Dissolved Air Flotation (DAF) operational parameters and limitations for wastewaters treatment with cost study

Industrial mineral oil wastewater from oil refineries and petrochemical processing poses a major environmental concern. Effluents from these processes is usually poor as it is heavily polluted, thus have high chemical oxygen demand (COD), soap oil and grease (SOG), turbidity, total suspended solids (TSS) amongst others. This wastewater, if discharge without treatment, causes severe pollution, oxygen depletion, and imbalanced ecosystem and human health risks. The main aim of this research was to modify, optimise and evaluate the performance of a continuous process using dissolved air flotation (DAF) pilot to treat wastewater from a local South African oil refinery wastewater treatment plant (WWTP) with the benefit of recovery of the oil from the wastewater. The study evaluated the feasibility of using different acids and coagulants. One factor at time (OFAT) approach was used on the DAF jar tester to identify the most important variables that affects the DAF treatability performance. The factors considered were; pH, flotation time, coagulant dosage, air to water ratio and air saturated pressure. The ranges considered for the factors were pH (4−6), flotation time (5−15 minutes), coagulant dosage (10−50 mg/L), air to water ratio (5–15%) and air saturated pressure (300–500 kPa). The key process operating parameters obtained from the OFAT were optimised using the Box Behnken design (BBD) adapted from response surface methodology (RSM). The BBD used had three levels, three factors and five centre points. This was employed to establish the relationship that existed between the water quality (contaminants) and the key interacting factors of the DAF jar tester, thus employing the most applicable combination of the factors on a continuous DAF pilot plant. The study was configured into two; Acid – Coagulation-DAF (pre-treatment) and Acid –DAF – Coagulation (post treatment). Three acids were investigated for their efficiency in the pre- treatment step, while four cationic inorganic coagulants and three polymeric organic coagulants were used both for the pre and post treatments. The OFAT experiments resulted in more than 75% removal efficiency of COD, SOG, TSS and turbidity. The removal efficiency was obtained at the following optimum values of pH 5, flotation time of 15 minutes at a coagulant dosage of 50 mg/L and an air to water ratio of 10% and finally, air saturated pressure was 350 kPa. On the other hand, BBD results showed 85% treatability performance at a lower coagulant dosage (30–45 mg/L), moderate air saturator pressure (300–425 kPa), and air-water ratio (8–12%) on the batch scale. While on the continuous process, the optimum coagulant dosage was around 100–180 mg/L. From the BBD results, the interacted factors for consideration were the air saturated pressure and coagulant dosage. These factors enhanced process control. The validation of all the response quadratic models were in good standing with the analysis of variance (ANOVA). The experimental results and the predicted models results agreed at 95% confidence level, finally, the models were significant and verified. Comparative studies of the pre and post treatment showed that 1 M H3PO4 was the most effective, economical and environmentally friendly acid to be used for both processes. Two cationic inorganic (alum and ferric chloride) and two polymeric organic (Z553D-PAC and Zetag32-FS/A50) coagulants were found to be effective with remarkable performance to destabilise and neutralise the oil droplets to coalesce larger flocs to enhance the oil-water separation. Far and above, the cationic inorganic coagulants were more cost effective than the polymeric organic coagulants, even though, the inorganic coagulants were cheaper they had higher conductivity (salts), thus raising environmental concerns. In conclusion, the pre-treatment of the DAF process yielded more recovery of water and oil, and hence this step was economically viable. The RSM demonstrated to be more effective and reliable in finding the optimal conditions of the DAF process than the OFAT method. Thus, the RSM offered a better option than the OFAT, because it included both the interactional and individual factors.

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 β.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, β.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 β.td values because a higher td (considering a fixed β) or a higher β (e.g., higher hydrophobicity of the flocs for a fixed td) would be necessary in the reaction zone to reach desired flotation efficiency.

Enhanced flotation technology using low-density microhollow beads to remove algae from a drinking water source

The fact that dissolved air flotation (DAF) (with 3–5 min of detention time) can replace sedimentation (with 2–3 h of detention time) for clarification has been overlooked by environmental engineers for many decades. Modern high-rate DAF clarifiers have advanced to such an extent that they could overshadow the conventional settling clarifiers in wastewater treatment. DAF hydraulic loadings increased from 1 to 2.5 L/m2/s and for a triple stacked unit to 7.5 L/m2/s; the detention time decreased from 30 to 3 min; air dissolving is improved and now requires only 10-s retention time in an air dissolving tube instead of the previous 60 s. This chapter discusses the field application of the DAF process for primary wastewater clarification, secondary flotation of aeration tank mixed liquor, and the design and operation parameters for a two-stage DAF operation.

