STUDENT RESEARCH PROJECT

SAND FILTERS.
COMPARISON OF THE MOST IMPORTANT
OPERATION PARAMETERS

María Graciela Vela Domínguez

MSc. Study Program WASTE

Supervisor: Martina Barabosz
Examiner: Heidrun Steinmetz

June 2013

INDEX
Introduction……………………………………………………........ 2
1. Sand Filtration……………………………………………..…... 3
1.1 Slow Sand Filtration………………………………………..…. 6
1.1.1 Filter Description…………………………………………..… 6
1.1.2 Operational Parameters…………………………………….. 8
1.1.3 Advantages and Disadvantages……………………….…. 11
1.2 Rapid Sand Filtration………………………………………..… 14
1.2.1 Filter Description……………………………..........................14
15
1.2.2 Operational Parameters………….…....................................

1.2.3 Advantages and Disadvantages……………………….….. 23
1.2.4 Rapid Sand Filtration Classification………………………. 23
1.2.4.1 Continuous Rapid Sand Filtration………………………. 24
1.2.4.1.1 Filter Description…………………………………………. 24
1.2.4.1.2 Filter Operation..………………………………………… 25
27
1.2.4.1.3 Operational Parameters………………………………..…………
28
1.2.4.1.4 Advantages and Disadvantages………………………….

Conclusion……………………………………………………….…. 29
Bibliography………………………………………………….…….. 31

1

Introduction
Sand filtration is a technology that has been used for wastewater treatment since a long time. Bed of granular material over a gravel bed is used to treat water. This process imitates a natural treatment that occurs as water runs through the soil. During the pass through, several processes such as physical, chemical as well as biological take place in the filter media.
Sand filtration can be classified by filtration modes according to the flow regimen slow sand filtration and rapid sand filtration. There are also other classifications available such as the operation conditions like under pressure or at atmospheric pressure.
Between the different sand filtration technologies in this project, the slow and rapid sand filtration parameters are going to be discussed. Also the new technology from rapid sand filters like Dynasand and Contiflow will be described and compared with the traditional sand filters.
Several parameters particularly the flow rate, filter media, working time influences the way the water is treated in the different sand filters. For example in slow sand filters the slow flow rate allows to create a biolayer which treats the water in a biological way. In order to have this special feature, specific size of filter media is needed.
Sand Filtration is capable to eliminate several organic materials, improve the turbidity

and

work

for

the

elimination

of

Giardia and Cryptosporidium, bacterias, or viruses. It is applied in the water treatment plants and also in the collection of rainwater. The sand filtration process is an environmentally friendly wastewater treatment technology; it is simple and also can be cheaper than other water treatment technologies.1

2

1. Sand Filtration

The principle of sand filters is to percolate water through a sand bed. The grains of sand form a layer or bed, the water penetrates this bed and the larger particles will be stuck in the intervals between grains acting like a sieve. 3
While working with sand packing bed, some properties from the packing bed can affect the filtration process. 3
• Grain shape: the shape of the grains can affect the design of the filter depending the way grains are packing together in the bed of the filter; also the hydraulic analysis is different if we have angular surfaces instead of spherical. • Material density: for designing multimedia filters, different layers of granular material of different density can be created to form a reverse graded filtration (larger filter grains on the top after the backwashing).
• Material hardness: during the backwashing certain materials can cause abrasion and breakdown.
• Granular bed porosity: It affects the head loss and the filtration efficiency. Filter bed porosity values are from 40 to 60 percent.
• Granular bed specific area: defined as the total surface area of the filter material divided by the bed volume.
The flow regime is an important aspect to observe the hydraulic behavior in the bed filter. For this characterization it is needed to have the undimensional number Reynolds. The Reynolds number (Re) is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for the given flow
3

conditions. The Reynolds number can be defined for a number of different situations where a fluid is in relative motion to a surface.3,5

Re =

ρ୴୐

(1)

μ

Where: ρ= Density of the fluid (kg/m³). µ= Dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s)) v= Mean velocity of the object relative to the fluid (SI units: m/s)
L=Characteristic linear dimension, (m)
For filtration, the flow regimes are different than in the ones that can be found in a flow through a tube. There are four regimes:
•

Darcy flow or creeping flow: Reynolds numbers less than one.

