Abstract

Aerobic granular sludge is seen as the future standard for industrial and municipal wastewater treatment. Through a Dutch research and development program, a full-scale aerobic granular biomass technology has been developed – the Nereda® technology – which has been implemented to treat municipal and industrial wastewater. The Nereda® system is considered to be the first aerobic granular sludge technology applied at full-scale and more than 40 municipal and industrial plants are now in operation or under construction worldwide. Further plants are in the planning and design phase, including plants with capacities exceeding 1 million PE. Data from operational plants confirm the system's advantages with regard to treatment performance, energy-efficiency and cost-effectiveness. In addition, a new possibility for extracting alginate-like exopolysaccharides (ALE) from aerobic granular sludge has emerged which could provide sustainable reuse opportunities. The case is therefore made for a shift away from the ‘activated sludge approach’ towards an ‘aerobic granular approach’, which would assist in addressing the challenges facing the wastewater treatment industry in Asia and beyond.

INTRODUCTION

Aerobic granular sludge has been extensively researched over the last two decades as a part of the search for more sustainable wastewater treatment solutions. Conventional activated sludge (CAS) systems have key disadvantages such as slow settling flocculent biomass necessitating large clarifiers and low reactor biomass concentrations (typically 3–5 kgMLSS/m3), large treatment system footprints and relatively high system energy usage. It has been shown at the lab, pilot and the full scale that aerobic granular sludge has distinct advantages, when compared to CAS systems, including improved settling characteristics, which in turn allows for higher biomass concentrations and hence more compact treatment systems.

A co-ordinated research partnership in the Netherlands led to the development of the Nereda® technology – a full-scale application of aerobic granular sludge. Currently, over 40 full scale Nereda® plants are operational or under design/construction across 5 continents. The operational full-scale plants have met effluent requirements whilst achieving more sustainable wastewater treatment with key advantages outlined below (compared to similarly loaded activated sludge systems):

  • 25–75% reduction in treatment system footprints as a result of higher reactor biomass concentrations and the non-use of secondary settling tanks;

  • 20–50% energy usage reduction and;

  • Associated capital and operational cost savings.

This paper highlights the different Nereda® design configurations which have been developed to meet requirements at different sites across the world. Furthermore, results from several full-scale treatment plants are presented and the potential to extract a high-value reuse product (alginate) from Nereda® excess/waste sludge is discussed.

AEROBIC GRANULAR BIOMASS AND THE NEREDA® TECHNOLOGY

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 & van Loosdrecht 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).

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 anaerobic) and multiple recycles between the zones or tanks (Wentzel et al. 2008).

In the early 2000's, lab-scale research at the Delft University of Technology (TU Delft), showed that aerobic granular sludge could be formed under a variety of conditions and that granular sludge could be used to achieve stable biological COD, phosphorus and nitrogen (de Kreuk et al. 2007). A collaborative public-private partnership was set up involving TU Delft, Royal HaskoningDHV, several Dutch District Water Authorities, STOWA (the Dutch Foundation for Applied Water Research). This partnership led to the development of the Nereda® wastewater treatment system, which is a full scale application of the aerobic granular sludge technology. Following initial pilot-scale research, the first full-scale Nereda® wastewater treatment plant was commissioned in 2006 at a cheese factory in the Netherlands (van der Roest et al. 2011). Subsequently, 18 full-scale Nereda® treatment plants have entered operation. Table 1 provides details of the operational plants as well as the full-scale plants under construction 11 plants) and in the final stages of design (11 plants).

