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Section 10: 10. - WaterNSW - WaterNSW

Section 10: Absorption Trenches and Beds 10. Absorption Trenches and Beds

10.6 Common technical issues ... to the absorption trench or bed for treatment and disposal. The effluent is typically distributed along the length of the trench or bed through slotted or drilled 100 millimetre distribution pipes, and then filtered through the ... fence the area or plant a protective barrier of shrubs to prevent access. ...

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Transcription of Section 10: 10. - WaterNSW - WaterNSW

1 Section 10: Absorption Trenches and Beds 10. Absorption Trenches and Beds

2 Design and Installation of On-site Wastewater Systems 119 10.

3 Absorption Trenches and Beds Design ..119 Operation ..125 Inspection ..134 common technical issues ..134 Case study ..136 Wick trenches ..137 In a conventional septic tank and absorption system, wastewater is gravity-fed or pumped from the septic tank to the absorption area. Trenches or beds (hereafter trenches refers to trenches and beds ) are usually built below ground and can be media-filled or consist of a durable self supporting arch resting on gravel (or occasionally coarse sand).

4 Trenches are usually relatively narrow and deep, while beds are wider and shallower. NSW Health requires non-disinfected effluent to be disposed of at a soil depth more than 300 millimetres for both trenches and beds (NSW Health, 2008). Trenches are usually 500 to 700 millimetres deep and up to around 600 millimetres wide. Beds are usually no deeper than 600 millimetres, but up to several metres wide and contain a number of distribution pipes or arches. The depth and overall basal area depend on soil type and anticipated wastewater volume, climate and site features.

5 In septic tank and absorption trench systems, household wastewater first flows into a primary septic tank where solids settle to bottom of the tank to form a sludge layer, and grease and fat float to the surface to form a scum layer. Clarified effluent then flows (or is pumped via a pump well) to the absorption trench or bed for treatment and disposal. The effluent is typically distributed along the length of the trench or bed through slotted or drilled 100 millimetre distribution pipes, and then filtered through the gravel and sand to the underlying soil.

6 A clogging layer or biomat develops along the bottom and sides of the trench and acts as a further filter. This filtering process helps remove pathogens, toxins and other pollutants. Nutrients in the effluent are taken up by vegetation (normally grass) planted across the absorption trench area, incorporated in the biomat, and, in the case of phosphorus, adsorbed onto clay particles in the soil. Absorption trenches and beds are typically used to manage primary treated effluent . They can also be used to manage secondary treated effluent , in which case higher loading rates are acceptable, as outlined in AS/NZS 1547:2012 On-site Domestic Wastewater Management.

7 Absorption trenches or beds used together with secondary treated effluent are small footprint systems. They are often used where there is limited space available for effluent disposal. Design The design of an absorption trench should consider the site and soil assessment outlined in Section 2. The absorption trench should be sized according to the recommendations of AS/NZS 1547:2012. Installing absorption trenches requires 119

8 Design and Installation of On-site Wastewater Systems council approval under the Local Government Act 1993.

9 The trench must comply with council and Sydney Catchment Authority requirements for buffer distances and setbacks from significant site features. The following points should also be considered. Where more than one trench is needed, trench lengths should be equal and effluent should be distributed evenly via a splitter box (Figure ) or sequencing valve (Figure ). The maximum number of trenches for any one design is 10. Trench lengths should be designed to ensure that effluent is evenly distributed and reaches the far end of each trench.

10 Individual trenches must be less than 20 metres for passive systems, or 25 metres for pressure dosed trenches. Figure Splitter box Figure Sequencing valve The linear loading rate is critical where soil is shallow (see Section 2 of this Manual). Distribution pipes and arches should be laid according to manufacturer s instructions. It is better for trenches or beds to be pressure dosed instead of allowing the effluent to trickle into the trench or bed. This provides better and more even distribution and use along the whole trench while prolonging the life of the trench.


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