In general, biological wastewater treatment processes are both complex and exceptionally varied. However, the number of instruments available for the detailed study of these processes is quite limited. This thesis focuses on the development of a new, advanced research tool suitable for the detailed study of wastewater treatment processes. The tool has been named the TOGA (Titrimetric and Off-Gas Analysis) sensor.
The main innovation of the sensor is the amalgamation of titrimetric and off-gas analysis techniques. The main measured signals are, among others, hydrogen ion production rate (HPR), oxygen transfer rate (OTR) and carbon dioxide transfer rate (CTR). While OTR is applicable to aerobic conditions, HPR and CTR are useful signals under all of the conditions found in biological wastewater treatment systems, namely aerobic, anoxic and anaerobic. The sensor is therefore a powerful tool for studying the key biological processes under all these conditions.
A major benefit from the integration of the titrimetric and off-gas analysis methods is that the acid-base buffering systems, in particular the bicarbonate system, can be properly accounted for. The TOGA methodology offers a new technique for interpreting HPR and CTR data. The integration of the two signals leads to a new relationship, whereby the measured data is grouped into a single variable (HPR*), which is correlated with the biological carbon dioxide production rate. In this way, the influence of the bicarbonate buffering system on the measured signals is removed, without requiring knowledge of the CO2 liquid /gas transfer coefficient. Experimental data resulting from the TOGA sensor was used to show that traditional methods for interpreting HPR data have a number of limitations, including that it is dangerous to assume constant contribution to HPR from CO2 transfer.
Experimental data resulting from the TOGA sensor in aerobic and anaerobic conditions demonstrates the potential application of the new sensor. The TOGA sensor was used to investigate the utilisation of external carbon in aerobic conditions by a mixed activated sludge culture. As expected, it was found that external carbon was utilised for both biomass growth and the formation of storage polymers. It was shown that the TOGA signals could be used to determine the rates of the two processes without requiring off-line laboratory analysis. This was achieved by developing and then identifying a general model describing external carbon utilisation under aerobic conditions. Simulation of the model showed that for the system studied, in which acetic acid was used as the carbon substrate, the majority of carbon was utilised for storage polymer formation. The simulation results compared very well with off-line measurement of acetic acid, ammonium and storage polymer (poly-β-hydroxybutyrate (PHB)).
The sensor was also used to study the behaviour of glycogen accumulating organisms (GAOs). Under aerobic conditions, the TOGA data, coupled with an established model describing the metabolism of GAOs, was used to quantify the rates of biomass growth and glycogen replenishment. Simulation of the identified model showed that the majority of substrate (poly-hydroxyalkanoate (PHA)) was used for glycogen replenishment. The simulation results compared well with off-line measurement of ammonium, glycogen and PHA concentrations. The semi-continuous nature of the TOGA data allowed investigation into the kinetics of these processes. It was found that Monod kinetics, with a high affinity constant, could adequately describe PHA degradation by GAOs in the system studied here.
In anaerobic conditions the TOGA sensor was also demonstrated as a powerful tool for the examination of GAOs. An identifiability study revealed that the TOGA data could be used to fully identify a recently developed model describing the metabolism of GAOs in anaerobic conditions. In a case study, it was found that the TOGA data contained sufficient information for the identification of the energy requirement for substrate transport, the maximum rate of substrate removal and the affinity constant with respect to acetate. Also, it was concluded that the data could be useful for the quantification of the maintenance coefficient, provided a sufficiently long experiment is conducted.
Additionally, the TOGA sensor was used to examine the two-step nitrification process, consisting of ammonia oxidation to nitrite and nitrite oxidation to nitrate. The sensor is particularly valuable for the study of these systems as the signals can be used to clearly differentiate between ammonia oxidation and nitrate production. It was shown (by comparison with off-line analysis) that the sensor is capable of accurately quantifying the rates of ammonia removal, nitrite accumulation and nitrate production in both activated sludge and biofilm systems. Furthermore, the set-up is such that the sensor can be used to examine the effect of operational parameters (e.g. pH and DO) on the reaction rates. For the biofilm system studied it was found that nitrite accumulation was favoured in high pH, low DO systems.
Overall, the thesis presents the development of the novel TOGA sensor for the study of biological wastewater treatment systems. The application of the sensor, in both aerobic and anaerobic conditions, shows that the TOGA sensor is a highly valuable tool for the detailed examination of biodegradation processes. It is therefore hoped that the outcome of this work will foster more efficient and accurate research into these complex and highly important biotechnology processes.