Pulp and paper mills use approximately 30 m3water per ton of produced paper. The process effluent has to be treated before being discharged. Mostly, the effluent is treated with aerobic biological processes using electricity for aeration, and added nutrients. The bio-sludge has low energy value and has to be disposed of. Here, we propose that effluent instead can be used as a feedstock for valuable products.
Some of the bacteria in the bio-sludge can accumulatepolyhydroxyalkanoates(PHAs). PHA is a biopolymer that has a commercial value and is an important building block for the bio-plastics industry. For an efficient PHA production, volatile fatty acids (VFAs) are needed as the feedstock substrate. Process streams rich in sugars can be fermented by the thermophilic bacteria Caldicellulosiruptor, providing acetic acid and hydrogen gas. The acetic acid can then be used as substrate for PHA accumulation.
In a case study conducted at a large integrated pulp and paper mill (>700 000 ADt/y) in Sweden, the theoretical production volume of hydrogen gas and PHA were estimated. The calculations were based on measured process effluent volumes and water quality parameters, biological process yields from laboratory and pilot scale testing, and practical experience for the unit processes. The results indicated that 0.3 kg of hydrogen gas and 1.6 kg of PHA can be produced per ADt paper or board. The additional associated benefits are significant for the mill because the demand for nutrients and energy for aeration may, at the same time, decrease by 15 to 50 percent.
By combining common biological treatment with fermentation and PHA accumulation, waste by-products can be transformed and upgraded to value-added hydrogen gas and PHA. Such a secondary side-process integration helps to shift toward the circular bioeconomy. At the same time, less energy and nutrients are needed for the wastewater treatment.