Improving membrane photobioreactor performance by reducing light path: operating conditions and key performance indicators

Abstract

Microalgae cultivation has been receiving increasing interest in wastewater remediation due to their ability to assimilate nutrients present in wastewater streams. In this respect, cultivating microalgae in membrane photobioreactors (MPBRs) allows decoupling the solid retention time (SRT) from the hydraulic retention time (HRT), which enables to increase the nutrient load to the photobioreactors (PBRs) while avoiding the wash out of the microalgae biomass. The reduction of the PBR light path from 25 to 10 cm increased the nitrogen and phosphorus recovery rates, microalgae biomass productivity and photosynthetic efficiency by 150, 103, 194 and 67%, respectively. The areal biomass productivity (aBP) also increased when the light path was reduced, reflecting the better use of light in the 10-cm MPBR plant. The capital and operating operational expenditures (CAPEX and OPEX) of the 10-cm MPBR plant were also reduced by 27 and 49%, respectively. Discharge limits were met when the 10-cm MPBR plant was operated at SRTs of 3-4.5 d and HRTs of 1.25-1.5 d. At these SRT/HRT ranges, the 2 process could be operated without a high fouling propensity with gross permeate flux (J20) of 15 LMH and specific gas demand (SGDp) between 16 and 20 Nm 3 airꞏm-3 permeate, which highlights the potential of membrane filtration in MPBRs. When the continuous operation of the MPBR plant was evaluated, an optical density of 680 nm (OD680) and soluble chemical oxygen demand (sCOD) were found to be good indicators of microalgae cell and algal organic matter (AOM) concentrations, while dissolved oxygen appeared to be directly related to MPBR performance. Nitrite and nitrate (NOx) concentration and the soluble chemical oxygen demand:volatile suspended solids ratio (sCOD:VSS) were used as indicators of nitrifying bacteria activity and the stress on the culture, respectively. These parameters were inversely related to nitrogen recovery rates and biomass productivity and could thus help to prevent possible culture deterioration.

Outdoor microalgae‐based urban wastewater treatment: Recent advances, applications, and future perspectives

“…Microalgae are versatile microorganisms that have shown the capacity of recovering nitrogen and phosphorus from sewage to values that can accomplish legal requirements (González‐Camejo et al, 2020). They can adapt to wide ranges of pH, irradiance intensity, temperature, and nutrient concentrations (Mantovani et al, 2020; Soares, Martins, & Gonçalves, 2019).…”

“…Consequently, microalgae in the deepest places of the reactor remain light‐limited or even in complete darkness, reducing microalgae productivity (Fernández et al, 2016; Raeisossadati, Moheimani, & Parlevliet, 2019). For this reason, light path in microalgae reactors tends to be short, that is, around 15–30 cm for raceway ponds (Acién et al, 2021; Arbib, de Godos, Ruiz, & Perales, 2017) and in the range of 2–10 cm for closed PBRs (González‐Camejo, Aparicio, et al, 2020; Slegers, Wijffels, van Straten, & van Boxtel, 2011). However, it must be considered that the shorter the light path, the higher the risk of overheating, which can be detrimental for microalgae growth (see Section 2.1.12).…”

“…Despite the plenty advantages of intensive microalgae-based wastewater treatment systems, they are not widely implemented yet due to several challenges such as high capital and operating costs and not being able to assure appropriate water quality in the long-term (Acién et al, 2021;González-Camejo, Aparicio, et al, 2020;González-Camejo, Barat, et al, 2020). To make microalgae cultivation feasible, the process has to focus on several issues: (a) to increase microalgae performance through the optimization of the cultivation process; (b) to apply CE principles by obtaining economic benefits from the microalgae biomass produced (Section 5.1); and (c) in the latter case, to reduce the high energetic demand of the harvesting system (see Table 2) as it can account for 0.2-5 kWh kg microalgae biomass −1 (Fasaei, Bitter, Slegers, & van Boxtel, 2018).…”

“…This refers to the reacting volume that is not exposed to light such as connecting pipes and distribution tanks. If a harvesting system (Table 2) is used to separate microalgae from the wastewater stream and recirculate it to the reactor as in membrane photobioreactors (MPBRs) (González‐Camejo, Aparicio, et al, 2020; Luo et al, 2018) or in membrane‐couple algal ponds (Robles et al, 2019), the volume of the membrane tanks should be minimized to increase microalgae performance. In this respect, Viruela et al (2018) obtained 15, 67, and 41% higher nitrogen removal rate, phosphorus removal rate, and biomass productivity (BP), respectively, when the membrane tank's volume was reduced from 27.2 to 13.6% of the total reacting volume.…”

“…In addition, low HRTs will increase the loading rates to the reactor, which can be detrimental for the wastewater treatment process due to excessive concentration of nutrients and/or other pollutants that can inhibit microalgae growth partially or totally (Assunção & Malcata, 2020). In this respect, González‐Camejo, Aparicio, et al (2020) observed a decrease in MPBR performance when operated at 1‐day HRT due to excessive nutrient loading rates, which promoted nitrifying and heterotrophic bacteria proliferations. Furthermore, Faleschini et al (2012) reported maximum biochemical oxygen demand (BOD 5 ) load of 60 kg BOD 5 ·ha −1 days −1 for a WSP operated in a temperate climate region.…”