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A two-stage electrodialysis (ED) method was used for lactic acid recovery. In the first step sodium lactate was concentrated with desalting electrodialysis using ion exchange membranes Ralex (Mega, Czech Republic). The second step was the electroconversion of sodium lactate to lactic acid by electrodialysis with bipolar membranes (EDBM) Neosepta (Tokuyama Corp., Japan). The lactic acid was recovered from model solutions and from real fermentation broth as well. The trials with model solutions were focused on determination of the suitable conditions for electrodialysis experiments and on investigation of the time course under different conditions. The fermentation broth from lactic acid fermentation had to be pretreated before electrodialysis experiments. The pretreatment consisted of ultrafiltration, decolourisation and removing of multivalent metal ions. In the first ED step the final lactate concentration of 175 g/l was obtained and afterwards the final lactic acid concentration of 151 g/l was reached in the second ED step.

Lactic acid separation from the fermentation broth was studied comprehensively and the results obtained showed very good agreement with the literature sources [6,19]. The results confirm that the two-stage electrodialysis is a suitable and efficient technique for the recovery of lactate ions from the pretreated fermentation broth and subsequent conversion into lactic acid with respect to environmental aspects. In the first ED step the final lactate concentration up to 175 g/l was obtained and the final lactic acid concentration of 151 g/l was reached in the second step. Total required energy in both electrodialysis processes consisting of energy Table 5 EDBM — experiments with model sodium lactate solutions Operating time, Initial lactate min concentration, g/l Final lactic acid Final lactate Final NaOH concentration, concentration, concentration, g/l g/l g/l Na+ transport, WS 1 WS 2 WS 3 WS 4 WS 5 WS 6 WS 7 WS 8 WS 9 82.5 14.0 58.1 Experiment Water transport Rate of Na+ transport, index, g/h ml/g Current Rate of lactic acid conversion, efficiency, % g/h Energy consumption, kWh/kg % of conversion WS 1 WS 2 WS 3 WS 4 WS 5 WS 6 WS 7 WS 8 WS 9 3.8 3.0 3.3 3.4 6.7 2.8 3.9 4.2 2.3 45.3 41.0 26.9 29.8 34.2 25.4 37.6 36.4 39.2 0.91 0.84 1.20 1.14 1.20 1.38 1.10 1.10 0.98 Experiment 11.6 10.5 6.9 7.6 8.7 6.5 9.6 9.3 10.0 0.5 1.3 1.5 2.0 2.3 1.8 1.0 2.1 5.1 g Table 6 EDBM — experiments with lactate recovered from the fermentation broth Experiment Operating time, min Na+ transport, Initial lactate concentration, g/l Final lactic acid Final lactate Final NaOH concentration, concentration, concentration, g/l g/l g/l g WM 1 WM 2 4.0 2.6 Experiment Water transport index, mg/l Rate of Na+ transport, g/h Rate of lactic Current acid conversion, efficiency, g/h % Energy consumption, kWh/kg % of conversion WM 1 WM 2 3.9 3.3 8.2 9.9 32.2 38.9 1.16 0.95 consumption for lactate transfer and for its electroconversion to lactic acid was about 1.5 kWh per 1 kg of lactic acid obtained.