Innovative wastewater treatment using aerobic granular sludge The Nereda technology

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1 Innovative wastewater treatment using aerobic granular sludge The Nereda technology Helle van der Roest, Andreas Giesen, André van Bentem and Struan Robertson Royal HaskoningDHV, Laan 1914 no. 35, 3818EX Amersfoort, The Netherlands Correspondence to: or 1 INTRODUCTION Since the development of Biological Nutrient Removal (BNR) systems using activated sludge, significant research has focused on improved separation techniques for activated sludge, either by improving sludge settling properties or by using physical separation techniques (i.e. Membrane Bioreactors). Fundamentally, improved settleability can be achieved using compact and larger particles with a higher specific density. This search for treatment solutions using larger, denser particles became the foundation for the research and development of aerobic granules by Delft University of Technology (DUT) under the guidance of Professor Mark van Loosdrecht. Based on the successful and pioneering fundamental aerobic granular sludge research at DUT, the aerobic granular sludge process has been engineered for full-scale commercial applications by Royal HaskoningDHV and has been commercially branded as the Nereda technology. Aerobic granular biomass has several key advantages over conventional activated sludge flocs that have been well-documented in literature over the last decade. These advantages include a good sludge settling ability that leads to better biomass retention and higher biomass concentrations, provision of a structured matrix for biomass growth, and an ability to withstand high load variations. The good settling ability of the granular biomass in a Nereda system has been translated into compact reactor designs that reduce plant footprints significantly and provide a sustainable treatment system that uses substantially lower amounts of energy and chemicals. The Nereda system therefore provides significant cost savings when compared to conventional activated sludge systems. In this paper the Nereda technology is discussed and the results from 4 full-scale installations with differing configurations are presented. The Nereda system performance is compared to parallel or earlier activated sludge systems with the same influent characteristics these comparisons prove the sustainability credentials of the innovative Nereda technology, with results exceeding the predictions made by aerobic granular sludge researchers over the last decade.

2 - 2-2 THE NEREDA TECHNOLOGY 2.1 Nereda development The research and development of aerobic granules commenced at DUT in Almost ten years later aerobic granular sludge was discovered and stable laboratory scale granulation was achieved. This lab-scale success was followed by the first pilot scale research at Ede WWTP and several pilot plants have been in operation for both industrial and municipal influent since 2003, both in and outside the Netherlands. The first industrial full scale Nereda prototype was implemented in 2005 by retrofitting an existing tank. In 2007 DUT, STW (Dutch Foundation for Applied Science), STOWA (Dutch Foundation for Applied Water Research), Royal HaskoningDHV and six Dutch Waterboards joined forces in the Dutch National Nereda Development Program (NNOP) to scale-up and implement aerobic granular biomass technology for municipal applications. In parallel two demonstration Nereda plants (Gansbaai, South Africa and Frielas, Portugal) were implemented and proved to be instrumental in the system development. Epe WWTP (a full-scale municipal plant in the Netherlands) was designed and built in 2010/2011 and has been in operation since After the successful design, construction and operation of Epe, many new Nereda installations with capacities up to 140,000 P.E. have been commissioned and the number of international applications in the pipeline is growing rapidly. The Nereda tank size at Gamerwolde WWTP (a municipal WWTP in the Netherlands commencing operation in 2013) is close to the largest SBR tank-size in the world, which shows that the technology has fully matured with regards to scale-up and is now ready for widespread implementation worldwide. 2.2 Aerobic Granular Biomass Aerobic granules were defined at the First Aerobic Granule Workshop 2004 in Munich, Germany as follows: 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. The main features defining aerobic granules are: a minimum particle diameter of 0.2 mm and; a SVI5 approaching SVI30 (i.e. rapid settling). Figure 1 illustrates the settling properties of aerobic granular sludge compared to activated sludge after 5 minutes of settling.

