USING JET MIXING TECHNIQUES TO ENHANCE DIRECT FILTTERATION

The present study investigates the effect of jet mixing techniques performed with two types of coagulants (Aluminum sulfate hydrate [Al2(SO4)3.16H2O] and Magnesium chloride MgCl2) using the polyacrylamide PAM (C3H5)n as flocculent aid, on the flocculation. The study include two parts, the first one is the numerical analysis using ANSYS Fluent and CFX program. The second part included the experimental work in which Kaolin particles used to simulate the suspensions in natural resource water. The results indicated that the mixing process using jet mixing tank system produce uniform semi spherical shape. The image analysis provided indications that the floc with manganese chloride are compacted and dense so that it can be more suitable for direct filtration procedure.


Introduction
In any water purification process or sewage treatment as well as industrial activity. The main objective of solid-liquid separation processes is the optimum removing of the suspended solids particles. Several sequential techniques can be applied for solids or particles removal such as filtration, flotation cyclone separation and settling [1].
In order to achieve efficiently removing of those suspended particles, it is important to collect adequately knowledge about their physical and often chemical properties. The most important physical characteristics of particles are size distribution, density, and Vol. xx, No. xx, Month 2xxx ISSN 2520-0917 www.jeasd.org structure/shape. The surface charge or the chemistry falls under chemical nature of particles that has great influence of the efficient of their removal [2].
The aim of coagulation and flocculation process is to collect the suspended particles into large aggregate for improving separation. Fundamental utilization of flocculation application in water treatment is purification. As well as flocculation is also substantial in other operation deals with suspensions and emulsions, such as in pharmaceutical and paint industries, due to its effect on suspension rheology [3].
1.1 Jet mixing turbulent jet is very common method which used for mixing miscible liquids in the chemical process. In jet mixing a fast-moving stream of primary liquid is injected into the bulk or secondary liquid that stationary or in slow-motion. The jet enters the bulk liquid and expanded at the jet angle δ, which is variable between about 15º and 25º for Reynolds ReJ >100.
The most common types of jet mixer are the side jet mixer and the coaxial jet mixer as show in figure1 [4]. The velocity difference between the secondary liquid and injected primary liquid caused to create a mixing layer at the jet boundary. The mixing layer growth is directly proportional to the jet direction flow leading to entraining and mixing the jet with bulk liquid.

Governing Equations
In order to analysis the fluid hydrodynamic in the mixing tank, the mass (continuity) equation and the momentum equation in additional to the renormalized group version of (k-ε) turbulent model equation by considering the infinite control volume with cylindrical coordinate.

Momentum Equation momentum
equations can be written in terms of shear stress τ governing the fluid motion for three dimensions in cylindrical coordinate [56] as: In zdirection: In rdirection: In θ -direction:

RESULTS AND DISCUSSION
Four nozzles arrangement at different height at the tank bottom used to provide the fluid flow motion. The fluids hydrodynamics in jet mixing tank are numerically are analyzed by ANSYS 15.0 -CFXand Fluent. In order to cover and trace the fluid behavior in the tank, many sections in the tank are analyzed and studied. One is at a vertical plane and extends along the center of the tank. The others are nine sections in horizontal plane at heights of 0. 1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and 0.55 m from the tank bottom. Figure 5 shows the velocity vectors in the vertical plane. It can be observed that fluid flow in all the zones of the tank, which is different from the stirred mixing tank. Therefore, eddies generated everywhere in tank as the fluid circulation in all direction. This can be considered as a significant improvement of the jet-mixing tank over the impeller-mixing tank as it increases the mixing efficiency.  Figure 6 illustrates the velocity contour at a vertical plane. It is obvious that there is very small poor mixing zones are located in the upper part of tank near the exit, where the flowing out of the mixing tank. It is useful that the low velocity zones existed at the upper zone because the floc do not exposed to a high shear stress and that may breaks them to smaller particles.

Floc and aggregate induced with aluminum sulfate hydrate coagulant [Al2
(SO4).14 H2O]. Figure 9 shows the image of aggregates as well as calibrated and analyzed images induced after 120s of the start of mixing process. It can be observed that the formed aggregate is compacted and tends to have spherical shape and less porosity. Therefore, it has a high specific weight that lead to and faster settling in the mixing tank. Images of flocculation process with alum after 120s of mixing.
Figure10. shows the settled aggregates at the end of mixing process. It is obvious that the settled aggregates are compacted, dense and some of them are agglomerated aggregates. The number of small parts and particles is very small because of the majority of these particles are tends to forms aggregates rather than affected with the flow in jet mixing tank.

Floc and aggregate induced with magnesium chloride coagulant MgCl2.
. Figure11 shows the aggregates formation after 120s of the start of mixing process with magnesium chloride. In figure11, it can be observed that the agglomerates are formed by jointing several compacted aggregates at the outer boundary as noted by the red circles. Theses agglomerates are compacted, dense and have high specific weight. In addition, they tend to form another agglomerates. Figure 11. Images of flocculation process with magnesium chloride after 240s of mixing. Figure 12 shows the evolution of flocculation process after 240s of the start of jet mixing process. It is obvious that the aggregates tend to grown up by jointing with other small floc. The aggregates are withstanding the shear stress after 240s of mixing because of the aggregate size is small and tend to spherical shape therefore it easily transport through flow stream in same velocity of stream. This behavior referred to the slow effect of magnesium chloride to enhance the particles to form floc. Figure (11) shows the calibrated image the analyzed image to estimate the average surface area of aggregates.

Figure12.
Image analysis of flocculation and aggregation process occurred in jet mixing tank with magnesium chloride as coagulant material after 240s of mixing process. Figure 13 shows the flocculation evolution after 600s of the start of mixing process. In The newborn aggregates have less tendency to re-flocculation a again because they are more stable in this stage of mixing process, or in other words it reaches steady state condition. Figure 13. image analysis of flocculation and aggregation process occurred in jet mixing tank with magnesium chloride as coagulant material after 600s of mixing process. Figure 14 shows the flocculation evaluation in jet mixing tank with two types of coagulants. It can observe the alum coagulant is faster effect in flocculation so the larger agglomerates induced after 120 second of mixing process while the coagulant manganese chloride is slower active so that the aggregates surface area induced with alum is larger than that induced with magnesium chloride. In drinking water treatment plant the sequence of purification process is chemical additive (coagulation) then flocculation and after that sedimentation process and the last procedure is filtration. Therefore, the alum coagulant is more suitable for flocculation process followed by sedimentation process.

Conclusions
1) The jet-mixing technique produces floc and aggregates have physical characteristics are suitable direct filtration process in water treatment plant.
2) The coagulant aluminum sulphate hydrate [Al2(SO4)3.14H2O] is faster indication in flocculation and produce floc with large surface area that is suitable for sedimentation process as next stage after flocculation process in drinking water treatment plant.
3) The coagulant manganese chloride [MgCl26H2O], although is not faster effected in flocculation process but, the floc induced is small, more dense, tends to have spherical shape, and compacted, that is more suitable for direct filtration process.