Aerobic Granular Sludge: Effect of Substrate on Granule Formation

Discharging untreated wastewater will contaminate the surface waters and can lead to spread of diseases and long term ecological damage. The most common method for treatment is by the activated sludge process. In this process, nutrients like nitrogen, phosphorus and COD are removed by bacteria grown in flocs. These bacterial flocs are separated from the treated water by settling. Due to the slow settling velocities of these flocs large settling tanks are needed. Settling tanks take up most of the required space for a wastewater treatment plant. Aerobic granular sludge is a compact technology designed to reduce area requirements, save energy while providing excellent effluent quality. Bacteria are grown in granules instead of flocs and have therefore a much higher settling velocity. This reduces the area requirement significantly. So much even so, that external settling tanks are completely omitted. To grow aerobic granules a few selection principles are needed. First, the influent is brought in contact with the biomass in an anaerobic environment. Here COD is converted by the bacteria into storage polymers. These storage polymers are then used for growth in the presence of oxygen hereby removing phosphorus and nitrogen from the bulk liquid. Secondly, a settling pressure is applied by which slow settling biomass is removed from the reactor, thus leading to the formation of granules. In previous research by the PhD students Janneke Beun (principle of aerobic granulation), Merle de Kreuk (basic process technology for granular sludge nutrient removal) and Mari Winkler and Joao Bassin (Microbiology and process engineering aspects of granular sludge) the basic concepts of the granular sludge technology were worked out. In this thesis the effects of several operational conditions on the conversion processes, formation and stability of aerobic granular sludge was studied. The quick implementation of the technology in practice also meant that several important subjects still needed further investigation. To ensure a well-functioning technology in domestic and industrial applications these subjects were studied in more detail (i.e. adsorption, effect of salinity, higher temperature and other substrates). Besides laboratory work also the start-up and performance of one of the first full-scale aerobic granular sludge reactors treating domestic wastewater is described. The ammonium adsorption properties of aerobic granular sludge, activated sludge and anammox granules have been investigated in Chapter 2. During operation of a pilot-scale aerobic granular sludge reactor, a positive relation between the ammonium influent concentration and the ammonium adsorbed was observed. Aerobic granular sludge exhibited much higher adsorption capacity compared to activated sludge and anammox granules. At an ammonium concentration of 30 mg N L-1, adsorption obtained with activated sludge and anammox granules was around 0.2 mg NH4+-N gVSS-1, while aerobic granular sludge from lab- and pilot scale exhibited an adsorption of 1.7 and 0.9 mg NH4+-N gVSS-1, respectively. No difference in the ammonium adsorption was observed in lab-scale reactors operated at different temperatures (20 and 30 ºC). In a lab-scale reactor fed with saline wastewater, we observed that the amount of ammonium adsorbed, decreased considerably when the salt concentration increased. The results indicate that adsorption or better: ion-exchange of ammonium should be incorporated into models for nitrification/denitrification, certainly when aerobic granular sludge is used. Salinity can adversely affect the performance of most biological processes involved in wastewater treatment (Chapter 3). The effect of salt (NaCl) on the main conversion processes in an aerobic granular sludge (AGS) process accomplishing simultaneous organic matter, nitrogen, and phosphate removal was evaluated in this chapter. Hereto an AGS sequencing batch reactor was subjected to different salt concentrations (0.2 to 20 g Cl- L-1). Granular structure was stable throughout the whole experimental period, although granule size decreased and a significant effluent turbidity was observed at the highest salinity tested. A weaker gel structure at higher salt concentrations was hypothesized to be the cause of such turbidity. Ammonium oxidation was not affected at any of the salt concentrations applied. However, nitrite oxidation was severely affected, especially at 20 g Cl- L-1, in which a complete inhibition was observed. Consequently, high nitrite accumulation occurred. Phosphate removal was also found to be inhibited at the highest salt concentration tested. Complementary experiments have shown that a cascade inhibition effect took place: first, the deterioration of nitrite oxidation resulted in high nitrite concentrations and this in turn resulted in a detrimental effect to polyphosphate-accumulating