Case Study

These experiments were carried out over a 12 month period on a test plant before the construction of a major 600,000 m3/day water treatment works. The design of the works called for Rapid Gravity Filtration with an expected filtration rate of 6.0m/hour through a bed depth of 0.75m of media.

The test plant tried several different combinations of filter media and measured the run time [hours] to a head loss of 1.8m.

Trial	Filter Media	Size [mm]	Depth [m]	Run Time
1	Fine Sand	0.42 - 1.00	0.60	2.5
1	Coarse Sand	1.00 - 2.06	0.15	2.5
2	Anthracite	1.25 - 2.50	0.15	7.0
	Fine Sand	0.42 - 1.00	0.45
	Coarse Sand	1.00 - 2.06	0.15
3	Anthracite	2.50 - 4.90	0.15	7.5
	Fine Sand	0.42 - 1.00	0.45
	Coarse Sand	1.00 - 2.06	0.15
4	Anthracite	1.25 - 2.50	0.30	10.5
	Fine Sand	0.42 - 1.00	0.30
	Coarse Sand	1.00 - 2.06	0.15
5	Anthracite	2.50 - 4.90	0.30	12.0
	Fine Sand	0.42 -1.00	0.30
	Coarse Sand	1.00 - 2.06	0.15

The experiments also showed

Colour removal 100%
No media breakthroughs
Maximum sand penetration 0.20m

Conclusions

Duel media filtration of anthracite and sand were required to obtain a reasonable run time of over 24 hours
Grade 2 anthracite [1.25 -2.50 mm] gave a much sharper interface between media layers.
A minimum of 0.30m of sand was required for filtration rates of up to 9m/hour.
No backwash problems were encountered up to rates of 54 m/hour.

Final Application

Filter Media	Size [mm]	Depth [m]
Anthracite	1.25 - 2.50	0.15
Fine Sand	0.42 - 1.00	0.45
Coarse Sand	1.00 - 2.06	0.15

Reasons

Grade 2 anthracite produced a much sharper interface with the sand media
0.45m of sand gave a greater margin against 'breakthrough' conditions.
The cost of anthracite is approx. twice that of sand [ by volume].

Notes:-

Since this water treatment works was built other specialist anthracite filter media producers have entered the market. Now a much wider range of media grades are available at prices little more than the cost of sand [by volume].