atv dvwk a 281 e libre

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GERMAN ATV-DVWKRULES AND STANDARDS

Standard

ATV-DVWK-A 281E

Dimensioning of Trickling filters and RotatingBiological Contactors

September 2001

ISBN 3-937758-36-4

Publisher/marketing:

ATV-DVWK German Association for Water, Wastewaterand Waste,Theodor-Heuss-Allee 17 D-53773 HennefTel. ++49-22 42 / 8 72-120 Fax:++49 22 42 / 8 72-100E-Mail: [email protected] Internet: www.atv-dvwk.de


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The main fields of activity of the ATV-DVWK are technical-scientific subjects and the economic as well asthe legal concerns of environmental protection. The politically and economically independent associationworks nationally and internationally in the fields of pollution control, wastewater, water-hazardous sub-stances, waste, hydraulic engineering, hydraulic power, hydrology, soil protection and contaminated sites.The ca. 16,000 members are active in municipalities, engineer offices, authorities, firms and associations

and also in universities. Of these there are 10,000 specialists with personal membership; these are engi-neers, scientists, lawyers, business persons, operating personnel and technicians. Via the corporate mem-bership in the ATV-DVWK there is access to ca. 160,000 specialists.

All rights, in particular those of translation into other languages, are reserved. No part of this Standard maybe reproduced in any form - by photocopy, microfilm or any other process - or transferred into a languageusable in machines, in particular data processing machines, without the written approval of the publisher.

Publisher: ATV-DVWK Deutsche Vereinigung fr Wasserwirtschaft, Abwasser und Abfall e.V.,Theodor-Heuss-Allee 17, D-53773 Hennef

Marketing: GFA Gesellschaft zur Frderung der Abwassertechnik e.V., Hennef

Setting and printing (German original): DCM, Meckenheim

GFA Gesellschaft zur Frderung der Abwassertechnik e. V., Hennef 2001

Die Deutsche Bibliothek [The German Library] CIP-Einheitsaufnahme

ATV-DVWK, German Association for Water, Wastewater and Waste:ATV-DVWK Rules and Standards [Media combination] / ATV-DVWK, Water Management,Wastewater, Waste. -Hennef : GFA, Ges. zur Frderung der AbwassertechnikPreviously under the title of: Abwassertechnische Vereinigung: ATV Set of Rules and Standards

StandardA 281E. Dimensioning of Trickling Filters and Rotating Biological Contactors.

ISBN 3-937758-36-4


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Foreword

The revision of ATV Standard ATV-A 135 (now ATV-DVWK-A 281) has become necessary as it no longercorresponds with the status of technology.

Compared with the issue of ATV Standard ATV-A 135 dated March 1989 the following important amend-ments have been made:

Basic validity for trickling filters and rotating biological contactors without limitation of the capacity (previ-ously 500 PT).

Removal of the determination of loading principles; a separate ATV-DVWK standard for all types ofwastewater treatment processes is being prepared.

The addition of a dimensioning approach for denitrification using trickling filters. Increase of tank surface area and reduction of tank depth of the secondary settling stage due to new tri-

als results.

The biological stage of wastewater treatment plants, employing trickling filters and rotating biological con-tactors without sludge return feed, is dealt with in this Standard. The standard applies only for rotating bio-logical contactors without artificial aeration for the supply of the biofilm with the required oxygen.

A detailed description of the theoretical basic elements and practical application of both the fixed bed proc-esses is contained in the ATV Handbook Biologische und weitergehende Abwasserreinigung [Biologicaland Advanced Wastewater Treatment] and Mechanische Abwasserreinigung [Mechanical WastewaterTreatment]. The development of the trickling filter process and the rotating biological contactor as well asthe factors on their treatment efficiency are covered in advanced literature.

