Sustainable recovery of nitrogen from sewage sludge
Saud, Ali (2023-12-18)
Väitöskirja
Saud, Ali
18.12.2023
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Ympäristötekniikka
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https://urn.fi/URN:ISBN:978-952-412-039-5
https://urn.fi/URN:ISBN:978-952-412-039-5
Tiivistelmä
The growing population and urbanization have increased the need for wastewater treatment alongside concerns about the disposal of the resulting sewage sludge. On the other hand, food scarcity and the reduction of fossil fuel-based fertilizers have increased the need for the recovery of nutrients from sewage sludge. This dissertation presents a comprehensive investigation into the feasibility of resource recovery from sewage sludge. Mostly focusing on nitrogen recovery, also taking into account phosphorus (P) and energy. The challenge posed by potential hazardous materials in sludge such as pathogens, pharmaceutical residues and microplastic has led to the selection of thermal treatments of sewage sludge. In thermal treatment, pyrolysis and combustion are used as the primary treatments while adsorption and stripping and scrubbing are often selected as post treatments to recover nitrogen from reject water, condensate, and the drying fumes resulting from the thermal drying of sewage sludge. This study examines the economic and environmental implications of these approaches and provides valuable insights for sustainable waste management practices.
The aim and objectives of the dissertation are achieved by process modelling, carrying out mass and energy calculations and by using a life cycle assessment (LCA) methodology. The research objectives of this dissertation are i) the comparative technical possibilities for recovering ammonia (NH3) from exhaust fumes of thermal drying, ii) the optimal integration approach for maximizing the NH3 recovery in wastewater treatment plants, sewage sludge treatment, and exhaust gases treatment processes, and iii) the assessment of environmental impacts which are associated with different NH3 recovery technologies, and strategies which are the most effectively minimize their environmental footprint.
In the light of the aims and objectives, four studies were conducted to address them. In Publication I, NH3 recovery from drying fumes during the thermal drying of sewage sludge is explored using packed bed scrubbers. The process is modelled for different ammonia concentrations (75 and 100 ppm) and a drying fumes inlet flow rate of 1,000 m3/h. The optimized parameters for scale-up are determined for 7,700 t/a sewage sludge treatment in Lappeenranta. The results indicate that a single scrubber with an inlet gas flow rate of 24,000 m3/h, 75 ppm ammonia concentration, 1.5 liquid to gas ratio, 100 °C liquid acid temperature, and pH of 3 achieves an impressive efficiency of more than 99%, reducing the outlet stream ammonia to 0.2 ppm. However, the initial economic analysis suggests that producing commercial-grade ammonium sulfate (AS) fertilizer from the recovered ammonia may be economically challenging.
Publication II focuses on heat and nutrient recovery through pyrolysis and combustion with gas scrubbing. Mass and energy balance calculations are made for a wastewater treatment plant (WWTP) with a capacity of 65,000 t/a of mechanically dewatered digestate (29% total solids). The nitrogen and phosphorus recovery from the digestate streams is evaluated, and the scenarios show potential for generating 3,500 t/a of ammonium sulfate (AS) fertilizer, along with producing 120 GWh/a of district heat and 9,700 t/a of ash with 500 t/a phosphorus in the combustion scenario, and 12,000 t/a of biochar with 500 t/a of phosphorus in the pyrolysis scenario. The nitrogen recovery requires additional electricity using a stripper and scrubber, and economic estimates reveal yearly investment expenses of 2–4 M €/a, as well as 2–3 M€/a for combustion and pyrolysis, respectively, with projected product revenues of 3–5 M €/a and 3–3.5 M €/a.
In Publication III, an LCA is used to evaluate composting, combustion, and pyrolysis options for dewatered sewage sludge digestate. Sewage sludge digestate combustion and composting outperformed pyrolysis in most effect categories. The pyrolysis of sewage sludge is currently under research, so additional data is needed to judge its performance. Publication IV used an LCA to investigate the environmental impact of nitrogen recovery for fertilizer from sewage sludge treatment in a municipal wastewater treatment plant (WWTP). Nitrogen was recovered from ammonium-rich reject streams from mechanical dewatering and thermal drying of anaerobically digested sewage sludge using air stripping or a pyrolysis-derived biochar adsorbent. The results varied by scenario and impact category. The global warming potential of nitrogen recovery based on biochar was the lowest, with net negative greenhouse gas (GHG) emissions of 22.5 kt CO2 eq./FU. Total GHG emissions were 2 kt CO2 eq./FU when NH3 was captured by air stripping and were 0.2 kt CO2 eq./FU in the base case without nitrogen recovery. The study also analyzed the potential environmental and health benefits of wastewater systems that incorporate integrated resource recovery.
