Author
 |
Brazil, Brian |
|
Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/9/2005 Publication Date: 4/4/2006 Citation: Brazil, B.L. 2006. Performance and operation of a rotating biological contactor in a tilapia recirculating aquaculture system. Aquacultural Engineering. Vol 34/3 pp 261-274. Interpretive Summary: In aquaculture, most bioreactors utilized fixed-film processes to accomplish ammonia reduction. Bioreactor configurations typically used in culture systems include, fluidized sand bed, trickling filters, and bead filters. Historical concerns over mechanical breakdown has limited wider acceptance of the rotating biological contactor (RBC) as an acceptable biofilter choice. Improved design and construction have nearly eliminated previous problems, yet there has been little examination of performance of full-scale RBC systems used in aquaculture. During this evaluation, industrial-scale RBC units installed in recirculating tilapia production systems were observed to achieve ammonia removal rates similar to other biological filter types. Simultaneously, the RBC reduced dissolved carbon dioxide concentrations by 39% and maintained dissolved oxygen levels at approximately saturation. Over the nearly 15 years of operation, routine regular maintenance, such as regular lubrication of bearings has been necessary to achieve nearly continuous operation. This study demonstrated that a well-constructed RBC can outperform other filter types as the RBC combines three unit processes, i.e. ammonia removal and carbon dioxide removal, and oxygen in a single vessels. Most other biofilter types require at least two vessels to accomplish these same results. While, the RBC is still relatively expensive, concurrent ammonia reduction and gas conditioning in addition to reduced pumping requirements potentially result in significantly lower operational costs making the RBC a more cost effective filtration device. Technical Abstract: In wastewater treatment, the rotating biological contactor (RBC) is used to achieve carbonaceous BOD removal and nitrification. Under these conditions, manufacturers have reliable performance data that has been used to established design criteria. In aquaculture environments, RBCs are primarily employed for nitrification. While some organic removal is achieved, the accumulation of dissolved organics is managed through ozonation and/or freshwater exchange. This paper describes the performance characteristics of an industrial-scale air driven RBC installed in a recirculating aquaculture system (RAS) rearing tilapia at 28 degrees C. Ammonia removal efficiency averaged 31.5% per pass for all systems examined, which equated to an average total ammonia nitrogen (TAN) removal rate of 0.42 g/m2/day. The RBC systems were configured as 3-stage unit where stages 1 and 2 possessing the same total surface area and the third approximately 25% smaller, providing a total surface of 13,380 m2. Within each RBC system, the ammonia oxidation rate (K1) for stages 1, 2, and 3 were 2.4, 1.5, and 3/hr. Nitrite conversion rate constants were much higher, averaging 16.2, 7.7, and 9.0 for RBC stages 1, 2, and 3, respectively. Dissolved organic carbon (DOC) constituents were not readily degraded, which resulted in low average removals of 6.6% across the RBC systems studies. However, increased DOC concentrations tended to decrease ammonia removal efficiency. Dissolved carbon dioxide (CO2) concentrations in the RBC effluent were approximately 39% lower than influent concentrations which ranged from 28 to 60 mg/L. During this evaluation, cumulative feed burden ranged between 5.5 and 7.3 kg of feed per m3 of freshwater; however, there was no detectable relationship to the RBC achieved ammonia removal rates similar to the predominant biofilter configurations (fluidized sand bed, bead filters, and trickling filters) used in aquaculture and the RBC stripped CO2 as efficiently as a cascade aeration column specifically designed for degassing. |