Table 1

List of full scale Nereda® treatment plants in operation, under construction and in the final phases of design

Operational plantsDaily average flow (m3/day)Peak flow (m3/h)Person Equivalent
(Calculated for p.e. a 54 g. BOD)
Start-upGreenfield/Retrofit CAS or SBR/Hybrid
Vika, Ede (NL) 50–250  1,500–5,000 2005 Retrofit 
Cargill, Rotterdam (NL) 700  10,000–30,000 2006 Retrofit 
Smilde, Oosterwolde (NL) 500  5,000 2009 Retrofit 
STP Gansbaai (RSA) 5,000 400 63,000 2009 Greenfield 
STP Epe (NL) 8,000 1,500 41,000 2011 Greenfield 
STP Garmerwolde (NL) 30,000 4,200 140,000 2013 Greenfield 
STP Vroomshoop (NL) 1,500 200 12,000 2013 Greenfield 
STP Dinxperlo (NL) 3,100 570 11,000 2013 Greenfield 
STP Wemmershoek (RSA) 5,000 468 39,000 2013 Greenfield 
STP Frielas, Lisbon (PT) 12,000 1,850 44,000 2015 Retrofit 
STP Ryki (PL) 5,320 465 38,600 2015 Greenfield 
Westfort Meatproducts, IJsselstein (NL) 1,250 330 43,000 2015 Greenfield 
STP Clonakilty (IRL) 4,896 622 23,000 2015 Greenfield 
STP Carrigtwohill (IRL) 6,750 844 41,000 2015 Greenfield 
STP Deodoro, Rio de Janeiro (BR) Phase I - 64,800
Phase II - 86,400 
4,590
6,120 
360,000
480,000 
2016
2025 
Greenfield 
STP Kingaroy (AUS) 2,625 450 11,000 2016 Greenfield 
STP Simpelveld (NL) 3,668 945 10,000 2016 Greenfield 
STP Cork Lower Harbour (IRL) 18,280 1,830 72,000 2017 Greenfield 
Plants under construction
 STP Highworth (UK) 1,719 197 10,000 2017 Greenfield 
 STP Jardim Novo, Rio Claro (BR) 24,166 1,806 152,000 2018 Greenfield 
 STP Hartebeestfontein (RSA) 5,000 208 52,000 2018 Greenfield 
 STP Alpnach (CH) 14,000 1,872 48,000 2018 Greenfield 
 STP Zutphen (NL) 10,128 550 237,000 2018 Greenfield 
 STP Utrecht (NL) 55,000 13,200 343,000 2018 Greenfield 
 STP Inverurie (UK) 10,871 544 47,204 2018 Retrofit 
 STP Kendal (UK) 26,000 1,749 103,000 2019 Greenfield 
 STP Österröd, Strömstad (S) 3,730 360 13,000 2019 Greenfield 
 STP Faro – Olhão (PT) 20,582 1,908 149,000 2019 Greenfield 
 STP Ringsend, Dublin (IRL) 600,000 50,000 2,670,000 2021 Retrofit 
Plants under design
 STP Morecambe (UK) 17,000 2,088 33,000 2018 Greenfield 
 STP Tatu, Limeira (BR) 57,024 3,492 322,000 2019 Greenfield 
 STP Tijuco Preto, Sumaré (BR) 19.900 1.492 110.000 2019 Greenfield 
 STP Breskens (NL) 3,500 1,000 31,300 2019 Greenfield 
 STP Jardim São Paulo, Recife (BR) Phase I – 22,792
Phase II – 67,764 
1,871
5,577 
109,000
325,000 
2019
2025 
Greenfield 
 STP São Lourenço, Recife (BR) Phase I – 18,842
Phase II - 25,123 
1,287
1,715 
105,000
140,000 
2020
2024 
Greenfield 
 STP Jaboatão, Recife (BR) Phase I - 109,683
Phase II - 154,483 
8,536
12,037 
609,000
858,000 
2020
2025 
Greenfield 
 STP Kloten (CH) 26,000 2,850 125,000 2023 Retrofit 
 STP Barston (UK) 21,784 1,424 86,000 Tbd Greenfield 
 STP Walsall Wood (UK) 7,176 646 29,166 Tbd Greenfield 