•

Forchheimer regime: Reynolds numbers between 1 and 100.

•

Transition zone: Reynolds numbers between 600 and 800.

•

Turbulent regime: Reynolds number more than 800.
As mentioned before the sand filtration can be classified according to their

flow regime, the slow sand filtration and the rapid sand filtration.3 The slow sand filtration stays in the Darcy flow regime. The rapid sand filters are found between the Darcy and the Forchheimer regimes and in special cases of high-rapid sand filtration, all giving a Reynolds number about 18. The highest Reynolds number that can be achieved is during the backwashing in a rapid sand filter with a value from 3 to 25.3
There are different classification criteria for sand filtration. For example by pressure operation in pressure and gravity filters. Pressure filters are closed vessels containing the bed of sand and in which a specific pressure is applied.
Gravity filters are open topped boxes where the raw water is fed above the sand bed and flows downwards under the action of gravity. 11
4

The filtration system has to be selected in order to comply with the quality water norms. Three basic regulatory requirements are: turbidity, color and biological contamination. Additional to these conditions, the frequency of cleaning cycle, chemical requirements, operational complexity and the sludge volume and toxicity have to be considered in order to select the appropriate filtration system.15 Another filtration technologies such as membrane filtration or reverse osmosis that compete with sand filtrations can achieve better quality results, but still the sand filtration has good results and a cheap invest. Also the sand filtration can be improved by addition of different materials such as for example activated carbon. The sand filtration technology has still many opportunities to be developed. 5

1.1 Slow Sand Filtration.

1.1.1 Filter Description
Slow sand filtration is a very old technology that has resurged in different research topics as a mean to remove special substances contained in the water.
For example to remove ferric and manganic compounds, that have been converted by the aeration from soluble ferrous and manganous salts in groundwater. 3,5,7
Slow Sand filters are open filters in which the water flows through the filter medium due to the pressure of the hydrostatic head by the aid of gravity.
Therefore a sufficient height of supernatant (water on the top of the filter) is needed. The sand used for slow sand filtration exhibit a fine grain size in which two ways of water treatment are done simultaneously, a combination of a physically and biologically removal of water pollutants. 3,5,7
The physically process involves the straining action of the shallow layer on the top of the bed in order to remove suspended particles. In the biological process pathogens, dissolved organic matter or nutrients will be adsorbed and decomposed through a microbial metabolism by an activated microbiological layer, the so called fixed biological film, which is created during the operation of the filter. 3,5,7
The fixed biological film is a gelatinous layer of microorganisms (bacteria, algae and protozoa) formed in the upper layers of the slow sand filters due to the accumulation of material caused by the small sand size. 3,5,7
Under the sand bed, an under drainage system is located to discharge the treated water and to offer support to the filter medium. This system consists in a false floor of porous concrete or a system of porous unjointed pipes surrounded and covered with graded gravel. 3,5,7

6

The gravel forms a supporting layer which supports and prevents the grains to being carried into the drainage pipes.

11

Another function of the pipes of

the under drainage system is to provide oxygen supply into the sand filter in order to control the right oxygenconcentration for the microorganisms contained in the biofilm when is needed. Slow sand filters have usually first a fine sand layer over another layer of coarse sand to prevent that the fine grained sand blocks the system of draining. 3,5,7

“The high efficiency of water treatment achieved by slow sand filters is partly explained by the slow filtration rate and fine effective size of the sand, but is also attributed to biological processes in the layer of slime material that accumulates above the sand surface (fixed biological film) and within the upper layers of the sand bed.“ 2

When the fixed biological film grows as much as that the head loss of the filter increases in an inadmissible way so that the rate of flow through the bed of sand will be reduced, the sand filter has to be cleaned. This is done by scrapping of the upper layer of the sand. 3,5,7

