Live Fast, Die Young: Optimizing Retention Times in High-Rate

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Live fast, die young: Optimizing retention times in high-rate contact stabilization for maximal recovery of organics from wastewater. Francis A. Meerburg, Nico Boon, Tim Van Winckel, Koen T. G. Pauwels, and Siegfried E Vlaeminck Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01888 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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Environmental Science & Technology

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Live fast, die young: Optimizing retention times in high-rate contact stabilization

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for maximal recovery of organics from wastewater.

3 4 5 6

Francis A. Meerburga, Nico Boona, Tim Van Winckela, Koen T. G. Pauwelsa and

7

Siegfried E. Vlaemincka,b,*

8 9 10 11 12 13 14

a

Center for Microbial Ecology and Technology (CMET), Ghent University, Coupure

Links 653, 9000 Gent, Belgium b

Research group of Sustainable Energy, Air and Water Technology, University of

Antwerp, Groenenborgerlaan 171, 2020 Antwerpen, Belgium *Corresponding author: Phone: +32 3 265 36 89; Fax: +32 3 265 32 25; E-mail: [email protected].

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Abstract

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Wastewater is typically treated by the conventional activated sludge process, which suffers

17

from an inefficient overall energy balance. The high-rate contact stabilization (HiCS) has

18

been proposed as a promising primary treatment technology to maximize redirection of

19

organics to sludge for subsequent energy recovery. It utilizes a feast-famine cycle to select

20

for bioflocculation and/or intracellular storage. We optimized the HiCS process for organics

21

recovery and characterized different biological pathways of organics removal and recovery.

22

Eight HiCS reactors were operated at 15°C at short solids retention times (SRT; 0.24-2.8 d),

23

hydraulic contact times (tc; 8 and 15 min) and stabilization times (ts; 15 and 40 min). At an

24

optimal SRT between 0.5-1.3 d, tc of 15 min and ts of 40 min, the HiCS system oxidized only

25

10% of influent chemical oxygen demand (COD) and recovered up to 55% of incoming

26

organic matter into sludge. Storage played a minor role in the overall COD removal, which

27

was likely dominated by aerobic biomass growth, bioflocculation onto extracellular

28

polymeric substances and settling. The HiCS process recovers enough organics to potentially

29

produce 28 kWh of electricity per population equivalent per year by anaerobic digestion and

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electricity generation. This inspires new possibilities for energy-neutral wastewater treatment.

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TOC/Abstract art

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Keywords 2 ACS Paragon Plus Environment

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Biosorption, Carbon capture and redirection, F/M ratio, High-rate activated sludge (HRAS), Sewage



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1. Introduction

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Conventional activated sludge (CAS) – in various process configurations – is by far the

40

most commonly applied technology to treat municipal wastewaters. Decades of optimization

41

have made it a reliable and efficient technology to remove organic matter and nutrients from

42

wastewater, although centralized CAS treatment suffers from disadvantages such as high

43

operational costs and dependency on continuous external electricity supply. Urged by

44

economic and societal pressure, innovation efforts focus on achieving energy neutrality and

45

sustainable recovery of resources; a term which can be coined as ‘wastewater mining’. A

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typical CAS plant requires an annual energy input in the order of 27 kWh per population

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equivalent (PECOD110),1 which corresponds to up to 1% of the electrical energy consumption

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in developed countries.2 Yet, in the case of domestic sewage, the chemical energy content

49

exceeds the required energy to operate a CAS plant by at least a factor nine.3 Anaerobic

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digestion (AD) of excess sludge is the most viable option to recover energy from wastewater

51

in the form of methane, except in limited cases where mainstream AD of raw wastewater is

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feasible.4 To achieve energy-neutral or even energy-positive wastewater treatment, it is

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necessary to minimize respiration losses to CO2, and maximize sludge production in an

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advanced primary treatment step combined with AD. Subsequently, secondary treatment

55

technologies should be optimized to polish residual organic matter and nutrients in an energy-

56

conservative manner.5

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A straightforward way to increase sludge production in wastewater treatment plants is

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chemically enhanced primary treatment (CEPT) with iron and aluminum coagulants.

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Removal efficiencies of particulate organics are as high as 85%,6 but the overall energy

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recovery potential of CEPT may be unsatisfactory if the wastewater has a large fraction of

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dissolved organics. The chemicals commonly used in CEPT, such as ferric chloride, alum

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and organic polymers, have been reported to inhibit biogas production during AD.7, 4 ACS Paragon Plus Environment

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Alternatively, high-rate activated sludge (HRAS) processes may be used to increase primary

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sludge production by combining a very short solids retention time (SRT) – typically several

65

hours to two days – with a high sludge-specific organic loading rate (SLR) – typically 2 to 10

66

kg biodegradable chemical oxygen demand (bCOD) per kg of volatile suspended solids

67

(VSS) per day.6 At this short SRT, only fast-growing microorganisms are retained, and

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sludge losses by grazing microorganisms and other secondary consumers can be avoided.9

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The high SLR minimizes sludge losses through cell decay and endogenous respiration.6