 STP Radcliffe (UK) 5,324 463 24,722 Tbd Greenfield 
Operational plantsDaily average flow (m3/day)Peak flow (m3/h)Person Equivalent
(Calculated for p.e. a 54 g. BOD)
Start-upGreenfield/Retrofit CAS or SBR/Hybrid
Vika, Ede (NL) 50–250  1,500–5,000 2005 Retrofit 
Cargill, Rotterdam (NL) 700  10,000–30,000 2006 Retrofit 
Smilde, Oosterwolde (NL) 500  5,000 2009 Retrofit 
STP Gansbaai (RSA) 5,000 400 63,000 2009 Greenfield 
STP Epe (NL) 8,000 1,500 41,000 2011 Greenfield 
STP Garmerwolde (NL) 30,000 4,200 140,000 2013 Greenfield 
STP Vroomshoop (NL) 1,500 200 12,000 2013 Greenfield 
STP Dinxperlo (NL) 3,100 570 11,000 2013 Greenfield 
STP Wemmershoek (RSA) 5,000 468 39,000 2013 Greenfield 
STP Frielas, Lisbon (PT) 12,000 1,850 44,000 2015 Retrofit 
STP Ryki (PL) 5,320 465 38,600 2015 Greenfield 
Westfort Meatproducts, IJsselstein (NL) 1,250 330 43,000 2015 Greenfield 
STP Clonakilty (IRL) 4,896 622 23,000 2015 Greenfield 
STP Carrigtwohill (IRL) 6,750 844 41,000 2015 Greenfield 
STP Deodoro, Rio de Janeiro (BR) Phase I - 64,800
Phase II - 86,400 
4,590
6,120 
360,000
480,000 
2016
2025 
Greenfield 
STP Kingaroy (AUS) 2,625 450 11,000 2016 Greenfield 
STP Simpelveld (NL) 3,668 945 10,000 2016 Greenfield 
STP Cork Lower Harbour (IRL) 18,280 1,830 72,000 2017 Greenfield 
Plants under construction
 STP Highworth (UK) 1,719 197 10,000 2017 Greenfield 
 STP Jardim Novo, Rio Claro (BR) 24,166 1,806 152,000 2018 Greenfield 
 STP Hartebeestfontein (RSA) 5,000 208 52,000 2018 Greenfield 
 STP Alpnach (CH) 14,000 1,872 48,000 2018 Greenfield 
 STP Zutphen (NL) 10,128 550 237,000 2018 Greenfield 
 STP Utrecht (NL) 55,000 13,200 343,000 2018 Greenfield 
 STP Inverurie (UK) 10,871 544 47,204 2018 Retrofit 
 STP Kendal (UK) 26,000 1,749 103,000 2019 Greenfield 
 STP Österröd, Strömstad (S) 3,730 360 13,000 2019 Greenfield 
 STP Faro – Olhão (PT) 20,582 1,908 149,000 2019 Greenfield 
 STP Ringsend, Dublin (IRL) 600,000 50,000 2,670,000 2021 Retrofit 
Plants under design
 STP Morecambe (UK) 17,000 2,088 33,000 2018 Greenfield 
 STP Tatu, Limeira (BR) 57,024 3,492 322,000 2019 Greenfield 
 STP Tijuco Preto, Sumaré (BR) 19.900 1.492 110.000 2019 Greenfield 
 STP Breskens (NL) 3,500 1,000 31,300 2019 Greenfield 
 STP Jardim São Paulo, Recife (BR) Phase I – 22,792
Phase II – 67,764 
1,871
5,577 
109,000
325,000 
2019
2025 
Greenfield 
 STP São Lourenço, Recife (BR) Phase I – 18,842
Phase II - 25,123 
1,287
1,715 
105,000
140,000 
2020
2024 
Greenfield 
 STP Jaboatão, Recife (BR) Phase I - 109,683
Phase II - 154,483 
8,536
12,037 
609,000
858,000 
2020
2025 
Greenfield 
 STP Kloten (CH) 26,000 2,850 125,000 2023 Retrofit 
 STP Barston (UK) 21,784 1,424 86,000 Tbd Greenfield 
 STP Walsall Wood (UK) 7,176 646 29,166 Tbd Greenfield 
 STP Radcliffe (UK) 5,324 463 24,722 Tbd Greenfield 