The first wave of evolutionary development of dissolved air flotation (DAF) is its application for drinking water treatment. Today, DAF has been adopted internationally including the world’s largest city, New York, for highly efficient and cost-effective potable water treatment. This book chapter introduces the DAF’s second wave of evolutionary development in wastewater treatment applications, such as primary flotation clarification, secondary flotation clarification, total physicochemical DAF wastewater treatment, multistage municipal wastewater treatment, tertiary wastewater treatment, combined municipal and industrial wastewater treatment, DAF process equipment development, and DAF process system development. Specific coverage of this publication includes conceptual development of independent physicochemical wastewater treatment systems (IPCWWTS), process glossary, commercial development process equipment, laboratory process evaluation, chemical optimization, pilot plant installation, biochemical oxygen demand (BOD) removal, total suspended solids (TSS) removal, nutrient removal, and pilot plant system operation and demonstration using actual screened raw wastewater from Lee Wastewater Treatment Plant, Lee, Massachusetts, USA. The IPCWWTS developed by the authors can be in any shape (circular or rectangular), can use any manufacturers’ products (Supracell, Sandfloat, KAMET, AquaDAF, Clari-DAF, etc.), and is for the entire world.

In this study, dissolved ozone flotation system with injecting ozone directly into a DAF pump developed was applied to remove turbidity and organic matter for water treatment. This ozone flotation system was evaluated on the basis of the efficiency of ozone transfer into water and the removal efficiency of turbidity and TOC in the Nakdong river water. The DAF pump system found to be more efficient than the conventional DAF system injecting ozone directly from a ozone generator. The ozone through the DAF pump process was dissolved in water much faster than through the general ozone generator due to the higher mixing efficiency through a pump impeller and high pressure inside the DAF pump. Also, the kinetics of ozone decay injected by the DAF pump was slower than injected by an ozone generator. The DAF pump flotation system shows that 30% difference of removal efficiencies is attained as the ozone dose increases from 1.2 mg/L to 8.2 mg/L. And the turbidity removal was increased with increasing ozone dose. Therefore it can be concluded that ozone in this flotation system facilitates improved removal of organic matter and turbidity due to enhanced destabilization of those.

The innovative combined physical-chemical and biological process system has been elaborated and proposed as an alternative technology for municipal wastewater treatment plants operation. In a series of laboratory bench-scale experiments, typical raw municipal wastewater from the Town of Lee, Massachusetts, USA, was treated directly or elevated to design concentrations (BOD5/TSS/NH3-N = 250 mg/L/250 mg/L/25 mg/L) before treatment. The current and future National Pollutant Discharge Elimination System (NPDES) effluent limitations were the goals for wastewater treatment in this research. The innovative dissolved air flotation (DAF) technology was applied for primary and secondary clarification. The improved activated single-sludge process was applied for biological treatment (carbonaceous oxidation, nitrification, and denitrification) under simulated cold weather conditions (5 °C). Three complex experiments were performed to prove the system efficiency. It is concluded that the combination of primary DAF, aerobic carbonaceous oxidation, aerobic nitrification, anoxic denitrification, secondary DAF-filtration, and chlorination is an effective innovative system for municipal wastewater treatment. The future NPDES effluent standards can be met. The addition of returned activated sludge (RAS) stabilization tank will increase the efficiency of biological treatment.