70

HRAS processes produce sludge with a higher anaerobic digestibility than CAS sludge,10, 11

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which makes their application suitable for energy recovery. HRAS systems are generally

72

used as a stand-alone process,12 as the A-stage of the Adsorptions-Belebungsverfahren (AB)

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process,9 or as high-rate membrane bioreactor (MBR) systems.13-15

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Conventional HRAS processes (i.e., HRAS processes with a conventional process

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configuration) struggle with the trade-off between minimizing respiration losses by lowering

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the SRT and maximizing effluent quality and bioflocculation performance by raising the

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SRT.12,

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optimization efforts should therefore primarily focus on enhancing bioflocculation and

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intracellular storage of organic matter at short SRT. To this purpose, high-rate contact

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stabilization (HiCS) was proposed as a novel HRAS technology.17 The HiCS process subjects

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the sludge to a feast-famine regime: return sludge is aerated in a stabilization phase (famine)

82

before it is brought into contact with wastewater under anaerobic conditions (feast).

16

To redirect a maximal amount of incoming organics into the sludge stream,

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Applying a feast-famine regime is a strategy to selectively favor bioflocculation18, 19 and

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production of storage polymers,20, 21 and its combination with high loading rates and short

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SRT in the HiCS process is hypothesized to raise organics recovery percentages further

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compared to conventional HRAS processes.17, 22 Bioflocculation, which occurs at the level of

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the extracellular polymeric substances (EPS) of sludge flocs, may be more influenced by



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loosely-bound EPS (LB-EPS) than by tightly-bound EPS (TB-EPS).23 Storage of organic

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carbon mainly occurs in the form of polyhydroxybutyrate (PHB).24

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It has been suggested that the HiCS process reaches higher COD removal efficiencies

91

compared to conventional HRAS systems.22 Previous experiments found that the energy

92

recovery by the HiCS system depends on the SRT and the hydraulic retention times during

93

the contact and stabilization phases, with a suggested minimum contact time of 5 min and a

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minimal stabilization time of 30 min.17, 25 Given the novelty of the HiCS process, research on

95

the process is scarce, and its status as an alternative to conventional HRAS processes has

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remained hypothetical. The HiCS process has not been optimized for maximal organics

97

recovery, nor is it known to what extent different biological pathways contribute to the

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overall removal of organics. In this study, the recovery of COD by means of bioflocculation

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and storage was investigated in the HiCS system at different combinations of SRT (0.24, 0.5,

100

1.3 and 2.8 d), contact time (8 and 15 min) and stabilization time (15 and 40 min) to identify

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an optimal operational strategy for recovering wastewater organics. Determination of the LB-

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EPS, TB-EPS and PHB content of the biomass was performed to estimate the relative

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contribution of bioflocculation and storage toward the overall removal of organic matter.


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2. Materials and Methods

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2.1.

Reactor operation

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Eight HiCS reactors were operated as sequencing batch reactors (SBR) with a working

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volume of 3 L. Synthetic wastewater was prepared according to Aiyuk and Verstraete26 with

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a complex organic fraction composed of acetate, peptone, starch, milk powder, dried yeast

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and soy oil, at a target concentration of 800 mg COD per liter26. At this COD concentration,

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the influent can be considered representative for high-strength domestic, or for industrial

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wastewater.6 The influent was prepared in batches that were kept at 15°C and used within 2

112

days. The reactors were inoculated with a 1:1 mixture of sludges from the high-rate and low-

113

rate stages of the wastewater treatment plant of Nieuwveer (Breda, NL). Samples were taken

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near-daily after an acclimation period of 3 to 6 days to reach a steady-state performance over

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several cycles of SRT. An orthogonal experimental design was implemented to optimize the

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SRT, contact time (tc) and stabilization time (ts) (Table 1). Wasting of sludge was performed

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during the contact phase. The waste flow rate was adjusted manually to maintain the desired

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SRT (calculations in Supplementary Information). The SBR cycle times were controlled by a

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microcontroller (Arduino, Turin, IT). All reactors had a contact phase of differing lengths,

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with a phase for filling and reacting and a phase for wasting, a settle phase of 40 min, a

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withdraw phase of 5 min, and a stabilization phase of differing lengths. After stabilization, an

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idle phase was implemented to synchronize the cycles of parallel reactors. Table 1 lists the

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operational conditions of each reactor.

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All experiments were performed in a temperature-controlled room at 15 °C (standard

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deviation 1°C) to mimic average wastewater temperatures of Western Europe. Peristaltic

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pumps and pressurized air were used for pumping and aeration, respectively. Stirring was

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achieved with a magnetic stir bar at a 800-1000 rpm rotational speed. Measurements of the

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dissolved oxygen (DO) concentration and pH in the reactors were made near-daily. Manual 7 ACS Paragon Plus Environment


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corrections of the air flow were made to ensure that DO levels were above 1.5 mg L-1 at the

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end of the stabilization phase, in order to remain well above the oxygen half-saturation

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coefficient of 0.2 mg L-1 for heterotrophic growth27 and minimize possible effects of low DO

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concentrations on the production of EPS28 and PHB29. The pH was between 7.2 and 8.7. It

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should be noted that stabilization, contact and settling processes in SBR systems occur

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sequentially. Therefore, the SLR and the removal rates of SBR reactors are inherently lower

135

compared to continuous systems, where these processes occur in parallel, and care should be

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taken when directly comparing rates between SBR and continuous systems.

