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8/6/2019 Slow Sand Filter Final
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INTRODUCTION
The unfortunate demise that Haiti has been subjected to has been a shock to the worldwide
community. As engineers we are entrusted with the responsibility to return some sense of
normalcy to the country that is relevant to our respective field. As civil engineers one of the
problems we can address, is the problem of providing clean water for different communities that
need it. One such community contains about 20 households and a well within the village. We
opted to provide clean water using a slow sand filter, instead of a rapid sand filter because of the
properties of the water source and its user friendliness.
Originating in Europe, slow sand filtration is classified as the first, modern water treatment
technology. This filtration process removes particles and microorganisms by the slow percolation
of water through a bed of porous sand, with the influent water introduced over the surface of the
filter (supernatant), and then drained from the bottom. Unlike other water treatment technology
(i.e. rapid sand filtration), conventional slow sand technology does not involve chemical or
physical pre-treatment applications.
The site was visited so that we can have a good idea of how much space was needed to construct
the facilities and also to do preliminary testing on the type of groundwater from the well that will
be fed into the slow sand filter. After this, research on the design had to be done.
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SAND FILTER DESIGN
This is the most important aspect of the community project in Haiti. A basic slow sand filter will
normally consist of a filter box, effluent flow control structure and a clear-well to store the
filtered water. The filter bed is where the main process takes place. The water source is fed intothe filter which consist from top to bottom, of the sand filter bed, support gravel and an under
drain. From there via a control valve it enters the effluent flow control structure which is a small
type of construction which alters the flow of water in some way. The effluent leaves the flow
control structure and enters the clear-well which houses the water for distribution. Other factors
that come into play will be the amount of persons being supplied, the amount of water consumed
on a daily basis and the capacity of the filter. The sand filter will be designed to meet the small
community’s need for approximately 7 years, which we believe is ample time for the country to
fully recover from the disaster. Figure 1 below shows a conventional slow sand filter.
Figure 1
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Figure 2 below illustrates an above view of our Slow Sand Filter setup
Figure 2
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According to the CIA World Factbook, Haiti’s annual population growth is 1.838% or approx
2%. If we take into consideration the Population Growth Factor chart, we can assume or
approximate the number of persons that will have to consume water during the seven years.
Table 1
From the table, if the small community’s annual growth rate for 100 persons is 2% and the
design period is seven 7 years, then the factor is 1.15. This number is multiplied by the
population size to determine the population in seven 7 years:
100 people x 1.15 = 115 people
Next we calculate the daily demand for water. If we assume that the average consumption of
each person is 70 litres per day, then the total daily demand is:
115 people x 70 litres/person/day = 8050 litres per day or approx 8.05m3/day
The hourly water demand is calculated when taking into consideration the design of the clear-
well. This hourly water demand takes into consideration the daily peak flows to ensure that water
is provided to the users without interruption. This is determined by taking 20% of the daily
demand:
8.05m3/day x .20 = 1.61m3/h
The total filter area is now determined by using the formula:
Area = Quantity of water per hour/Velocity of water
A= Q/VThe velocity normally ranges between 0.1m/h to 0.2m/h. For our slow sand filter, we will
assume a filtration rate of 0.1m/h, therefore our total filter area is:
Area = 1.61m3/h = 16m2
0.1m/h
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We can construct our two filters from the area given: 16m2 ÷ 2 = 8m2. The filters ratio of length
to width will be between one to four. The length will be 4m and the width 2m.
FILTER BOX
The filter box consists of four parts:
• The water reservoir above the filter bed (supernatant)
• The filter bed,
• The underdrain system, and
• The filter control system
Table 2 below lists the important design parameters needed for the filter box.
Table 2 – Filter Box Design Parameters
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ADVANTAGES OF A SLOW SAND FILTER SYSTEM
1. Sludge handling problems are minimal.2. They can remove from 91 to 99.999 percent of harmful bacteria and viruses from water.
3. They improve water clarity.
4. They do not require the addition of chemicals to function.
5. They can be designed to function using only the power of gravity.
6. They are inexpensive.
7. They do not pollute.
8. Maintenance is simple and easily understood.
9. Their construction is easily understood.
10. They can operate in remote areas where no electrical power or petroleum energy is
available.
