Measurement of the sediment oxygen demand in selected stations of the Pasig river using a bench-scale benthic respirometer

• Main Author: Belo, Lawrence Phoa
• Format: text
• Language: English
• Published: Animo Repository 2008
• Subjects: River sediments—Quality; Environmental Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/5876; https://animorepository.dlsu.edu.ph/context/etd_masteral/article/12736/viewcontent/CDTG004438_F_Partial.pdf
• Institution: De La Salle University

Sediment oxygen demand (SOD) is the rate at which dissolved oxygen is removed from the water column in surface waters mainly due to the respiration of benthic organisms and decomposition of organic matter in the riverbed or bottom sediments. Several studies have shown that SOD can contribute from 30 to 90% of the total oxygen uptake especially in shallow and slow-moving waters. In a slow moving river with highly organic sediment like the Pasig River, SOD can be a major cause for the constantly low dissolved oxygen (DO) concentrations in the water column. However, despite its potential consequences on the water’s DO level, nothing has been done in the Philippines to investigate the possible effects of SOD. This study focused on the measurement of the oxygen demand exerted by the riverbed sediments on the overlying water. The reconnaissance was conducted late December to early March which is the local dry season in the Philippines, where the flowrate becomes relatively low and water quality becomes worse. Three stations namely, Marikina (upstream), Sanchez (middle-stream) and Jones (downstream) were selected from among 9 stations along the stretch of the Pasig River. For the laboratory measurement of SOD, benthic respirometers were fabricated to isolate a known volume of water to be recirculated over a known area of collected river sediments. Changes in DO concentrations were monitored using DO probes and were recorded. The SOD obtained were then compared against water quality parameters temperature and TSS and sediment quality parameters (grain characteristics and organic content) while varying the water flowrate and amount of sediment contained within the chambers. Measured SOD25 rates observed in the study ranged from a low 0.04 g/m2day in Marikina Station to a high 9.53 g/m2day for Sanchez station. Marikina station averaged at 0.80 g/m2d, Jones station at 3.0 g/m2d and Sanchez station at 2.97 g/m2d. The observed impact of SOD was significant and accounted for as low as 22% to as high as 85% of the total oxygen demands of Marikina and Jones stations, respectively. Positive correlations were observed between SOD and flowrate, organic content and total suspended solids and negative but minimal correlation between SOD and sediment depth (amount of isolated sediment). A notable increase in measured SOD was observed from the low flow to the high flow. When the flowrates were doubled, an SOD increase of 29%, 18% and 7% were observed from Marikina, Sanchez and Jones, respectively. Whereas when the flowrates were tripled, an increase in measured SOD of 17% in Marikina, 124% in Sanchez and 36% in Jones were observed. Organic content of sediment was found to highly influence exerted SOD with an overall correlation of 0.6374 (n = 81) with Marikina station exerting the lowest SOD and Sanchez exerting the highest SOD, as expected from the quality of their sediments. Sediment type was also found to highly influence SOD wherein measured SOD from sandy sediments are less (Marikina SOD = 0.58 ± 0.44 g/m2d) than those measured from muddy or sandy-silty sediments (Jones SOD = 2.19 ± 0.77g/m2d and Sanchez = 2.15 ± 1.53 g/m2d). Temperature differences of 1 to 20C within the same season were found to be insignificant with the measured SOD. The effect of algal respiration on the overall SOD was found to be significant; decreasing the effective SOD to half its original value. Several factors affect the variability in measuring SOD. Regression analysis was done in order to isolate the significant parameters / variables directly affecting SOD. Correlation, regression analysis and ANOVA were then done to the principal variables (S = TSS, %C = organic content and Q = flowrate) left and lead to an empirical SOD equation: SODT = 0.035(S) - 27.9973(%C) + 0.050348(Q) - 2.55263the SOD empirical equation was found useful in estimating the SOD of the stations although errors measured from as low as 0% to as high as 177% in measured SOD. The high variability between the predicted and the measured SOD were attributed other variables such as algal and biological population, BOD and COD, and seasonal variability.