6. Dimensioning and Design of Nitrogen_Stefan Rettig

Dimensioning and design of nitrogen removal technologies

Dipl.-Ing. S. RettigTU Berlin, Department of Urban water management

Gustav-Meyer-Allee 25, D - 13355 Berlin

Phone: +49 / (0) 30 / 314 72356; Fax: +49 / (0) 30 / 314 72248e-mail: [email protected]

Departement of urban water management 2

Introduction

Biological wastewater treatment: Fixed film (trickling filter, rotating diskfilter) or suspended biomass (activated sludge system)

Predominantly the activated sludge system is practiced

Municipal and industrial wastewater treatment

Basic of the activated sludge system:Combination of aeration tank

+ subsequent sedimentation (clarifier) + return of the separated biomass (return sludge)

Carrier of biological treatment - activated sludge -

Invention of the activated sludge system 1914 (Ardern, Lockett)


Basis of dimensioningbasic possibilities

1. Evaluation of available data (normal case)2. Additional specific investigations (series of

measurements), if data is insufficient3. Mathematical determination with known (exceptional

case) Consideration of future development

(mostly with the help of characteristic values) Demography (population development) Residential areas Industry Tourism


Cumulative frequency / Undercut frequency

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600

Qo in m3/d

Sum

men

häuf

igke

it [%

]

Undercut [%]

COD-load, inflow

Cum

ulat

ive

frequ

ency

[%]

Bd,COD[1000 kg/d] COD-load

Inflow rate


Dimensioning Flows and Loads

The following values are required from the influent to the biologicalreactor:

lowest and highest wastewater temperature

organic load (Bd,BOD Bd,COD), load of suspended solids (Bd,SS) and of phosphorus (Bd,P) for the determination of the sludge production and thus the calculation of the volume of the aeration tank

organic load and nitrogen load for the design of the aeration facilityfor (as a rule) the highest relevant temperature

–Loading condition: BOD/N; highest saisonal peak

maximum inflow rate with dry weather QDW,h (m³/h) for the design of the anaerobic mixing tank and the internal recirculation flow rate

dimensioning inflow rate QWW,h (m³/h) for the design of thesecondary settling tanks


Biological Standard ProcessesActivated sludge processes

Elimination of: COD, BOD5, NH4-N, NO3-N, P

Effluent

Aeration

Clarifier

Return sludge

Excess sludge

anoxic zonedenitrification

aerobic zoneCOD-elimination

& nitrification

Recirclation sludge

Influent(primary-treated)


Inhabitant-specific loads

g/(Inhabitant·d)

German standard (ATV-DVWK A 131)During biological wastewater treatment process for each kg BOD5 about 0,04-0,05 kg Nitrogen and about 0,01 kg Phosphorus are needed for the development of biomassand discharged in the waste sludge. *) The share returned in the sludge liquor has to be concerned. Thus the loads in theinfluent of the biological treatment stage can increase up to 20 %.


Dimensioningof the activated sludge process I

1. Determination of the relevant flows and loads2. Selection of the treatment process

=> Nitrification/Denitrification

Activated sludge tank (Part 1) Set up of a Nitrogen-balanceSelection of the treatment process

Nitrification/Denitrification; P-Elimination; SelectorSelection of the return sludge ratio;

intermitting DN timeDetermination of the denitrification capacityDetermination of the required sludge ageCalculation of the sludge production


N in influent:Co,N = Co,org.N + Co,NH4-N+ Co,NO3-N+ Co,NO2-N

Nitrogen balance

Effluent

anoxic zonedenitrification

aerobic zoneCOD-elimination

& nitrification

Excess sludge

aeration

Clarifier

Return sludge

Recirculation sludge

Influent(primary-treated)

N in effluent:Ce,N = Ce,org.N + Ce,NH4-N+ Ce,NO3-N+ Ce,NO2-N

N in sludge:CNWS = 0,04 - 0,05 · Co,BOD5

Co,TKN


German effluent regulations for municipal sewage; monitoring values

113 **)10 15 75 > 100.0005218 **)10 20 90 10.000 bis < 100.0004

––10 20 90 5.000 bis < 10.0003

–––25 110 1.000 bis < 5.0002

–––40 150 < 1.0001

tot P

mg/l

Ntot anorg.*

mg/l

NH4-N *)

mg/l

BOD5

mg/l

COD

mg/l

PE based on BOD inlet

60 g BOD5/(PEd)

