Taking advantage of autotrophic biomass: potassium and phosphorus recovery from municipal wastewater
- Autores: Sara Johansson
- Directores de la Tesis: Jesús Colprim Galcerán (dir. tes.), Maël Ruscalleda Beylier (codir. tes.), Bart Saerens (codir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2019
- Idioma: español
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología del Agua por la Universidad de Girona
- Materias:
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- Resumen
Each year, millions of tons of mineral fertilizer are applied in agriculture. Despite enormous losses on the way between field and plate, municipal wastewater is a concentrate of the nutrients consumed by a society and a point source of nutrients to receiving water bodies. Conventional wastewater treatment has focused on the removal of nutrients to avoid detrimental environmental effects, but increasing awareness on the limited nature of the raw materials for mineral fertilizer is pushing for a shift from removal to recovery of nutrients at municipal wastewater treatment plants.
Within the wastewater treatment plant, digested sludge liquor (centrate) is a small stream rich in nutrients. During the past decade, two technologies has been implemented for the treatment of this stream. Struvite precipitation for phosphorus recovery and partial nitritation-anammox (PNA) as an energy-efficient alternative for nitrogen removal. However, the implications of coupling of these two technologies has not been fully explored. In this PhD thesis, two routes taking advantage of autotrophic PNA for nutrient recovery are investigate.
Chapter 4.1 focuses on biologically induced precipitation occurring inside PNA granules. Granules with a high inorganic content formed in a lab-scale PNA reactor fed with centrate and were found to have a phosphorus content similar to phosphate rock. The mineral was concluded to be hydroxyapatite and the content of heavy metals complied with proposed EU limits for fertilizer, as well as requirements from the phosphorus industry. The precipitation was induced by the physical and biochemical properties of PNA granular sludge without the addition of chemicals and represents a novel alternative to phosphorus recovery from wastewater. Due to the high inorganic content of harvested granules, their removal do not interfere with demands for PNA sludge for inoculation purposes, nor with the bioactivity of the reactor. Harvest is easy due to gravitational settling.
Chapter 4.2 is focused on the recovery of potassium together with phosphorus in the form of potassium struvite, a mineral that can be returned to soil as fertilizer. Ammonium is inhibitory to the formation of potassium struvite and precipitation needs to be preceded by a nitrogen removal step. In this chapter PNA is suggested and proven to be a suitable technology for nitrogen removal prior to potassium struvite precipitation from centrate. Lab- and pilot-scale reactors removed up to 85% of ammonium, which allowed for potassium struvite formation. Co-precipitation of ammonium struvite resulted in the recovery of a multi-nutrient product containing all three macronutrient N, P and K. Bicarbonate consumption by the autotrophic biomass reduced the alkalinity by up to 90%, which far surpasses the capacity for CO2 stripping through aeration. PNA prior to struvite precipitation could therefore drastically lower alkali dosing for pH control.
Due to the limited attention paid to potassium recovery, the faith of potassium within wastewater treatment plants in not well-documented. Chapter 4.3 presents the results of a sampling campaign conducted over a Bio-P plant, with the aim to map nutrient flows with special attention to potassium. Contrary to phosphorus that mainly end up in the sludge fraction, potassium was shown to preferably be present in the water vi fractions. The results also served as a basis to calculate the amount of potassium that can be recovered from centrate, e.g. through precipitation of potassium struvite.
PNA today is successfully implemented as an energy-efficient alternative for nitrogen removal in the treatment of centrate. This PhD dissertation strives to expand the view on what PNA granular sludge can do. The autotrophic nature of PNA sludge serves as a biological CO2 stripper of high efficiency, while the biochemistry and physical properties of granular PNA sludge functions as a biological crystallizer. Taking advantage of autotrophic nitrogen removal can lead to both energy and chemicals savings while producing nutrient-rich compounds that can be returned to soils as fertilizer. Thus PNA can play an important role in the conversion of wastewater treatment plant into resource recovery facilities.
