Short-term biochemical oxygen demand (BODst) estimation using an oxygen uptake rate measurement method

The objective of this research was to estimate short-term biochemical oxygen demand (BOD_st) using an oxygen uptake rate (OUR) measurement method. Activated sludge samples were taken from a return sludge pipe in an oxidation ditch at Maharaj Hospital, Nakhon Ratchasima Province, Thailand. Sludge samples were filtered and aerated overnight in the preparing chamber until reaching the endogenous respiration phase, which provided the initial sludge concentration of 5,489 mg/L. Sludge sample concentrations were diluted to 880, 2,160, and 5,297 mg/L. Sodium acetate of 20 mg/mL (13 mg COD/mL) was used as a readily biodegradable COD substrate (rbCOD) and dosed to the sludge sample. Final COD concentration was varied into 4 dilutions; 7.21, 14.40, 28.71 and 57.70 mg/L. Temperature, pH and dissolved oxygen (DO) were controlled throughout the experiment at 28±0.5 °C, 7.8±0.1 and 6-7 mg/L, respectively. DO and OUR values were collected using an ultimate hybrid respirometer, which was developed by Saensing and Kanchanatawee [1]. The results showed that the linear relationship between BODst and COD substrate in all sludge dilutions were high and similar (R² = 0.9891-0.9957). The area under the OUR curve can be used to easily estimate the BOD_st and linearly related with biodegradable organic substances. Average R² throughout the experiment was 0.9940. The results were consistent with the studies of Gernaey et al. [2], Saensing and Kanchanatawee [1] and Premanoch [3] which had average R² values of 0.9990, 0.9952 and 0.9964, respectively. Estimation of BODst from an OUR respirogram can provide rapid results and also give a linear relationship for the amount of organic matter concentration. It was apparent that the OUR measurement method using an ultimate hybrid respirometer was effective for determination of BODst with less chemical and time consumption.