Water treatment system design and layout

1. Introduction

The North Carolina Fish Barn project at North Carolina State University NCSU has been active in the evaluation and development of recirculating technology for the intensive production of fish since 1989. In 1993, with funding from AGA Gas, Inc. Cleveland, OH, AGA AB Stockholm, Sweden, AquaOp- tima AS Trondheim, Norway, and the Energy Division of the North Carolina Department of Commerce Raleigh, NC, a ‘third generation’ Fish Barn recirculat- ing system was designed and tested. The Fish Barn system incorporated a tank and particle trap technology referred to as ECO-TANK and ECO-TRAP, respectively. The ECO-TANK and ECO-TRAP technology are described by Skybakmoen 1989 and more recently by Timmons et al. 1998 and are products of AquaOptima AS. The North Carolina Fish Barn system is described by Twarowska et al. 1997 and is referred to as the ECOFISH™NCSU system. The system consisted of a tank with a circular flow pattern, a particle trap, a sludge collector, a drum screen filter, a trickling biofilter gravity fed, referred to as a BioSump, a downflow oxygen contactor, and a vertical manifold water inlet in the culture tank. In 1994, Carolina Power Light Company CPL became a corporate sponsor of work ongoing at the NC Fish Barn. In 1996, NC State and CPL proposed the development of the large-scale recirculating fish production demonstration system to be funded by the Electric Power Research Institute Palo Alto, CA. The facility was designed by NCSU and CPL personnel based upon the ECOFISH™NCSU system. The overall layout of the facility is described in Hobbs et al. 1997. The fish production system consists of a 5.1 m 3 quarantine tank, a 13.3 m 3 secondary quarantine or nursery tank, and four 60 m 3 growout tanks. The tank systems are housed in a 39.5 m long × 9.75 m wide agricultural barn structure, with much of the treatment equipment contained in two 3 m × 6 m shed type ‘mechanical’ rooms. Referred to as The CPLEPRI Fish Barn, the facility is designed to produce approx. 45 metric tonnes mt of tilapia fish annually based on 1 g fingerlings growing to market size of 580 g in approx. 210 days. This paper details the design and operational characteristics of the main growout tank system and associated water treatment components.