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Table 1: Overview of reactor operation, sludge characteristics and COD concentrations of

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the influent and effluent. SLR: sludge-specific loading rate, VER: volume exchange ratio.

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Standard errors are shown behind values, where applicable. (*) The bCOD/COD ratio was

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0.99.

React or

SRT

tc:ts

Steady -state operati on

Influent flow rate

SLR (*)

Stabili zation volum e

VE R

Contactor VSS

VSS/ TSS ratio

Influent COD

Effluent COD

mg COD

mg COD

%

L-1

L-1

g bCOD g-1 VSS

min:mi d

n

d

L d-1

d-1

L

%

A

0.24 ± 0.01

15:40

21

16.0

8.1 ± 1.5

1.83

39

856 ± 97

95

919 ± 86

448 ± 40

B

0.46 ± 0.03

15:40

26

21.4

4.7 ± 0.6

1.83

39

1160 ± 106

91

660 ± 31

249 ± 15

C

1.31 ± 0.08

15:40

26

20.9

1.7 ± 0.1

1.83

39

2790 ± 130

92

668 ± 30

234 ± 22

D

2.82 ± 0.38

15:40

18

21.0

1.5 ± 0.1

1.47

51

3920 ± 189

89

843 ± 52

322 ± 10

E

0.53 ± 0.02

8:40

25

22.3

3.9 ± 0.2

1.53

49

1650 ± 95

93

834 ± 24

387 ± 27

F

0.50 ± 0.01

15:15

25

22.3

4.8 ± 0.1

1.53

49

1300 ± 35

95

834 ± 20

464 ± 18

G

0.88 ± 0.03

8:40

25

20.9

2.4 ± 0.1

1.62

46

2470 ± 100

92

844 ± 21

381 ± 16

H

0.85 ± 0.05

15:15

25

20.9

3.2 ± 0.1

1.62

46

1850 ± 74

95

835 ± 12

504 ± 15

142

2.2.

mg L-1

Coagulation and settling control experiments


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Physicochemical coagulation and settling experiments of the synthetic wastewater were

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performed as controls to assess the performance of a primary settling treatment and a CEPT

145

treatment under identical flow rates and reaction times as in the HiCS reactors B and C. For

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the primary settling experiment, influent was stirred in a 3 L batch reactor for 15 min, and left

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to settle for 40 min. Prior to the CEPT experiment, the coagulants Al2(SO4)3 and FeCl3 were

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tested for their optimal concentration in a series of batch tests with a dosage between 0 and

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300 mg L-1 for Al2(SO4)3 and between 0 and 700 mg L-1 for FeCl3. After 15 min of stirring

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and 40 min of settling, the transmission at 610 nm was measured spectrophotometrically. A

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dosage of 122 mg L-1 Al2(SO4)3 resulted in the clearest supernatant, and was used to perform

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the CEPT experiment. Influent was stirred in a 3 L batch reactor with addition of this optimal

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coagulant dose, stirred for 15 min and left to settle for 40 min. The physicochemical

154

experiments were performed in triplicate.

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2.3.

Extraction of EPS

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A heat extraction protocol was modified from Li and Yang30 and Morgan et al.28 to extract

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the LB-EPS and TB-EPS fractions from the sludge. Sludge samples for EPS measurement

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were taken at the end of the contact phase. Sludge samples were immediately centrifuged at

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4000g for 5 min and resuspended in 10 mL of Ringer’s solution (9 g L-1 NaCl, 0.42 g L-1

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KCl, 0.48 g L-1 CaCl2 and 0.20 g L-1 NaHCO3, adjusted to pH 7.0) diluted to 25% with

161

distilled water and preheated to 60°C to ensure that the suspensions reached an immediate

162

temperature of 50°C. After vortexing for 1 min, the samples were centrifuged at 4000g for 10

163

min to isolate the LB-EPS fraction in the supernatant. The pellets were resuspended in 10 mL

164

diluted Ringer’s solution, and heated at 60°C for 30 min. After centrifugation at 4000g for 15

165

min, the TB-EPS fraction was recovered from the supernatant. The EPS fractions and pellets

166

were stored at -20°C until further processing.



167

2.4.

comparisons

168 169 170

PHB extraction, analytical procedures, calculations and statistical

All analyses, calculations and statistical comparisons were carried out as described in the Supplementary Information.

171 172

3. Results

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3.1.

