2. Methods

as shown in the Amazon River sampling of suspended sediments [1]. A characteristic of cohesive sediments is to form aggregates (flocs) that behave in a very different way that non-cohesive sediments. Measuring in-situ flocs settling velocities in rivers is not possible with conventional sediment sampling instruments. Recently, in-situ optical instruments are being used for floc size measurement, particularly in the field of oceanography [2, 3]. Those instruments are very expensive because they use laser illumination and are not used in common river engineering practice. In this chapter, a method based on suspended concentration sampling and laboratory particle size analysis in a rotating annular flume is used to obtain flocs size, and with an appropriate settling velocity model deduce the settling velocity to be used in the Rouse equation. The method is validated with sediments from Usumacinta and Grijalva rivers, the two major rivers in México, whose basins are located in the states of Chiapas and Tabasco near the Guatemalan border.

The settling velocity of cohesive sediments is an important design parameter in aquatic environments such as water treatment plants, storm water ponds, sediment filling in lakes, sedimentation in estuaries, dredging in rivers, and sediment removal in aquaculture devices especially when shortage of water is a concern [4]. The reuse of water is the main characteristic

The efficient removal of solids is a main concern in these systems because of the accumulation of non-used food and fish excreta. These solids are generally less than 65 μm in diameter and behave as cohesive sediments [5]. These sediments form flocs or aggregates, made of water, inorganic particles, and organic particles [6–8]. To obtain adequate settling models for these particles is an active field of research [9]. Some researchers include variable fractal dimension functions that depend on a characteristic size of flocs that is difficult to obtain over a large population of flocs [10]. Other authors use geometrical parameters of flocs like floc perimeter, which is not easy to measure in engineering practice [11, 12]. The flocs' settling velocity model proposed in this study uses parameters that are possible to average, using optical methods with some floc samples.

The tanks most widely used are circular [13, 14]. Water is supplied in these tanks by means of diffusers at the walls. In this project, a small scale circular water recirculation tank was used in order to study the solids behavior in the tank. There is a central settling device in order to remove the solids (Figure 1). The settling device functions according to the hydrociclons principle [15].

Two optical techniques were used in this work, Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV), to measure fluid and particle velocities, respectively. Polyamide tracers 25 μm in diameter were used to obtain fluid velocities using PIV, and flocs were tracked as particles in the PTV technique. PTV also allowed us to measure particle size and shape. Digital holography for particle image velocimetry (DHPIV) has also been used to determine

The attempts to model settling velocity as a function of floc size, shape, and density demonstrated that density varies with floc size. Later work demonstrated that floc density depends on the fractal nature of flocs [16]. Recently, the effect of shear rate on floc density was

the size and shape of flocs considering their volumetric nature.

of the latter systems.

112 Applications in Water Systems Management and Modeling

demonstrated [11].
