| Electrodialysis (ED) with ion exchange membranes as core component, is honored as clean production technology. Ion exchange membranes (IEM) includes three types: anion exchange membrane (AEM), cation exchange membrane (CEM) and bipolar membrane (BM). Conventional electrodialysis (CED) usually contain anion and cation exchange membranes, its function is mainly for concentrating or diluting a solution. Electrodialysis with bipolar membranes (EDBM) is developed in the base of CED by introducing the use of BM. A bipolar membrane is a composition of CEM and AEM, and its typical function is splitting water (or methanol) into H+and OH-(H+and CH3O-) under a reverse potential bias without generation of gas. Therein an electrolyte salt solution can be converted into its corresponding acid and base without adding any chemicals. It can be flexibly coupled with many other technologies and obtain a better function by means of technological symbiosis, with improving the producing efficient. There has accumulated some academic and industrial experience about applications of electrodialysis in industrial production, environmental protection and bio-production. Here we will propel the further application of electrodialysis and focus on the improvements on the operation. So this work will be mainly carried out on integrated operation of ED with other producing units, aiming to lay the foundation for in-depth study, promotion and applications. The two main application fields were selected: fermentation production of organic acid; recovery of nitrogen and phosphorus from excess sludge. The main interest focus on the innovative research on integration of ED and existed producing technology, exploiting the advantages of ED, replacing or updating old-fashioned and poor producing technology. In the specific operation, we investigated the effects of different operating parameters on the performance. This dissertation comprises six chapters, and the content are detailed.Chapter1begins with an introduction of modern industry production and put forward the application of ion exchange membrane. And then, there is a detailed description of IEM and membrane stack of ED. It ends with a brief summary on the applications of ED, on the base of which, the strategy of coupling ED with other units is proposed. It is just the research contents of this dissertation.Chapter2investigates the operating feasibility of in situ combination of ED and fermenter under batch operation model. The in situ combination is base on the relationship between the alkali producing by EDBM and alkali consuming by fermenter. If the alkali amount gained from EDBM can guarantee the normal running of fermenter is the key to the success of the operation. To this end, according to the production capacity and the amount of base required for one fermenter period, we calculate the number areas and units for constructing the EDBM stack. And in this way, the operational compatibility and uniformity of the integrated operation can be guaranteed. The results indicated that effective transformation lactate into lactic acid and alkali by in situ combination of fermentation and EDBM with operational compatibility and uniformity could be achieved. The integration can achieve a lactic acid recovery ratio of86.05%at the current density60mA/cm2, while a complete fermentation was accomplished. This combination could shorten the shut-down period of EDBM process, save a part in.tial investment on membrane materials, and achieve a synchronous management of the fermenter and EDBM process.In Chapter3, the batch operation model was replaced by a continuous one, and the feasibility of in situ combination of EDBM and fermenter running in this model was investigated. As we known, batch operation consist of input of feedstock(s), reaction, output of product(s), then such time sequential steps will be repeated over again and again. So batch operation is labor-intensive and time-consuming compared with the continuous operation. The advantage of continuous operation are:production capacity, higher equipment utilization, easier operation with other units to form integration, automatic operation and so on. We investigated the stability of the fermenter under continuous operation first, and the maximum flow rate was determined. Then separate operation of continuous EDBM and integration of EDBM and fermenter under continuous operation were investigated. The results indicated that effective transformation lactate into lactic acid and alkali by in situ combination of fermentation and EDBM under continuous operation model could be achieved. The main results concluded as follows:(1) the maximum feed rate (fermenter can afford) is0.24L/h which is also equal to the feed rate of the subsequent individual continuous EDBM;(2) while converting lactate into lactic acid, a sufficient amount of alkali could be provided to the fermenter for adjusting pH;(3) The net end concentration and total molar