Divergence between respirometry and physicochemical methods in the fractionation of the chemical oxygen demand in municipal wastewater.

For activated sludge modeling purposes, the methods used to evaluate the readily biodegradable chemical oxygen demand (RBCOD) in the influents are by biological or via physicochemical assays. However, there has not been sufficient wide comparison between these methods. The main goal of this study was to investigate the performance of the main chemical oxygen demand (COD) fractionation protocols, considering a representative wastewater in the context of tropical and developing countries. Different physicochemical characterization procedures, respirometric tests, and chemical analyses were performed. The fate of the soluble COD in the aeration tanks was studied. The results of the study showed that a marked difference may exist, in municipal wastewaters, between the estimates of the RBCOD fractions measured by respirometry and by any of the physicochemical methods. The evaluated influent showed a rather large fraction of COD that was passing the filters without being rapidly biodegradable, but which was removed quickly by enmeshment in the bioflocs. The consequences of such divergences and behavior are discussed.

Full activated sludge model no. 1 calibration experience at a medium-size WWTP in Mexico.

As part of the efforts done to introduce the practice of modeling in Latin America, this research carried out at the North-East WWTP of Monterrey represents the first comprehensive modeling case in Mexico. The main objective was to reproduce the organic carbon removal and sludge production rates of the plant, based on ASM1. Different intermediate studies were performed prior to the calibration of the model: influent characterization, tracer tests and hydraulics modeling, sludge settling tests and respirometry. Two fractionation methods (STOWA protocol and Influent-advisor) were compared, showing no equivalent patterns. A stepwise sequence of calibration was developed and successfully applied. The hydraulics of the reactors at the plant was reproduced by use of a series of 3 to 5 CSTRs. The waste and return activated sludge flowrates (Q(WAS) and Q(RAS)) were corrected based on the inorganic and total suspended solids mass balances. The Vesilind settling constants were measured (V(o) and r(hind)), while the flocculent zone settling parameter (r(floc)) was adjusted to calibrate the secondary clarifier. In ASM1, the adjusted parameters were the COD soluble inert fraction (frS(I)) and the particulate substrate fraction (FrX(S)). All other ASM1 parameters were kept at their default values. The steady-state calibrated model (in GPS-X) adequately described the quality of the effluent (carbon and nitrogen) as well as the sludge composition (M. Liquor and WAS). This case study provides voluntarily detailed data to allow its wide use for training and teaching purposes.

Design of a tracer test experience and dynamic calibration of the hydraulic model for a full-scale wastewater treatment plant by use of AQUASIM.

The setup of the hydraulic model structure of wastewater treatment plants (WWTPs) is an important step in the calibration of activated sludge models. The hydrodynamics of a full-scale municipal WWTP (Monterrey, Mexico) has been studied by means of the use of tracer tests and of a commercial simulator. A presimulation approach allowed the authors to quantify the appropriate rhodamine mass, set up a sampling plan, and evaluate the anticipated visual effect of the tracer test in the receiving river. The hydraulic behavior of the aeration tank for the first treatment line, a 7-cell plug-flow reactor, was shown to be best represented by 5 virtual mixed-tanks-in-series. The second treatment line, which included a vertical loop reactor (VLR), was best modeled as 3 tanks-in-series. The VLR, alone, was shown to be similar to a continuously stirred tank reactor, and not a circuit of tanks, as generally used to represent oxidation ditch reactors.