The prediction of water quality in terms of variables like dissolved oxygen (DO), biochemical oxygen demand (BOD), pH value, total dissolved solids (TDS) and salinity etc. is useful for evaluating the use of water for various related purposes. The widely used Streeter and Phelps models for computing biochemical oxygen demand and its impact on dissolved oxygen do not account for the settleable component of BOD and related implications. The model also does not account for the impact of storage zone on the stream's DO. In the present work an attempt is made to develop a model which simultaneously accounts for the settleable component of BOD and the effect of storage zones on river's DO. An application of the model to real field data suggests that the cumulative impact of settleable BOD and presence of storage zone in the river is to shift the critical deficit closer to the point source and magnify its amount.

Mathematical models have often been accepted as a useful tool to assess and manage water quality in water bodies. Dissolved Oxygen (DO), a surrogate variable for the general health of an aquatic eco-system and Biochemical Oxygen Demand (BOD) are two important parameters to assess the level of pollution in river/stream. Various models developed to date accounts for only that portion of BOD which is in dissolved form and not the least in settleable form. These models also do not account for the storage zone in a scattered way in rivers and hence do not represents the actual situation caused by the discharge of partially treated/ untreated waste water, which contains a significant portion of BOD in settleable form, into the water body with a large width where the water becomes stagnant anywhere, due to rag/garbage in rivers. The present work represents a model to predict the concentration of total BOD when partially treated/untreated waste water is discharged into the river having staggered storage zones and thus address the above stated situation. Nitash Kaushik et al