As with all aerobic processes for biological wastewater treatment, the contact between biomass andwastewater is to be established and the biomass is to be supplied with oxygen. With the trickling filter proc-

ess the wastewater is spray irrigated over the filter material so that, during the dripping process, the contactbetween biomass and wastewater is established. In general, aeration is without application of further en-ergy. With rotating biological contactors the partially submerged filter material is rotated about its longitudi-nal axis with the application of energy. During the emergent phase of the material the biofilm can take upoxygen from the surrounding air and in the submerged phase the pollutants from the wastewater.

The following are to be mentioned as favourable characteristics of trickling filters and rotating biologicalcontactors:

in general they are simple and stable to operate. no activated sludge return is necessary. trickling filter and rotating biological contactor facilities enable the colonisation of micro-organisms which

have long generation times. Thus even compounds which are difficult to degrade can be eliminated withlittle loading.

in general the energy requirement is small.


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Authors

This standard has been prepared by the ATV-DVWK Working Group KA-6.3 Trickling filters and contac-tors, within the ATV-DVWK Specialist Committee KA-6 Aerobic biological wastewater treatment proc-esses, and the ATV-DVWK Specialist Committee KA-5 Settling processes.

The ATV-DVWK Working Group KA-6.3 Trickling filters and biological contactors has the following mem-bers:

Dr.-Ing. Jrgen Bever, Oberhausen (Chairman) Dr.-Ing. Georg Mehlhart, DarmstadtProf. Dr.-Ing. Harro Bode, Essen Dr.-Ing. Manfred Roth, StuttgartDr.-Ing. Bernd Dorias, Stuttgart Dr.-Ing. Sigurd Schlegel, EssenProf. Dr.-Ing. Werner Gebert, Planegg Dipl.-Ing. Gert Schwentner, SindelfingenDr.-Ing. Hans-Dieter Kruse, Bad Zwischenahn Dr.-Ing Gerald A. Steinmann, Weienburg

The members of the ATV-DVWK Specialist Committee KA-5 Settling processes are:

Prof. Dr.-Ing. Ernst Billmeier, Mnchen Dr.-Ing. Helmut Resch, Weissenburg (Chairman)Dipl.-Ing. Winfried Born, Kassel Prof. Dr.-Ing. Karl-Heinz Rosenwinkel, HannoverDr.-Ing. Andrea Deininger, Weyarn Dr.-Ing. Reinhold Rlle, StuttgartDr.-Ing. Thomas Grnebaum, Essen Dr.-Ing. Andreas Schulz, EssenProf. Dr.-Ing. F. Wolfgang Gnthert, Neubiberg Prof. Dr.-Ing. Carl Franz Seyfried, HannoverDr.-Ing. Karl-Heinz Kalbskopf, Dinslaken Dr.-Ing. Andreas Stein, EmsdettenProf. Dr. Peter Krebs, Dresden

The ATV-DVWK Specialist Committee KA-6 Aerobic biological wastewater treatment processes has thefollowing members:

Dipl.-Ing. Reinhard Beer, Cottbus Dr. Dipl.-Biol. Hilde Lemmer, Mnchen

Dr.-Ing. Jrgen Bever, Oberhausen Prof. Dr.-Ing. Jrg Londong, WuppertalProf. Dr.-Ing. Harro Bode, Essen Prof. Dr.-Ing. Norbert Matsch, Wien/sterreichDr.-Ing. Reiner Boll, Hannover Dipl.-Ing. Anton Peter-Frhlich, BerlinProf. Dr. Lothar Huber, Neubiberg Prof. Dr.-Ing. Karl-Heinz Rosenwinkel, HannoverProf. Dr.-Ing. Dr. Rolf Kayser, Braunschweig Dipl.-Ing. Peter Schleypen, Mnchen(Chairman) Dr.-Ing. Burkhard Teichgrber, EssenProf. Dr.-Ing. Karlheinz Krauth, Stuttgart Dipl.-Ing. Volker Ziess, HaanDr. rer. nat. Joachim Richard Lemke, Leverkusen


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Contents

Foreword .................................................................................................................................................. 3

Authors ................................................................................................................................................... 4

User notes ................................................................................................................................................ 7

1 Area of application ................................................................................................................... 7

