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In 2007 when I attended the Onsite Wastewater Course in Tullamore I asked the course lecturers what I thought was a very simple question.

The Question was "Is there a direct correlation between a T-Value obtained on a site and the sizing of a sand filter". Unfortunately no one that day was able to answer my question except to state that the EPA 2000 Manual stated that for soil covered sand filters the hydraulic loading should be in the range of 40 - 60 l/m².d.

In the current CoP Manual (pg 44) it states "The hydraulic loading should not exceed 60 /m²/day."

Over the years there has been huge confusion in relation to what is the correct procedure for sizing Sand Polishing Filters. In certain instances if a site recorded a high T-Value then this would require a large soil polishing filter... Low permeability, therefore, spread the effluent ut over a larger area. However, Site Assessors, Architects and Engineers have at times decided to ignore the recorded high T-Value and propose a sand polishing filter based on the 60 l/m²/day. This has resulted in a smaller plan area.

Example 1: T= 49, PE6,

There are 2 options:

Option A:- Required Soil Polishing filter 180sq.m (based on 05 l/m²/day)

Option B:- Required Sand Polishing Filter 15sq.m (based on 60 l/m²2/day) (Incorrect Design)




Of course we now all know that the sizing of a polishing filter should take into consideration the rate of permeability of the subsoil but this was not always taken into consideration hence from time to time an attitude of "Throw in a sand filter on-top of the Clay, Sure t'will all be ok!!" was adopted.

Over the years the EPA has received a number of queries from local authorities and practitioners and I am delighted (Yes I'm sad) to see that the EPA has over the last few weeks published “Clarification on disposal of effluent from Polishing Filters, Tertiary Treatment Systems".

It states: (full clarification can be downloaded from EPA Manuals section of this website under INFORMATION>DOWNLOADS)

Reason for clarification;

In Section 10 Tertiary Treatment Systems of the Code of Practice there is currently no guidance on what to do with the effluent discharging from tertiary treatment systems (sand filters, reed beds or package treatment systems). According to the under-riding principles of the Code, the tertiary treated effluent has been treated to a high enough standard such that it can discharge to the groundwater. However, the hydraulic issue still needs to be accounted for such that the effluent does not back up and create problems to the tertiary treatment process itself. Hence, some calculations have been carried out to discharge an appropriate percolation area for the discharge of such clean effluent depending on the T-value of the subsoil into which it is being discharged.

These calculations (which include a safety factor of 3.5) show that the area (A) of subsoil required for the discharge of tertiary treated effluent,

Area (A) that is required per person is calculated as follows.

A=0.125 X T-value (or P-value) m² per p.e.

I have decided to draw a few examples detailing the clarification.

Example 2: T= 49, PE6, based on CoP Clarification Feb 2012

The required area (A) of subsoil underneath a Tertiary Treatment Systems is therefore calculated as follows 0.125X49X6= 36.75m²

Furthermore final effluent discharging at the base of the polishing filter shall be discharged to 300mm deep gravel distribution layer (Pea Gravel, 10-20mm).



So in plain English.

The sand polishing filter can be calculated based on 60 l/m2/day i.e. 150X6/60=15m² but this sand filter must sit on-top of a 36.75m² stone bed which is 300mm deep and a minimum depth of 300mm of subsoil is to be maintained between the point of infiltration and the bedrock/water table.

If your T-Value is between 03-20 then “No distribution area is required” i.e. all that is required is a minimum depth of 300mm of subsoil to be maintained between the point of infiltration and the bedrock/water table.

Example 03: T= 19, PE6, based on CoP Clarification Feb 2012

Option A:- Required Soil Polishing filter 45m² (based on 20 l/m²/day)

Option B:- Required Sand Polishing filter



The sand polishing filter can be calculated based on 60 l/m2/day i.e. 150X6/60=15m². This sand filter does not require a gravel bed on which to sit.

It is important to remember in all proposed sand solutions :

• There is at least 0.3m of naturally occurring soil above the bedrock.
• The maximum high groundwater level is at least 0.3m below the natural ground surface.
• For T=20-75 The base of constructed gravel bed will correspond to the location of the T/P invert.
• For T=03-20 The base of the constructed sand filter will correspond to the location of the T/P invert.

It is also worth noting that even the CoP allows a loading rate of 60 l/m2/day the EPA2000 manual quoted a loading rate of 40 l/m2/day.

Costings:

Sand to complete a 20sq.m filter bed can be supplied to site for €1150 X Vat

Transport cost also need to be factored into the equation.

Contact 087-9903697 if you require a site specific quotation.