## 1 Introduction  
Washing water and wastewater quality control in on-farm packhouses is of great importance for the quality of agricultural production as well as the environment. Producers require rapid and efficient methods for water quality monitoring. Conventional standard analytical methods are expensive, time-consuming and require special laboratory conditions and equipment. Therefore they are inconvenient and impossible to use for the water quality monitoring at the on-farm packhouses. Ultraviolet/visual spectrometry (UV/Vis) can have significant advantages over other in situ methods for this application due to its wide availability and low application costs. Previously, various UV/Vis methods were used, for example, to determine the particle size of polystyrene in solution, the size distribution of quantum particles of a ZnO semiconductor, the size of gold-coated palladium clusters in solution, and to study the aggregation of b-carotene particles [1]. UV/Vis technology is rapid and can be used for the monitoring of the most important water quality parameters: total suspended solids, dissolved organic carbon, the chemical oxygen demand, the biochemical oxygen demand, and measure of detergents and nitrates, since these materials absorb electromagnetic radiation at different wavelengths (\(\lambda\)). Consequently, spectrophotometry has been already proposed for the industrial wastewater quality monitoring, characterization of the organic and inorganic compounds, nitrates, detergents, dyes, as well as elements (Fe, Mn, Cr) as a fast, simple, non-destructive and inexpensive analytical method [2; 3; 4; 5; 6]. Moreover, using indirect methods of calculations can be useful in the research of wastewater microbiological activity, biomass structure, growth and biochemical processes of dissolved organic matter (DOM) reduction [7]. Recently there has been an emergence of commercially available UV/Vis spectrophotometers for the continuous optical measurements on-line and in-situ. However, the lack of selectivity and reliability of the methods remains the general restriction of these instruments for wider application [8; 9; 10]. UV/Vis spectrophotometry is based on Beer-Lambert law [11]:  
\[\text{Log}\;\text{I}_{\text{o}}\text{/I}=\alpha\text{-}\text{c}\text{-}\text{l}; \tag{1}\]  
where I is the intensity of light at specific wavelength \(\lambda\) passing through a sample of length l, I\({}_{\text{o}}\) is the light intensity incident on the sample, c is the concentration, and \(\alpha\) is the absorptivity of the sample at the specific wavelength. Between spectral and chemical data, the most common are linear and quadratic models of interactions. A linear model is as follows:  
\[\text{Y}=\varepsilon_{1}\text{c}_{1}1+\varepsilon_{2}\text{c}_{2}1+\cdots+ \varepsilon_{\text{p}}\text{c}_{\text{p}}\text{l}+\text{E}, \tag{2}\]  
where Y is the response of a particular sample at wavelength \(\lambda\), \(\varepsilon_{\text{p}}\) is the molar extinction coefficient for the p sample, c\({}_{\text{p}}\) is the corresponding concentration and E is the spectral error. If \(\varepsilon\) is measured at several wavelengths, then the equation obtained is:  
\[Y_{j}(\lambda_{i})=\sum_{k=1}^{p}\varepsilon_{k}\varepsilon_{k}j+E_{y}, \tag{3}\]  
where \(j\) is the index for samples, \(i\) is the index for wavelengths, \(p\) is the number of components to be determined and \(E_{y}\) is spectral errors. In matrix notation, Equation (3) can be written as follows:  
\[\text{Y}=\text{C}\text{-}\text{K}+\text{E}_{\text{Y}}; \tag{4}\]  
where Y is the j-i matrix of calibration spectra, C is the j-l matrix of component concentration, K is the l-i matrix of absorptivity-path length products, and \(\text{E}_{\text{Y}}\) is the j-i matrix of spectral errors. The limitation of this model is that it can only be applied to up to three variables. Multivariate calibration methods enable analyzing those analytes that interact with each other and predict their concentration from spectral data [12; 13; 14].