Enhancing Biosolids Drying Through Solar Thermal Applications

The use of the sun for drying biosolids has been a time-tested method to reduce water and pathogen content. Biosolids are traditionally spread out over an open drying bed while the sun naturally dries them and reduces water content. However, the odour this process produces has made it difficult to manage in dense urban environments. The high cost of transporting biosolids to processing sites has also contributed to the challenge of wastewater management. While conventional thermal drying has proven to be an effective solution in reducing volume, the use of solar thermal applications continue to be appealing, as a renewable low-cost, and low-energy alternative.

One possible solution for drying biosolids is the construction of greenhouse facilities where the odours can be controlled, yet still allow the solar energy to pass through and create the drying effect. A parameter that should be regulated is the temperature brought about by the trapped heat from the sun. Some facilities have even included automated mixers to enhance the drying process. Biosolids can be loaded into a greenhouse in batches or continuously loaded through a conveyor. The greenhouse setup provides a low-energy option with operational simplicity and reduced cost. The large carbon footprint and low water evaporation rates however, make it a non-viable option for large wastewater treatment plants in urban areas due to limited space.

Evaporation is a function of outdoor solar radiation, air temperature, relative humidity, and ventilation rate. These factors can significantly affect the water evaporation rates of biosolids in greenhouses and can likewise be influenced by weather changes throughout the year.

A study initiated by Domènec Jolis and Natalie Sierra from the San Francisco Public Utilities was conducted to evaluate whether recent advances made in solar thermal technology created sufficient water evaporation rates that make solar drying of biosolids feasible. The study was demonstrated by constructing a solar drying chamber with warm air from a solar thermal panel being routed to the chamber to aid in evaporation. Experiments were done with water alone to measure water evaporation rates in a variety of weather conditions and to create a regression model for evaporation. They also conducted the experiment with digested, dewatered biosolids to measure evaporation rates when drying biosolids.

The paper noted a doubling of temperature in the chamber during daylight hours reaching as high as 38 degrees Celsius. Total solids concentration in biosolids samples reached 42.3% after 102 hours in the dryer. The data showed that evaporation rates not only correlated with the temperature inside the dryer chamber but also with the mixing of biosolids. The evaporation rates measured were more than twice of those previously reported in the literature for solar dryers which implies that with the right setup optimized for mixing, humidity control and energy recovery, higher rates could still be achieved. If the ideal setup can be achieved in larger scale demonstration projects, the results from this study would result in the development and deployment of compact solar dryers located in urban areas.

If you are a municipality in Ontario and in need of a biosolids management solution, please feel free to contact us at 1 (877) 479-1388.

Fundamentals of Renewable Energy and Applications

Enhancing Biosolids Drying Through Solar Thermal Applications
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