Direct Filtration of Municipal Wastewater using Flat-sheet Ceramic Membrane

for pollutant removal and resource recovery

Yan-xia Zhao, Pu Li and Xiao-yan Li Department of Civil Engineering The University of Hong Kong

2 November 2019

Wastewater Treatment

2

Problems in municipal wastewater treatment

High energy consumption (aeration) for biological organic oxidation: >0.3 kWh/m3

Resources in wastewater are wasted: Energy, water, nutrients and organics (into sludge)

Pollutants in wastewater

Organics: Oxygen depletion in water

Nutrients (N and P): Algal blooms

Difficult to meet stringent discharge limits Methanol addition for N removal, Fe(III) dosing for P removal

An energy-intensive, end-of-the-pipe operation

(http://www.rznews.cn/news/)

Secondary sludge

3

New Technology - Enhanced separation and sludge refinery (ESSR)

Primary sedimentation

tank

Activated sludge aeration basin (organic degradation and N removal)

Secondary clarifier

Sewage Effluent

Flocculant Ceramic

membrane unit

Primary sludge

Food waste

Organic- & P-rich sludge

Side-stream module for resource recovery Valuable products

Liquid for denitrification

Methanol

Conventional Technology

II. Side-stream process for resource recovery Co-fermentation of sludge & food waste for P recovery, and caporate and bioplastic (PHA) productions.

Waste sludge Innovative wastewater treatment I. Chemically-enhanced membrane filtration Use of ceramic membranes to concentrate pollutants into the sludge and reduce the load on treatment.

Year

P fe

rtili

zer p

rice

inde

x

www.ers.usda.gov

Fossil fuels: deplete by 2150. (http://www.eco-info.net/fossil-fuel-depletion.html)

Fossil P reserve: depletes by 2080.

Resource (P & VFAs) Recovery Phosphorus (P-fertilizer)

Volatile fatty acids (VFAs-organic carbon source)

4

Denitrification (for N removal) Polyhydroxyalkanoates (PHAs)

• Bioplastics: complete biodegradability • Produced by biosynthesis from organic

substrates (e.g. starch) • Increasing demand, but still expensive

(>HK$40/kg) • Production cost can be greatly reduced

if waste organics were used

Membranes and Membrane Fouling

Polymeric membrane

Ceramic membrane

Flat sheet

tubular

Hollow fiber

Flat sheet

Flat sheet

tubular

Fouled membranes

Chemically-facilitated flocculation for the enhanced organic and P removals. Novel flat-sheet ceramic membranes for solids-liquid separation.

Flocculant Ceramic membrane

Influent Effluent

Chemically-enhanced membrane filtration with ceramic membranes

√

6

7

Coagulation

High COD, N and P

Low COD

Nitrogen Removal/Recovery

Water Reuse

Concentrated COD and P

Fermentation for organics and P Recovery

Flat-sheet ceramic membrane (FCM)

Sewage

Add-on (side-stream) module

Test water: Stanley Sewage Treatment Works (Stanley STW)

Coagulants: FeCl3, Polyaluminum chloride (PACl)

Flat-sheet ceramic membrane: Pore size: 100 nm Size: 25 x 25 x 5 mm Area: 0.04 m2

α-Al2O3 Meidensha corporation, Japan Outside-in filtration mode

Membrane and materials

Sludge

8

Pollutant Removal

Pollutant removal performance by flat sheet ceramic membrane (FSCM) filtration, with the chemical enhanced primary treatment.

Long-term operation of the coagulation – FSCM filtration system with different cleaning strategies.

Long-term filtration

performance

Resource recovery Mass balance analysis and Resource recovery

Results and Findings

I.

II.

III.

1. Pollutant removal by coagulation-membrane filtration

Fig. Performance of direct sewage filtration with pre-coagulation with FeCl3 or PACl: (a) COD removal, (b) TP removal. (Filtration flux was set at 1.0 m/d (41.7 LMH) )

9

0

100

200

300

23.9 3.757.3 1.1

With PAClWith FeCl3

COD

conc

entra

tion

(mg/

L)

Sewage

246.0 23.0(a)

With PAClWith FeCl3Sewage0

2

4

6

8

0.02 0.0040.005 0.0003

(b)

4.8 1.3

TP c

once

ntra

tion

(mg/

L)

Influent Effluent

1. Performance of the coagulation-membrane process

Fig. Reduction of the membrane fouling rate by pre-coagulation with FeCl3 or PACl: (c) increase in TMP and (d) the related MFR values.

