Wastewater treatment from pulp and paper industry coupled with nitrogen gas fixation using purple non-sulfur bacteria

Abstract

This study investigated the wastewater treatment efficiency from pulp and paper industry by coupling with nitrogen gas fixation using purple non-sulfur bacteria (PNSB). Wastewater from the pulp and paper industry (PPW) contains limited nitrogen, making the development of an effective treatment process necessary. PNSB have been gaining attention to treating wastewater as they exhibit a high growth rate and ability to simultaneously remove carbon and nutrients. However, nitrogen deficiency in PPW limits PNSB growth. In this study, PPW was treated with PNSB under varying conditions, including different HRTs and nitrogen gas supplementation. Four main stages (A, B, C, and D) were established to evaluate the effects of HRT and nitrogen supply on PNSB growth and treatment efficiency. Batch culture was performed in stage A, and PNSB biomass reached 0.46 g L-1. During batch culture, TOC, COD, and TN removal efficiencies were 22.2%, 15.2%, and 20.4%, respectively. In stage B, the HRT was set to 4 days, and PNSB biomass did not increase, resulting in low removal efficiencies. Although HRT changed to HRT 8 days in stage C, the biomass concentration remained low due to limited biomass accumulation in stage B and short operation time (4 days), preventing significant increases in both biomass and removal efficiencies. Finally, nitrogen gas was supplied continuously to confirm the effectiveness of nitrogen gas on growth (stage D). After supplying nitrogen gas, biomass and removal efficiency increased. PNSB biomass reached 0.72 g L-1, and TOC, COD, and TN removal efficiency were 68.6%, 69.5%, and 33.5%, respectively. Throughout the experiment, dissolved total nitrogen was almost removed from the PPW, but TOC and COD removal efficiencies were low, which indicates carbon to nitrogen ratio was not optimal for PNSB, and carbon was left in PPW. Moreover, protein content in PNSB was 68.9%, 27.4%, 29.2%, and 45.8% in stage A, B, C, and D, respectively. This value was lower than generally reported (> 50% w/w), indicating that the nitrogen content in PPW was insufficient to support PNSB growth. This study suggests that, when treating PPW with PNSB, HRT should be optimized, and the C/N ratio also should be adjusted to a range that supports optimal growth.