Scientists at the Tennessee Valley Authority (TVA), and in collaboration with the U.S. Environmental Protection Agency (EPA), are continuing to develop and refine an innovative wastewater treatment system referred to as reciprocating subsurface-flow constructed wetlands. Reciprocation relates to patented improvements in the design and operation of paired subsurface-flow constructed wetlands, such that contiguous cells are filled and drained on a frequent and recurrent basis. This operating technique turns the entire wetland system into a fixed-film biological reactor, in which it is possible to control redox potential in alternating aerobic and anaerobic zones. Reciprocating systems enable manipulation of wastewater treatment functions by controlling such parameters as hydraulic retention time, frequency of reciprocation, reciprocation cycle time, depth of reciprocation, and size and composition of substrate. These improved wetland technologies have been used for treating municipal/domestic wastewater, high strength animal wastewater, and mixed wastewater streams containing acids, recalcitrant compounds, solvents, antifreeze compounds, heavy metals, explosives, and fertilizer nutrients. Results from selected treatability studies and field demonstrations will be summarized with respect to conceptual design and treatment efficacy.

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Muscle Shoals, Alabama 35660, USA Growth and biomass accumualtion of selected nutrients and trace metals were monitored for six species of aquatic macrophytes during June, August and November, 1993. Plant species were cultivated in two polyculture treatments, each replicated three times. Polyculture I consisted of Scirpus acutus (hardstem bullrush), Phragmites communis (common reed), and PhaZaris mmiinaceu (canary grass). Polyculture II consisted of Trpha spp. (cattail), Scirpus atrovirens (green bullrush), and Scirpus cyperims (wool grass).Each of the six cells ( 6 x 9 x 0.6 m), was operated as a gravel-substrate, subsurface-flow wetlands in a continuous recirculating mode. At six week intervals, macro, micro and trace elements were dissolved and added to the sump of the recirculating system . On each of three sampling dates, replicate shoot and root samples were collected, segregated by species and tissue type (roots, rhizomes, stems and leaves), and prepared for gravimetric biomass estimates and chemical analysis. Tissue specific concentrations of N, P, K, Ca, Mg, Fe, Mn, Zn and Cu. were determined on each date for each species and tissue type. Results will be discussed with respect to species specific growth rates, biomass accumulation, and seasonal uptake and translocation of plant nutrients. KEYWORDSAquatic macrophytes; biomass; constructed wetlands; heavy metals; mesocosm; nutrients; root to shoot rat io; translocation; NTRODUCTION Emergent aquatic plants help to regulate water quality and concentrations of nutrients and other dissolved compounds in natural and constructed wetlands.Nutrient regulation is via plant uptake, biomass accumulation and modification of the rhizosphere to facilitate microbially mediated oxidation-reduction reactions , Brix 1993, Armstrong et ai. 1990, and Gersberg et al. 1986). Although uptake and storage of nutrients by aquatic macrophytes is relatively minor compared to storage and retention in wetland soils (Johnston 1991), their ability to assimilate and transform dissolved nutrients and other substances into stable organic compounds can result in significant improvements in the quality of surface waters. According to Reddy and DeBusk (1987), desirable traits of emergent macrophytes for waste water treatment include rapid growth, high biomass potential and the ability to assimilate and store nutrients for prolonged periods of time. Extensive data bases have been compiled which provide information related to rates of nutrient uptake, tissue specific nutrient concentrations and nutrient storage capacity for many of the floating and emergent macrophytes (Johnson 1991, Reddy andDebusk 1985). However, much of the information is derived from surface flow and natural wetland studies conducted at different locations, during different seasons and under widely differing environmental condition. In a recent technology assessment of subsurface flow constructed wetlands it was noted that there is a critical need for evaluating wetland plants other than cattails reeds and rushes (Reed 1993

Manganese (Mn) is a difficult metal to remove in acid mine drainage because of high pH requirements for oxidation of Mn to form Mn oxide precipitates. However, Mn removal can be quickened in a gravel system providing a large surface area and high pH for Mn oxidation to occur with microbiological mediation. An experiment was designed with 12 gravel bed mesocosms that were 9.8 m3 × 6.7 m3 × 0.6 m3 to determine the best design criteria for optimizing Mn removal. Treatments consisted of two Mn loading rates (1.1 and 2.7 g/m2·d) and two gravel types (limestone and river gravel) and were replicated three times.
Water flowed through the experimental wetlands for 734 days. Manganese removal was more effective in limestone than in river gravel. Manganese removal was not affected by water temperature ranging from 5 to 30 °C in either rock material. Manganese removal rates ranged from 100 to 600 m/a in the limestone wetlands and 10 to 60 m/a in the river gravel wetlands. Greater pH in limestone (approximately 6.9) compared with river gravel (approximately 5.5) favored Mn oxide precipitation. Greater pH, coupled with oxidation–reduction potential (redox) values greater than 430 mV in the limestone, resulted in water chemistry near conditions predicting manganite to be the dominant Mn phase. Ideal pH and redox conditions for Mn removal are pH from 6.8 to 7.2 and redox greater than 500 mV.
The range of dissolved oxygen (DO) required to remove Mn in the various wetlands ranged from 3 to 5 mg/L, with approximately one‐half of the DO loss caused by Mn hydroxide formation and one‐half ascribed to biological consumption. The influent DO should be at least 0.35 mg/L for every 1 mg/L Mn removed. Removal rates for Mn ranged from 1 to 17 g/m2·d in limestone and from 1 to 2 g/m2·d in river gravel. Limestone is the material of choice for subsurface flow wetlands for Mn removal. Removal rates and required chemistry determined in this study can be used to design subsurface flow wetlands for optimum Mn removal.

Two‐year‐old blue tilapia, Oreochromis aureus (Steindachner), were stocked into five replicate breeding hapas (1·2 m × 1·2 m × 2·7 m) at a density of six males and six females per hapa. At stocking, males and females averaged 415 and 225g respectively. During the 50‐day trial (July‐August), 62900 seed (eggs and sac‐fry) representing 78 spawns, were collected from 30 females. In comparative treatments, prestocking surgical removal of the male's maxillary bone did not influence hatchery productivity. However, removal of the female's maxillary bone significantly reduced frequency of repeat spawning and clutch size (P < 0·05). Removal of the maxillary bone also influenced weight gain of broodstock. It is hypothesized that these significant responses were due to the surgical removal of the maxillary bone and the subsequent breakdown in dominance hierarchies.