Enhancement Of Fenton Sludge On Anaerobic Treatment And Its Mechanisms

The Fenton process uses the catalysis of Fe2+and H2O2 under acid conditions(pH=3-4)to produce reactive hydroxyl radicals(·OH).It can decompose most organic matters which is widely used in the treatment of refractory organics wastewater.However,when adjusting the pH to neutral at the end of the process,a lot of iron contained sludge,termed Fenton sludge will generate due to the flocculation of the iron ions.Fenton sludge contains lots of organic matter and iron ions which leads to serious pollution if it is directly discharged into the environment.At present,it lacks effective disposal measures to limit the further application of the Fenton process.Anaerobic biological treatment is considered one of the most promising technology to treat organic solid waste.It can decompose complex organic matter into methane and carbon dioxide to realize the recycling of resources and energy.Considering that iron oxides can induce dissimilatory iron reduction(DIR)to accelerate the decomposition of organic matters,the possibility of anaerobic digestion(AD)for the treatment of Fenton sludge as well as the role of iron in Fenton sludge on improving AD efficiency via inducing DIR were explored.We expected to provide a new method for the resource utilization of Fenton sludge.The main results are as follows:(1)In order to investigate the effect of AD on removing organic matter in Fenton sludge and the role of Fenton sludge on enhancing the anaerobic performance,Fenton sludge was introduced to AD treatment of waste activated sludge(WAS).Results showed that approximately 60%-70%of the organic matters in Fenton sludge were decomposed and converted into methane and carbon dioxide through AD.Meanwhile,the ferric iron contained in Fenton sludge effectively promoted the degradation of polysaccharide and protein in WAS via DIR.It provided a lot of small-molecule organic acids(acetate,etc.)for methanogens to use.As a result,adding Fenton sludge to the anaerobic reactor could enhance methane production by 21.0%and sludge reduction by 9.4%.Microbial community analysis showed that the abundance of iron-reducing bacteria(IRB),such as Clostridium was improved in Fenton sludge added reactor.Together with amounts of ferrous iron detected in the anaerobic reactor,it confirmed the appearance of the DIR process.(2)The Fe(?)contained in the Fenton sludge added to the anaerobic reactor can generate different kinds of iron(hydrogen)oxides to affect the AD performance.To explore these differences,Fe(OH)3(the amorphous iron oxide)and Fe2O3(crystal iron oxide)were used to investigate their mechanism for improving AD.The study found that both two iron(hydrogen)oxides promoted AD performance,but the mechanisms involved were different.Amorphous Fe(OH)3 was easier to be reduced due to its higher redox potential,and the reduction rate of DIR was 25.65%higher than that of Fe2O3,which could accelerate the decomposition of organic matter and provided more substrates available for methanogens.While crystalline iron oxides were not easy to be reduced due to their lower redox potential.Thus,DIR was difficult to take place.However,crystalline iron oxides increased the protein and humic substances content of the extracellular polymer substances(EPS)secreted by microorganisms,which lead to the higher electron transfer efficiency of the EPS,then improving the methane production process.(3)In order to improve the efficiency of DIR,redox humic substance(HS)was introduced into the AD system to enhance the decomposition of organic matter.The experimental results showed that the DIR rate was increased by 14%with HS addition.The quinone group of HS could obtain electrons from the oxidation process of organic matter to produce hydroquinone,hydroquinone could further use the electrons to reduce ferric iron and itself converted back to quinone group,that is,HS acted as an electron shuttle to promote electron transfer between microorganisms and iron oxides.On the other hand.HS is rich in hydroxyl and carboxyl functional groups which can complex with the surface of iron oxide.It effectively inhibited the deposition of free ferrous iron on the surface of iron oxides,thus delaying the mineralization of iron oxides,maintaining iron oxides at high reactivity to make the DIR continuously happen.(4)In order to further improve the hydrolytic acidification efficiency of AD and the interspecies electron transfer between microorganisms,an iron-containing biochar with both good conductivity and charge-discharge ability was prepared using Fenton sludge as raw material,then it was applied to AD.Under the condition of high-temperature pyrolysis,the ferric iron in Fenton sludge was reduced to magnetite,which enriched iron-reducing bacteria(IRB)to accelerate the decomposition of complex organic matter.Meanwhile,magnetite with excellent conductivity was used as a conductor to accelerate the direct interspecies electron transfer(DIET)of the microbial community.The flocculated organic matter in Fenton sludge was charred to form biochar during the pyrolysis process.The surface of the biochar was rich in quinone group/hydroquinone group and other functional groups,which acted as electron shuttles to mediate the electron transfer of microorganisms to the surface of iron oxides.After optimizing the preparation conditions,it showed that the iron-containing biochar prepared at pyrolysis temperature of 400? owned the best conductivity and charge/discharge properties.The iron-containing biochar overcame the shortcomings of biochar that it was difficult to obtain both capacitive and electrical conductivity at one pyrolysis temperature by traditional method.When adding this iron-containing biochar to the anaerobic reactor,methane production was increased by 22.6%,which effectively promoted the anaerobic performance.