The results from the present study showed that the addition of waste iron shavings increased the nitrogen removal efficiency from the biological and catalytic ozonation treated dyeing and finishing wastewater. For AnSBR-C, the removal of nitrogen could be related with the organics in the wastewater, which was thought to be refractory organics since they have undergone biological and chemical oxidation. However, it seems that this part of organics could still be utilized by denitrifying bacteria. The addition of waste iron shavings provided additional electrons and therefore obviously increased the TN removal efficiency. The low ammonia concentration in the effluent indicated that nitrate was mainly removed by biological denitrification (Shin and Cha 2008), but not by the abiotic reactions as shown in Eq. 1 (Suzuki et al. 2012), further showing the advantage of biological denitrification based on iron.
$$ {\text{NO}}_{ 3}^{ - } + 3 {\text{Fe}}^{0} + {\text{H}}_{ 2} {\text{O}} + 2 {\text{H}}^{ + } \to {\text{NH}}_{ 4}^{ + } + {\text{Fe}}_{ 3} {\text{O}}_{ 4} $$
(1)
The effluent COD of AnSBR-C was lower than that of AnSBR-Fe. The reason could be that the bacteria preferred to use iron instead of the refractory organics in the wastewater. In AnSBR-C, the bacteria had to use the refractory organics, and therefore less COD was left in AnSBR-C compared to AnSBR-Fe. The ratio of removed COD to the theoretical COD needed for nitrate and nitrite removal was calculated and is shown in Table 1. The ratio in AnSBR-C was higher than 1, and there were two possible reasons. First, part of the COD from suspended solid in the wastewater might be absorbed by the sludge. Second, the growth of microorganisms might also consume part of the COD. The above mentioned COD were not used for denitrification although it was accounted for the total removed COD, which thereby resulted in the ratio higher than 1. However, the ratio in AnSBR-Fe was 0.46, which means the electrons provided by the removed COD was not enough for denitrification, and therefore the additional electrons should be mainly derived from the waste iron shavings. The lower concentration of soluble Fe ion in the effluent of AnSBR-Fe (4 mg/L) compared to that in the influent could be due to that both ferrous and ferric salts were good flocculant which precipitated easily in neutral pH (Rodrigues et al. 2013; Wang et al. 2008; Zhao et al. 2011). The ratio of MLVSS/MLSS was only 0.24 in AnSBR-Fe, while it was about 0.36 in AnSBR-C, and the result indicated that more inorganic compounds were present in the MLSS of AnSBR-Fe, which could be related with the precipitation of ferric and ferrous salts. The concentration of MLVSS represents the concentration of microorganisms in the system, and it should be noted that MLVSS in AnSBR-Fe did not include all the microorganisms since some microorganisms could be attached to the waste iron shavings and therefore not be quantified by the measurement of MLVSS.
In the batch experiments, the presence of waste iron shavings improved the TN removal efficiency by 3% (the TN removal efficiencies increased from 14 to 17%) in the batch experiments (Fig. 6b), while the TN removal efficiency was improved by 8% (the TN removal efficiencies increased from 12 to 20%) with the addition of waste iron shavings in the continuous experiments (Fig. 3). The reason might be that the microorganisms utilizing waste iron shavings were mostly attached in the surface of waste iron shavings, while the sludge used for the batch experiment was obtained from the liquid phase. The observation of biofilm formation during iron corrosion was also reported before (Lee and Characklis 1993). As previously mentioned, the TN removal by heterotrophic denitrification should be lower than 12% (the value in AnSBR-C) in AnSBR-Fe since the effluent COD of AnSBR-Fe was higher than that of AnSBR-C. Since the TN removal by waste iron shavings via chemical reaction could be negligible (Fig. 6c), the TN removal efficiency by autotrophic denitrification should be higher than 8% in AnSBR-Fe considering the total TN removal efficiency of 20%.
3DEEM fluorescence analysis shows that protein like compounds were not degraded in both AnSBR-Fe and AnSBR-C. By comparing the 3DEEM fluorescence of the effluent from the two reactors (Fig. 7), it was obvious that the intensity of fulvic acid-like peak in the sample obtained from AnSBR-C was higher than that of AnSBR-Fe, which could be due to that more organics were degraded in AnSBR-C as shown in Table 1 and therefore more fulvic acid-like compounds were formed.
The effluent of AnSBR-C contained 580 species by UHPLC-QTOF analysis, which was slightly higher than that in the raw wastewater, and it could be related with the formation of new species in the reactor. For the effluent of AnSBR-Fe, the number of detected species was only 547, which was lower than that in both raw wastewater and effluent from AnSBR-C. The above results clearly showed that the organic transformation in AnSBR-C and AnSBR-Fe were different. It should be noted that decreased organic species in the effluent of AnSBR with waste iron shavings does not necessarily mean the low organic concentration in the effluent as can be seen from the COD results (Table 1).
GC–MS analysis showed that 1-Dodecanamine, N,N-dimethyl- was dominant in all the three samples, which was also detected in the wastewater in our previous study (Wu et al. 2016). A better degradation efficiency was observed in AnSBR-C compared to AnSBR-Fe, which might be related with the higher degradation of COD in AnSBR-C. Similar results were also found for other organic species including 2-Propanone, 2-(2-N-Benzyl-N-methylaminoethyl)-4,5-dimethoxyphenylaceticacid, methyl ester, Octadecane etc. The above results clearly showed that the degradation efficiency of some organic species might be reduced due to the addition of waste iron shavings. The rest organic species were not degraded or even enriched. It could be due to that these organics species were recalcitrant to biological degradation or produced by the transformation of other organic compounds.
The results from the present study clearly showed that waste iron shavings could be used to improve the nitrogen removal efficiency from the biological and catalytic ozonation treated dyeing and finishing wastewater, where the residual organics were not enough to be utilized for biological denitrification. The nitrogen removal efficiency was around 20% in AnSBR-Fe, and the corresponding TN in the effluent were in the range 35–45 mg/L, which was still higher than the discharge requirement (<20 mg/L). In order to further increase the TN removal efficiency, pretreatment of the waste iron shavings to increase the surface area may be a solution, which would increase the contact between waste iron shavings and microorganisms and therefore increase the electron transfer rate. For instance, NZVI was used as electron donor for biological denitrification, which resulted in high nitrogen removal efficiency since it has huge surface area (An et al. 2009; Shin and Cha 2008). However, NZVI was not feasible for wastewater treatment in practice due to its relatively high cost, difficulty to be stored and easiness to be oxidized. Therefore, the increase of the surface area of waste iron shavings would be a cost-effective method for biological denitrification. Another way to further increase the TN removal efficiency would be the addition of organic carbon sources (e.g. acetate, methanol) in order to meet the discharge requirement.