Whey Management for the Dairy Industry: Acid and Salty Whey Treatment and Processing Using Membrane Technology

© 2020 Sahar TalebiAcid whey and salty whey have presented a major disposal issue for the dairy industry. The processing of acid whey has proven challenging due to the presence of lactic acid and high levels of minerals, while salty whey is underutilized due to the high levels of salt. The treatment of these two types of whey will allow the production of high value products, including whey powders, lactose powder, and concentrated salt solutions for use in cheese salting or the chlor-alkali industry. The use of membrane technology has been studied for the treatment and processing of these whey streams. Electrodialysis has been shown to be effective for the treatment of acid whey since the process can achieve high removal of lactic acid and minerals when compared to pressure driven membrane processes, such as nanofiltration. However, electrodialysis is not used widely due to the high operating costs associated with membrane replacement and electrical consumption as a result of membrane fouling and poor process performance. In this thesis, the fouling of ion-exchange membranes during the electrodialysis of fresh sweet and acid whey was investigated. Although the fouling of ion-exchange membrane has been examined by many researchers, the feed solution was generally made using resolubilised powders. It has been demonstrated by other researchers that using fresh solutions provide results and outcomes closer to industrial applications. Furthermore, process optimization is a key parameter to reduce the cost of the treatment process. As a result, the effects of concentrate pH and applied current density were investigated to determine the optimum operating conditions that would minimize membrane fouling and enhance ion removal. Although membrane fouling occurred in all experiments, the effects on system performance were limited. Reductions in the current during pure sodium chloride circulation fell to a minimum of 80% of the original value after 5 hrs of whey processing. The use of an alkaline concentrate resulted in the strongest increase in system resistance, but the mineral deposits formed appeared to detach readily, thereby reducing these effects. The use of an acidic concentrate gave significantly greater rates of lactic acid removal, thus reducing the total membrane area required. A solution of hydrochloric acid with a pH of 1.0 was effective for in-situ cleaning of the mineral deposits. However, protein deposits were not readily removed when using the recommended base cleaning formula of 3% sodium chloride at a pH of 9.2. The concentrate stream in an electrodialysis process is considered a waste stream thus adding to the total volumes of waste generated by the dairy factory. Therefore, the use of salty whey permeate as the concentrate stream was investigated during the electrodialysis of sweet whey. The use of salty whey permeate is expected to reduce freshwater update and the volumes of wastewater generated from the treatment process. The type of concentrate (0.1M sodium chloride or salty whey permeate) did not affect the rate of sweet whey demineralization or the energy consumed per tonne of sweet whey processed, but less sodium and more divalent cations were removed when salty whey permeate was used. In addition, the use of electrodialysis for the demineralization of salty whey permeate was investigated to generate a lactose rich stream as the diluate stream and a concentrated salt solution as the concentrate stream. It was observed that salty whey permeate could be effectively demineralized using either 0.1M sodium chloride or a second stream of salty whey permeate as the concentrate stream. The concentrate purity could be enhanced by using monovalent selective membranes without increasing the energy consumption of the process (3.2 kWh per kg of sodium chloride removed from the diluate at 15 V across 2 cell pairs). Furthermore, combining different membrane technologies can assist in enhancing the treatment of acid whey to produce high quality whey powder. Three process combinations were examined at pilot scale, namely, (1) ultrafiltration and electrodialysis; (2) ultrafiltration, nanofiltration, and electrodialysis; and (3) ultrafiltration, dia-nanofiltration, and electrodialysis. Although all three combinations were successful in reducing the levels of lactic acid and minerals in acid whey, the lowest ratio between lactic acid and lactose (0.017 g lactic acid/g of lactose) was obtained with the process that utilized dia-nanofiltration. The energy required for the electrodialysis of the ultrafiltration permeate and dia-nanofiltration retentate were comparable (7.5 and 7.8 kWh/tonne of feed, respectively). However, the dia-nanofiltration retentate was at least 3.5 times more concentrated than the ultrafiltration permeate, thus reducing the annual energy consumption and capital investment of the electrodialysis unit. The product of the nanofiltration and electrodialysis process was successfully dried to produce a powder with an ash and moisture content of 4% and 2.5%, respectively. To further add value to the acid whey treatment process, the possibility of recovering lactic acid from a salt solution was investigated using either loose reverse osmosis membranes or an electrodialysis process. The recovered lactic acid could be reused in the cheese making process thus reducing fresh acid intake. Partial separation between lactic acid and salts was achieved at low applied pressures and feed pH in the reverse osmosis process, as a greater permeation of salts was observed under these conditions. Furthermore, lactic acid retention was enhanced by operating at room temperature with low feed pH. Partial separation between lactic acid and potassium chloride was also achieved in the electrodialysis process. However, the final concentration of potassium at 70% demineralization of the diluate stream was relatively high resulting in a low purity of lactic acid. Furthermore, the observed losses in lactic acid increased with the addition of sodium chloride to the feed solution. This indicates that the separation becomes more challenging as the complexity of the feed solution increases. Although electrodialysis has been widely studied for the treatment of sweet and acid whey, other electrically driven membrane processes, such as membrane capacitive deionization, have never been investigated. Three different pre-treated acid whey solutions were processed through a lab scale membrane capacitive deionization unit, namely, ultrafiltration permeate, nanofiltration retentate, and dia-nanofiltration retentate. Although a lowest demineralization rate was calculated for the nanofiltration retentate, a higher removal of lactic acid and cations was achieved when compared to the ultrafiltration permeate. Furthermore, similar molar concentration of ions were removed from the ultrafiltration permeate and dia-nanofiltration retentate (41 mEq/L and 43 mEq/L, respectively), however, the total energy consumption was lower for the dia-nanofiltration retentate (0.0122 Wh/mEq of cations removed Vs 0.0243 Wh/mEq of cations removed from the ultrafiltration permeate). Finally, it was found that the energy consumed for the treatment of acid whey ultrafiltered permeate using membrane capacitive deionization was comparable to the energy reported for the electrodialysis process. Overall, the results presented in this thesis have demonstrated that both acid whey and salty whey can be treated and transferred into valuable products for the dairy industry. Future work on this topic could include: (1) investigating the feasibility of using electrodialysis reversal for acid whey treatment; (2) performing a pilot scale assessment of the membrane capacitive deionization process; (3) undertaking an economic evaluation to justify using pressure driven membrane process prior to electrodialysis and membrane capacitive deionization processes; and (4) reassessing the possibility of recovering lactic acid from a salt solution by using other available technologies/ processes such as electrodeionization and selective crystallization of lactic acid