This report is a literature review on microalgal harvesting and processing submitted as partial fulfillment of subcontract XK-3-03031-01. The work was performed under subcontract to SERI with funds provided by the Biomass Energy Technology Division of the U.S. Department of Energy.~t
The performance of an upflow anaerobic sludge blanket (UASB) reactor and a hybrid UASB-filter reactor was investigated and compared for the treatment of domestic wastewater at different operational temperatures (28, 20, 14 and 10 degrees C) and loading rates. For each temperature studied a constant CODt removal was observed as long as the upflow velocity was lower than 0.35 m/h. At these upflow velocities similar removals were observed for both reactor types at 28 and 20 degrees C, 82 and 72% respectively. However, at 14 and 10 degrees C the UASB reactor showed a better COD removal (70% and 48%, respectively) than the hybrid reactor (60% and 38%). COD removal resulted from biological degradation and solids accumulation in the reactors. At 28 degrees C, a constant 200 g sludge mass was observed in both reactors and COD removal was attributed to biological degradation only. At lower temperatures, solids accumulation was observed in addition to biological degradation with an increase in reactor sludge as the temperature decreased. The decrease in biological degradation at lower temperatures was offset by solids accumulation and explains the similar overall COD removal efficiency observed at 28 degrees C, 20 degrees C and 14 degrees C. The decrease in temperature was also followed by an increase in the effluent TSS concentration in both reactors. At 14 and 10 degrees C a lower effluent TSS concentration and better performance was observed in the UASB reactor.
A typical steady state bulk pH of about 5 was established in a nitrifying fluidized bed with chalk as the only buffer agent. In spite of the low pH, high rate nitrification was observed with the nitrification kinetic parameters in the chalk reactor similar to those of biological reactors operating at pH>7. Various methods were used to determine the reasons for high rate nitrification at such low pH including (i) determination of bacterial species, (ii) microsensor measurements in the biofilm, and (iii) comparison of nitrification performance at low pH with a non-chalk fluidized bed reactor. Fluorescence in situ hybridization (FISH) using existing 16S rRNA-targeted oligonucleotide probes showed common nitrifying bacteria in the low pH chalk reactor. The prevalent nitrifying bacteria were identified in the Nitrosomonas oligotropha, Nitrosomonas europeae/eutropha, Nitrosospira and Nitrospira related groups, all well known nitrifiers. Microelectrode measurements showed that the pH in the biofilm was low and similar to that of the bulk pH. Finally, reactor performance using a non-chalk biofilm carrier (sintered glass) with the same bacterial inoculum also showed high rate nitrification below pH 5. The results suggest that inhibition of nitrification at low pH is highly overestimated.
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