Assessing the suitability of solar dryers applied to wastewater plants: A review

Sludge dewatering and drying are the main processes related to sludge management in wastewater treatment plants (WWTPs). Sludge disposal is a high-cost activity, and drying the sludge reduces its mass and volume, resulting in savings in storage, handling and transportation. The discoveries regarding the use of solar energy in agricultural studies provided valuable information for using in sewage sludge drying. Some studies have reported that dry sludge has met the EPA Class A requirement for biosolids using only solar energy as an energy source. The proper sludge mixture, manual or mechanical, and the dewatering process can significantly increase the drying rate, reducing drying time and the surface area needed. The environmental conditions and the sludge's type greatly influence the drying system. A solar dryer system may be suitable to dry different types of wastewater sludge. Modeling techniques can predict the behavior of the solar drying system and, thus, save time and money in experimental steps. CFD modeling of the sludge drying system is usually done by adopting specific boundary conditions and solving the Navier Stokes equations for air and sludge. There is no standard methodology for comparing solar dryers and common methodologies, such as system efficiency and thermal efficiency, disregards different dryers in different operational conditions. A SWOT (strengths, weaknesses, opportunities, and threats) analysis indicated that, in general, the chapel-type greenhouse with mixed-mode drying has higher drying rates, resulting in reduced drying time and can be scaled to any size. Thus, this type of dryer emerges as a more economical alternative to commercial solar dryers. Based on a systematic review, this work points the SWOT analysis as a useful tool for selecting solar dryers.