Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Comparative approach to the performance of direct and indirect solar drying of sludge from sewage plants, experimental and theoretical evaluation Billal Ameri a, ⁎ , Salah Hanini a , Amina Benhamou b , Dihya Chibane b a Biomaterials and Transport Phenomena Laboratory (LBMPT), University of Yahia Fares, Ain d’hab, 26000 Medea, Algeria b Materials Research Unit, Process and Environment (URMPE), University of M'Hamed Bougara, Boumerdes 35000, Algeria ARTICLE INFO Keywords: Drying kinetics Moisture Sludge Solar drying ABSTRACT The objective of this work is to contribute to the enhancement of sludge of wastewater treatment plants and solar energy in Algeria. A kinetic comparison was made to the direct and indirect solar dryers operated with natural convection. Two models have been developed to simulate drying. Under natural conditions and at ambient temperatures below 32 °C, the indirect dryer gives high temperatures and efficiency compared to the direct dryer. Drying curves show that the higher the temperature is, the shorter drying time is and the faster drying speed is. From the drying curves, only the decreasing speed phase or slowdown (phase 2) is observed in the two types of drying, which is similar to results obtained in the literature with plant products. Solar dryers have reduced the time needed, a moisture reduction of 71% which increases the dryness of the mud from 14.47% to 85.53%. Thirteen mathematical models existing in the literature were tested and compared with the proposed models. The proposed models and the modified Henderson and Pabis model showed the best smoothing qualities of all indirect and direct dryer drying kinetics, with an advantage for the proposed models as they are di- mensionless (independent models of units of measurement), they contain fewer parameters and verify the boundary conditions. 1. Introduction Today, more than ever, waste management has become a priority of the environmental policy in the world (Hort et al., 2009). Wastewater from various urban activities cannot be rejected as such in the en- vironment because it contains various organic and inorganic pollutants. They must undergo, before their release into the natural environment, a sewage treatment which leads the production of sludge (Roux et al., 2010). The sludge purification plants are considered as waste by several regulations. They are a bulky waste (95–99% water) whose composition is problematic: highly fermentable substances and high disease burden (viruses, bacteria, parasites, etc.) (Gavala et al., 2003). In Algeria, the volume of wastewater discharged nationwide is currently estimated at nearly 750 million m 3 and will exceed 1.5 billion m 3 by 2020. The current situation (operation) number of wastewater treatment plant is 102 (52 PLANTS+50 lagoons), current installed capacity is 570 h m 3 /year (1999: 28 PLANTS to a processing capacity of 98 million m 3 /day). Location of being implemented program: number of wastewater treatment plants is 176 (87 PLANT+89 lagoons), in- stalled capacity is 355 h m 3 /year (Kessira, 2013). The problem of dis- posal of sludge from sewage treatment plants is more delicate given the increased production of wastewater and regulations that is becoming more demanding. The use of sewage sludge in agriculture has been widely practiced in most developed countries (Laube and Vonplon, 2004). Sludge processing pathways have always had goals; reduction of volume and fermentability, namely their stabilization (Bennouna and Kehal, 2001). In this perspective, the drying is often a necessary plant to facilitate subsequent management of sludge as recovery or storage (Rayan, 2007). There are several methods for sludge dewatering, among which mentioned is mechanical dewatering and thermal drying. The latter will be a development because it is the one that was chosen as part of this work. Drying is a dewatering method for the extraction of a solid, a semi-solid or a liquid by evaporation (Al-Masri and Kahline, 2013). The dryers are different because of the energy source that can be used as a fossil fuel, gas, electricity or solar power and the nature of the transfer of heat that is transmitted to the product (Fohr and Figueiredo, 1987; Lahsasni et al., 2004). Energy has always been a vital issue for humans and human societies. Human behavior is strongly induced by the availability or non-availability, abundance or its scarcity. These beha- viors will arise new challenges, particularly in the environment and the socio-economic balance (Bekkouche, 2009; Angélique, 2002). https://doi.org/10.1016/j.solener.2017.11.032 Received 17 November 2016; Received in revised form 10 November 2017; Accepted 12 November 2017 ⁎ Corresponding author. E-mail address: billel.ameri@hotmail.fr (B. Ameri). Solar Energy 159 (2018) 722–732 Available online 23 November 2017 0038-092X/ © 2017 Elsevier Ltd. All rights reserved. T