1.1 Preamble.................................................................................................................................... 71.2 Objective .................................................................................................................................... 71.3 Scope......................................................................................................................................... 8

2 Symbols...................................................................................................................................... 8

3 Basic elements of dimensioning .............................................................................................. 103.1 Loading with wastewater............................................................................................................ 93.2 Loading from sludge liquor and external sludge........................................................................ 11

4 Pre-treatment ............................................................................................................................ 11

5 Trickling filters ......................................................................................................................... 11

5.1 Description of the the process ................................................................................................... 115.1.1 General....................................................................................................................................... 115.1.2 Filter material ............................................................................................................................. 125.2 Dimensioning ............................................................................................................................. 13

5.2.1 General details on dimensioning................................................................................................ 135.2.2 Wastewater treatment without nitrification ................................................................................. 145.2.3 Wastewater treatment with nitrification ...................................................................................... 145.2.4 Wastewater treatment with nitrification and denitrification......................................................... 15

6 Rotating biological contactors ............................................................................................... 16

6.1 Description of the process.......................................................................................................... 166.1.1 General....................................................................................................................................... 166.1.2 Material and types...................................................................................................................... 176.2 Dimensioning ............................................................................................................................. 186.2.1 General details on dimensioning................................................................................................ 18

6.2.2 Wastewater treatment without nitrification ................................................................................. 196.2.3 Wastewater treatment with nitrification ...................................................................................... 19

7 Phosphorus removal ............................................................................................................... 20

8 Waste sludge production ........................................................................................................ 20

9 Secondary settling tanks......................................................................................................... 20

9.1 General....................................................................................................................................... 209.2 Dimensioning of the secondary settling tank of single-stage trickling filters and rotating

biological contactors................................................................................................................... 21

9.3 Notes on tank shape and design ............................................................................................... 21


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User Notes

This Standard is the result of honorary, technical-scientific/economic collaboration which has beenachieved in accordance with the principles appli-cable therefor (statutes, rules of procedure of theATV-DVWK and the Standard ATV-DVWK-A400E). For this, according to precedents, there e-xists an actual presumption that it is textually andtechnically correct and also generally recognised.

The application of this Standard is open to everyo-ne. However, an obligation for application can ari-se from legal or administrative regulations, acontract or other legal reason.

This Standard is an important, however, not thesole source of information for correct solutions.With its application no one avoids responsibility forhis own action or for the correct application in spe-cific cases; this applies in particular for the correcthandling of the margins described in the Standard.

1 Area of Application

1.1 Preamble

The treatment of the stormwater in the sewer net-work and of wastewater in the wastewater treat-ment plant form one unit for the protection of sur-face waters. For the dimensioning of thewastewater treatment plant and the stormwateroverflows the planning periods are to be matchedto each other. The planning period should com-prise not more than 25 years.

In the case of special conditions the dimensioningcan often be carried out more correctly with the aid

of trials and operating results of existing plants.Under certain circumstances costs can be savedthrough this. The trials plants for this are to be es-tablished at least on a semi-industrial scale andoperated for not less than half a year under practi-cal operating conditions with the inclusion of thecold season.

1.2 Objective

Using the dimensioning values recommended inthis standard for municipal wastewater one can

meet or undercut the achievable minimum effluentrequirements which correspond with the require-ments of the German Wastewater Ordinance dated09.02.1999, Appendix 1, and the associated sam-pling specifications.

It is pointed out that short-term ammonium dis-charge peaks, in particular with combined waste-water in plants with large preliminary settling tanks,are unavoidable and more marked than with acti-vated sludge plants.

If commercial or industrial wastewater with highfractions of slowly biodegradable and/or inert or-ganic substances is discharged, a higher residualCOD than with domestic wastewater can arise.The same applies for areas with low water con-

sumption and low infiltration rate, as then the inertCOD concentration increases.

In this Standard technical regulations are drawn upfor the dimensioning both for

carbon removal as well as the nitrification anddenitrification using trickling filters as well as for

carbon removal and nitrification using rotatingbiological contactors.

