What will happen if the sand sieve analysis results are too high or too low?

The sieve analysis tells you important things about the sand you are using.  The effective size of the sand is the diameter for which 10% of the sand is smaller, and 90% is bigger.  The uniformity coefficient describes how similar in size the sand particles are.  The silt content is the percentage of sand with a diameter < 0.1 mm. 
 
If your effective size is below 0.15 mm, that means too much of the sand is very small sand particles. This will lead to a low flow rate.  If the effective size is greater than 0.20 mm, then there aren't enough small sand particles and the flow rate will probably be too high. 
 
If the uniformity coefficient is below 1.5, that means that all of the sand particles are very similar in size.  This is illustrated in the first diagram below.  In this case the sand particles won't fit very close together, leaving larger pores for water to flow through.  When sand particles are different sizes, they fit closer together, and work better to filter water (like in the second diagram).   
 
If the silt content is too high (above 4%), the silt will cause the water flowing out of the filter to be cloudy (turbidity).  This will reduce over time, but it could take months, after which the effective size of your sand will have increased, and the uniformity coefficent decreased. 
 
Effective size can sometimes be increased, and silt content decreased, by washing the sand more.  The uniformity coefficient is more difficult to correct.  It is determined by the sand source.  For example, river sand often has a low uniformity coefficient.
 
Another important check on the sand media, in addition to the sieve analysis, is to test the flow rate in an installed filter.  The flow rate of the filter should be close to 0.4 liters per minute.  It should not be > 0.5 liters/minute.  The flow rate is very important.

After the filter has been used 3 or 4 times (to remove the cloudy water from installing the filter), the filtered water should be clean looking - no cloudiness. 

If the sieve analysis results are off or it isn't possible to achieve the target flow rate, you may need to find a new sand source.
 
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