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There is a recognized need for mineralizing aquaculture-derived sludge in aquaponics systems in order to reduce waste production. Many recent studies of aquacultural waste treatment have focused only the production of biogas as opposed to the potential for mineralization of nutrient-rich sludge. Upflow anaerobic sludge blanket (UASB) reactors provide one possible solution for breaking down sludge into bioavailable nutrients that can subsequently be delivered to plants. As such, this study examines the mineralization performance of sequential UASB reactors that are designed with an expanded granular sludge bed (EGSB) and compared to standard aerobic and anaerobic batch reactors. Results of our experiments demonstrate that only chemical oxygen demand reduction is significantly different. An unexpected drop in pH of one of the three reactor systems revealed that a pH below 6 was able to significantly increase the mineralization and mobilization of nutrients. Approximately 25% of phosphorus, potassium, and calcium could also be recovered from the sludge under lower pH conditions, as compared to the mineralization performance of standard UASB reactors running at a higher pH. However, the opposite effect was observed with respect to organic sludge reduction, where diminished performance was observed in the low-pH reactor. The current study implies that anaerobic reactors operating at low pH can potentially contribute towards improved nutrient recovery in multi-loop aquaponics systems and reduction of additive agents for pH control of the hydroponic subsystem.

The effect of a high and low non-starch polysaccharide diet (HNSP and LNSP diet) and ultrasound treatment on particle size distribution and carbon bioavailability in fecal waste of rainbow trout (Oncorynchus mykiss) was studied. Feces were collected from four flow-through fish tanks, two tanks fed the HNSP diet and two the LNSP diet. The collected feces were sonicated (disintegrated) in duplicate with high-intensity (0.6 W/ml), low-frequency (f = 20 Hz) ultrasound at five different energy levels (0.6 W/ml for 0, 0.25, 1, 4, and 16 min). The particle size distribution of the treated feces samples was measured by wet sieving (1000, 500, 200, 100, 63, 36, 1.2 μm screen size) and total suspended solids (TSS) measurement. Carbon bioavailability in sonicated fecal waste samples was determined with oxygen uptake rate (OUR) tests. The results showed that: (1) feces from the HNSP diet contained significant more particulate material and bigger particles; (2) carbon bioavailability was almost three times higher in untreated LNSP feces when compared with HNSP feces; (3) almost 50% of HNSP feces could have been recovered on a microscreen of 36 μm after wet sieving, whereas it was only 10% for LNSP feces; (4) the production of small particles (1.2–36 μm), which could pass a drum filter screen and potentially accumulate in RAS, was approximately 50 g/kg feed, showing no significant differences between diets; (5) sonication increased fecal dry matter below 36 μm (p = 0.015), but it had no significant effect on the median particle size; (6) sonication increased carbon bioavailability with 7–10% for the HNSP feces (p = 0.037); (7) fecal particles withstood up to 16 min sonication at an intensity of 0.6 W/ml and a frequency of 20 Hz corresponding to specific energy input of 20,000 kJ/kg DM without major changes in particle size distribution. The results of this study indicate that the applied ultrasound treatment of fecal waste is not an effective method to increase short-term carbon bioavailability.

The kinetics and design of a suspended bacteria growth reactor, which can be integrated in a 100MT African catfish farm, were determined. Such a reactor converted nitrogen (N) and phosphorus (P) from RAS effluents into heterotrophic bacteria biomass. The determined kinetics were: yield = 0.537 g VSS/g C; endogenous decay coefficient = 0.033 h −1 ; maximum specific growth rate = 0.217 h −1 ; half-velocity constant = 0.025 g/l; maximum rate of substrate utilization = 0.404g C/g VSS h. A reactor integrated in a 100MT farming facility would have a volume of 11 m 3 , based on a minimum HRT of 6 h. The kinetics and reactor design were integrated in a model to predict the VSS production (volatile suspended solids as measure of bacteria biomass) and nutrient conversions. The VSS production was on average 187 ± 2 g VSS/kg feed and the inorganic nutrients (N and P) were removed with an efficiency of 85 ± 3.0% and 95 ± 2.5%, respectively. A carbon (C) supplementation level of 455 g C/kg feed was required to ensure optimal C:N ratios for heterotrophic bacteria production. The production of heterotrophic bacteria biomass is, therefore, a prospective tool to lower nutrient discharge and to increase nutrient retention and sustainability of RAS in the future.

This paper analyses nutrient conversions, which are taking place in integrated intensive aquaculture systems. In these systems fish is cultured next to other organisms, which are converting otherwise discharged nutrients into valuable products. These conversions are analyzed based on nitrogen and phosphorous balances using a mass balance approach. The analytical concept of this review comprises a hypothetical system design with five modules: (1) the conversion of feed nutrients into fish biomass, the “Fish-Biomass-Converter”; (2) the separation of solid and dissolved fish waste/nutrients; the “Fish-Waste-Processor”; (3) the conversion of dissolved fish waste/nutrients, the “Phototrophic-herbivore-Converter”; (4 and 5) the conversion of solid fish waste, the “Bacterial-Waste-Converter”, or the “Detrivorous-Converter”. In the reviewed examples, fish culture alone retains 20–50% feed nitrogen (N) and 15–65% feed phosphorous (P). The combination of fish culture with phototrophic conversion increases nutrient retention of feed N by 15–50% and feed P by up to 53%. If in addition herbivore consumption is included, nutrient retention decreases by 60–85% feed N and 50–90% feed P. This is according to the general observation of nutrient losses from one trophic level to the next. The conversion of nutrients into bacteria and detrivorous worm biomass contributes only in smaller margins (e.g. 7% feed N and 6% feed P and 0.06% feed N 0.03 × 10−3% feed P, respectively). All integrated modules have their specific limitations, which are related to uptake kinetics, nutrient preference, unwanted conversion processes and abiotic factors.