The biological wastewater treatment systems should be managed to achieve minimum sludge production and maximum methane production while achieving satisfactory biological nutrient removal. However, the achievement of the above goals is often hindered by the poor biodegradability of secondary sludge. The overall objective of this thesis is to develop a novel strategy based on treatment of secondary sludge using free nitrous acid (FNA i.e. HNO2) for reducing sludge production and enhancing methane production without deteriorating nitrogen removal performance.
The feasibility of achieving sludge reduction based on sludge treatment using FNA was investigated through long-term laboratory tests. By treating part of the secondary sludge with FNA at 2.0 mg N/L for 24 h along with the subsequent recirculation of the FNA-treated sludge to the activated sludge system for degradation, it was revealed that sludge production was reduced by 28%. Also, the addition of the FNA-treated sludge did not negatively affect the treatment performance and sludge properties of the reactor, namely the organic carbon and nitrogen removal, nitrous oxide (N2O) emission and sludge settleability. Endogenous denitrification batch tests indicate that FNA-treated sludge is available as a carbon source for denitrification.
The feasibility of enhancing methane production based on pretreatment of secondary sludge using FNA was investigated through laboratory biochemical methane potential tests. It was demonstrated that methane production increased with increased FNA concentration used in the pre-treatment step. Model-based analysis indicated FNA pretreatment improved both hydrolysis rate and biochemical methane potential, with the highest improvement being approximately 50% (from 0.16 to 0.25 d-1) and 27% (from 201 to 255 L CH4/kg VS added; VS: volatile solid), respectively, achieved at 1.78-2.13 mg HNO2-N/L.
In order to further improve methane production from secondary sludge, another technology option based on combined FNA and heat pre-treatment was studied. Secondary sludge from a full-scale plant was treated for 24 h with FNA alone (0.52-1.43 mg N/L at 25 °C), heat alone (35, 55 and 70 °C), and FNA (0.52-1.11 mg N/L) combined with heat (35, 55 and 70 °C). The pre-treated secondary sludge was then used for biochemical methane potential tests. Compared to the control (no FNA or heat pre-treatment of secondary sludge), biochemical methane potential of the pre-treated secondary sludge was increased by 12-16%, 0-6%, 17-26%, respectively; hydrolysis rate was improved by 15-25%, 10-25%, 20-25%, respectively, for the three types of pre-treatment. Heat pre-treatment at 55 and 70 °C, independent of the presence or absence of FNA, achieved approximately 4.5 log inactivation of pathogens, thus capable of producing Class A biosolids.
The feasibility of achieving the nitrite pathway (i.e. NH4+→NO2-→N2) based on sludge treatment using FNA was investigated through long-term laboratory tests. With part of the return sludge treated with FNA at 1.4 mg N/L for 24 h, the nitrite pathway was rapidly (in 15 d) established in a reactor treating synthetic domestic wastewater. The average nitrite accumulation ratio (NO2--N/(NO2--N+NO3--N)×100%) was above 80% in the above reactor, whereas no nitrite accumulation was observed in the reactor without FNA treatment. The nitrite-oxidizing bacteria (NOB) population in the reactor with FNA treatment was 80% lower than the reactor without FNA treatment, indicating that the majority of NOB were eliminated. The FNA-based strategy for establishing the nitrite pathway substantially improved total nitrogen removal, and did not increase N2O emission or deteriorate sludge settleability. The achievement of the nitrite pathway reduces the carbon requirement for biological nitrogen removal by 40% theoretically, and therefore could increase availability of carbon source for methane production via the reinstallation of the primary settler. This enables enhanced methane production while achieving satisfactory nitrogen removal.
FNA required for the goals proposed in this thesis can be produced on site through nitritation of anaerobic sludge digestion liquor. However, nitrite accumulation has been frequently reported to stimulate the production of N2O, which is a potent greenhouse gas. N2O emissions from nitritation reactors receiving real anaerobic digestion liquor have been reported to be substantially higher than those from reactors receiving synthetic digestion liquor. The causes for the above differences were investigated, and strategies to reduce N2O emissions from reactors treating anaerobic sludge digestion liquor were also developed. It was found that heterotrophic denitrification supported by the organic carbon present in the real digestion liquor was the key contributor to the higher N2O emission from the reactors receiving real digestion liquor. Dissolved oxygen (DO) at 1 mg/L or above suppressed heterotrophic nitrite reduction thus reduced aerobic heterotrophic N2O production. It was recommended that DO in a nitritation system receiving anaerobic digestion liquor should be maintained at approximately 1 mg/L to minimise N2O emission.
The FNA-based strategy is potentially economically and environmentally attractive. It has the potential to be applicable to any biological wastewater treatment systems utilizing activated sludge, with the above listed benefits achievable depending on the design and function of the plants.