• Energy Research

Hydrazine is an intermediate product of the anaerobic ammonium oxidation (Anammox) process where both ammonium and nitrite in wastewater are converted to nitrogen gas by bacteria. In this study the effect of external hydrazine addition (5, 10, 15, and 20 mg/L) on the start-up period of the Anammox process was studied using sequencing batch reactors (SBRs). The SBR with an addition of 10 mg/L hydrazine took only 7 weeks to stabilize and achieve the maximum removal of ammonium and nitrite, whereas the SBR without the addition of hydrazine took 12 weeks. The amount of Heme C extracted from the biomass indicated that externally added hydrazine accelerated the growth of Anammox bacteria and reduced the release of nitrous oxide gas from the reactors.

The inhibitory effect of free ammonia (FA;NH(3)) on the metabolism of Nitrobacter is investigated using a method that allows decoupling energy generation from growth processes. A lab-scale sequencing batch reactor (SBR) was operated for the enrichment of Nitrobacter. Fluorescent in situ hybridization (FISH) analysis showed that 73% of the bacterial population in the reactor was Nitrobacter, while no Nitrospira was detected. Batch tests were carried out to measure the oxygen uptake rate (OUR) by the culture at various FA levels, in the presence (OUR with CO(2)) or absence (OUR without CO(2)) of inorganic carbon (CO(2), HCO(3)(-) and CO(3)(2-)). The FA inhibition on the respiration initiated at below 1mgNH(3)-NL(-1) in both cases. OUR without CO(2) gradually decreased by 12% when the FA concentration increased from 0 to approximately 4mgNH(3)-NL(-1) and remained at the same level till an FA level of 9mgNH(3)-NL(-1) (the highest FA concentration applied in this study). This indicates that FA has a limited inhibitory effect on the respiratory capability of Nitrobacter. Starting from a level that is 15% higher than OUR without CO(2) when no FA was present, OUR with CO(2)decreased more rapidly than OUR without CO(2) reaching the same level as OUR without CO(2) when FA was between 6-9mgNH(3)-NL(-1). This implies that in this range of FA the presence of inorganic carbon did not cause any increase in the respiration activity of Nitrobacter. The results suggest that, while still oxidizing nitrite at approximately 75% of the non-inhibited rate, Nitrobacter likely ceased to grow at an FA level of above 6mgNH(3)-NL(-1). While the real mechanisms remain to be identified, this study indicates that the FA inhibition on Nitrobacter is likely much more serious than suggested by previous studies where OUR with CO(2) (or the equivalent nitrite oxidation rate) was used as the sole measure of the inhibitory effects.

Microbial fuel cells (MFC) haves drawn much attention as a potential approach for sludge treatment and renewable energy production. In this study, a single chamber air‐cathode in a membrane‐less MFC (ML‐MFC) was operated in batch mode with dewatered sludge from a municipal wastewater treatment plant as the substrate. Electrogenic bacterial species in the sludge acted as a catalyst for the generation of electricity. The performance of the ML‐MFC was evaluated using one‐factor‐at‐a‐time (OFAT) method followed by response surface methodology (RSM) via Central Composite Design using a quadratic model. In the preliminary OFAT study, the highest voltage generation (852.7 mV) and COD removal (149.2 mg/L) were obtained when the pH, electrode distance, moisture content, and temperature were at 6.0, 3 cm, 30% (vol/wt), and 35°C, respectively. After incubation of the ML‐MFC using optimum conditions suggested by the RSM, the voltage was successfully increased to 927.7 mV (improved 9%), while COD removal increased to 170.8 mg/L (improved 15%). This showed that optimization using RSM gave better results than the OFAT method. The maximum power density recorded from the optimum conditions was 41.3 mW/m2. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 208–219, 2019

Microalgae-based biodiesel has gained widespread interest as an alternative energy source. Low-cost microalgae harvesting technologies are important for economically feasible biodiesel production. This study investigated, for the first time, the impact of adaptation period and height to diameter (H/D) ratio of a reactor on the growth and self-flocculation of microalgae, without the addition of bacteria. Six reactors were grouped into three sets of experiments, and each reactor was operated for 30 days at similar operating conditions (volume exchange ratio = 25% and settling time = 30 min). In set 1, two 8-L reactors, H5a (H/D ratio: 5) and H8a (H/D ratio: 8), were operated under batch operation. In set 2, reactors H5b and H8b were operated as sequential batch reactors (SBRs) without an adaptation period. In set 3, the reactors H5c and H8c were operated as SBRs with an adaptation period. The findings showed a threefold improvement in biomass productivity for the higher H/D ratio (H8c) and a reduction in biomass loss for microalgae. The H8c reactor exhibited 95% settling efficiency within 5 days, in comparison to 30 days for the H5c reactor. This study demonstrated that a higher H/D ratio and the introduction of an adaptation period in SBR operation positively influences growth and self-flocculation of enriched mixed microalgae culture.