Evaluation of the performance of a pilot-scale solar still for olive mill wastewater treatment

Abstract

A pilot-scale solar still was constructed to evaluate the use of solar distillation for olive mill wastewater management. Conventional distillation experiments were initially conducted and showed that the increase of temperature resulted to higher concentrations of total phenols, tyrosol and hydroxytyrosol in the distillates. Afterwards, four experiments were performed in the pilot system with different types of olive mill wastewater to examine the effect of temperature and the role of underfloor heating system on its performance. The use of the heated underfloor resulted to higher average and maximum daily temperatures in the still and increased the production rates of the distillates, which reached up to 4.10 L/d m2. Liquid chromatography – high resolution mass spectrometry analysis detected concentrations of tyrosol and 4- hydroxybenzoic acid in the collected distillates at concentrations up to 0.70 and 0.10 mg/L, respectively. At the same time, a strong correlation was observed between the average daily temperature in the solar still and the concentrations of total phenols in the recovered distillates. The implementation of mass balances showed that almost 33% of the total input phenols found in olive mill wastewater remained in the solid residue. Eight phenolic compounds were detected in this material at concentrations ranging between 0.04 mg/kg (quinic acid) and 8.8 mg/kg (hydroxytyrosol), while its calorific value ranged from 15.28 MJ/kg up to 25.12 MJ/kg. Economic evaluation was implemented for the case of upscaling the said pilot system for operation at a medium size three-phase Greek olive mill. Taking into consideration that the distillate could be reused for the washing and malaxation of olive fruits, while solid residual could be sold as biofuel, the operational and overall costs were estimated to 0.60 and 1.21 €/m3, respectively.