Treatment strategies for coupled biostimulants and polyhydroxyalkanoates production from agri-food waste

The increase in the global human population means a greater demand for food and plastic goods. Alternative solutions are therefore needed to improve crop production and reduce plastic pollution. This work focused on the conversion of agri-food waste into volatile fatty acids (VFAs, yield 0.3 kgCODVFA/kgCOD) and into bacterial mass rich in polyhydroxyalkanoates (PHAs, yield 0.1 kgPHA/kgCOD), biobased and biodegradable polymers that can be used in place of fossil-based plastic. The PHA-rich biomass was subject to improved hydrolysis, including alkali, acid and enzymatic treatments, for the concomitant extraction of PHAs for the plastic industry and the production of biostimulants for agriculture. While NaOH was most effective in bacterial cell disruption (hydrolysis up to 45.4 %) and nitrogen extraction (63.0 %), the greener and gentler enzymatic treatment still allowed reasonable hydrolysis (33.8 %) and nitrogen extraction (58 %), while preserving the molecular weight of the recovered biopolymer. The seed priming test allowed the selection of the four most efficient protein hydrolysates (NaOH 0.5 M, HCl O.1 M, alcalase and papain without pretreatment) to apply to tomato seedlings via foliar spray and drench, to test for any possible biostimulant effect. The foliar spray treatment resulted in both root and shoot growth promotion (up to 29 %), with no statistical difference between treatments. The drench application was less effective, with only roots showing improved growth (alcalase-treated plants up to 23 %), while shoots were not statistically better than the controls. The results suggest the potential use of enzymatic approaches for the retrieval of PHAs and biostimulants from bacterial biomass.