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

Sammanfattning: 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.