Abstract
Based on mass and energy balance calculations, this work investigates the possibility of recovering heat and nutrients (nitrogen and phosphorus) from municipal sewage sludge using pyrolysis or combustion in combination with a gas scrubbing technology. Considering a wastewater treatment plant (WWTP) with 65,000 t/a of mechanically dewatered digestate (29% total solids), 550 t/a nitrogen and 500 t/a phosphorus were recovered from the 4900 t/a total nitrogen and 600 t/a total phosphorus that entered the WWTP. Overall, 3600 t/a (73%) of total nitrogen was lost to the air (as N2) and clean water, while 90 t/a (15%) of total phosphorus was lost to clean water released by the WWTP. Both in combustion and in pyrolysis, the nitrogen (3%) released within thermal drying fumes was recovered through condensate stripping and subsequent gas scrubbing, and together with the recovery of nitrogen from WWTP reject water, a total of 3500 t/a of ammonium sulfate fertilizer can be produced. Furthermore, 120 GWh/a of district heat and 9700 t/a of ash with 500 t/a phosphorus were obtained in the combustion scenario and 12,000 t/a of biochar with 500 t/a phosphorus was obtained in the pyrolysis scenario. The addition of a stripper and a scrubber for nitrogen recovery increases the total electricity consumption in both scenarios. According to an approximate cost estimation, combustion and pyrolysis require annual investment costs of 2–4 M EUR/a and 2–3 M EUR/a, respectively, while 3–5 M EUR/a and 3–3.5 M EUR/a will be generated as revenues from the products.
Highlights
The global population is estimated to increase from 7.7 billion to 9.7 billion by 2050 [1]
The pyrolysis scenario S1.1 and combustion scenario S2.1, where nitrogen is recovered from the drying fumes but not from the reject water, are presented in Figures 6 and 7, respectively
The integration of pyrolysis and combustion processes with a gas scrubbing technology at a large-scale wastewater treatment plant (WWTP) was investigated through mass and energy balance calculations, in order to maximize the recovery of nutrients and energy from municipal sewage sludge
Summary
The global population is estimated to increase from 7.7 billion to 9.7 billion by 2050 [1] This expected rapid increase, with the concentration of people in cities, raises several problems in terms of waste, sewage sludge, which is the semi-solid byproduct generated during municipal and industrial wastewater treatment [2]. Sewage sludge contains nutrients such as phosphorous and nitrogen [5] It contains harmful substances, including heavy metals, aromatic hydrocarbons, pathogens, and, as was recently discovered, microplastics and medical residues. Due to this fact, sewage sludge utilization as it is, and even after biological treatment, risks soil contamination [6]
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