Water reuse is a key feature of inland, brackish-water, recirculating aquaculture systems (RAS) and mineralization of thickened waste may play a role in reducing water discharge. In clear-water systems solids filtration is robust; hybrid systems in comparison, operate with less filtration. By allowing the collected sludge to remain in contact with an external water body, the process of mineralization or decomposition of solids, can release nutrients and minerals important for plant production in brackish-water aquaponics. It is unclear how waste from the two system types might differ and what effects aerobic versus anaerobic environments may have on mineralization of the material. This trial compared aerobic and anaerobic mineralization of marine shrimp sludge from two system types over a four-week period to determine changes in concentrations of dissolved nitrogenous compounds, phosphate and mineral content over time.
Sludge was collected from separate clear-water (CW) and hybrid (HY) shrimp RAS settling chambers. A 28-day trial examined each water type in aerobic (AE) and anaerobic (AN) conditions. The two water types and two oxygen levels created four treatments: CW-AE, CW-AN, HY-AE, HY-AN. Each treatment was randomly assigned to four replicate, 18-L containers. The water in each container was analyzed weekly for total ammoniacal nitrogen (TAN), nitrite-nitrogen (NO2-N), nitrate-nitrogen (NO3-N), phosphate (PO4) and alkalinity (ALK). Additional water samples were collected weekly and filtered, then sent to an independent lab for elemental analysis via inductively coupled plasma mass spectrometry (ICP-MS).
Concentrations of ALK, TAN and PO4 tended to be higher overall and NO3-N tended to be lower overall in anaerobic treatments. Because the denitrification process reduces NO3 and produces alkalinity, results indicated denitrification occurred in anaerobic treatments. Plants generally prefer uptake of NO3 over ammonia and use less energy to do so; however, both compounds can be assimilated into plant tissues. The CW-AE treatment showed a gradual increase in PO4 concentration that may continue past the duration of this project. However, anaerobic treatments generated far more PO4 than aerobic treatments in the four weeks of this trial. The difference (initial – final) in elemental concentrations showed an increase of dissolved Ca and decrease of Mn and Fe concentrations in aerobic systems. There was an overall decrease in all treatments of Na, Mg, Ca, K and Sr concentrations, and an overall increase in all treatments of S, Li, Ni, Si, P and Br concentrations. A two-week mineralization time in anaerobic treatments yielded the highest TAN, NO3 and PO4 concentrations, suggesting additional mineralization time is unwarranted for these nutrients. However, it is likely that other plant essential nutrients could be optimally mineralized at different time intervals, or could become more bioavailable over time. Results indicate that system type may not be very impactful with regard to waste mineralization; however, aerobic versus anaerobic environments, and perhaps time, can make large impacts on what compounds are mineralized into the water column and at what concentrations. This information may be used by aquaponics producers to select for particular compounds best suited to the nutritional needs of their plant species being cultivated.