The flow structure in the separation zone of a DAF pilot plant and the relation with bubble concentration

Abstract

Fluid dynamics is believed to be of crucial importance for the removal mechanism in Dissolved Air Flotation. The paper is a presentation of experimental studies on velocity and air-content profiles in a DAF pilot plant. The correlation between air content and flow patterns is discussed. Two flow structures are detected. A stratified flow structure is defined by a horizontal transport of the water in a upper, less-dense layer, and a lower, downwards-vertical transport in the lower, dense layer. Between the layers a return flow is found. The short-circuiting flow is defined as an immediate downward deviation towards the outlet arrangement at the bottom of the tank, probably causing decreasing effluent quality. Density gradients, caused by differences in air content in the tank, are believed to generate a stratification of the water body, thus causing the water to be transported in layers. Measurements of air-content profiles correlate to the measured flow structures, also indicating a separation of the water body in two layers with a distinct boundary.

A model of particle removal in a dissolved air flotation tank: Importance of stratified flow and bubble size

“…By including the effect of bubble aggregation for the first time in a CFD model of DAF, the model was able to identify conditions under which stratified flow occurs, and to quantitatively predict the benefit of the stratified flow in terms of better bubble removal. The results of the model were in good agreement with the experimental measurements of Lundh et al (2001). The inclusion of only two phases (air and water), however, was a limitation in the model, as it could not be used to directly extrapolate the impact of the stratified flow on particle removaldwhich is the ultimate goal of DAF treatment.…”

“…Table 1 e Particle removal efficiency for varying air fractions and bubble sizes. Loading rate 11.8 m/hr, particle size 10 mm, particle density 1100 kg/m 3 , particle volume fraction 0.0001. w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 2 6 2 e2 7 2 form stratified flow (bubbles are needed to create stratified flow since the stratified flow is caused by flow density differences, as explained in Lundh et al (2001)). In general, therefore, an increase in bubble size and air fraction up to a certain point promotes stratification of the flow, whereas an increase beyond this bubble size and air fraction can lead to the quick removal of all of the bubbles from top of the tank, thereby preventing the formation of stratified flow in the separation zone.…”

“…Some researchers have attributed the discrepancy between the current high surface loading rates and the earlier model predictions to the presence of stratified flow in the separation zone of a DAF tank (Haarhoff and Edzwald, 2004;Edzwald, 2007). Stratified flow, shown in the experimental pilot-scale measurements of Lundh et al (2001), is defined as water traveling in a horizontal layer from the top of the inlet baffle towards the far wall of the separation zone, and then traveling back to the baffle in a horizontal second layer below the first (Fig. 1).…”

Measurement of bubble bed depth in dissolved air flotation using a particle counter

“…Edzwald et al (1999) reported that, as the loading rate increased, the bubble bed (blanket) went deeper, but the relationship between the bubble depth and removal efficiency was not addressed. Lundh et al (2001) tried to measure the bubble concentration in a pilot plant with a turbidity meter, but the exact bubble bed depth could not be measured.…”

Influência da taxa de aplicação superficial e da concentração de ar na eficiência de clarificação de água de abastecimento em unidades de flotação por ar dissolvido

“…Ainda assim existem muitas lacunas a serem preenchidas, sendo consenso na indústria de água a necessidade de mais pesquisas para a otimização do processo de FAD, bem como obtenção de critérios confiáveis de projeto, aumento dos níveis de segurança e qualidade, aumento da eficiência na clarificação, diminuição do tempo de floculação e, ainda, redução dos custos de investimento e operação. Neste sentido, a falta de experiência e de pesquisas acerca das características hidrodinâmicas do processo é notável (LUNDH et. al., 2001).…”