Abstract

Atmospheric concentration of carbon dioxide (CO2), a major component of greenhouse gases, has increased significantly since the beginning of the industrial revolution. Many attempts have been aimed at capturing, sequestering, and reducing carbon dioxide emissions, but they have not been very efficient and economical.
The objective of this work is to develop a sustainable system to reduce the carbon dioxide emissions by applying anaerobic treatment of wastewater. In this method, industrial emissions containing CO2 are injected into a wastewater stream entering the anaerobic reactor where CO2 is biologically converted to methane as a biogas. This conversion is based on the final step of anaerobic degradation in which methanogenic bacteria produce methane from acetic acid or CO2 and hydrogen. Consequently, with the addition of carbon dioxide after wastewater pollutant degradation (that provides acetic acid and hydrogen), methane with a high efficiency can be produced through a highly sustainable process.
To investigate the feasibility of this process for CO2 removal, two series of batch tests using the Chemi-Thermomechanical Pulping (CTMP) and recycled pulp and paper wastewater were performed. In this project pulp and paper wastewater was selected since this industry produces a large amount of wastewater and is responsible for a large portion of CO2 emissions. In order to determine the optimal conditions, the effect of different parameters such as pH (5.5 - 7.5), and temperature (30 - 35ºC) on the efficiency of CO2 and COD removal and methane production was investigated.
As a conclusion of the first part of this work, it was shown that anaerobic treatment can be used to remove carbon dioxide by bioconversion to methane. By applying this method, CO2 concentration will be reduced and methane will be produced simultaneously. The results show that at all pH values examined in the present work, CO2 removal by bioconversion into methane is higher in samples with CO2 injection compared to the control samples with no CO2 injection. Results also showed that CO2 removal was higher at lower pH values. For example, for CTMP wastewater at pH 7.5 and 35°C the CO2 removal by bioconversion into methane in samples with CO2 injection was 13 mg/l (19%) higher than the control sample, while this value at pH 5.5 was 515 mg/l (29%) higher than the control sample. At 35°C and at all pH values, the increase in CO2 removal by bioconversion to methane in samples with CO2 injection compared to the control samples with no CO2 injection was 3-6% higher than that obtained at 30°C. The best efficiency for CO2 removal occurred at pH 5.5 and 35ºC.
Operating pH and temperature and injection of CO2 didn’t show significant impact on COD removal, although higher COD reduction rates were achieved at higher temperature. It was shown that COD reduction rates were almost similar at different pH values. Temperature has a significant impact on methane generation at all operating pH. Injection of carbon dioxide had a positive impact on methane production and in samples with CO2 injection more methane generation was observed.
Although at higher pH values methane generation is higher than that at lower pH values, the increase in methane generation by the injection of CO2 to the wastewater is lower. The reason for this observation is that at a higher pH of 7 and 7.5, only a small amount of CO2 was dissolved in the wastewater and was later converted to methane. Therefore, at higher pH values, the difference in methane generation in the presence and absence of CO2 injection was less than that observed at lower pH values.
In CTMP wastewater at 35ºC, the injection of CO2 into the wastewater increased methane generation by 162 ml (108%) at pH 5.5 and by 22 ml (3%) at pH 7.5. For the recycled paper experiment at 35ºC, the injection of CO2 into the wastewater increased methane generation by 54 ml (93%) at pH 5.5 and by 8 ml (4%) at pH 7.5.
The continuous experiments were performed following the batch tests in the UASB reactors at three organic loading rates (OLR of 1, 2, and 3g COD/l.d.) using CTMP wastewater for 115 days. Results showed that regardless of CO2 injection and initial pH of wastewater, COD removal was almost equal from the reactor with CO2 injection in the feed (R2) and control reactors (R1). The COD removal, equal to 70% was achieved at OLR=1 g COD/l.d, and its value gradually decreased to 65% at OLR=3g COD/l.d.
Methane generation in R2 with CO2 injection was higher than the control reactor with the same pH (pH 5.5) in its influent wastewater. However these values were less than the methane generation in the control reactor without pH adjustment (with influent wastewater of pH 6.5) that was more suitable for methanogenic activity. Methane generation in R2 at OLR 1, 2 and 3 was approximately equal to 400-570, 960-1120, and 700-1700 ml/d, respectively, while these values for R1 reactor with pH adjustment were approximately equal to 200-300, 460-700, and 370-920 ml/d. The higher methane generation in R2 compared to R1 with pH adjustment is attributed to the bioconversion of CO2 to methane. Results showed that approximately 83-97% of the injected dissolved CO2 in R2 was removed by the proposed pathways.
The potential GHG reduction and economic feasibility of the developed process was evaluated by applying detailed calculations. GoldSETTM software was applied to compare the proposed developed process with conventional hybrid and aerobic treatment processes based on the sustainability aspects. The results of GoldSET software confirmed the higher sustainability of the developed hybrid treatment process compared to the conventional hybrid and aerobic treatment processes. Application of developed hybrid treatment process instead of conventional hybrid treatment process can annually save up to 3 million dollars in annual costs of treatment plants and will reduce GHG emissions by 100,000 tCO2e/y.
A numerical method based on the Runge-Kutta fourth-order method was developed to investigate the controlling kinetic parameters for anaerobic digestion of carbon dioxide. Results showed that the values of kinetic parameters estimated from the two experimental setups were very close. The obtained values for Ks and m for Cascades wastewater were 0.4g/l and 0.02/d, while these values for CTMP wastewater were 0.6 g/l and 0.025/d respectively. Results of both experiments showed that simulated values were in compliance with the experimental data and similar pattern during different experimental conditions were observed.