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BACKGROUND The influence of pH, sludge retention time (SRT) and organic loading rate (OLR) were evaluated in the fermentation of cheese whey to produce volatile fatty acids (VFA) in a sequencing batch reactor (SBR). The reactor performance was studied at SRT of 4, 6 and 10 days and the influence of pH (5, 5.5 and 6) was determined at each SRT. The OLR effect was also studied in the range 3 to 12 g COD L‐1 d‐1, while keeping the other parameters mentioned above constant. RESULTS A more acidic pH , improved the degree of acidification at short SRT (4 and 6 days), reaching 76 and 84%, respectively, meanwhile at pH 6 the degree of acidification (DA) improved and reached 85% at longer SRT (10 d). Regarding the VFA profile, a pH shift from 5 to 6 stimulated the production of acetic and propionic acids, whereas the production of butyric and valeric acids decreased irrespective of the SRT applied. Moreover, the increase of SRT from 4 to 10 d boosted propionic and valeric acids production regardless of the pH applied. Lower OLR favoured the production of propionic and valeric acids, while higher OLR increased the production of butyric acid. Lactose degradation and VFA production followed the Michaelis–Menten model. CONCLUSION A selective VFA distribution can be reached by modifying parameters such as pH, SRT and OLR in acidogenic reactors, therefore, they could be suitable for further valorisation purposes in the production of polyhydroxyalkanoates from such VFA. © 2017 Society of Chemical Industry

Volatile fatty acids (VFA) production and degree of acidification (DA) were investigated in the anaerobic treatment of cheese whey by comparison of two processes: a continuous process using a laboratory upflow anaerobic sludge blanket (UASB) reactor and a discontinuous process using a sequencing batch reactor (SBR). The main purpose of this work was to study the organic loading rate (OLR) effect on the yield of VFA in two kinds of reactors. The predominant products in the acidogenic process in both reactors were: acetate, propionate, butyrate and valerate. The maximum DA obtained was 98% in an SBR at OLR of 2.7 g COD L d, and 97% in the UASB at OLR at 15.1 g COD L d. The results revealed that the UASB reactor was more efficient at a medium OLR with a higher VFA yield, while with the SBR reactor, the maximum acidification was obtained at a lower OLR with changes in the VFA profile at different OLRs applied.

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At the request of the authors, the article titled "Effect of the solid retention time in the obtention of polyhydroxyalkanoates" [version 1; peer review: awaiting peer review]. F1000Research 2021, 10:864 (https://doi.org/10.12688/f1000research.28852.1) has been retracted from F1000Research. Since publication of the article, the corresponding author has notified the Editorial team that this work was originally performed at University of A Coruña with the involvement of María C. Veiga and Christian Kennes, who were not notified regarding the submission of this manuscript or listed as authors. After discussion with all parties, including contact with the corresponding author’s institution, it was agreed that the article would be retracted in order to correct the academic record. As the article contains content which potentially should not have been made publicly available the content of the article has been removed. The authors apologise for this honest error.