On-Site Wastewater Treatment System Performance

5 On-Site Wastewater Treatment System Performance

The EN 12566 series of standards consists of a number of parts – refer to Fig. 5.1 for their applications. The normative requirements of the standards, at the date of publication, have been incorporated into this CoP.

A treatment system should meet the requirements of I.S. EN 12566-3:2005 and be followed by a disposal system designed to

prEN 12566-7 or as per the guidance provided within this code. Alternatively a treatment system should consist of a product meeting the requirements of I.S. EN 12566-1:2000/A1:2004 or I.S. EN 12566-4:2007 followed by a disposal system meeting the requirements of I.S. CEN/TR 12566-2:2005 or I.S. CEN/TR 12566- 5:2008 or followed by a product meeting the requirements of prEN 12566-6 or as per the


Environmental Protection Agency 7

Code of Practice: Wastewater Treatment and Disposal Systems Serving Single Houses (p.e. ≤ 10)

guidance provided within this code. A tertiary treatment system meeting the requirements of prEN 12566-7 might be added to the total system where higher levels of treatment are required by the local authority.

The performance of septic tank systems in treating domestic effluent relies primarily on the soil attenuation capability of the percolation area. Contaminant attenuation begins in the septic tank and continues through the distribution pipework, the surface of the biomat, the unsaturated soils and in the saturated zone. Research in the US indicates that filtration, microstraining, and aerobic biological decomposition processes in the biomat and infiltration zone remove more than 90% of BOD and suspended solids (SS) and 99% of the bacteria (University of Wisconsin-Madison, 1978) and similar results were found by the Colorado School of Mines (Van Cuyk et al., 2005). These findings are supported by Irish EPA funded research projects (2001-MS 15- M1 and 2005-W-MS 15) undertaken by TCD. These septic tank systems are designed on a prescriptive basis (see Section 7), and are considered to achieve a satisfactory effluent quality, and treatment efficiency is usually not stated.

In general, wastewater treatment systems do not provide for the removal of significant amounts of nitrogen or phosphorus.

While septic tank systems can remove a limited amount of nitrogen but high-density installation of wastewater treatment systems can cause contamination (Wakida and Lerner, 2005).

The Colorado School of Mines, Golden, Colorado (Van Cuyk et al., 2005) observed high removals of phosphorus within soil infiltration systems throughout their study. As the finite sorption capacity of the upper layers of soil becomes exhausted, soils at greater depths will become increasingly more important for phosphorus attenuation as operational time extends for several years. Irish research by Gill et al. (2009a) also supports these findings.

For package wastewater treatment plants, compliance with phosphorus limits is usually achieved by dosing chemical coagulants into influent to precipitate phosphates, which settle out in the downstream settlement tank. Research shows that plants are capable of removing more than 90% of the total phosphorus load with adequate coagulant dosing and chemical precipitation (Hellström and Jonsson, 2003).


Leach System Trial Hole Cork
Leach System Trial Hole

The absorption capacity of gravel media of reed beds becomes exhausted after an extended period (e.g. 6 months to 1 year). Soluble phosphorus can pass forward with the treated effluent flow unless special media with a high absorption capacity are used (Molle et al., 2003; Zhu et al., 2003; Gill et al., 2009b) and it is replaced regularly (e.g. every 5 years).

As phosphorus removal is dependent on the natural mineralogy of the soil into which the effluent is being discharged (both percolation area and polishing filter) and there is a finite capacity in the soil, this should not alone be relied upon in nutrient-sensitive areas. Secondary treatment systems may be modified to specifically improve their nutrient removal capacity. In addition, there are a number of proprietary (tertiary treatment) systems on the market that provide enhanced nutrient removal for nitrogen and phosphorus. These should be tested in accordance with the requirements of prEN 12566-7.

O.S.C Environmental Engineering and Percolation Testing