Assessment of plastic materials for bio elements in RAS MBBRs’

There have over the last couple of years been cases where bio film build-up on bio elements have been inhibited in RAS bio filters. One case was second phase of Grieg Seafood’s in 2018 in Finmark, Norway. The MBBRs’ were inoculated with nitrifying bacteria from RAS Right from NOVA-Q. Normally, it would take a week or two and one would see a good and healthy bio film development. In the said case, there was no bio film build-up at all. It took several months before noticeable film occurred. This was never seen before. The bio elements were made of Poly Propylene (PP). However, as they were for a saltwater application, chalk had been added to increase density to 0.98. The bio elements were of a slight greenish color. It later emerged that they had been made from re-cycled PP of unknown origin.

Plastic surfaces, as well as nitrifying bacteria, are negatively charged. In principle, a plastic surface and negatively charged bacteria or particles should repel each other, and in principle they do. There are temporarily other mechanisms that influence the adhesion process of biofilm-forming bacteria.

There is a correlation between the hydrophobic properties of a plastic substance (intermolecular forces -mJ/m2) and the adhesiveness of nitrifying bacteria. The more hydrophobic a surface is (a “low energy surface”), the greater its attractiveness for particles or bacteria. A high energy surface or hydrophilic surface has low attractiveness.

PP and PE surfaces are low energy surface plastic materials compared to many other plastic species and are thus good plastic materials for bio elements. In the literature, PP appears slightly better than PE in terms of adhesion of nitrifying bacteria and formation of biofilm. This is despite the fact, that PE has greater surface roughness, but PE has higher surface energy with associated less attractiveness compared to PP.

In addition to the above conditions that have an impact on biofilm formation, there could potentially be inhibition caused by substances added to the plastic material. In an interview with the Plastics Department at the Danish Technological Institute, anti-biofilm additives are frequently used to avoid fungi, bacteria, algae coatings depending on the area of ​​use of the actual plastic. If these products are used later in connection with recycled plastics for production of bio elements, a problem with inhibition may arise. Another additive for plastic is Carbon Black which is used for UV protection.

Additives make the plastic material more expensive and will normally not be added from plastic material suppliers unless specified. There is thus only the possibility of contamination if recycled plastic is used or incorporated, which often can be the case. It is not known with certainty what Carbon Black addition means for the properties of the plastic, i.e., how much it changes the inter molecular forces, only that it has an actual toxic effect. Carbon Black is standard for PE pipes, whether used above or below ground. With KSK’s Saddleback with Carbon Black, low bio film formation has been identified.

In a test carried at Danish Technical University (DTU), it showed that the Saddleback bio element with Carbon Black had least nitrification rate compared to a RK Plast bio element with non-addition. This despite the RK had been added a density adjusting additive. See below:

Another study on various forms of plastic shows that clean recycled HDPE plastic had good adhesion, whereas HDPE used for potable water had extremely poor adhesion. See below:

At the IAA’s delivery of PP bio elements at Grieg Seafood, in Finnmark, where there was a great inhibition of biofilm formation, chalk had been added to the bio elements. Similarly, PP with added talc in the study: ”Characterization of nitrifying bacteria in marine recirculation aquaculture systems with regard to process optimization ” showed also to have inhibition and reduced biofilm formation. This can be attributed to the fact that the added talc changes the surface from a low energy to a very high energy surface. This is also apparent from: “Effects of Surface Modification of Talc on Mechanical Properties of Polypropylene / Talc Composites”, Keyan Liua *, Wolfgang Stadlbauera, Gernot Zitzenbachera, Christian Paulikb and Christoph Burgstaller.

Another aspect of importance is how a bio element is fabricated, even with same raw material. It has shown that there is a difference in bio elements that are extruded, and injection molded. In a test conducted by DTU, an example is shown below. The orange arrow shows the injection molded concentrations of the tested water, in this case for Nitrite.

In another experiment conducted at FREA A/S, a Danish RAS producing trout, following bio elements were assessed:

• IAA PP green, with chalk
• IAA PE no addition
• Saddleback, black carbon
• Saddleback, clear no addition
• RK, black carbon
• KK K5, clear
• Warden, PE

It showed that all bio elements containing Carbon Black had little bio film build-up. Detected by standard surface scrape procedure. The same was the case with the IAA PP with added chalk (to increase density).

All clear bio elements had adequate bio film build-up quite similar. Detected both visually and with standard surface scrape procedure.

There was no clogging of the IAA bio element. Most probably due to the open cylinder profile, see below:

Results

The above is shown with controls (except Warden). The IAA HDPE elements had a smooth film with no clogging occurrences. Both on the K5 as well as the Saddleback elements occurred with clogging. The Warden bio elements also had a smooth biofilm formation, but a bit of a tendency to clogging, though significantly less than the K5 and Saddleback bodies.

Other observations:

There has been seen break-up of certain extruded bio-elements. One case was IAA PP bio elements at Grieg Seafood and MOWI, Canada. This can be because of extrusion mal functioning; it can be storage in sub-zero temperatures over prolonged periods or physical impact pressures.

Another example of bio film break-up is the K 5 at SAF, Switzerland. It does not seem to happen with PE or HDPE.

Conclusions:

Different plastic materials have distinctive characteristics in connection with nitrifying bacteria’s ability to adhere to the surface.

With same plastic material, there seem to be a difference in adhesion depending on the element being extruded or injection molded, with least adhesion with the latter.

Some bio elements clog up easier than others. This has implications in reduced active surface area. It has also implications in that there is chance of anaerobic niches. Finally, it can host Geosmin and MIB producing bacteria.

Bio elements with Carbon Black should be always avoided.

Bio elements should not be added chalk or other additives to adjust density. If that is the case, the bio elements must be assessed before adding to bioreactors.

There seem to be little difference in PP, PE and HDPE. PP has a density of 0.92 and will be suitable for freshwater situations. HDPE has a density od 0.98 and is suitable for saltwater. All materials can, however, be used in freshwater.