**2.1 Materials**

To ensure the repeatability of experiments, we modelled the biowaste (mixture of sewage sludge and VFG) generated in the settlements with a material mixture of fixed ratio as

affect the oxidation of the hydrogen, decreasing the reaction efficiency and increasing the

To keep the degradation process balanced, seeding material is needed. A significant effect in balancing the process can be achieved by properly setting the ratio of the methanogen seeding material. However, the determination of the optimal amount of the seeding material is a complex issue which is of great importance to the operation of batch reactors being economical. Low seeding material ratio, in extreme cases, can lead to acidification of the reactor or, in better cases, moderates the process rate. This can be compensated by extending

The increase of the biogas production and the decrease of the treatment time can be achieved also by increasing the quantity of the seeding material since this way a more effective degradation can be counted on. However, increasing the quantity of the seeding

The optimal waste to seeding material ratio in the case of municipal solid organic wastes is 1:2.3 in laboratory, while less than 1 : 1 in full scale conditions (Brummeler, 1993). According to related literature data, the duration of the treatment in cases of municipal organic waste is around 30-36 days (Brummeler et al., 1991, 1992); however in cases of a low seeding material ratio, the duration of the treatment can be 50 days or more (Brummeler et al., 1992). There is no published data about seeding material demand for the dry, batch anaerobic co-treatment

We assume that the combined dry batch treatment of VFG waste generated on settlements and of sewage sludge has many advantages. As a result of the co-digestion, because of the different easily degradable organic contents of the sewage sludge and VFG, we can count on the increase of the gas yield projected to reactor volume, compared to a separate treatment of the VFG and sludge. We can assume as an advantage that a more balanced quality of the sewage sludge can have a positive effect on the co-digestion with organic wastes having quality varying with time and space. A further advantage can be, from the aspect as a potential of anaerobic treatment of the sewage sludges generated on smaller settlements, that the increased waste flow with VFG can make it economical. Nevertheless, it is necessary to investigate the appropriate seeding material ratio, the determination of which does not depend only on the achievable methane yield but on the required duration of the treatment

Our aim is to study the combined dry batch treatment of VFG and sewage sludge. Our goal is to evaluate the aspects of determination of the optimal seeding material ratio, besides the study of the avoidance of acidification of the reactors, the achievable greater degradation

In order to achieve our goals, we carried out laboratory experiments with dry batch reactors.

To ensure the repeatability of experiments, we modelled the biowaste (mixture of sewage sludge and VFG) generated in the settlements with a material mixture of fixed ratio as

partial pressure of the hydrogen (Gujer & Zehnder, 1983).

the retention time associated with increased reactor volume.

material can result in the increase of the reactor volume, too.

and on the targeted stabilization goal of organic material, too.

rate of organic material and the maximal gas yield.

**2. Materials and methods** 

**2.1 Materials** 

of the biowaste and waste water sludge.

follows: 50% municipal excess sludge, 50% VFG consisting 25% fresh grass and 25% kitchen waste. The excess sludge came from the activated sludge technology of a municipal waste water plant, which can be characterized with a 20-day sludge retention time. The kitchen waste consisted of 25% potato peel, 15% lettuce, 15% bread, 15% cucumber peel, 10% cabbage, 10% paper and 10% coffee grounds.

The amount of total solids (furthermore as TS), volatile solids (furthermore as VS) and the value of chemical oxygen demand (furthermore as COD) of the waste and the sludge are presented in Table 1.


Table 1. Characteristics of waste and sludge used for the experiment

To characterize the seed, we defined its stability and methanogenic activity. The seed was not stable, it could be degraded by a further 13%. The organic degradation occurred mostly within the first 30 days. The digested sludge came from a completely stirred tank reactor operated with 20 days hydraulic retention time. The methanogenic activity of the seed was 0.026 CH4-COD . g VS-1 . d –1, which shows the maximum methane production measured in chemical oxygen demand (COD) of digested sludge for a unit of volatile solid in a unit of time.
