Sustainable Marine Aquaculture

EVALUATION OF SOLID REMOVAL AND NITRIFICATION PERFORMANCE OF A RAS WITH A PROP-WASHED BEAD FILTER AND FLUIDIZED BED SAND FILTER

Research Team:
Tim Pfeiffer (USDA) - Principle Investigator
Ken Riley (HBOI) - Principle Investigator
Todd Lenger

The design and operational characteristics of Recirculating Aquaculture Systems (RAS) are among the most important factors determining the success or failure of an aquaculture operation. In this study, a small-scale RAS was designed and developed to optimize use of water, energy and technology in a demonstration system for tilapia production. The goal was to use a two-phase RAS to produce multiple crops of tilapia (Oreochromis aurea) without the use of supplemental oxygen. The specific objectives were to 1) evaluate solids removal using dual-drain technology, a swirl separator and a floating bead bioclarifier; 2) evaluate nitrification across a floating bead bioclarifier and fluidized sand filter; 3) monitor oxygen consumption in the culture units and filtration components, and 4) examine the integration and reliability of RAS components and automated system controls.

A RAS with two types of solids removal devices (swirl separator and floating bead bioclarifier) and two biofilters (floating bead bioclarifier and fluidized sand filter) was evaluated for performance at four different feed rates in this study. The system consists of two 3.65 meter diameter panel fiberglass circular tanks with a sloping bottom for fish culture. Culture volume for each tank is approximately 12,500 L. Solids removal is accomplished in part by using a swirl separator to capture the low-volume, high solids effluent from the center bottom drain of each tank. Flow from the swirl separator joins the high volume flow from the elevated sidewall drain of each tank into a wastewater sump. Flow from the sump is pumped to a propeller-washed floating bead bioclarifier (PWBF). Backwashing of the bioclarifier is an automated process which is activated daily. Additional biofiltration is provided by a fluidized bed sand filter. Water returns to the culture tanks by gravity flow.

The two production tanks were stocked with a total of 1280 tilapia fingerlings (average stocking density = 45 fish/m3). Throughout the production cycle the nitrification performance of the biofilters is being evaluated and the PWBF and swirl separator are being sampled for solids removal ability.

Tank 1 was provided with 450 g of the 3.1-mm floating pellet (32% CP) and Tank 2 was provided with 350 g of a the floating 2-mm pellet (45% CP) at each feeding. After four weeks the filters and swirl separator were sampled for nitrification and solids removal performance. Dissolved oxygen, pH, and temperature are measured daily, and alkalinity, CO2, total ammonia nitrogen (TAN), nitrite, and nitrate are measured weekly.

Feed rates have been increased stepwise throughout the study. To evaluate the filter performance at each feed level the volumetric total ammonia nitrogen conversion rate (VTR) is calculated six-hour intervals during each sampling period. The VTR equation is as follows:

VTR = [ K_c x (TANI + TAN_E) x Q ] / V

where,

VTR = volumetric TAN conversion rate (g TAN/ m3 media/ day)
K_c = unit conversion factor, 1.44
TANI = Influent TAN concentration (mg/L)
TAN_E = Effluent TAN concentration (mg/L)
Q = flow rate through the filter (Lpm)
V = volume of filter media (L)

Solids capture by the swirl separator and PWBF are analyzed by measuring the total volume of sludge removed by each device on a daily basis. Sludge samples are analyzed for TSS on a daily basis during the four-day sampling period. The volume of the sludge and the result from the total suspended solids (TSS) analysis of the sludge samples were used to determine the weight of solids removed from the system by these two components.