Nereda® operates a cyclical process with three cycle components or stages: simultaneous influent fill and effluent withdrawal; aeration/reaction and settling – all of which occur in a single reactor without partitions (Giesen et al. 2013). Granulation can be achieved via an incremental start-up process using activated sludge for seeding or alternatively granular seed sludge from other Nereda® plants can be used. The enhanced sludge settleability of aerobic granular sludge is evident from a comparison of typical full scale SVI (sludge volume index) values – for aerobic granular sludge the SVI5 (5 minutes) tends towards the SVI30 (30 minutes), with typical values at operational Nereda® plants around 30–60 ml/g (Giesen et al. 2013), whereas for activated sludge the SVI30 is typically in the range of 110–160 ml/g and the SVI5 is not measured because activated sludge exhibits minimal settling after 5 minutes (Tchobanoglous et al. 2004).

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.

RESULTS FROM NEREDA® TREATMENT PLANTS

New insights have emerged since implementing the first full-scale Nereda® installations allowing for further innovation, system development and design optimisation. Several system configurations have been developed to suit a variety of scenarios experienced from site to site. Two ‘greenfield’ or parallel extension approaches have been used, whilst two ‘brownfield’ approaches have also been developed – these configurations are detailed in Table 2 below. For ‘brown field’ Nereda applications, it is often possible to reuse existing infrastructure and implement a significant increase in biological treatment capacity against low investments. Examples of such applications in Table 1 are the retrofit of the existing SBR's of Cargill's wastewater treatment facility in Rotterdam (The Netherlands) and Irish Water's Ringsend STP. The Nereda® at Lisbon's Frielas STP is an example where conventional continuous activated sludge tanks were retrofitted.

Table 2

Nereda® configurations

Nereda® ConfigurationTypical LayoutConfiguration characteristicAdvantagesReference examplesPotential Applications
Continuous feed, 3 + reactors 3 reactors  At least 1 reactor in feed phase at any given time Scalable for application to large (>100 ml/d) and mega (>500 ml/d) treatment plants Epe STP (Netherlands)
 
‘Greenfield sites’; or extension to existing plants with parallel Nereda® system 
Influent buffer followed by X reactors 1 buffer +2 reactors  Buffer stores influent between feeds to reactors Optimised investments (2 versus 3 reactors) Wemmershoek STP (South Africa)
 
‘Greenfield sites’; or extension to existing plants with parallel Nereda® system 
Hybrid 1 or more Nereda® reactors with excess sludge connection to activated sludge system  Waste Nereda® sludge to activated sludge system Enhance activated sludge system performance; Optimal use of existing infrastructure Vroomshoop STP (Netherlands)
 
‘Brownfield sites’;
Extension/optimisation scenarios, utilising existing infrastructure 
Retrofit Convert existing continuous activated sludge reactor, SBR or any suitable tank  Use existing tanks or CAS reactors Cost-effective capacity and performance enhancement using existing infrastructure Frielas STP (Portugal)
 
‘Brownfield sites’; Limited space or budget but require enhanced capacity and/or performance 
Nereda® ConfigurationTypical LayoutConfiguration characteristicAdvantagesReference examplesPotential Applications
Continuous feed, 3 + reactors 3 reactors  At least 1 reactor in feed phase at any given time Scalable for application to large (>100 ml/d) and mega (>500 ml/d) treatment plants Epe STP (Netherlands)
 
‘Greenfield sites’; or extension to existing plants with parallel Nereda® system 
Influent buffer followed by X reactors 1 buffer +2 reactors  Buffer stores influent between feeds to reactors Optimised investments (2 versus 3 reactors) Wemmershoek STP (South Africa)
 
‘Greenfield sites’; or extension to existing plants with parallel Nereda® system 
Hybrid 1 or more Nereda® reactors with excess sludge connection to activated sludge system  Waste Nereda® sludge to activated sludge system Enhance activated sludge system performance; Optimal use of existing infrastructure Vroomshoop STP (Netherlands)
 
‘Brownfield sites’;
Extension/optimisation scenarios, utilising existing infrastructure 
Retrofit Convert existing continuous activated sludge reactor, SBR or any suitable tank  Use existing tanks or CAS reactors Cost-effective capacity and performance enhancement using existing infrastructure Frielas STP (Portugal)
 
‘Brownfield sites’; Limited space or budget but require enhanced capacity and/or performance 

Detailed treatment performance of various industrial and municipal Nereda plants has been reported before (e.g. Giesen et al. 2013; Pronk et al. 2015) and below operation results of Ryki STP, Prototype Utrecht and hybrid Vroomshoop will be presented.

