Utilization of treated sewage for carbon dioxide capture and nutrients removal by microalgae cultivation in a membrane photobioreactor

Dr. Ryo Honda (Japan)

Assistant Professor, Kanazawa University

Aiming carbon dioxide capture and utilization, we proposed a concentrated microalgae cultivation process utilizing nutrients in treated sewage. Submerged-membrane filtration was installed in a photobioreactor to achieve high nutrients loading and to maintain a high concentration and production of microalgae. Chlorella vulgaris, Botryococcus braunii and Spirulina platensis were continuously cultivated with simulated treated sewage and 1%-CO₂ gas. As a result, the carbon dioxide capture rate and volumetric microalgae productivity were largest when the reactor was operated under 1-day hydraulic retention time (HRT) and 18-days solids retention time (SRT) conditions. Difference in the microalgae productivity and cell concentration was smaller than difference in nutrients loading from the past studies using high-nutrients media. Consequently, independent control of HRT and SRT, which was enabled by installation of submerged membrane, was effective for high-efficient microalgae cultivation and carbon dioxide capture utilizing treated sewage. Improvement of nutrients loading expectedly improves process performance.

Degradation of Wasted Sludge Using Sulfate Reduction

Dr. Eri Nakakihara (Japan)

Project Officer of Environment/ an Eco-technological Special Course, Faculty of Environmental Design, Kanazawa University

Abstract: Although anaerobic digestion of sewerage sludge is effective energy recovery process, low digestive efficiency of waste activated sludge is weakness of this process. We reported that sulfate was reduced and acetate was produced when activated sludge was stored in the room temperature (RT) before. Sulfate reduction might be useful for pretreatment of sludge digestion. In this study, degradation of wasted sludge in the sulfidogenic conditions were examined in a laboratory scale continuous reactor.

The reactor was made up of a glass flask (500mL). First, concentrated activated sludge (TS 6000mg/L) was put into the reactor. In Run 1, the concentrated sludge was added to the reactor, continuously. In Run 2, the same sludge and sulfate (500mg/L) was added to the reactor and operated concurrently with Run 1. HRT was set at 72 hrs (Period 1) and 24 hrs (Periods 2 and 3). Temperature was regulated at 35℃(Period 1) and 40℃ (Period 2). In Period 3, the reactor was operated at RT (15-20℃). In Run 3, the reactor was operated at 30℃ and 24 hrs of HRT in sulfidogenic condition.

In Run 1, decrease in the sludge concentration was small and organic acids were not produced. On the contrary, in Runs 2 and 3, sulfidogenic condition, the sulfate concentration decreased remarkably. Bicarbonate concentrations increased with sulfate reduction in the all temperature. In Period 3 of Run 2, in which the reactor was operated at RT，high concentration of acetate and propionate was detected. On the other hand, organic acids were not accumulated when the temperature was over 30℃. Produced sulfide could be utilized for denitrification.

In Run 3, DNA was extracted from the sludge and the microbial community of sulfate-reducing microorganisms was elucidated by a dissimilatory (bi)sulfite reductase gene-targeted nested PCR-DGGE.The DNA sequence of a band of the influent sludge was most closely related to Desulfobulbus elongates(79%). The DNA sequences of two bands of the effluent sludge of SR reactor were most closely related to Desulfobulbus propionicus (79%) and Desulfomicrobium escambiense (94%), respectively.Desulfobulbus spp. and Desulfomicrobium spp. can utilize several different type of organic matter and oxidize organic substrate incompletely. These results suggested that methanogenic archaea might use acetate as a substrate, when the temperature was over 30℃.