Test No. 302A: Inherent Biodegradability: Modified SCAS Test

This Test Guideline describes a method which is an adaptation of the Soap and Detergent Association semi-continuous activated sludge (SCAS) procedure for assessing the primary biodegradation of alkyl benzene sulphonate. The test does not simulate those conditions experienced in a sewage treatment plant.

Activated sludge from a sewage treatment plant is placed in an aeration (SCAS) unit. The test compound (non-volatile, water soluble, organic, non-inhibitoring to bacteria at the test concentration) and settled domestic sewage are added, and the mixture is aerated for 23 hours. The aeration is then stopped, the sludge allowed to settle and the supernatant liquor is removed. The sludge remaining in the aeration chamber is then mixed with a further aliquot of test compound and sewage and the cycle is repeated. The above fill and draw procedure is repeated daily throughout the test. A high concentration of aerobic micro-organisms is used. The length of the test for compounds showing little or no biodegradation is indeterminate, but experience suggests that this should be at least 12 weeks. Biodegradation is established by determination of the dissolved organic carbon content of the supernatant liquor. This value is compared with that found for the liquor obtained from a control tube dosed with settled sewage only.

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