28
] statement, together with the vetiver performance described in the previous paragraph, can also explain the high variation of the unfiltered BOD caused by the uneven detachment of biofilm layers from different stages of degradation.
was the filter that showed the best results in terms of DO (Figure 2
B), with a statistically significant average of 2.11 mg L−1
against 0.66, 0.23, and 0.75 mg L−1
, and BTFG
, respectively. It is known that DO is an important parameter for the oxidation of nitrogen, influencing the performance of BTFs.
In Table 3
, the presence of vetiver grass when analyzed individually had no improvement on the performance of BTFs, concerning BOD removal. According to Ali et al. [1
], better aeration of the system contributes to the greater removal of organic matter in BTFs. However, the results found in the present study indicated the opposite: between BTFLV
, the second option showed to be more efficient in removing organic matter. Queluz [29
] found a lower BOD content in the final effluent for non-vegetated CW than for CW vegetated by Typha latifolia
. According to the author, it is possible that this result occurred due to the release of exudates by the roots. Such a prerogative can explain what happened in the present study. There was no significant difference between both filters with gravel due to the solids retention previously mentioned.
Mendonça et al. [30
] also did not find a significant influence of plant species for the removal of BOD. The authors state that BOD removal is more significantly related to other mechanisms such as physical (removal of particulate BOD) and microbiological (removal of soluble BOD) mechanisms. However, they emphasized the importance of vegetation for other processes.
3.2. Performance for Phosphorus Removal
Analyzing the phosphorus average in Figure 2
D, a minimum difference between influent and effluent was observed. Unsatisfactory removal values were consistent with Kishimoto et al. [27
] that used a BTF system with OLR ranging from 0.2 to 0.9 kg m−3
of BOD and HLR from 1 to 5 m3
(ranges that include the operational parameter employed in the present study) with influent concentration 3.0 mg L−1
of phosphorus and did not obtain removal of this parameter. The average concentrations of phosphorus effluent on the media and vegetation are shown in Table 4
. Statistical analysis shows no significant interaction between factors; therefore, its unfolding was not necessary.
As can be seen in Table 4
, the gravel media (BTFGV
) differed statistically from LECA (BTFLV
), wherein the gravel provided better results for total phosphorus removal. BTFG
had an average removal efficiency of 28% against 19%, 18%, and 19% of BTFLV
, respectively. Regarding the support media performance, these results agree with those obtained by Lima et al. [31
] in CWs. Comparing the performance of CWs without gravel vegetation and LECA as support media, the authors found close efficiency of 12%, 26%, 12%, and 25% for CW with gravel only, gravel with vegetation, LECA only, and LECA with vegetation, respectively, at a retention time of 24 h. However, at a retention time of 48 h and 72 h, Lima et al. [31
] concluded that the absorption through plants in CWs was probably the most significant mechanism of phosphorus removal in gravel and LECA.
It is known that BTFs usually have a lower retention time than CWs, causing less contact time between the wastewater and the vegetation. It was mentioned that a rougher media as gravel allows a longer retention time than LECA. Therefore, just as described for BOD, in the present study the difference in the efficiency of BTFG
) may have occurred because it stayed longer retained on the media gravel than in LECA, increasing its retention time.
Considering the vegetation factor (BTFGV and BTFLV: with the presence of vetiver grass and BTFG and BTFL: without the presence of vetiver) no statistical difference was noticed. This indicates that vegetation did not affect significantly phosphorus removal in BTF. It was noticed a reduced retention time of BTFs compared to CWs due to the high capacity of biomass assimilation and growth rate of vetiver. Despite that, a positive interference of vegetation in phosphorus removal in BTFs as in CWs did not occur, even on a small scale, as expected.
3.3. Performance for Nitrogen Series Removal
As can be seen in Figure 2
E,F, the TKN removal efficiency for BTFLV
was 27%; 20%; 12% and 31%, respectively. For TAN, these values were 24%; 7%; 7% and 22%, respectively.
The results of statistical analysis applied to TKN and TAN are shown in Table 5
. No significant difference to the packing media or vegetation was observed. However, the interaction of these factors had a significant difference. Nonparametric statistics were used for nitrate due to no fulfillment of the assumptions for the parametric test.
