Each year, millions of tons of mineral fertilizer are applied in agriculture. However, despite large losses on the way between field and plate, municipal wastewater is still a concentrate of the nutrients consumed by the society and a point source of nutrients to receiving water bodies. Conventional wastewater treatment has so far 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. During the past decade, two technologies have been implemented for the treatment of digested sludge liquor from wastewater treatment plants, the so-called ‘centrate´: struvite precipitation for phosphorus removal and recovery, and partial nitritation-anammox (PNA) as an energy-efficient alternative for nitrogen removal. However, no one has fully explored how to couple them. Sara Johansson’s PhD thesis, entitled “Taking advantage of autotrophic nitrogen removal: Potassium and phosphorus recovery from municipal wastewater”, investigates two routes taking advantage of autotrophic PNA for the recovery of nutrient-rich products.

Route 1: Biologically induced precipitation occurring inside PNA granules – Unlike chemical precipitation of struvite, which is induced by addition of a magnesium source and where the pH is controlled either by CO₂ stripping through aeration or addition of a base, biologically induced precipitation is induced by pH and substrate gradients created by the physical and biochemical properties of the biomass. The researcher harvested granules with a high inorganic content in a lab-scale PNA reactor fed with centrate, and characterized their chemical composition. Analysis by emission spectroscopy showed that the granules had a phosphate content of 36 wt% P₂O₅, similar to that of phosphate rock, and a Ca/P ratio close to that of hydroxyapatite. Crystalline hydroxyapatite was confirmed by X-ray diffraction. Emission spectrometry further showed that the content of heavy metals complied with proposed EU limits for fertilizer, as well as requirements from the phosphorus industry. Furthermore, due to the high inorganic content of harvested granules, their removal does not interfere with demands for PNA sludge for inoculation purposes, nor with the bioactivity of the reactor, and harvest is easy due to gravitational settling. As the mineral forms without the addition of chemicals, this route represents a novel alternative to phosphorus recovery from wastewater.

Route 2: Recovery of potassium together with phosphorus in the form of potassium struvite – Precipitation of potassium struvite needs to be preceded by a nitrogen removal step, as ammonium has a negative effect on the formation of potassium struvite. In this thesis, PNA has proven to be a suitable technology for nitrogen removal from centrate. Lab- and pilot-scale PNA 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 macronutrients N, P and K. Besides, 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. Although commonly growth-limiting in terrestrial ecosystems, potassium is not considered to contribute to eutrophication and is therefore, unlike phosphorus and nitrogen, not regulated at the European, nor at the national level. Consequently, the fate of potassium within wastewater treatment plants in not well-documented. A sampling campaign was conducted over the sludge line of a Bio-P plant, with the aim to the map nutrient flows, especially potassium, in order to better understand where to best implemented potassium recovery. Results showed that the three macronutrients take three distinctly different routes within the plant due to the characteristics of each compound. Potassium is a small ion that is easily leached; nitrogen can take many forms, including gaseous states; and phosphorus in the form of phosphate readily forms salts with a low solubility in water. Sara Johansson estimated that from the daily load, 80% of the incoming potassium leaves the plant with the effluent while 85% of incoming phosphorus ends up in the sludge fraction and exits the plant through the biosolids. Incoming nitrogen is to 80% removed in the biological step and leaves the plant as nitrogen gas. Finally, a solids mass balance was used to calculate the flow of centrate to 198 m³ d^-1 (<1% of the incoming flow to the plant) and a daily flow of 49 P^–PO₄^3-, 241 N-NH⁴⁺ and 85 kg K⁺.

PNA today is successfully implemented as an energy-, carbon- and cost-efficient alternative for nitrogen removal in the treatment of centrate. This PhD 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. Therefore, 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. It is thus undeniable that PNA can play an important role in the conversion of wastewater treatment plant into resource recovery facilities within the circular economy framework. Moreover, this thesis seeks to broaden the discussion on nutrient recovery beyond phosphorus and nitrogen, while raising awareness on the current heavy dependency on imported minerals for fertilizer production and advocating a more sustainable food production and consumption system. Carried out at UdG LEQUIA research group and Aquafin facilities in Belgium, the thesis is part of Marie Sklodowska Curie’s Industrial Doctorate Training Network “TreatRec” (Horizon 2020, GA 642904). Thus, it was directed by Dr Jesús Colprim (UdG), Dr Maël Ruscalleda (currently at Createch360) and Dr Bart Saerens (Aquafin).