Recovery Of Sodium Hydroxide From Chitin Processing Alkaline Wastewater By Couple-Membrane Techonology

China has become the world largest manufacturer and exporter of chitin and its derivatives, which are more and more gaining broad applications. The large amount of acidic and alkali wastewater discharge accompanying the production processes has severely hampered the development of chitin industry. In fact, the resulting wastewater is hard to treat thus increasing the processing cost. To curb this issue, we based ourselves on the success of membrane technology in the wastewater treatment, a stainless steel ultrafiltration membrane (SSF) and an alkali-resistant nanofiltration membrane (NF) to recover the caustic sodium and concentrated protein from the alkali wastewater (AWW) discharged from chitin factories in this paper. The aim is to explore a low-cost, highly efficient and resource reusable method of AWW treatment. This study has not only an academic value for the development and utilization of membrane technology in chitin industry; but also has an important application value in treatment of AWW resulting from the chitin industry.In order to understand the characteristics of AWW from chitin processing and provide basis for the application of membrane technology, we respectively analyzed the main components of AWW discharged from two most representative domestic chitin factories: shrimp shell and crab shell as raw materials. The separation study of AWW from shrimp shell by SSF and NF was performed. Results showed that the average caustic concentration of shrimp AWW was 3.4%(wt/wt), suspended solids (SS) was 0.11% (wt/wt); total protein content was 1.7% (wt/wt), approximately 87% (wt/wt) of which with the molecular weight cut-offs (MWCO) lower than 1000; the alkaline concentration of AWW from crab shell was 4.0% (wt/wt), SS is 3.093% (wt/wt); total protein content was 4.32% (wt/wt), while above 85% of the total protein molecular weight in the crab's AWW were with the MWCO above 10,000. Two kinds of AWW were both complete in the amino acid component, and the main component of the wastewater generated by same raw material in the same factory was stable. The average flux of SSF and NF for the shrimp's AWW were respectively 125 L·m-2·h-1and 27.65 L·m-2·h-1. The volume recovery ratio for caustic sodium was 96%, and the quality indicators of recovered caustic sodium was in norm within the first-class product as defined in IS-IT-ⅡN ational standard. The flux of SSF membrane can be completely regenerated through "alkali-acid-base" cleansing process, consisting of 2% (wt/wt) NaOH, 70℃, 40min and 0.5% (wt/wt) nitric acid, 60℃, 30min; NF membrane can be fully regenerated with water rinse. The NF recovered permeate is caustic sodium solution has a reusable value. The recovered concentrated liquid is the protein concentrates with also a reusable value, thus achieving Zero-emission, proving the feasibility of the couple-membrane technology.To explore the membrane resistance and flux decline rule of SSF during the treatment of AWW system and establish corresponding mathematical model, the membrane separation parameters in actual system and the separation characteristics of SSF in AWW were analyzed by pilot-scale experiment. Results showed that: the peak SSF membrane flux was reached when the trans-membrane pressure (TMP) was at 3.1 bar, and the increase of membrane fouling resistance is in accordance with the growth model. The correlation coefficient (CC) between the predicted value obtained from the flux model and tested data was higher than 0.96, demonstrated that the flux model and membrane resistance model established in this paper can well describe the SSF membrane filtration characteristics for this type of AWW. The concentration differential equations model can quantitatively describe the varying trend of non-alkaline liquid solids concentration of retention following the time change, and quantitatively describe the SSF membrane treatment process and results to the chitin AWW. It can be used as a basic tool for evaluation of the feasibility, economy and on-line control.In order to understand the separation characteristics of the alkali-resistant NF membranes in alkali system, membrane resistance and flux decline rules in filtration of the AWW, we performed analysis of combining S-K model and artificial neural networks model (ANNs) by the methods of determination and analysis of the retention rate of glucose and NaCl under certain conditions. This allowed to establish a NF flux mathematical model for practical AWW. The results showed that: the pore radius and separation layer effective thickenss (Δx/Ak) of the alkali-resistant NF membrane was about 0.380nm and 65.63nm, respectively. Membrane swelling studies demonstrated that the NF membrane is fit for treatment of high concentration caustic sodium solution for a long time. The correlation coefficient of the predict values as simulated by the flux decline model and experimental data was higher than 0.98. This proves that NF model can quantitatively describe the process and results for the NF process of AWW, and laid a theoretical basis for industrial applications and evaluation tools for this technology.To verify the economics and the feasibility of technology that combines SSF and NF membrane processes for recovery of alkali waste, simulation analysis by mathematical methods was carried out based on economic principles, in combination with the actually established engineering data, SSF and NF flux models. Results showed that: the cost model established is reliable. In the composition of the total cost of the process, the investment costs of membrane accounted for the largest proportion, among which the SSM cost accounted for 0.28CNY/L, thus above 56% the total cost; NF membrane cost was 0.11CNY/L, accounting for slightly above 26% of the total cost. The membrane cost exhibited"platform areas", and the proportion to total cost of two kinds of membranes was in reverse proportion; The increase in the fluid to be treated increased the amount of membrane demand, and also the membrane area utilization ratio. This resulted reduction in the operation cost per unit of fluid volume treated. This was more apparent when daily running time was longer, resulting in more and significant total costs reductions for the same daily processing capacity. This model is cost-effective for large-scale industrial production. The investment cost of membrane equipment significantly increases with the growth of daily capacity. The investment cost is less than 1.5 million CNY for a 100 m3 daily capacity of factory's discharge.