Comparing the concentrations of TKN, the BTFG
was statistically different from the others, providing the best results for its removal (Table 5
). The solids retention factor afforded by gravel helped in this result since the TKN composition is present in the nitrogen organic forms, which is more susceptible to removal physical processes, such as the retention of solids through the media.
Metcalf and Eddy [22
] affirm that DO concentrations of 0.5 mg L−1
B) approximately, may cause denitrification due to the repression of nitrate-reducing enzymes. Considering TAN, both BTFLV
differed significantly (Table 5
). Based on this result and according to the DO concentrations in Figure 2
B, we can suggest that there was significant nitrification for BTFLV
in comparison with the other BTFs. Therefore, lower aeration of BTFL
may have affected the DO levels which were insufficient to promote suitable conditions for nitrification.
In Lima et al. [31
] study, the influent used by the authors had a concentration of BOD and TKN close to the values found in the present study. The authors found average efficiencies for TKN removal of 12%, 21%, 9%, and 19% to CW with only gravel, gravel with vegetation, only LECA and LECA with vegetation, respectively. They affirmed that there was no difference in the performance of CWs with or without vegetation for this parameter. Onodera et al. [32
] operating a BTF with TKN influent of 30 mg L−1
obtained an effluent with 21 mg L−1
of TKN, and further by opening a window in BTF, the concentration reduced to 4 mg L−1
provided by the larger input of DO.
Moreover, competition between nitrifying and heterotrophic bacteria may also interfere in the nitrification process. Conditions of high concentrations of organic matter can provide a favorable environment predominance of heterotrophic bacteria, as these bacteria compete for oxygen and space, restricting the nitrifying microorganisms [33
Regarding the N-NO3−
was the most promising among the BTF, which differed significantly from the others by the Kruskal–Wallis test at a 5% significance level (Table 5
). Distinct behavior of vetiver grass over the inserted media was observed. In the filter where vetiver grass was associated with LECA (BTFLV
), it was noticed a positive effect of vegetation in the BTF in two of the three forms of nitrogen measured. As exposed previously, BTFLV
had also the highest concentration of DO (Figure 2
Ye and Li [34
] emphasize that complete nitrification is only able to occur for DO above 1.5 mg L−1
, which was achieved only by BTFLV
to the detriment of the other BTFs. This prerogative can explain the results for BTFG
, which despite presenting TAN removal statistically significant from BTFL
, the N-NO3−
levels were not statistically equivalent to BTFLV
Angassa et al. [20
] also obtained a significant increase in DO in CW planted with vetiver, even with decreasing hydraulic retention time. According to the authors, this result may have occurred due to oxygen release into the root zone area in the massive root hairs, resulting in more aeration. In the present study, the largest voids of LECA when associated with vetiver grassroots may have provided higher aeration of the filter and consequently a more favorable environment for the nitrification denoted by the higher N-NO3−
concentration (0.35 mg L−1
Vetiver is an easily adaptable plant in harsh environments. The cost-to-benefit of its implementation is high since the plant is able to assimilate a lot of biomass with a low initial cost [35
]. Furthermore, the plant can survive for decades with little maintenance [36
]. Regarding LECA, despite its high initial cost compared to gravel, the cost of acquisition can be compensated with the lifetime of this material, which is equivalent to the system lifetime. In this way, vetiver can be implemented in BTFs, together with LECA, when aiming for higher levels of nitrification at an affordable cost.
Given the vetiver seedling cost and considering a plant density equal to 15 units per m² of the area in biofilter plant, according to the adopted by Darajeh et al. [37
] for wetlands, an additional cost of implantation of US $
6.00 per m² of planted area for implementation of BTFLV
in relation to BTFL
is estimated. In general, each filter has a different behavior, being more suitable according to a chosen variable. For BTFLV,
the dissolved oxygen variable stood out with the highest concentration among BTFs. Moreover, it reached the best result for TAN and N-NO3−
as well. The BTFL
excelled with the lowest values for BOD. In addition, the BTFG
had the lowest settleable solids and TKN values, besides good performance for TAN removal as well. Both BTFLV
had more variables that excelled, indicating the good settings from the studied filters.