The operation of wastewater treatment plants results in direct emissions, from the biological processes, of greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as indirect emissions resulting from energy generation. In this study, three possible ways to reduce these emissions are discussed and analyzed:(1)minimization through the change of operational conditions,(2)treatment of the gaseous streams, and(3)prevention by applying new configurations and processes to remove both organic matter and pollutants. In current WWTPs, to modify the operational conditions of existing units reveals itself as possibly the most economical way to decrease N2O and CO2emissions without deterioration of effluent quality. Nowadays the treatment of the gaseous streams containing the GHG seems to be a not suitable option due to the high capital costs of systems involved to capture and clean them. The change of WWTP configuration by using microalgae or partial nitritation-Anammox processes to remove ammonia from wastewater, instead of conventional nitrification-denitrification processes, can significantly reduce the GHG emissions and the energy consumed. However, the area required in the case of microalgae systems and the current lack of information about stability of partial nitritation-Anammox processes operating in the main stream of the WWTP are factors to be considered.

Anaerobic digestion (AD) is commonly used for the stabilization of agro-food wastes and recovery of energy as methane. Since AD removes organic C but not nutrients (N and P), additional processes to remove them are usually applied to meet the stringent effluent criteria. However, in the past years, there was a shift from the removal to the recovery of nutrients as a result of increasing concerns regarding limited natural resources and the importance given to the sustainable treatment technologies. Recovering N and P from anaerobically pretreated agro-food wastes as easily transportable and marketable products has gained increasing importance to meet both regulatory requirements and increase revenue. For this reason, this review paper gives a critical comparison of the available and emerging technologies for N and P recovery from AD residues.

Sludge is a by-product of municipal wastewater treatment plants (WWTPs) and its management contributes significantly to the operating costs. Large WWTPs usually have anaerobic sludge digesters to valorize sludge as methane and to reduce its mass. However, the low methane market price opens the possibility for generating other high value-added products from the organic matter in sludge, such as polyhydroxyalkanoates (PHAs). In this work, the economic feasibility of retrofitting two types of WWTPs to convert them into biofactories of crude PHAs was studied. Two cases were analyzed: (a) a large WWTP with anaerobic sludge digestion; and (b) a small WWTP where sludge is only dewatered. In a two-stage PHA-production system (biomass enrichment plus PHAs accumulation), the minimum PHAs cost would be 1.26 and 2.26 US$/kg PHA-crude for the large and small WWTPs, respectively. In a single-stage process, where a fraction of the secondary sludge (25%) is directly used to accumulate PHAs, the production costs would decrease by around 15.9% (small WWTPs) and 19.0% (large WWTPs), since capital costs associated with bioreactors decrease. Sensitivity analysis showed that the PHA/COD (Chemical Oxygen Demand) yield is the most crucial parameter affecting the production costs. The energy, methane, and sludge management prices also have an essential effect on the production costs, and their effect depends on the WWTP’s size.

The application of autotrophic nitrogen removal processes in the main line of wastewater treatment plants will contribute to achieve its self-energy-sufficiency. However, the effective suppression of nitrite oxidizing bacteria (NOB) activity at the conditions of low temperature and low ammonium concentration (mainstream conditions) was identified as one of the main bottlenecks. In this study, stable partial nitritation at 16 ºC and 50 mg NH4 + -N/L was achieved maintaining inside the reactor free nitrous acid (FNA) concentrations inhibitory for NOB (> 0.02 mg HNO2-N/L), without dissolved oxygen concentration control. The FNA inhibitory concentration was generated by the partial nitritation process, and its stimulation was studied with two different inhibitors: sodium azide and nitrite. The microbiological analysis revealed that, throughout the operational period with inhibitory FNA levels, the NOB populations (dominated by Nitrospira) were effectively washed out from the reactor. This is an advantage that allowed maintaining a good stability of the process, even when the FNA concentration was not enough to inhibit the NOB, taking about 40 days to develop significant activity. The observed delay on the NOB development is expected to enable the establishment of corrective actions to avoid the partial nitritation destabilization. The use of the FNA to achieve a stable partial nitritation process is recommended to profit from the natural pH decrease associated to the nitritation process and from its favoured accumulation at low temperatures as those from the mainstream. In this research study an analysis about the influence of ammonium and alkalinity concentrations was also performed to know in which scenarios the FNA inhibitory concentration can be achieved.

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