In addition, information is given for phosphorus

removal.

In accordance with the requirements under [Ger-man] water law, the structural and operating re-quirements and the sensitivity of the surface wa-ters through parallel units, reserve equipment etc.is to be oriented towards an appropriately high op-erational safety.

A prerequisite for the secure function of the plantplanned in accordance with this standard is thatsufficient qualified, trained and permanently tech-

nically supported operating personnel are em-ployed and are involved in the planning process,comp. ATV Advisory Leaflet ATV-M 271 Person-albedarf fr den Betrieb kommunaler Klranlagen[Personnel requirement for the operation of mu-nicipal wastewater treatment plants, currently notavailable in English].

With the systems dealt with in this Standard one isconcerned with fixed bed reactors with very differ-ent types of construction and process technology.

Therefore, in this Standard, trickling filters (Chapter


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5) and rotating biological contactors (Chapter 6)are dealt with separately. The subjects Basic ele-ments of dimensioning (Chapter 3), Pre-treatment (Chapter 4), Phosphorus removal(Chapter 7), Waste sludge production (Chapter 8)and Secondary settling tanks (Chapter 9) arepresented jointly.

1.3 Scope

This Standard applies basically for the dimension-ing of single-stage trickling filter and rotating bio-logical contactors and for pre-anoxic denitrificationtrickling filters. Some advice is given for trickling fil-ters and rotating biological contactors in the sec-ond stage. Attention is drawn to the ATV Report

Multi-stage biological wastewater treatmentplants [3] [currently not available in English] withregard to multi-stage facilities. ATV-A 257E appliesfor wastewater lagoons with intermediate tricklingfilters and rotating biological contactors.

Due to the peculiarities of small wastewater treat-ment plants attention is drawn to the ATV StandardATV-A 122E. For small scale wastewater treatmentplants with a wastewater inflow up to 8 m3/d, DIN4261 applies. For hospital wastewater treatment

plants DIN 19250 is to be additionally taken intoaccount. ATV Standard ATV-A 129 [currently notavailable in English] applies for the disposal ofwastewater from recreation and tourist facilities.

The Standard applies for wastewater which origi-nates from households or from facilities whichserve commercial or agricultural purposes insofaras the harmfulness of this wastewater can be re-duced by means of biological processes with thesame success as with wastewater from house-holds.

2 Symbols

[Translators note: the symbols/indices below inEnglish are in line with the general rules of ATV-DVWK Standard A 198E. Where these differ fromthe original German the latter are shown in squarebrackets.]

a - number of rotary distributorarms

ASST [ANB] m2 surface area of the secon-

dary settling tankARC [ART] m

2 theoretical surface area of

the rotating biological contac-tor (sum of the surfaces ofthe trickling material)

ARC,C m2 theoretical surface area of

[ART,C] the rotating biologicalcontactor for carbon removal

ARC,N m2 theoretical surface area of

[ART,N] the rotating biological con-tactor for nitrification

ATF [ATK] m2 surface area of the trickling

filterBA,BOD g/(m

2.d) BOD5 surface loading of the

[BA,BSB] rotating biological contactorBA,TKN g/(m

2.d) TKN surface loading of therotating biological contactor

Bd,BOD,InB kg/d daily BOD5 load in the[Bd,BBS,ZB] influent to the biological reac

torBd,NO3,D kg/d daily nitrate-nitrogen load to

be denitrifiedBd,N,WS kg/d daily load of nitrogen which[Bd,N,S] is removed through the

waste sludge from the trick-

ling filter or rotating biologicalcontactor facilityBd,TKN,InB kg/d daily TKN load in the influent[Bd,TKN,ZB] to the biological reactorBd,inorgN,InB kg/d daily load of inorganic nitro-[Bd,anorgN,AN] gen in the effluent of the

secondary settling stageBd,orgN,SST kg/d daily load of organic nitrogen[Bd,orgN,AN] in the effluent of the second

