The research team, led by Luca Gerdes genannt Janßen of the University of Applied Sciences Bremen together with colleagues from Tanta University, the University of Montpellier, and the Helmholtz Centre for Polar and Marine Research, collected tilapia sludge from a commercial recirculating aquaculture system and subjected it to a battery of treatments rarely compared side by side under identical conditions. These included simple sealed storage, gradual acidification to pH 5, ultrasonication, aerobic mineralization with continuous aeration, anaerobic digestion at 37 degrees Celsius, and two sequential approaches that combined digestion with post-treatment steps. The goal was to quantify how each strategy shifted nutrients from the solid-bound pool, where they are inaccessible to plants, into the dissolved pool, where they become bioavailable.

The starting point was sobering. In untreated sludge, more than 75 to 90 percent of nitrogen, phosphorus, calcium, magnesium, and most trace elements were locked in the solid phase, while potassium was the exception, largely dissolved at over 80 percent. On a dry matter basis, the sludge contained 37.5 grams of total nitrogen and 18 grams of total phosphorus per kilogram, along with substantial iron, zinc, manganese, and copper. These numbers confirm what aquaponic practitioners have long observed: aquaculture process water is deficient in several macro- and micronutrients compared to optimal hydroponic requirements, with iron consistently limiting and phosphorus frequently bound to particles.

The treatment comparison revealed pronounced differences in nutrient mobilization pathways. Anaerobic digestion variants achieved the highest nitrogen mineralization, reaching up to approximately 47 percent, with ammonium-dominated supernatants containing roughly 460 to 480 milligrams of ammonium-nitrogen per liter. In contrast, aerobic mineralization promoted nitrification and yielded nitrate-dominant nitrogen at approximately 257 milligrams per liter, though its overall nitrogen mineralization efficiency remained lower at around 25 percent. Purely physicochemical treatments, such as ultrasound and acidification alone, mobilized less than 4 percent of nitrogen, suggesting that nitrogen release is primarily governed by microbial decomposition rather than solubility equilibria.

Phosphorus followed a different logic entirely. Its mobilization was primarily pH-driven, peaking under acidification at approximately 165 milligrams of phosphate-phosphorus per liter and increasing further after post-acidification of anaerobic digestion supernatants. Aerobic mineralization actually reduced dissolved phosphorus, likely because phosphate released during organic matter degradation was rapidly immobilized through microbial assimilation or adsorption onto newly formed iron and manganese oxides. Manganese increased markedly under acidified and unaerated conditions, reflecting its redox-sensitive behavior, whereas dissolved iron remained low across all treatments, precipitating as hydroxides under oxic conditions or as sulfides under anoxic ones.

When the resulting supernatants were compared to ideal hydroponic nutrient media, none produced a directly usable solution. Most required dilution due to elevated ammonium, with anaerobic digestion and storage-derived streams needing 22 to 32 times dilution to meet a hypothetical ammonium ceiling of 15 milligrams per liter. Iron supplementation remained necessary in every case, as dissolved iron stayed far below typical hydroponic requirements of approximately 2.5 milligrams per liter. Potentially toxic elements, however, remained in the low microgram-per-liter range and well below FAO irrigation guidelines and reported algal toxicity thresholds, offering reassurance about contaminant mobilization.

The most compelling demonstration came from growth trials with Arthrospira platensis, the cyanobacterium better known as spirulina. Selected sludge-derived media were adapted by dilution and supplementation with sodium bicarbonate, potassium nitrate, and potassium phosphate to match the nitrogen and phosphorus composition of a standard SAG medium. Among these, the non-acidified anaerobic digestion supernatant supported the strongest biomass and pigment performance, reaching up to 1.32 grams of dry weight per liter, with a chlorophyll a content of 21.7 milligrams per liter and a C-phycocyanin content of 24.6 milligrams per gram of dry weight. These figures compare favorably with previously reported dry weights for this species grown in fortified synthetic media.

Beyond nutrient recovery, anaerobic digestion also produced biogas, with specific yields of approximately 368 to 370 normal liters per kilogram of volatile solids. This dual recovery of energy and nutrients positions digestion as a central component of circular aquaculture, since the biogas can offset part of the substantial energy demand of warm-water recirculating systems, particularly for water heating. The steeper initial biogas production observed with ultrasonic pretreatment suggests enhanced early hydrolysis kinetics, though final yields converged, indicating that the total biodegradable fraction was not substantially altered by sonication.

The study’s broader message is that sludge mineralization can recover substantial nutrient fractions from solid waste streams, but process selection must be aligned with the intended downstream application. Aerobic mineralization was most effective for organic matter degradation and nitrate formation, anaerobic digestion achieved the highest nitrogen mineralization, and acidification steps were most effective for phosphorus mobilization. Interestingly, the low-technology sealed storage treatment mobilized both nitrogen and phosphorus to a degree comparable to more complex approaches despite requiring no aeration, heating, or chemical addition, highlighting that treatment selection should consider not only maximum nutrient release but also process intensity in terms of time, infrastructure, energy, and chemical inputs.

As aquaculture continues to grow as one of the world’s fastest-growing food sectors, and as waste management of nutrient-rich byproducts remains a substantial economic burden, this research demonstrates that the same waste stream concentrates much of the nutrient load it is costly to dispose of. By closing the loop between fish production and plant or cyanobacterial biomass cultivation, circular recirculating aquaculture and aquaponic systems can convert a costly disposal problem into two usable products in a single step, moving the industry closer to genuine resource self-sufficiency.

Subject of Research: Nutrient recovery from aquaculture sludge for circular aquaponics and bioeconomy

Article Title: Closing the loop in aquaponics: biological and physicochemical recovery of nutrients from aquaculture sludge for circular bioeconomy

Article References: Gerdes genannt Janßen, L., Elshobary, M., Ramos, N., Schwenkler, T., Ende, S., & Noke, A. (2026). Closing the loop in aquaponics: biological and physicochemical recovery of nutrients from aquaculture sludge for circular bioeconomy. Frontiers in Aquaculture, 5, Article 1842840. https://doi.org/10.3389/faquc.2026.1842840

Image Credits: AI Generated

DOI: 10.3389/faquc.2026.1842840

Keywords: aquaculture sludge, aquaponics, nutrient recovery, circular bioeconomy, anaerobic digestion, Arthrospira platensis, spirulina, recirculating aquaculture systems, phosphorus recovery, sustainable aquaculture, Closing, loop