Indigenous Microalgae-activated Sludge Cultivation System for Wastewater Treatment

“Everything is everywhere, but the environment selects the best duo’’; natural selection holds the key to the relationship between the micro-algal communities in Lake Mälaren and the activated sludge organisms in the municipal wastewater of Västerås. Interest in microalgae-based activated sludge wastewater treatment has been increasing in recent decades as a way to meet the demand for sustainable wastewater treatment. Bacteria assisted microalgae cultivation is vital for reducing the emission of greenhouse gases (GHG) in the long term and for the prevention of excess biological and chemical sludge in conventional wastewater treatment sites. This licentiate thesis focuses on process parameters such as light intensity, retention time and nutrient composition and the influence of micro-algal activated sludge (MAAS) on waste water treatment efficiency. The laboratory investigations will later be used to design a pilot scale approach. Results from a semi-continuous MAAS process using raw wastewater showed combined removal of nitrogen (N) and phosphorous (P) at 2, 4 and 6 days of residence time. The removal of P was stable and a maximum removal efficiency from untreated wastewater of 50-65 % was achieved. The average N removal efficiencies at 6, 4 and 2 days of residence time were 82, 65 and 39 % respectively. In general, the removal of nutrients increased as expected with increased residence time. In addition, the MAAS sampled at different residence times showed a high biogas potential during batch tests. Most importantly, no lag phase was observed during the anaerobic mono-digestion. Addition of P to the iron flocculated wastewater (treated wastewater) increased the N removal efficiency to 88 and 92 % at 4 days of residence time. Overall, the consumption of iron will be reduced in the future by using the MAAS process. The flocculation step may also be shifted to after the MAAS process. This will retain the P that increases N removal in the MAAS process and thus also helping to reduce chemical sludge. Photosynthetic active radiation studies showed that the required light intensity increases with the concentration of biomass in a PBR operated at 14°C (water temperature). The MAAS floc is key for efficient utilisation of light and oxygen synthesis. However, the evaluation of the process needs to be verified by comparing continuous and discontinuous (i.e. intermittent lighting hours) artificial lighting conditions. This will enable the safe discharge of wastewater with minimal power consumption. Additionally, rapid settleability of the MAAS was observed in all the semicontinuous studies. Therefore, expensive harvesting techniques can be avoided with the help of MAAS. The experiments described here only cover laboratory operation, and the results have to be verified in pilot scale operation in prevailing Swedish weather conditions. The primary focus will be to achieve the discharge limits set by Mälarenergi AB (Nitrogen: <15 mg L-1; Phosphorous: <0.5 mg L-1) with minimal residence time. Popular scientific summary Municipal wastewater is mainly composed of water containing anthropogenic wastes that are rich in nutrients such as carbon, nitrogen and phosphorous. The cost of biological treatment of wastewater is increasing globally due to the population growth in urban areas. In general, the activated sludge (AS) process is a biological nutrient removal process used in wastewater treatment plants (WWTPs). The AS is composed of different microorganisms among which bacteria play a crucial role in wastewater treatment (WWT). During the process, air is bubbled through the medium to supply oxygen, and methanol is added to improve removal of nitrogen, which is released as a gas. Phosphorous is removed using chemical precipitation. The current process requires electrical energy, precipitation chemicals and handling of excess sludge, and it emits carbon dioxide (CO2) as a by-product. This process has been used in WWTPs since 1914, although numerous modifications have been implemented to meet the stringent regulations in the European Union and other locations. Microalgae are microorganisms that, like plants, also perform photosynthesis. They are green and reproduce quickly using available nutrients (nitrogen and phosphorous) and CO2 from their environment in the presence of light. As a result of photosynthesis, oxygen is released as waste gas. The oxygen synthesised during this process can be utilised by the AS bacteria, resulting in the microalgae activated sludge (MAAS) process. The main advantage of this process is the combined removal of nutrients from the aqueous phase. The goal of this research is to implement indigenous microalgae cultivation in the activated sludge process to consume CO2 and recover nutrients from wastewater. This study was performed to improve the understanding of processes such as light utilisation, nutrient removal and recovery of biomass from wastewater in closed photo-bioreactors. Photo-bioreactors are vessels where cultivation is carried out in the presence of light. In this study, initial experiments investigate the influence of the light spectrum on photosynthetic growth in micro-algal cultivation. This is followed by a look at operational challenges of the microalgae cultivation during the AS process. Experiments are then performed on the process in the photo-bioreactors with different wastewater treatment times. The results show that 2-6 days of treatment can be used to reduce nutrients in wastewater if the process is optimised further. Nutrient ratio is also analysed for the availability for micro-algal growth. The recovered MAAS after the wastewater treatment subjected to anaerobic mono digestion showed biogas yields of about 60-80% within 5 to 9 days. The experimental verification of chemically precipitated wastewater shows that phosphorus was limiting for micro-algal growth. Additionally, the optimal oxygen supply as a function of light response was verified for photo-bioreactors. The outcome of this study shows that identification of the right conditions can reduce the treatment time. In so doing, stable nutrient removal and reduction of precipitation chemicals can be achieved, as well as improved recovery of valuable nutrients like phosphorous and nitrogen.