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.
A basic study on vacuum flotation of waste activated sludge
A vacuum flotation of waste activated sludge was investigated experimentally. The results showed almost equal or higher flotation efficiency compared with usual dissolved-air flotation. The growth rate of separated liquor zone was approximated by the equation that was similar to Roberts' equation of batch settling. Some additional treatments, such as, thermal treatment, mixing with water and/or dissolution of carbon dioxide enhanced the flotation rate. In addition, the small amount of air was sufficient to raise the sludge, since minute air bubbles were made under a partial vacuum condition. Furthermore, the small space in the flotation cell saved a great amount of energy to obtain the vacuum pressure. These results show that the vacuum flotation is promising process for practical use.
The mechanism of dissolved air flotation for potable water: basic analysis and a proposal
The principal mechanism of flotation of suspended solids in natural waters is by collision of free micro-bubbles with flocs and attachment at hydrophobic sites. Such sites are formed by adsorption of naturally-occurring organic compounds on to the precipitated alumina (or ferric “hydroxide”) derived from the coagulant added. 
 
Although effective, the current flotation process uses supersaturated water inefficiently, mainly because the average bubble size obtained is above the optimum. Unfortunately, no method is known for generating a dense cloud of micro-bubbles (< 50 μm) without adding surfactants. 
 
A more economical procedure might be based on mixing bubble-free super-saturated water with the chemically treated water, followed by conditioning in a flocculator under moderate shear. Under these conditions bubbles grow from nuclei within the flocs, which can subsequently be floated in a quiescent tank. Bench tests indicated that the method is feasible, the growth time of the bubbles being comparable with that needed for good flocculation.