Modified scheme for one-dimensional BOD-DO models.

An alternative numerical scheme for predicting the BOD variation with time at various successive distances from a wastewater outfall in a stream, is presented in this paper. The presented scheme (for the solution of differential equations incorporating the advection, dispersion, and biochemical decay) removes unnecessary restrictions imposed on the grid size ä(x) and ä(t) in the existing MAD scheme.The concept of numerical dispersion is exploited to model the physical dispersion process, and the presented scheme manifests explicity and stability. The robustness of the presented scheme in real life situations is demonstrated through an illustrative example based on hypothetical but rational and realistic data.

Nutrient contributions by benthal sludge deposits.

Settled solids from effluents discharged into a river system, undergoing decomposition at the river bottom, form an appreciable internal nutrient source for the biological activities in the river system. During the stabilization of benthal deposits, a variety of nutrients are released into the overlying waters. The exchange between sediment and overlying waters is a major component of the nitrogen and phosphorous cycles in the natural waters. The releases of such nutrients is a surface phenomenon, regulated by the conditions of benthal sludge layers, flow rate of overlying waters, etc. The rate of ammonia nitrogen release manifested an optimum low value when benthal sludge depth was 0.2 m, but was not influenced by the flow rate of overlying water and h/d ratios. The rate of phosphate release from benthal sludge was independent of depth of benthal sludge, flow rate and h/d ratios. The nutrients in the benthal sludge layers were increasing with time, and were concentrated at a layer 10 cm below the top surface. The nutrients release (percent of nutrient remaining in top benthal sludge layers) decreased with time and became almost constant after about 40 days. The nutrients release under continuously accumulating conditions of benthal sludge and the effects of frequency of addition have been discussed in this paper. The nutrients release was less when the frequency of addition was less.

Polynomialized model for critical DO deficit.

The existing DO sag models of Streater-Phelps have become obsolete in the present day context of polluted streams in which a part of the BOD removal necessarily takes place through sedimentation. The Streater Phelps models do not consider this aspect. Bhargava's theoretically derived model for the critical DO deficit rests on an almost linear removal of the settleable BOD and an exponential decay of non-settleable BOD. However, the Bhargava's model has two independent but complex expressions, one each for times less than and greater than the transition time. A polynomialized form of Bhargava's models for critical DO deficit has been developed as a single expression and universally applicable without any regard to the transition time. Unlike, the Streater-Phelps or Bhargava models, the presented polynomial form of Bhargava models, for critical DO deficit has an additional advantage of evaluating the critical dissolved oxygen deficit concentrations directly and without first determining the time of occurrence of such a deficit. The material presented would thus add to the exiting literature on the subject.