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AGROIWATECH Résumé de rapport

Project ID: ICA2-CT-2002-10010
Financé au titre de: FP5-INCO 2
Pays: Bosnia and Herzegovina

Technical and economical evaluation of the processes of agro-industrial solid waste biodegradation

The technical and economical evaluation of the anaerobic processes and technologies can not be completely evaluated only from laboratory data since scale up factors can play an effective role during full scale plant design. To evaluate the performance of an inclined plug flow reactor a pilot plant has been designed and constructed according to the experience gained from laboratory scale reactors and similar reactors installed at other research institutes. The plant has been equipped with on line record of temperature inside of reactor, effluent and gas quantity and gas quality, while measurement of total and volatile solids, pH, volatile fatty acids, alkalinity, ammonia and organic nitrogen was carried out at the HEIS laboratory.

Solid waste obtained from the potato processing industry, was past through mechanical treatment prior to treatment process. Treatment temperature was set and controlled on 36°C. Samples for laboratory analyses were taken from three sampling point along reactor chamber.

The plant was operated to evaluate the performances of the treatment in terms of maximal loading, biogas production, and possible inhibitions on anaerobic process. In that direction, different amounts of solid wastes were used for the purpose of loading the reactor.

The results obtained can be summarized as follows.

Anaerobic treatment of solid waste is a very sensitive process on changing the loading regime. That is why this process takes a lot of time (at least 8 months) to achieve maximum loading and efficiency. In the same time, possible over loadings can vary causing fast negative impacts on whole process, with very long period for recovery, but sometimes without any possibilities to go back in steady state.

Process imbalance in an anaerobic digester will normally lead to accumulation of VFA resulting in decrease in pH. The increase in acid concentration may not be immediately registered as a drop in pH if the buffer capacity of the material in the reactor is high. The organic acid accumulation therefore has to reach a high level before it is detected as a drop in pH. At that point the organic acids significantly inhibit the process. pH is easy and inexpensive to measure, and it is effective indicator in systems with low buffer capacity. In any case, measurement of VFA together with alkalinity is much better for the purpose of control and to avoid VFA inhibitions.

Demonstrated treatment requires continuous supervision, due to the need to install on-line measurement of: loading quantity, biogas quantity, biogas quality (at least content of methane), effluent quantity, temperature inside of reactor, outside temperature. In addition it is necessary to analyse pH, alkalinity and VFA on daily basis.

Continuous loading of pilot plant with smaller quantities of waste would be preferable comparing to the loading of once per day and decreases sensitivity of the system
Similar results on samples from three different sampling points suggesting that pilot plant was not a real plug flow reactor, but more like CSTR reactor. Mixture inside was more liquid then slurry.

By the end of test period, it can be considered that pilot plant was still at start-up period. Hydraulic retention time decreased from 135 days down to 54, and in overloading period reached the value of 27 days.

Reported by

Hydro Engineering Institute of Civil Engineering Faculty University of Sarajevo
Stjepana Tomica 1,
Bosnia and Herzegovina