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Characteristics of a Good Carrier Media
Characteristics of a Good Carrier Media

    Tom owns a production factory. Every day, the factory produces 10 kg of a certain product. Based on his experience, Tom knows that, on average, one trained worker with correct tools could manage to pack and ship out 1 kg of the product daily. Being a predictive man, Tom considers the probability that one worker might get sick. To be on the safe side, he has 11trained workers instead of 10. This helps him prevent an overcapacity situation in the warehouse in the event that one of them is sick or the factory has to increase its production capacity. With his 11 trained workers and correct tools, Tom’s factory operates smoothly every day without trouble.

    Now, let’s compare this with the biological wastewater treatment process. The product is the organic load to be removed by means of wastewater treatment; the trained workers and tools are equivalent to the carrier media and biomass in the wastewater treatment process, respectively. Similar to the workers who cannot pack and ship out the products without tools, the carrier MBBR media cannot remove the organic load from the wastewater without biomass.

    If one day Tom’s factory starts producing another product that requires a different packing method, he doesn’t dismiss his 11 workers and hire a new group. Rather, he allows his workers time to become familiar with their new working tools. The same applies to biological wastewater treatment: The biomass that grows on the carrier MBBR bio carrier media will gradually adapt itself to remove a different type of organic load or concentration.

    In order for Tom to ensure his factory can run smoothly, he needs both skilled workers and a good set of tools for them to use. The lack of either will slow down the packing speed. For biological wastewater treatment with MBBR, the biomass as well as the carrier media that acts as the housing for the bacteria are very important.

    A good MBBR carrier media provides more than just a protected habitat for the bacteria to grow; it also ensures that all bacteria that grow on it are sufficiently supplied with nutrients for their metabolism. During the biological treatment process, the bacteria consume dissolved organic substances. Without sufficient nutrients, the growth of the bacteria is hindered, or worse, the bacteria die off. These phenomena will reduce the removal efficiency and lead to an unqualified wastewater discharge. Hence, a proper selection of the carrier media is essential. This decision will affect both the organic removal performance and the cost required to run the plant.

    A good MBBR carrier has the following characteristics:

    ? A large protected surface area to maximize the amount of biomass;

    ? A porous surface to strengthen the biomass’s adhesion;

    ? An optimal substrate diffusion depth to ensure the metabolism;

    ? Wear-resistance for durability.

    In terms of treatment, a good MBBR carrier aquaculture filter media ensures that all biomass is active to remove the organic substances from the water. From the user’s perspective, a good MBBR carrier media eases the operation and provides a variety of savings, such as in construction and operation. 

    In a wastewater treatment application, the required amount of MBBR carrier media depends on the organic load that needs to be removed by means of the bacteria’s metabolism, the rate of which is influenced by water temperature and the type of substrate.

    Although MBBR carrier media might just be a little piece of plastic (or some other material), its role in wastewater treatment is vital to keep the biomass active in order to deliver the best possible organic removal performance. WW

    Moving Bed Biofilm Reactors (MBBR) are used increasingly in closed systems for farming of fish. Scaling, i.e. design of units of increasing size, is an important issue in general bio-reactor design since mixing behaviour will differ between small and large scale. Research is mostly performed on small-scale biofilters and the question is to what extent this can be upscaled to a commercial level. Therefore, the objective of this research was to establish the effect of mixing and scale on MBBR performance. The research was done in two major parts; firstly effects of scale-sensitive factors were studied in small reactors. Secondly, performance of these small reactors was then compared to increasingly large reactor sizes, using the same inlet water quality and biofilm.

    Firstly, a 200 L MBBR (medium scale) was operated continuously using a synthetic feed solution. Biofilm carriers from this reactor was used for short-term experiments in 0.8 L reactors (small scale) and compared with the performance of the 200 L medium scale reactor. Reactor geometry and superficial air velocity (m h?1) were identical in these experiments. Subsequently, the small reactors were incubated with biofilm carriers from three commercial farms and performance compared with these large scale reactors. In a number of additional experiments the effect of mixing and Total Ammonia Nitrogen (TAN) was tested at small and medium scale.

    The results showed that MBBR scale has a significant effect on TAN removal rate. In general, the larger the scale the better the performance. TAN removal (rTAN) at small scale (0.8 L) is about 80% compared to that at medium scale (200 L). The difference between small scale and large scale (>20 m3) is even higher. These findings warrant further studies on whether a plateau is reached in rTAN at a certain scale; a study which will have considerable importance for optimal design and dimensioning of commercial scale RAS. It was further found that superficial air velocity is not a good scaling factor for MBBRs. Upscaling while maintaining geometry implies increasing air injection depth and therefore increased energy input will be required at a comparable superficial air velocity, which is not incorporated in the superficial air velocity term (m h?1). Superficial air velocity and plastic media filling% were found to have a strong effect on mixing time at small scale. An air velocity below a threshold of 5 m h?1 decreased TAN removal at both small and medium scale. Intense mixing at small scale increased TAN removal at low TAN concentration. However, at a high TAN concentration, the small scale MBBR always performed at not more than 80% of the capacity of the medium scale system, irrespective of the mixing conditions. Hence, the capacity of full scale systems will be under-estimated when based solely on small scale experiments.

