An evaluation of methods for assessing the removal efficiency of a grit separation device

Abstract

The paper considers the many problems which can beset any attempt to verify or measure the performance of a grit removal device, including contractual ambiguities, the difficulties of assessing any flow dependency, sampling difficulties, uncertainty about analytical techniques and efficiency definitions. The different definitions of efficiency are discussed and it is concluded that the ‘partial penetration’ would be the most applicable to the situation. The usefulness is discussed of attempting to separate grits which do not damage machinery and improve the handleability of the primary sludge. In the measurement of efficiency only two process streams need be sampled. Different protocols are considered which specify which streams should be sampled, and the advantages and disadvantages of each are set out. Solid stream sampling of recovered grit and primary sludges over a lengthy period is rejected as an unwieldy technique which may be impracticable on certain sites. The analysis of sludges for a small quantity of grit is problematic and subject to error, but this is also true of settled liquid stream samples. Rather it is concluded that supplementary grit addition coupled with liquid effluent stream sampling would give an efficiency subject to little error. The results of recent significant work carried out in the testing of a Grit KingR Separator, both in-situ and under controlled conditions, using different methods are presented in this paper. The recommended technique is described together with the sampling methods and further work required to verify/validate the procedure.