Aquaculture America 2024

February 18 - 21, 2024

San Antonio, Texas

COMPARING AEROBIC AND ANAEROBIC MINERALIZATION OF SLUDGE FROM CLEAR-WATER AND HYBRID BRACKISH-WATER RAS

Rayne C. Pollack*, Jill C. Fisk, Andrew J. Ray

Center for Aquaculture and Aquatic Sciences, Kentucky State University, Frankfort KY, 40601 USA

rayne.pollack@kysu.edu

 



  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, h ybrid 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 plants used in brackish-water aquaponics.  It is unclear how the waste from the two system types might differ and what the effects of aerobic versus anaerobic environments may have on mineralization of the material. T his 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 ammonia 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 inductively coupled plasma mass spectrometry (ICP-MS)  for  elemental analysis.

 The 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,  these results indicate that denitrification was occurring in the anaerobic treatments . P lants  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, the  anaerobic  treatments generated  far more PO4  than the 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 fter only two weeks,  mineralization had yielded the highest TAN, NO3 and PO4 concentrations, suggesting that different compounds could be generated depending on the amount of time the material was processed .  Results of this study indicate that system type may not be very impactful with regard to waste mineralization. However, aerobic versus anaerobic mineralization environments , and perhaps time, can make large impacts on what compounds become dissolved in the water. This information  may be used  by producers  to select for particular compounds best suited to the needs of their particular aquaponics plants.