Composting and biogasification for senna pod waste stabilization

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Courtesy of ORBIT e.V.

A medical factory in Finland extracts sennocid from pods of a senna plant with a solvent containing methanol and water. Sennocid is the active compound in some constipation medicines (Otavan Iso Kasvitietosanankirja, 1981). The extraction process produces 50 m3 of pod waste weekly. In 1998, 757 tons of pod waste was produced. The water and methanol contents of the waste are approximately 25 w/w-% both.

The plant used by the medical factory is Cassia senna L. Other names in use are C. acutifolia Del., Senna acutifolia (Del.) Batka, and S. alexandrina Miller. (Lebrun J-P and Stork 1992) Cassia senna belongs to leguminous plants, and is a perennial shrub. It is cultivated in Northern Sudan. Cassia senna can grow up to 3 m. The pods are oblong, flattened, 16...26 mm long, and brown. (El Amin 1990; Otavan iso kasvitietosanankirja 1981) The pods are manually collected from the shrubs, dried in the sun, and shipped to Finland.

Composting and anaerobic digestion are widely used in solid waste management. Each process has its advantages and disadvantages. In general, anaerobic process is more complex than aerobic process. However, in an anaerobic process energy is recovered as methane gas. Aerobic processes are net energy consumers because oxygen must be supplied. Both methods significantly reduce the volume of the waste. The end products of aerobic processes are humus, CO2, and H2O, and of anaerobic processes digested sludge, CO2, and CH4. Composting is the most commonly used biological process for the organic fraction of municipal solid waste. (Tchobanoglous et al. 1993) Anaerobic treatment is widely used as the stabilisation method for wastewater sludge.

The aim of this study was to examine composting and anaerobic treatment as the pod waste treatment alternatives. The large quantity and the high methanol concentration of the pod waste make the choice of the treatment method challenging. Composting with different materials was examined in a field-scale experiment. Anaerobic degradation of the pod waste was studied in a batch bioassay.

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