A respirometer with improved sensitivity for ready biodegradation testing

In this paper we describe a respirometry system with improved sensitivity, that was used to test materials at concentrations down to 3 mg C/l. Data obtained with this system, operating under the conditions of the OECD ready biodegradability tests 301B, C, D and F, is presented and compared with CO2-production data obtained under 301B conditions in the Sealed Vessel Test. For simple materials, the shape of the curves, not easily seen with CO2 production or DOC removal data, is clearly biphasic with a sigmoidal first part — suggesting growth of the inoculum on the test materials. For a selection of rapidly and readily biodegradable materials, the removal of soluble carbon was complete at the end of the first, sigmoidal part of the biodegradation curve; It is speculated that the remainder of the curve probably represents the mineralisation of part of the biomass. The respirometer generated many hundred data points per degradation curve and Monod or exponential growth models were fitted and growth parameters were derived. Growth rates under ready test conditions were derived for six materials as follows: aniline μ = 0.31 h−1, ethanolamine μ = 0.13 h−1, 1,6-hexanediol μ = 0.05 h−1, pentaerythritol μ = 0.04 h−1, C12.8EO μ = 0.13 h−1 and commercial LAS μ = 0.05 h−1.