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Optimering av bio-P-processen vid Västra strandens avloppsreningsverk i Halmstad

Eriksson, Amanda LU (2018) VVAM05 20181
Chemical Engineering (M.Sc.Eng.)
Abstract
As eutrophication of lakes and oceans is increasing, regulations regarding phosphorus effluents becomes stricter. Consequently, the majority of Sweden’s wastewater treatment plants employs some method for phosphorus removal. Increased efforts for sustainable purification methods has also led to phosphorus removal by chemical means starting to be replaced by biological phosphorus removal. A common biological treatment method for phosphorus removal is Enhanced Phosphorus Removal (EBPR), in which specific process conditions are used to drive microorganisms to accumulate excessive phosphate. Such a method was introduced at Västra stranden’s wastewater treatment plant in the years of 2005-2006, which worked satisfactorily for several years.... (More)
As eutrophication of lakes and oceans is increasing, regulations regarding phosphorus effluents becomes stricter. Consequently, the majority of Sweden’s wastewater treatment plants employs some method for phosphorus removal. Increased efforts for sustainable purification methods has also led to phosphorus removal by chemical means starting to be replaced by biological phosphorus removal. A common biological treatment method for phosphorus removal is Enhanced Phosphorus Removal (EBPR), in which specific process conditions are used to drive microorganisms to accumulate excessive phosphate. Such a method was introduced at Västra stranden’s wastewater treatment plant in the years of 2005-2006, which worked satisfactorily for several years. However, in the year of 2015 the efficiency of the process started deteriorating which forced the plant to start adopting chemicals in order to not exceed effluent regulations. The purpose of this degree project is thus to, in accordance with the plant’s current design, evaluate the EBPR-process in an attempt of optimizing it. Hence, recommendations with regards to process changes believed to stabilize the process will be included.
Various process parameters known for influencing EBPR were evaluated by examining, for example, the phosphate release rate, COD/TP-ratio and by looking into carbon source availability. The way phosphate and nitrate varied in different zones in different process steps were studied and the project came to also include evaluation of the biological nitrogen separation. Moreover, sludge age, residence time and various design ratios provided additional basis for evaluation.
It was shown how the EBPR-startup resulted in significant expansion of polyphosphate accumulating organisms, indicating the sludge having high potential for EBPR. Once the chemicals used for precipitation of phosphorus had been washed out, the main problem were reoccurring phosphorus peaks which though started declining towards the end of the project. Furthermore, the results indicated that the EBPR-function seemed to be hindered both by insufficient amounts of VFA and by the occurrence of, in certain process steps, impeding nitrate. The low amounts of VFA originates from the inadequately functioning primary sludge hydrolysis. This process step had been deemed necessary as the side stream hydrolysis along with incoming VFA does not provide the process with sufficient amounts of VFA. It is therefore recommended to attempt increasing the sludge level and even out the flow distribution between the basins. Similarly, an attempt to establish a more continuous recirculation could improve the hydrolysis. In addition were undesired oxygenation shown to take place in a few anaerobic process steps, which perhaps were most pronounced for the first zone in the second biological treatment line. Therefore, it is also recommended that the waterfall resulting in this oxygenation is removed by, for example, introducing a ramp.
As a result of incomplete denitrification, nitrate levels primarily originating from the ARP were on several occasions proved high enough to negatively impact the EBPR-process. Since the sludge’s denitrification rate appeared to be acceptable, the problem is suspected to originate from a too low C/N-ratio. One solution could be to increase the flow used as carbon source in the ARP but not before performing a more complete sampling to further determine potential daily and weekly variations of NO3-N and COD. Similarly is further monitoring of, for example, incoming VFA/PO4-P-ratios and more continuous monitoring of outgoing nitrate from ARP advocated. In addition, the use of modeling should be considered as a tool to further assess how change in parameters such as COD, VFA, PO4-P, NO3-N etc. influence the optimal process configuration. Lastly, it should be mentioned that as the water temperature starts to decrease again, the zone distribution should be reviewed once more. This since ATV-guidelines suggest that the aerated sludge age deemed necessary for nitrification is too low at lower water temperatures. (Less)
Popular Abstract (Swedish)
Hur kan vi tackla problemet med övergödning utan att våra reningsverk ska behöva använda massor med kemikalier för att ta bort ämnena skyldiga till detta fenomen?