Removal efficiencies and sludge production

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With an increasing SRT from 0.24 to 2.8 d (reactors A to D), the total COD removal

175

efficiency increased significantly from 52% at SRT 0.24 d to 65% at SRT 1.3 d, after which

176

it decreased slightly to 60% at SRT 2.8 (not significant) (Figure 1A). At an SRT of 0.5 d,

177

decreasing tc from 15 (reactor B) to 8 min (reactor E) did not result in a lower CODtot

178

removal, while a decrease of ts from 40 to 15 min (reactor F) significantly lowered the

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removal efficiency to 44%. The same trend was observed at higher SRT (reactors C, G and

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H), but since the effective SRT of these reactors did not equal their designed SRT of 1 d,

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quantitative comparison is preliminary. Correlation analysis revealed that only the ts was

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significantly correlated to the removal efficiency of CODtot and CODdiss, with a positive

183

correlation coefficient (Table S1). It should be noted that correlation analysis may only

184

detect linear or monotonic associations between variables, and no non-linear associations, as

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may be the case between the SRT and the removal efficiencies for CODpart, CODcoll and

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CODdiss, which showed an optimum at an intermediate SRT.

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Removal of CODpart contributed most to the total COD removal, with removal percentages

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between 80 and 93% which did not differ significantly among the reactors. Regarding the

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influence of SRT and tc:ts ratio, the removal percentages of dissolved COD followed the same

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trends as for CODtot; the highest CODdiss removal efficiency was observed at an SRT of 1.3 d

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(34%, reactor C) and lowering the ts from 40 to 15 min significantly lowered the CODdiss 10 ACS Paragon Plus Environment

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removal efficiency to a mere 5%. The lowest and most variable removal efficiencies were

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observed for colloidal COD, which was even negative in reactor A (Figure 1A).

194



195


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Figure 1: (A) COD removal efficiencies for the different reactors. Error bars show

197

standard errors (SE). (B) Average balance of COD fractions leaving each reactor, relative to

198

incoming COD. Error bars show standard errors.

199 200

The observed sludge yield showed a decreasing trend with increasing SRT (Figure S1).

201

The lowest yield was observed at an SRT of 2.8 d (reactor D), which was significantly lower

202

than in the reactors with a shorter SRT, where the observed yield approximated the

203

theoretical maximum of 1 g CODsludge g-1 CODremoved. Changes in tc and ts had no significant

204

effect on the observed sludge yield. This was confirmed by correlation analysis (Table S1),

205

where the yield was significantly correlated to the SRT (ρ = -0.88) but not to the tc or ts.

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The influent contained, on average, 0.04±0.08 mg L-1 NO2--N, 2.4±0.8 mg L-1 NO3--N, and

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4.1±2.9 mg L-1 PO43--P. Whereas no nitrite and nitrate were added to the synthetic

208

wastewater, these values were consistent with the nitrite and nitrate concentrations in the tap

209

water used for the synthetic wastewater. Removal efficiencies of combined nitrite and nitrate

210

ranged between 59% and 92%, presumably due to denitrification, with the highest removal in

211

reactors C and G. Removal of orthophosphate ranged between 20% and 63%, with the

212

highest removal in reactor B.

213

It should be noted that there were variations in mixed-liquor VSS concentrations among

214

reactors as a result of the different SRTs, and this resulted in a range of different SLRs

215

(Table 1). Therefore, care should be taken when directly comparing performances between

216

reactor A, which had a very high SLR, and reactors C and D, whose SLR was in the lower

217

range of values obtained. For the same reason, any effects of SRT on reactor performance

218

described in this study may, in part, also be due to the coupled differences in SLR.

219

220

3.2.

Recovery of organic matter



221 222

An overall COD balance was made over each reactor to monitor the fractions of COD that

223

entered and exited the reactor systems via waste sludge, effluent and losses to oxidation, as

224

presented in Figure 1B.

225 226

The best performance was observed at the three lowest SRTs (reactors A, B and C), with

227

average COD recovery percentages of 54, 53 and 55% and oxidation percentages of 10, 10

228

and 14%, respectively, which did not differ significantly. Only at the longest SRT of 2.8 d

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(reactor D), a clear effect of the SRT on reactor performance could be observed, with a

230

significantly lower recovery of 18% and an oxidation of 44%. While changes in tc and ts did

231

not significantly change the fraction of COD oxidized, a decrease in tc from 15 to 8 min (in

232

reactors E and G) resulted in a significantly higher fraction of colloidal COD in the effluent

233

and a decrease in ts from 40 to 15 min (reactors F and H) resulted in a significant reduction in

234

the fraction of recovered sludge and significant increases in the fractions of colloidal and

235

dissolved COD in the effluent. Correlation analysis did not show strong associations between

236

the COD recovery percentage and operational variables (Table S1), but the fraction of

237

CODdiss leaving the reactors was negatively correlated to the ts.

238 239

Physicochemical coagulation and settling experiments were performed to assess the COD

240

removal efficiency and organic matter recovery of a primary settling treatment and a CEPT

241

treatment under identical conditions of influent composition, flow rates and reaction times as

242

in the HiCS reactors B and C. The results are presented in Table 2 and compared to the HiCS

243

reactors B and C, which had the best performance of all reactors in terms of COD removal

244

efficiency and organic matter recovery. Primary settling of the influent resulted in a poor

245

removal efficiency and organic matter recovery, whereas coagulation with Al2(SO4)3


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followed by settling resulted in an organic matter recovery equal to that in the optimal HiCS

247

reactors. However, the HiCS reactors performed significantly better in terms of COD

248

removal efficiency since, besides recovery, an additional part of the incoming COD was

249

oxidized.

