Water quality in Imbang river, Negros Occidental: effluents and pollutant loads from agriculture, sugar mills, households, and shrimp farms
Share
Abstract
An ecological assessment of Imbang River in Negros Occidental was undertaken from December 1992 to February 1995. The effluents from sugar mills, households, shrimp farms, sugarcane plantations and rice fields were characterized and their pollutant loads estimated. Water quality and invertebrate assemblages were analyzed at several sites along the river to determine the environmental status. Results showed significant seasonal and site variations in water quality along Imbang River. The dry season, coinciding with the milling season, was the more critical time of the year as water quality tended to deteriorate. The segments of the river near the sugar mills and households had the poorest water quality. Sugar mill effluents had high water temperature (average 33oC but as high as 50oC), low dissolved oxygen, high total solids, the highest settleable solids (average 2.5 and as high as 17 m/l), and the highest biochemical oxygen demand (average 259 ppm but as high as 14,800 ppm BOD). Domestic effluents had low pH, high ammonia, very high BOD, plus detergents or surfactants and high levels of fecal coliform bacteria. Agricultural runoff had high nitrate, high total solids, and the highest total suspended solids (average 296 ppm but as high as 5,095 ppm TSS). Shrimp ponds used saline water of average 23 ppt, and had the highest total solids (average 23,456 ppm and as high as 57,400 ppm). By far the major contributor of pollutant loads into Imbang River was agriculture, due to its huge areal extent and huge volume of water use and run-off. Agricultural run-off carried the highest annual loads of 7,858 kg phosphate; 6,495 kg ammonia; 794 kg nitrite; 67,212 kg nitrate; 16,987 metric tons settleable solids; 16,800,000 mt total solids, and 11,890,000 mt total suspended solids; but only 297 mt BOD. Sugar mill effluents had the highest BOD load (1,583 mt/yr) and also had high nutrient loads. Household effluents contributed the second largest loads of solids next to agriculture, and also added surfactants (966 kg/yr) and fecal coliforms into the river. The six shrimp farms at the lower reaches of Imbang River were a minor contributor of pollutants into the river, annually adding about 891 kg ammonia; 1,077 kg phosphate; and 181,325 mt total solids.
Suggested Citation
Gonzales, G. A., Gonzales, H. J., Sanares, R. C., & Taberna, E. T. (2008). Water quality in Imbang river, Negros Occidental: effluents and pollutant loads from agriculture, sugar mills, households, and shrimp farms. In T. U. Bagarinao (Ed.), Research Output of the Fisheries Sector Program (Vol. 2. Reports on Fisheries and Aquaculture, pp. 35-51). Quezon City, Philippines: Bureau of Agricultural Research, Department of Agriculture.
Type
Book chapterISBN
9718511776Related items
Showing items related by title, author, creator and subject.
