nutrient removal from sewage
5.6 nItrouS oxIdE EMISSIonS In Bnr
During the operation of a WWTP several off gas emissions can occur, including nitrous oxide (N2O), carbon dioxide (CO2), methane (CH4) and nitric oxide (NO).
N2O is of particular environmental concern, since it has a global warming potential that is 298 times higher than that of CO2. In terms of CO2 equivalents (eq) nitrous oxide contributes by 7.9% to the total anthropogenic greenhouse gas (GHG) emissions. The global production of N2O emissions from WWTPs corresponds to approximately 3.2% of the total estimated anthropogenic N2O emissions (IPCC, 2001; Kampschreur et al. 2009). The N2O emissions from wastewater management contribute by 26% to the total greenhouse gas (GHG) emissions of the water chain. The guidelines of the Intergovernmental Panel on Climate Change (IPCC, 2006) have decreased the standard N2O emission factor from 1%
to 0.5% of the influent nitrogen load of the WWTP influent (Kampschreur et al.
2009). It is estimated that N2O emissions from wastewater correspond to 100 Mt CO2 eq., while CH4 emissions to 630 Mt CO2 eq. for 2010 (Monni et al. 2006).
Several practical design and operating decisions in WWTPs (including the BNR processes) have considerable impact on the overall environmental performance, including the GHG emissions (Keller & Hartley, 2003). At the level of a BNR treatment plant, the N2O emissions can reach up to 83% of the operational CO2
footprint (Desloover et al. 2011).
N2O emissions during BNR occur during the biological processes of nitritation and denitritation (Figure 5.7). During the nitritation process, N2O can be formed via two routes: the first pathway is as a by-product of the incomplete oxidation of hydroxylamine (NH2OH) to nitrite. Hydroxylamine is formed through the oxidation of ammonium by ammonium oxidizing bacteria (AOB) and then NH2OH is oxidized to nitrite a biochemical reaction which produces nitrous oxide. The second pathway of N2O formation is attributed to the process known as nitrifier denitrification. In this biochemical process nitrite is used as electron acceptor instead of oxygen; this can occur during nitritation under limiting dissolved oxygen (DO) conditions. In this pathway, the reduction of NO2− to NO and to N2O by AOB takes place. The third pathway occurs during the anoxic conditions. In this process, NO and N2O are produced as process intermediates of nitrate reduction to N2. This is the only stage in which N2O is also consumed as it is reduced to N2
(Ni et al. 2011).
Figure 5.7 Simplified representation of biochemical processes responsible for nitrous oxide production during nitrification and denitrification: Route 1: aerobic ammonium oxidation by AOB, Route 2: nitrifier denitrification by AOB and Route 3:
heterotrophic denitrification.
It is important to determine whether the implementation of the advanced bioprocesses described in the previous section, could increase the potential carbon footprint of the WWTP. Desloover et al. (2012) performed an overview of the quantified N2O emissions from full scale BNR plants that apply the conventional nitrification/denitrification and advanced nitritation/anoxic ammonium oxidation (anammox) and nitritation/denitritation processes; they concluded that nitritation is the bioprocess that mainly contributes to N2O emissions. Full-scale measurements also point to nitrite as a factor in N2O production (Ahn et al. 2010). Taking into account the much higher greenhouse gas impact of N2O compared to CO2, it is necessary to determine whether nitrogen removal bioprocesses based on transient nitrite accumulation are systematically greater contributors of N2O than full nitrification processes (IPCC, 2001; Ahn et al. 2011). N2O emission rates can vary considerably due to the differences in the wastewater composition, the applied treatment process, the operating parameters and the environmental conditions. The most important parameters that affect the N2O emissions include the DO concentration, the nitrite in the mixed liquor and the COD/N ratio during denitrification. Nitritation/denitritation is increasingly being applied, particularly for the treatment of nitrogenous effluents, due to the lower energy and organic carbon source requirements compared to conventional nitrification/denitrification.
However, such processes can result in significant N2O emissions (Kampschreur et al. 2009). Furthermore, the use of SBR technology, particularly when combined with the treatment of highly nitrogenous effluents such as sludge reject water can enhance the nitrous oxide emissions. The implementation of strategies to mitigate N2O emissions in the BNR processes via nitrite can increase their sustainability.
5.7 concluSIon
Several configurations are currently applied for BNR in full scale WWTPs. Both attached and suspended growth processes are implemented. The adoption of the
Integration of energy efficient processes 91 most appropriate BNR scheme is closely related to the treated effluent requirements for phosphorus and nitrogen. In cases where treated effluents are discharged to sensitive water bodies, the nutrient limits can be very low and the technology selection is critical. Nitritation/denitritation and the completely autotrophic nitrogen removal have emerged as alternative BNR processes compared to the conventional nitrification/denitrification. Such processes have the advantages of lower aeration requirements, resulting in lower operating expenses. They are increasingly being adopted for the treatment of the nitrogenous sludge reject water. In BNR processes it is also important to consider the GHG emissions and particularly nitrous oxide emissions. The aforementioned innovative processes should be implemented with care in order not to increase the overall carbon footprint of the WWTP.
5.8 AcknoWlEdgEMEnt
This work was supported by the Marie Curie FP7-PEOPLE-2012 project with title Low environmental footprint biological treatment processes for waste and wastewater treatment, LEF-BIOWASTE (Number 322333).
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