Solids Residence Time Impacts Carbon Dynamics and Bioenergy Feedstock Potential in Phototrophic Wastewater Treatment Systems

The use of wastewater-grown microalgae
has the potential to reduce
the cost of algae-derived biofuels while simultaneously advancing
nutrient recovery at water resource recovery facilities (WRRFs). However,
a significant barrier has been the low yield and high protein content
of phototrophic biomass. Here, we examine the use of solids residence
time (SRT) as a selective pressure in driving biochemical composition,
yield, biofuel production, and WRRF nutrient management cost. We cultivated
mixed phototrophic communities in controlled, laboratory-scale photobioreactors
on the local WRRF secondary effluent to link SRT with biochemical
composition and techno-economic analysis to yield insights into biomass
composition and downstream processing effects on minimum fuel selling
price. SRT significantly impacted biochemical composition, with total
and dynamic carbohydrates the highest at low SRT (total carbohydrates
being 0.60 and 0.32 mg-carbohydrate·mg-protein–1 at SRT 5 and 15 days, respectively). However, there were distinct
differences between extant, steady-state performance and intrinsic
potential, and longer SRT communities were able to accumulate significant
fractions (51% on an ash-free dry weight basis, AFDW %) of carbohydrate
reserves under nutrient starvation. Overall, hydrothermal liquefaction
(HTL) was found to be more suitable than lipid extraction for hydrotreating
(LEH) and combined algal processing (CAP) for conversion of biomass
to fuels, but LEH and CAP became more competitive when intrinsic carbon
storage potential was realized. The results suggest that the use of
algae for nutrient recovery could reduce the nutrient management cost
at WRRFs through revenue from algal biofuels, with HTL resulting in
a net revenue.

利用废水培养的微藻，兼具降低藻类生物燃料制备成本与推动水资源回收设施（water resource recovery facilities, WRRFs）营养盐回收的双重潜力。然而，光合生物质普遍存在产率偏低、蛋白质含量过高的显著瓶颈。本研究探讨了以固体停留时间（solids residence time, SRT）作为选择压力，对菌群生化组成、产率、生物燃料生产以及WRRF营养盐管理成本的调控作用。我们以当地WRRF二级出水为培养基质，在可控的实验室规模光生物反应器中培养混合光合菌群，旨在建立SRT与生化组成的关联，并通过技术经济分析，揭示生物质组成与下游工艺对最低燃料售价的影响机制。研究结果表明，SRT对菌群生化组成具有显著调控作用：低SRT条件下总碳水化合物与动态碳水化合物含量最高（当SRT为5天与15天时，总碳水化合物含量分别为0.60与0.32毫克碳水化合物·毫克蛋白质⁻¹）。但现有稳态运行性能与菌群内在潜力存在显著差异：较长SRT培养的菌群在营养盐匮乏条件下，可积累占无灰干重基准（ash-free dry weight basis, AFDW %）51%的碳水化合物储备。总体而言，相较于加氢精制提脂（lipid extraction for hydrotreating, LEH）与藻类联合处理工艺（combined algal processing, CAP），水热液化（hydrothermal liquefaction, HTL）更适用于将生物质转化为燃料；但当菌群的内在碳储存潜力得以充分发挥时，LEH与CAP的市场竞争力将显著提升。本研究结果显示，利用藻类开展营养盐回收，可通过藻类生物燃料带来的收益降低WRRF的营养盐管理成本，其中HTL工艺可实现净正收益。