250 251 252 253

Table 2: Comparison of COD removal efficiency and organic matter recovery of the primary settling and CEPT experiments, and the best-performing HiCS reactors (B and C).

254 Reactor SRT tc:ts

COD removal efficiency Particulate

Colloidal

Dissolved

Organic Total

matter recovery

d

%

%

%

%

%

Settling

81.6 ± 1.2

20.7 ± 5.5

-1.0 ± 0.8

30.1 ± 1.0

30.6 ± 1.0

Coagulation + settling

79.2 ± 2.8

60.0 ± 2.0

22.4 ± 0.6

49.8 ± 1.4

50.5 ± 1.3

HiCS B

0.46

15:40 90.6 ± 1.6

19.8 ± 14.2

29.7 ± 5.4

61.8 ± 2.1

53.2 ± 6.9

HiCS C

1.31

15:40 87.6 ± 4.8

14.9 ± 15.3

34.1 ± 6.6

64.6 ± 3.0

55.1 ± 3.4

255 256

257

3.3.

Extracellular polymeric substances and poly-β-hydroxybutyrate

258 259

The concentrations of proteins and carbohydrates in the tightly bound EPS fractions were

260

always between two to eight times higher than in the loosely bound EPS fractions (Figure 2).



261 262 263

Figure 2: Specific protein and carbohydrate concentrations of the loosely (top) and tightly bound (bottom) EPS fractions. Error bars show standard errors (SE).

264 265

In the LB-EPS fraction, carbohydrate concentrations showed a gradual decline with

266

increasing SRT, to a minimum of 2.3 g g-1 VSS at an SRT of 2.8 d (reactor D). In the TB-

267

EPS fraction, the carbohydrate concentrations remained stable around a level of 18 g g-1 VSS,

268

and none of the reactors showed significant differences. Protein concentrations were more


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variable, and were the main factor determining changes in the protein-to-carbohydrate (P/C)

270

ratio. The P/C ratio significantly increased with increasing SRT to a maximum of 6 mg mg-1

271

at an SRT of 1.3 d (reactor C) in both the LB-EPS and TB-EPS fractions, after which the

272

ratio declined again at an SRT of 2.8 d (reactor D). At an SRT of 0.5 d, a decrease of the tc to

273

8 min (reactor E) and ts to 15 min (reactor F) had no significant influence on the protein-to-

274

carbohydrate ratio in the LB-EPS fraction, but caused a strong decrease in the TB-EPS

275

fraction, compared with a tc:ts ratio of 15:40 min (reactor B). Correlation analysis showed

276

that the EPS composition did not monotonously correlate to operational variables (Table S1).

277

Correlations with COD removal efficiency were generally negative for the carbohydrate

278

fractions and positive for the protein fractions of the EPS. The strongest correlations were

279

between the protein content in the TB-EPS fraction and the CODtot (ρ = 0.90) and CODdiss

280

removal efficiency (ρ = 0.74), between the protein content in the LB-EPS fraction and the

281

CODcoll removal efficiency (ρ = 0.79), and between the carbohydrate fraction in the LB-EPS

282

fraction and the CODpart removal efficiency (ρ = -0.83).

283 284

The intracellular PHB concentration at the end of the contact phase showed a strong

285

influence of the SRT, with a significant increase from a concentration of 2.6 to 21 mg g-1

286

VSS with increasing SRT up to 1.3 d (reactor C), followed by a strong decline by 41% at an

287

SRT of 2.8 d (Figure S2). A change in the tc did not result in differences in PHB

288

concentration, but a decrease in ts from 40 min (reactor B) to 15 min (reactor F) caused a

289

significant PHB decrease by 60% at an SRT of 0.5 d. Correlation analysis showed no

290

monotonous correlation between the PHB content and operational variables (Table S1). The

291

PHB content was, however, positively correlated to the removal efficiencies of total (ρ =

292

0.95), particular (ρ = 0.71) and dissolved (ρ = 0.74) COD. Furthermore, the PHB content was

293

negatively correlated to the carbohydrate fraction of LB-EPS (ρ = -0.83) and positively to the



294

protein fraction of the TB-EPS (ρ = 0.86). Measurements with a ten-minute interval within

295

each reactor cycle showed that PHB concentrations never changed considerably from feast to

296

famine phase.

297 298 299

4. Discussion

300

4.1.

