Biogas upgrading, CO2 valorisation and economic revaluation of bioelectrochemical systems through anodic chlorine production in the framework of wastewater treatment plants.

Biogas production in wastewater treatment plants (WWTPs) plays a decisive role in the reduction of CO2 emissions and energy needs in the context of the water-energy nexus. The biogas obtained from sewage sludge digestion can be converted into biomethane by the use of biogas upgrading technologies. In this regard, an innovative water scrubbing based technology, known as ABAD Bioenergy® is presented and considered in this work. The effluents resulting from this system consist of biomethane and treated wastewater with a high CO2 concentration. Therefore, the study explores the feasibility of using this CO2-containing effluent in the cathode of a bioelectrochemical system (BES) for the transformation of CO2 into methane. Techno-economic assessment of the process is presented, including the valorisation of anode reactions through the production of chlorine compounds. Finally, the potential impacts of applying this technology in a WWTP operated by FCC Aqualia are (i) increasing biomethane production by 17.4%, (ii) decreasing CO2 content by 42.8% and (iii) producing over 60 ppm of chlorine compounds to disinfect all the treated wastewater of the plant.

Nitrite and free nitrous acid sludge pre-treatments to enhance methane production in continuous anaerobic digestion: Comparing process performance and associated costs.

Secondary sludge pre-treatment with free nitrous acid (FNA) has been proven to enhance methane production during anaerobic digestion. However, it is still unclear if the same enhancement can be achieved only using nitrite, without sludge acidification. In this paper, secondary sludge was pre-treated during 5 h with nitrite within the range of 50-250 mg NO2--N/L at neutral pH (6.7). Results obtained from biochemical methane potential tests (BMPs) indicated that sludge pre-treatment at 150 mg NO2--N/L presented the best enhancement of methane production (24% as compared to the control). These conditions were used to pre-treat sludge added in a continuous lab-scale anaerobic digester that operated in parallel to another digester receiving sludge pre-treated with FNA (250 mg NO2--N/L at pH 5.5). Results showed a very similar performance in terms of methane enhancement in both reactors, indicating that sludge acidification is not needed to improve methane yield. A preliminary economic assessment also highlights the need for assessing real chemical costs and national power prices before the implementation of these pre-treatment steps as the associated benefits can significantly change depending on the country where the wastewater treatment plant is located.

Optimization of free nitrous acid pre-treatment on waste activated sludge.

The effectiveness of the Free Nitrous Acid (FNA) sludge treatment was tested in the range from 0 to 3.0 mg N-HNO2/L with acidified and neutral pH. 4 h pre-treatment times were used and the specific methane production (SMP) investigated. Results show that between 50 and 100 mg/L of N-NO2-/L disappeared during the FNA pre-treatment, reducing its effectiveness. A minimum level of nitrite (174 mg N-NO2-/L tested in this study), independently of pH/FNA, was necessary to assure the presence of the chemical throughout the duration of the pre-treatment. Sludge viability was compromised while WAS solubilization and SMP were enhanced with nitrite concentrations of 174 mg N-NO2-/L or higher, even at low FNA levels (<0.15 mg N-HNO2/L). Results show that acidified pH is not needed to enhance methane production, making the pretreatment more economically and environmentally attractive.