11. Close operator supervision is not necessary
DISADVANTAGES OF A SLOW SAND FILTER
1. They do not remove 100 percent of the colour or odour from water.
2. They require relatively non-turbid water to function properly.
3. They cannot function if they are frozen.
4. Water must be added regularly and the sand must stay covered by water continuously.
5. During construction care must be taken to prevent air pockets from forming in the sand -
this will foul the filter and require removing the sand and removing all anaerobic bacteria
and odours.
6. Industrial pollution is only partially removed from water by these filters.
7. These filters must be allowed to "ripen" (have water flow through them for at least 3
weeks) before they will purify water.
8. They are VERY heavy (1000 pounds plus) and much thought must go into where they
will be located - it is almost impossible to move them once they are set up.
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MAINTENANCE
Slow sand filters are very easy to maintain. Once the bed becomes clogged, the top layer of the
sand is removed. To do this the water in the bed is drained and the top layer scraped off. When
this is done, filtered water is allowed back through the system from the under-drains up to cover the sand layer. As this water flows through the bed, raw water may be re-introduced. It will,
however, take 1 – 2 days before the bed is functioning properly. It may therefore a good idea to
re-filter the first raw water filtered after cleaning. The sand which is removed should be washed
immediately to prevent putrefying (rotting) and then stored for re-use.
When the depth of sand in the bed has reached the minimum level of 0.7 metres, the bed must be
re-sanded. An extra 0.3 metres of sand should be removed before the fresh sand is placed on the
bed. Once the new sand is installed, this old sand can be replaced on top to promote the growth
of bacteria. Tasks can be broken down into several categories:-
Daily: 1. Check the raw water intake
2. Check and adjust the rate of filtration
3. Check water level in the filter
4. Check water level in the clear-well
5. Sample and check water quality
6. Check any pumps
7. Enter observations in a logbook
Weekly: 1. Check and grease any pumps and moving parts
2. Check the distribution networks and taps
3. Clean the site of plant
Monthly: 1. Scrape the filter beds
2. Wash the scrapings; store the retained sand
Yearly: 1. Clean the clear-well
2. Check the filter and clear-well are watertight
Every Two Years: 1. Re-sand the filter units
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CONCLUSION
In conclusion, we were given the task of providing a community with clean water, with the
use of a well. It was decided that a slow sand filter will be best suited for this operation as it can
remove 91% - 99.9% of harmful bacteria and viruses from the water and it is inexpensive and
easily maintained. Our purification system was designed to provide the community of growing
population (approximated up to 115 people) with an average consumption of 70 liters per person,
with clean water for approximately 7 years. This included two filter tanks, one weir and two
clear wells and space left for further expansion. Although there are both pros and cons that come
with using a slow sand filter system, we believe that it is most suitable, considering the recent
natural disaster, the size of the community, the water source and the funds available.
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REFERENCES
Logsdon, G., Kohne, R., Abel, S., & LaBonde, S. (2002). Slow sand filtration for small water
systems. Journal of Environmental Engineering & Science, 1(5), 339-348. Retrieved March 2,
2010 from Academic Search Complete database: http://search.ebscohost.com/login.aspx?
direct=true&db=a9h&AN=10579509&site=ehost-live
References
Tech Brief: A National Drinking Water Clearing House Fact Sheet. Retrieved March 2, 2010
from:
http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/slow_sand_filtration_DWFSOM40
Slow Sand Filter Conceptual Design: Technical Report No. 101 September 2003. Retrieved
March 2, 2010 from: http://www.weriguam.org/docs/tr_101.pdf
Slow Sand Filtration: Fact Sheet 2.12. Retrieved March 2, 2010 from:
http://www.aguadechuva.com/download/whoslowsandfilter.pdf
Slow Sand Filter FAQ: Advantages and Disadvantages. Retrieved March 2, 2010 from:
http://www.slowsandfilter.org/ssf_faq.html
Water For The World: Designing a Slow Sand Filter – Technical Note No. RWS. 3.D.3.
Retrieved March 17, 2010 from: http://www.lifewater.org/resources/rws3/rws3d3.pdf
The World Factbook: Population Growth Rate. Retrieved March 15, 2010 from:
https://www.cia.gov/library/publications/the-world-factbook/geos/ha.html