Evaluation of settling chamber hydraulic retention time in a Sunshine bass biofloc production system

Authors: Green, Bartholomew - Bart; Ray, Candis

Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2021
Publication Date: 12/22/2021
Citation: Green, B.W., Ray, C.L. 2022. Evaluation of settling chamber hydraulic retention time in a Sunshine bass biofloc production system. North American Journal of Aquaculture. 84:165-171. https://doi.org/10.1002/naaq.10229.
DOI: https://doi.org/10.1002/naaq.10229

Interpretive Summary: In the outdoor biofloc technology production system a complex of living organisms, including phytoplankton and bacteria, closely associated with particulate organic matter that is maintained in suspension in the water column by continuous aeration metabolizes ammonia excreted by the intensively fed fish to nitrate. Consequently, high fish yields can be obtained in response to high fish stocking and feeding rates; however, the quantity of suspended solids that accumulate in the water column increases with the amount of feed consumed daily by fish. Water from biofloc production tanks can be diverted to an associated settling chamber where solids settle out of the water column and are removed from the system. The length of time that tank water spends in the settling chamber, also known as the hydraulic retention time, affects solids removal efficiency. The effects on solids removal, water quality dynamics, and Hybrid Striped Bass (Sunshine Bass) performance of 18-, 43-, and 87-minute settling chamber hydraulic retention times were evaluated in this 126-day study in biofloc tanks. Once settling chambers were activated, concentration of suspended solids decreased faster and the quantity of solids removed increased as hydraulic retention time increased. Biotransformation of excreted ammonia to nitrate (nitrification) was not affected by any of the hydraulic retention times tested. Additionally, there was no evidence that biotransformation of nitrate to nitrogen gas (denitrification) occurred in settling chambers in any treatment. Sunshine Bass performed well in all treatments with overall production averaging 4.4 kg/m3 at 81.9% survival. Although Sunshine Bass were grown successfully to market size in biofloc culture, future research should address higher stocking rates and settling chamber operational parameters to maintain higher total suspended solids concentrations and promote denitrification.

Technical Abstract: High total suspended solids concentrations that result from the high feeding rates used in the biofloc technology production system can impact fish performance negatively. Excess total suspended solids are removed from biofloc production tanks by side-stream settling chambers. The effects on solids removal, water quality dynamics, and Hybrid Striped Bass (Sunshine Bass; Morone chrysops x M. saxatilis) performance of 18-, 43-, and 87-minute settling chamber hydraulic retention times (designated HRT18, HRT43, and HRT87, respectively) were evaluated in this 126-day study in biofloc tanks. Upon activation of settling chambers total suspended solids concentration decreased faster in the HRT87 treatment than in the other two treatments. Dry matter mass of solids discharged each time settling chambers were drained and total dry matter mass of solids discharged each increased linearly with hydraulic retention time. Compared to the HRT43 and HRT 87 treatments, total suspended solids accumulated about 17% more slowly and 25% less sodium bicarbonate was needed in the HRT18 treatment which suggests that nitrification was impacted negatively by weekly solids removal. Denitrification was not detected in settling chambers given the absence of significant differences between influent and effluent dissolved inorganic nitrogen concentrations in all treatments. Sunshine Bass performed well in all treatments with overall production averaging 4.4 kg/m3 at 81.9% survival. Although Sunshine Bass were grown successfully to market size in biofloc culture, future research should address higher stocking rates and settling chamber operational parameters to maintain higher total suspended solids concentrations and promote denitrification.