Removal of iron ore slimes from a highly turbid water by DAF.

This paper addresses Dissolved Air Flotation (DAF) process variables, such as the flocculation parameters and the recycle water addition, as well as the pretreatment chemical variables (coagulation conditions), to determine the optimal values for the flotation of iron ore slimes found in a highly turbid water sample from the Gualaxo do Norte River, a tributary of the Doce River Basin in Minas Gerais, Brazil. This work was conducted using a flotatest batch laboratory-scale device to evaluate the effectiveness of DAF for cleaning the water polluted by the Samarco tailings dam leakage and determine the ability of DAF to reduce the water turbidity from 358 NTU to values below 100 NTU, aiming to comply with current legislation. The results showed that the four types of tested coagulants (PAC, ferric chloride, Tanfloc SG and Tanfloc SL) provided adequate conditions for coagulation, flocculation and flotation (in the range of 90-99.6% turbidity reduction). Although the process variables were optimized and low residual turbidity vales were achieved, results revealed that a portion of the flocs settled at the bottom of the flotatest columns, which indicated that the turbidity results represented removal caused by a combination of flotation and sedimentation processes simultaneously.

The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit in the light of a mathematical model.

The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit was investigated in the light of a known mathematical model. The following design and operational parameters were considered: the hydraulic detention time (tdcz) and hydraulic loading rate in the contact zone, the down-flow loading rate in the clarification zone, the particle size distribution (d F), and the recirculation rate (p). As a reference for DAF performance analysis, the proposed ß.td parameter from the above mentioned mathematical model was employed. The results indicated that tdcz is an important factor in DAF performance and that d F and floc size are also determinants of DAF efficiency. Further, ß.td was sensitive to both design and operational parameters, which were varied in the DAF pilot plant. The performance of the DAF unit decreases with increasing ß.td values because a higher td (considering a fixed ß) or a higher ß (e.g., higher hydrophobicity of the flocs for a fixed td) would be necessary in the reaction zone to reach desired flotation efficiency.

Characterization of micro-bubble size distribution and flow configuration in DAF contact zone by a non-intrusive image analysis system and tracer tests.

This paper aims to investigate the influence of some dissolved air flotation (DAF) process variables (specifically: the hydraulic detention time in the contact zone and the supplied dissolved air concentration) and the pH values, as pretreatment chemical variables, on the micro-bubble size distribution (BSD) in a DAF contact zone. This work was carried out in a pilot plant where bubbles were measured by an appropriate non-intrusive image acquisition system. The results show that the obtained diameter ranges were in agreement with values reported in the literature (10-100 microm), quite independently of the investigated conditions. The linear average diameter varied from 20 to 30 microm, or equivalently, the Sauter (d(3,2)) diameter varied from 40 to 50 microm. In all investigated conditions, D(50) was between 75% and 95%. The BSD might present different profile (with a bimodal curve trend), however, when analyzing the volumetric frequency distribution (in some cases with the appearance of peaks in diameters ranging from 90-100 microm). Regarding volumetric frequency analysis, all the investigated parameters can modify the BSD in DAF contact zone after the release point, thus potentially causing changes in DAF kinetics. This finding prompts further research in order to verify the effect of these BSD changes on solid particle removal efficiency by DAF.

Influence of flocculation conditions in the performance of an experimental domestic sewage treatment plant consisting of an anaerobic expanded bed reactor followed by dissolved air flotation.

This paper presents the results of a study performed with an experimental domestic sewage treatment plant (240 m3 x d(-1) flow) consisting of expanded bed anaerobic reactor (EBAR) followed by dissolved air flotation (DAF) unit. For the flotation step, the anaerobic reactor effluent was previously coagulated with 50 mgFeCl3 x l(-1) and flocculated under different conditions (mean velocity gradient, Gf, and flocculation time, Tf). The Gf values were from 60 to 100 s(-1) associated with 13 and 20 min Tf values. During the tests, the following operational conditions of the flotation unit were maintained: chemical addition (50 mgFeCl3 x l(-1)), 18% recirculation rate associated with a pressure of 450 +/- 10 kPa in the saturation chamber and overflow rate of 180 m3 x m(-2) x d(-1). Temperature ranged from 23.8 degrees C to 30.0 degrees C. Best results were achieved for Gf = 80 s(-1) and Tf = 20 min. For these conditions, the DAF unit removal efficiencies were: 94.4% for chemical oxygen demand (with 53 mg x l(-1) COD residual), 87% for phosphorus (with 0.80 mgP x l(-1) residual), 96.7% for total suspended solids (with 9 mg x l(-1) TSS residual) and 96.4% for turbidity (with 12.9 NTU residual), when the anaerobic reactor effluents have worst quality during the whole day.

Sewage treatment by anaerobic biological process associated with dissolved air flotation.

This paper presents the results of a study performed with a lab-scale batch DAF unit fed with previously coagulated (with FeCl3 or cationic polymer) effluent from a pilot scale up-flow anaerobic sludge blanket (UASB) reactor treating domestic sewage. The adequate coagulation/flocculation conditions--chemical dosage, time (Tf) and mean velocity gradient (Gf) in the flocculation step--and air requirements for flotation process were investigated. Best results were achieved for 65 mg.l-1 of FeCl3 at Tf around 15 min and Gf of 80 s-1. In the assays where only polymer was applied, 7 mg.l-1 of cationic polymer dosage gave optimum removals with Tf around 15 min and Gf of 30 s-1. Air requirements ranged from 9.5 to 19.0 g of air.m-3 wastewater. Best TSS (95% and residual of 2 mg.l-1), COD (85% and residual of 20 mg.l-1) and total phosphate (95% and residual of 0.6 mg.l-1) removals were obtained when applying FeCl3, although the use of cationic polymer also produced good level of TSS (74% and residual of 14 mg.l-1) and COD (75% and residual of 45 mg.l-1) removals. For the UASB-DAF (batch) system and FeCl3, global efficiencies would be 97.2% for COD, 97.9% for phosphate and 98.9% for TSS.

Ozonation followed by coagulation/flocculation and flotation as post-treatment of the effluent from an anaerobic baffled reactor treating domestic sewage.

This paper reports on the use of ozonation and dissolved air flotation as a post-treatment of the effluent from an anaerobic baffled reactor treating domestic sewage. After preliminary essays, the present experiment was performed fixing coagulant doses and, to all of them, some ozone doses were investigated. Later, the pH value and the ozone dose which provided the best removal efficiencies of all the parameters involved were tested, changing the coagulant dose and varying, for each of them, two ozone doses: the zero one and the optimum. Considering the best conditions of coagulation/flotation (ferric chloride dose of 65 mg.L-1 and pH around 5.5), the ozone dose application of 6 mg.L-1 led to a significant level in the removal efficiency of COD (80.4%), BOD (79.0%), total phosphate (93.4%), apparent color (91.9%) and turbidity (97.0%), demonstrating that the system seems to be efficient and capable of promoting a high degree of sewage post-treatment, reducing the coagulant dose until 30%, with a consequent reduction in the sludge generation.