Study On Enhanced Anaerobic Biological Treatment Performance Of High Salinity Organic Wastewater With Rice Straw Ash

Many industrial sectors generate a large amount of high salinity organic wastewater（HSOW）,such as mustard tuber wastewater,heparin sodium pharmaceutical wastewater,membrane concentrate,etc.In addition to high salt content（>1.0%）,this type of wastewater also contains a large amount of organic matter（COD>2000 mg/L）.For the treatment of this type of wastewater,the removal of high concentration organic pollutants is the primary requirement for meeting the discharge standards.Anaerobic biological treatment was widely used,due to its advantages of low operating cost,resource recycling,and less secondary pollution.However,high concentration of salt would inhibit the metabolic activities of anaerobic microorganisms,leading to the reduction of anaerobic digestion efficiency.The use of potassium ion to regulate osmotic pressure could alleviate the inhibitory effect of high salt concentration on microorganisms,and improve anaerobic treatment performance.However,the traditional cost of adding potassium chloride is high.In practical application,it is necessary to find a cheaper potassium source.Based on this,rice straw ash（RSA）was selected as a potassium source in this study to alleviate the inhibition of high salinity on anaerobic microorganisms,and the feasibility of improving the anaerobic digestion performance of HSOW was evaluated.Firstly,the effect of straw ash addition on anaerobic digestion performance of HSOW was analyzed,and the optimal dosage of RSA was obtained.The mechanism of RSA promoting anaerobic digestion of HSOW was clarified.Then,two methods were used to start-up the anaerobic reactors,namely under high organic loading rate（OLR）and gradually increase of OLR,to evaluate the impact of RSA addition on the start-up performance of the reactors.Finally,RSA was applied to the anaerobic treatment of mustard tuber wastewater aiming at promoting the anaerobic digestion performance of mustard tuber wastewater.It could provide a reference for the practical engineering application of this method.The main results of this study are as follows:（1）Different dosage of RSA were added to the anaerobic digestion system of HSOW with 3.0%Na Cl content,and their effect on the anaerobic treatment performance of this kind of wastewater were analyzed.The results showed that,the addition of RSA promoted the anaerobic digestion performance of HSOW,and the optimal dosage of RSA was 0.88g/L.At this dosage,methane production increased by 221.60%and TOC removal rate reached 66.42%.The addition of RSA increased the proportion of anaerobic microbial active cells and the activities of key enzymes in high salinity environment,and promoted the growth of acetoclastic methanogen bacteria Methanothrix.The results of metagenomics analysis showed that,under high salinity environment,the methanogenesis pathway of microorganisms was mainly hydrogenotrophic methanogenesis.After RSA addition,the methane production pathway changed from hydrogenotrophic methanogenesis to acetoclastic methanogenesis.When the RSA dosage was 0.88 g/L,the abundance of functional genes related to substrate transporter,electron transfer and ion transporter increased,which was crucial to the resistance of microorganisms to high saltinity inhibition and the promotion of anaerobic digestion of HSOW.（2）Two sets of anaerobic reactors with a volume of 5 L were used to treat HSOW with a salt content of 3.0%.The reactors were directly started under high OLR conditions to verify the promoting effect of RSA addition in the process.The results showed that microorganisms were strongly inhibited in the reactor without RSA addition,resulting in poor methane production performance and reactor start-up failure.The addition of 0.88 g/L RSA in the reactor resulted in a significant increase in methane production and TOC removal rate,and VFAs were consumd in a relatively short period of time.After stopping the addition of RSA,the system could maintain stable operational performance.The addition of RSA increased the activities of dehydrogenase,acetate kinase,and coenzyme F₄₂₀in the anaerobic reactor,and as the microorganisms adapted to high salinity environments,the activities of the three key enzymes gradually increased,with the largest increase in coenzyme F₄₂₀concentration.The addition of RSA promoted the accumulation of acetoclastic methanogen Methanothrix.（3）Two sets of anaerobic reactors were used to treat HSOW,and the start-up of the reactor was carried out by gradually increasing the OLR.The effect of RSA addition on the start-up performance of the reactor was explored.The result showed that,in the early stage of reactor start-up,the COD removal rate in the control reactor（without RSA addition）was low and VFA accumulation occurred,resulting in poor methane production performance.The addition of RSA in the experimental reactor improved the treatment performance of HSOW,promoted the consumption of VFA,and maintained a high COD removal rate which was above 70%.Afterwards,the OLR of the reactor was gradually increased,and the experimental reactor stopped adding RSA.During the process of achieving the same OLR in both reactors,shorter time was needed for the experimental reactor.In addition,a higher OLR can be reached for the experimental reactor.When the OLR increased to 7.62 kg COD/（m³·d）,the COD removal rate of the control reactor decreased to 50.11%.Under this OLR,the COD removal rate of the experimental reactor was above 70%.When the OLR increased to 9.04 kg COD/（m³·d）,the COD removal rate of the experimental reactor was still higher than 60%.This indicated that,after acclimatization,the microorganisms in the reactor with RSA addition had higher capacity for HSOW treatment.（4）Anaerobic reactor was used to treat mustard tuber wastewater.In the early stage during the reactor start-up period,the COD removal rate in the control reactor（without RSA addition）was relatively low,and it reached 75.33%by the 26th day.Although the start-up OLR of the experimental reactor was twice that of the control reactor,but the COD removal rate reached 75.50%on the 16th day.The addition of RSA promoted the degradation of organic matter in mustard tuber wastewater,accelerated the consumption of VFA,shortened the acclimatization time of microorganisms in high salinity environment,and improved the anaerobic digestion performance of mustard tuber wastewater.When the OLR increased to9.22 kg COD/（m³·d）,the COD removal rate of the control reactor decreased to 52.52%,while that of the experimental reactor was 61.16%.This indicated that,after acclimatization,the microorganisms in the reactor with RSA addition had stronger treatment performance for mustard tuber wastewater.