Övergödning av hav och sjöar, resulterandes i ökad algblomning och syrebrist, sker på grund av utsläpp av för mycket näringsämnen såsom fosfor. Sverige har därav strikta reningskrav för vattnet vi släpper ut och de flesta reningsverk bedriver därmed någon form av fosforrening. Fosforrening kan bedrivas genom att använda så kallade fällningskemikalier vilka klumpar ihop sig med fosfor och annat material där klumparna sedan sjunker till botten som slam. Sådana kemikalier är dock dyra och eftersom de även påverkar miljön negativt har intresset för andra reningsmetoder ökat.
... (More)
Hur kan vi tackla problemet med övergödning utan att våra reningsverk ska behöva använda massor med kemikalier för att ta bort ämnena skyldiga till detta fenomen?
Övergödning av hav och sjöar, resulterandes i ökad algblomning och syrebrist, sker på grund av utsläpp av för mycket näringsämnen såsom fosfor. Sverige har därav strikta reningskrav för vattnet vi släpper ut och de flesta reningsverk bedriver därmed någon form av fosforrening. Fosforrening kan bedrivas genom att använda så kallade fällningskemikalier vilka klumpar ihop sig med fosfor och annat material där klumparna sedan sjunker till botten som slam. Sådana kemikalier är dock dyra och eftersom de även påverkar miljön negativt har intresset för andra reningsmetoder ökat.
Biologisk fosforavskiljning (bio-P) nyttjar, istället för kemikalier, mikroorganismer som naturligt kan ta upp fosfor från vattnet. Då inget syre finns tillgängligt släpper dessa bakterier fosfor som de har lagrat i sina celler samtidigt som de tar upp lättåtkomligt kol. Om bio-P-bakterier sedan ges tillgång till syre börjar de använda det kol de har lagrat i sina celler för att växa och tar samtidigt upp fosfor från vattnet runt omkring dem. Eftersom bakterierna växer tar de upp mer fosfor när syre finns tillgängligt än de släpper när syre inte finns tillgängligt. Alltså fås ett nettoupptag av fosfor och eftersom bakterierna sedimenterar till botten som slam avskiljs fosfor från vattnet.
Som har nämnts behöver alltså bakterierna tillgång till kol för att kunna ge ett nettoupptag av fosfor men det är inte alltid säkert att det finns tillräckligt mycket lättillgängligt kol i inkommande avloppsvatten. Avloppsvattnet brukar dock alltid innehålla en stor mängd totalt organiskt kol vilket kan omvandlas på reningsverket till sådant lättillgängligt kol genom något som kallas hydrolys. Hydrolys är kort beskrivet en process där mikroorganismer i frånvaro av syre omvandlar svårtillgängligt organiskt material till mer lättillgängliga kolföreningar.
Västra strandens avloppsreningsverk i Halmstad använder sig av sådan beskriven biologisk fosforavskiljning och eftersom det inkommande avloppsvattnet har för lite tillgängligt kol bedriver de även hydrolys. Faktum är att de bedriver hydrolys med både primärslam, vilket är det slam som bildas genom sedimentering av inkommande vatten, och med returslam, som är det slam som bildas vid den kväve- och fosforreningen. Då den biologiska fosforavskiljningen inte fungerat som önskat de senaste åren utvärderades processen genom att titta på olika parametrar som inverkar på processen.
Även fast returslamshydrolysen visade sig vara relativt bra dimensionerad var den inte så effektiv och resultaten tydde på att primärslams¬hydrolys kan vara nödvändig för att bilda tillräcklig mängd lättillgängligt kol för processen. Primärslamshydrolysen indikeras dock producera mindre lättillgängligt kol än förväntat vilket kan vara anledningen till att processen ibland går dåligt. Dessutom tydde resultaten på att det ibland förekom ämnen som nitrat i processteg där nitrat kan störa bio P funktionen. Detta föreföll vara en konsekvens av att delar av den biologiska kvävereningen ibland inte fungerade fullständigt. Beräkningar indikerade att bakterierna ansvarig för kvävereningen också hade tillgång till för lite kol med avseende på hur mycket kväve de skulle omvandla. Kvävereningen skulle därför eventuellt kunna förbättras genom att öka flödet som innehåller det lättillgängliga kolet.
