Operating regimes
They can work with ascending or descending fluids, the latter being much more common. In the case of downflow devices, the fluid can flow under pressure or by gravity alone. Pressure sand bed filters are commonly used in industrial applications and are often referred to as rapid sand bed filters. Gravity-fed units are used in water purification, especially drinking water, and these filters have found wide use in developing countries (slow sand filters).
In general, there are several categories of sand bed filters:
The diagram illustrates the general structure of a rapid pressure sand filter. The sand in the filter takes up most of the space in the chamber. It sits on a floor of nozzles or on a drainage system that allows filtered water to escape. The pretreated raw water enters the filter chamber from the top, flows through the filter media and the effluent is drained through the drainage system at the bottom. Large process plants also have a system in place to evenly distribute raw water to the filter. In addition, a distribution system that controls air flow is usually included. This system allows a constant distribution of air and water and avoids excessively high water flow rates in specific areas. A typical distribution of grains comes out due to frequent backwashing. Smaller diameter grains predominate in the upper part of the sand layer, while coarse grains dominate in the lower parts.
Two processes that influence the functionality of a filter are maturation and regeneration.
At the beginning of a new filter operation, the filter efficiency increases simultaneously with the number of particles captured in the medium. This process is called filter maturation. During filter maturation, the effluent may not meet quality criteria and must be reinjected in previous steps of the plant.[6] Regeneration methods allow the reuse of the filter medium. Solids accumulated in the filter bed are removed.[6] During backwashing, water (and air) is pumped back through the filter system. The backwash water can be partially reinjected ahead of the filtering process and the wastewater generated must be disposed of. The backwash time is determined by the turbidity value behind the filter, which must not exceed a set threshold, or by the pressure loss through the filter medium, which must not exceed a set value.
Smaller sand grains provide more surface area and therefore greater decontamination of the inlet water, but also require more pumping energy to drive the fluid through the bed. A compromise is that most rapid pressure sand bed filters use grains in the 0.6 to 1.2 mm range, although for specialized applications other sizes may be specified. Larger feed particles (>100 micrometers) will tend to block the pores of the bed and turn it into a surface filter that clogs quickly. Larger sand grains can be used to overcome this problem, but if there are significant amounts of large solids in the feed, they must be removed prior to the sand bed filter by a process such as decantation:[5] 302-303.
It is recommended that the depth of the sand bed be about 0.6-1.8 m, regardless of the application. This is related to the maximum performance discussed below.[5].
Guidance on the design of rapid sand bed filters suggests that they should operate with a maximum flow rate of 9 m/m/hour.[7] From the required flow rate and maximum flow, the required bed surface area can be calculated.
The last key design point is to ensure that the fluid is properly distributed throughout the bed and that there are no preferred fluid paths where sand can be entrained and the filter compromised.
Rapid pressure sand bed filters typically operate with a feed pressure of 2 to 5 bar(a) (28 to 70 psi(a)). The pressure drop across a clean sand bed is usually very low. It increases as solid particles are captured in the bed. Solid particles are not captured uniformly with depth, but are captured more at the top of the bed and the concentration gradient decays exponentially.[5].
This type of filter captures particles down to very small sizes and has no actual cut-off size below which particles always pass. The shape of the filter particle size efficiency curve is a U shape with high particle capture rates for the smallest and largest particles with a drop in the middle for medium-sized particles.[7]
The accumulation of solid particles causes an increase in the pressure lost through the bed for a given flow rate. In a gravity fed bed, when the available pressure is constant, the flow rate decreases. When pressure loss or flow rate is unacceptable and the filter no longer functions effectively, the bed is backwashed to remove accumulated particles. In the case of a pressurized rapid sand bed filter, this occurs when the pressure loss is about 0.5 bar. The backwash fluid is pumped back through the bed until it fluidizes and expands by up to 30% (the sand grains begin to mix and, when rubbed, expel the solid particles). Smaller solid particles are removed with the backwash fluid and are typically captured in a settling tank. The fluid flow required to fluidize the bed is typically 3 to 10 m/m/hr but is not run for long (a few minutes).[5] Small amounts of sand may be lost in the backwashing process and it may be necessary to replenish the bed periodically.
As the title indicates, the filtration rate is modified in the slow sand filter, however, the biggest difference between the slow and fast sand filter is that the top layer of sand is biologically active, as microbial communities are introduced into the system. The recommended and usual filter depth is 0.9 to 1.5 meters. The microbial layer is formed within 10 to 20 days from the start of the operation. During the filtration process, raw water can filter through the porous sand media, stopping and trapping organic matter, bacteria, viruses and cysts such as Giardia and Cryptosporidium. The regeneration procedure for slow sand filters is called pigging and is used to mechanically remove dried particles from the filter. However, this process can also be carried out underwater, depending on each system. Another limiting factor for the water to be treated is turbidity, which for slow sand filters is defined as 10 NTU (Nephelometric Turbidity Units). Slow sand filters are a good option for budget-conscious operations, as filtration uses no chemicals and requires little to no mechanical assistance. However, due to continued population growth in communities, slow sand filters are being replaced by fast sand filters, mainly due to the length of the operating period.
The continuous backwash or upflow sand filter is the newest operating regime. The clearest difference with respect to the previous ones is that the water to be filtered is fed from the bottom and the filtered water is obtained at the top. This reverse flow allows the countercurrent washing process to be integrated into the filtration process, thereby reducing the amount of washing water to be used and reducing cleaning time. The maximum head is about 5.4 lps/m with a constant head loss of 0.6 m.[8].
Filters that have different filter layers are called mixed bed filters or multimedia filters. Sand is a common filter material, but anthracite, granular activated carbon (GAC), garnet, and ilmenite are also common filter materials. Anthracite is a harder and less volatile material than other coals. Ilmenite and garnet are heavier than sand. Garnet is made up of several minerals, which causes a changing red color. Ilmenite is an oxide of iron and titanium. GAC can be used in the adsorption and filtration process at the same time. These materials can be used both alone and in combination with other media. But the filter layers will always be ordered by density, the heavier compounds will settle at the bottom, while the lighter ones will sit at the top. Different combinations give a different filter rating and also a different porosity throughout the filter, which translates to a different pressure drop. A very common arrangement for these filters is composed of: anthracite on the top, sand and garnet, with a gravel support at the bottom. The depth of these filters is usually between 0.6-1 m, above 1 m the pressure loss increases greatly and below 0.6 m the thickness of each filter layer is reduced, thus reducing its effectiveness. Rated operating flow and pressure drop are between 3-7 gpm/ft and 3-7 psi. When the pressure drop increases above 10 psi, a backwash operation is necessary, which consists of reversing the flow (water goes upwards) to remove particles trapped in the filter media, and this will exit through the top of the filter with the backwash water. Typically, the backwash is about 3 times the normal filtrate flow (it should be high enough to lift the filter media and remove particles trapped in it). Monomedia is a single layer filter, commonly composed of sand and is today replaced by newer technology. The deep bed monomedia is also a single layer filter consisting of anthracite or GAC. The deep bed monomedia filter is used when water quality is consistent and provides longer run time. Double media (two layers) usually contain a layer of sand at the bottom and a layer of anthracite or CAG on top. Trimedia or mixed media is a filter with three layers. Trimedia usually have garnet or ilmenite in the bottom layer, sand in the middle, and anthracite in the top.