Abstract:
The objectives of the study was to examine the self-purification capacity of the Rangtubtap Canal through і) calibrating recovery coefficient (k) of parameter obtained in field and laboratory conditions; іі) validating field recovery coefficient and ііі) developing a relative equation between field and laboratory conditions. The study was carried out during the stagnant water period of the Rangtubtap Canal in November
2004. The water temperature, pH, salinity, conductivity, dissolved oxygen (DO), biochemical oxygen demand (BOD5) were collected everyday, except total coliform bacteria (TCB) were collected every other day. The k values of DO and BOD5 are determined using a first order differential equation of Streeter and Phelps by Least Square Method. For TCB, a first order differential equation of Chick is used for k determination. Water qualities of the Rangtubtap Canal during the study period were in the class 4 of the surface water quality standard of Thailand. It was found that the retention times of DO, BOD5 and TCB were in linear relation in both field and laboratory conditions. The k values of each parameter under field conditions varied in the range from 0.003 - 0.010, 0.073 - 0.137 and 0.327 - 0.374 per day for DO, BOD5 and TCB, respectively. Under laboratory condition, the k values were from 0.006 - 0.021, 0.058 - 0.071 and 0.313 - 0.384 per day for DO, BOD5 and TCB, respectively. Thus the self-purification capacity of the water sample under laboratory conditions was higher than that under field conditions. When using k of DO, BOD5 and TCB obtained from field conditions to predict these three parameters, the error values varied from 7.51 - 32.80 %, 29.60 - 72.90 % and 77.84 - 80.74 % for DO, BOD5 and TCB, respectively. The values of DO, BOD5 and TCB under field and laboratory conditions are positively correlated with R = 0.760, P < 0.01; R = 0.847, P < 0.01 and
R = 0.993, P < 0.01, respectively. For the k values of DO and TCB are positively correlated with R = 0.815, P < 0.05 and R = 0.680, P < 0.05, respectively. However, the k value of BOD5 is positively correlate but is not significantly different with R = 0.305, P = 0.555. From the results of the study, the k values of DO, BOD5 and TCB in the Rangtubtap Canal can be applied for determining self-purification capacity in monitoring programs and water quality surveillance for the rivers in Thailand.