3 - 3 - Figure 1 - Settling properties aerobic granular sludge (left) compared to activated sludge (right) The Nereda technology utilises design and control mechanisms to encourage biomass to form granules rather than activated sludge flocs. The agglomerates formed allow simultaneous anaerobic, aerobic and anoxic conditions to exist throughout the granules and hence, reduces the need for multiple tanks and recirculation. 2.3 Nereda process The Nereda process operates intermittently, with the fill and decant phase occurring simultaneously. Due to the good settleability of the aerobic granules, the process does not require mechanical decanting features to ensure low solids in the effluent. As a consequence of the unique characteristics of granules, the Nereda technology uses an optimised SBR-cycle, in which the 4 steps of a typical SBR cycle are reduced into 3 steps (see Figure 2): Simultaneous fill / draw: during this cycle step the wastewater is pumped into the reactor and at the same time effluent is withdrawn. Aeration: during the aeration phase the biological conversion processes take place. The outer layer of the granules is aerobic and here nitrifying bacteria accumulate. The formed nitrate is denitrified in the anoxic core of the granules. And last but not least, phosphorous uptake occurs. Sedimentation: after the biological processes a sedimentation phase is required for separation of clear effluent and granular sludge. This time is (very) short due to the excellent settling properties of the sludge. Then the system is ready for a new cycle.

4 Fill 2. Areation Influent 4. Draw 3. Settling Effluent Figure 2 - SBR cycle (left) and Nereda cycle (right) 3 PROJECT REFERENCES The first full-scale Nereda was launched in 2005 by retrofitting a storage tank into a reactor, treating a waste water flow of 250 m 3 /day at a cheese factory in The Netherlands. The success of this first milestone plant confirmed the potential of this innovative technology. Currently more than 10 full-scale Nereda installations are in operation for the treatment of industrial and municipal wastewater, with a large number of plants in the planning and construction phase. Results from several of the full-scale operational Nereda installations with differing configurations are further discussed below. 3.1 Epe WWTP, The Netherlands The Epe WWTP (Figure 3) was designed and constructed by Royal HaskoningDHV in and has been fully operational since September Prior to design, a pilot trial was carried out for four years and the data collected from this pilot was used to design the full scale plant. The plant consists of the following main processes: inlet works with 3mm screens and grit removal, followed by three Nereda reactors and effluent polishing via gravity sand filters. The Nereda reactors are designed to receive an average daily flow of 8,000 m 3 /day and a peak flow of 36,000 m 3 /d. If necessary metal salt dosing is added during effluent polishing to ensure P-levels meet the stringent effluent requirements. The waste sludge stream is thickened via a gravity belt thickener and transported offsite. The WWTP was designed to meet the stringent effluent criteria presented in Table 1 within a temperature range of 8 to 25 C.

5 - 5 - Figure 3 - Nereda Epe WWTP Table 1 - Epe WWTP Design Criteria Parameter Influent Effluent limits Comments (kg/d) (mg/l) COD 5, BOD 2,230 7 Average. Maximum value 15 mg/l Kj-N TN / 8 Summer and winter average TP / 0.5 Summer and winter average TSS 2, Maximum value Starting in September 2011 the influent to the plant was progressively increased to 100% flow over a period of four months. The previous activated sludge plant continued to process the influent alongside the Nereda reactors whilst granular sludge levels were built-up. It should be noted that part of the granulation period was over the winter months when the average wastewater temperature was below 10 C. The process verification period for Epe WWTP was completed between March - May The average effluent quality achieved during the verification phase is summarised in Table 2.

6 - 6 - Table 2 - Epe WWTP Performance during process verification in March - May 2012 (process temperature C) Parameter Influent Effluent (Average) (mg/l) (mg/l) COD BOD 333 < 2.0 NKj NH4-N N-total - < 4.0 P-total Suspended Solids 341 < 5.0 The stringent effluent requirements for nitrogen and phosphorous were (and continue to be) comfortably met, along with all other effluent criteria. With an actual load of approximately 60-70% of the design load, the granular sludge concentration is kept at a significant lower sludge concentration of approximately 2.5 kg/m 3 compared to the design concentration of 8 kg/m 3. The Epe excess sludge is anaerobically co-digested and dewatered at a WWTP nearby. The dewatered sludge is incinerated at a centralized facility. The dry solids content of the excess sludge is maintained at 4-5% to ensure that it is easily transportable. For this concentration a low polymer dosing rate of g/kg dry solids is sufficient. With higher dosing rates the excess sludge concentration can be thickened to concentrations up to 9%. Another remarkable aspect is that the influent characteristics at Epe WWTP fluctuate extensively and ph shocks occur fairly frequently ph as low as 3 and as high as 10, due to the discharge of a significant portion of industrial wastewater to the system. In the past, after ph shocks, the previous conventional activated sludge system often suffered from a major loss of sludge and consequently reduction of biological treatment capacity lasting several weeks. In comparison, the Nereda system receiving the same influent ph shocks was found to resume smooth operation after a few cycles and was back to normal operation in 1-2 days after the shock occurred this shows the improved system resilience achieved by granular sludge. The energy consumption was reduced significantly once the old activated sludge plant was taken off-line (mid February 2012). The plant s overall energy consumption decreased from 3,500 kwh/d to 2,250 kwh/d. The average daily energy consumption

