**2.2 Methods**

The TS content and the volatile solids (VS) content of the samples were determined by drying and burning to constant weight at a temperature of 105°C and 650°C, respectively. The chemical oxygen demand (COD) of the sludges was measured by the standard method MSZ 21976-10:1982.

The amount of biogas generated, was measured by an "A1" type, Schlumberger wet gas meter. The methane and hydrogen content of biogas was measured by a Shimadzu 2014 gas chromatograph. The temperature of the column was 60°C, the temperature of the injector was 170°C, and the temperature of the detector was 250°C. As carrier gas we used nitrogen with 20 mL/min gas flow. In the 3.0-m long, 3.00-mm internal diameter glass column, Supelco Molecular sieve filling was put. The detection was done with TCD detector. We measured the quantity and the methane content of the biogas every day at the beginning, and then, when the amount of the biogas decreased, every other and then every fifth day.

Co-Digestion of Organic Waste

the calculation of the results.

tangent to the fitted curves.

**3. Results and evaluation** 

material ratios; and

**2.4 The quantification of anaerobic degradation** 

material was defined by the formula below:

up phase, pursuant to the Monod and the Briggs-Haldane model.

and Sewage Sludge by Dry Batch Anaerobic Treatment 103

Each reactor was filled with an equal amount and quality (TS=22%) of waste. In order to prevent the disturbing effects caused by the oxygen, we flushed the heads of the reactors with nitrogen gas after the sampling. The diluting effect of the head-space was considered at

The COD of methane produced in the anaerobic degradation of organic substrate corresponds with the COD of the removed organic mass (Lettinga & Hulshoff Pol, 1990). The amount of organic matter removed during the anaerobic treatment, the degree of degradation, was determined by measuring the total amount of methane produced during the period (T), which was converted to COD, taking into account that 1 Ndm3 methane is equivalent to 2.86 g CH4-COD. Based on this, the degree of degradation of the organic

 DT% = (∑CH4 CODT / sludge COD) × 100 (1) We fitted a logistic function-relation (DT% = Dmax / (1 + e-k(t-t0))) to the measuring results with SPSS 14.0 software. We used sludge as seeding material and the substrate for the tests after storing at 5°C, therefore we had to calculate with the lag phase in the beginning by choosing the logistic function-relation. The logistic curves take into consideration the start-

We determined the value of maximum degradation (Dmax) in case of biowaste and sludge for the fitting as 65% and 50% respectively, which values were based on our former own measuring results (Rózsané et al, 2011) and on technical literature data (Haug, 1980). We determined the *k* invariant of reaction speed and the *t0* time defining the inflexion point in a

In the case of methane production projected to the volume of reactor, we did not deduce the methane production of the seeding material, but we used the results for the whole volume of the mixture of waste and seeding sludge. In case of the measuring results used for the volume of the reactor, we fitted the function-relation in a way identical with the previous, where the maximal degree of methane production (CH4max) was determined with the account of substrate to seeding material mixing rates and the maximal degradability. To characterize the speed of the degradation process, in the case of both measuring results, we determined the values of the starting v10d and v30d degradation speed as the direction

We assumed that the balance of the multi-stage anaerobic digestion process can be influenced by setting the ratio of the seeding material which results in greater degradation


of the organic content of the treated waste, as well as in greater methane production.

We evaluated the experimental results based on two aspects:

way that the function-relation would have the best fit (R2) of the measuring results.

To determine the methanogenic activity of the seeding material we used neutralized acetic acid as a substrate. To decrease the retardatory effects we added macro- and micro-nutrients (Biotechnion, 1996), and incubated the samples on the temperature of 35°C. We used liquidphased mixed reactors to decrease the substrate-gradient. The amount of biogas generated, was calculated on the basis of pressure changes in the head-part of the 1.5 dm3 reactors. To remove the generated CO2, NaOH pellets were placed in the heads-part of the reactors. Specific methanogenic activity of the seed was calculated on the basis of cumulative methane production graphs by taking the tangent of the deepest slope of the curve.

The acidity of the sludge was checked by a pH meter (340i WTW) pH/mV measuring device, to which a SenTix 41 type electrode was connected.
