CTB3365x – Introduction to Water Treatment W2b – Grit removal design

Jules van Lier

After the screenings, the sewage, with all its suspended and soluble pollutants, is directed to the subsequent unit. How do we remove the sand particles that may disturb the well-functioning of the bioreactors? In this lecture we will discuss the principles and design of sand & grit removal units. Together with the sewage, a substantial fraction of sand is transported to the entrance of the sewage treatment plant. Sand has a higher density, and settles much faster than the organic suspended solids. To prevent that the sand particles enter the bioreactor, which eventually leads to mall-functioning, sand and grit is removed in the so-called pre-treatment units. Generally, sand removal is directly positioned after the screening devices. Lowering of the liquid flow velocities in screens is restricted to 0.4 m/s to prevent that sand settles in the screens. In the grit removal device, we lower the liquid velocity a bit further to make use of gravity separation to settle the sand. Gravity separation, or settling or sedimentation, is one of the most common approaches to pre-treat, or partially treat, the domestic and municipal sewage flows. During gravity separation, we clarify the sewage flows from suspended matter, resulting in low turbidity liquid effluents, and a concentrated stream of settled solids. Basically, we can distinguish two types of settling: Discrete settling and Flocculent settling. In Discrete settling, the particles’ size, shape, and specific density do not change in time. Discrete settling is regarded as a non-interactive settling of particles from a dilute suspension: For instance, the settling of grit and sand in water. During the settling, the particle velocity accelerates until a constant velocity is reached.

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Discrete settling can be mathematically described using the classical sedimentation laws of Newton and Stokes, as illustrated in the slide. In fact, there are 2 main forces enacting upon the particle, which are the gravity force, and a drag force.

The gravity force results from the particle characteristics, and is directly dependent on the density difference between the particle and the liquid, multiplied by the gravitational acceleration, and the volume of the particle.

The drag force results from the motion of the particle through the liquid. And is thus dependent on the particle’s velocity, as well as on the prevailing drag coefficient, the size of the particle, and the liquid density.

As the particle increases in velocity, eventually, the drag force and the applied force will approximately equate, causing no further change in the particle's velocity. This velocity is known as the terminal velocity, settling velocity or fall velocity of the particle. This is readily measurable by examining the rate of fall of individual particles. The terminal velocity of the particle is affected by many parameters. In fact, anything that will alter the particle's drag. In addition to the particle density and the liquid viscosity and density, the terminal velocity is most notably dependent upon the particle size and shape, symbolized by the so-called sphericity factor. For ideal spheres this factor is 1 and for sand particles the factor is 2. Factors exceeding 20 are used for fractal flocs.

In non-stagnant medium, the drag force ‘Cd’ is dependent on the liquid flow regime, or the level of liquid turbulence, as indicated by the upper 3-term equation. The symbol NR stands for Reynolds number, which describes the liquid turbulence.

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Under laminar flow conditions, with Reynolds number < 1, the drag coefficient is reciprocally correlated to Reynolds number as illustrated in the slide.

At low Reynolds number, only the first term of the equation determines the outcome.

In the transition area, with Reynolds number between 1 and 2000, the drag coefficient should be calculated using the full 3 term equation.

At very high Reynolds numbers, larger than 2000, turbulence is not further impacting the drag force, which is then fixed at 0.34.

The other type of settling is flocculent settling. During flocculent settling, particles agglomerate, or coalesce, or flocculate, and have no constant characteristics.

The particles vary in size and increase in mass, resulting in increased velocities during settling. Flocculent settling generally occurs with organic suspended solids, which are largely present in the domestic sewage and in our sewage treatment reactor. Flocculent settling can be divided in 2 subgroups, dilute suspension settling and hindered settling. In dilute suspension settling, particles are present in low concentrations, but generally higher than 50 mg/l. Particles are free to coalesce with any other particle on the route of sedimentation, increasing their settling velocity. During the settling, the particles’ motion do not cause significant water displacement. 3

The settling efficiency is directly related to the hydraulic surface load, and the hydraulic retention time. There are no mathematical formulas to describe the process, and sedimentation characteristics are determined by laboratory tests. The other type of flocculent settling is the so-called hindered settling.

In principle, the same mechanisms occur as in dilute suspension settling. But at high suspended solids concentrations, the large quantities of particles cause multiple interactions, resulting in a slow downward movement of particles. Upward water movement further hinders settling. A clearly marked interface between sludge and the supernatant liquid occurs. When time passes, the settled solids at the bottom further compress, leading to a density gradient from the bottom to the sludge liquid interface as illustrated in the graph. How can we apply the settling theory to the design of our grit chamber? Let’s imagine we are going to design a rectangular grit chamber with a certain Length, Height, and Width.