7 - 7 - achieved by the Nereda system is approximately 35% less than all types of similar sized conventional plants in the Netherlands (taking into account similar effluent requirements). 3.2 Frielas WWTP, Portugal The Frielas WWTP is operated by Simtejo, a wastewater concessionaire responsible for the operation of the main sewers and treatment plants of the Greater Lisbon area. It is a 70,000 m 3 /day plant, currently at 70% of its biological design capacity and receives, mainly, domestic wastewater from 250,000 inhabitants. Regarding effluent quality, the WWTP has carbon removal and disinfection requirements (i.e., COD < 125 mg/l and TSS < 35 mg/l) and no specific discharge limits for nitrogen and phosphorous. Since commissioning, back in 1997, the Frielas WWTP suffered from several operational constraints related to technological decisions made during the design phase, and also because the influent wastewater characteristics changed significantly from those used for the original plant design. To validate if Nereda could improve the plant performance under realistic field conditions, a portion of one of the six continuous activated sludge reactors was retrofitted into a Nereda reactor with a volume of approx. 1,000 m 3 (25% of the volume of one of the 6 existing reactors), which was then run in parallel to the remaining five activated sludge reactors. Besides achieving robust and efficient operation during all influent conditions, another key driver for the retrofit was evaluating the possibility to substantially lower the electricity demand of the existing WWTP. Another important motivation for the implementation of Nereda was the possibility of working at higher hydraulic loads and achieving nutrient removal without the (eventual) future need for increasing reactor volume. For start-up the demo reactor was seeded with normal activated sludge from one of the adjacent parallel conventional reactors. In the Nereda demo reactor, Simtejo reached a steady state SVI30 around 40 ml/g, a SVI5 as low as 60 ml/g, a granule fraction (percentage of TSS larger than 0.2mm) above 80% and an increased biomass concentration in the range of 6 to 8 g/l. It is important to note that the startup was made partially during winter with diluted wastewater (i.e. COD levels below 250 mg/l) contributing to a slower biomass growth and granulation. After more than one year of practical plant operation the results are outstanding; the effluent quality is significantly better and is far more consistent than the quality obtained in the original continuous AS system, due to improved sludge quality (see Figure 4).

8 SVI30 (ml/g) SVI A.S. vs SVI Nereda AT1 AT2 AT3 AT4 AT5 Nereda Figure 4 - Settleability of the biomass from Nereda reactor, compared to activated sludge (AS) from the five biological reactors (AT) at Frielas WWTP A significant decrease in energy consumption was also observed in the Nereda reactor. Since the Nereda reactor at Frielas is operated in parallel with the conventional aeration tanks using the same water depth and utilising the existing and common air supply equipment, the comparison between the two technologies is reliable and representative. During a two month monitoring period the air consumption of both systems was measured with the dissolved oxygen (DO) levels similar and nitrification in Nereda fully suppressed to mimic the biological performance of the AS reactors. It was observed that the average specific consumption for Nereda amounts to 0.35 kwh/kg COD removed, representing approximately 30% electricity savings compared to aeration for the AS system. Combining this with the energy saving that granular sludge brings by not requiring settling tanks, sludge recirculation pumps and postfiltration units, the overall electricity saving potential for the Frielas WWTP was computed to 50% if fully converted to Nereda. Following the positive results obtained with the Nereda demonstration reactor, Simtejo requested Royal HaskoningDHV to design and implement an extension of the demo to a full-scale reactor operated parallel to the existing continuous Activated Sludge treatment reactors. The full-scale reactor has a treatment capacity of 12,000 m 3 /d (40,000 PE) and was brought online on 14 July With this upgrade the Frielas WWTP will be operated as a hybrid Nereda plant, with the granular excess biomass from the Nereda reactor being pumped to the continuous AS lines. By this inoculation process, the sludge characteristics and settling performance of the

