Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal Mixotrophic algal system for centrate treatment and resource recovery I.S.A. Abeysiriwardana-Arachchige a , G.W. Chapman a , R. Rosalez b , N. Soliz b , Z. Cui b , S.P. Munasinghe-Arachchige a , H.M.K. Delanka-Pedige a , C.E. Brewer b , P.J. Lammers c , N. Nirmalakhandan a, ⁎ a Civil Engineering Dept., New Mexico State University, Las Cruces, NM, USA b Chemical and Materials Engineering Dept., New Mexico State University, Las Cruces, NM, USA c Arizona Center for Algae Technology & Innovation, Arizona State University, Phoenix, AZ, USA ARTICLE INFO Keywords: Centrate treatment Nitrogen recovery Phosphorus recovery Energy recovery Algal sewage treatment ABSTRACT In traditional activated sludge-based wastewater treatment plants equipped with anaerobic digesters, the nu- trient-rich centrate from the digester is typically recirculated to the headworks of the plant exerting a parasitic load. As an alternative, mixotrophic algal cultivation in a blend of the centrate and primary efuent is proposed to yield energy- and nutrient-rich biomass from which, liquid biofuel and fertilizers could be recovered. Core components in the proposed sewage treatment and resource recovery (STaRR) system include i) algal sewage treatment/biomass cultivation; ii) hydrothermal liquefaction of algal biomass for energy recovery, and iii) re- covery of struvite and concentrated ammonia from the hydrothermal liquefaction byproducts. Results from a 700 l pilot-scale STaRR system run with four diferent blends of centrate and primary efuent are presented to validate the proposed approach. A 20: 80 centrate-to-primary efuent blend enabled discharge standards for ammoniacal‑nitrogen, phosphate phosphorus, and biochemical oxygen demand to be met in < 7 days; volu- metric removal rates at this blending ratio were 4.0 mg/l-d, 0.70 mg/l-d and, 10.50 mg/l-d, respectively. Hydrothermal liquefaction of the resulting biomass (20% solids content, 350 °C, and 60 min holding time) yielded 77.5 g light bio-crude oil and 108.0 g heavy bio-crude oil per kg of dry algae. It is estimated that the STaRR system can produce 6.75 kg of struvite and 3.42 kg of concentrated ammoniacal nitrogen per 100 m 3 of blended wastewater. 1. Introduction Sewage treatment plants (STPs) across the world have long been operated with the specifc goal of meeting the promulgated discharge standards for suspended solids, organic carbon, nutrients, and patho- gens. To comply with the standards, most STPs have employed a se- quence of processes, whose carbon- and ecological-footprints have now come under scrutiny. For instance, water and wastewater utilities in the U.S. use 3–4% of the total electrical energy to render the service; energy demands in other developed countries are similar [1,2]. Aside from the energy demand, disposal of the waste sludge produced by biological processes accounts for up to 50% of the plant's operating costs [3]. To reduce the carbon-footprint and net sludge generation, medium-to- large-sized STPs have resorted to anaerobic digestion (AD) of the waste sludge [2]. According to the Water Environment Federation database, 1286 of the 14,748 STPs in the US incorporate AD to convert waste sludge to methane [4]. Of them, ~33% generate electricity from the methane (~27 kW per MGD); 2% purify the digester gases and inject methane into natural gas pipelines, and the rest either fare the gases or use them to generate heat for plant activities [4,5]. Following the di- gestion process, the stabilized sludge is typically dewatered and the solid fraction is incinerated, used as fertilizer, or disposed of in landflls [6]; the nitrogen-rich liquid fraction, referred to as centrate, is often returned to headworks of the treatment plant [7,8]. The practice of returning the centrate to the headworks has been a concern since it increases the nitrogen (N) load to the plant (up to 30%) and increases the overall cost of treatment [7,8]. The lack of organic carbon in the centrate prevents the removal of excess N using con- ventional heterotrophic bacteria-based treatment [8,9]. As a remedy to this, several ways to “short-circuit” nitrifcation-denitrifcation process through nitrite, such as the SHARON process [10], have been proposed [11]. These processes for N removal can reduce the carbon requirement for denitrifcation from 3.5–4.0 to 2.0–2.5 mg COD/mg of ammo- nia‑nitrogen [12]. With the discovery of the Anammox bacteria that can https://doi.org/10.1016/j.algal.2020.102087 Received 17 June 2020; Received in revised form 24 September 2020; Accepted 25 September 2020 ⁎ Corresponding author. E-mail address: nkhandan@nmsu.edu (N. Nirmalakhandan). Algal Research 52 (2020) 102087 Available online 16 October 2020 2211-9264/ © 2020 Elsevier B.V. All rights reserved. T