In general, it is considered that the characteristics of the stillage produced from lignocellulosic materials are comparable to those of the stillage obtained when conventional feedstocks are employed, thus the treatment and utilization methods of this latter type of stillage can be applied to the lignocellulosic stillage (Wilkie et al., 2000). The flowsheets proposed for the treatment of the stillage from bio­mass contemplate its evaporation and centrifugation in order to obtain the thin stillage and solid materials (Wooley et al., 1999). The solids recovered have high lignin content so they are sent to the burner of the boiler for generation of the steam required by the overall ethanol production process. The thin stillage is par­tially recirculated, along with the bottoms of the rectification column, as water for washing the pretreated lignocellulosic biomass (see Chapter 11). The remaining thin stillage is directed to the evaporation train in order to concentrate it into a syrup containing about 60% water. Due to this high moisture content, this syrup can undergo anaerobic digestion though its incineration.

Besides the stillage, other liquid waste streams are generated in the process for fuel ethanol production from lignocellulosic biomass. For the treatment of this wastewater, different alternatives have been evaluated that include its anaerobic digestion to remove 90% of its organic matter content, as well as the utilization of the biogas produced for steam generation. The effluent of anaerobic digestion can be sent to an aerobic digestion pond where another 90% of the organic matter content is removed. The aerobic sludge formed in this process is withdrawn by clarification and filtration. The filtered sludge can also be sent to the burner for steam generation. The water from the clarifier can be recirculated in the process (Merrick & Company, 1998; Wooley et al., 1999).

Case Study 10.1 Preliminary Comparison of Effluent Treatment Alternatives

Process synthesis procedures should provide insight on the more suitable tech­nologies for effluent treatment during fuel ethanol production. In a previous work (Cardona et al., 2006), three effluent treatment configurations were analyzed. The effluent studied was the thin stillage generated during the production of fuel etha­nol from lignocellulosic biomass as well as other liquid effluents (wastewater from

pretreatment and detoxification). In addition, the solid residues generated during the centrifugation of the whole stillage were also studied. These residues are mainly represented by the lignin nontransformed during the process, residual cellulose and hemicellulose, and the cell biomass (in this case Zymomonas mobilis fragments). The configurations were simulated using Aspen Plus® v11.1 applying the general guideline for simulations described in Chapter 8, Case Study 8.1. To analyze the biological transformations taking place during the degradation of the organic mat­ter contained in the stillage, the stoichiometric approach was employed in the framework of the process simulator. The percentage of organic matter removal (degradation) was evaluated based on the mass balance results calculated. In addi­tion, preliminary data on generation of electricity was also taken into account. The aim of this case study was to investigate different flowsheet configurations and their combinations for the treatment of the liquid and solid effluents obtained during fuel ethanol production from lignocellulosic biomass.

The treatment procedures analyzed consider the anaerobic digestion of the thin stillage, the aerobic digestion of this same stream, the combination of these two treatments, and the incineration of solid residues in order to obtain electricity. These options are depicted in Figure 10.1. The anaerobic digestion was analyzed through the following chemical reactions mediated by a consortium (mixed culture) of anaerobic bacteria in the corresponding reactor:

+ h2o ^ СбН12Об

Cellulose Water Glucose

(C5H8O4)„ + H2O ^ C5H„O5 Hemicellulose (xylan) Water Xylose

CHL8Oa5Na2 + 0.5 H2O ^ 0.167 C6H12O6 + 0.2 NH3 + 0.1 H2 Z. mobilis Water Glucose Ammonia Hydrogen

C6H12O6 ^ 2 C2H5OH + 2 CO2 Glucose Ethanol Carbon dioxide

3 C5H10O5 ^ 5 C2H5OH + 5 CO2 Xylose Ethanol Carbon dioxide

2 C2H5OH + CO2 ^ CH4 + 2 CH3COOH Ethanol Carbon dioxide Methane Acetic acid

CH3COOH ^ CH4 + CO2

Acetic acid Methane Carbon dioxide

4H2 + CO2 ^ CH4 + 2 H2O

Hydrogen Carbon dioxide Methane Water

These reactions represent some of the main stages of anaerobic digestion of organic matter. The first three reactions correspond to the hydrolytic processes, the following three reactions are carried out by fermentative bacteria, the sixth reaction represents the acetogenesis stage, and the last two reactions globally describe the action of methanogenic bacteria. The formation of anaerobic bacteria is not shown in this reaction scheme although it has been taken into account.

The aerobic digestion was simulated considering the oxidation reaction for each one of the organic compounds present in the liquid effluent. As an example, the complete oxidation of glucose is presented as follows:

C6H12O6 + O2 ^ 6 CO2 + 6 H2O Glucose Oxygen Carbon dioxide Water

Under aerobic conditions, a large portion of the organic matter contained in wastewater may be oxidized biologically by microorganisms to carbon dioxide and water, thus the formation of activated sludge (aerobic bacteria) was also consid­ered. Approximately 50% reduction in solids content can be achieved through this treatment.

Finally, the incineration of the solid residues was simulated through a stoichio­metric approach where all the organic compounds are completely oxidized into carbon dioxide and water without formation of any cell biomass. The products of this process are the combustion gases and the remaining ash. The main combustion reaction is the burning of lignin whose energy content achieves 25.4 MJ/kg.

The first configuration contemplates the anaerobic digestion of the liquid efflu­ent (mostly thin stillage) in an anaerobic reactor producing biogas and a suspension of anaerobic sludge that is settled in a decanter. Then, the sludge is mixed with the predried solids residue in order to be burnt in the co-generation system. This

system consists of a burner where the combustion reaction is carried out, a cyclone for removing the ash, a heat exchanger representing the boiler, and a turbogenera­tor where the electricity is produced employing the high-pressure (HP) steam from the boiler. The low-pressure (LP) steam can be used to cover the thermal energy required in the overall ethanol production process. This configuration is schemati­cally depicted in Figure 10.2.

The second configuration is based on the aerobic digestion of the wastewater.

As in the previous case, the aerobic sludge generated and the solids residue are sent to the cogeneration system (Figure 10.3). Finally, the third configuration comprises the combination of anaerobic and aerobic digestion as well as the delivery of the generated sludge and solids residue to the cogeneration unit, as can be observed in Figure 10.4.

The preliminary results obtained are presented in Table 10.1. The scheme involv­ing the anaerobic digestion of the liquid effluents followed by the aerobic digestion of the liquid stream exiting the anaerobic digester represents higher removal of the organic matter contained in these effluents, as suggested in the work of Merrick & Company (1998). If only one type of digestion is considered, the anaerobic diges­tion gives better results. In the Table 10.1, the sole incineration of the two types of effluent streams (solid and liquid) is also considered as a fourth configuration.

In this case, the power regenerated was considered as the comparison standard

(100%). The configurations involving the separate treatment of the wastewater show a reduced generation of electricity due to the lower availability of organic compounds entering the cogeneration unit. However, the incineration implies that the liquid streams are evaporated before their mixing with the solid residues. For this process, energy is required and the need for thermal energy should be supplied by the low-pressure steam extracted from the turbogenerator. This means that the net energy (thermal and electric) produced can be lower than the net energy of the schemes involving the separate treatment of wastewater. Undoubtedly, process simulation tools are invaluable to assess all these configurations and the energy surplus that may be obtained from these treatment schemes.