2. Water treatment system design and layout

The water treatment systems for all of the tanks within the facility are of similar layout but sized appropriately for the daily amount of feed provided to each system. The quarantine tanks each have separate water treatment systems to provide some isolation of potential disease causing organisms that could be introduced into the growout system. The four growout tanks are identical in size and capacity 6.40 m diameter × 1.98 m deep. Growout Tanks 1 and 2 share a common water treatment system, as do growout Tanks 3 and 4. A flow diagram in plan view for one growout system is shown in Fig. 1. Water exits from the two culture tanks A through the particle traps B to the sludge collectors C and a common standpipe well D. The water then passes through a drum screen filter E to the biofilter F. The water is returned to the culture tanks by centrifugal pumps G via downflow oxygen contactors H, which add pure oxygen to the flow stream. The water re-enters the culture tanks through Fig. 1. Flow diagram in plan view of the layout of the growout tank system. Fig. 2. Elevation view of tank system, particle trap, sludge collector, standpipe well and drum screen filter after Hobbs et al. 1997. two vertical manifolds I per tank. Water from the main treatment flow stream is diverted at a rate of 230 liters per minute l min − 1 through two foam fractionators J. System piping cross connections provide operational flexibility and heating capabilities via a heat pump 5. Table 1 provides a detailed description of the equipment used in the main growout system. A typical growout tank system layout is shown in ‘elevation’ view in Fig. 2. This diagram shows the fiberglass tank A with the particle trap B set in the concrete tank foundation floor. Using the tank water level as a reference elevation, water flows from the particle trap through two separate pipes to the sludge collector C and the standpipe well D where the flows are rejoined. The flow proceeds through the drum screen filter E towards the BioSump, which is not shown on this diagram. Gravity flow is used as much as possible to carry water through the treatment processes. While this can be viewed as an energy savings feature, the greater advantage is the improvement in solids removal. In general, solids are larger and more easily removed by gravity settling andor mechanical screening processes before they are subjected to the shearing forces of a centrifugal water pump impeller. Settleable solids are removed rapidly within the culture tank by the particle trap which is shown in detail in Fig. 3 ECO-TRAP 300; AquaOptima AS, Trondheim, Norway. Studies have shown that fecal solids and uneaten feed are removed from the tank within minutes of settling to the flat bottom of the culture tank Twarowska et al., 1997. The ECO-TRAP has two outlets in which water flows from the culture tank. Settleable solids are captured by the particle trap as they slide beneath a plate located in the tank center just above and parallel to the tank bottom. The uneaten feed and fecal solids are collected in a bowl within the particle trap and are removed via a 30 l min − 1 flow stream designated B in Fig. 3. The settleable solids that are captured by the particle trap B are removed from the flow stream in a ‘sludge collector’ or settling cone external to the tank as shown in Fig. 4 as flow B. Clarified water overflows from the sludge collector C and goes 7 T .M . Losordo et al . Aquacultural Engineering 22 2000 3 – 16 Table 1 Specifications of components in one 2-tank growout system in the CPLEPRI fish barn Suppliermanufacturer Quantity Component function Description Model NA Glass Boat Works, PO Box 674, Exmore, 60 m 3 at 6.4 m dia.×1.98 m deep, fiber- 2 Tanks A glass VA 23350 USA 300 ECO-TRAP, stainless steel polyethylene 2 AquaOptima AS, Kjøpmannsgata 35, Particle trap B sludge collector C 7011Trondheim, Norway PVC Custom, local materials Standpipe well D 54 cm dia.×122 cm high, 2-15.24 cm in- 1 lets, 1–25.4 cm outlet, PVC Hydrotech w40 micron screen Water Management Tech., PO Box 66125, 802 Drum screen filter E 1 Baton Rouge, La. 70896 USA NA Corrugated steel pipe, 2.44 m diameter× Contech Construction Products, Inc., PO Box 2 Trickling filter BioSump 800, Middletown, OH 45042, USA F 5.66 m deep, concrete bottom BioBlok 200 EXPO-NET Danmark AS, Georg Jensens 15.4 m 3 BioBlok, plastic media, 55×55×55 cm Biofilter media Vej 5, DK-9800 Hjørring, Denmark blocks, 200 m 2 m − 3 4 PS 6 Aquatic Eco-Systems, Inc., 1767 Benbow Sweetwater, 2 hp, 8.9 full load amps at 230 Centrifugal pumps G VAC, 8.07 running amps at 230 VAC Court, Apopka, FL 32703 USA 1.856 kW Aquatic Eco-Systems, Inc. 4 Oxygen Saturator fabricated from welded OY 110 Downflow oxygen contac- tors H polyethylene pipe AquaOptima AS Vertical manifold I ECO-FLOW, PVC 4 110 2 Top Fathom, PVC venturi driven TF12 Foam fractionator J Aquanetics Systems, Inc., 5252 Lovelock, San Diego, CA 92110, USA Aquatic Eco-Systems, Inc. S-45 1 Regenerative blower 1 Sweetwater, 1.5 hp, 10.4 full load amps at 230 VAC, 8.87 running amps at 230 VAC 2.04 kW 1 Dayton, High Pressure, Direct Drive Model 6K481B W.W. Grainger, Inc., 4820 Signett Dr., High volume blower 3 Blower, Stock No. 7C483, 2.7 full load Raleigh, NC 27604 USA amps at 230 VAC, 2.77 running amps at 230 VAC 0.637 kW Model GT026 FHP Manufacturing, Inc., 601 Northwest Florida heat pump, 10.25 kW heating, 10.5 1 Heat pump water heater 5 65th Court, Fort Lauderdale, FL 33309 USA full load amps at 230 VAC, 9.77 running amps at 230 VAC 2.247 kW to the adjacent standpipe well. Flow stream A shown in Fig. 3 carries suspended solids through the elevated strainer of the particle trap B at a design rate of 800 l min − 1 per tank. Fig. 3. The ECO-TRAP particle trap showing high solidslow flow stream B and high flowlow solids stream A, after Hobbs et al. 1997. Fig. 4. Sludge collector that works in conjunction with the ECO-TRAP to remove settled waste solids from the flow stream B after Hobbs et al. 1997. Fig. 5. Tank, sludge collector, drum screen filter and BioSump biofilter layout after Hobbs et al. 1997. The settleable solids and suspended solids flow streams from each tank come together in the standpipe well D where the flows from both tanks combine and are carried to a drum screen filter E Hydrotech, Water Management Technologies, Baton Rouge, LA at a combined rate of 1660 l min − 1 . At this point, all solids larger than the size of the screen on the drum screen filter 40 microns are removed by the screen and then by the intermittent high-pressure rinse spray to a waste stream. The filtered water leaves the drum screen filter E and exits through the discharge pipe which then divides the stream in two, flowing to the two 2.44 m diameter BioSumps F shown in Fig. 5. The water is distributed over the top of the BioSump biofilter media with a single rotating distribution nozzle Balanced AquaSystems, 1051 Swanston Drive, Sacra- mento, CA. The water falls through 1.65 m of plastic biofilter media BioBlok 200, EXPO-NET which has a specific surface area of 200 m 2 m − 3 . The ammonia is converted to nitrate at a design rate of approx. 0.45 g TAN m − 2 day − 1 by the bacteria attached to the media. Carbon dioxide is removed from the downward cascading water stream with a counter-current air flow generated by a 0.62 kW ‘high-volume blower’ Dayton, Model 6K481B, W.W. Grainger, Inc. which pro- vides a total of 6.8 m 3 min − 1 of air at 10 cm H 2 O pressure. This air is introduced in each BioSump just below the biofilter media but above the water level in the bottom of the BioSump each BioSump receives 3.4 m 3 min − 1 . At the bottom of the BioSump, the water collects to a depth of approximately 1 m and has a residence time of 5.4 min. An array of 16 flexible membrane diffusers FlexiDisc, EnviroQuip International, Cincinnati, OH provides aeration in each BioSump with a supply of 1.4 m 3 min − 1 of air. A 2.0 kW regenerative blower Sweetwater Model S-45, Aquatic Eco-Systems, Inc. provides the air to the aeration arrays in both BioSumps at a combined rate of 2.8 m 3 min − 1 at 76 cm H 2 O pressure, to liberate carbon dioxide and add dissolved oxygen. Water is then pumped from the bottom of each BioSump with two-2 kW centrifugal pumps G Sweetwater, Aquatic Eco-Systems, Apopka, FL at a rate of 415 l min − 1 each through oxygen injection components to each tank Fig. 6. For each tank, two downflow oxygen contactors H Oxygen Saturator Model OY110, Aquatic Eco-Systems, Apopka, FL are used. Water flows into the top of the downflow oxygen contactors, is mixed with gaseous oxygen, and exits the bottom in a pressurized 0.5 – 1.0 bar flow stream for delivery to the culture tank. The oxygenated water re-enters the culture tank through two vertical manifolds I ECO-FLOW Model 110, AquaOptima AS, Trondheim, Norway that allow for even distribution of the water from top to bottom in the tank water column.

3. Water treatment system design and operational characteristics