การบำบัดฟอสฟอรัสในน้ำจากการเลี้ยงกุ้งโดยแบคทีเรียสังเคราะห์แสงสีม่วงที่ไม่สะสมซัลเฟอร์ที่คัดแยกได้จากบ่อเลี้ยงกุ้ง

Abstract

Shrimp cultivation not only produces high income but also causes environmental impacts. One of the key environmental concerns for shrimp cultivation is the discharge of rearing water with high levels of nutrients, especially phosphorus into waterways, resulting in eutrophication. To solve this problem, biological treatment is well recognized for applying; and the use of purple non-sulfur bacteria (PNSB) is one of alternative attractive choices because of their high removal efficiency in wastewater treatment with various metabolic growth conditions. Therefore, this study aimed to investigate water quality of rearing water from shrimp cultivation, to isolate and screen PNSB from shrimp ponds with their ability for reducing phosphorus in water from shrimp cultivation, and to optimize conditions for phosphorus removal. It was found that water quality of rearing water had phosphorus level that was dramatically higher than the standard level for discharge whist other parameters were in the optimal range. A total of 83 PNSB strains were isolated from water and sediment samples collected from various shrimp ponds located on Phang-nga and Songkhla provinces. For primary screening, there were 42 strains (51%) grew well (OD660> 1.0) in glutamate-acetate broth supplemented with 1.5% (w/v) NaCl, under conditions of microaerobic-light and aerobic-dark. However, in secondary screening only two strains (W12 and W48) could grow well in sterile rearing water collected from shrimp ponds. Both strains were selected for tertiary screening to investigate their ability to remove phosphorus in sterile rearing water under both incubating conditions. They produced no significant differences for phosphorus removal efficiency ( 50%) with exception under microaerobic-light conditions as strain W12 roughly reduced 46% phosphorus. These 2 strains had the possibility to be used as inoculants for removal of phosphorus from rearing water in shrimp ponds. The molecular identification using 16s RNA gene found that both selected isolates were Afifella marina W12 and Rhodovulum sulfidophilum W48. The optimization for phosphorus removal using Response Surface Methodology with Central Composite Design by a mixed culture of both selected strains (ratio 1:1) in sterile rearing shrimp water under aerobic-dark conditions for 96 h found that the optimal conditions were 5% inoculum, initial pH 7.5 and 2.5% salinity. Confirmation of the results, non-sterile rearing shrimp water was used under the the optimized conditions at 72 h removals of phosphorus and nitrate by a mixed culture were 28.33 and 49.27%, respectively. On the other hand, native control as no inoculation reduced 22.77% for phosphorus and 48.19% for nitrate. However, no significant difference was found for removal of sCOD (soluble Chemical Oxygen Demand) at 24 h by inoculated PNSB and control sets as roughly 90% reduction (< 10 mg/L sCOD). In contrast, unsuitable conditions for shrimp growth were observed in the native control as higher levels of nitrite > 1 mg/L) and pH. The mixed culture, throughout the experiment, could control pH and nitrite in ranges of standard water quality that suitable for shrimp cultivation, which requires pH value in a range of 7.5 - 8.2 and nitrite level less than 1 mg/L. The removal efficiency of the mixed cultive was high at the 24 h of treatment. Methylene blue and DAPI (4.6-diaminido-2-phenylindole) stains confirmed that the mixed culture removed phosphorus from rearing water and then accumulated in the form of polyphosphate granules as their increasing with longer treating times.