Ryki STP – Poland

In the city of Ryki (Lublin Province, Poland) a new Nereda® wastewater treatment plants entered operation in February 2015. This is the first Nereda® installation located in the eastern part of Central Europe and also the first Nereda® plant that has to contend with low process temperatures during the winter period. The Ryki Nereda® plant is designed to treat 5,320 m3/d (dry weather), corresponding to 38,600 PE. In addition to the challenging winter temperatures, the plant has to treat a range of different incoming sewages (domestic, septic tanks and industrial) and has to handle extended industrial peak load periods. The combined pre-treated influent is fed to an influent buffer tank (500 m3) from where two Nereda® reactors (2,500 m3 each) are separately fed by three submersible pumps (‘1 buffer +2 reactors configuration’). Biological treated wastewater is discharged to surface water via an existing pond. Table 3 shows the design loads for the plant, Figure 1 the wastewater temperatures experienced at the plant and lastly Table 4 shows the effluent performance compared to the effluent requirements.

Table 3

Design loads for the Ryki Nereda® plant

ParameterDesign values
DomesticSeptic tankersIndustrialTotal
Daily dry weather flow (m3/d) 2,400 120 2,800 5,320 
Daily wet weather flow (m3/d) 3,418 120 2,800 6,338 
COD (kg/d) 1,680 384 2,500 4,564 
BOD5 (kg/d) 960 156 1,200 2,316 
TSS (kg/d) 1,200 144 400 1,744 
Total N (kg/d) 192 22 112 326 
Total P (kg/d) 48 28 80 
ParameterDesign values
DomesticSeptic tankersIndustrialTotal
Daily dry weather flow (m3/d) 2,400 120 2,800 5,320 
Daily wet weather flow (m3/d) 3,418 120 2,800 6,338 
COD (kg/d) 1,680 384 2,500 4,564 
BOD5 (kg/d) 960 156 1,200 2,316 
TSS (kg/d) 1,200 144 400 1,744 
Total N (kg/d) 192 22 112 326 
Total P (kg/d) 48 28 80 
Table 4

Effluent performance at the Ryki Nereda® plant

ParameterEffluent requirementsEffluent quality (average from April 2015 to February 2016)
Reactor 1Reactor 2Pond Outlet
COD (mg/l) 125 43 46 39 
BOD5 (mg/l) 15 5.5 6.3 4.4 
TSS (mg/l) 35 13 13 4.5 
Total N (mg/l) 15 5.7 5.5 5.0 
Total P (mg/l) 0.9 0.8 0.8 
ParameterEffluent requirementsEffluent quality (average from April 2015 to February 2016)
Reactor 1Reactor 2Pond Outlet
COD (mg/l) 125 43 46 39 
BOD5 (mg/l) 15 5.5 6.3 4.4 
TSS (mg/l) 35 13 13 4.5 
Total N (mg/l) 15 5.7 5.5 5.0 
Total P (mg/l) 0.9 0.8 0.8 
Figure 1

Temperatures at the Ryki WWTP.

Figure 1

Temperatures at the Ryki WWTP.

The Nereda® installation at Ryki has been operational for more than two years and continues to achieve effluent compliance, despite the low winter temperatures and highly variable seasonal loading.

Vroomshoop STP – the Netherlands

A hybrid Nereda® configuration was selected for the upgrade of the Vroomshoop STP (the Netherlands) and the new plant entered operation in 2013. The main feature of the hybrid configuration (see Figure 2) is that the Nereda® waste sludge is fed into a parallel activated sludge system. The plant is designed with a dry weather hydraulic capacity of 156 m3/h and rain flow of 1,000 m3/h, whilst the design pollution load is 22,600 PE (population equivalents at 150 gTOD/PE).

