Aerobic granulation is seen as the future standard for industrial and municipal wastewater treatment and subsequently research efforts are quickly developing in this field. As an outcome of a concerted Dutch program, an aerobic granular biomass technology has been scaled-up and implemented for the treatment of urban and industrial wastewater. This Nereda® technology is considered being the first aerobic granular sludge technology applied at full-scale. Operating data from the first municipal full scale plant confirm the projected advantages with regard to treatment performance, energy-efficiency and cost-effectiveness. The technology, now applied at tank sizes similar to the world’s largest SBR tanks, is considered proven and applicable for even the largest applications. During the presentation the latest results and lessons learned will be presented..IntroductionOne of the most critical aspects of the activated sludge process has always been the separation of biomass and treated water. Besides the development of physical separation techniques (membrane bioreactors) the improvement of settling properties of the activated sludge has been an important research topic. The basic requirement for biomass with good settling properties is a granular structure based on compact, dense, large particles with a high specific gravity.Discovered in 1995 and further developed by Mark van Loosdrecht from the Delft University of Technology (DUT), the process of using aerobic granular biomass for wastewater treatment has been scaled up and engineered to suit commercial applications by Royal HaskoningDHV, a Dutch E+C company and has been commercially branded as Nereda® Technology. WABAG signed a License agreement with RHDHV for applying this Technology in India and Switzerland.The Nereda® technology has been applied in various industrial and municipal applications and demonstrated its robustness and stability. The first full-scale industrial applications date back to 2005, while in parallel the technology was further scaled-up for municipal application. Following the first demonstration plants in South Africa and Portugal, a full-scale municipal Nereda® was started up in 2011 at the WWTP of Epe (59,000 PE) followed in 2013 by the WWTP of Garmerwolde (140,000 PE). On both plants significant improvements regarding process stability, effluent quality (e.g. Epe meets TN <5 mg/L, TP <0.3 mg/L) and energy savings (>30 %), compared to traditional activated sludge processes, have been demonstrated. Meanwhile a total of 25 Nereda® plants and 8 process improving units are in operation or under design, with capacities ranging from 15,000 to 950,000 PE..Aerobic Granular Biomass TechnologyAerobic granules were defined at the First Aerobic Granule Workshop 2004, Munich, Germany which stated “Granules making up aerobic granular activated sludge are to be understood as aggregates of microbial origin, which do not coagulate under reduced hydrodynamic shear, and which subsequently settle significantly faster than activated sludge flocs.” Starting with activated sludge, aerobic granular sludge can be formed by applying specific process conditions such as selectively wasting slow settling biomass and retaining faster settling sludge (de Kreuk et al, 2005). Furthermore, favouring slow growing bacteria such as Poly-phosphate Accumulating Organisms (PAOs) has been shown to enhance granulation (de Kreuk et al, 2006). Aerobic granular sludge consists of bio-granules, without carrier material, of sizes typically larger than 0.2 mm. The granular biomass can be used to biologically treat wastewater using similar processes to activated sludge system, however the granular sludge has a distinct advantage of faster settling velocities when compared to activated sludge, which allows for higher reactor biomass concentrations (e.g. 8-15 g/l) (de Kreuk et al, 2007). SVI5 of aerobic granules being comparable to SVI30 of activated sludge. Figure 1 illustrates the settling properties of aerobic granular sludge compared to activated sludge after 5 minutes of settling. Furthermore the particles formed provide a structured matrix for biomass growth, containing spheres with anaerobic, aerobic and anoxic conditions (de Kreuk et al, 2007) which are populated by different microorganisms including nitrifiers, denitrifiers, and Glycogen Accumulating Organisms (GAOs) along with PAOs. This allows for a simultaneous execution of the processes required for nutrient removal, and provides the foundation for a process that is both simple and requires minimal space..Figure 2 shows a pictorial representation of the distribution of biological organisms within aerobic granules compared to activated sludge. As we see, compared to normal activated sludge there are much more nitrifiers, PAOs and GAOs. This explains the superior BNR performance noticed in Aerobic Granular Reactor (AGR). Also this picture illustrates why it is experienced that granular biomass is more stable and less sensitive towards toxicity and fluctuations. The bacteria/water surface is much smaller and toxicity will mainly effect the bacteria at the outer shell. The higher bacteria population makes them to recover quickly. More importantly, especially the GAO and PAO produce Excellular Polymeric Substances (EPS) or biopolymers that actual form the backbone of the granule and are the house for the microorganisms. This backbone is chemically very stable. By the way, recently research has started how in future to recover the biopolymer as valuable byproduct.When aerated, an oxygen gradient forms within aerobic granules whereby the outer layers are aerobic and the inner core is anoxic or anaerobic (de Kreuk et al, 2007). Nitrifiers and heterotrophic bacteria proliferate in the aerobic outer layer of the granules, enabling the degradation of organics (COD removal) and nitrification (conversion of ammonia to nitrite/nitrate) respectively (de Kreuk et al, 2007). A simultaneous nitrification-denitrification process occurs whereby the formed nitrates (from nitrification) are denitrified (conversion of nitrate to nitrogen gas) in the anoxic core of the granules (Pronk et al, 2015). PAOs in the aerobic granules enable enhanced biological phosphorus removal whereby phosphate uptake occurs during aeration and phosphate rich waste sludge is subsequently removed from the system (de Kreuk et al, 2005). Aerobic granular sludge can therefore achieve biological nutrient removal in a single