wastewater treatment & energy production

7/23/2019 Wastewater Treatment & Energy Production

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Wastewater treatment & energy

production - Pragues experience

ICT Prague,

CZECH REPUBLIC

Pavel JENICEK


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Transformation of pollution into

biogas in WWTP

aerobicWWT

BM

anaerobicstabilization

WWWWT

BGanaerobic


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Main benefit of anaerobic digestion of sewage sludgeis production of energy in biogas.

Produced energy can cover not only the energy needfor the sludge treatment, but a substantial part of the

energy consumption of the whole treatment plant.

Combined heat and electricity production from

biogas is mostly applied in Europe (subsidized

electricity from renewable sources).

Energy from sludge


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co-digestion of sludge with external organic wastes

improvement of anaerobic digestion

(minimization of energy consumption)

Ways to energy self-sufficiency in

municipal WWT plants


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Interdependence of specific biogas

production and potential energy production

specific biogas

production

(L/kg VS)

annual biogas

production

(m3/person)

potential annual

electricity production

(kWh/person)

300 6.1 15.8

400 8.2 21.1

500 10.2 26.4

600 12.3 31.6

700 14.3 36.9

Annual energy

consumption per person

at WWT 31-47 kWh(Balmer,2000)


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higher amount of treated sludge

sludge pretreatment to enhance anaerobic

biodegradability

optimization of the digestion technology

Ways to higher biogas production


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Excess activated sludge

not beneficial (energy, degradability)

Primary sludgeby intensification of primary sedimentation

more pollution removed by AD

worse COD/N(P) for BNR processes

Increase of treated sludge amount


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Pre-treatment techniques mainly applied

mechanical disintegration ultrasonic treatment

thermal or chemical hydrolysis

Waste activated sludge stream is pretreated mostly,

because its degradability is low.There are two technological options:

a) pre-treatment of total sludge flow

b) pre-treatment of partial sludge stream only


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For the selection of pre-treatment method

especially investment and operational costs

(mainly energy consumption) are crucial.

Most promising technologies:

mechanical disintegration by lysate centrifugecombining thickening of waste activated sludge anddisintegration

thermal process applied to total sludge input,replacing digester heating


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Optimization

of technological conditions of the digestion

based on the improvement of:

the homogeneity of fermentation mixture indigesters

the way and frequency of feeding

the total solids concentration in the raw sludge

the process temperature the process configuration - staging

the AD conditions - microaerobic AD


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Many of the mentioned measures

have been gradually applied in

the Central Waste Water Treatment Plant of Prague

With the aim of

- higher capacity of digesters

- higher efficiency of digestion- higher biogas production

- higher self-sufficiency in electricity


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Central Wastewater Treatment Plant of Prague

Q - 3.5 m3/s

1 600 000 PE

Sludge treatment


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Combined heat and power production

Thickening centrifuges

Dewatering centrifuges

I.stage digesters

II.stage digesters

12 digesters , 4800 m3each


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I. Stagemixedheated

II. Stage

Gasholder

55oC

52oC


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I.stage

1) Improvement of mixing efficiency

by installing a better mixing device

TheoreticalHRT

Real HRT beforechange of mixing

Real HRT afterchange f mixing

11 days 8 days 10 days

100 % 72 % 91 %


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0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

totalsolids

(t/y

r)

PS

TES

RS

Total solids input into digesters.

PS primary sludge, TES thickened excess sludge, RS raw sludge

2) Increase of treated sludge amount

addition of iron salts to pre-precipitation of primary sludge


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3) Sludge disintegration by lysate centrifuge

By anaerobic degradability tests it was proved

the increase of specific biogas production 11 31 %

- improves degradability of the biological cell material,

which is difficult to degrade, breaks up floc structure andcell walls and releases cell lysate


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4) Change of operational temperaturemesophilic (38 oC) thermophilic (55 oC)

0

5000

10000

15000

20000

25000

0 10 20 30 40 50

lo ading rate of VS t /d.tank)

b

o

p

o

o

m

3

d

a

thermophi l ic

mesophi l ic

Loading rate of mesophilic digesters: 13.3 VS [ t/d.tank]

thermophilic digesters: 20.7 VS [ t/d.tank]


