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Characteristics of a Good Carrier Media
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Characteristics of a Good Carrier Media
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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 aquaculture 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 μm) and bacterial activity in freshwater RAS operated at increasing

intensity of recirculation (feed loading from 0.043 to 3.13 kg feed m?3 make-up water). Four independent

sets of water samples from different systems were analyzed and compared including samples from: (i) two

individual constructed wetlands treating the effluent system water from two commercial,

freshwater rainbow trout (Oncorhynchus mykiss) farms of different recirculation intensity; (ii) an

8.5 m3 pilot scale RAS; and (iii) twelve identical, 1.7 m3 pilot scale RAS assigned one of four micro-

screen treatments (no micro-screen, 100, 60, or 20 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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