Simultaneous removal of carbon, nitrogen and phosphorus from municipal wastewater through continuous-flow oxygen-controlled moving bed biofilm reactors

Eutrophication is an imbalance of aquatic system functioning due to an uncontrolled discharge of nutrients, i.e. nitrogen (N) and phosphorus (P), in water bodies. European legislation has defined discharge limits for BOD5, COD and TSS for wastewater treatment plants (WWTPs) serving >2000 population equivalent (PE). Moreover, stringent nutrient-discharge limits have been set for treated effluents from WWTPs larger than 10000 PE discharging into sensitive areas. Nevertheless, for small communities (below 2000 PE), the current national directive (D. Lgs. n°152/2006) and European legislation does not oblige to respect the discharge limits required for larger communities, although strict purification values can be set at local level, mainly depending on the receiving water body (e.g. lake or river). Italy is characterized by the existence of a huge amount of low-populated locations, as almost 44% of the municipalities count less than 2000 inhabitants. Centralised wastewater treatment is not always feasible or the most cost-effective option for all sites due to geographical conditions and dispersed settlements. The aim of this doctoral thesis was to study a compact solution for the removal of C, N and P for the treatment of low- and medium-strength municipal wastewater that may help small communities to implement efficient and low-cost wastewater treatment. The combined removal of carbon and nutrients via simultaneous nitrification and denitrification (SND) coupled to P removal in moving bed biofilm reactors (MBBRs) was chosen as a potentially advantageous system for the scope as biofilm reactors enable the formation of a stratified biofilm with different microbial families depending on wastewater composition and operating conditions. Although satisfactory removal efficiencies have been achieved by operating single-stage MBBR as sequencing batch reactors (SBR) alternating anaerobic and aerobic/anoxic conditions, the MBBR-SBR technology is best suited to treat industrial wastewater, being discontinuously produced during the day, while a continuous-mode operation would be more suitable for municipal wastewater treatment. In this PhD work, SND coupled to P removal was investigated in continuous-flow single-stage MBBRs adopting different aeration strategies, i.e. stable microaerobic conditions and intermittent aeration (IA) conditions. The microaerobic conditions were set by maintaining a dissolved oxygen (DO) of 1.0 (±0.2) mg L-1, resulting in a simultaneous removal of COD, total inorganic nitrogen (TIN) and dissolved phosphorus (P-PO43-) with average efficiencies of 87%, 58% and 66% respectively, at feed C/N ratio of 4.2. At feed C/N ratio of 2.7, average TIN RE was 46% due to lack of electron donor for denitrification. At the same time, at feed C/N ratio of 5.6, excess overgrowth of heterotrophic aerobic bacteria (HAB) led to poor nitrification, determining an average TIN RE of 51%. Subsequently, aeration strategy was shifted from a continuous aeration mode to a microaerobic/aerobic IA condition. SND process and P removal were studied in a continuous-flow IAMBBR under different DO regimes (0.2–2, 0.2–3 and 0.2–4 mg L-1). Simultaneous removal of dissolved organic carbon (DOC), TIN and P-PO43- removal efficiencies of 100%, 62% and 75% were achieved at DO range at 0.2–3 mg L-1 and at feed C/N ratio of 3.6. The bacterial community composition of the IAMBBR biofilm was investigated by Illumina sequencing, revealing the coexistence of nitrifiers (Nitrosonomas and Nitrospira) and denitrifiers with P-accumulating ability indicated as putative PAO (Hydrogenophaga), which were abundantly detected throughout the entire study. As last step, the feasibility of coupling a simultaneous partial nitrification and denitrification (SPND) to P removal were studied in two continuous-flow IAMBBRs. The effect of feeding two different carbon sources, i.e. ethanol and acetate, on the reactor performances and microbial community composition was also investigated. The inhibition of nitratation phase (NO2-→NO3-) allowed to reach average DOC, TIN and P-PO43- REs of 100%, 81–88% and 83–86% at DO range of 0.2–3 mg L-1 and feed C/N ratio of 3.6. Finally, the effect of different feed C/P ratios (22 and 11) on P removal and microbial community of both biofilm and suspended biomass was studied. Illumina sequencing displayed the presence of putative PAOs such as Hydrogenophaga and Acidovorax, while typical NOB were never detected. MBBRs performing combined SPND and P removal achieved a 20% higher TIN RE compared to previous SND process under similar DO and C/N conditions. This work shows that efficient, simple and low-cost C, N and P removal can be achieved in MBBR by controlling the aeration pattern and at certain C/N/P ratios. Therefore, microaerobic/aerobic MBBRs are proposed as an interesting solution for implementing wastewater treatment in small communities.