Fig.1 Slow Sand Filter20.
7

There are different kinds of designs for Slow Sand Filters (SSF), some of them are listed below:
• Oxfam slow sand filter: that uses a synthetic fabric on the top on the filter
• Dug and lined slow sand filter: uses an excavated basin covered with a plastic sheet and over this basin the filter media is collocated.
• Drum Filters: drums that are situated one after the other to give different filtration phases.
There are special cases in the slow sand filter when the addition of granular activated carbon is used to adsorb dissolved matter and micropollutants.
Also projects about the efficiency of slow sand filtration for the removal of cysts of
Giardia and Cryptosporidiumenteroparasites and also organic material which is in dissolved state have been done. 1,3,11
Slow sand filtration provide a great improvement in the physical, chemical and biological quality of waters, by removing the suspended solids and at the same time offering a biological treatment.1,3,11
1.1.2 Operational Parameters
Filter Media
In a slow sand filter the sand bed has to be a finer filter material, usually quartz sand.
The sand size required has a diameter between 0.3 and 0.45 mm. The grain size of the sand is selected depending on the size of the suspend particles contained in the water to treat. If the suspended particles are small, then the grain size has also to be small in order to trap the suspended particles from the water. 3,5,7

8

Filtration Rate
In the slow sand filters it is important to maintain the flow rate slow enough so the microorganisms can grow in the media. To achieve the right water flow in the filter, the filtration rate has to be between 0.05-0.3 m/h. 3,5
The filtration rate v is composed as follows:

v= v= (2)

Flow rate [m³/h]

A=

୅

Filtration rate [m³/m²h]

Q=

୕

Surface Area [m²]
The sand size is a way to achieve slow flow rate. Using small sand grains

more surface area can be created. As it can be seen in equation (2), the filtration rate is indirectly proportional to the area. That means, the smaller the grains of the sand used as filter medium, the higher the surface area and finally the lower the filtration rate. 3,5
But beside of the grain size of the sand the filtration rate is also bounded by the supernatant water and the head loss.
The supernatant water creates hydrostatic pressure due to the water column over the filter bed. The higher this water column is, the higher the hydrostatic pressure acting over the filter bed. With this hydrostatic pressure increase, the filtration rate will increase as well.
In the case of the head loss, when it increases a lower filtration rate can be observed. Low filtration rate causes long retention time (more than rapid sand filters), and the need of bigger filters in order to treat more amount of water.
If the filtration rate is too low, the fixed biological film developed would increase into the deep of the filter and cause clogging. On the other hand, if the speed is too high then this biological layer will not grow properly because it would be washed out every time. 3,5
9

Oxygen Content
The effectiveness of the fixed biological film depends not only on the organic material in the water and the filter rate but also on the oxygen content at a good temperature. 2,3
Oxygen is used in the metabolism of biodegradable components and for consumption of pathogens. The oxygen content of the filter should be above 3 mg/l. If the oxygen content falls to zero, anaerobic decomposition occurs, with consequent production of hydrogen sulphide, ammonia, and other substances.
Sometimes the oxygen content of the raw water is not enough, then an aeration system is used (oxygen supply through the under drainage pipes system). 2,3,11
Filter Lifetime
The working period of the filter can be from 1 to 12 months it will depend on the quality of the water to treat (turbidity, concentration of suspended solids, etc). Once the head loss is in the maximum, and the filter start to get clog the filter will need to be clean it (scrapping).
In the slow sand filtration the expected removal of the different substances are high. For sediment is between 70 to 90% and a removal of BOD of 70%. The following table describe averaging values of BOD, COD,nitrogen and phosphorus in the filterable substances.21
Tabla 1. Average values of BOD,COD,nitrogen and phosphorous in filterable substances.19
BOD5 Sludge loading of pre-deposited biological stage,B DS

g BOD5 / g
FS

g COD / g FS

g P / g FS

g N/ g FS