sewage treatment in cork and munster
 

Septic Tank Design

Conventional septic tanks comprise of a septic tank and percolation area. Septic tanks are primary settlement tanks providing a limited amount of anaerobic digestion. The majority of the treatment occurs in the percolation trenches and the underlying subsoil. These systems provide effective treatment and disposal of domestic wastewater when properly sited, installed and maintained.

EN 12566 Small Wastewater Treatment Systems up to 50 PT – ‘Part 1: Prefabricated septic tanks’ is a product standard developed by CEN and published by NSAI. The standard specifies a range of requirements and test methods in relation to septic tank design and performance, some of which are referred to in the following paragraphs.

The following guidance on the general design of conventional rectangular septic tanks should help ensure best performance.

  • Septic tanks should comprise of two chambers and have a minimum length to width ratio of 3:1 in order to promote settlement of suspended solids;
  • Larger septic tanks are better than smaller tanks because of greater settlement of solids and larger hydraulic retention time for liquid and solids;
  • Properly designed baffles provide quiescent conditions and minimise the discharge of solids to the percolation area;
  • The inlet and outlet of the septic tank should be separated by a long flow path for the wastewater; if the outlet is too close to the inlet, solids settlement and grease separation may be inadequate; and
  • Access and inspection openings should be incorporated into the roof of the septic tank. The opening should be constructed to such standard and in such a manner that unintended access (for example by children) cannot occur.
  • T-pieces to be installed as they assist to prevent odours.

Septic tanks should be able to:

  • Withstand corrosion;
  • Safely carry all lateral and vertical soil pressures; and
  • Accommodate water pressure from inside and outside the tank without leakage occurring.
  • Septic tanks should be watertight to prevent wastewater escaping to the soil outside, and to prevent surface water and groundwater from entering the tank.

The septic tank should be of sufficient volume to provide a retention time for settlement of the suspended solids while reserving an adequate volume for sludge storage. The volume required for sludge storage is the determining factor in sizing the septic tank and this sizing depends on the potential occupancy of the dwelling, which can be estimated from the maximum number of people that the house can accommodate, and this is determined from the number and type of bedrooms. The minimum size for a single bedroom can be taken as 6.5m2 and of a double bedroom as 10.2m2.

The tank capacity may be calculated from the following formula:
C = 180 P + 2000

Where
C = the capacity of the tank (litres)
P = the design population with a minimum of 4 persons

A minimum capacity of 2720 litres (2.72 m3) should be provided. This assumes that de-sludging of the septic tank is carried out at least once in every 24-month period
When kitchen grinders are installed, additional sludge solids are discharged with the wastewater and de-sludging intervals of septic and primary tanks need to be increased, therefore these are not recommended for single houses.

Septic Tank Design Features

A septic tank should be watertight up to the top of the tank. Methods employed to test such tanks should be in accordance with EN 12566:1. All joints in the tank should be sealed properly, including tank joints (sections and covers), inlets, outlets and risers. The joints should be clean and dry before applying the joint sealer.

The volume of water filled up to the outlet should be at least the nominal capacity claimed by the manufacturer.

Septic tanks should be securely covered to prevent unauthorised access and ensure operational safety. Access should be given to the inlet and outlet areas for routine maintenance sampling, removal of sludge, maintenance etc.

All materials used in the construction of the works should comply with the requirements of the Building Regulations, 1991 (and subsequent amendments) and the relevant Technical Guidance Documents.

In addition to the general requirements above prefabricated tanks should be manufactured from suitable materials (e.g. pre-cast concrete, glass reinforced plastic, glass reinforced concrete) and the requirements stated above for capacity, hydraulics, strength and water-tightness should be observed. In the case of light prefabricated tanks, attention should be paid to the risk of flotation of the tanks as a result of groundwater pressure or surface run-off gaining access to the excavation.

Quality control for compliance with EN 12566:1 is required to be demonstrated by the manufacturer of the septic tank.

The construction of block work in-situ tanks is not recommended due to the difficulty in carrying out water tightness tests on-site and the difficulties in their general