    Suspended particles in recirculating&nbsp;aquaculture&nbsp;systems (RAS) provide surface area that can be colonized by bacteria. More particles accumulate as the intensity of recirculation increases thus potentially increasing the bacterial carrying capacity of the systems. Applying a recent, rapid, culture-independent fluorometric detection method (Bactiquant?) for measuring bacterial activity, the current study explored the relationship between total particle surface area (TSA, derived from the size distribution of particles >5&nbsp;μm) and bacterial activity in freshwater RAS operated at increasing intensity of recirculation (feed loading from 0.043 to 3.13&nbsp;kg&nbsp;feed&nbsp;m?3&nbsp;make-up water). Four independent sets of water samples from different systems were analyzed and compared including samples from: (i) two individual&nbsp;constructed wetlands&nbsp;treating the&nbsp;effluent&nbsp;system water from two commercial, freshwater&nbsp;rainbow trout&nbsp;(Oncorhynchus mykiss) farms of different recirculation intensity; (ii) an 8.5&nbsp;m3&nbsp;pilot scale RAS; and (iii) twelve identical, 1.7&nbsp;m3&nbsp;pilot scale RAS assigned one of four micro-screen treatments (no micro-screen, 100, 60, or 20&nbsp;μm mesh size micro-screens) in triplicate. There was a strong, positive, linear correlation (p&nbsp;<&nbsp;0.05) between TSA and bacterial activity in all systems with low to moderate recirculation intensity (i.e. feed loading ≤1&nbsp;kg&nbsp;feed&nbsp;m?3&nbsp;make-up water). However, the relationship apparently ceased to exist in the systems with highest recirculation intensity (feed loading 3.13&nbsp;kg feed&nbsp;m?3&nbsp;make-up water; corresponding to 0.32&nbsp;m3&nbsp;make-up&nbsp;water&nbsp;kg?1&nbsp;feed). This was likely due to the accumulation of dissolved nutrients sustaining free-living bacterial populations, and/or accumulation of suspended colloids and fine particles less than 5&nbsp;μm in diameter, which were not characterized in the study but may provide significant surface area. Hence, the study substantiates that particles in RAS provide surface area supporting bacterial activity, and that particles play a key role in controlling the bacterial carrying capacity at least in less intensive RAS. Applying fast, culture-independent techniques for determining bacterial activity might provide a means for future monitoring and assessment of microbial water quality in aquaculture farming systems.

    Microbial water quality in&nbsp;recirculating aquaculture systems&nbsp;(RAS) is important for successful RAS operation but difficult to assess and control. There is a need to identify factors affecting changes in the bacterial dynamics – in terms of abundance and activity – to get the information needed to manage microbial stability in RAS. This study aimed to quantify bacterial activity in the water phase in six identical, pilot scale freshwater RAS stocked with&nbsp;rainbow trout&nbsp;(Oncorhynchus mykiss) during a three months period from start-up. Bacterial activity and dynamics were investigated by the use of a patented method, BactiQuant?. The method relies on the hydrolysis of a fluorescent&nbsp;enzyme-substrate&nbsp;and is a rapid technique for quantifying bacterial&nbsp;enzyme activity&nbsp;in a water sample. The results showed a forty-fold increase in bacterial activity within the first 24&nbsp;days from start-up. Average BactiQuant?&nbsp;values (BQV) were below 1000 at Day 0 and stabilized around 40,000 BQV after four weeks from start. The study revealed considerable variation in initial BQV levels between identically operated and designed RAS; over time these differences diminished. Total ammonia nitrogen, nitrite and nitrate levels were very similar in all six RAS and were neither related to nor affected by BQV.&nbsp;Chemical oxygen demand&nbsp;(COD) and biological oxygen demand (BOD5) were highly reproducible parameters between RAS with a stable equilibrium dynamic over time. This study showed that bacterial activity was not a straightforward predictable parameter in the water phase as&nbsp;e.g. nitrate-N would be in identical RAS, and showed unexpected sudden changes/fluctuations within specific RAS. However, a bacterial activity stabilization phase was observed as systems matured and reached equilibrium, suggesting a successive transition from fragile to robust&nbsp;microbial community&nbsp;compositions.

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