Sensor	Manufacturer	Optical System	Measured Wavelength	Measured Parameter	Advantages	Accuracy	Operating Range	Source
AMI SAC254	SWAN, Switzerland	Two-wavelength photometer with one optical channel, light-emitting diode (LED) light	254 nm	Surrogate parameter to determine dissolved organics	Measuring interval: 30 s to 3 min	±1% m−1	UV absorption: 0–6 mg/L DOC, TOC: 0–6 mg/L SAC254: 0 to 300 m−1 Temperature: 5–30 °C	[28]
ProPS-UV Photometer	Trios GmbH, Germany	Detector type: UV spectrometer, light source: deuterium lamp	200–385 nm	nitrate, CODeq and TOCeq	Customize path lengths, Spectral analysing software, Additional calibration functions	±0.01% mg/L	Temperature 0–30 °C, 32–86 °C; Measurements: 0.62–600 mg/L	[29]
IQ Sensor NET	WTW GmbH, Germany	256 channel silicon photodiode array detector, deuterium lamp	200–720 nm	A range of parameters, e.g., SAC, UVT	Data logger	±3% mg/L	SAC: 0.0–3000 m−1 Temperature: 0–45 °C	[30]
spectro::lyser	s::can Messtechnik GmbH, Austria	256-pixel photodiode array detector, xenon flash lamp	200–720 nm 220–390 nm	Various parameters	Various parametersDiffer path lengths	±2% mg/L	Temperature: 0–45 °C; TOC: 0–180 mg/L; NO2-N: 0–40 mg/L; NO3-N: 0–100 mg/L; UV254: 0–500 abs/m	[31]
Real UV254 probe	RealTech, Germany	Mercury UV lamp and LED lamp	253.7 nm	SAC254	Various parametersVarious path lengthsField calibration	±5% m−1	Temperature: 0 to 45 °C UV254: 0–20 abs/cm	[32]
UV absorption sensor	Endress+Hauser, Switzerland	Hotovoltatic cells detector, low-pressure mercury lamp	254 nm	SAC254	Data logger	±3% m−1	0–2.5 abs/cm 0 to 90 °C	[33]
IQ SensorNet system	YSI, Germany	Detector: LED and photodiode	254 nm	UVT-254 and SAC254	Has a controller	±2% m−1	Temperature 0 to 45 °C; UVT-254: 0–100; SAC254 0–3000 m−1	[34]

2.2. Water Quality Measurements with Proprietary Algorithms

As reported earlier, some advanced full-spectrum online UV-Vis spectrophotometers can determine a range of water quality parameters, including UV₂₅₄, colour, DOC, turbidity and nitrate. The parameters can be computed based on the applications via the instrument’s built-in proprietary algorithms. According to the manufacturer literature, these algorithms were developed based on chemometrics techniques, such as partial least squares (PLS) and multiple linear regression, to establish the relationship between UV-Vis spectra and laboratory measurements of water samples ^[7]. The algorithms were created by hundreds of global datasets containing both UV-Vis spectra and reference laboratory data obtained from a wide range of water quality ^[7]. Instruments have default configurations to apply the generic calibration for particle compensation to the raw spectral data using the built-in algorithms. UV₂₅₄ measures the amount of light absorbed by conjugated organic compounds, which have been widely used as a rapid water quality measurement technique to control water treatment processes [36]^[35]. UV₂₅₄ from the online UV-Vis instruments based on built-in algorithms generally performs well for the treated water as less interference exists [27]^[26]. However, performing site-specific calibrations may be needed if it is used for source water with a complex matrix. It is more difficult to judge the accuracy of the colour measurement than UV₂₅₄ using the online UV-Vis instruments, as the standard laboratory colour measurement method relies on SW measurement. Besides, the wavelength selected to measure colour may be different based on regions and water sources; for instance, 456 nm is used by Australia and the USA, whereas 410 nm is used by Russia to measure colour in natural water [37]^[36]. To measure the colour of water, the online instrument needs to be set up according to the required wavelength. DOC is used to monitor water quality from catchment to tap water, and measurement is usually carried out using the laboratory-based standard method. Online UV-Vis instruments with built-in algorithms can be used as alternative measurements; however, they are often reported as water-specific and lacking accuracy and thus need additional calibrations against different water matrices [38]^[37]. Turbidity determined by the online UV-Vis spectrophotometers with the generic built-in algorithms is comparable to the turbidity results analysed in the laboratory [39,40]^[38][39]. Nitrates generated from the online instruments with the generic built-in algorithms are generally not satisfactory [39]^[38], but another group reported that the results are comparable with a laboratory analysis [40]^[39]; this could be related to the specific algorithm used and the monitoring application. The measurements of water quality parameters using the online UV-Vis spectrophotometers are often source-water dependent. Thus, additional site-specific calibrations are needed to improve the accuracy of measurements [7,18,41,42,43]^{[7][18][40][41][42]} which is an instrument function provided to enter the laboratory-determined value of the collected grab sample measured at the same time by the online instrument. The site-specific calibration process involves modifying the slope and intercept of the built-in regression function using laboratory data from the reference grab samples [18,41]^[18][40]. To achieve the best calibration results, grab samples are needed to be representative and cover the whole measurement range of the water. Measurement accuracies can be enhanced with the use of an increased number of grab samples for the calibrations of the online UV-Vis spectrophotometers. Applications of online UV-Vis instruments can reveal that the significant fluctuations of water quality could affect the accuracy of the measurements, and long-term monitoring required regular calibration to compensate for the variation of the particle character issue [16,27,41]^[16][26][40]. A site-specific calibration was conducted for a submersible UV-Vis instrument to monitor water quality in a forested catchment, and comparable results were achieved [42]^[41]. In contrast, a site-specific calibration was performed for a UV-Vis submersible instrument to measure the water quality of stream water, but concentrations were overestimated because of inaccurate particle compensation [43]^[42]. Therefore, accurate site-specific calibration of the UV-Vis instrument is crucial to obtain measurements for water quality monitoring. Calibrations of online UV-Vis instruments should be performed as needed for situations such as an instrument being installed for the first time, changing the monitoring locations of the instrument, or the instrument being inaccurate to the reference water samples. Routine calibration may not be necessary for monitoring less variable source water or stable water quality, such as treated water, but routine verification of the measurements using lab references is recommended to ensure the accuracies of the instruments [38]^[37]. Site-specific calibrations have been approved to achieve the desirable measurement outcomes and can adequately account for the differences in large water quality changes or between different types of water [13,15,44,45,46]^{[13][15][43][44][45]}.

2.3. User-Developed Algorithms for Spectral Absorbance Measurements

When there are difficulties to obtain accurate continuous measurements using online UV-Vis spectrophotometers with the instrument built-in algorithms or when the instruments do not come with built-in algorithms, alternative particle compensation (calibration) techniques can be developed by end-users. To better utilise the online instruments for water quality monitoring and water treatment process control, as well as lower the maintenance costs, more importantly gaining in-house experience and knowledge of the instruments, researchers and water utilities have been seeking techniques for particle compensation alternative to the built-in particle compensation methods. The particle compensation techniques based on the UV-Vis spectra can be categorised into direct subtraction compensation and chemometric modelling. Table 2 summarises particle compensation techniques from the literature for online water quality monitoring using the UV-Vis instruments.