Sand filtration and backwashing for water treatment and purification

Backwashing and Sand Filter Medium

Filters are the much-in-need tools for getting clean fluids when purity is declining in all the substances. Whether water or milk, fluids can be filtered using a porous material. Where there are even many other methods of filtration like crystallization and evaporation, the filtration through sand as medium is highly effective especially while filtering water. Filtration plants remove suspending particles from water at commercial as well as domestic level. These plants are easy to operate as well as maintain. These plants can be backwashed thus cleaning the medium of filtration i.e. sand. The water as well as size of sand particles decide when the sand should be cleaned through backwashing. While paper and cotton can also be used in filtration process, they have much lesser life than sand. The regular cleaning of sand keeps the waste management plants operational. The sewage water is the most challenging for filtration purposes and such water will require the sand to be cleaned much more often and the head loss will also be higher in this case. Backwashing decreases the cost of replacement of the plant as well as mass accumulation in the tank. It enables reuse of not only sand but the whole water tank and the treatment plant. It is recommended to design the tank and valves in such a way to reduce head loss and increase efficiency of system.

Introduction

The water treatment is gaining increasing importance given the declining quality of drinking water globally. The quality of water is decreasing because of the industrial growth and addition of pollutants to the natural water reservoirs. The backwashing and sand media use is a method in which water is passed through porous medium so that the pollutants are absorbed and pure water is left behind. Thus the quality of water increases as it is circulated through sand or asbestos. The sand or porous material can be reused and thus the method is quite cheap. In the world where economic conditions are tight and mainly poor regions face the problem of undrinkable water, the backwashing process can be a cheap water treatment at commercial level. The water treatment can be used for drinking water, washing purpose water and swimming pools (Cleaning the pool filter sand, 2013) where using chemicals might not be appropriate.

Steps in Backwash

The method of back washing often involves several different steps. The duration and intensity of the steps depend on the quality and size of sand particles as well as the water. These include following:

1. Spread a bed of sand in a tank

2. Enter water in the tank from bottom

3. Reverse water flow i.e. pass the water a few times in the tank to ensure all suspended particles hang in sand

4. Particles of sand scrub against each other

5. Waste and pollutant solids deposit in sand

6. Depending on quality of water, water is circulated

7. Water is collected in clean container

8. Wastes are washed from sand

9. Sand is put pack in container

Satterfield (2005) says that the duration of backwash should depend on when the water comes out clean. The sand bed should have appropriate expansion because too much expansion or too little expansion would not be effective. The bed also expands when the water is pumped up the sand media. The expansion of filter bed must be in the range of thirty to fifty percent. About 30 inches sand bed expands about 4.5 to 6 inches (p. 3).

Head Loss

When water is moved through sand for filtration purposes, the direction is bottom up. Thus pressure is required to suck the water up. The pressure difference, that is required to pump water through sand, while filtration is called head loss. When the head loss increases, this is a symbol that now the filter i.e. sand should be cleaned (Soil & Aquifer Properties and Their Effect on Groundwater, 1998). The head loss depends on properties of sand and tank like depth, sand's texture, and porousness and permeability. The geology and vegetation in an area also affects the porousness of the sand. Thus the filtration of water does not only mean absorbing the solid particles but also chemicals. The head loss should not be very high because then the water will pass through with great speed that is not good. The sand particles have limited ability to absorb particles and thus the speed of water should be optimum as it flows up sand.

Sand Particles

The size of the sand particles matter a lot in the filtration process, head loss and maintenance. If the particles of the sand are very small, they will have very tiny air spaces between them. Subsequently, there will be very little space for the water molecules to pass through the sand. This will decrease the speed of water and will raise the requirement of pressure that could pull water up. Hence, the head loss will be higher. Also the bigger size of sand particles is not good. When the size of sand particles is too big, the water will easily make its passage through the sand but the problem is that then water will be moving along with solid suspension and it will not be removed from water. For water to get cleaned when passing through the sand, sand particles should be of optimum size so that they allow only water to pass and not the solid suspension. With time, as solid particles are left in sand, the spaces are covered and water does not find easy passage through sand and spaces are locked. At this time high pressure is required to move water and sand needs to be cleaned.

Washing Sand

While there is no price tag attached to the sand used in the treatment plant, the efficiency of sand decreases with time as the water passes through it and leaves back solid particles. Therefore, if the treatment is desired to work properly for a long time, it should be cleaned in time. The quality of sand effects the time sand needs before washing. Sand can be washed by back washing in which an amount of water can be circulated to absorb the waste solid material and leaves back clean sand that can again be used to filter water. It should be noted that dirty water is never used for cleaning sand. The dirty water does not have the capacity to hold any further amount of solid suspensions. The water has limited capacity and cannot hold very heavy solid particles in it. Therefore, the clean water which was already processed is used to clean the sand. Such water is light and can absorb particles that are present in the sand. The water in the tank that was processed earlier can be used. The process of cleaning sand should be such that the filter media expands and water absorbs particles from it. The rate of backwash should be very high to wash the sand. The need for backwashing can be identified when the head loss increases very high or it takes too long until clean water comes out of the tank (Water and wastewater Engineering, 2012).

Discussion

The water treatment is important not only for the drinking water but also in the water used for washing and swimming. The filtration of water is inexpensive and thus used by the pool maintenance bodies as well (Cleaning the pool filter sand, 2013). The water cleaning through sand however should take care of a few things. Sand should be timely cleaned so that it can be used for a long time effectively. The water should not be passed very slowly or very fast because an optimum speed is required to leave the solid suspensions in sand. The sand particles should not be too small or too large that they hinder filtration process.

Conclusion

Water treatment is increasingly becoming important as well as expensive. The increases population and increased pollutants in water require very high speed processing plants that can serve the need of population. In this scenario, the sand filtration plants for waste water management are important. These plants are cheap as well as easy to maintain. These plants can work in any part of the world since the planet is rich in sand. A slight modification in the design of plants can be made to adjust to needs of people. It should be noticed that there is a limit to capacity of sand for holing solid suspensions in water. Thus sand should be cleaned through backwashing in order to use it again for filtration purposes. The water is filtered through sand and it helps clean the sand during backwash. Thus the porous nature of sand serves dual purpose.