9 - 9 - existing AS plant will improve, resulting in a further increase of hydraulic and biological treatment capacity at the plant (see Section 3.4 for further details on the advantages of the hybrid configuration. 3.3 Garmerwolde WWTP For the upgrade of the 375,000 p.e. Garmerwolde WWTP of Waterboard Noorderzijlvest (northern Netherlands), the consortium of contractors GMB / Imtech and their consultant Witteveen+Bos selected the Nereda technology as the most suitable alternative. In 2013, the Nereda extension at Gamerwolde, the largest Nereda system built to date, commenced operation (see Figure 5). Figure 5 Garmerwolde WWTP, The Netherlands (note the compact Nereda system in the foreground and much larger old AB system in the background) Following a retrofit into an AB-system in 2005, the Garmerwolde WWTP was not able to meet nutrient removal targets. The Waterboard decided to use a design-build tender to modify and optimize the plant. Based on the evaluation of more than twenty options, the winning consortium selected Nereda as the best solution to extend the biological nutrient removal capacity. The addition of a 140,000 PE unit in parallel to the existing plant convincingly out competed a full range of different solutions provided by other bidders. The hydraulic capacity of the extension is 20,000 m 3 /day, with a peak flow of 4,200 m 3 /h. Each Nereda tank at Garmerwolde has a volume of 9,500 m 3, which is similar to the world s largest SBR-tanks. Considering that the Nereda installation treats

10 % of the daily influent flow and the existing installation (AB-system) 59%, Figure 5 clearly shows Nereda s advantage in terms of system footprint. Making use of a 4,000 m 3 buffer and two Nereda reactors, the extension performance requirement for nutrient removal is as follows: Total-N of 7 mg/l (yearly average) and; Total-P of 1 mg/l (average of ten successive samples). It should be noted that the Nereda extension has no subsequent filtration step (unlike Epe WWTP). The requirements for the AB system are set at Total-N = 12 mg/l in order to achieve an average effluent concentration for the total plant of Total- N = 10 mg/l. After a year-long monitoring period the Nereda extension was found to fully comply with the stringent effluent requirements. The Garmerwolde WWTP offers the possibility to directly compare the performance of the Nereda and activated sludge technologies. The energy consumption of the Nereda installation including intermediate pumping is significantly lower than the energy consumption of the AB system. Furthermore, the AB system requires chemical dosing, including C-source (denitrification), coagulants (sludge properties) and iron salts (phosphorous removal). Apart from the high chemical costs incurred, the dosing also results in a sludge production more than two times higher than the more sustainable parallel Nereda extension. Consequently the overall operational costs (energy, chemicals, sludge treatment) of the activated sludge plant are significantly higher than the new Nereda extension. 3.4 Vroomshoop, WWTP In the late 2000s, the Dutch Waterboard Regge en Dinkel (now Vechtstromen) was interested in helping with the development of the promising and innovative Nereda technology as a part of their strategic commitment to advancing the wastewater treatment technology field. At the Vroomshoop WWTP an opportunity emerged to use Nereda for an expansion and replacement of the existing treatment plant, which consisted of an ageing oxidation ditch system which would not be able to meet future effluent requirements, especially with regard to nutrient (nitrogen and phosphorus) removal. Several options were considered including: using only conventional activated sludge (CAS), using only Nereda or: a hybrid combination of CAS and Nereda. The latter hybrid option was selected because it proved an effective means to make use of an existing settling tank at the site, an optimal way to deal with the high rain weather to dry weather flow ratio at the plant and most importantly this option provided an opportunity to meet innovation and sustainability targets such as reducing energy consumption. The hybrid arrangement of the treatment plant is schematically shown in Figure 6 below.