Drinking water treatment options for taste and odor control

Application of dissolved air flotation (DAF) in semi-aerobic leachate treatment

Particle-bubble collisions in dissolved air flotation (DAF) systems play a crucial role in the removal of total suspended solids (TSS). DAF particle-bubble collision models incorporate factors such as particle diameters, charge and density, bubble diameters, and collision factors. The challenge lies in accounting for the wide range of particle and bubble sizes and obtaining complex model inputs. To address this, a simplified model for TSS removal in DAF units was established using low-cost laboratory measurements, including particle size distribution and density. Additionally, microbubble diameter profiles were derived from bubble velocities using particle image velocimetry software (PIV). Six independent variables, encompassing influent particle characteristics (such as particle size distribution and density) and DAF running characteristics (temperature, contact zone detention time, inflow and recycle flows), were employed in the simplified model. The model's accuracy was evaluated using a laboratory-scale DAF system with two different influents: Delft canal water and anaerobic sludge. The predicted TSS removal from the simplified model aligned well with the laboratory-scale DAF results, yielding removal efficiencies of 68 ± 1 % and 77 ± 3 % for Delft canal water and anaerobic sludge, respectively. Furthermore, when the simplified model was applied to two full-scale DAF systems, it successfully identified an underperforming system (DAF2) with a TSS removal efficiency of 91 %, contrasting with the theoretical removal model-predicted efficiency of 98 %. This study highlights the utility of combining bubble size distribution measured by PIVlab and particle size distribution obtained using FIJI-ImageJ as an economical and efficient approach to acquiring the necessary inputs for predicting TSS removal in DAF systems.

Effluent of wastewater treatment plant is to be disinfected to protect drinking water sources. DOF(dissolved ozone flotation) was developed to meet this purpose. A DOF is strengthened system with ozone to a DAF(dissolved air flotation) system. A DAF system has good floating power by numerous micro-bubbles, and ozone has strong oxidation capability. And DOF system has good floating power and strong oxidation capability simultaneously. When DOF was applied to secondary wastewater effluent, color of 11CU in raw water that was secondary effluent was reduced to 1CU by the DOF system. Removal rate of other water quality parameters treated by DOF were also higher than that by DAF, which were proved the strength of oxidation capability of ozone. When ozone concentration of 6.1 mg/l were applied in DOF system, general aerobic bacteria were reduced to 5 CFU/ml from TNTC (too many numbers to count). With the same ozone concentration, total coliform were not detected at all. These figures are under the numbers of drinking water regulation. These microbes were the target major parameters of a DOF. It was proved that a DOF was very effective in disinfection of wastewater treatment plant effluent as well as in removal of color, turbidity, and T-P.

The global demand for petrochemical and petroleum industry products unavoidably generates large volumes of oil refinery wastewater (ORW). The complete treatment, reclamation and disposal of the ORW to an acceptable environmental limit is currently a challenge to most of the petroleum industries. With the current development in conventional treatment methods viz. coagulation, dissolved air flotation (DAF), and biological and membrane separation processes. DAF, which is well-established separation process, effectively employs microbubbles as a carrier phase for separation. Although, DAF is frequently used in combined water and wastewater treatment plants, its fundamental characteristics and operational parameters have not yet been fully investigated for the treatment of ORW. In this study, the correlation and effects of the parameters understudy (coagulant dosage, air saturator pressure, air-water ratio and rising rate) on chemical oxygen demand, soap oil and grease, turbidity and total suspended solids removal from ORW were examined experimentally using a laboratory DAF system. The results showed that increasing the saturator working pressure and the rising rate had less effect on the system, than increasing the air-water ratio. The agglomeration of the oil droplets was found to depend solely on the polyaluminum sulphate (PAS) dosage to destabilize the oil droplets. The DAF treatability performance showed over 80% removal of the contaminants at optimum conditions of pH of 5, PAS dosage of 10 mg/L, rising rate of 15 minutes, air saturator pressure of 300-500 kPa, and air-water ratio of 5-15%. The PAS dosage was found to be the most significant factor. Therefore, a moderate increase of the PAS dosage under these optimum conditions will increase the DAF efficiency in the treatment of ORW.

Enhanced efficiency of dissolved air flotation for biodiesel wastewater treatment by acidification and coagulation processes

Flotation as a wastewater treatment technique is designed to remove all particles generally encountered as very fine emulsions, suspended solids, and colloids from wastewater. Historically, Dissolved Air Flotation (DAF) has been used to achieve this removal. More recently, other flotation techniques such as induced air, electro, cavitational, and Centrifugal Flotation Systems (CFS) have been applied in wastewater treatment. CFS use centrifugal force to enhance mixing of particles and bubbles with treatment chemicals and accelerate solid/liquid separation. In the most recent design, centrifugal hydrocyclone mixing was combined with small dissolved-air flotation bubbles leading to the development of the hybrid, dissolved-air centrifugal flotation.

Factorial evaluation of operational variables of a DAF process to improve PHC removal efficiency