-
Sugar mill effluents and water quality in Imbang River and Malisbog River, Negros Occidental
(Bureau of Agricultural Research, Department of Agriculture, 2007)The effluents of two sugar mills and the effects on water quality in the receiving rivers were studied. Sugar mill A was located in Barangay Dos Hermanas in Talisay and discharged directly into Imbang River. Sugar Mill B was located in Barangay Hawaiian in Silay City and discharged into Malisbog River, a tributary of Imbang. Both sugar mills had sedimentation tanks and lagoons for partial wastewater treatment prior to discharge. Water sampling was done at 13 stations at effluent discharge sites and also upstream and downstream of these sites. The sugar mill effluents were particularly high in biochemical oxygen demand (BOD 109–419 mg/l), total suspended solids (168–384 mg/l), and total solids (1,185–1,234 mg/l), also high in ammonia (0.2–0.5 mg/l) and water temperature (31–38°C), but low in dissolved oxygen (2–5 mg/l). Measured stream flows varied at the different stations and were generally lowest at stations near sugar mill A and at stations near sugar mill B. At these sites, the depth of Imbang River varied from about 10 cm during low water flow in December–May to about 2 m during high water flow in June–November. During normal low flows, the sugar mill effluent comprised 75–85% of the total stream volume, causing highly polluted conditions immediately below the outlets. Sugar mill A discharged high annual loads of solids, BOD, nitrate, and phosphate into Imbang River, whereas sugar mill B loaded plenty of solids, BOD, ammonia, and phosphate into Malisbog River. The sugar cane milling season in Negros Occidental started in October and ended in May, coincident with the dry season. Significantly higher levels of BOD and nutrients, but lower DO, were observed in the river during the milling season (see figures in Gonzales et al., this volume), both because of greater discharge and lower dilution by lower stream flows. River water quality was better at the stations upstream than downstream of the sugar mills. Stations near the sugar mills had BOD, ammonia, and solids at concentrations exceeding the allowable limits set for river water by the Department of Environment and Natural Resources. -
Households, agriculture, industry, fishing, and fish farming along Imbang River, Negros Occidental
(Bureau of Agricultural Research, Department of Agriculture, 2007)Interviews were conducted among respondents identified from the households, agriculture farms, sugar mills, and fish farms along the whole stretch of Imbang River, Malisbog River, and Muyao Creek, down to Barangay Balaring at the coast of Silay City in Negros Occidental. Among the 1,073 households, 11% used river water for washing clothes, but 20% also used the rivers for disposal of waste waters, 11% for human wastes, and 13% for animal wastes. Among the 30 respondents from the agriculture sector, 70% discharged water into the river. The two sugar mills in the area treated waste waters partially before release into the rivers; one sugar mill also released wastes in a nearby rice field. Milling wastes such as bagasse, molasses, and mud press were reused and not dumped into the river. Imbang River was both the water source and wastewater sink for seven fish farms. -
Shrimp pond effluents and water quality in Imbang River, Negros Occidental
(Bureau of Agricultural Research, Department of Agriculture, 2007)The contribution of shrimp farm effluents to the pollution load in Imbang River, Negros Occidental was measured during the period May 1993 to February 1995. Shrimp pond effluents were characterized and the pollution load estimated. The pond effluents had low average nitrite (0.0025 ppm) and nitrate (0.06 ppm) and optimum (for shrimp culture) pH 7.9, phosphate 0.15 ppm, dissolved oxygen 5.20 ppm, and salinity 23.3 ppt. Ammonia was 0.13 ppm on average in most farms, above the safe level for shrimp, and total suspended solids was 23 ppm, about 2.5x the allowed limit for effluents. Biochemical oxygen demand (20 ppm) and settleable solids (0.15 ppm) were still with acceptable limits. Residues of organochlorine pesticides were present at very low concentrations, well below the safe levels for aquatic life. Most of the pollution load came from the regular water exchanges over the 4-month crop cycle, at least every two weeks in low-density farms and more frequently in the high-density farms. The total draining of pond water at harvest contributed a minor load. Total solids from shrimp farms contributed a huge load, about 181,325 mt/yr. Total suspended solids contributed 1,285 mt/yr and settleable solids <1 mt/yr. The total BOD load was 154,367 kg/yr. The phosphate load was about 1,080 kg/yr, and the total nitrogen load was 1,225 kg/yr. The effects of effluent release from farms were localized. Upstream water quality and other uses of the river were not affected. Since most of the shrimp farms were located 1.5–2 km from the sea, the release of effluents during water exchange and at harvest did not adversely affect water quality downstream of these farms. Where such draining increased the levels of ammonia, phosphate, and total suspended solids in the river, the effect was significant only within 250 m from the release point, and the pollutants were dissipated about 550–800 m downstream The other water quality variables were at low levels in the pond effluents and did not affect the river water during draining. Often the concentrations of pollutants in the river were higher before than during draining of pond effluents. Stations upstream of the release sites of pond effluents often had high pollutant concentrations from other upstream sources.
