Evaluating toxicity in the anaerobic treatment of paper pulping effluents and the biological performance of EGSB reactor

Abstract

Pulp and paper industries are water-intensive industries and composed of complex production processes. Untreated pulping wastewater is very toxic and lethal to aquatic life if discharged untreated. Anaerobic treatment technology has gained interest in treating these types of wastewater by reducing organic compounds. This thesis research aimed to evaluate the potential toxicity which might present in chemithermomechanical pulp (CTMP) wastewater, through the biological performance of a lab-scale expanded granular sludge bed (EGSB) reactor.
In the theoretical part CTMP process, characteristics of wastewater, toxicants, the functionality of an EGSB reactor, and any limiting factor that influences the treatment were investigated. In the experimental part, CTMP wastewaters from a mill in Sweden were analyzed using a lab-scale EGSB reactor for 182 days. Anaerobic biodegradability and toxicity test were done to measure the extent of anaerobic digestion in this wastewater. Different parameters such as chemical oxygen demand (COD) removal efficiency, volatile fatty acids (VFA), alkalinity, pH, and nutrient uptake were measured to access the biological performance of the EGSB reactor starting from unacclimated anaerobic granular biomass. Data analyzing tools including Excel and PHREEQC modeling were used in this research study. Excel tool was used to plot the graph and curve fitting whereas the PHREEQC model was done to understand the corrosivity of the biogas and calcite precipitation in the effluent discharge pipe.
The results showed that about 60% of the organic compounds in the wastewater were biodegradable, and no significant toxicity was found. Also, the performance of the EGSB was good with COD removal of roughly 50% at the stable phase. The presence of wood cellulose fiber in the wastewater had a negative impact on the performance of the reactor more specifically blockage in the recirculation and reduce methane production. However, based on the experimental results, EGSB alone would not be enough to remove most of the organic pollutants, which require additional post treatment such as aerobic system and membrane filtration to meet the discharge limit.