The USEPA and the OECD have both provided guidance on how a “pass” results from a biodegradability test should be applied (i.e., using specific half-lives) in modeling wastewater treatment plant fate models. For compounds that are readily biodegraded, the half-lives range from 1 hr (US EPA) to 0.69 hrs (OECD). Based on the recommended half-lives (0.69-1 hr) for modeling readily biodegradable chemicals, is the assumption of Rolsky and Kelkar (2021; Table 2) valid that 1.5 – 20% of PVA will be degraded from the various treatment sections of a wastewater treatment plant? If no, please indicate if you think the actual percent of PVA biodegraded would be much higher, higher, lower, much lower?

Answer Explanations

• No

  Expert 5

  Actual percent PVA biodegration is likely higher.

• No

  Expert 3

  Judging from the available information on this issue, I am of the opinion that the actual percent would be higher. It is difficult to provide a number on this, but I also think that additional information is needed on the validity of the half-lives mentioned for the specific case of readily degradable polymers. This recommendation is based on the fact that the OECD recommendation is based on data for small soluble chemicals, whereas it needs to be verified that the guidance is indeed applicable to polymers.

• Yes

  Expert 8

  Based on the half-lives range of 0.69-1 hrs, the percentage of degradation in a WWTP will be much higher than the range 1.5-20% reported by Rolsky and Kelkar. The 0.69-1 hrs values provide very fast kinetics that would decrease concentration of PVA to a half in just one hour, that considering the hydraulic residence time in the activated sludge reactor, typically 8-12 h will decrease PVA concentration to very low values lower than 1% of the input, i.e. > 99% removal.
  However, these fast rates also appear to be overoptimistic. In any case, I would recommend pilot tests under relevant operational conditions to evaluate the real removal rates of PVA. The degradation rates can be based on acclimated or non-acclimated organisms and the results should be better with acclimated microorganisms.
  Anyway, I already considered in my previous review that 1.5 – 20% of removal appeared to be very low for a full-scale urban WWTP.

• No

  Expert 6

  I believe the actual percent of PVA biodegraded would be much higher than what is proposed by Rolsky & Kelkar. However, Rolsky & Kelkar include literature references that indicate that e.g., the degree of crosslinking can impact the sorption behavior of PVA. While I would not expect a hydrophilic polymer like PVA to become strongly sorbtive in this context, it should be pointed out that direct conversion of a readily biodegradable result to an expected WWTP half-life is less straightforward for compounds that do show (strong) sorption.

• No

  Expert 7

  Gerike and Fischer (1979, 1981) were the first to demonstrate in a study with 44 chemicals that readily biodegradable substances are removed in activated sludge plants. Nearly all the chemicals which passed the readily biodegradability tests were removed (>80%) in their simulation test of wastewater treatment plants. Based on their results and many other high removal percentages found by others, it is safe to assume high biodegradation percentages of PVA in properly operated and designed (SRT >10 days) activated sludge plants. The ⪂94% of EPA is in line with the science.
  Schonberger at al (1997) found >90% PVA removal in a full-scale plant. Less degradation of PVA at low temperatures due to slower growth rates. This has also been found with the oxidation of ammonium ito nitrate (nitrification) in primarily small plants. Plants should be designed to nitrify in both summer and winter.

  Gerike P, Fischer, WK, (1979) A Correlation study of biodegradability determinations with various chemicals in various tests. Ecotox Environ Safety, 3, 159-173
  Gerike P, Fisher, WK, (1981) A correlation study of biodegradability determinations with various chemicals in various tests II Additional results and conclusions. Water Res., 5(1), 45-55.

  ECHA SimpleTreat model (substance; readily bidegradable and time window criterion fullfilled) 87% removed by biodegradation SimpleTreat is conservative

• No

  Expert 1

  The degree of polymer degradations reported by Rolsky et. al. (2021) was based on compilation of literature review where a wide range of polymer degradation numbers was given for different wastewater treatment plant as the data is highly dependent on the location and technology of the wastewater treatment plant. It was not clear how Rolsky et. al. processed the statistics of this literature data which yield the final values reported in the Rolsky et.al (2021)’s paper. The assumption seems invalid since some of the equations term involved show unclear source and undefined. These degraded PVA values in various treatment sections of the waster water treatment plant are likely dependent on the technology and years of operation of the waste water treatment plant. Due to this, it is also impossible to estimate whether the actual percent would be higher or lower. Hence, I cannot conclude whether the actual PVA degraded values should be higher or lower unless more information about the location or technology of the wastewater treatment plant is provided.

• I cannot answer

  Expert 2

  I don’t understand the question. Given the long latency period before degradation commences, PVA clearly fails to meet the USEPA or OECD half-life guidelines for being regarded as ‘readily biodegradable’. How this links to the proportion surviving treatment in WWTPs is not clear – the temporal scales involved are so different that they are not directly comparable.

• No

  Expert 4

  If we accept that the outcome of the OECD 301B Ready Biodegradability Test provided in the peer-reviewed publication by Menzies et al. (2022) qualifies PVA as readily biodegradable according to USEPA and OECD guidelines, then the recommended half-lives of 0.69-1 hour is applicable to PVA in wastewater treatment facilities. In this context, the crucial information needed to interpret the modeling results is the Hydraulic Retention Time of 18-24 hours which leads is estimated to lead to 20% PVA degradation during Activated Sludge process; and estimated 30 minutes HRT leading to 1.5% degradation during disinfection process. In my opinion, according to these data and assumptions, the actual percentage of PVA that will be degraded during the Activated Sludge Process will be "higher". The reason that I do not say "much higher" is because there are so many other variables including other polymers beside PVA in real Activated Slide, and there may be competition for the bacterial enzymes responsible. However, these were taken into consideration, to some extent, by the US EPA-recommended half-lives. The estimated degradation of 1.5% degradation of PVA during disinfection process is more difficult to access because the chemical process is difference from Activated Sludge, and since this is a sequential concentration-dependent process, the rate of degradation is expected to be very small, and perhaps negligible given the very brief retention time.