Study On Advanced Treatment Of Mid-stage Waste Water From Alkali Straw Pulping By Membrane Bio-reactor (MBR)

The national standard on paper industrial pollutants discharging has been revised from on and on with the rapid development and innovation of this industry,which is first released in 1983 and revised in 1992 and 1999 respectively.In 2001,the index number of GWPB2-1999 was replaced by GB 3544-2001.And in 2003,SEPA has made some amendment on GB3544-2001.The indicators in the former＜Discharge Standard of Water Pollutants for Paper Industry＞(GB3544-2001) were nearly to these of such discharge standard in developed countries and/or regions in early 1990s,such as the discharge standard for COD_Cr of non-wood paper industry is 450mg/L.However,the threshold of COD_Cr for V-class water in current＜Environmental Quality Standard for Surface Water＞(GB3838-2002) is 40mg/l.The big difference between the two data will lead to contamination of receiving water body caused by highly concentrated wastewater.In terms of the requirements of＜Outline of the 11^th Five-year Plan on Economical and Social Development in P.R.CHINA＞and＜The Notice of Publishing Comprehensive Work Plan Energy Conservation and Emission Reduction(No.15[2007]of the State Council)＞,the COD_Cr should be decrease from 14.14 million tons to 12.73 million tons by 2010,which is tough to complete.Pulp & Paper is one of the most important industries in China,and it also contributes a lot for water pollution.In 2006,the amount 1.553 million tons COD_Cr discharged by paper-making industry,ranks No.1 among all the industrial COD_Cr emissions,accounting for 33.6%of the total amount.The latest＜Discharge Standard of Water Pollutants for Paper Industry＞(GB3544-2001) has been released on June 25^th,2008 and will be put into effect on Aug l^st,2008.The new standard will play an important role in promoting industrial structure optimization,stabilizing discharge pollutants to meet relevant standards to fulfill the target of 11^th "Five-year" Plan.The latest＜Discharge Standard of Water Pollutants for Paper Industry＞(GB3544-2001) defines discharging thresholds for existing factories and new factories concerning pulp-making sector,pulp & paper sector and paper-making sector.Furthermore,the draft standard for approval also defines specific thresholds for some areas with high-density exploitation,subdued environment carrying capacity,less water environment capacity,weak ecological environment,easy to cause serious water pollution etc.Compared with current＜Discharge Standard of Water Pollutants for Paper Industry＞,new parameters such as N,P,chroma,dioxin are added, AOX(adsorbable organic halogens) is adjusted as an indicator for pollution control,and the discharge amount and concentration has been strengthened to a higher level as well.Take an example of straw pulp & paper making factory:The permitted wastewater discharge amount is reduced from 300m³/t to 80t/t by 73%for existing straw pulp & paper factories,for new straw pulp & paper factories,this figure is reduced from 300m³/t to 40t/t by 87%.The concentration of COD_Cr for existing straw pulp & paper factories is reduced from 450mg/l to 150mg/l,which is only 33%compared to the former standard,for new straw pulp & paper factories,this figure is reduced from 450mg/l to 90mg/l by 80%.The concentration of BOD₅ for existing straw pulp & paper factories is reduced from 100mg/l to 30mg/l,which is decreased by 70%in the new standard,for new straw pulp & paper factories,this figure is reduced from 100mg/l to 20mg/l by 80%.The concentration of SS for existing straw pulp & paper factories is reduced from 100mg/l to 50mg/l,which is stricter than the former by 50% in the new standard,for new straw pulp & paper factories,this figure is reduced from 100mg/l to 30mg/l by 80%.The concentration of ammonian,total nitrogen,total phosphorus, AOX and chroma for existing straw pulp & paper factories in the current standard is 10 mg/l,15mg/l,1.0 mg/l,15mg/l,50 times respectively,which is decreased by 50%;for new straw pulp & paper factories,this figure is reduced from 450mg/l to 90mg/l by 80%.The water pollutants discharge standard for pulp and paper factories is also stricter than before on some extent.According to the new standard,the factory should take measures to control and treat the pollution to meet relevant standard before discharging.For wastepaper pulp & paper factories,they are facing bigger challenges than straw pulp factories and wood pulp factories.Take straw pulp factory for example,the COD_Cr concentration of grey water is around 2000mg/l.However,in the new standard,it is request to reduce to 150mg/l before discharging,which means the COD_Cr removal should reach to 92.5%.This is quite difficult for pulp & paper wastewater,as it's less biodegradable.Chroma concentration for most straw pulp & paper factories is around 80～100.In accordance of the new standard,it should be reduce to 50.Thus,decoloring process will be needed or designed into current processes.This is also difficult.Ammonia,total nitrogen,total phosphorus is a little difficult to remove for ammonium sulfite pulp factories,but it's easier for alkaline technology.It is surveyed that discharge concentration of ammonia,total nitrogen,total phosphorus could be below 10mg/l,15mg/l and 1mg/l respectively for more than 70%enterprises in Shandong province, approximately to the requirements in the new standard.In order to meet the requirement in the new standard,wastewater treatment facilities should be improved besides technical process optimization,structure adjustment etc.Technologies for advanced treatment for grey water in alkaline wheat straw pulp process,wood pulp process and wastepaper pulp process is mature and popular.Most of these technologies are used to include aerobic biological treatment and coagulation sedimentation process,with COD_Crremoval efficiency could reach above 80%.Whereas it could not reach to the requirement in the new standard with these technologies,new process should be introduce to deal with this problem.MBR(Membrane Biological Reactor) is a potential solution for current and