Figure 2

Schematic depiction of the Vroomshoop STP.

Figure 2

Schematic depiction of the Vroomshoop STP.

The discharge of the Nereda® waste or excess sludge into the activated sludge system has been found to significantly improve the sludge settleability of the activated sludge. Figure 3 shows how the SVI in the activated sludge system steadily decreased as a result of the addition of the Nereda® waste sludge, indicating improved sludge settleability.

Figure 3

Comparison of SVIs of the Nereda® and activated sludge systems at Vroomshoop STP (data from end-user: Waterschap Vechtstromen).

Figure 3

Comparison of SVIs of the Nereda® and activated sludge systems at Vroomshoop STP (data from end-user: Waterschap Vechtstromen).

Improved settleability in an activated sludge system could allow for an increase in MLSS (mixed liquor suspended solids) concentrations in the activated sludge system and therefore increase the biological treatment capacity and/or; the possibility to allow higher hydraulic loading on the secondary settling tanks since the sludge settling rates are improved. Another potential advantage of this hybrid configuration is an improvement in biological phosphorus removal in the activated sludge system, since Nereda® waste sludge contains higher concentrations of PAOs when compared to activated sludge.

Between June and November 2014, energy usage monitoring at the Vroomshoop STP showed that the Nereda® side of the plant used on average 35% less energy than the activated sludge side. Furthermore, effluent performance monitoring in 2014 showed the compliance of the plant under full loading conditions (see Table 5).

Table 5

2014 Effluent performance at the Vroomshoop WWTP (data from end-user: Waterschap Vechtstromen)

ParametersAverage Influent (mg/l)Average Effluent (mg/l)Requirement (mg/l)Regulatory Compliance Criteria
Organics COD 720 55 125 Limit (3× per year up to 250) 
BOD5 263 10 Limit (3× per year up to 20) 
Nitrogen TN – 7.2 10 Yearly Average 
TKN 66 5.2 – – 
NH4-N – Summer = 1.4; Winter = 3.0 Summer = 2
Winter = 4 
Average (1 May - 1 Nov.)
Average (1 Nov. - 1 May) 
NO2/NO3-N – 2.0 – – 
Phosphorus TP 8.9 0.9 Moving average of 10 successive samples 
PO4-P – 0.6 – – 
Suspended Solids TSS 317 10 30 Limit 
ParametersAverage Influent (mg/l)Average Effluent (mg/l)Requirement (mg/l)Regulatory Compliance Criteria
Organics COD 720 55 125 Limit (3× per year up to 250) 
BOD5 263 10 Limit (3× per year up to 20) 
Nitrogen TN – 7.2 10 Yearly Average 
TKN 66 5.2 – – 
NH4-N – Summer = 1.4; Winter = 3.0 Summer = 2
Winter = 4 
Average (1 May - 1 Nov.)
Average (1 Nov. - 1 May) 
NO2/NO3-N – 2.0 – – 
Phosphorus TP 8.9 0.9 Moving average of 10 successive samples 
PO4-P – 0.6 – – 
Suspended Solids TSS 317 10 30 Limit 

Prototype Nereda® Utrecht (PNU)

In 2013 a project specific Nereda® prototype (PNU) was installed at the existing Utrecht STP in order to investigate the potential of utilising Nereda® for the replacement of the existing 430,000 PE plant which is aging and utilises the non-optimal AB type activated sludge process. The prototype consist of a single 1,000 m3 reactor which is designed to treat an average flow of 1,500 m3/day (9,000 PE), however the plant can be fed up to 600 m3/hr for test purposes. After successful demonstration and optimization of the design parameters for the Utrecht STP specific conditions, the PNU is operated by Royal HaskoningDHV as test and training facility. Whereas testing full-scale plant performance beyond the plant design conditions is often not possible because at operational plants effluent quality is a priority and the plant receives influent defined by the incoming sewer system, at the PNU facility it is possible for test purposes to operate well beyond the normal conditions. PNU is also used to validate usability and reliability of instrumentation and equipment design optimizations.

