Chapter 3 - Integrated algal-based sewage treatment and resource recovery system

Abstract

An integrated sewage treatment and resource recovery (STaRR) system is proposed as a greener alternative to the current practice where valuable components in the sewage are destructively removed at the expense of significant energy input. The STaRR system entails mixotrophic cultivation of algal biomass in primary effluent, followed by hydrothermal liquefaction (HTL) of the resulting biomass to recover its energy- and nutrient contents. Historically, photoautotrophic algal systems have been engineered for sewage treatment, either as a dedicated process for nutrient removal from secondary effluent or as a coprocess in concert with heterotrophic bacteria for organics and nutrient removal. In contrast, the STaRR system utilizes a novel mixotrophic algal strain that affords reduction of biochemical oxygen demand, nutrients, bacteria of concern, and viruses in sewage in a single step to yield discharge-ready effluent. Results from pilot scale field demonstration of the STaRR system at a local sewage treatment facility are presented documenting its performance in treating primary effluent to better than secondary treatment levels, and in recovering high percentage of its nutrient content. Reductions of bacteria and virus indicators by the STaRR system are found to be superior to those in a parallel secondary system. Higher heating value (HHV) of the algal biomass cultivated in the STaRR system averaged 24.2MJ/kg. HTL of this algal biomass resulted in light biocrude yield of 5.4–10.8 dry wt% and heavy biocrude yield of 5.8–7.8 dry wt%. HHV of the resulting biocrude averaged 37.6MJ/kg. The STaRR system on average recovered 70% of phosphate and 51% of nitrogen from the primary effluent, yielding 1.62kg struvite, 0.34kg calcium phosphate, and 3.92kg ammonium sulfate per 100m3 of primary effluent.