Production of lactic acid from source-sorted organic household waste

The valorization of organic waste into valuable chemicals is an attractive approach to reducing fossil fuel dependence and greenhouse gases emissions, meeting sustainable society development targets and offsetting the carbon footprint.
Lactic acid (LA), a versatile platform molecule, has been widely applied in the chemical, cosmetics and food industries. Its promising application is used as a precursor for the synthesis of bioplastic. Moreover, source-sorted organic household waste (SSOHW), discarded by the domestic sector, is a promising candidate for fermentative LA production, due to the high content of sugar and nutrients (e.g. nitrogen and phosphoric). As for LA production, pure culture fermentation is mainly used; thus, the pre-treatment of feedstock is usually required to decompose complex organic matter, due to the limited ability of lactic acid bacteria (LAB) to metabolise oligosaccharides. Additionally, feedstock and equipment need to be sterilised, in order to avoid contamination, which is an intricate and a costly process. In order to ensure a sustainable fermentation, higher LA yield and concentration are essential factors in lowering overall costs. Therefore, the aim of this PhD thesis is to 1) develop a bio-augmentation approach to improve LA production from SSOHW, 2) explore LA production at acidic conditions (i.e. pH 4.0), using acid-tolerant LAB to reduce neutralising agent utilisation and fermentation costs, 3) investigate the effect of storage duration on LA production and 4) up-concentrate LA from fermentation broth, using forward osmotic (FO) technology to improve the techno-economic feasibility of downstream process.

First, the effect of bio-augmentation on LA production from SSOHW was investigated by comparing pure and indigenous culture fermentation. The results showed that bio-augmentation with Pediococcus acidilactici boosted LA concentration by 22.3% and 109% compared to indigenous and pure culture fermentation, respectively. Moreover, fermentation parameters (i.e. initial pH, temperature and total solid loading) were optimized by response surface methodology. The highest LA concentration and yield of 39.3 ± 0.5 g/L and 0.75 ± 0.02 g/g_-sugar, respectively, were achieved at initial pH of 8, temperature of 32.4 °C and total solid loading of 105 g/L. In addition, consecutive batch fer mentation was conducted to examine the adaptation of bio-augmented P. acidilactici to SSOHW. The addition of P. acidilactici led to an increment above 18% compared to abiotic augmentation for three consecutive generations, and at the same time it decreased the formation of by-products.

Subsequently, adaptation and isolation were used to develop an acid-tolerant culture and to improve LA production from SSOHW in acidic conditions. To this end, different adaptation strategies (i.e. maintaining pH at 4.0 and stepwise decreasing the pH from 5.0 to 4.0) were investigated in consecutive batch fermentations. Both adaptation strategies were capable of significantly increasing LA production at pH 4.0. However, stepwise pH decrease was a more efficient strategy in terms of increased LA concentration and decreased by-product formation. Moreover, bio-augmentation with autochthonous acid-tolerant LAB was also evaluated to improve LA production. Four acid-tolerant LAB were isolated from SSOHW. According to their growth performance and LA production, the isolate belonging to Lactobaccilus reuteri was selected as the bioaugmentation strain and boosted LA concentration by 29.0% from SSOHW atpH 4.0, compared to abiotic augmentation.

Furthermore, natural fermentation usually occurs during the collection and storage of SSOHW, which can significantly influence ensuing LA production. Thus, the effect of storage duration on characteristic changes and fermentative LA production was investigated. The results indicated that 68% of total sugar was consumed over 15 days’ storage, and the largest consumption was at a storage duration of 7-15 days. Despite LAB dominating indigenous microbiota, only 12.3% of consumed sugar was converted to LA on the 15th day of storage. With regards to the following fermentation performance, a short length of storage had a positive effect, due to the increase in biodegradability, while prolonged storage (i.e. 7-15 days) significantly decreased LA production potential due to a high amount of sugar “losses” in storage period. Hence, storage time should be limited to approximately one week.
Lastly, up-concentration of LA in fermentation broth was conducted using forward osmotic technology to reduce energy consumption and improve economic sustainability of the following the LA separation step (e.g. distillation). The
result showed that providing the fermentation broth with a lower pH was beneficial for LA up-concentration. Likewise, different draw solutions (i.e. NaCl, MgCl₂ and deep eutectic solvent) were tested to improve up-concentration efficiency.
The easy recovery of deep eutectic solvent with higher LA up-concentration performance made it the ideal draw solution.

Overall, this Ph.D. study provided a feasible approach to improving LA production from SSOHW, and by adapting/isolating an acid-tolerant culture, LA production in acidic conditions was achieved, thereby reducing fermentation costs. Moreover, the up-concentration of LA significantly reduced energy consumption and improved the economic sustainability of downstream separation process.