Biogas production in breweries

Breweries produce typically 2 - 6 hl of wastewater per hl produced beer. The Chemical Oxygen Demand (COD) of this wastewater varies between 2000 and 6000 ppm, with a BOD (Biological Oxygen Demand)/COD ratio of 0.5-0.7. The COD consists mainly of easily biodegradable organic compounds such as sugars, ethanol and soluble starch. Because of its high biodegradability, biological treatment (anaerobic and aerobic) is the most favorite treatment method for brewery wastewater.

During aerobic treatment, organic compounds are oxidized to carbon dioxide and water by aerobic bacteria, in the presence of air. The energy requirement in an aerobic system is typically 0.7-1 kWh/kg CODand the applied volumetric loading rate 0.5 - 2.0 kg COD/m³/d. During anaerobic treatment, organic compounds are reduced to methane gas by anaerobic bacteria in an environment free of oxygen. The energy requirement in anaerobic treatment is a factor 10 lower compared to aerobic treatment, 0.07-0.1 kWh/kg COD. Volumetric reactor loading rates can go up to 25 kg COD/m³/d, resulting in smaller reactor volumes. Produced anaerobic biomass can be sold as a seed source for quick start-up of new anaerobic plants, so there are no significant disposal costs as for aerobic sludge.

Often in brewery wastewater treatment anaerobic and aerobic treatment are combined. Firstly, in the anaerobic reactor the bulk of the COD, 70-85%, is converted into biogas on a small surface area. Secondly, in an aerobic/anoxic post-treatment step up to 98% of the COD, and nutrients are removed.

When biogas is burned in brewery boilers or in a combined heat and power unit (CHP), anaerobic treatment can create a positive energy balance. If utilization of biogas directly in the boiler of the brewery or in a separate CHP unit is more economical, must be evaluated for each project and depends on local conditions. In a CHP unit, both electrical and thermal energy are produced. . In some countries, e.g. in Germany, this kind of renewable energy production is subsidized by the state. If public subsidies are available, the installation of a CHP unit can be economically feasible, even though this is related to higher investment costs. In breweries, direct utilization of biogas in a boiler is usually the preferred solution. The reason for this is that Investment costs for a CHP unit are higher and more extensive biogas treatment is required. Conventional boiler fuel can partly be replaced by utilization of biogas, resulting in operational cost saving. In the context of decreasing fossil fuel reserves, anaerobic wastewater treatment makes a brewery more independent from external fuel supply. Futhermore it contributes to a more sustainable brewing process.

Of all anaerobic processes those technologies based on high-rate, compact, granular biomass technology, such as UASB, EGSB and IC, have a leading position. Biothane, a company part of Veolia Water Solutions & Technologies, applied worldwide more than hundred UASB (medium rate) and EGSB (high rate) systems at breweries.

A cross section of a typical Biobed® EGSB reactor of Biothane is shown in Figure 1. Before introduction of wastewater in a Biobed® EGSB reactor, the conditions of the wastewater are optimised for anaerobic biomass in a'conditioning tank'. In the conditioning tank pH and temperature are corrected and micronutrients can be added to the wastewater if required. After conditioning the wastewater is pumped into the Biobed® reactor, and evenly distrubuted over the reactor bottom through an influent distrubution system. The wastewater passes through a dense bed of anaerobic granular biomass, typically consisting of self forming black pellets with a diameter of 2-4 mm. The granular biomass is characterized by a high settling velocity of >50 m/h and a densitiy of 40-80 kg Dry Solids/m3. In top of the reactor, the biomass, produced biogas and treated water are seperated by a three-phase separator (settler). The treated water is discharged to the conditioning tank where part of it is mixed with the feed water and recirculated to the Biobed® reactor. Produced biogas is also conveyed to the gas chamber of the conditioning tank before further utilization. By recirculation of the treated water and removing of degassing CO2 with the biogas stream from the conditioning tank the required caustic use for wastewater neutralization is reduced by recycling of alkalinity. Since both the conditioning tank and the Biobed® EGSB reactor are operated gas tight and under pressure, the system is odour free and gas holder for gas storage is not necessarily required. After compression, the biogas can directly be used in a boiler or CHP unit. Additionally an emergency flare is always located close to the anaerobic reactor.