Relative hydrophobicity (RH)

RH has been used in biotechnology as an indicator of physiological state of microorganisms. RH is measured by the test called microbial adhesion to hydrocarbons (MATH) test. MATH-test is performed with solvents such as n-hexane, n-octane, n-octanol, n-dodecane, etc., where the ratio of microbes in the aqueous phase and in the solvent phase are measured based on light absorbances. Separation funnels are used to perform the test as shown in Fig. 1.

   In the original version of MATH-test, 4 ml sample is mixed with 1 ml n-dodecane and vortexed for 2 minutes followed by resting for 15 minutes to allow the phase separation. After taking 0.75 ml of aqueous sample, absorbance (Se) is measured at 400-600nm. The absorbance of the original aqueous sample (Si) is also measured at the same wavelength. Finally, RH is calculated using the following equation (Saini, 2010).


RH of biosolids can be used to estimate to qualitatively estimate the the membrane fouling potential in MBR. There are many variations in MATH-test depending on the solvent used, voltexing method and duration, the wave length used, etc. In some cases, the amount of dissolved materials in solvent phase is used as an indicator of RH after evaporating the solvent.

Hydrop2Fig. 1. Separation funnel (drawing is taken from here).

   The RH-value alone cannot be used as an indicator of good or bad sludge filterability (Van den Broeck, 2011). In the following figure, the RH of MBR 6 sludge decreases from 83% in winter to 60% in summer but the filtration resistance, R20, defined here decreases from 0.71 ×1012m−1 to 0.07 × 1012m−1. On the contrary, MBR 8 sludge was more hydrophobic in summer than in winter, e.g. 62% versus 53%, but the filtration resistance, R20, increases from 6.28 ×1012m−1 to 9.90 ×1012m−1.

Hydrop1Fig. 2. Relative hydrophobicity (RH) of activated sludge samples (Van den Broeck, 2011).


© Seong Hoon Yoon