|M.Sc Student||Bar Massada Iris|
|Subject||Study of the Parameters that Affect Anaerobic Ammonia|
Oxidation (Anammox) as Part of a Process Aimed at
Complete Removal of Nitrogen Species
from Recirculating Aquaculture
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Ori Lahav|
|Full Thesis text - in Hebrew|
The banning of fish-cage production in the Eilat-bay resulted in an urgent need for the development of land-based aquaculture. A possible alternative is RAS (Recirculating Aquaculture Systems). Water treatment in RAS typically consists of solid substances removal, ammonia oxidation and CO2 stripping, whereas nitrate in not treated. The current research employs a denitrification reactor in which organic matter produced in the ponds ("intrinsic organic matter" that contains nitrogen in its structure) is used as the electron donor. This procedure results in ammonia release, as a function of both ammonification and ammonia assimilation. In a previous work it was found that the actual ammonia concentration in the reactor was much lower than that expected, indicating another ammonia removal mechanism.
In the present research, it was found that the “ammonia disappearance” phenomenon in the denitrification reactor is a result of ammonia oxidation by anammox bacteria, which are present in the reactor at varying concentrations, depending on the operational conditions, particularly the solids retention time (SRT). It was found that the anammox population arrives with the seawater to the pond, where they are enriched/concentrated in the intestines of fish and supplied to the denitrification reactor following the separation of the feces by the solids filter.
Since the Denitrification reactor is fed on a daily basis with solids backwashed from the filter, it is practically fed with anammox bacteria together with the organic matter that stimulates the process. The results suggest that at SRTs shorter than six days the anammox bacteria population does not grow in the system, thus their concentration in the reactor tends toward their concentration in the solids filter, and their activity rate is rather slow. At SRTs >6 days, the concentration of the bacteria in the reactor increases and so does their activity (maximum removal of 35 mgN/l/d of ammonia). The relative fraction of anammox out of the total bacterial concentration increases with increased SRT as follows: 13±9, 38±9, 40±10, and 57±8 at SRT of 4, 6, 8, and 12.5 days, respectively. As the ammonia oxidation rate remains roughly constant although the relative concentration of anammox bacteria increases with increased SRT, there is no benefit in applying SRTs higher than six days. At the expected operational conditions of the denitrification system in Eilat, effluents are expected to contain 1 mgN/l. Since future regulation is 2 mgN/l, a polish treatment following the denitrification process is not expected to be necessary.