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Operational parameters of digesters

Specific loading rate of

mesophilic digesters: 2,8 kg /m3.d VS

thermophilic digesters: 4,3 kg /m3.d VS

Specific biogas production per volatile solids added

Thermophilic Mesophilic

Operational

temperature

55 oC 38 oC

(m3/kg) 0.71 0.54

(Nm3/kg) 0.61 0.48


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higher stability of the process

higher biogas production

improvement of the energy balance of the WWTP

high resistance against foaming

less problems with odor

higher efficiency of pathogens destroying

Benefits of intensification measures


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Daily course of biogas production (------) and

volatile solids input () at CWWTP

Increase of

sludge amountImproved digestion


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The increase of specific biogas production

as a consequence of the intensification measures


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The increase of specific biogas production

as a consequence of the intensification measures


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Coverage of electricity requirements by biogas

in Prague's central wastewater treatment plant

0

10

20

30

40

50

60

70

80

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

(%)


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Conclusions I

Increasing temperature to thermophilic level and

improvement of mixing efficiency have led to abetter utilization of the existing facilities, more

efficient destroying pathogens and avoided digester

overloading and foaming.

The destruction of activated sludge bacteria cellsduring the thickening increased the portion of more

easily degradable constituents of the sludge.

The overall higher process efficiency is associatedwith the higher biogas production and improvement

of the energy balance of the process.


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Conclusions II

Energy self-sufficiency of municipal wastewater

treatment plant can be achieved due to the advanced

anaerobic digestion of the sludge produced during

the treatment process.The necessary condition for energy balance

improvement is optimization of the total energy

consumption at the wastewater treatment plant.


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Thank youfor your attention.

Wastewater treatment & energy

production - Pragues experience


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Literature

Jenicek, P., Bartacek, J., Kutil J., Zabranska J. and Dohanyos M. (2012) Potentials and Limits of Anaerobic Digestion of Sewage Sludge:

Energy Self-Sufficient Municipal Wastewater Treatment Plant? Water Science and Technology accepted for publication.

Zabranska, J., Dohanyos, M., Kutil, J., 2006. Disintegration of excess activated sludge - evaluation and experience of full-scale applications.

Water Science and Technology 53, 229-236.

Dohanyos, M., Zabranska, J., Kutil, J., Jenicek, P., 2004. Improvement of anaerobic digestion of sludge. Water Science and Technology 49,

89-96.

Zabranska, J., Dohanyos, M., Jenicek, P., Ruzicikova, H., Vranova, A., 2003. Efficiency of autothermal thermophilic aerobic digestion and

thermophilic anaerobic digestion of municipal wastewater sludge in removing Salmonella spp. and indicator bacteria. Water Science and

Technology 47, 151-156.

Zabranska, J., Dohanyos, M., Jenicek, P., Zaplatilkova, P., Kutil, J., 2002. The contribution of thermophilic anaerobic digestion to the stable

operation of wastewater sludge treatment. Water Science and Technology 46, 447-453.

Zabranska, J., Dohanyos, M., Jenicek, P., Kutil, J., 2000. Thermophilic process and enhancement of excess activated sludge degradability -

two ways of intensification of sludge treatment in the Prague central wastewater treatment plant. Water Science and Technology 41, 265-272.

Dohanyos, M., Zabranska, J., Jenicek, P., Stepova, J., Kutil, V., Horejs, J., 2000. The intensification of sludge digestion by the disintegration ofactivated sludge and the thermal conditioning of digested sludge. Water Science and Technology 42, 57-64.

Zabranska, J., Stepova, J., Wachtl, R., Jenicek, P., Dohanyos, M., 2000. The activity of anaerobic biomass in thermophilic and mesophilic

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Dohanyos, M., Zabranska, J., Jenicek, P., 1997. Enhancement of sludge anaerobic digestion by using of a special thickening centrifuge. Water

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Dohanyos, M., Zabranska, J., Jenicek, P., 1997. Innovative technology for the improvement of the anaerobic methane fermentation. WaterScience and Technology 36 333 340 IF 0 896

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