Allt som allt drogs också slutsatsen att det kan vara mycket användbart att ta hjälp av datoriserade matematiska modeller. Detta för att bättre få en uppfattning om hur ändringar i olika parametrar påverkar olika delar av processen. (Less)
Please use this url to cite or link to this publication:
author
Eriksson, Amanda LU
supervisor
organization
course
VVAM05 20181
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Enhanced phosphorus removal, EBPR, bio-P, wastewater treatment, PAO, avloppsteknik
language
Swedish
id
8944539
date added to LUP
2018-06-04 15:54:48
date last changed
2018-06-04 15:54:48
@misc{8944539,
  abstract     = {As eutrophication of lakes and oceans is increasing, regulations regarding phosphorus effluents becomes stricter. Consequently, the majority of Sweden’s wastewater treatment plants employs some method for phosphorus removal. Increased efforts for sustainable purification methods has also led to phosphorus removal by chemical means starting to be replaced by biological phosphorus removal. A common biological treatment method for phosphorus removal is Enhanced Phosphorus Removal (EBPR), in which specific process conditions are used to drive microorganisms to accumulate excessive phosphate. Such a method was introduced at Västra stranden’s wastewater treatment plant in the years of 2005-2006, which worked satisfactorily for several years. However, in the year of 2015 the efficiency of the process started deteriorating which forced the plant to start adopting chemicals in order to not exceed effluent regulations. The purpose of this degree project is thus to, in accordance with the plant’s current design, evaluate the EBPR-process in an attempt of optimizing it. Hence, recommendations with regards to process changes believed to stabilize the process will be included. 
Various process parameters known for influencing EBPR were evaluated by examining, for example, the phosphate release rate, COD/TP-ratio and by looking into carbon source availability. The way phosphate and nitrate varied in different zones in different process steps were studied and the project came to also include evaluation of the biological nitrogen separation. Moreover, sludge age, residence time and various design ratios provided additional basis for evaluation. 
It was shown how the EBPR-startup resulted in significant expansion of polyphosphate accumulating organisms, indicating the sludge having high potential for EBPR. Once the chemicals used for precipitation of phosphorus had been washed out, the main problem were reoccurring phosphorus peaks which though started declining towards the end of the project. Furthermore, the results indicated that the EBPR-function seemed to be hindered both by insufficient amounts of VFA and by the occurrence of, in certain process steps, impeding nitrate. The low amounts of VFA originates from the inadequately functioning primary sludge hydrolysis. This process step had been deemed necessary as the side stream hydrolysis along with incoming VFA does not provide the process with sufficient amounts of VFA. It is therefore recommended to attempt increasing the sludge level and even out the flow distribution between the basins. Similarly, an attempt to establish a more continuous recirculation could improve the hydrolysis. In addition were undesired oxygenation shown to take place in a few anaerobic process steps, which perhaps were most pronounced for the first zone in the second biological treatment line. Therefore, it is also recommended that the waterfall resulting in this oxygenation is removed by, for example, introducing a ramp.
As a result of incomplete denitrification, nitrate levels primarily originating from the ARP were on several occasions proved high enough to negatively impact the EBPR-process. Since the sludge’s denitrification rate appeared to be acceptable, the problem is suspected to originate from a too low C/N-ratio. One solution could be to increase the flow used as carbon source in the ARP but not before performing a more complete sampling to further determine potential daily and weekly variations of NO3-N and COD. Similarly is further monitoring of, for example, incoming VFA/PO4-P-ratios and more continuous monitoring of outgoing nitrate from ARP advocated. In addition, the use of modeling should be considered as a tool to further assess how change in parameters such as COD, VFA, PO4-P, NO3-N etc. influence the optimal process configuration. Lastly, it should be mentioned that as the water temperature starts to decrease again, the zone distribution should be reviewed once more. This since ATV-guidelines suggest that the aerated sludge age deemed necessary for nitrification is too low at lower water temperatures.},
  author       = {Eriksson, Amanda},
  keyword      = {Enhanced phosphorus removal,EBPR,bio-P,wastewater treatment,PAO,avloppsteknik},
  language     = {swe},
  note         = {Student Paper},
  title        = {Optimering av bio-P-processen vid Västra strandens avloppsreningsverk i Halmstad},
  year         = {2018},
}