Ability to remove nutrients and organic load from the water of intensive cultivation of marine shrimp by means of static sedimentation tanks with and without the presence of the halophyte grass Spartina alterniflora Loisel
The alteration of the water quality of shrimp crops throughout the fattening period can provide unfavorable conditions for development, or even pathologies to animals, due to the elevated concentrations of nitrogenous and phosphate compounds, suspended and dissolved organic matter, in addition to accumulation of microorganisms. Brazilian environmental legislation provides that carcinoculture projects in the coastal zone have sedimentation systems, a simple and low-cost technological option for maintaining crop water quality and effluent control. Water treatment systems with rooted plants can be integrated with sedimentation systems for greater efficiency in the purification and generation of by-products from effluents.This work evaluated the capacity of removing nutrients and the organic load from the water of an intensive cultivation of marine shrimp Litopenaeus vannamei by means of Static Sedimentation Tanks with (TSEV) and without (TSE) the presence of floating rafts with Spartina halophyte grass. alterniflora Loisel. Three identical tanks of each type were operated. Between February / March 2012, 3 consecutive incubation cycles (C1, C2 and C3) of 500L of saline water from shrimp culture were carried out. During each incubation, water analyzes were performed at 4 different retention times (0, 24, 48 and 72 hours) to quantify the levels of total ammoniacal nitrogen (NAT), nitrite, nitrate and phosphate, in addition to the organic load, estimated by suspended particulate matter (MPS) and chlorophyll content a.The temporal variation of treatment systems between cycles and times of nutrient retention, organic load and monitored physical-chemical parameters was analyzed through Analysis of Variance. C1 showed a higher initial organic load in the water (TSE and TSEV averages; MPS = 212-273 mg / L, chlorophyll a = 372-469μg / L) and a fast phosphate regeneration in both types of tanks (average increase of 1376% after 72h), which was associated with the removal of MPS and chlorophyll a from the water column and mineralization / desorption of phosphorus in the sedimented material near the bottom. Under lower average organic loads (MPS <182 mg / L, chlorophyll at <186 μg / L), C2 and C3 showed average removal rates for phosphate of up to 18%. A significant negative correlation was detected between the levels of nitrate and nitrite (Spearman's correlation rs = -0.46).In C3, the average concentration of nitrate dropped continuously and the vii of nitrite increased continuously over the 72 h of retention reaching, respectively, a removal of 31% and an enrichment of 251% of the contents in the water. Both the nitrate denitrification process and nitrification (conversion of ammonia to nitrite) seem to be involved in the temporal variation of these nutrients. In all incubation cycles, nitrate concentrations in TSEV were 8-19% lower than in TSE, possibly due to the peripheral community attached to the roots of S. alterniflora. The mean values of NAT fluctuated daily in all incubation cycles, but were less than 15 μg / L, a common feature in biofloc culture systems where high rates of ammonium absorption by bacteria and phytoplankton occur.Both sedimentation tanks tested provided a reduction in the organic load, making the water with characteristics closer to those required by environmental standards and would make it possible to minimize the impacts on aquatic environments around the shrimp farming enterprises. The results found in this work demonstrate that the capacity of removing or releasing compounds from the water coming from intensive carcinoculture nurseries by static treatment systems can vary depending on the characteristics of the culture water. The occurrence of few differences in the treatment of organic load and nutrients between TSE and TSEV was attributed to the depleted growth of S. alterniflora, requiring further research in the development of floating support structures and the acclimatization of this halophyte to hydroponics.