Towards energy neutral WWTPs: piloting mainstream partial nitritation AGS and anammox

ENG- Wastewater treatment plants (WWTPs) play a crucial role in safeguarding public health by treating wastewater before discharge into the environment. However, conventional facilities consume substantial amounts of energy to power the treatment processes. According to estimates, wastewater treatment accounts for about 3% of the total electricity consumption worldwide, with significant associated greenhouse gas emissions and operational costs. To address these challenges, WWTPs are adopting energy-efficient technologies and using renewable energy sources such as the biogas produced by the anaerobic digestion (AD) of the sludge. However, WWTPs are still energy-demanding facilities and new process configurations are being studied to achieve more operational self-sufficiency and sustainability. In this context, this thesis addresses a new configuration in the mainstream line of the WWTP aiming to maximize the energy recovery with biogas while minimizing the energy consumption of the process. Such configuration is based on a first high-rate activated sludge (HRAS) reactor followed by a two-stage autotrophic nitrogen (N) removal. In the HRAS reactor, organic matter is redirected to the sludge line to maximize the biogas production trough AD, while, in the proposed two-stage autotrophic N removal, a first partial nitritation (PN)-aerobic granular sludge (AGS, PN-AGS) reactor is followed by anaerobic ammonium oxidation (anammox). Compared with conventional biological N removal, the oxygen requirement can be lowered by 63% and the organic carbon consumption for denitrification by up to 100%. A lab-scale AGS reactor was used, while a pilot plant of the hole proposed configuration was designed, constructed and operated in La Garriga WWTP to test the technology. Only real wastewater was used. The objectives of the thesis were, first, to develop a robust and replicable operational strategy to achieve aerobic granulation by treating HRAS effluent. Second, to adapt the AGS process to achieve PN, producing a suitable effluent with NO2- and NH4+ to feed the final anammox process. Thereafter, to study the performance of the final anammox process when the AGS is stably operated aiming to PN, and, finally, to quantify the energy consumption and recovery of the new treatment configuration vs. the conventional La Garriga WWTP, assessing the energetic viability of the new technology. In the AGS reactor, a selective wasting from the top of the settled sludge bed was found to be a successful strategy to effectively trigger the formation and establishment of aerobic granules in the system. However, full granulation was not seen as crucial to the proper performance of the AGS reactor. The maintenance of a low solids volumetric index (SVI < 100 mL/g) with a mixture of granules and flocs was helpful enough for an improved operation with high organic matter removal and nitratation repression. Stable PN was achieved under mainstream conditions by treating real HRAS effluent. Nitrite oxidizing bacteria (NOB) activity was successfully repressed by the free nitrous acid (FNA) produced in the reactor. In the anammox reactor, an effluent with a suitable quality (i.e., total nitrogen, TN, < 8 mg/L) could be achieved in the reactor by controlling the previous PN conversion at NO2-/NH4+ ratios lower (0.80 g N/g N) than the theoretical 1.32 g N/g N, which implies a lower oxygen requirement. Finally, according to the energy balance, estimated energy savings (kWh/m3) of 44 % could be reached by applying the HRAS + PN-AGS + anammox configuration