11 Figure 6 Flow scheme at Vroomshoop, with full hydraulic routing options The new Vroomshoop WWTP is designed for P.E and to receive m 3 /day of wastewater the plant entered operation in mid The introduction of Nereda excess/waste sludge into the CAS system has proven to be a highly effective way to improve the performance of the CAS system. The settleability of the CAS sludge showed a marked improvement (lower SVI) as can be seen in Figure 7 below. June 2013 start-up Jan Summer 2014 Dec Figure 7 SVI comparison of the Nereda and CAS systems at Vroomshoop

12 The hybrid system at Vroomshoop has resulted in the CAS sludge settleability improving to previously unattainable levels. At the same time, the performance of the plant in terms of effluent quality has also been excellent (see Table 3). Table 3 Vroomshoop WWTP treatment performance (entire plant) in 2014 (data from Waterboard Vechtstromen) Parameter (mg/l) Influent Effluent Requirement COD BOD TN 7,2 10 TKN 66 5,2 - NH 4 -N - 2,2 Summer = 2 Winter = 4 NO x -N - 2,0 - TP 8,9 0,9 2 PO 4 -P - 0,6 TSS Furthermore, energy consumption monitoring at Vroomshoop (June-November 2014) showed the following specific energy use under dry weather conditions: CAS system 0.58 kwh/m 3 Nereda system 0.37 kwh/m 3 The Nereda side of the treatment plant therefore uses approximately 35% less energy than the CAS side. It should be noted that the energy consumption of both the CAS and Nereda systems can be optimised even further. 4 CONCLUSIONS Nereda is an innovative and sustainable technology for wastewater treatment. Over the last decade, an extensive research and development program in the Netherlands has led to the aerobic granular biomass technology being scaled-up and for the first time being implemented at the full-scale for the treatment of municipal and industrial wastewater. Since 2006, the technology has successfully been used in various industrial and municipal full-scale applications around the world, thereby demonstrating system robustness and stability. A number of system configurations have been developed to cover a wide spectrum of treatment scenarios and requirements these configurations and their applicability can be summarised as follows: Multiple Nereda reactors with at least one reactor receiving influent at any given time e.g. implemented at Epe WWTP o Similar approach to classic SBR designs

13 Influent buffer in front of Nereda reactors e.g. implemented at Garmerwolde WWTP, Netherlands (140,000 PE) o Allowing for lower capital costs (tank volume savings and less equipment) Retrofit of existing CAS or SBR reactors to Nereda e.g. implemented at Frielas WWTP, Portugal o Increasing plant capacity, whilst making use of existing infrastructure Expansion of capacity/ effluent improvement for existing CAS systems hybrid integration with new Nereda reactors e.g. at Vroomshoop WWTP, Netherlands ( PE) o Integration of Nereda into CAS systems to expand plant capacity and improve overall plant effluent performance whilst optimally using existing infrastructure The full-scale results presented in this paper show the success of the abovementioned configurations. The full-scale results also prove that the Nereda technology is compact (small system footprint) and requires significantly lower amounts of chemicals and energy i.e. a sustainable and cost-effective solution when compared to conventional activated sludge systems. With full-scale applicability proved for a broad range of scenarios across the world, the next step is widespread implementation as a means to move wastewater treatment towards improved sustainability, in line with environmental and societal demands. References Water Research Morgenroth, E., Sherden, T., Van Loosdrecht, M.C.M., Heijnen, J.J, and Wilderer, P.A., Aerobic granular sludge in a sequencing batch reactor, Volume 31, Issue 12, December 1997, Pages [Dissertation], Delft University of Technology, The Netherlands De Kreuk, M.K., Aerobic Granular Sludge - Scaling-up a new technology, 2006, ISBN Water Science and Technology Kreuk, M.K de., Kishida,N., Van Loosdrecht, M.C.M, (2007) Aerobic granular sludge - state of the art. Vol 55 No 8 pg 75-81, IWA publishing Water Research Bassin, J.P., Pronk, M, Kraan, R., Kleerebezem, R., Van Loosdrecht, M.C.M. (2011) Ammonium adsorption in aerobic granular sludge, activated sludge and anammox granules., Vol 45(16), Water Research Kreuk, M.K. de, N. Kishida, S. Tsuneda, M.C.M. van Loosdrecht (2010). Behavior of polymeric substrates in an aerobic granular sludge system. Vol 44, Water Research Winkler, M.-K.H., J.P. Bassin, R. Kleerebezem, L.M.M. de Bruin, T.P.H. van den Brand, M.C.M. van Loosdrecht (2011). Selective sludge removal in a segregated aerobic granular biomass system as a strategy to control PAO-GAO competition at high temperatures., Vol 45,