Dewatering waste-activated sludge using dissolved gas-flotation followed by filtration
After the successes of flotation of waste-activated sludge (WAS), using the new technique utilizing CO 2 gas as well as using model gas (80% N 2 +20% CO 2 ) in previous research because of the high water solubility of CO 2 gas, the aim of this study is to develop a simple method for dewatering WAS for easier reuse and safe disposal. The paper introduces a laboratory model for dewatering WAS in two stages: flotation followed by filtration. The first stage enables recycling a mixture of greenhouse gases containing 20% of CO 2 and 80% of N 2 gases by volume. The second stage uses a simple compression cell for dewatering WAS. Experiments were carried out to reduce the moisture content and volume of WAS. This was executed using compression force introduced by a low value of air pressure. Using the experimental dewatering model, promising results were obtained. Furthermore, other data were obtained, such as the effect of temperature on the efficiency of dewaterability. It is hoped that the results of this study will lead to further study of collecting industrial CO 2 gas emissions from burning fossil fuels for use in wastewater treatment to decrease the resulting harmful effects of global warming.
Evaluation of dissolved air flotation process for water clarification and sludge thickening
Dissolved air flotation was evaluated using pilot and bench tests for water clarification and sludge thickening. A DAF pilot study was conducted on one water source with low turbidity, high color, high organic content and algal blooms. For this water, coagulation with alum, cationic polymer and pH around 6.1 resulted in removal of turbidity, color and algae. THMFP and TOC were removed by approximately 25 and 50 percent, respectively. A river water source was used in another pilot study to identify limits on source water turbidity for the DAF process. Tests conducted with induced high turbidity values resulted in filtered water quality with low turbidity and complete removal of color. TOC was removed by approximately 35 percent, however, inconclusive results were obtained for THMFP removal. Based on these pilot test, DAF is a viable clarification process, especially for source waters with low turbidity (infrequent spikes upto 100 NTU), high algal blooms and high color. Bench-scale DAF assisted sludge thickening resulted in performance similar to gravity thickeners. High recycle ratios (around or greater than 100 percent) were required for effective sludge thickening.
Rational design of packed saturators
The paper describes the development of a mathematical model for predicting the mass transfer efficiency of packed saturators used for dissolved air flotation. The first part of the paper deals with the equilibrium condition, i.e. the maximum attainable air concentration with a packed saturator at 100% transfer efficiency. The second part derives a kinetic model based on concentration-driven diffusion and the Lewis-Whitman two-film theory. The transfer rate constant across the laminar liquid film and the wetted packing area are estimated with the Onda correlations. The model is finally used to demonstrate the effects of hydraulic loading, packing depth, packing size and temperature on saturator efficiency.