Efforts at Clemson University (from 1990-2001) focused on intensification of fish culture and reduction in water discharge by enhancing algal photosynthesis in a “high-rate algal pond’ combined with raceway fish culture in a technique entitled the Partitioned Aquaculture System (PAS) . Work at Clemson in the earliest 1/40-acre prototype PAS units suggested maximum algal productivity of 9.9 gm-C/m2-day at 0.65 m water depth and water velocity of 0.29 ft/sec (12.5 cm/s) , with average algal density of 66 mg/l. At a water velocity of 0.072 ft/sec algal productivity averaged 6.5 gm-C/m2-day at an algal density of 48 mg/l. One third acre and 2.0-acre PAS prototypes used slowly rotating paddlewheels to circulate water through shallow (1.5 ft) channelized ponds (95% of area), increasing algal photosynthesis and enhancing treatment rate of nitrogenous waste (NH3). Catfish were confined to 5% of the total area in high-density raceways (6.0 lb/ft3) at mixing/aeration energy of 4.5 hp/acre. At th at time, conventional catfish ponds (CP) were utilizing 100% of pond volume/area for fish culture at aeration energy of 2.0 hp/acre. The Split-Pond (SP), developed at the National Warm-water Aquaculture Center (NWAC) beginning in 2002, represented a lower cost adaptation of Clemson’s PAS. Prototype SPs at NWAC range in size from 5.0-7.0 acres utilizing 5.7-7.0 hp/acre mixing/aeration, with fish culture confined to 20-25% of surface area with 75-80% devoted to waste treatment. NWAC also reported on evaluation of prototype 2.0-4.0 acre, intensively aerated catfish ponds (IP) utilizing 100% of pond volume for fish culture at elevated aeration energy levels of 6.5-7.9 hp/acre.
A performance analysis of prototype CP, SP and IP and PAS units suggested fish carrying capacity in CP of 5,000-7,500 lb/acre, PAS at 15,000-18,000 lb/acre, SP at 12,330-19,000 lb/acre and IP at 9,200-18,245 lb/acre. Average/maximum sustained feed loading was 100/150 lb/acre-day for CP, 160/250 for PAS, 110/280 for SP, and 84/270 for IP. The PAS, SP and IP are all highly photosynthetic systems however, the PAS provides the highest degree of net photosynthesis and ammonia removal. Average daily photosynthesis in the PAS was projected at 5.9 gm-C/m2 with 4.6 in the SP, 5.1 in the IP and 1.0 in the CP. Maximum photosynthesis in the PAS (fully loaded) at 250 lb feed/acre-day) was projected at 8.2 gm-C/m2-day (81% of small PAS units) . The PAS aggressively re-mineralizes settled algal biomass, in which the released nutrients are recaptured as additional algal biomass. The SP aggressively removes algal biomass (via sedimentation in the waste treatment zone, WTZ) and promotes nitrification and denitrification in a WTZ anoxic layer. The IP provides a higher degree of settled algal storage in the sediment. Consequently, the IP is more subject to sporadic release of sediment ammonia driven by climatic changes, whereas the PAS and SP provide more consistent operator control of oxygen and nitrogen flux.
The PAS was designed to provide maximum nitrogen treatment via enhanced algal photosynthesis. Capital costs for PAS are similar to in-pond raceways at $22,630/acre with a breakeven catfish production cost of $1.32/lb. In contrast, SP capital cost is projected at $6,904/acre with BEC of $0.92/lb and IP at $8,380/acre with BEC of $0.93/lb. The SP provides 78% of the net photosynthetic capability of the PAS at 70% of the production cost/lb. The IP provides 87% of the net photosynthesis at 69% of the production cost/lb. The IP requires minimal modification of existing ponds, the major cost being addition/maintenance of aerators. The SP requires substantial modification of existing ponds, but provides more consistent/reliable treatment of ammonia nitrogen. In addition, SP provide advantages in fish feeding/harvesting and predator control. Fish farmers have reported excessive accumulation of ammonia (3-6 ppm) in IP, particularly during winter months.