future wastewater treatment,depended on its own features: membrane filtration could extremely improve the separation efficiency, also the high concentration of activated sludge in aerobic pond and specific bacteria in the sludge would improve the bio-chemical treatment efficiency.Membrane Bio-Reactor(MBR) has been gradually applied to wastewater treatment of paper making industry.Han Huaifen and her colleagues tried aerobic MBR to treat pulp wastewater(black liquid,grey water,white water),and compared it with conventional active sludge technology and bio-contact oxygenation technology.It is shown that MBR treatment could decrease the COD_Cr to an amount of less than 100mg/L (the hydraulic retention time is 18 hrs),and the total removal rate of the whole reactor could reach to 90%;while the other two failed to meet the requirement even the hydraulic retention time lasts for 40 hrs,with COD_Cr concentration of 149.3mg/L and 197.3mg/L respectively.The result is well proved that MBR is a better way for refractory biodegradation wastewater than the other two methods.Jin Mantong's experiment on Hollow-fiber Membrane Bio-reactor shows that this reactor can reach a higher removal rate of COD_Cr on refractory biodegradation wastewater such as paper making wastewater.When the sludge concentration is around 6,000mg/L,the removal efficiency can reach to 85%,with COD_Cr concentration of 50-60mg/L in the effluent.There are researches abroad that disposing the waste liquor and it is showed that MBR is more efficiency to remove COD_Cr,TOX and SS than activated sludge process.Other than the conventional bio-treatment methods,MBR has absolute advantages as follows:higher separating efficiency;help on the increasing of microbe concentration in the reactor; good for improving the reaction rate of the microbe;higher oxygen transfer efficiency.Besides,this system is auto-controlled,easy to manage and has better treatment results,more stable water quality and occupies smaller room.The research hereby is focusing on disposing middle-stage wastewater with Hollow-fiber Membrane Bio-reactor,discussing the optimal indicator,removal rate,film pollution control,biology regime and its economic indexes,etc.Research on deep disposal of secondary effluent of paper making wastewater has been done.It was found that since the influent water contains low amount of COD_Cr and sludge,very limited nutrition could be supplied to microbe,thus it was hard to reach the expected sludge concentration of more than 3,500mg/L.,and MLVSS/MLS is only about 0.3.If no additional nutrition is put in,the COD_Cr removal rate would be very low.Under the condition that the residence time is 10 hours and dissolved oxygen is 3.5mg/L,the removal efficiency of COD_Cr is only 30%,turbidity removal rate can reach the number of 90%;the dispersion degree of diameter of particles is too small and over 80%of the smaller-than-100um particles cumulated.So the film is very slightly polluted,and 60 days would be a cleaning cycle.The application of MBR on bio-chemical tank effluent of paper making wastewater has also been tested.Piping the effluent directly into the reactor helps to solve the problem of low mud concentration due to lower nutrition and improves the COD_Cr removal efficiency.Under the condition of 10 hours residence time and sludge concentration 8,000mg/L, the removal efficiency of COD_Cr can reach 45%.On a basis of record on membrane fouling and cleaning in frequency under the study of MBR treatment on effluent from biochemical pool,it is shown that if the back wash could be carried out every day,it would extend the chemical cleaning cycle of membrane.When the membrane is cleaned in physical method,the chemical cleaning cycle could extent to 60 days;when the physical method is not used,the chemical cleaning cycle is only 15 days.And the cleaning cycle will shorten with the increase of sludge concentration.Once the sludge concentration increases above 10000mg/l,membrane pollution will be aggravated.In the study of MBR treatment on effluent from biochemical pool,by measuring sludge communities,microorganism quantity and kinds,OUR (Oxygen Uptake Rate),grain dispersity,EPS/SMP to compare physics properties between sludge in MBR and sludge in raw wastewater.The results show that the microorganism in MBR has increased by almost 3 times,hay bacillus increase to 3.4 times than before,the OUR also speed up,sludge particular above 200μm increase,EPS is cumulated with time, only SMP does not vary much.According to the study on sludge balance status and sludge concentration variations after continuous aerobic process,it is shown that internal consumption is existed in sludge.The record on molecular weight distribution curve of influent and effluent shows that molecular weight over 30000 could be trapped partly.Research on adding activated carbon into MBR to treat wastewater from bio-chemical pool shows that the removal effect of COD_Cr and chroma would be better for activated carbon power other than activated carbon particular.Adding activated carbon is helped to extend the cleaning cycle to reduce membrane pollution.Research on the primary effect for MBR combined with anaerobic wastewater treatment of effluent from biochemical pool and MBR combined with reverse osmose-membrane,it is calculated the adequate amount of ozone to be added is 25mg/1min.The reverse osmose part could meet the regular requirement when the ratio for clean water and wastewater is 1:4 and SDI value is 1.45.Based on different technologies and treatment effect on chroma,this study also investigates some material possible to cause chroma variation by GC-MS.This paper has raise up an optimized advanced treatment technology with relevant analysis on economic indicators.In conclusion,the application of MBR to treat the grey water from alkli straw pulp technology could remove COD_Cr more than 45%.The MBR could be operated normally in long-term by regular membrane cleaning.If other measures could be taken together,the effluent could be possible to meet the requirement stated in the new standard.And this should be proved by further engineering test.