The invention of conventional activated sludge about 100 years ago enabled humanity to safely treat its wastewater, protecting the environment while supporting society. This sewage treatment plant (STP) has become more advanced, and efficiently removes wastewater pollutants like nutrients (nitrogen, phosphorus), organic carbon, pathogens.
This advanced treatment has however come with a cost of about 60-120 euro pp-1 y-1 .
More cost-effective sewage treatment can be achieved in more circular approach, with reuse of water, energy and nutrients from wastewater.
An energy neutral STP fits within this circular approach, and is feasible by the separation of different processes. In the first stage, an energy and organic carbon rich fraction is
harvested from the wastewater and transformed to biogas. In a second stage, the remaining nutrients, including nitrogen, need to be treated. Conventional nitrogen removal technologies, like nitrification/denitrification, need organic carbon to remove nitrogen. Partial nitritation/anammox (PN/A) does not have this ‘carbon-hunger’, and is therefore highly suitable as a post-treatment to achieve an energy neutral STP that guarantees
effluent quality. PN/A depends on the interplay of two bacterial groups, with partial nitritation as first step, in which about half of the incoming ammonium is oxidized to nitrite by the aerobic
ammonia-oxidizing bacteria (AerAOB). The produced nitrite is then reduced by the anaerobic ammonium-oxidizing bacteria (AnAOB) with the remaining ammonium to nitrogen gas, and some nitrate, the so-called anammox conversion. Nitrite oxidizing bacteria (NOB), who convert nitrite to nitrate with oxygen, are unwanted in this process, because they lower the removal efficiency. The process should therefore be developed so that the growth conditions are optimal for both ammonium-oxidizing bacteria (AerAOB and AnAOB), while NOB are suppressed. Suppressing NOB is a challenge for the control of PN/A for the treatment of domestic sewage (mainstream PN/A), because lower temperatures (10-25°C), lower influent
ammonium concentrations (<50 mg N L-1) and daily fluctuations in wastewater quality challenge process control. For successful operation, a new framework was developed with
ON/OFF control: stimulation/suppression of the desired/undesired bacteria and IN/OUT control: retention/removal of the desired/undesired bacteria. A combination of ON/OFF + IN/OUT control should lead towards a working process. In this PhD thesis, several strategies for successful ON/OFF control were mechanistically researched upon in Chapter 2, 3 and 4, while in Chapter 5, operational strategies (ON/OFF and IN/OUT) were tested to obtain a well-working mainstream PN/A. AnAOB inhibition by oxygen (ON/OFF)
The presence of partial nitritation in PN/A has the consequence that AnAOB are always in the proximity of oxygen. The influence of oxygen on the strictly anaerobic AnAOB, who perceive stress/inhibition by oxygen, is ambiguously described in literature. Therefore, a
study was executed to describe the recovery of AnAOB after oxygen exposure. In a reactor, a highly enriched community of AnAOB was obtained without any nitrifiers present. A range of oxygen concentrations (0.05-2 mg O2 L-1) were added over different exposure times (1.4-24h) in a highly-controlled reactor, after which the activity response of AnAOB was followed up in high temporal resolution with an ammonium sensor (Δt = 10 min). After short exposure, i.e. 1.5-8h, AnAOB directly resumed their activity, which was lower than the initial activity prior to exposure. Dependent on the perceived stress, AnAOB gradually recovered their activity in 5-37h, until steady-state was achieved. The experimental data did not fit well with a conventional ‘instant recovery’ Monod-type inhibition model. These results indicate that recovery of AnAOB after oxygen exposure was previously overlooked. It is recommended to account for this effect in the intensification of partial nitritation/anammox.
