By Bart Dewaele and Laetitia d’Ursel, Waterleau Group NV
Beer brewing and processing is complex and energy intensive. And despite significant technological improvements, energy consumption remains a major environmental challenge in the brewing industry, together with wastewater and produced byproducts.
Nowadays, almost every large brewery has a wastewater treatment plant (i.e. anaerobic pre-treatment and aerobic post-treatment). A first option to reduce the breweries energy footprint is to use the biogas generated through this process and convert it into electrical and thermal power.
But, apart from the wastewater, the brewery industry also generates different byproducts such as spent grains, kieselguhr, and excess yeast from the fermentation process. Excess yeast is actually also a valuable energy source when submitted to anaerobic treatment.
Due to its characteristics, the total excess yeast flow cannot be mixed with brewery wastewater for anaerobic treatment in the same reactor. The yeast produced during the fermentation process is only partially reused for the pitching of subsequent brews. This excess yeast has a rather high nutrition value after autolysis and is generally used for animal feed. Its disposal however, often presents a considerable problem. Direct discharge often causes logistic issues and pre-dewatering is rather expensive and only possible on the non-autolyzed yeast.
Taking into account sustainability and cost-effectiveness, anaerobic treatment offers the best treatment process for brewery excess yeast: the energy conservation aspect is indeed the main driver in choosing for anaerobic treatment as this solution avoids energy loss during drying and/or dewatering of the yeast, while biogas is produced during its anaerobic treatment process.
All in all, the anaerobic digestion of the excess yeast helps reducing the energy footprint of the brewery.
For anaerobic treatment, the high suspended solids (SS) and nitrogen (N) concentration in the yeast are to be taken into account. The characteristics of the brewery excess yeast are shown in the table below.
Due to the high amount of suspended solids, the total yeast flow cannot be mixed with the brewery wastewater flow as brewery wastewater is generally pre-treated in a UASB (Upflow Anaerobic Sludge Blanket) reactor. This type of anaerobic reactor is dependent on the growth of well-settling granular sludge. However, too important suspended solids concentrations and too low COD/SS ratios are known to interfere with the granulation of anaerobic sludge.
Consequently, a separate and other type of reactor is to be provided for the anaerobic treatment of yeast. Because of the very high COD concentration of the yeast flow and the corresponding hydraulic retention time, a CSTR (Continuously Stirred-Tank Reactor) type of reactor is to be used.
Practical example: SABMiller brewery in Uganda
Quick facts: 3.000 m³/d wastewater - 10.500 kg COD/d - Biogas production: 4.200 Nm³/d
At the Nile Breweries site in Uganda, a SABMiller brewery, Waterleau designed and built an adapted CSTR yeast digester. The yeast digester is part of a complete anaerobic/aerobic wastewater treatment plant build in a fast mounting concept, allowing 25% less construction time.
It also combines the wastewater treatment with biogas-to energy (digestion of organic waste) as the effluent of the yeast digester is send towards the aerobic post-treatment. The biogas produced is mixed with the biogas from the UASB reactor and is being reused in a dual fuel boiler after desulfurization.