The INGREEN project is one of the important efforts that are being made to reduce the pollutant levels and water consumption of the paper mill industry. Treatment of paper mill wastewater typically includes removal of suspended solids and diminishing the organic matter. Aerated ponds and activated sludge methods are the most common treatment systems in the paper industry. In an aerated pond, wastewater is treated through a combination of physical, biological and chemical processes. They have large residence times between 3 and 20 days, and consequently a large volume. The largest secondary treatment system is activated sludge. Compared to the aerobic ponds the aerated activated sludge systems have the advantage that the process is more efficient and requires less surface.
The composition of paper mill wastewater (PMW) is characterized by a high chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total suspended solids (TSS), which cannot be discharged into the environment. Fortunately, the volatile fatty acids (VFAs) contained in these effluents could be easily utilized for value-added products such as polyhydroxyalkanoates (PHA).
In the EU-funded INGREEN project one of the targets is to produce PHAs from paper mill wastewater by fermentation. This approach offers a great opportunity to improve the environmental impact of a paper mill plant in combination with production of useful biopolymers. The PHA-enriched biomass developed in the INGREEN project will be used to produce new bio-based prototypes of feed with increased levels of dietary fiber and biodegradable packaging applications. With respect to circularity this is a great approach as the material from the side stream stays in the same supply chain (packaging) or is even upgraded (feed). Furthermore, PHA production from such wastewaters is accompanied by considerable reductions in organics and nitrogenous compounds. In the INGREEN project a mixed microbial culture is used benefitting from bacteria that are already present in small amounts in the starting material.
The production of PHA-enriched biomass is achieved by two main phases: selection and accumulation. The first one allows to select the PHA-producing bacteria from the mixed microbial culture, by performing daily cycles of dynamic feeding, using the paper-mill wastewater as carbon source. During the accumulation phase, the selected biomass is then fed with fresh paper-mill wastewater, performing multiple feeding cycles in order to maximize the amount of PHA stored within the bacterial cells.