Food Industry Water Purification: The Key Challenge of Achieving Zero Liquid Discharge

The raw water for food factories is mostly sourced from municipal tap water or groundwater. However, this raw water still contains trace suspended solids, colloids, residual chlorine, hardness ions, and microbial risks. Therefore, such water quality cannot meet the requirements for high-end beverages, dairy products, and brewing water. The water treatment equipment first processes the raw water through a combined "multi-media filtration + activated carbon adsorption + ultrafiltration" process. The goal of this treatment is to reduce turbidity to below 0.2 NTU and remove the vast majority of colloids, organic matter, and bacterial spores. This is intended to eliminate fouling and microbial growth risks for the downstream reverse osmosis system. Ultrafiltration serves as the protective barrier for reverse osmosis. Its effluent Silt Density Index  must be stably maintained below 3. Once this index is exceeded, organic fouling and microbial biofilm will rapidly destroy the separation performance of the membrane elements. This leads to fluctuations in the effluent water quality. The main difficulty of this pretreatment stage lies in the significant seasonal variation in water temperature and the drastic fluctuation in the organic content of the water source. Therefore, the system needs to rely on the linkage between online turbidity meters, oxidation-reduction potential meters, and intelligent chemical dosing systems to accurately dose flocculants and biocides. Only in this way can it provide stable pretreatment assurance for the downstream system.

Dairy, slaughterhouse, and beverage processing wastewater contains high concentrations of proteins, fats, lactose, starch, and blood. Such wastewater is an extremely challenging organic wastewater to treat. To treat this type of wastewater, the purification system first adopts an enhanced biochemical process consisting of "oil separation and dissolved air flotation—anaerobic hydrolysis—anoxic denitrification—aerobic biological treatment." This process can remove most biodegradable organic matter and ammonia nitrogen. Afterwards, the system further connects a "membrane bioreactor + nanofiltration" for advanced treatment. This advanced treatment stage retains and separates residual suspended solids and macromolecular organic matter. Among this, the technical difficulties lie in the anti-biofouling capability of the membrane bioreactor and the high-load inhibition tolerance of the anaerobic reactor. These technical challenges directly determine whether the effluent can consistently meet reclaimed water reuse or discharge standards. This biochemical and advanced treatment stage is the core link for degrading high-concentration organic pollutants and ensuring effluent water quality.

Achieving zero liquid discharge for high-salinity wastewater is the final stage of food industry water purification. After being deeply concentrated by reverse osmosis and nanofiltration, the salinity and refractory organic matter concentrations in the wastewater are greatly increased. Subsequently, the concentrate enters a mechanical vapor recompression evaporator or a falling film evaporative crystallizer. At this point, the equipment faces severe challenges, including heat transfer surface coking caused by protein charring and fat polymerization/saponification, as well as pitting corrosion of stainless steel by chloride ions. To address this, the system must use 316L or duplex stainless steel materials, combined with scale inhibition dispersants and online chemical cleaning, to control the scaling rate. Simultaneously, the system utilizes condensate polishing filtration to achieve distillate reuse. These measures truly realize the closed-loop utilization of water resources and the resource recovery of organic concentrates. How to maintain a high concentration factor under low energy consumption and stably produce reclaimed water that meets sanitary standards is the direction of long-term industry research.