Design characteristics of perforated pooled circular stepped cascade (PPCSC) aeration system

In the present study, an improvised design over circular stepped cascade (CSC) and pooled circular stepped cascade (PCSC) aerator, named the perforated pooled circular stepped cascade (PPCSC) aerator, has been conceptualized and tested for its suitability as an aerator for small intensive aquaculture ponds. Based on dimensional analysis, dimensionless geometric parameters – ratio of width of consecutive steps (Wi/Wi+1) and ratio of perforation diameter to bottom-most radius (d/Rb) and dimensionless dynamic parameters – Froude (Fr) and Reynolds (Re) number were proposed. Initially, aeration experiments were conducted to optimize the geometric parameters, keeping the dynamic parameters constant. Keeping the optimized values of Wi/Wi+1 = 1.05 and d/Rb = 0.0027 as constants, aeration experiments were further conducted at different discharges (Q) and different bottommost radius (Rb) to study the characteristics of oxygen transfer and power consumption of PPCSC aerator at different dynamic conditions. Based on the optimized results, four prototype PPCSC aerators with Rb = 0.75 m, 0.90 m, 1.05 m and 1.20 m were fabricated for their aeration performances. The results showed that the standard aeration efficiency (SAE) values of the prototype PPCSC aerators based on brake power ranged between 3.36 and 4.98 kg O2/kWh, with the average being 4.45 ± 0.741 kg O2/kWh. This shows that the SAE of the PPCSC aerator is many more folds higher than that of the other available cascade aerators, viz., PCSC (SAE: 2.873 ± 0.342 kg O2/kWh) and CSC (2.470 ± 0.256 kg O2/kWh) aerators. The study clearly indicates that this PPCSC aerator may very well be used as pre-aeration or post-aeration units in water or wastewater treatment plants and small-scale intensive aquacultural ponds, replacing the other existing aerators.

Biological Control of Excessive Phytoplankton Growth and the Enhancement of Aquacultural Production

A method is proposed to control phytoplankton biomass in aquacultural ponds, using both zooplankton and filter-feeding silver carp (Hypophthalmichthys molitrix). The technique maintains co-existence of zooplankton and filter-feeding fish by excluding the fish from part of the water column. Zooplankton, which feed on smaller algal species, and silver carp, which feed on large algae and zooplankton, together can consume all sizes of phytoplankton, thus controlling algal biomass. This technique was tested in 1000-L tanks, some containing channel catfish (Ictalurus punctatus) alone, some both catfish and silver carp, and others catfish and silver carp with a zooplankton refuge. The refuge permitted coexistence of high densities of large zooplankters with the filter-feeding fish. This combination of filter-feeders reduced algal biomass by as much as 99%, increased phytoplankton diversity, and showed a trend toward improved silver carp growth compared with treatments without a refuge. The proposed technique could be applied to both intensive and extensive aquacultural systems.