**4. Conclusion**

Based on the test results we stated that the sewage sludge can be well degraded also through co-digestion by dry batch treatment together with VFG waste. We stated that in the case of 1:0.5 and 1:1 biowaste to seed ratios, the reactors became acidified. Even in the case of higher seeding ratios 9-day initial "lag" phase can occur. The hydrogen content of the biogas and the pH in the reactors indicate an initial accumulation of fatty acids in the reactor. We measured the highest, 54% organic material degradation in the case of 1:3 biowaste to seed ratio. Comparing our measurement results with literature data, it can be stated that the total methane production projected to one unit of organic material and the organic material degradation is nearly the same in total. Our laboratory-scale experiment, however, was influenced by the relatively long "lag" phase. Based on our tests it can be stated that it is a complex task to determine the optimal seeding ratio and retention time where a universal value cannot be given. In practice, the optimal values have to be determined one by one, taking into consideration the degradation target together with the specific gas yield projected to the reactor. From the aspects of costs reduction regarding the investment and operation, based on the values of the gas productions referred to the reactor volume, the 1:1.5 biowaste to seed ratio seemed to be the most efficient. This lets the conclusion be drawn that it is not worth to recycle the seeding material in the reactors in a higher ratio than this. According to the above, the compromising waste to seed ratio taking into consideration the different aspects is minimum 1:1.5 which takes into account the higher degradation of the organic material, as well as the quantity of the methane producible from one unit of reactor volume and the demand for low investment costs.

In the case of the same seed ratio, we experienced great difference in the efficacy of the treatment in case of biowaste containing VFG and the sewage sludge. During a co-digestion of sewage sludge and VFG wastes, because of the VFG waste having a quality varying in space and time, it is advisable to determine the suitable seed ratio through degradation tests in advance.
