Title: Microalgal-bacterial photobioreactor for the removal of carbon dioxide from biogas engine flue gases
Author: Ann Njeri Numi
Supervising Institution: IHE Delft - Institute for Water Education
This study involved the use of a consortium of microalgae and nitrifying bacteria for the co-treatment of carbon dioxide (CO2) from biogas engine flue gases and NH4+- N rich wastewater from an Up flow Anaerobic Sludge Blanket (UASB) treating cattle manure. The research was aimed at determining the effects of CO2 supply on NH4+- N removal rates of microalgae and nitrifiers. It also focused on studying the effects of CO2 supply on NH4+- N removal rates of mixed culture, microalgae and nitrifiers separately. The biomass was grown in a base reactor under CO2 limiting conditions. Determination CO2 removal rates by mixed culture, microalgae and nitrifiers and was also a priority of the study.
The study involved one continuous experiment and three batch experiments. The first experiment was conducted in a 1L photobioreactor which was operated as a Sequencing Batch Reactor (SBR). It was fed with artificial wastewater from the UNESCO-IHE laboratory containing 35 mg/L NH4+- N to study the NH4+- N removal rates of a mixed culture, microalgae and nitrifiers. It was used as a base-reactor for the cultivation of biomass to be used as inoculum in batch experiments 2a, 2b and 2c. Batch experiments 2a, 2b and 2c were conducted in 1 L serum bottles. The serum bottles were fed with a mixture of 10 % CO2 and 90 % N2 from 10 L tedlar bags at a flow rate of 1.7 L/h. In experiment 2a, the flask was artificially illuminated and contained a mixed culture. In experiment 2b, the flask was artificially illuminated to promote microalgae growth and nitrifier inhibitor (N-Allythiourea/ATU) was added. Experiment 2c was conducted in the dark to prevent microalgae growth. This was done to determine the effects of CO2 enrichment on the NH4+- N removal rate by microalgae and nitrifiers separately. The rates of CO2 removal were also determined through comparing CO2 concentrations in the inflow and outflow (head space).
The maximum NH4+- N removal rates observed in the base reactor was only 1.1 mg/L.h (day 42). The highest average NH4+- N removal rates observed were 0.3 ± 0.2 mg/L.h which was relatively low compared to similar studies. The low NH4+- N removal rates were expected due to the carbon limitation in the base reactor. Batch experiment 2a (mixed culture) had the highest CO2 removal rate of 1.9 ± 0.2 mg/L.min. The lowest was 0.1 ± 0.03 mg/L.min which was observed in Batch experiment 2c (nitrifiers active culture). According to the carbon balance, only a small percentage was fixed into the biomass (4.91 ± 0.0 % and 9.91 ± 3.4) in batch experiment 2a and 2b respectively. There was no fixation in batch experiment 2c. From the results obtained in this study, it was clearly evident that CO2 enrichment increased NH4+- N removal rates. Maximum average NH4+- N removal rates of 0.8 ± 0.1 mg/L.min were observed in batch experiment 2a while the lowest were observed in batch experiment 2b (0.2 ± 0.1 mg/L.min). However, from the nitrogen balance, it was seen that the main NH4+- N removal pathway was microalgae uptake. Nitrification levels were very low.
Keywords: carbon dioxide, ammonium, microalgae, nitrifying bacteria, biofixation, nitrification, consortium