quantity for lactic acid can reach1.46mol/L and2.18mol, and for OH-are1.32mol/L and1.70mol, respectively;(4) this continuous operation mode of production can indeed reduce the workload and improve productivity Compared with batch operation. However, continuous production process also has its shortcomings, which is the lactate utilization (ie, recovery efficiency) was relatively low. For example, in continuous EDBM process, the recovery ratio was only59.0%and69.5%under operating current density30and40mA/cm2, respectively.Phosphate recovery from excess sludge by electrodialysis was investigated in chapter4. Too much phosphorus in the surface water will cause eutrophication, which has a bad impact on the water quality and ecosystem balance and so eutrophication is a thorny issue in the environmental field. Activated sludge processing is a common and effective method employed for treating wastewater containing phosphorus and nitrogen, where the biodegradable organics in it as the growing substance for organisms. Enhanced biological phosphorus removal (EBPR) process can effectively remove phosphorus. The amount of removed phosphorus from the wastewater in one single cycle will be accumulated into discharge sludge (the aggregative state of microorganisms living) which called as excess sludge. This study was carried out on the basis of a reformation of the EBPR technology and the core of reformation is the introduction of an electrodialysis unit into the EBPR. The results indicate the following:(Ⅰ) in the batch operation, the solution rich in phosphate contains nearly no phosphate after the treatment of CED;(Ⅱ) in the continuous operation, low phosphate concentration in the effluent can be achieved under a low feed rate; for example, a phosphate removal ratio of95.8%can be achieved under10mL/min;(Ⅲ) both single CED operation and integrated electrodialysis operation are feasible ways to recover phosphate from a solution rich in phosphate. The current efficiency for producing H3PO4by EDBM can reach80.3%with an energy consumption about5.3kWh/kg under the operating current density10mA/cm2. The effective recovery of phosphate suggests that there is a possibility for the reformation of EBPR process. In that case, this study could bring us two benefits:the phosphorus content in the bothersome excess sludge becomes low and scarce phosphorus can be recovered effectively.Chapter5investigated the feasibility of simultaneous recovery of ammonia salt and phosphate from excess sludge. The principle of "Anaerobic phosphorus release" is not able to gain a fully release phosphorus from microorganisms into the outside water and microorganisms containing nitrogen resources at the same time, so we should look for other ways for capturing nitrogen and phosphorus resources in the sludge at the same time. Anaerobic digestion process can achieve the degradation of organic matters and it has been widely applied into the treatment of excess sludge. Sidestreams, or "return liquors", are typically very high in ammonia and phosphorus and it can be take as resources rich in phosphorus for recovery and reuse of N&P. In order to recover and reuse the nutrients in the solution, struvite formation and precipitation is considered as a promising technology. The main interests of this study focus on the recovery of ammonia and phosphorus from Sidestreams of anaerobic digestion by coupling of electrodialysis and struvite reactor. In addition, an operation of gas stripping of NH3was investigated also. The results indicate:(Ⅰ) the Sidestreams solution contains nearly no N&P after individual continuous CED operation, both the recovery ratio ranges from90%to100%and the concentrations were increased higher than10times;(Ⅱ) under integrated operation of CED with struvite reactor, the amount of N&P in concentrated chamber of CED does have a big reduction, about60to70%;(Ⅲ) For the integrated operation of CED, struvite reactor and gas tripping of ammonia, the excess nitrogen resource could be also recovered excepting the reaction of phosphorus into struvite precipitation. The nitrogen content in the concentrate chamber can be further reduced by about50%compared to the condition of (Ⅱ). In one word, the integrated operation of these three units indeed could gain an effective recovery of N&P.The sixth chapter is a summary of the full text of this dissertation, and outlook was made for the application of electrodialysis technology.The results of these chapters prove the feasibility of applying ED into biological production of organic acids and recovery of the resources of N&P existed in excess sludge. However, these preliminary studies are laboratory studies, in order to achieve the level of actual industrial production, there needs further study. I hope this study can be helpful to the in-depth application ED technology and the promotion of the development of ED towards modern industry. |
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