10

0 10 20 30 40 50 60 700

10

20

30

40

0 5 10 60 62 64 66 68 70 720

20

40

60

80

3

3

With PAClWith FeCl3

(c)TM

P (k

Pa)

Operation duration (h)

No coagulant (d)

MFR

(kPa

⋅h-1)

Operation duration (h)

No coagulant

With FeCl3 With PACl

11

2. Long-term operation and performance

Cycle Cleaning strategy Membrane recovery rate

Duration Chemicals Backwash period Backwash flux (m/d)

1st - - - - 92.7 h/3.9 d 2nd 1.5% NaClO 5 min 5.4 87% 45.0 h/1.9 d 3rd 1.5% H2O2 5 min 5.4 90% 69.3 h/2.9 d 4th 0.1 M citric acid 5 min 5.4 80% 66.7 h/2.8 d 5th 0.1 M NaOH, 0.1 M HCl 10 min 5.4 94% 91.2 h/3.8 d 6th 0.1 M NaOH bath, 0.1 M HCl bath b4 h - 95% 106.7 h/4.4 d 7th 1.5% NaClO, 0.1 M citric acid c10 min 1.8 98% 101.1 h/4.2 d 8th 1 5% N ClO 0 1 M HCl c10 i 1 8 98% 124 6 h/5 2 d

0 100 200 300 400 500 600 700

-30

-20

-10

0

10

20

30

40

TMP (

kPa)

Time (h)Fig. Change in TMP with filtration time during the continuous operation of the coagulation-FSCM filtration process.

Table. Membrane cleaning strategies.

Flux: 1.0 m/d (41.7 LMH) Coagulation HRT : 2min Filtration tank HRT : 36 min

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2. Membrane fouling and cleaning

~ 20μm Separation layer

New Ceramic membrane

Surface Cross section

Polluted membrane and its cleaning by different methods

Polluted After brushing After Sonication After NaOH and HCl bath

Fig. The SEM photos of the new ceramic membrane (surface and the cross section )

Fig. The SEM photos of the polluted ceramic membrane surface before and after varied cleaning strategies.

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2. Membrane fouling and cleaning strategies

LAP: Loosely Attached Pollutant, causing Ref-l

SAP: Strongly Attached Pollutant, causing Ref-s

IRP: Internal Reversible Pollutant, causing Rif-r

IIRP: Internal Irreversible Pollutant, causing Rirr

if if r irrR R R−= +ef ef l ef sR R R− −= +

t m f m ef ifTMPR R R R R R

Jµ= = + = + +

Ref-l = (TMP1-TMP2) /(µ J); Ref-s = (TMP2-TMP3) /(µ J)

Rif-r = (TMP3-TMP4) /(µ J); Rirr = (TMP4-TMP0) /(µ J)

Fig. Membrane fouling resistance with/without coagulation: (a) quantitative analysis; (b) percentage distribution analysis.

0

10

20

30

40

1.10.2

1.30.20.8

4.00.41.5

4.36.46.3

24.3 19.1

With PAClWith FeCl3Mem

bran

e fil

tratio

n re

sist

ance

(*10

11 m

-1)

Rm Ref-l Ref-s Rif-r Rif-i

No coagulant

24.0

(a)

01020304050607080

(b)

Rif-iRif-rRef-s

Perc

enta

ge (%

) No coagulant With FeCl3 With PACl

Ref-l

14

0 200 400 600 800 1000-22-20-18-16-14-12-10-8-6-4-2

TMP

(kPa

)

Time (min)

Hot water Sonication NaOH-HCl NaClO H2O2

Citric acid

(a)

0 50 100 150 200 2500

5

10

15

20

25

30

Hot water Sonication NaOH-HCl NaClO H2O2

Citric acid

MFR

(*10

-3 k

Pa⋅s-1

)

Time (min)

(b)

2. Membrane fouling and cleaning strategies

Fig. Selection of backwash solution influenced membrane anti-fouling performance: (a) change of TMP with filtration time; (b) change of membrane fouling rate with filtration time.

