the cod balance of a wwtp: possibilities and limitations for improving … ·  · 2017-06-13the...

The COD Balance of a WWTP:

Possibilities and Limitations

for Improving the Energy Efficiency

2nd IWA Conference on Holistic

Sludge Management

Malmö, June 9th 2016

Martin Kaleß

Energy content of wastewater

Energy demand of WWTPs

Processes for carbon recovery (examples)

Possibilities and limitations of providing energy

from wastewater by calculation of a COD

balance

Conclusions and outlook

Outline

Energy in wastewater occurs in different forms

Chemically bound energy expressed by COD:

Energy content of wastewater

Authors / reference

COD load

(g COD / (PE * d))

Almeida et al. (1999) 112 (A)

Andreottola et al. (1994) 116 (A, rural) – 120 (A, urban)

ATV DVWK A 131 (DWA 2000) 120 (P)

Hansen et al. (2011) 120 (A)

Hartwig et al. (2010) 120 (A)

Henze (1997) 130 (A)

Jardin (2012) 120 (A)

Jönsson et al. (2005) 134 (A)

Kroiss and Svardal (2009) 110 (A)

Schmidt et al. (2003) 108 (A) / 143 (P)

Stommel (2011) 130 (A) / 175 (P)

A: Average; P: 85-Percentil

Average energy content of municipal

wastewater from literature: 120 g COD/(PE*d)

Assuming 1 kg COD corresponds to 3.5 kWh

(Olsson, 2015), (Mergelmeyer and Kolisch, 2014)

153 kWh/(PE*a) are available in wastewater

Average energy demand of a German WWTP:

34 kWhel/(PE*a)

Energy content of wastewater /

energy demand of its treatment

Average energy content of municipal

wastewater from literature: 120 g COD/(PE*d)

Assuming 1 kg COD corresponds to 3.5 kWh

(Olsson, 2015), (Mergelmeyer and Kolisch, 2014)

153 kWh/(PE*a) are available in wastewater

Average energy demand of a German WWTP:

34 kWhel/(PE*a)

Fate of chemically bound energy

Influent 100%

to bio-

logical

step

70% Effluent 8% Excess

sludge

25%

Primary

clarifyer

Respiration 37%

Digested sludge 28 %

Activated

sludge

process

Biogas 27%

digestion According to Svardal, 2014

r eturn sludge screenings s and

influent

p rimary sludge e xcess sludge

a ctivated

sludge

tank

f inal

sedimentation

tank

screen g rid chamber

and

g rease trap

p rimary

s ettlement

tank

r eceiving

water

Classification of processes for carbon recovery

Carbon recovery from

wastewater

possibilities for carbon

recovery

particulate dissolved

physical sedimentation yes no

sieving yes no

flotation yes no

adsorption yes yes

biological biological

incorporation no yes

chemical precipitation /

flocculation yes partly

Conventional sedimentation processes

remove 30 % of the incoming COD at a retention time

of 0.75 – 1 h (DWA 2000)

Sedimentation performance can only be slightly

improved by prolonging the retention time

Enhancement of sedimentation processes in

primary clarifiers by adding chemicals

Chemically enhanced primary

treatment (CEPT) (1/3)

Laboratory studies within the project are

ongoing

CEPT (2/3)

Results from literature study

CEPT: Removal efficiency (3/3)

Author Total COD (or BOD5)

removal [%]

Conditions

Aiyuk et al. (2004) 73 laboratory scale

Bourke (2000) 55-65 (BOD5) full scale

De Feo et al. (2007) up to 70 laboratory scale

Gerges et al. (2006) 53 laboratory scale

Ismail et al. (2011) 56-61 laboratory scale

Leentvaar et al. (1977) 72 laboratory scale

Muzenda (2012) 58 (BOD5) laboratory scale

Ødegaard (1992) 73 full scale (87 Norwegian

WWTPs)

Screenings usually not considered in terms of

energy

Washing of screenings can help to recover

carbon from screenings

Amount and composition of screenings depend

on the sewer system and the bar spacing of the

screen

Carbon rich wash water can be used for

digestion

Washing of screenings (1/5)

Washing of screenings (2/5)

Washing of screenings (3/5)

Full scale investigations at three German

WWTPs

Record of the hourly amount of raw screenings

within one day

Washing the screenings from 24 hours in

several washings

Determination of the COD load of the wash

water

Comparison of the COD load in the wash water

to the COD load of the influent

Washing of screenings (4/5)

Plant

COD load of the

wash water (kg COD) obtained from

incoming screenings (time span 24h)

COD load (kg COD) of

the WWTP influent

(assumption:

120 g COD / (PE*d))

Ratio (%)

A 11 1,944 0.57

B 10 3,480 0.29

C 17 4,200 0.40

Washing of screenings: results

(5/5)

Faeces easily elutable

COD recovery by treating screenings

excess sludge

from first stage

return sludge

intermediate

settling

excess sludge

from second stage

return sludge

influent

internal recirculation

effluent

high load

stage

low load

stage

final

sedimentation

first stage second stage

Two-stage processes (1/3)

Two-stage processes (2/3)

Advantages

high concentration of substrate in the first stage

two separate biocenoses provide better conditions to

separated microorganisms than one can do

adsorption of dissolved substances in the first stage

less energy consumption (higher load means less

respiration) therefore more energy rich sludge from

first stage

Semi-scale pilot tests were conducted to

quantify COD-elimination of first stage

Two-stage processes: results (3/3)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 d 0.25 d 0.5 d 1 d 0.125 d 0.25 d 0.5 d 1 d

total dissolved

Eli

min

ati

on

of

CO

D

sludge age tSRT

maximum value

minimum value

75-%-percentile

25-%-percentile

median

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Eli

min

ati

on

COD balance: possibilities and

limitations (1/2)

Assumptions:

120 g COD / (PE*d)

CEPT: 73 % removal efficiency

8 % still remain in the effluent as in conventional

processes (dissolved inert)

From the remaining 19 % -> 7% are transformed in

the excess sludge (same ratio as in conventional

activated sludge systems)

80 % of the influent COD load could enter the

digestor

COD balance: possibilities and

limitations (2/2)

50 % conversion of sludge into gas

40 % efficiency of combined heat and power

plant

153 kWh/ (PE*a)*0,8*0,5*0,4= 24.5 kWhel/ (PE*a)

Conclusions (1/2)

Holisitic approach combines sludge treatment

as well as sludge providing

Presented numbers will be specified within the

project period

CEPT shows high elimination efficiency for

COD

Washing of screenings has a minor impact on

carbon recovery

Two-stage activated sludge systems may enjoy

comeback due to elimination of dissolved COD

by adsorption

Conclusions (2/2)

COD recovery and transformation into electrical

energy leads to 24.5 kWhel/ (PE*a) in optimistic

case

Compared to 34 kWhel /(PE*a) average energy

consumption on German WWTPs

Outlook

Gap could be closed when considering lower

energy demand due to lower aeration because

of carbon recovery

Denitrification problems may occur and must

be solved by introducing autotrophic

processes

Thank you for your attention!

Acknowledgement: The authors would like to express their sincerest

thanks to the Federal Ministry of Education and Research for funding the

project E-Klär (reference 02WER1319).

Thanks to the project partners

and the co-authors!

Furthers questions: please feel free to ask or send it to

kaless@isa.rwth-aachen.de