Lessons learned from the first BioDryer installation at Silicon Valley Clean Water

Introduction: This presentation will offer valuable insights into the performance and optimization of the first BioDryer™ technology in the United States (US), currently operating at Silicon Valley Clean Water (SVCW). Attendees will learn how this novel process works and performs using actual data. The authors will describe the process in details, discuss the energy efficiency and operational benefits of the BioDryer™ and the practical lessons gained from two years of operation and maintenance The presentation will also highlight the challenges and solutions encountered during the implementation and optimization phases, offering a comprehensive understanding of how the BioDryer™ can be a viable solution for managing biosolids in water resource recovery facilities (WRRFs).

Abstract: As disposal costs rise and market pressures increase, options for biosolids end use are becoming more limited. Thermal drying of dewatered sludge is emerging as an attractive solution for WRRFs. This process significantly reduces the volume and weight of sludge, thereby helping mitigate risks associated with hauling costs and evolving regulations, such as per- and polyfluoroalkyl substances (PFAS). Traditionally, three main types of thermal dryers have been used for biosolids applications in the US: belt, indirect, and rotary drum. However, the BioDryer™ technology by BioForceTech (BFT) has recently emerged in the US wastewater market, with the first units developed at SVCW, CA (Figure 1) While the BioDryer™ shares some similarities with traditional thermal dryers, it is a hybrid of an advanced composting system and a low-heat drying system that allows for significant thermal and electrical energy saving (Table 1).

SVCW and BFT entered into an agreement in 2013 to develop and pilot the first BioDryer™ and pyrolysis system at their facility. By 2019, BFT built and installed three full-scale BioDryers™ and a pyrolysis unit. Each BioDryer™ is designed to process up to 1,000 wet tons of biosolids per year, drying the product from 20% total solids (TS) to 80% solids, resulting in a four-fold reduction in mass. The three units were designed to dry approximately 20% of SVCW's dewatered biosolids all year long. In 2023, after a year of working alongside BFT staff, SVCW staff assumed full responsibility for operation of the BioDryers™. Since then, SVCW has worked with BFT to continue to optimize the process and has gained valuable insights along the way. To enhance the system's efficiency, several improvements were made: the hot water loop temperature was increased to boost heat transfer, loading and offloading automation was implemented to minimize downtime, heat exchangers were redesigned, gate opening mechanisms were improved and programming was refined. With these optimizations, SVCW has occasionally operated the BioDryers™ at over 100% capacity (Figure 2). As table 2 shows, on average, the BioDryers™ are capable of reliably drying the biosolids from 21 %TS to 84%TS

BFT's BioDryer™ uses bacteria that breakdown organic matter and generate a portion of the heat required to evaporate water. External heat is supplied by a hot water loop that is operated at 180-200oF. This system can use wasted heat recovered from a combined heat and power or pyrolysis system, leading to a process that might need external heat source. The BioDryers™ dry biosolids through a three-phase process. Once the loading of wet biosolids is complete, heated air is blown through the wet biosolids to quickly provide an ideal temperature for mesophilic or thermophilic bacteria to grow while feeding on the organic matter available in the biosolids. Because there is no requirement to produce Class A certified biosolids, SVCW operate its BioDryers™ in the mesophilic range (90-100 degrees F). As bacteria breakdown the organic matter, additional heat is generated. This heat is sufficient to evaporate the water, which is then removed by the continuous airflow and the strategic rotation of the BioDryer™. As the contents of the BioDryer™ get beyond 60 %TS, bacterial activity declines. However, moisture continues to be removed through the continuous heated air flow. The BioDrying™ cycle automatically stops when the content reaches the target %TS, after which the unloading of the dried biosolids is initiated. The authors will describe the technology in detail using actual performance data (Figures 3-5).

In addition to being more energy efficient than other dryer systems, safety risks are minimized compared to other systems because the BioDryer™ process operates at low temperatures. The system also utilized a contained aerobic process, which results in low odor emissions, and SVCW staff report no significant dust issues. However, it is crucial to avoid mixing wet material with dry material during storage to prevent the risk of smoldering and fire.

SVCW staff has demonstrated that this BioDrying™ technology is viable and can be a good option to consider for small to medium scale plant that have enough footprint available to install these units. The system also provides modularity and operating flexibility needed to adapt to different dewatering operation scenarios and performance target.