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Subject :Sand acceptable.. 2010-12-03 12:18:42

Darragh Donnelly

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Joined: 2009-12-28 15:43:51
Posts: 20
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Hi can someone tell me is Sand acceptable by Local authorities as a final filter pre discharge to ground.

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Subject :Re:Sand acceptable.. 2011-08-17 13:48:57

Jamie Wallace

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Joined: 2011-04-08 12:29:26
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I believe that according to the EPA Code you need to discharge through the minimum depth of unsaturated permeabble subsoil below all septic tank or secondary treatment systems outlined in Table 6.2. I understand this to mean that if you use a sand filter as secondary treatment you have to discharge through a minimum of 900mm of unsaturated natural subsoil after the sand filter. If I understand your question correctly then sand is not acceptable to discharge through, directly to the ground water.

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Subject :Re:Sand acceptable.. 2011-08-17 16:27:34

Nick Bailey

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Joined: 2011-03-30 15:55:18
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Yes, Jamie is spot on here, a soil polishing filter will still be required after the sand filter

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Subject :Re:Sand acceptable.. 2011-08-17 18:17:04

Tim Clifford

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Joined: 2009-11-02 22:18:21
Posts: 29
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This is an excellent question that Iâ€™d love clarification from the EPA because I have a different view from above. This is my opinion for what its worth.

Letâ€™s take the Septic Tank aspect first.

According to page 29 of the CoP http://screencast.com/t/T6EED7ZzZ the intermittent sand filter needs to be 0.9m and sit ontop of 0.9m subsoil with a T-value between 3-50. The intermittent sand filter is then sized in accordance with Table 8.2 on page 30.

Sizing the intermittent sand filter: A 06 Population house will generate 900L daily. Site has a recorded T-value of 25 and I am only able to achieve the 0.9m of required subsoil due to rock. Solution: using the 10 l.sq.m.d as per Table 8.2 with the T=25 this would result in a 90.sq.m intermittent sand polishing filter sitting ontop of the 0.9m existing subsoil. The new GL would then be in the region of 1632mm above existing GL.

In the instance of a Secondary WWTU:
When a sand filter is placed after a secondary WWTU it is considered a tertiary treatment system according to page 43 of the CoP. http://screencast.com/t/MlewNNmLwmr

Furthermore under section 10.1 Polishing Filters it states that all polishing filters should have a minimum thickness of 0.9 m of free-draining unsaturated soil OR sand between the point of infiltration of the effluent and the water table and bedrock. They may be below, at ground surface or partially or totally above ground surface

Sizing the sand filter: A 06 Population house will generate 900L daily. Site has a recorded T-value of 25 and I am only able to achieve the 0.9m of required subsoil due to rock.

Solution:
Soil = using the 10 l.sq.m.d as per Table 10.2 with the T&P= 25-39 range this would result in a 90.sq.m soil polishing. The new GL would then be in the region of 732mm above existing GL.

Sand: According to page 44 of the CoP a sand polishing filter can be loaded at max 60 l.sq.m.d. http://screencast.com/t/GVxmyLrfNqG In this instance the existing subsoil can be removed and totally replaced with 0.9m of a sand bed. The new GL would then be in the region of 732mm above existing GL but would only be 15 sq.m in size.

Also to back up my argument if you revert to table 6.3 and look under a P<3 there is a line that states â€œHowever, if effluent is pre-treated to tertiary state then the site will be hydraulically suitable to assimilate the hydraulic loadâ€ I appreciate that this refers to fast P rates but as I am proposing to remove all the subsoil and replace with sand i.e. tertiary treatment then the site the site will be hydraulically suitable to assimilate the hydraulic load.

Personally Iâ€™d love to see clarification of this question from the EPA as I know each local authority interpret this issue their own way.

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