The influence of wastewater parameters on AerAOB and NOB activity (ON/OFF) Two distinct STP, Blue Plains (BP), Washington DC, US and Nieuwveer (NV), Breda, NL,
were compared to study the influence of often fluctuating and location-specific wastewater parameters on the AerAOB and NOB activity. Both STP have a similar temperature profile,
yet vary in operational and wastewater parameters, resulting in an opposite AerAOB/NOB potential activity ratio at 20°C (BP = 0.6; NV = 1.6). To understand what factors steer the
activity of AerAOB and NOB, a methodology based on known activity models was developed, called ‘add-on mechanistic modelling’. This add-on mechanistic model allowed
us to separate the influence of different wastewater parameters, i.e., temperature, inorganic carbon (from alkalinity), ammonium, nitrite, and phosphate concentrations, on the activity of AerAOB and NOB. The results showed that AerAOB and NOB reacted similar on temperature in both STP, despite the significant differences in AerAOB
community. For BP, the presence of sufficient inorganic carbon (~3 mM C) appeared to be crucial for higher AerAOB activity and growth. Relieving this limitation would lead
towards a similar AerAOB/NOB potential activity ratio in both STP. Mainstream integrated fixed-film activated sludge (IFAS) PN/A reactor at 26°C: operational strategies for an optimal nitrite source and sink (ON/OFF + IN/OUT)
An IFAS reactor was operated, in which floccular sludge that performs mainly the aerobic conversions (e.g., AerAOB, NOB), resides together with an AnAOB rich biofilm that grows
on a carrier material. In this biofilm, the slow growing AnAOB have a long residence time, and are well protected from oxygen by an aerobic bacterial layer on top of the biofilm. In
contrast, when the right process conditions are given, NOB can be gradually washed out by precisely controlling the aerobic floccular sludge retention time (AerSRTfloc) (IN/OUT
control). Variation of the aeration strategy and nitrogen loading rate were further used to select for the desired bacteria (ON/OFF control). The best operational period was with
continuous two-point aeration, with alternation of a low (0.05 mg O2 L-1 for 10 min.) and high (0.27 mg O2 L-1 for 5 min.) dissolved oxygen setpoint, combined with a low but
sufficient AerSRTfloc of ~ 7d. At this moment, good NOB-suppression and a nitrogen removal rate of 122±13 mg N L-1 d-1 was obtained, which makes the technology feasible
for implementation in countries with warm wastewater temperatures. Under these conditions, the floc acted as a nitrite-source (AerAOB), while the carrier acted as nitritesink (AnAOB, NOB). For a successful nitrite source, maintaining a higher floccular sludge concentration (~0.5-1 g VSS L-1) allowed sufficient aerobic ammonium conversion, while
keeping the AerSRTfloc sufficiently short (<7d) at low oxygen setpoints (0.05-0.3 mg O2 L1 ) enabled higher floccular AerAOB/NOB activity ratios (7-29 vs. 1). For the carrier as a
nitrite sink, lower DO setpoints (0.05 vs. 0.15 mg O2 L-1) and higher loading rates (150-200 vs. 60 mg N L-1 d-1) resulted in higher AnAOB/NOB activity ratios, while the carrier biofilm
thickness might not play a significant role. The operational strategies highlighted within the source-sink framework can serve as a guideline for successful operation of mainstream
PN/A reactors. Novel return-sludge treatment for NOB-suppression (ON/OFF) A sewage treatment facility has a return-sludge line that returns sludge, when it is
separated from the cleaned wastewater, back to the reactor. The idea was to design a return-sludge treatment that exposed floccular sludge, e.g. located in the IFAS reactor
described above, towards stress conditions that favor AerAOB over NOB. Different combinations of known stress factors; sulfide (0-600 mg S L-1), anaerobic starvation (0-
8d), and a free ammonia (FA) shock (30 mg FA-N L-1 for 1h), were tested for immediate stress response and long-term recovery. The best combination was 150 mg S L-1, 2d
anaerobic starvation and a FA-shock. Despite no positive change observed in the immediate-stress response, AerAOB recovered much faster than NOB, with a nitrite
accumulation ratio (effluent nitrite on nitrite + nitrate) peak of 50% after 12 days. Studying long-term recovery may therefore be crucial for the design of an optimal NOB-suppression
treatment, while applying combined stressors regularly may lead towards an implementable NOB-suppression treatment.
This PhD thesis aids to the development of mainstream PN/A, by its focus on mechanistic insights and operational strategies, linking operational conditions with microbiology
(abundance, taxonomy, and morphology) and bacterial activities. Further development of the technology might lead towards an energy neutral STP, with good effluent quality, who
protects health and environment, while addressing parts of the challenges that we face in the 21st century.