Next, we are going to apply a certain flow rate ‘Q’. Now let’s imagine a certain particle, with a certain settling velocity. Based on the applied Q, the Width and the Height of the grit chamber, the particle will flow with a certain horizontal velocity. Now let’s sediment the particle at the given flow rate. Great, the particle is removed from the liquid. Now let’s take a new particle with a lower settle ability and/or a higher horizontal flow. Ooops our tanks is apparently too small.

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One more try with another particle and a proper settle ability and an appropriate flow rate.

A particle with discrete settling is retained in the grit chamber, when the time required for vertical settling is less than the lasting time that the particles flow to the end of the tank.

In other words, the ratio settling velocity to Height must be larger than the ratio horizontal velocity to Length. Since the horizontal velocity is determined by the Q and the vertical surface, or Height Width dimensions, the settling velocity must be larger than the surface velocity, which is the Q divided by the horizontal surface. The maximum allowable horizontal surface velocity or hydraulic surface load, is also called the Hazen velocity. Interestingly, this Hazen velocity is independent on the Height of the grit chamber, as can be deduced from the above formulas.

Having the Hazen velocity, or maximum hydraulic surface load, determined, we may understand that all particles with a settling velocity higher than the Hazen velocity will be removed from the water line. Since the various particles are characterized by different settling velocities, often the feed inlet of the grit chamber is distributed over the height. What will be the impact of such fed inlet? Yes indeed: also particles with a lower settling velocity than the maximum hydraulic surface load will settle. And thus: a higher removal efficiency!

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The most important design features of a rectangular grit chamber are: The maximum hydraulic surface load or Hazen velocity should be below 40 m3/m2.h, or 40 m/h, or 0.011 m/s. The critical horizontal velocity that may lift the sand from the bottom is 0.3 m/s. This 0.3 m/s is therefore called, the critical scouring velocity or slip velocity. In our design, the resulting horizontal velocity must always be below this value. A design horizontal velocity close to the critical scouring velocity will result in an excellent separation of sand particles from the organic fraction. Note that the critical scouring velocity of organic material is much lower. For instance, for primary sludge the critical scouring velocity is 0.03 m/s. The above design features will result in an efficient washing of the sand, which can be subsequently used for other purposes, like construction works. How much sand will be recovered at an average STP? This will depend on the type of sewerage applied, the extent of the sewer network, and size of the area served. For STPs treating the sewage of more than 100.000 inhabitants the sand production is about 2 -12 liters per person per year! So, a few containers per week. Owing to the high variations in sewage flow, generally, various rectangular grit chambers are designed next to each other, which can be disconnected when not needed. The final design results in a number of, for instance, 4 long gutters receiving a constant flow rate, having a constant discharge to the next process units. The maximum hydraulic surface load is 40 m/h, whereas the horizontal velocity or scouring velocity is approximately 0.3 m/s. The generally applied Length – Width ratio ranges between 10:1 to 15:1. The clean sand is generally collected in a mechanized way, as indicated in this slide. Instead of rectangular grit chambers, also square grit chambers are applied.

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In this design, the sand is moved to one lateral side by a slowly moving hopper. The collected sand, is subsequently lifted and washed from organic solids using the same incoming flow. Clean sand is subsequently collected, whereas the washed organic solids proceed their way to the next process units. Some design features of a square grit chamber are: Water depth is 0.8 -1 m Tanks can be square or circular Maximum hydraulic surface load is 30 m3/m2.h or 30 m/h The tank is characterized by a fluctuating discharge And is equipped with an external sand washer, continuously washing the solids. Knowing the fundamentals of discrete settling, one can also use the liquid turbulence to increase the terminal velocity of the sand particles, enhancing separation. This is done in a so-called aerated grit chamber. Most organics will be separated from the sand particles. But the produced slurry will still contain a mixture, that needs to be separated outside the tank. An interesting advantage of an aerated grit chamber is the accumulation of fat, oil, and grease at the top of the chamber. This floating material can then be skimmed off, preventing possible problems in the bioreactors. The collected slurry from the grit chamber can be further cleaned by applying a hydro-cyclone, which creates a concentrated downward stream with sand for discharge, and a more light upward stream with organic suspended solids. The liquid overflow is returned to the main water stream.

With the screens and grit removal units we removed most inorganic matter from the sewage. It’s now time to have a look to the biological part of the system.

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