Reaching sufficient effluent quality

301

The HiCS system is designed to be the primary stage of a two-stage activated sludge

302

system, because the operational strategy to achieve maximal redirection of organic carbon in

303

HRAS systems (i.e., lowering the SRT and raising the SLR) typically result in a deterioration

304

of overall COD and nutrient removal efficiencies.12, 16, 31 In this study, the COD removal

305

efficiency increased with SRT, but declined again at the longest SRT of 2.8 d, primarily

306

because of lower removal of colloidal and dissolved COD. This may be due to the combined

307

effects of improved biosorption with increasing SRT, and a higher incidence of hydrolysis or

308

release of soluble microbial products (SMP). Indeed, experiments with high-rate membrane

309

bioreactors (MBR) at short SRT showed an increase in SMP with increasing SRT between

310

0.5 and 2 d.32 Within the range of short SRTs tested here, an optimum was found between 0.5

311

and 1.3 d.

312

To allow organic matter removal in activated sludge by the relatively rapid processes of

313

sorption and bioflocculation, studies have shown that a minimal contact time of 5 or 10

314

minutes was required.33,

315

significantly when the tc was lowered from 15 to 8 min, but a higher fraction of colloidal

316

COD left the system through the effluent. Possibly, the bioflocculation process was not

317

complete after 8 min. A longer contact time of 15 min is therefore recommendable to obtain a

318

more stable operation of the HiCS system and effluent quality.

34

In this study, the fraction of recovered COD did not differ


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319

A strong decrease in COD removal was observed when the ts was lowered from 40 to 15

320

min. This indicates that a stabilization period of 15 min is not sufficient for the sludge to

321

deplete its adsorbed or stored COD and regenerate, and is in accordance with Huang and Li25

322

who found that stabilization times should be at least 30 min. Tsang et al.35 found that the

323

relative ratio of feast-to-famine retention times determines the strength of the selection

324

pressure toward bioflocculation. According to the kinetic selection theory,20 both

325

bioflocculation and storage should therefore be favored when stabilization times are longer.

326

An optimal ts for the HiCS system is therefore more likely to be found above 35 min17 or 40

327

min, as observed in this study.

328

Comparison between physicochemical coagulation and settling tests demonstrated that the

329

HiCS system at these optimal SRTs achieves a higher overall COD removal efficiency

330

compared to CEPT, especially in the particulate and dissolved COD fractions. The CEPT

331

control experiment removed 22% CODdiss, and is in concurrence with previous studies who

332

found that alum-based CEPT can remove some CODdiss by adsorption onto the formed

333

Al(OH)3 gel.36 However, the CEPT experiment was not optimized for factors such as shear

334

force and reaction times, and a fully optimized CEPT system may achieve a better COD

335

recovery than what was obtained in this study.

336

The best effluent quality was obtained by reactor C, with an average COD concentration

337

of 234 ± 22 mg L-1. Whereas absolute effluent COD concentrations will differ according to

338

the specific influent strength and composition, this value is far above the effluent standards of

339

many urbanized regions, such as the standard of 125 mg L-1 COD set forth by the European

340

Union.37 Moreover, the potential for biological nitrogen and phosphorus removal is limited in

341

high-rate systems, due to the short SRT.38, 39 Therefore, high-rate systems typically require

342

further polishing of organics and nutrients from the effluent during secondary treatment.40 To

343

attain energy-neutral wastewater treatment, optimization efforts should combine the



344

optimization of organics recovery in HRAS systems with the development and

345

implementation of energy-efficient technologies for downstream nutrient removal. For

346

example, anammox-based deammonification requires 60% less energy for nitrogen removal

347

compared to conventional nitrification/denitrification,41 and much work is being performed to

348

improve the feasibility of anammox-based nitrogen removal under mainstream conditions.42

349

For efficient deammonification, a low bCOD/N ratio is desirable,43 presumably below 1.4 g

350

bCOD g-1 N44, 45. Since raw domestic wastewater typically has a ratio above 9 g bCOD g-1 N

351

6

, optimization of the HiCS system should primarily focus on redirecting organic carbon.

4.2.

352

Improving the bioflocculation and storage response

353

The removal of CODtot peaked to more than 60% in the reactors with an SRT of 0.5 d and

354

1.3 d. Because the extent of oxidation only varied around 10%, these reactors effectively

355

removed organic matter by means of conservational mechanisms, such as bioflocculation and

356

storage. The COD removal efficiencies were correlated with higher concentrations of

357

proteins and lower concentrations of carbohydrates in the EPS. The positive correlation

358

between the EPS protein concentrations and COD removal supports the hypothesis that the

359

hydrophobic moieties of EPS proteins are responsible for the bioflocculation process,46 and

360

concurs with the experimental results of Xie et al.47 who suggested that EPS proteins play a

361

more important role in bioflocculation than carbohydrates. The protein-to-carbohydrate ratio

362

peaked at an intermediate SRT of 1.3 d. This is similar to what was observed by Faust et al.14

363

and may explain why both positive and negative correlations with SRT have been reported.14,

364

48

365

the results of Yang and Li49 who found that the TB-EPS content of activated sludge reactors

366

remained relatively constant under changing conditions of carbon source in the synthetic

367

wastewater, SRT and loading rate. The TB-EPS proteins varied among reactors and showed

368

strong positive correlations with the CODtot and CODdiss removal efficiencies (Table S2).

