# Solar Concentration for Wastewaters Remediation: A Review of Materials and Technologies
## 2 Solar Collector Systems Employed for Wastewater Treatment  
The starting point for the use of solar collectors in wastewater remediation can be traced back to parabolic systems, originally developed for thermal energy applications, and then adapted in 1989 in Albuquerque, NM, USA for water purification. Immediately afterwards, in 1990, a dedicated facility started operations at the _Plataforma Solar de Almeria_--Spain. Ten years later, dedicated research on wastewater treatments started to be effectively performed [19]. Solar collectors can be concentrating or non-concentrating systems. The former can be classified depending on the principle adopted for focusing sunlight and based on whether they use a fixed or moving receiver [20], while the latter generally consist of flat panels which can be fixed or movable, i.e., following the sun. While in thermal solar applications all wavelengths of the sunlight spectrum are concentrated onto an absorber to produce an increase in temperature, for a solar AOP, the most effective photons are those on the high-energy side of the spectrum, in the UV or near UV range (300-400 nm wavelength), due to the prevalent use of wide band-gap semiconductors as catalyst materials. Wavelengths up to 600 nm are, to date, effectively exploited only by photo-Fenton reactions or by emerging catalyst materials designed ad-hoc for visible light absorption [21]. This is therefore an opportunity and a challenge for the materials science field. The use of light at wavelengths higher than 400 nm would also allow the use of silvered mirrors, which are simple and robust, but which present poor UV reflectance.  
In fact, given that, to date, the range between 300 and 400 nm has been considered to be of exceptional importance, conventional silver mirrors have not been considered suitable as reflectors. This is mainly due to their low average reflectance in this wavelength interval, with a minimum at around 320 nm related to an interband transition [22]. Furthermore, the glass covering commonly present on silver mirrors contains iron impurities, which further contribute to reductions in UV reflected radiation.  
Mirrors based on aluminum are generally considered a better option when working with processes requiring UV photons [23]. In fact, their reflectance is high (around 93%) and almost constant in the 300-400 nm range. As proposed by [PERSON] and [PERSON] [24; 25], the best reflective surface for solar AOPs should be: (i) efficient in the UV range, (ii) weather resistant, (iii) reasonably cheap. At present, the benchmark solution is based on anodized and electro-polished aluminum surfaces. An available alternative is using aluminum mirrors protected with an acrylic coating [26].  
The design of a solar AOP system requires consideration of a number of factors: the mirror materials and shaping, the catalyst, wastewater loading method (batch or once-through), flow type and rates, pressure drops, eventual pretreatment, eventual oxidant loading method, pH control and the use of a tracking system to enhance the direct solar radiation [21]. These criteria are presented in Table 1.  
Most existing literature reports are based on different combinations of these factors, giving rise to a variety of configurations [13; 19; 21]. Some of the main design choices for both concentrating and non-concentrating systems are reviewed in the following sections.