In wastewater treatment plants, the amount of ammonium is one example of a measure to determine the quality of the effluent wastewater. Ammonium is regarded as a hazardous chemical for aqueous ecosystems and can cause eutrophication due to its high nitrogen content. The ammonium content in the treated wastewater is controlled by aeration of the biological treatment stage, in which ammonium is converted to nitrate. The aeration process often accounts for the largest energy consumption of the wastewater treatment plant, which motivates automatic control solutions that can both aid in reducing the discharge of ammonium in the effluent and improve the energy efficiency of the aeration process. One such control technique currently used by several large municipal wastewater treatment plants in Sweden is ammonium-based aeration control. In this technique, the aeration process is controlled based on measurements of the effluent ammonium concentration. The purpose of the thesis was to study an extension of ammonium-based aeration control that could better adapt to daily, and often large, fluctuations in the influent load. The proposed method is to use an iterative algorithm to tune the set-point of the ammonium feedback controller. The objective is to, over a given time interval, achieve a flow-proportional mean of the effluent ammonium concentration close to a desired value for a wide range of influent loads. The method was tested by extensive simulations, and the results indicate that the iterative set-point tuning algorithm has the potential to offer a superior ability to achieve a desired flow-proportional mean at the end of a given evaluation period and, in some instances, energy savings compared to standard ammonium feedback control.

The aeration process is often the single largest consumer of electricity in a wastewater treatment plant. Aeration in biological reactors provides microorganisms with oxygen which is required to convert ammonium to nitrate. Ammonium is toxic for aqueous ecosystems and contributes to eutrophication. The importance of aeration for the treatment results in combination with the high costs motivates automatic control of the aeration process. This thesis is devoted to ammonium feedback control in municipal wastewater treatment plants. With ammonium feedback control, the aeration intensity is changed based on a measurement of the outlet ammonium concentration. The main focus of the thesis is design, implementation, evaluation and improvement of ammonium PI (proportional-integral) controllers. The benefits of ammonium feedback control are established through long-term experiments at three large wastewater treatment plants in Stockholm, Sweden. With ammonium feedback control, energy savings up to around 10 % were achieved compared to keeping the dissolved oxygen concentration constant. The experiments generated several lessons learned with regard to implementation and evaluation of controllers in full-scale operation. The thesis has established guidelines on how to design ammonium feedback controllers for situations when cost-effective operation is the overall aim. Simulations have demonstrated the importance to limit the dissolved oxygen concentration in the process and under what conditions the energy saving with ammonium feedback control is large. The final part of the thesis treats improvements of ammonium PI control through minor modifications to the control structure or controller. Three strategies were studied: gain scheduling control, repetitive control, and a strategy reacting to oxygen peaks in the last aerobic zone. The strategies all had their benefits but the ammonium feedback controller was the key factor to improved aeration control.

Lustgas är en växthusgas, ca 300 gånger mer potent än koldioxid och som har potential att bryta ned ozonlagret med sin livslängd på 120 år i atmosfären. Lustgasutsläppen beräknas ha ökat med mellan 20–30% sedan förindustriell tid vilket tros bero på en ökad användning av konstgödsel i jordbrukssektorn men en del av all lustgas som bildas kommer från avloppsreningsverk vilka står för ca 1,6% av lustgasavgången till atmosfären där Henriksdals reningsverk bidrar med lustgasproduktion. Under 2021 beräknade man en lustgasavgång från Henriksdals reningsverk på ca 37 ton, vilket kan jämföras med klimatavtrycket på 10 000 nyregistrerade bilar vid körningar på ca 1500 mil/år. Syftet med rapporten var att undersöka om det gick att beräkna mängden kväve i avgående lustgas och jämföra det med hur mycket av den beräknade lustgasen som avgår till luft av andelen renat ammonium, samt att undersöka olika strategier för minimering av lustgasproduktion. Det finns flera vägar till lustgasbildning i de biologiska reningsprocesserna i verket. Tre huvudspår finns dock till lustgasbildning varav autotrof nitrifikation var aktuell för detta examensarbete. Flera driftsstrategier finns att tillgå för att undersöka hur lustgasproduktion påverkas av olika faktorer. Under detta examensarbete undersöktes en av dessa, syrebörvärden, där ett fast syrevärde i den luftade bassängen antingen skulle minska eller öka lustgasproduktionen beroende på luftningsintensitet och ammoniumkoncentration i vattnet. Lustgasavgången beräknades bestå till 0,19% av kväve renat från ammoniumbelastningen. Resultatet visade att vid låga syrehalter i vattnet fanns en lägre andel lustgasbildning jämfört med högre luftningsintensitet. Nackdelen blev dock att andelen renad ammonium sjönk i blocket. Vid högre luftningsintensitet var lustgasavgången högre i ett av huvudscenariona. I det andra scenariot med hög luftning var andelen renat ammonium lägre än i första scenariot, men med låga lustgashalter. Slutsatsen blev att lustgas kan bildas under olika förutsättningar men att under just den här studien var lustgasproduktionen som högst när nitrifikationen var som mest effektiv. / Nitrous oxide is a greenhouse gas, almost 300 times more potent than carbon dioxide, and which has the potential to deplete the ozone layer with its lifetime of 120 years in the atmosphere. Nitrous oxide emissions are estimated to have increased between 20–30% since pre-industrial times, which is believed to be due to increased use of artificial fertilizers in the agricultural sector, but part of all nitrous oxide that is formed comes from wastewater treatment plants, which account for approx. 1.6% of nitrous oxide emissions to the atmosphere where Henriksdals treatment plants contribute to nitrous oxide production. In 2021, nitrous oxide emissions from Henriksdal's treatment plant were calculated to be approximately 37 tons, which can be compared to the climate footprint of 10,000 newly registered cars driven approximately 15 000 km per year. The aim of the report was to investigate whether it was possible to calculate the amount of nitrogen in outgoing nitrous oxide to compare it with how much of the calculated nitrous oxide is emitted to air from the proportion of purified ammonium, as well as to investigate different strategies for minimizing nitrous oxide production. There are several routes to the formation of nitrous oxide in the biological purification process in the plant. There are, however, three main tracks for nitrous oxide formation, of which autotrophic nitrification was relevant for this degree project. Several operational strategies are available to investigate how nitrous oxide production is affected by various factors. During this thesis, one of these, oxygen set-point, was investigated, where a fixed oxygen value in the aerated basin would either decrease or increase nitrous oxide production depending on aeration intensity and ammonium concentration in the water. The nitrous oxide discharge was calculated to consist of 0.19% nitrogen purified from the ammonium load. The result showed, that at low oxygen levels in the water, there was a lower proportion of nitrous oxide formation compared to higher aeration intensity. The disadvantage, however, was that the proportion of purified ammonium decreased in the block. At higher aeration intensity, nitrous oxide emissions were higher in one of the main scenarios. In the second scenario with high aeration, the proportion of purified ammonium was lower than in the first scenario, but with low levels of nitrous oxide. The conclusion was that nitrous oxide can be formed under different conditions, but during this study, nitrous oxide production was highest when nitrification was most efficient.