This study promotes multifunctional wastewater systems with integrated resource recovery for environmental and health benefits. In conclusion, this dissertation contributes significant insights into the potential and challenges of resource recovery from sewage sludge, promoting sustainable and environmentally conscious wastewater treatment practices. It also highlights the utility of recovered products such as ammonium sulfate, biochar, and district heat. Biochar has demonstrated its significance as a valuable product to utilize as an adsorbent and to capture carbon. The integration of different nitrogen recovery technologies can enhance resource efficiency, reduce environmental impacts, and improve the circular economy in sewage sludge treatment with the development of sustainable solutions to handle sewage sludge volumes safely.
The aim and objectives of the dissertation are achieved by process modelling, carrying out mass and energy calculations and by using a life cycle assessment (LCA) methodology. The research objectives of this dissertation are i) the comparative technical possibilities for recovering ammonia (NH3) from exhaust fumes of thermal drying, ii) the optimal integration approach for maximizing the NH3 recovery in wastewater treatment plants, sewage sludge treatment, and exhaust gases treatment processes, and iii) the assessment of environmental impacts which are associated with different NH3 recovery technologies, and strategies which are the most effectively minimize their environmental footprint.
In the light of the aims and objectives, four studies were conducted to address them. In Publication I, NH3 recovery from drying fumes during the thermal drying of sewage sludge is explored using packed bed scrubbers. The process is modelled for different ammonia concentrations (75 and 100 ppm) and a drying fumes inlet flow rate of 1,000 m3/h. The optimized parameters for scale-up are determined for 7,700 t/a sewage sludge treatment in Lappeenranta. The results indicate that a single scrubber with an inlet gas flow rate of 24,000 m3/h, 75 ppm ammonia concentration, 1.5 liquid to gas ratio, 100 °C liquid acid temperature, and pH of 3 achieves an impressive efficiency of more than 99%, reducing the outlet stream ammonia to 0.2 ppm. However, the initial economic analysis suggests that producing commercial-grade ammonium sulfate (AS) fertilizer from the recovered ammonia may be economically challenging.
Publication II focuses on heat and nutrient recovery through pyrolysis and combustion with gas scrubbing. Mass and energy balance calculations are made for a wastewater treatment plant (WWTP) with a capacity of 65,000 t/a of mechanically dewatered digestate (29% total solids). The nitrogen and phosphorus recovery from the digestate streams is evaluated, and the scenarios show potential for generating 3,500 t/a of ammonium sulfate (AS) fertilizer, along with producing 120 GWh/a of district heat and 9,700 t/a of ash with 500 t/a phosphorus in the combustion scenario, and 12,000 t/a of biochar with 500 t/a of phosphorus in the pyrolysis scenario. The nitrogen recovery requires additional electricity using a stripper and scrubber, and economic estimates reveal yearly investment expenses of 2–4 M €/a, as well as 2–3 M€/a for combustion and pyrolysis, respectively, with projected product revenues of 3–5 M €/a and 3–3.5 M €/a.
In Publication III, an LCA is used to evaluate composting, combustion, and pyrolysis options for dewatered sewage sludge digestate. Sewage sludge digestate combustion and composting outperformed pyrolysis in most effect categories. The pyrolysis of sewage sludge is currently under research, so additional data is needed to judge its performance. Publication IV used an LCA to investigate the environmental impact of nitrogen recovery for fertilizer from sewage sludge treatment in a municipal wastewater treatment plant (WWTP). Nitrogen was recovered from ammonium-rich reject streams from mechanical dewatering and thermal drying of anaerobically digested sewage sludge using air stripping or a pyrolysis-derived biochar adsorbent. The results varied by scenario and impact category. The global warming potential of nitrogen recovery based on biochar was the lowest, with net negative greenhouse gas (GHG) emissions of 22.5 kt CO2 eq./FU. Total GHG emissions were 2 kt CO2 eq./FU when NH3 was captured by air stripping and were 0.2 kt CO2 eq./FU in the base case without nitrogen recovery. The study also analyzed the potential environmental and health benefits of wastewater systems that incorporate integrated resource recovery.
This study promotes multifunctional wastewater systems with integrated resource recovery for environmental and health benefits. In conclusion, this dissertation contributes significant insights into the potential and challenges of resource recovery from sewage sludge, promoting sustainable and environmentally conscious wastewater treatment practices. It also highlights the utility of recovered products such as ammonium sulfate, biochar, and district heat. Biochar has demonstrated its significance as a valuable product to utilize as an adsorbent and to capture carbon. The integration of different nitrogen recovery technologies can enhance resource efficiency, reduce environmental impacts, and improve the circular economy in sewage sludge treatment with the development of sustainable solutions to handle sewage sludge volumes safely.
Kokoelmat
- Väitöskirjat [1046]