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Should mulch be used for bioretention systems ?

In this photo, the mulch on the left is ‘new’/ ‘un-used’ mulch that we recommend placing on the surface of Filterra bioretention systems – and replacing approximately every 12 months.  The mulch on the right is mulch that has been on the filter media surface of a Filterra bioretention system in Melbourne, Victoria for approximately 8 months – clearly demonstrating one of the key reasons Ocean Protect uses mulch on the filter media surface for Filterra bioretention systems – capturing particulate matter that could otherwise migrate into the filter media bed.

As a biofiltration surface layer, mulch is the first line of defence for treatment via physical filtration and chemical complexing, but also protects the underlying treatment media from scour and occlusion. The media stays protected and infiltration rates are maintained while most of the sedimentation occurs on the surface of the mulch within a biofiltration practice.  In addition to trapping particulate matter that could migrate into the media bed, it also provides many benefits thought to improve long-term functionality. For example, supporting the biological community by providing organic replenishment to the media, pollutant treatment, adsorption site regeneration, and moderating temperature and moisture within the media bed (Hills et al 2023).

Ocean Protect recommends the use of a 75mm depth of a double shredded, hardwood variety.  Mulch for Filterra biofiltration systems is tested for leaching, floatability, fertility and hydraulic capacity to ensure proper flow characteristics for permeability and water retention, and to ensure pollutant discharge does not occur and that no materials are present that could harm the vegetation.

For further information, please contact us.


Hills M, Allen V, Pedrick J, Macleod A, 2023, Filterra – analysis of long-term performance. Prepared by Contech Engineered Solutions. (here)

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