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For carrying out sludge lab-scale tests, and interlaboratory trials for standards validation, the availability of samples with "certified and constant" characteristics is required. When "dried sludge" samples cannot be used, problems arise because (i) most fresh sludge characteristics change with storage time, also because some preservation practices are not applicable or their effects unknown, (ii) some fresh sludge characteristics are strongly affected by handling, and (iii) fresh sludge requires particular precautions and authorization for transportation. This means that using "fresh sludge" samples cannot guarantee reliability and reproducibility of results, so a valid alternative consists in testing "synthetic suspensions" samples to be on-site prepared. This should also give the possibility to compare results obtained in different places and times. Within this framework, Task Group 3 (TG3) of Working Group 1 (WG1) of Technical Committee 308 (TC308) of European Standardization Committee (CEN) undertook the preparation of a Technical Report dealing with the preparation of synthetic suspensions, both inorganic and organic, in repeatable conditions, able to describe the behaviour of a real sludge. This paper discusses a proposal for ingredients and modalities to prepare such synthetic suspensions that will be systematically tested in already planned lab experiments to define an optimal recipe.

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