Given that most wastewater treatment plants are expected to have constant exposure to polyvinyl alcohol from household use, do you agree with the assumption of Rolsky and Kelkar (2021) that the adaptation of polyvinyl alcohol-degrading organisms will be minimal? (1=strongly disagree; 10=strongly agree)

Answer Explanations

• 5

  Expert 1

  I do not agree with the assumption by Rolsky et. Al (2021) where they claimed that the adaption of polyvinyl alcohol degrading organisms will be minimal in the waste water treatment plant. Although you will not expect the amount of PVA entering into a wastewater treatment plant to change a lot with time, the condition of the medium into the wastewater treatment plant carrying the PVA might change (e.g. pH and other dissolved organics in the medium) from time to time. Hence, I am not 100% convinced that alcohol-degrading organisms will behave/respond exactly the same regardless of time. More studies such as toxicity test need to be done to evaluate the adaptation of polyvinyl alcohol-degrading organisms.

  The assumption was also made based on outdated references as mentioned by the authors in the supplementary section: “While aqueous environments are considerably more favorable for the degradation of PVA, research suggests that it degrades poorly within terrestrial ecosystems. In soil for example, PVA has shown to only lose ~10% of its mass over a period of either 74 days or up to two years [29,37,38].” The references (29, 37, 38) were the studies reported in 1999. In fact, there are recent studies that suggested PVA degradation by bacteria can be realized within a week, for example: https://doi.org/10.1080/09593330.2017.1349189 (Stenotrophomonas sp. strain SA21 degraded 90% of the PVA present in the culture medium after 4 days.) and https://doi.org/10.1007/s13205-019-1882-6 (optimal temperature and pH for PVA biodegradation by strain RA23 was 28 °C and 7.0, respectively, and 85% of 0.1% PVA was degraded after 5 days under these conditions). Hence, the assumption made by Rolsky and Kelkar (2021) was probably underestimated where they should have referred to more works before coming to this assumption.

• 4

  Expert 5

  Uncertain on this aspect - constant exposure is given, but it is unclear if there is sufficient PVA to have bacteria adapt to it, given that in a typical WWTP, there is plenty of carbon available for digestion.

• 3

  Expert 3

  In principle, I am inclined to respond '1'- strongly disagree. There is, however, the minor reservation of common information/data on kinetics of biodegradability of polymers currently missing. (see also the response to the previous question).

• 1

  Expert 6

  As already pointed out in my responses to the previous set of questions, the direct translation of lab-scale data to WWTP-scale removal efficiencies by Rolsky & Kelkar is highly questionable. Their reasoning is not in line with the generally accepted scientific consensus regarding persistency of readily biodegradable materials and how activated sludge adaptation works in (semi-)continuously exposed large-scale WWTP systems.

• 1

  Expert 7

  Adaptation may refer to a single microorganism or to a community of microorganisms present in for instance a test vessel, an ecosystem or wastewater treatment system. Adaption of a single microorganism to a substance as for instance acetate is never required (enzymes necessary to degrade this substance are constitutively expressed). Other microorganisms synthesize enzymes needed to degrade a substance upon exposure (inducible expression). Induction takes only a few hours. Adaptation in wastewater treatments systems, ecosystems and tests as for instance those described in OECD 301 and 302 guidelines is a process that takes place in microbial communities i.e. the increase of the number of competent microorganisms upon exposure (growth of microorganisms). Adaptation will only occur when a community and/or inoculum of a test does contain microorganisms capable of growing on the test item. Microorganisms capable of growing on PVA do exist as demonstrated by almost all test results described (also those showing little degradation for whatever reason). In view of the widely dispersed use of pods it is very likely that communities of microorganisms all wastewater treatment plants are exposed and consequently adapted.

• 2

  Expert 8

  I think that the adaptation of microorganisms is possible, as the PVA degradative experiments of Menzies et al clearly demonstrate that they are biodegradable compounds even without using specifically adapted microorganisms. A priori PVA does not appear to be an excessively complex molecule with biodegradability problems. Therefore, it is to be expected that some PVA degrading microorganisms will be able to oxidize these compounds under aerobic conditions in the activated sludge system.

• Expert 2

  We don’t have enough information to assess whether the constancy of PVA loading, or the absolute amount of PVA in input water is important for driving adaptation among bacteria. It’s not reasonable to assign a score when there is insufficient understanding to make a call.