Engineering feasibility, economic viability and environmental sustainability of energy recovery from nitrous oxide in biological wastewater treatment plant

Approaches to energy and resource recovery from municipal wastewater

“…As such, extensive efforts have been devoted to developing novel processes for enhanced nitrous oxide production from wastewater (Lin et al, 2018;Myung et al, 2015). As illustrated in Figure 2.8, nitrous oxide can be generated through biological or coupled biological-chemical pathways in wastewater treatment (Zhang et al, 2019). In biological nitrogen removal processes, nitrous oxide can be generated via three pathways, i.e.…”

“…Using the CANDO process to treat AD liquor with an ammonium concentration of 1,000 mg N/L, the generated nitrous oxide was estimated at 1,194.3 g N 2 O/m 3 (Zhang et al, 2019). Given a unit chemical energy of nitrous oxide of 1.9 kJ/g, the net recoverable energy from nitrous oxide could stand at 1.9 kJ/g N 2 O × 1,194.3 g N 2 O/m 3 = 2,269.2 kJ/m 3 , equivalent to 0.22 kWh generated from one cubic meter of AD liquor through CHP at an electrical conversion efficiency of 35%.…”

“…It should also be noted that anaerobic digester liquor only accounts for about 1% of the mainstream inflow in a WWTP (Fux & Siegrist, 2004), indicating that only about 0.22 kWh/m 3 anaerobic digester liquor × 1% = 0.0022 kWh/m 3 of wastewater treated could be generated, which accounts for about 0.5% of total energy consumption in WWTPs. Moreover, harvesting and further purification of the produced nitrous oxide are necessary prior to subsequent combustion for energy recovery, while the potent environmental risk related to the residual dissolved nitrous oxide should also be carefully assessed (Zhang et al, 2019). Consequently, it appears reasonable to consider that the production and recovery technologies for nitrous oxide as a fuel are still at the early developmental stage, while further efforts will be needed to explore feasible engineering options for high-efficiency and cost-effective recovery of nitrous oxide with the aim to improving the overall energy efficiency of WWTPs.…”

“…Wastewater treatment coupled with CANDO process. The dotted line box indicates the CANDO process: (1) partial nitrification of ammonia to nitrite; (2) nitrous denitritation of nitrite to nitrous oxide; and (3): co-combustion of nitrous oxide with methane Zhang et al (2019),. with permission.…”

Perspectives and actions moving forward

“…Currently, phosphorus is mainly recovered from the sludge digestion liquor in the sidestream in the form of struvite, hydroxyapatite and vivianite (Campos et al, 2018). Similarly, a process known as coupled aerobic-anoxic nitrous decomposition operation (CANDO) has recently been reported in the production of nitrous oxide from sludge digestion liquor (Vasilaki et al, 2019); however, technical feasibility, economic viability and environmental sustainability have not yet fully demonstrated (Zhang et al, 2019). So far, ammonium can be recovered from anaerobic digestion liquor as a fertilizer in the form of ammonium sulphate, via steam stripping, distillation and reversible chemsorption (Zarebska et al, 2015), but energy consumption appears to be significantly high.…”