The specific methanogenic activity (SMA = gDQO-CH4-gVSS-1-d-1) is defined as the rate of methane production, expressed as COD, regarding biomass expressed as the content of volatile suspended solids (VSS). In anaerobic degradability test measures the rate of degradation of a compound relative to a standard compound that is acetic acid determining [36].

COD

Where: Slope = m=LCH4; Biomass = X [=] gVSS/L; Methane conversion у COD =0.35

Methanogenic activity and toxicity

Methanogenic activity were performed using the pressure transducer technique, which involves the monitoring of the pressure increase developed insealed vials fed with non­gaseous substrates or pressure decrease in vials pressurised with gaseous substrates. Strict anaerobic conditions must be maintained. The same technique can be used to perform the methanogenic toxicity tests. The fifty percent inhibition concentration (IC50) was defined as the methanogenic concentration that caused a 50% relative activity loss [35, 37].

The technique is as follows:

1. The sludge is left in mineral medium for 24 hours at 30-35 °C in order to consume the entire carbon source which may have been brought into water of the plant.

2. Methanogenic activity tests were conducted in 160 mL in serology bottles with an operating volume of 150 mL. The volume of volatile suspended solids was set at 2 g/L and COD concentration used was varied from 0.25, 0.5, 1, 2, 3 y 5 g/L using acetate as a carbon source, and staying a relationship of 0.125, 0.25, 0.5, 1, 1.5 y 2.5 gCOD/gVSS respectively.

3. The bottles were sealed with rubber stoppers, and incubated 24 h at 35°C.

4. Methane was determined by the displacement volume of a solution of 3% NaOH [38].

Figure 2 show a schematic representation of methane measuring.