M. Kuglarz, K.GrĂĽbel
University of Bielsko-Biala, Faculty of Materials and Environmental Sciences, Institute of Environmental Protection and Engineering, Bielsko-Biala, PL

CO-DIGESTION OF SEWAGE SLUDGE AND RAPESEED CAKES COUPLED WITH EFFECTIVE TREATMENT OF POST-DIGESTION LIQUORS


The aim of this study was to develop the technological concept ensuring: (1) high efficiency co-digestion of by-products from the production of biodiesel and sewage sludge as well as (2) an effective method of post-digestion effluents treatment, - to the level allowing its discharge to the natural reservoir. The digesters were fed with sewage sludge as well as co-digestion mixture of 2% rapeseed cakes + 98% sewage sludge. The addition of rapeseed cakes to sewage sludge influenced positively the degree of organic matter biodegradation and the quantity and quality of biogas produced. The co-digestion mixtures performed in optimal conditions (HRT = 22-24 days) generated the double amounts of biogas - containing about 10-12% more CH4, compared to the samples including the only sewage sludge. Under these conditions, biogas yield increased by about 48-82%, - depending on the amount of co-substrate used. Taking into account the high content of organic matter (COD) and nutrients (NH4+, PO43-) in post-digestion effluents, an attempt was made to treat them by the application of chemical precipitation and membrane filtration. The treatment of post-digestion liquors with a struvite precipitation and a subsequent RO process turned out to be effective and allowed to decrease contaminants’ concentration below discharge limits, allowing direct release of permeate to a natural receiver. However, it was necessary to apply an increased ratio of magnesium and phosphates during struvite precipitation (Mg2+: NH4+: PO43- = 1.5:1:1.5), preceding reverse osmosis treatment.

Keywords: co-digestion, biodiesel, post-digestion effluents, chemical precipitation, reverse osmosis