The nearly constant level of TB-EPS carbohydrates in all reactors may be consistent with


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369

This suggests that the formation of TB-EPS proteins may be associated with the metabolic

370

conversion of soluble substrates. Conversely, the LB-EPS protein content showed a strong

371

positive correlation with the CODcoll removal efficiency. This corresponds to the observation

372

that the bioflocculation performance of sludge is mainly associated with the LB-EPS fraction,

373

as has been shown in literature.23, 49 Further work should be performed to determine the

374

contribution of different EPS fractions towards the removal of CODpart, CODcoll and CODdiss.

375

Whereas the complex synthetic wastewater in current study was selected to reflect realistic

376

amounts of CODpart and CODdiss, organic and ammonia-nitrogen, as found in raw domestic

377

sewage,26 differences in influent composition and strength may result in changes in LB- and

378

TB-EPS production when the HiCS system is operated with real wastewater of medium to

379

low strength. It should also be noted that EPS measurement is sensitive to the specific

380

extraction method50, and the existence of several often-used methods may prevent direct

381

comparison among reported measurements in literature.

382

The settling characteristics of the sludge were addressed indirectly through monitoring the

383

effluent quality and organics recovery performance of the reactors. In all reactors, the

384

constant settling time of 40 min and maximum travel distance of 7.5 cm subjected the sludge

385

to an equivalent upflow velocity of 0.11 m h-1, which is far below the range of 1.3-3 m h-1

386

typically employed in primary treatment stages with sludge return,6 and was deemed

387

sufficiently low to minimize effects of differences in sludge settleability on reactor

388

performance in this study. However, additional optimization of the settleability and

389

bioflocculation properties could further improve the solids/liquid separation and organics

390

recovery in the HiCS system, as evidenced by the presence of particulate and colloidal COD

391

in the effluent of all reactors.

392

The COD removal efficiency strongly correlated with intracellular PHB concentrations.

393

The PHB production did change with variations in the SRT and stabilization time, but no



394

temporal changes were observed within the feast-famine cycles of each reactor (data not

395

shown). Feast-famine cycling is known to select for sludge with a strong storage response,

396

even at very short SRTs below 1 d.21 The highest level of PHB observed in this study was 21

397

mg g-1 VSS (reactor C), which was considerably lower than the 40 mg g-1 VSS that may be

398

achieved in low-rate wastewater treatment systems optimized for PHA production.51

399

Considering the COD/weight ratio of 1.67 g COD g-1 PHB and COD/VSS ratio of 1.58 g

400

COD g-1 VSS in reactor C, the production of PHB only accounted for 2.3% of the recovered

401

COD. This suggests that production of storage polymers was not the predominant route of

402

COD removal in these experiments, and that bioflocculation, biomass growth and settling

403

were responsible for most of the COD removal. Beun et al.52 reviewed a number of studies

404

that subjected activated sludge to feast-famine conditions with acetate as carbon source, and

405

demonstrated that the fraction of consumed acetate going to PHB production remains

406

constant at a range of SRTs >2 d, but that PHB production is less predictable at lower SRT.

407

In a pilot HRAS study with real wastewater at an SRT of 0.5 d, Haider53 demonstrated that

408

PHB only accounted for 2% of the influent organics. Possibly, the SRTs in this study were

409

too low to selectively favor micro-organisms with a storage-dependent growth cycle.

410

4.3.

Reaching energy neutrality

411

The overall redirection of COD is determined by the product of COD removal and

412

observed sludge yield. Up to an SRT of 1.3 d, the yield approached 1 g COD g-1 COD, which

413

is well above the maximum yield of 0.4-0.7 g COD g-1 COD for heterotrophic growth.6

414

Yields higher than the heterotrophic growth yield may indicate that COD is removed by

415

processes other than cell growth, such as storage54, and it is not uncommon that growth yields

416

reported for HRAS processes approach the upper limit of 1 g COD g-1 COD.12 In this work,

417

the sludge yield was dependent on the SRT, which is a well-described phenomenon,6 but not

418

by the tc and ts. At extremely short SRTs below 0.5 d, it is difficult to maintain a viable 22 ACS Paragon Plus Environment

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419

sludge mass in an activated sludge reactor. Given these findings, the optimal operational

420

conditions of the HiCS system in terms of carbon redirection and recovery are a tc of 15 min,

421

a ts of 40 min and an SRT between 0.5 and 1.3 d. The carbon recovery percentages of 53% to

422

55% are higher than for conventional HRAS systems with an SRT between 0.5 to 1 d, which

423

achieve a recovery between 27% and 41% of incoming COD to sludge,12 although direct

424

comparison of these values is difficult because of differences in influent composition and

425

operational conditions.

