Enhancement Of The Two-step Preparation Of Environmentally Friendly Snow-melting Agents From Food Wastewater

Food wastewater was a high concentration organic wastewater generated from the food waste treatment process,with the characteristics of high solids content,good biodegradability and high chemical oxygen demand（COD）.If food wastewater was discharged directly,it would cause serious water pollution.Anaerobic fermentation technology was an efficient resource treatment method for food wastewater.Due to the high concentration of organic matter in food wastewater which was easily degradated,anaerobic fermentation technology could be used to convert the wastewater into volatile fatty acids（VFAs）such as acetic acid,propionic acid and butyric acid.Acetic acid could be further converted into environmentally friendly snow-melting agents（ESA）with acetate as the main component.At present,the preparation cost of environmentally friendly snow-melting agents was high,mainly because the source of acetic acid was from industrial,which was costly.Food wastewater was used as a substrate for acetic acid generation by anaerobic fermentation,and then was converted to environmentally friendly snow-melting agents,which could not only achieve the resource utilization of food wastewater,but also reduce the cost.This paper studied a two-step technology for the preparation of environmentally friendly snow-melting agents from food wastewater.Firstly,anaerobic fermentation of food wastewater by the immersed membrane bioreactor（i MBR）was indicated to produce substrate for environmentally friendly snow-melting agents,and anaerobic acid production from food wastewater was enhanced by optimizing the reaction operation parameters and exogenous materials addition.Afterthat,VFAs were separated and concentrated by microfiltration membranes and then to prepare environmentally friendly snow-melting agents.The effect of organic loading rate（OLR）of i MBR on the anaerobic acid generation was investigated to obtain the optimal organic loading condition.To increase the VFAs yield,the enhanced effect of biochar addition on anaerobic acidogenic fermentation from food wastewater was investigated,and the residue produced in the acidogenic fermentation process was reused.Different concentration methods were used to condense the anaerobic fermentation broth and obtain the best method.The environmentally friendly snow-melting agents were obtained by evaporation and crystallization after adding different proportions of calcium and magnesium ions,and the composition and characteristic of the environmentally friendly snow-melting agents were analyzed.The main research conclusions were as follows:（1）High concentration of VFAs from food wastewater could be achieved by controlling the organic loading rates.i MBR reactors were operated at different OLR（2.5,5,8 and 7 kg COD/m³/d）to treat food wastewater.The results showed that the highest VFAs of 45.9 g/L could be obtained when OLR was 5 kg COD/m³/d,of which acetic acid was 30 g/L,accounting for 65%.The acidification rate of the reaction system reached the maximum of98%.The activities of dehydrogenase and acetate kinase reached the maximum,with an increase of 74%and 263%,compared to the initial activities,respectively.The increase of OLR provided sufficient substrate for microbial,however,the further increase of OLR was not benefit to the system stability,the excessive OLR led to a decrease of VFAs,with acetic acid of 33.3 g/L,accounting for 78%of the total VFAs.Considering the both VFAs yield and system stability,an OLR of 5 kg COD/m³/d for i MBR was considered to be the optimal condition for VFAs gengeration from food wastewater.（2）The addition of biochar could promote the anaerobic acidogenic fermentation performance from food wastewater and increase the yield of VFAs.The VFAs generaion could be increased after biochar addition under the optimal organic loading rate based on the previous chapter,the maximum VFAs concentration reached 55.6 g/L,with an increase of21.3%,of which the acetic acid was 46.5 g/L,with an increase of 55.2%.The biochar addition increased the dehydrogenase and acetate kinase activities by 54%and 233%,alkaline protease andɑ-glucosidase activities increased by 171%and 109%,respectively.The biochar enriched Firmicutes,a microorganism associated with anaerobic acidgeneration,and the electron supply and acceptance capacities of anaerobic sludge were increased by 10.1%and 32.7%,respectively,electron transfer capacity was improved and thus enhanced VFAs generation.The residual residue from the acid production stage was pyrolyzed to sludge-based biochar with rich functional groups and good heavy metal removal efficiency,among which BC500 was the best,with Pb²⁺removal of 100%after 10 min,Zn²⁺removal of97.3%and Cd²⁺removal of 73.5%after 2 h,respectively.（3）Four concentration methods,including forward osmosis membrane,rotary evaporation,nitrogen blowing and vacuum parallel concentration,were used to concentrate the anaerobic fermentation broth which was separated from the acidogenic fermentation process.The results showed that all the four methods could effectively concentrate the anaerobic fermentation broth.Forward osmosis was considered to be the best concentration method in this study based on the concentration time,concentration degree and energy consumption.The type of drawing solution affected the forward osmosis.Due to the high molecular weight of magnesium chloride,it could reduce the reverse osmosis trend and provide and maintain high water flux.The optimal concentration effect could be obtained by using magnesium chloride solution as the drawing solution,with a maximum concentration time of 3.5,the final VFAs concentration of 154 g/L,acetic acid concentration of 106.44 g/L,with an recovery of 97.8%of VFAs.The forward osmosis concentration process took the shortest time of 8 minutes,with the best concentration efficiency,and the final concentration degree was not significantly different from rotary evaporation method.With the osmotic pressure on both sides of the selective osmosis membrane as the driving force,the energy consumption of forward osmosis membrane was lower.（4）The XRD and FTIR spectra showed that the anaerobic fermentation broth was synthesized to an acetic acid based environmentally friendly snow-melting agent.The mainly components of the environmentally friendly snow-melting agents were potassium acetate and potassium propionate,with 76%and 10.5%based on mass,respectively.The freezing point of environmentally friendly snow-melting agents was in the range of-33.5～-3.7℃,with good snow and ice melting ability,the ice melting ability could reach 114.6%of sodium chloride,and it was not significantly different from sodium chloride and calcium chloride.The environmentally friendly snow-melting agents had lower corrosion compared with sodium chloride and calcium chloride,the corrosion rate on carbon steel and concrete was significantly reduced by 94.7%and 78.4%.In addition,the environmentally friendly snow-melting agents had good eco-friendliness,which had less impact on plant seed germination,seed germination rate could reach 85%under 0.45%mass concentration,the relative damage rate to plant seeds was 15%,which was 35%lower than the sodium chloride.