SizeCategory

PE: population equivalent Abwasserverordnung (AbwV vom 2004)


Nitrogen removal procedures (DWA-A 131e, 2000)


The operating conditions in aeration tank and secondarysettling tank are influenced through Mixed-liquor suspended solids concentration in the influent to the

secondary settling tank SSEAT

Mixed-liquor suspended solids concentration of the return sludgeSSRS

Return sludge ratio RS = QRS/Q.

Suspended solids mass balance (neglecting XSS,EST)

Return Sludge Ratio I


Effluent

Return sludge

Influent QWW,h

QRS = 0.75·QWW,h

max.QRS = 1.0 ·QWW,h


Return Sludge Ratio II


Denitrification capacity

(DWA-A 131e, 2000)


Sludge age tSS: [d]Average retention time of activated sludge in the activated sludge system

Amount of sludge in the aeration tank MLSSAT VATtSS = = Removed amount of sludge QES MLSSES + Q MLSSE

tSS = 1/(SPd·BSS) tSS = 1/µmax tSS …..10 to 12 days

MLSSAT: Total amount of solids (MLSS) (measured) [g/l or kg/m3]MLSSES: Total amount of solids in the excess sludge

Important design parameterfor activated sludge system

Enough time for the growth of microorganisms

Dimensioning sludge age in days dependent on the treatment target and thetemperature as well as the plant size (intermediate values are to be estimated)


Required Sludge Age (DWA-A 131e, 2000)


Sludge production SPd

SPd = SPd,BOD + SPd,P SPd,BOD= SPC,BOD * Bd,BOD,ZB

SPd,P [kg/d] = Qd [m³/d] (3 XP,BioP + 6,8 XP,Prec, Fe + 5,3 XP,Prec,Al)/1000(DWA-A 131e, 2000)


Dimensioningof the Secondary Settling Tank

1. Selection of the sludge volume index2. Selection of the sludge thickening time tTh; dependent on the biological process

selected3. Determination of the return sludge suspend solids concentration (SSRS)4. Selection of the return sludge ratio (RS) and estimation of the permissible

suspended solids concentration of the activated sludge in the biological reactor(SSAT).VAT reduces with increasing SSAT.AST and tST rises with increasing SSAT.

5. Determination of the surface area of the scondary settling tank (AST) from thepermissible surface overflow rate qa or the sludge volume loading rate qav

6. Determination of the depth of the secondary settling tank from partial depths forthe functional zones and other sepcifications

7. Dimensioning of the sludge removal (scraper)8. Verification of the selected thickening time by the sludge removal (scraper)

performance9. Dimensioning of the return sludge and excess sludge pumps

Transfering: SS-content


Sludge volume SV: Volume of sludge after 30 min. settling of 1000 ml activated Volume of sludge after 30 min. settling of 1000 ml activated

sludgesludge Measured value > 250 ml Measured value > 250 ml dilution the sample (factor)dilution the sample (factor) Common values: Common values: 200 200 –– 600 ml600 ml

Sludge volume index SVI: Quotient of sludge volume and liquor suspended solids SVI = SV / MLSS Common values: Common values: 75 75 -- 180 ml/g180 ml/g Bulking sludge Bulking sludge SVI > 150 ml/gSVI > 150 ml/g

Mixed liquor suspended solids MLSS: Content of biomass Content of biomass Common values: Common values: 3 3 -- 6 6 g/lg/l

Characteristic parameters

(Steinke, 2009)


Approximate values for theMLSS concentration in thebiological reactor dependent on the sludge volume index forSSRS = 0.7·SSBS

= MLSS

Standard values for the sludge volume index

(DWA-A 131e, 2000)


Settlement in horizontal flow tanks

(Austermann-Haun, 2011)


2.0 - (2.5)Activated sludge plants with denitrification

1.0 - 1.5Activated sludge plants with nitrification

1.5 - 2.0Activated sludge plants without nitrification

Thickening time tTh [h]Type of wastewater treatment

An exceeding of the thickening time of tE = 2.0 h requires a very advanced denitrification in the biological reactor.

