A synergistic combination of algal wastewater treatment and hydrothermal biofuel production maximized by nutrient and carbon recycling

Abstract

This study introduces and analyzes a novel system for algal biofuel production that synergistically integrates algal wastewater treatment with hydrothermal liquefaction (HTL) of wastewater biosolids and algae into bio-crude oil. This system maximizes the biofuel potential of wastewater inputs by internally capturing and recycling carbon and nutrientsa powerful concept referred to as multi-cycle nutrient reuse, which amplifies waste nutrients into multiple cycles of algal biomass and bioenergy production. We call this system "Environment-Enhancing Energy" (E 2 -Energy) because it can simultaneously improve conventional wastewater treatment by nutrient removal and production of a large amount of biofuel co-products.Moreover, E 2 -Energy resolves several key bottlenecks commonly associated with large-scale algal biofuel production including: contamination of target high-oil algal cultures, high nutrient costs/usage, unsustainable fresh water usage, and large energy inputs for dewatering/extraction. A series of algal cultivation and HTL experiments were conducted to confirm the primary steps and performance characteristics of the E 2 -Energy system. These experiments showed: (1) low-oil, mixed algal-bacterial biomass can be successfully cultured with the recycled HTL aqueous product; (2) both organics and nutrients are removed from wastewater during algal-bacterial biomass production (63-95% reduction);(3) this low-oil, algal-bacterial biomass can be converted into bio-crude oil via HTL with a high yield ($50%) and a net positive energy balance; and (4) the HTL step re-releases nutrients to an aqueous phase product that can be recycled back to step (1). This repeating loop of steps 1-4 facilitates multi-cycle reuse of nutrients and thus provides biomass amplification. ‡ A mathematical model was also developed using STELLAâ to simulate mass balances for long-term E 2 -Energy operations with internal recycling of nutrients and carbon. The model results showed that E 2 -Energy can amplify the biomass and biofuel production from wastewater by up to 10 times, which gives it the potential to replace all US petroleum imports using only current wastewater feedstocks and carbon dioxide from the atmosphere or point sources. Thus, E 2 -Energy represents a major paradigm shift-where wastewater treatment systems become optimized biofuel producers with enhanced effluent quality, which provides a viable and advantageous pathway to sustainable, carbon-neutral energy independence.

Simultaneous removal of inorganic and organic compounds in wastewater by freshwater green microalgae

“…Bacteria have the ability to degrade organic pollutants to a form that algae are capable of utilizing. 41 Also, algae can provide the O 2 necessary for aerobic bacteria to biodegrade organic pollutants, consuming in turn the CO 2 released from bacterial respiration. 14 …”

An investigation of reaction pathways of hydrothermal liquefaction using Chlorella pyrenoidosa and Spirulina platensis

“…Additionally, Tables S3 and S4 illustrated that nitrogenous organic compounds (NOCs) were detected in all aqueous fraction samples, such as 2-pyrrolidinone (RT 7.58), 2-piperidinone (RT 27.03) and 3-pyridinol (RT 31.80), which are typical NOCs detected in the HTL process of microalgae [7,11,12,30]. The results indicated that many classes of NOCs with methyl groups were detected, such as pyrazine, methyl-(RT 7.58), 2,5-PyrrolidinSedione, 1-methyl-(RT 22.67) and 3-pyridinol, 6-methyl-(RT 31.51) in aqueous fraction of CP and SP.…”

“…Aqueous fraction is a wastewater that containing most of the nutrient and organic matter from the feedstock [30,31]. The total chromatograms for the aqueous fraction of CP and SP were shown in Fig.…”

Characterization of aqueous phase from the hydrothermal liquefaction of Chlorella pyrenoidosa

“…Consequently, higher temperature with a shorter retention time or lower temperature with a longer retention time is recommended in this study. Previous studies (Zhou et al, 2013;Li et al, 2014) showed that the aqueous products from HTL process are mostly used to cultivate microalgae. The pretreatment of heavy dilutions are generally viewed as necessary for algae growth because the aqueous co-product of the HTL process contain high concentration of nutrients such as N and P. Biller et al (2012) applied different dilution multiples of 50, 200, 400 and 600 to the recycled process water from HTL process.…”

“…This team dedicated to studying a novel system for biofuel production that synergistically integrates nutrient recycling for algae cultivation with HTL of microalgae into bio-crude oil, which is referred to as ''Environment-Enhancing Energy (E 2 -Energy)''. This approach has been discussed in detail in our previous publication (Yu et al, 2011a;Zhou et al, 2013;Gai et al, 2014;Chen et al, 2014a;Chen et al, 2014b). It showed that E 2 -Energy can amplify the biomass and biofuel production from wastewater by up to 10 times, which gives it the potential to replace all U.S. petroleum imports using only current wastewater feedstocks and carbon dioxide from the atmosphere or point sources.…”