## 3 Results and Discussion
### Effect of the Initial Hexavalent Chromium Concentration
There are several ways of handling sludge from water treatment processes, such as dewatering, stabilizing and disposal to landfills, but the most important aspect is to reduce the amount of sludge produced, which can be potentially achieved by the multiple dosing suggestion in a unit operation such as that proposed in the present study. The chromium removal results from all the experiments in this study were in agreement with the results from previous studies, examining Cr(VI) removal in batch mode operation [PERSON] et al. (2011) [PERSON] et al. (2011) found that for pH 7.5-8 and for ratio [Fe(II)]/[Cr(VI)] equal to 5, the chromium removal is  
Figure 4: Influence of starting hexavalent chromium concentrations on Cr(VI) removal. Experimental conditions: initial Cr(VI) concentration 50, 100, 200, 400 \(\mu\)g/L, flocculant dose 0.5 mg/L, linear velocity 4 m/h, pH 7.3.  
up to 40-60%, a result that has also been achieved in this study. [PERSON] et al., 2011 [15], claimed that the optimal ratio [Fe (II)]/[Cr (VI)] for Cr(VI) removal equal or greater than 90% is 15-20, at pH 7.5 and 25 \({}^{\circ}\)C. Our experiments are also in agreement with this claim to achieve chromium removal rate more than 90% and as previously discussed with other similar studies [14]. A previous study by [PERSON] et al., 2017 [20], examined the removal of Cr(VI) by Fe(II) using pipe flocculation followed by ultrafiltration, with part of the sludge to be recirculated into the pipe reactors. This unit was similar to that examined in present study, with the difference that in the present study, the sludge was not recirculated. In this study it was shown that Cr(VI) concentrations in the order of 300 \(\upmu\)g/L could be removed to lower than 10 \(\upmu\)g/L by a dosage of 1 mg/L Fe(II) at neutral pH values. This corresponds to a molar ratio Fe(II)/Cr(VI) somewhat higher than 3, which is much below the respective values obtained in the present study and the available publications for standard Cr(VI) removal techniques by Fe(II). Therefore, another aim of this study was to prove indirectly that the recirculation of the sludge in units with plug flow reactors could act in a catalytic way and enhance the kinetics of the Fe(II) reaction with Cr(VI), thus reducing the respective competition by the reaction of Fe(II) with oxygen and finally reducing the amount of Ferrous sulphate needed for the optimized Cr(VI) removal from water. In addition, this process contributes to the reduction of the chromium-containing sludge produced. This is very important, especially for treatment units designed to operate at the household level, because the sludge handling for small units is a matter of concern, since it is expected to increase the operational costs to a disproportionate degree. Moreover, this is particularly significant, because this method has the overall disadvantage of the difficulty in capturing the end moment of Cr(VI) reduction to Cr(III), thus in conventional treatment (i.e., all reagent dosing at once and once through filtration without any recirculation) the end point cannot be captured and more reagent than needed is usually dosed. In these cases, the process can be monitored closer resulting in less reagent requirements, thus making it more possible to dose closer to the end point of reduction reactions.