## 2 Materials and Methods
### Sampling and Laboratory Analyses  
The results of parameters monitored are presented in the same order as samples taken from marked sampling points (Figures 1-3). Temperature (T, \({}^{\circ}\)C) was monitored in-situ. In all cases, water samples were collected by the grab method and kept for one day at 4 \({}^{\circ}\)C prior to laboratory analyses. The collected samples were analyzed for the water quality parameters, such as color, suspended solids (SS), total dissolved solids (TDS), pH, electrical conductivity (EC), biological oxygen demand in 5 days (BOD\({}_{5}\)), chemical oxygen demand (COD\({}_{\rm Cr}\) and COD\({}_{\rm Mn}\)), dissolved organic carbon (DOC),  
Figure 3: Flowchart and view of the carrots washing and wastewater treatment system D.  
Figure 2: Flowchart of carrots washing and processing system C.  
Kjeldahl nitrogen (Nkj), total phosphorus Ptot, and inorganic nitrogen N(NH4-N + NO2-N + NO3-N). Deionized water samples were analyzed as standards. All analytical methods are in accordance with standard methods [27] of the water and wastewater analysis, such as ISO 11923:1997, ASTM D5907--13, ISO 10523:2008, ISO 7888:1985, ISO 5815-1:2003, ISO 5814:2012, ISO 6060:2003, EN 1484:1997; ISO 8245:2000; EN 12260:2003, BS 6068-2.83:2003; ASTM D7573--09, ISO 7887:2011; ISO 6878:2004; ISO 7150-1:1984; ISO 6777:1984; ISO 7890-3:1988 [27]. For the estimation of the uncertainties in the lab analysis, a triplicate analysis was performed, and the mean results, as well as standard deviation, were calculated. Measurements of the UV/Vis absorbance were performed with UV/Vis spectrophotometer Helios \(\gamma\) (Thermo Scientific, USA) in the range of 210 to 660 nm in a quartz cell of 10 mm path length after appropriate dilution with deionised water in order to overcome signal saturation in the UV region (absorbance values above 1 a.u.). In addition, blank samples of deionized water were measured to adjust the 100% transmittance signal. The data set was composed of 672 spectra of washing water and wastewater samples collected over a period of 12 months and of deionized water (control) at specific wavelengths of the UV/Vis spectra. Absorbance at A210 nm is used for roughly evaluation of nitrates, A225 nm--detergents, A240 nm--organic matter OM + suspended solids SS, A254 nm--dissolved organic carbon DOC, A320 nm--suspended solids SS, A390 nm--color, A660 nm--turbidity [2; 6; 28]. The dissolved organic carbon (DOC) quality was evaluated by specific ultraviolet absorbance (SUVA). SUVA254, which indicates the proportion of aromatic compounds in the DOC, was calculated according to equation [29]:  
\[\text{SUVA}_{\text{254}}\ \text{(L/mg\text{:}M)}=\text{A254}\ \text{(cm}^{-1})/ \text{DOC}\ \text{(mg/L)}\cdot\text{100}\ \text{cm/M} \tag{5}\]  
Normally SUVA index of surface waters ranges from 1 to 6. As SUVA index increases, a larger concentration of aromatic compounds are in the solution when SUVA \(<\) 3, mainly hydrophilic low molecular weight compounds, are present. When SUVA \(>\) 4, organic matter is hydrophobic and high in aromatic compounds. Also, the bigger SUVA index can be resulted by iron, colloidal, or other impurities [7; 30; 31]. In order to use A254 nm as alternative parameters to estimate BOD5 and COD, suspended solids have to be removed by filtering with a 0.45 um pore size membrane filter. However, it is inconvenient, time-consuming and expensive. Chevakidagarn [2] studies of wastewater UV/Vis measurements showed a strong linear correlation between absorbance at A550 and the difference of filtered and non-filtered UV absorbance at A260 nm. Some filtered and unfiltered samples were measured at A240 nm, A320 nm, A254 nm and A660 nm and strong linear relationships were obtained (R2 0.7-0.9).