AS 4276.23:2016 Water microbiology
8.2.2 Liquid samples ( ≤ approx. 7% solids)
An adequate degree of homogenization of liquid samples should have been achieved in the initial mixing step. If not, a subsample is homogenized using stomaching or blending. The minimum subsample volume required for a liquid sample is 300 mL. Check the pH of homogenized liquid samples. Sample pH shall be adjusted to 7.0–7.5 by addition of 1.0 N HCl or NaOH prior to removal of further subsamples.
8.2.3 Solid samples (> approx. 7% solids)
The procedure for solid samples shall be as follows: weigh out 30 ±0.1 g of well-mixed sample into a receptacle to be used for homogenization (blender jar, stomacher bag, sample container) and add 270 mL SEB. Cover or otherwise seal the sample in the container and homogenize. It may be beneficial, especially if the sample is particularly hard or dry, to allow it to soak in the SEB prior to homogenization (0.5–1.0 h). Check pH and adjust to 7.0–7.5 by addition of 1 N HCl or NaOH. This homogenate represents 0.1 gram wet weight sample per millilitre.
8.2.4 Direct addition to enrichment broths
When well mixed sample is directly added to enrichment broths, the sample is in effect being homogenized in the broth rather than extraction buffer. The same procedural details apply except that the weight of sample added to an enrichment broth is according to the applicable method, with pro rata adjustments as necessary. The tolerance on any respective masses and volumes is ±5%. Liquid samples will have been pH adjusted prior to addition and require no pH adjustment unless a different pH is specified in the applicable method. For solid samples, adjust the pH only after the subsample is added to the enrichment broth.
8.4.1 Manual and/or mechanical mixing
It is common in sludge matrices for aggregation to occur. Therefore simple manual or mechanical mixing, as generally employed to prepare microbiological matrices, may not be sufficient to achieve a homogenous sample. More robust containers may be required to adequately and safely process these samples. Take care when opening receptacle lids as foaming can occur during mixing. Consider aerosol minimizing strategies to avoid biological safety concerns, including inhalation.