tank without the need for separate anaerobic and anoxic compartments or tanks. Comparatively, activated sludge systems capable of biological nitrogen and phosphorus removal require at least 3 tanks or zones (anaerobic, anoxic and anaerobic) and multiple recycles between the zones or tanks (Wentzel et al, 2008).Nereda® systems are preceded by conventional pre-treatment consisting of screening, grit removal and, depending on the application, FOG (fats, oils and greases) removal; whilst primary sedimentation is optional. Typical reactor depths range from 5.5 to 9 m, with lower and deeper depths possible; whilst secondary settling tanks and major sludge recycles are not required for the Nereda® system similar to that of Sequencing Batch Reactor (SBR). .The ProcessThe Nereda® process uses an optimized SBR cycle in which the 4 steps of a typical SBR cycle are reduced into 3 steps (Figure 4) and can be called an Advanced SBR:1. Simultaneous fill/draw: During this stage the wastewater is pumped into the reactor and at the same time the effluent is drawn.2. Aeration: During the aeration phase, biological conversion take place. The outer layer of the granules are aerobic and it is here where nitrifying bacteria accumulate. This forms nitrate that is then denitrified in the anoxic core of the granules. In the final step phosphorous uptake occur3. Sedimentation: Following the biological processes, a sedimentation phase separates the clear effluent from the sludge. The time for phase separation is short due to the excellent settling properties of the sludge. The system is then ready for a new cycle..The key advantages of Nereda® are summarized as follows:Cost-effective• Compact and uncomplicated tank design• Less mechanical equipment• No separate clarifiers neededEasy to operate• Robust and reliable process performance• Fully automated plant operation possibleSustainable• High effluent purity and efficient nutrient removal• No or minimal use of chemicals• Significantly lower energy consumption.Controlling the ProcessTo meet the effluent demands and energy efficiency requirements of the WWTP, optimisation of the Nereda® process can be controlled by online process analysers measuring ammonium, ortho-phosphate, oxygen, and the oxidation reduction potential (ORP). For less stringent effluent requirements, typically the main process control parameters are oxygen and ORP. Like for all advanced controls it is desirable that the measurement values are highly reliable. At the Epe wastewater treatment plant reliability of the ammonium, phosphate and nitrate measurements is ensured by a predictive diagnostic system called Prognosys that monitors and interprets the instrument’s internal signals to inform the user of the instrument condition. The reading is expressed as a percentage value and is designed to inform operators about upcoming maintenance needs before measurements become questionable and might affect the process.Ammonium and phosphate are measured using the outdoor versions of the Amtax sc (NH4+) or Phosphax sc (PO43) analyser respectively. These analysers do not measure directly in the process medium, but the sample for analysis is pumped from the Nereda® reactor, pre-filtered (<0.45 microns) in a selfcleaning module and transported to the analyser. Both analysers have an analysis time of approximately 5 minutes. Oxygen (LDO sc) and pH/ORP (pHDS sc) sensors can be directly placed in the medium thereby delivering measurement values in real time. .In figure 5 the different measurement signals of sensors and analyzers during the aeration phase are shown as a trend line. It can be seen that during the aeration cycle, the oxygen concentration is kept constant and there is a decrease of the ammonium and the orthophosphate concentration. The ORP signal increases in accordance with the increasing ratio of oxidised to reduced species. At Epe and Garmerwolde, a significant parameter for process control in the aeration phase of the Nereda® process, is the NH4+ concentration value delivered by the Amtax sc analyser. The reliability of the NH4+ and other measurement values is constantly monitored by Prognosys and classified in percentage values as the so-called measurement indicator. In case the value of the measurement indicator starts decreasing from 100% there is still enough time to take action before results get questionable. If the value should drop below 50%, an alternative strategy to control the aeration is activated using the mV value delivered by the ORP sensor as backup signal..Data Transfer & CommunicationAll measurement signals from one reactor are captured by a single SC1000 controller. TCP/IP is used for the communication between the controller and the AquaSuite® Nereda® PLC. Controller and attached instruments can be remotely monitored via the network, i.e. measurement values as well as the status of the instruments provided by Prognosys can be retrieved and maintenance steps like a calibration can be remotely started.Results from Epe WWTP, The NetherlandsEpe WWTP is a full scale Nereda® plant which was designed and constructed by Royal HaskoningDHV in 2010-2011 and has been operational since September 2011. The plant consists of the following main processes; inlet works with screens and grit removal, followed by three Nereda® reactors and effluent polishing via gravity sand filters. The Nereda® reactors are designed to take average daily flows of 8,000 m3/day and a peak flow of 36,000 m3/d. The waste sludge is thickened via a gravity belt thickener and transported off-site. The performance of the plant is outlined in table 1. One key advantage of Nereda® is reduced power consumption. At Epe, the original plant energy consumption was approximately 3,500 kWh/d. With Nereda®, the average daily consumption is now 2,000 kWh – 2,500 kWh. This is approximately 35% less than all types of similar sized conventional plants in the Netherlands.ConclusionsExisting Nereda® plants demonstrated that the technology is capable of effectively treating wastewater for removal of ammonia, total nitrogen and phosphorus. The process is effective at removing these parameters to low levels, in line with future effluent consent limits that might be put in place by the EU water framework directive. Notably, the technology is delivering wastewater treatment at a significantly reduced CAPEX (plant size, footprint) and OPEX (energy, chemicals) compared with conventional technologies on the market.