organisms (PAOs). By preventing the occurrence of the nitrification process and therefore avoiding the nitrite accumulation, the effect of salt concentrations on the bio-P removal process was shown to be negligible up to 13 g Cl- L-1. Salt concentrations equal to 20 g Cl- L-1 or higher in absence of nitrite also significantly reduced phosphate removal efficiency in the system. When aerobic granular sludge is applied for industrial wastewater treatment different soluble substrates can be present. For stable granular sludge formation on volatile fatty acids (e.g. acetate), production of storage polymers under anaerobic feeding conditions has been shown to be important. This prevents direct aerobic growth on readily available COD, which is thought to result in unstable granule formation. In Chapter 4 we investigate the impact of acetate, methanol, butanol, propanol, propionaldehyde and valeraldehyde on granular sludge formation at 35 °C. Methanogenic archaea, growing on methanol, were present in the aerobic granular sludge system. Methanol was completely converted to methane and carbon dioxide by the methanogenic archaeum Methanomethylovorans uponensis during the one-hour anaerobic feeding period, despite the relative high dissolved oxygen concentration (3.5 mg O2 L-1) during the subsequent two-hour aeration period. Propionaldehyde and valeraldehyde were fully disproportionated anaerobically into their corresponding carboxylic acids and alcohols. The organic acids produced were converted to storage polymers, while the alcohols (produced and from influent) were absorbed onto the granular sludge matrix and converted aerobically. Our observations show that easy biodegradable substrates not converted anaerobically into storage polymers could lead to unstable granular sludge formation. However, when the easy biodegradable COD is absorbed in the granules and/or when the substrate is converted by relatively slow growing bacteria in the aerobic period stable granulation can occur. The influence of sludge age on granular sludge formation and microbial population dynamics in a methanol- and acetate-fed aerobic granular sludge system operated at 35 °C is investigated in Chapter 5. During anaerobic feeding of the reactor, methanol was initially converted to methane by methylotrophic methanogens. These methanogens were able to withstand the relatively long aeration periods. Lowering the anaerobic solid retention time (SRT) from 17 to 8 days enabled selective removal of the methanogens and prevented unwanted methane formation. In absence of methanogens, methanol was converted aerobically, while granule formation remained stable. At high SRT-values (51 days) γ-Proteobacteria were responsible for acetate removal through anaerobic uptake and subsequent aerobic growth on storage polymers formed (so called metabolism of glycogen accumulating organisms). When lowering the SRT (24 days), Defluviicoccus-related organisms (cluster II) belonging to the α-Proteobacteria outcompeted acetate consuming γ-Proteobacteria at 35 ºC. DNA from the Defluviicoccus-related organisms in cluster II was not extracted by the standard DNA extraction method but with liquid nitrogen, which showed to be more effective. Remarkably, the two glycogen accumulating organisms (GAO) types of organisms grew separately in two clearly different types of granules. This work further highlights the potential of aerobic granular sludge systems to effectively influence the microbial communities through sludge age control in order to optimize the wastewater treatment processes. Recently, aerobic granular sludge technology has been scaled-up and implemented for industrial and municipal wastewater treatment under the trade name Nereda®. With full-scale references for industrial treatment application since 2006 and domestic sewage since 2009 only limited operating data have been presented in scientific literature so far. In this study performance, granulation and design considerations of an aerobic granular sludge plant on domestic wastewater at the WWTP Garmerwolde, the Netherlands were analysed (Chapter 6). After a start-up period of approximately 5 months, a robust and stable granule bed (> 8 g L-1) was formed and could be maintained thereafter, with a sludge volume index after 5 minutes settling of 45 mL g-1. The granular sludge consisted for more than 80 % of granules larger than 0.2 mm and more than 60 % larger than 1 mm. Effluent requirements (7 mg N L-1 and 1 mg P L-1) were easily met during summer and winter. Maximum volumetric conversion rates for nitrogen and phosphorus were respectively 0.17 and 0.24 kg (m3 d)-1. The energy usage was 13.9 kWh (PE150∙year)-1 that is 58 – 63 % lower than the average conventional activated sludge treatment plant in the Netherlands. Finally, this study demonstrated that aerobic granular sludge technology can effectively be implemented for the treatment of domestic wastewater.