Molasses and oxygen unlock nutrients in catfish farm sludge

Nutrient-rich sludge from intensive African catfish farms can be transformed into a more useful fertilizer by combining aeration with an organic carbon source, according to a study conducted in the Philippines. Researchers working with a decoupled aquaponics system compared three ways of mineralizing sludge collected from a recirculating aquaculture system: anaerobic digestion without aeration, aerobic treatment with aeration, and aerobic treatment supplemented with commercial molasses. The molasses-amended treatment produced the strongest release of nitrogen, phosphorus, potassium, calcium and magnesium into the liquid phase, offering a potential route to recovering nutrients that would otherwise remain trapped in solid fish waste.

The experiment took place at the Freshwater Aquaculture Station of the University of the Philippines Visayas in Miagao, Iloilo, where the researchers operated an integrated system consisting of a recirculating aquaculture unit, a lettuce hydroponic unit and separate mineralization chambers. The fish component held 500 African catfish, Clarias gariepinus, with a mean body weight of 120 grams each. Stocking density was 10 kilograms per cubic metre, and the animals were fed commercial feed containing 34% crude protein at a rate equivalent to 3% of body weight per day. Water continuously circulated through a settling tank, biofilter and sump, with total water exchange reaching 400% per day.

Most of the nutrients entering a recirculating aquaculture system originate in fish feces and uneaten feed. Although these materials contain elements essential for plant growth, many remain bound to particles and organic matter in the sludge. Conventional single-loop aquaponics can also force compromises on fish, plants and microbial communities because the connected compartments require different operating conditions. A decoupled design allows the fish system, crop system and sludge-mineralization unit to be controlled independently, while keeping mineral-rich liquid separate from the water returned to the fish tank. This separation allows recovered nutrients to be concentrated for hydroponic use rather than immediately diluting them in the recirculation loop.

For the study, sludge accumulated in the settling tank for seven days before being siphoned and allowed to settle for another 24 hours. The resulting 10.5 litres of concentrated sludge was divided into three 3.5-litre portions and diluted to a total-solids concentration of 10 grams per kilogram. Anaerobic treatment, designated T1, was held in a sealed 20-litre bucket without aeration or mixing. Aerobic treatment, T2, received continuous aeration through an airstone. The third treatment, T3, was also aerated but received molasses as a carbon source. The amount was calculated from the carbon and nitrogen content of the dry sludge and the 24% carbon content of the molasses to establish a carbon-to-nitrogen ratio of 20 to 1, a ratio previously associated with improved mineralization of African catfish sludge.

Samples were taken before treatment and on days 5, 10 and 15, with pH and temperature monitored daily. Total nitrogen was measured using the Kjeldahl method, while calcium and magnesium were determined by EDTA titration. Phosphorus was analyzed with the vanadomolybdate method and potassium by flame atomic absorption. The experiment used a one-way analysis of variance and, where appropriate, a Tukey post-hoc test to identify significant differences among treatments. These measurements allowed the researchers to track both changes in nutrient concentration and the environmental conditions associated with nutrient release.

The three treatments experienced broadly similar temperatures, ranging from 25.6 to 32.6°C, but their pH behavior differed sharply. The molasses-amended treatment fell into an acidic range of 3.36 to 5.91, while the anaerobic and aerobic treatments remained between 5.8 and 7.42. By day 15, T3 contained 2,700 milligrams per litre of nitrogen, 100 milligrams per litre of phosphorus, 720 milligrams per litre of potassium, 12,115.6 milligrams per litre of calcium and 3,391.9 milligrams per litre of magnesium. Nitrogen rose sharply by day 5 and then changed only slightly, whereas the unamended aerobic treatment produced a marked nitrogen increase only by day 15, reaching 2,160 milligrams per litre. Anaerobic treatment left nitrogen close to its starting concentration, at 1,072 milligrams per litre.

Phosphorus showed some of the clearest treatment effects. In T3, phosphate-phosphorus increased significantly by day 15, whereas concentrations in both T1 and T2 fell below 5 milligrams per litre and remained at similarly low levels. Potassium was already relatively soluble before treatment, but its response to molasses was striking. T1 and T2 remained near their initial range, whereas T3 rose to 670 milligrams per litre on day 5 and peaked at 830 milligrams per litre on day 10 before easing to 720 milligrams per litre on day 15. Calcium reached 12,115.6 milligrams per litre in T3 on days 10 and 15, compared with 7,269.3 milligrams per litre in T2. Magnesium in T3 climbed from below 170 milligrams per litre to 1,695.9 milligrams per litre by days 5 and 10 and then doubled to 3,391.9 milligrams per litre by day 15.

The authors linked the unusual performance of T3 to the combined effects of carbon, oxygen and acidity. Aeration supplies oxygen for heterotrophic microorganisms that break down organic matter, including proteins, and release nitrogen into the liquid. Molasses provides readily available carbon that can accelerate microbial growth, while respiration produces carbon dioxide and lowers pH. The acidic conditions appear particularly important for phosphorus, calcium and magnesium because they help dissolve mineral compounds that would otherwise remain insoluble or precipitate at higher pH. The parallel increases in phosphorus and magnesium are consistent with the dissolution of struvite, a mineral containing ammonium, magnesium and phosphate. The molasses itself also contributed potassium to the system.

Relative to untreated sludge supernatant, the aerobic molasses treatment produced percentage increases of 155% for nitrogen, 400% for phosphorus, 3,500% for potassium, 150% for calcium and 1,895% for magnesium. The result is notable because earlier work on simple aeration had reported prominent increases only after longer treatment periods. However, the recovered liquid did not match the complete nutritional needs of hydroponic lettuce: nitrogen, phosphorus and potassium concentrations remained below their optimum levels, even though calcium and magnesium entered sufficiency ranges recommended for lettuce. The findings therefore support molasses-assisted mineralization as a partial source of plant fertilizer, rather than a complete replacement for formulated nutrient solutions.

Overall, the study indicates that properly managed oxygen and carbon inputs can turn a difficult waste stream from intensive catfish production into a more valuable agricultural input. The strongest performance came from aerating sludge with molasses, a treatment that reduced organic solids while concentrating several plant-essential elements in the recovered liquid. Further optimization would be needed to control the nutrient balance and potentially supply supplemental inorganic fertilizer, but the results demonstrate a promising mechanism for reducing waste and nutrient discharge while improving the productivity of decoupled aquaponics. The approach could be particularly valuable in regions with intensive catfish farming, limited water resources and a need to produce vegetables without relying entirely on conventional fertilizers.

Subject of Research: Nutrient recovery from African catfish sludge through aerobic mineralization with molasses

Article Title: Comparison of the nutrient composition of sludge under aerobic and anaerobic mineralization from African catfish, Clarias gariepinus ( Actinopterygii : Siluriformes : Clariidae ), reared in an intensive recirculating aquaculture system

Article References: Guyapale, R. M., Dionela, C., & Huervana, F. (2024). Comparison of the nutrient composition of sludge under aerobic and anaerobic mineralization from African catfish, Clarias gariepinus (Actinopterygii: Siluriformes: Clariidae), reared in an intensive recirculating aquaculture system. Acta Ichthyologica et Piscatoria, 54, 275-282. https://doi.org/10.3897/aiep.54.131982

Image Credits: AI Generated

DOI: 10.3897/aiep.54.131982

Keywords: aquaponics, catfish aquaculture, sludge mineralization, nutrient recovery, molasses, recirculating aquaculture systems, hydroponics, wastewater reuse, plant fertilization, water sustainability, Comparison, nutrient