Influence of the cleaning method on the membrane anti-fouling propensity

De-fouling effect: H2O2 > Citric acid > NaClO > Hot water > Sonication > NaOH-HCl

15

3. Mass balance analysis and resource recovery

VFAs

Biosynthesis Polyhydroxyalkanoate

(PHA) production Bioplastics

(high value-added products)

Concentrated Sludge Volatile fatty acids

Sludge fermentation for organic hydrolysis and P recovery. Organic recovery from the sludge supernatant for PHA production. Use of the residual organic in the sludge liquor for N removal by denitrification.

Sludge fermentation for P and VFA release and biosynthesis for PHA production Co-Fermentation

P recovery

Food waste

10 20 30 40 50 60 70 80

Inten

sity

2θ (degree)

Synthetic vivianite

Vivianite-containing recovery precipitates

Application 1: Co-fermentation of sludge and food waste for P recovery

3. Resource recovery

Effluent Denitrification

or PHA recovery

CEPT sludge

Anaerobic reactor

Anaerobic supernatant

(P-rich)

P-containing precipitates

Precipitation &

Crystallization

Fig. X-Ray Diffraction (XRD) analysis of the P recovery products

17

1st-effluent

Raw sewage

Effluent

Aerobic reactor

Anoxic reactor Anaerobi

c reactor

CEPT sludge

Primary sedimentation

tank

Nitrification effluent

Supernatant (VFAs)

Nitrification: NH4+ + 2O2 → NO3

- + H2O + 2H+

Denitrification: NO3- + VFAs → N2 + H2O + CO2

Sludge liquor

0 2 4 6 8 10 120

5

10

15

20

25

30

N-Co

ncen

tratio

n (m

g/L)

Time (d)

NO3--N in

NO3--N out

0

20

40

60

80

100

Removal Efficiency

Deni

trific

atio

n Ef

ficie

ncy

(%) Stage V(An-sup)/V(N-eff)

1 1:4 2 1:10 3 1:6

Effective denitrification with a V(sludge liquor)/V(nitrified effluent) >1:6.

Application 2: VFAs in sludge liquor for denitrification

3. Resource recovery

Fig. Denitrification (N removal) results.

18

TEM image of intracellular PHAs found in the PHA-producing sludge from R1 1 2 3 4 5 6 7 8

0

4

8

12

16

20

24

28

32 CEPT liquor (high load) CEPT liquor (low load) Glucose solution

PHA

con

tent

(g P

HA

/g V

SS) %

Time (d)PHA content of activated sludge in the bioreactors

during enrichment process

Application 3: VFAs in sludge liquor for PHA biosynthesis

3. Resource recovery

Bioplastics

PHA (Polyhydroxyalkanoates)

19

Flat-sheet ceramic membrane (FSCM) filtration replacing the primary sedimentation: Improves effluent quality, reducing organic load on the down-stream treatment process Concentrates sludge with organic recovery of 78.3%, facilitating the follow-up resource recovery

Stable operation of the system was achieved. The sludge was concentrated by >10 times with the high COD and P contents for potential resource recovery. Co-fermentation for P release for fertilizer, VFA for denitrification and PHA production.

PACl-FSCM filtration has a much reduced fouling rate, and the membrane fouling is mainly caused by loosely attached pollutants (LAP) that can be readily cleaned.

Conclusions

Coagulant Raw wastewater

Air

Coagulation

Membrane separation

Overflow

Permeate

Suction Pump

High COD, N and P

Low COD, high N

Flat-sheet ceramic membrane (FCM)

Concentrated COD and P

Fig. Schematic sof the coagulation and flat-sheet ceramic membrane (C-FSCM) filtration process

Acknowledgements

Funding Support T21-711-16R and C7044-14G from the Research

Grants Council (RGC) of Hong Kong SAR Government.

20

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The University of Hong Kong