ULTRA-COMPACT BIOFILM REACTOR FOR CARBON AND NITROGEN REMOVAL
ZHAO YONG CHANG
ZHAO YONG CHANG
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Abstract
The Ultra-Compact Biofilm Reactor (UCBR) is a relatively new technology in wastewater treatment. Like other biofilm reactors, it can maintain a high biomass concentration and possess good pollutant removal ability. Moreover, the separation of hydraulic retention time from sludge retention time enables it to accumulate slow growing bacteria like nitrifiers in the biofilm. Its ability to remove nitrogen is therefore expected to be high.
Comparing with other biofilm reactors, UCBR uses very small particles as the biofilm carriers. These carriers have a very high specific surface area (2000 to 4000 m2/m3). The biomass concentration in the reactor can reach up to 10,000 mg/L in bench-scale reactors and 40,000 mg/Lin full-scale reactors even when the biofilms are thin.
With a mixed culture used for both carbon and nitrogen removal, the biofilm formation and development was found to have induction, growth, mature and sloughing stages. Once the biofilms sloughed, the whole cycle would repeat itself. The performance of the reactor would then be unstable. In order to overcome instability caused by biofilm sloughing, carriers can be introduced into the reactor sequentially. The carrier would then be in different development stages and consequently an overall stable performance is attainable.
Carrier washout was a problem in the bench-scale experiment. The carrier washout was caused by an inadequate overflow channel or air bubbles escaping through the effluent channel. A properly designed three-phase separator does not allow air bubbles to escape through the effluent channel having enough space to avoid wall attachment.
Carbon removal in the UCBR was good. Using ethanol as the carbon source and at 20 kg/tn3-d COD loading rate, the effluent COD was only 20 mg/L. This represented more than 95% COD removal efficiency at 45 minutes hydraulic retention time. Furthermore, the immediate reactor response when loading was changed from 5 to 10 kg/tn3-d and from 10 to 20 kg/m3·d showed that the reactor had good damping characteristics.
When the reactor biomass concentration fluctuated, the nitrification performance was also unstable. The nitrification rate then varied between 0. I 5 and 2.0 kg N/m3·d. After the carrier biofilm growth and loss was stabilised by sequential carrier addition, nitrification became more stable. An average ammonia removal rate of 2.1 kg N/m3·d was obtained when bulk ammonium nitrogen concentration was 16 mg/L and the COD loading rate was 5 kg/m3·d. At 10 kg/m3·d COD loading rate, 1.77 kg N/m3.d ammonium nitrogen removal rate was obtained with a stable effluent ammonium nitrogen concentration of 3 to 4 mg/L. Ammonia, inorganic carbon and dissolved oxygen were the nutrients in the nitrification process. Their concentrations in the bulk liquid could affect the nitrification capacities of the UCBR. Among these factors, dissolved oxygen could probably be the main limiting substrate because of the competition from heterotrophes. UCBR with thin biofilms on smaller particles minimises the effect of oxygen deficiency and therefore, leads to higher nitrification rates.
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Date
1999
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