426

An estimation of the potential energy recovery by means of AD can be made, considering

427

that the methane yield during AD of HiCS sludge with an SRT of 1 d was reported to be 0.73

428

g CODmethane g-1 CODsludge-fed.17 Thus, digestion of recovered sludge from the HiCS reactors

429

in this study would result in an energy recovery of 39±5% at an SRT of 0.5 d and 40±3% at

430

an SRT of 1.3 d, expressed as percentage of influent COD that is finally recovered as

431

CODmethane. With a conversion factor of 203 g COD kWh-1 - a conservative estimation,55 - a

432

methane energy content of 3.86 ×10-3 kWh g-1 COD, and an electricity conversion efficiency

433

of 0.35 kWhel hWhmethane-1 in a combined heat and power (CHP) installation,56 this would

434

allow an annual recovery of 27.1 and 27.9 kWh of electricity per population equivalent

435

(PECOD110), respectively. Müller and Kobel57 determined that typical large CAS plants in

436

Germany equipped with a CHP unit can produce 16.4 kWh PEBOD60-1 y-1 from combustion of

437

biogas. This is equivalent to about 13.3 kWh PECOD110-1 y-1 for sewage with standard

438

characteristics.6 Thus, a full-scale HiCS system could potentially achieve an energy recovery

439

of more than twice as high as that of a CAS system. It should be noted that the methane yield

440

from HRAS sludges may differ depending on the type of sludge, wastewater and anaerobic

441

digestion conditions, and values between 44% and 73% CODmethane CODsludge-fed-1 have been

442

reported.17, 58-60 Digestibility experiments of HiCS sludge should therefore be performed to

443

estimate of the energy recovery potential of the HiCS system under various conditions.



444

However promising, the results of this work should be validated at various conditions to

445

reflect full-scale wastewater treatment, where there may be variations in wastewater flow

446

rate, strength, composition, temperature and other characteristics. In case of sewage treatment

447

from a combined sewer system, additional resilience is required of the activated sludge

448

system to deal with periodic dilution of sewage and increases in flow rates due to rainfall.

449

Fluctuations in wastewater flow rate create changes in the reactor HRT, which may

450

negatively impact the COD recovery performance, for example by limiting the extent of

451

substrate adsorption in the reactor and causing particle washout from the settler when the

452

HRT becomes too short, or by increasing the extent of COD oxidation in the reactor and

453

causing substrate hydrolysis in the settler when the HRT becomes too long. A continuous

454

HiCS system may provide better protection against load variations and shocks than a

455

conventional HRAS system, because at any time, a major fraction of the sludge resides in the

456

stabilization reactor and is not in contact with the influent.61 Compared to the stability of

457

controlled laboratory experiments, the fluctuating conditions of full-scale systems may result

458

in a higher diversity of the activated sludge community,62 which may in turn favor functional

459

redundancy and resilience of the system.63 However, the presence of fluctuating conditions

460

may warrant the use of safety margins when designing a full-scale HiCS system, and the

461

choice of SRT, tc, ts and other operational parameters should be made both in function of

462

maximizing organics recovery as well as maintaining a sludge community dense and resilient

463

enough to withstand perturbation.

464

To move toward a mature technology that economically competes with CEPT, primary

465

settling, and conventional HRAS processes, the HiCS process should be further optimized for

466

COD recovery. Different primary treatment technologies should be compared in terms of

467

COD recovery performance, energy balance, and operational and investment costs when

468

coupled to secondary treatment, to determine the advantages and disadvantages of each


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469

technology. For example, CEPT may have a lower investment cost compared to a HiCS

470

system, but higher operational costs due to chemical dosage, and a lower sludge digestibility

471

due to the presence of coagulants in the sludge.8

472 473

In conclusion, this study showed that high-rate contact stabilization as a primary

474

wastewater treatment technology was able to recover up to 55% of incoming organic matter

475

as sludge, which could be used for electrical energy recovery. The major share of organic

476

matter removal was attributed to conservational mechanisms such as bioflocculation, biomass

477

growth and settling, as opposed to oxidation. To reach maximal energy recovery, optimal

478

operational conditions for the HiCS process were identified as a sludge age between 0.5 and

479

1.3 d, a contact time of 15 min and a stabilization time of 40 min. Future research should

480

focus on further improving colloidal and dissolved COD recovery efficiencies, and validating

481

the performance of the HiCS system under various full-scale wastewater treatment

482

conditions, in comparison with other primary treatment technologies.

483 484

Acknowledgements

485

F.A.M and S.E.V were supported, respectively, as doctoral candidate (Aspirant) and

486

postdoctoral fellow by the Research Foundation Flanders (FWO-Vlaanderen). N.B. was

487

supported by the Inter-University Attraction Pole (IUAP) ‘µ-manager’ funded by the Belgian

488

Science Policy Office (BELSPO, P7/25). We thank Bert Bundervoet for the provision of

489

sludge inocula, Dries Seuntjens and Samnang Nop for their aid in developing the Arduino

490

controller, Veerle De Smedt, Bavo Vanneste and Greet Van De Velde for practical work and

491

Jo De Vrieze for feedback on the manuscript.

492 493

Supporting Information Available 25 ACS Paragon Plus Environment


494

PHB extraction; Analytical procedures; Calculations and statistical comparisons;

495

Observed yield for the HiCS reactors; Intracellular PHB concentrations; Correlation

496

coefficients for operational variables and performance indicators. This material is available

497

free of charge via the Internet at http://pubs.acs.org.

498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533

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