Permitted Thickening Time (tTh)


Suspended Solids Concentrationin the bottom sludge

Achievable suspendedsolids concentration in the bottom sludge SSBS

can be estimatedempirically in dependence on the SVI and tTh

(DWA-A 131e, 2000)


Surface Overflow Rate and Sludge Volume Surface Loading Rate

The surface overflow rate qA is calculated from thepermitted sludge volume loading rate qSV and the dilutedsludge volume DSV as:

(DWA-A 131e, 2000)


QWW,h (m³/h) - Max. inflow rate

SVI (l/kg) - Sludge volume index

SSEAT (kg/m³) - Suspended solidsconcentration in the influent to settling tanks

Effluent

Return sludge

QRS

Dimensioning of the Secondary SettlingTank


The required surface area of the secondary settling tank results as follows:

For vertical flow secondary settling tanks the effectivesurface area at the mid-point between inlet aperture and water level is to be set

With this the geometry of normal tank shapes is taken intoaccount

Settling Tank Surface Area

(DWA-A 131e, 2000)


Settling Tank Surface Area

Horizontal flow circular secondary settlingtanks

(DWA-A 131e, 2000)


Dimensioningof the activated sludge process II

Activated sludge tank (Part 2) Calculation of the volume of the biological reactorDimensioning of aeration (O2-demand; daily peak)Dimensioning of circulation units; design of circulation pumpsType of biological reactorChecking of acid capacity and pH

Takeover: concentration of SS


Required mass of suspended solids in biological reactor:MSS,AT = tSS,Dim · SPd [kg]

The volume of the biological reactor is obtained as follows:

As comparative figures the BOD5 volume loading rate (BR) and the sludge loading rate (BSS) can be calculated:

Nitrification BSS= 0,10 kg BOD5/(kgSS·d)

Bd = BOD5 QoNitrification BR =0,35 kg BOD5/(m³·d)

Usual values of MLSS,AT: 2 - 6 g/l

Volume of the Biological Reactor


Design of recirculation

t

RS

t

RZRStRC

t

RZ

t

RS

BMorgNeorgNONNNH

ANNO

NNH

QQRF

QQbzwQRFQQ

QQ

QQRC

therefore

XSCSwithSS

RC

.*][

:

1

,,,,4

,3

,4

Effluent

Return sludge

Influent QWW,h



Dimensioning of aeration (O2-demand)

Oxygen consumption for C-EliminationOUd,C [kg O2/d] = OUC,BOD,spez · Bd,BOD,I

Oxygen consumption for Nitrification

OUd,N [kg O2/d] = Qd * 4,3*(SNO3,D – SNO3,IAT + SNO3,EST)/1000Oxygen consumption for Denitrification (+)

OUd,D [kg O2/d] = Qd * 2,9 * SNO3,D / 1000

Specific oxygen consumption OUC,BOD [kg O2/kg BOD5, validfor CCOD,IAT/CBOD;IAT ≤ 2.2

(DWA-A 131e, 2000)


Dimensioning of aeration (Daily peak OUh)

Load cases:

I fN = 1 with fC = x

II fN = x with fC =1

24*)(*

]/[ ,,,2

NdNDdCdCh

OUfOUOUfhkgOOU

with fC and fN = Peak factors for load peaks (appear at different times)

-

1.25

6

2.0

-

1.2

8

1.8

-

1.2

10

-

1.3

4

Sludge age in d

1.5

2.5

1.15

15

2.0fN for BC,BOD,I ≤ 1.200 kg/d

-fN for BC,BOD,I > 6.000 kg/d

1.1fC

25


Summary

The calculated tank volumes are highly influenced by thecorrect evalution of the loads (Q, COD, BOD, N- and P-load)

Design based on sludge age

Design of denitrification: capacity of denitrification

Design of secondary settling tanks based on sludge volumeload

Simple calculation according to A 131

Computer-based models according to A 131 or equivalentapproaches

Documents

6. Dimensioning and Design of Nitrogen_Stefan Rettig