Biosolids and Algae--Two Problematic Birds, One Sustainable Stone

With rising costs and increased pressure on landfilling and land application of biosolids, many utilities are evaluating alternative and innovative processes for managing biosolids. These drivers are being further pushed based on ever growing concerns with nutrient loading and emerging contaminants, specifically perfluorinated compounds (ie. PFAS), which is increasing interest in innovative technologies. One particular game changing technology is Hydrothermal Liquefaction (HTL) which has the potential to destroy PFAS. HTL technology uses high-pressure (200 bar) and high-temperature (350 °C) process produces a biocrude product from sludge fed to the system at solids concentrations between 15 percent and 25 percent solids. The residual liquid from the HTL process can be further treated using Catalytic Hydrothermal Gasification (GHG) to generate renewable natural gas (RNG) to provide a sustainable closed-loop system with minimal waste. Nutrient recovery from the process is also possible with the recovery of ammonia from the effluent water and recovery of phosphorus from the precipitate. Metals in the sludge accumulate in a precipitate byproduct. A high-level flow diagram showing how HTL can be integrated into a WWTP is provided in Figure 1. A more detailed system PFD is provided in Figure 2. For another concern, Harmful Algal Blooms (HABs), threaten our natural water supplies due to their impacts on health, tourism, industry and ecosystems and cost an estimated $1 billion in economic damages and losses each year in the United States alone. The EPA has noted that HABs are 'one of our nation's most widespread, costly and challenging environmental problems'. With continued nutrient enrichment and the effects of climate change, HABs are now occurring more frequently, becoming more toxic, lasting longer and geographically expanding across the globe. Use of algae harvesting as a tool to mitigate HABs has been challenged by scalability to rapidly treat large volumes of water, mobility to address the dynamic nature of wild algae populations in waterbodies, and issues with algal toxins and recovered biomass disposal. To combat these issues, AECOM developed an innovative and effective solution to these challenges using a patent pending Hydronucleation Flotation Technology (HFT). HFT is a liquid solid separation technology that physically separates and removes intact algae cells from the water column along with the key nutrients (phosphorus and nitrogen) that fuel algal growth, carbon and potent algal toxins. The versatility and scalability of HFT for real world application has been proven effective in numerous demonstration tests conducted in Florida and New York, including the award winning federally funded USACE Engineer Research & Development Center (ERDC) Harmful Algal Bloom, Interception, Treatment and Transformation System (HABITATS) research program. Like biosolids, the dewatered Algae can also be used as a feedstock in HTL for biocrude production. The biosolids from a wastewater treatment plant and harvested seasonable algae dewatered both provide suitable feedstocks for HTL to produce biocrude. Locating these facilities at a local wastewater treatment plant can then be used to solve two issues with one sustainable solution. Biosolids are produced year-round while Harmful Algal Blooms are more seasonal. To further test this option a mobile pilot unit was set up at the Altamonte Springs WWTP near Orlando, Florida. Pictures of this demonstration along with biocrude production is provided in Figures 3 and 4 along with performance data showing 40-percent energy product yield by mass. The presentation will highlight the results and challenges noted during the pilot exercise. Next steps will also be presented for further process development and understanding of feed stock attributes associated with readily pumpable biosolids that are amenable to HTL processing and biofuel production.