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| | | Junior Member
 
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| | Can anyone find an explication for the following: BNR removal plant, 800cu m anaerobic reactor, 3.000 cu m anoxic reactor, 7.000 cu m oxic reactor. Flow 20.000 cu m /day, COD 400 ppm, BOD 180 ppm, SS 125 ppm, TKN 70 ppmN, NH4 53 ppmN, P 6ppm settled sewage mainly domestic ww. MLSS 3.000 - 4.000 ppm, BOD loading <0,1 Kg / KgMLSS. Internal recycle 200%, external 80%. Plant complies to discharge limit of 10 ppm TN, except during wet spells. The plant is in a normally dry climate, when it rains the temperature in the reactor drops 2 - 3ºC, last episode from 22 to 19ºC. The hydraulic load is kept constant, excess flow is diverted after primary sed. and plant loading is reduced by approx. 40%, but ammonia levels in the effluent increase. nitrite and nitrate remain constant. When rain ceases results return to normal after about 5 days. |
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| What happens to mixed liquor pH and DO levels before and during rain events? Are your mixed liquor concentations relatively stable during these events?
Wiff Peterson |
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| | | Supreme Being
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| | There is a chance that the pH lowers substantially with infiltration as rain water is pH 5.7 or thereabouts. Nitrifiers generally don't like rapid changes in pH. The temperature drop probably is related to infiltration as well. Regards TerryF |
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| | Thanks for your posting We only measure pH in the settled sewage and effluent. Normal levels are between 7,4 - 7,9, and we dont observe drastic changes during wet spells. Our ww is very well buffered, conductivity 3.500 micro siemens (reduced during wet spells). Would you recommend registering pH in the aeration basins or perhaps alkalinity in the effluent? DO is automatically controlled and possibly is higher during wet spells. We keep MLSS relativley stable, and reduce sludge wasting during wet spells. Our wet spells could last, at most, two days. |
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| | TerryF Thanks for your reply. We dont have separate sewerage systems, so the temperature drop is due to the colder rain water. Do you think a 2 - 3ºC drop could affect the nitrifiers? |
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| | Several things could be happening:eg the lower temp will slow down the rate of nitrification; the rainwater could be introducing DO into the anoxic zone, decreasing denitrification...is the increase in effleunt TN due to increases in ammonia or nitrate or both? The measurement of these should give you a clue to what is happening. To understand problems like these you really need to meodel your system on a model such as Biowin.
David
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| | | Supreme Being
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| | A drop of that magnitude will only slow the process down , but won't be too drastic. I would be concerned about all the other things that will be happening when you get a large amount of infiltration , flow diversions etc. Even though your numbers look okay at face value there will be things happening with pH , alkalinity , and conductivity as well as interactions between hetertrophic and nitrifying bacteria that change because the loading is totally different. A biomass in any treatment plant tends to become acclimatised and the population dynamics adjusted to the unique load charateristics of that plant. A major rain event changes those characteristics by dilution of the sewerage with water that is clean low pH and low condictivity and sometimes carrying substantial amounts of nitrogen. Addittionally infiltration often brings with it silt , pollutants and nutrients as run off from urban areas which will again change the whole dynamics of the load. Only closer monitoring of plant conditions during such events would a definitive answer be able to be given. REgards TerryF |
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| | TN increases due to higher ammonia levels, nitrite and nitrate remain constant. I´m looking at different models to simulate plant conditions. Thanks |
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| | | Junior Member
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| | Just a note to inform people interested in my posting. We have just had a wet spell, once again we observed an increase in the ammonia levels in the effluent, but not as pronounced as past events. Conductivity and alkalinity values were reduced in the primary effluent but ammonia levels remained relatively high. This seems to agree with TerryF´s posting and we feel it could be the root of all our problems. We will intensify our monitoring during wet and dry spells. One question, where in the plant and which bacteria are responsible for the conversion of organic nitrogen to ammonia? (ammonification). |
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| Sounds like you're on top of the right things. Organic N is converted to NH3 as part of BOD breakdown early in the activated sludge process...
Wiff Peterson |
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