Study On Removal Of Nitrate From Groundwater Using Solid Organic Carbon Sources And Embedding Fungus

In recent years, along with the rapid development of industry and agriculture, worldwide groundwater is suffering relatively serious pollution of various pollution factors. Among them the nitrate pollution in the groundwater are particularly common and serious. Nitrates exist in the water in the form of NO₃^- ion ,and the existence is long-term and stable and it cann't form precipitation.Because the nitrates can transform into nitrite after entering biology,and it cause great harm for humans and other biological health.So it is gradually from domestic and international attention. So we can not only control and manage the nitrate pollution source and should also actively look for economic and effective method to repair contaminated groundwater. The removal methods of nitrate in the groundwater are mainly physical and chemical method, chemical method and biological methods.But thinking of completely removing nitrate in the groundwater and reducing denitration cost, the biological methods basing on denitrification are the most economical, effective and most widely used techniques to remove nitrate in the groundwater. According to characteristics of the lack of organic carbon and denitrifying bacteria and enriching difficultly in the groundwater, the traditional biological methods of adding liquid carbon source have two problems of high cost and causing easily secondary pollution. Solid organic matter rich in cellulose and immobilized cell technique are gradually researched and applicated in biological denitrification.The natural organic rice straw and the paper plates saled in the market are mainly selected as solid organic carbon sources in this research. cellulose is rich in rice straw. Paper plates is made of sugarcane, reed and pulp raw materials and also contains a large number of cellulose. Activated sludge and immobilized embedding bacteria particles are respectively used as denitrifying bacteria. Four groups of experiments were designed: Rice straw is used as solid carbon sources and medium and activated sludge as denitrifying bacteria; Rice straw as solid carbon sources and medium and embedding bacteria particles as denitrifying bacteria; the paper plates as solid carbon sources and medium and activated sludge as denitrifying bacteria; the paper plates as solid carbon sources and medium and embedding bacteria particles as denitrifying bacteria. By changing the environmental factors (hydraulic retention time(HRT), pH, temperature and nitrate nitrogen concentration),each test denitrification is studied and its denitrifying process is analyzed.Some conclusions can be gotten through the experimental study.(1) The rice straw for solid carbon source and some sand whose particle is 5mm are hybrided are used as medium to fill in the reactor which is inoculated activated sludge.The reacter startups fast and can efficiently remove NO₃^--N in the groundwater,don't accumulate NO₂^--N and happen phenomenon of N₂ chocking. Biological denitrification process in the reacter is influenced by HRT ,pH and temperature and nitrate nitrogen concentration in the different level,but its ability of against acids fluctuation and low temperature resistance is still strong. When the initial NO₃^--N concentration is 24mg/L, pH 7.5, temperature 20 30℃between,in the effluent water ,the NO₃^--N concentration is low to 1mg/L, the removal rate of the NO₃^--N above 98%,NO₂^--N not accumulated,and the pH tends to be neutral ,the COD is 30mg/L or so.(2) When the embedding bacteria particles are domesticated with CH2COONa for carbon source, the embedding bacteria particles are easily domesticated and its denitrification performance is good and lessly influenced by HRT.When HRT is 6h,the initial NO₃^--N whose concentration is 42mg/L basically is completely removed and NO₂^--N don't accumulate.As the initial NO₃^--N concentration increases, denitrification rate will also increase in the reacter, NO₃^--N load Processed can achieve to 87mg/(L·h) maximum,but the alkalinity is bigger in the outflowing water.(3) In the reacter ,whose mediums are rice straw for carbon source and embedding bacteria particle domesticated ,the denitrification process are influenced by temperature and HRT in different level,but its ability of against acids fluctuation is strong and it is not basicly affected by pH. When the initial NO₃^--N concentration is 25mg/L, temperature 20℃,in the effluent water ,the NO₃^--N concentration is low to 0.5mg/L, the removal rate of the NO₃^--N above 98%,NO₂^--N not accumulated,and the pH tends to be neutral ,the COD is 25mg/L or so.(4) The two reactors can startup fast when the paper plate is used as solid organic carbon source,they are influenced by environmental factors such as HRT, NO₃^--N concentration,pH and temperature,but the influence degree is different. Overall,1# reacter with paper plates and embedding bacteria as medium influenced by environmental factors is less than 2# reacter with paper plates as medium and activated sludge vaccinated。When HRT is 7.2h, temperature 25℃,pH 7.5, 100mg/L or so is the biggest NO₃^--N concentration which can be handled in the 2# reacter , however, in the same conditions, 120mg/L or so is the biggest NO₃^--N concentration which can be handled in the 1# reacter. When water temperature is below 25℃or pH not neutral, nitrate nitrogen removal rate in1 # reactor is higher about 15% than in the 2 # reactor.(5) The rice straw and paper plate of solid organic carbon sources contain the soluble small molecules organic matter which can quickly dissolve into water,and slow-release carbon source by their own hydrolysis and microbial and enzyme degradation function to provide organic electron donor for denitrification. With solid organic carbon source for medium, the reacter is vaccinated activated sludge and successly startuped.Its denitrification is mainly through biological film adherent growth at solid carbon source surface ,and the reaction kinetics model conforms to level the dynamics of single substrate Monod equation. With solid organic carbon sources and embedding bacteria mixed particles as medium,the reactor stable operates. The denitrifying processes includes two parts:one part is that the biofilm adherent growth at solid carbon source surface main degradates solid carbon source,and accompanied by certain denitrifying function.Its reaction kinetics accord with single substrate of Monod equations.Another part is that embedding bacteria particles assume major denitrifying function, its reaction kinetics model conforms to the double bottom of content Monod equation.To sum up research shows:The rice straw and paper plate is respectively used as solid organic carbon source ,the preferable denitrification effect can be both gotten.When activated sludge and embedding bacteria particles use solid carbon source to do denitrificate,the reacter with activated sludge inoculated is more influenced by environmental factors, the reacter with embedding bacteria particles added is less influenced by environmental factors and not basically pH.In the same conditions,the denitrification effect in the reacter with embedding bacteria particles added is better than in the reacter with activated sludge inoculated.But the embedding bacteria exists substrate mass transfer problems in the denitrification process.So the methods is feasible which solid organic matter for rice starw and paper plate rich in cellulose are used as the denitrification carbon sources .And the advantage of the embedding bacteria's strong environmental adaptability make the method have development potential which solid carbon sources and embedding bacteria are combined to use to remove the nitrate in the groundwater.