Treated wastewater is decanted from Nereda® using a fixed overflow weir, similar to a conventional clarifier. In the design of the first municipal Nereda® plants, it was decided to discharge any particles that might lead to scum with the treated effluent as the obtained water quality fully meet the discharge requirements. To investigate the achievable effluent quality when – like in many clarifiers – scum forming particules are kept in the reactor, baffles were added to the PNU effluent launders in 2015. Figure 4 shows how the effluent suspended solids were reduced to below 10 mgTSS/l. Based on these results the optional use of scum baffles has been introduced in various full-scale designs where stringent requirements apply for suspended solids or total-P.

Figure 4

Effluent suspended solids performance at the PNU facility with baffles (no primary clarification).

Figure 4

Effluent suspended solids performance at the PNU facility with baffles (no primary clarification).

FURTHER DEVLOPEMENTS – ALE RECOVERY

Research at TU Delft uncovered the ability to extract alginate-like exopolysaccharides (ALE) from aerobic granular sludge (Lin et al. 2010). Alginate is currently produced from seaweed at relatively high costs and is used in a variety of industries as a thickener or gel and as a basis for coatings. Aerobic granular sludge has been found to contain between 20 to 30% of ALE. Extracted ALE could potentially be used in the chemical sector, as a soil enhancer in agriculture or as a brick additive (van der Roest et al. 2015). The recovery of ALE from Nereda® excess sludge (aerobic granular sludge) is a potential re-use opportunity, whereby a waste stream could be converted into a product with a high resale value. Combining ALE extraction with the existing excess sludge treatment processes at wastewater treatment plants could also improve sludge treatment efficiency because ALE extraction reduces sludge volumes and the remaining (non-extracted) sludge has a higher digestibility and an improved dewaterability. The National Alginate Research Programme (NAOP) has been set up in the Netherlands to further research and develop this promising sustainable re-use concept. The NAOP is a public-private sector collaborative research initiative with the goal of developing sustainable and commercially viable ALE-extraction from Nereda® excess sludge (van der Roest et al. 2015). The NAOP is similar to the public-private collaborative partnership that successfully developed Nereda®. During the summer of 2017 a pilot study was carried out and based on the results two demo installations will be designed and realized in 2019.

DISCUSSION AND CONCLUSIONS

Results from full-scale Nereda® treatment plants over the last decade have shown that Nereda® has numerous advantages when compared to similarly loaded activated sludge systems, including:

  • 25–75% reduction in treatment system footprints as a result of higher reactor biomass concentrations and the non-use of secondary settling tanks;

  • 20–50% energy usage reduction and;

  • Associated capital and operational cost savings.

Nereda® treatment plants have been shown to achieve similar or improved enhanced biological nutrient (nitrogen and phosphorus) removal when compared to similarly loaded activated sludge systems. Furthermore, the possibility to recover ALE from Nereda® waste sludge has the potential to generate a reuse product with high commercial value.

Four main Nereda® configurations have been developed for a wide range wastewater treatment scenarios ranging from ‘green-field’ systems to retrofits at ‘brown-field’ sites. The hybrid configuration (e.g. Vroomshoop STP) whereby Nereda® waste sludge is fed into a parallel activated sludge system has the potential to increase the loading capacity of the activated sludge system through improved sludge settleability. This configuration could therefore be applied advantageously for the extension of existing plants with an activated sludge line.

The results achieved at full-scale Nereda® treatment plants show that aerobic granular sludge has clear and significant advantages over CAS systems. Currently sustainability requirements (including cost-effectiveness) are driving technological advancement and innovation. The advantages of Nereda® in comparison to activated sludge systems ultimately translate into more sustainable and cost-effective wastewater treatment. A shift away from the ‘activated sludge approach’ towards an ‘aerobic granular approach’ would assist in addressing the challenges facing the wastewater treatment industry in Asia and beyond.

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