dary settling stageBR,BOD kg/(m

3.d) BOD5 volumetric loading of[BR,BSB] the trickling filter

BR,TKN kg/(m3.d) TKN volumetric loading ofthe trickling filter

DSST m diameter of the secondary[DNB] settling tankPTXXX I total number of inhabitants[EWXXX] E and population equivalents

referred to the parameterXXX, e.g. BOD5

hSST[hNB] m depth of the secondary set-tling tank

hTF[hTK] m height of the trickling filter

filler material


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n 1/h rotationsrevolutions per hourof the rotary distributor

qA,SST m/h surface loading rate of the[qA,NB] secondary settling tankqA,TF m/h surface overflow rate of the

[qA,TK] trickling filterqWO[q] m3/(m.h) weir overflow rate

QDW,d m3/d daily wastewater inflow with

[QT,d] dry weatherQComb,h m

3/h dimensioning peak flow from[QM,h] combined or separate sys

temsQSST[QNB] m

3/h dimensioning inflow of thesecondary settling tank

QRF m3/h recirculation flow

QDW,2h m3/h maximum dry weather flow

[Qt] rate as 2 hourly mean

QTF[QTK] m3/h influent to the trickling filter:QDW+QRF

RRm[RVm] - recirculation ratio QRF toQDim,In

RRDW[RVt] - recirculation ratio QRF toQDW,2h

FF[SK] mm/arm flushing forcetSST[tNB] h retention period in the sec-

ondary settling tankVSST[VNB] m

3 volume of the secondary set-tling tank

VTF[VTK] m

3

volume of the trickling filterVTF,C m3 volume of the trickling filter

[VTK,C] for carbon removalVTF,N m

3 volume of the trickling filter[VTK,N] for nitrificationVTF,D m

3 volume of the trickling filter[VTK,D] for denitrification

Pollution parameters and concentrations:

CXXX mg/l concentration of the parame-ter XXX, in the homogenised

sampleSXXX mg/l concentration of the parame-ters XXX, in the filtered sam-ple (0.45m membrane filter)

XXXX mg/l concentration of the filterresidue, XXXX= CXXX SXXX

Indices for the location or purpose of the sampling(always last):

In [Z] sample from influent to thewastewater treatment plant

InB [ZB] sample from influent to bio-logical reactor

EB [AB] sample from the effluent ofbiological ractor

ESST [AN] sample from the effluent ofthe secondary settling tank

WS [S] sample from the waste

sludgeRF sample from the recirculationflow

MV [W] monitoring value [Authors af-ternote: here, effluent re-quirement with defined sam-pling procedure]

Frequently used parameters:CBOD,InB mg/l average BOD5 concentration[CBSB,ZB] with dry weather from daily

inflow Qd without recirculation

c flow in the influent to thebiological reactor

CBOD,InB,RF mg/l average BOD5 combined[CBSB,ZB,RF] concentration with dry weather

from daily inflow Qd andrecirculation flow at therotary distributor

CN,InB mg/l concentration of the total[CN,ZB] nitrogen in the homogenised

sample in the influent to thebiological reactor

SinorgN,MV mg/l monitoring value for inor-[SanorgN,W] ganic nitrogen in the effluentsample as N

SNH4,ESST mg/l concentration of the ammo-[SNH4,AN] nium in the effluent sample

as NSNO3,ESST mg/l concentration of the nitrate in[SNO3,AN] the effluent sample as NSNO3,D mg/l concentration of nitrate-nitro-

gen to be denitrifiedSorgN,ESST mg/l concentration of the organic[SorgN,AN] nitrogen in the effluent of the

secondary settling tankXorgN,BM mg/l organic nitrogen incorporated

in the biomassXSS,ESST mg/l concentration of suspended[XTS,AN] solids in the effluent of the

secondary settling tank


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3 Basic Elements ofDimensioning

3.1 Loading with Wastewater

The BOD5load (Bd,BOD,Inin kg/d), undercut on 85 %of the dry weather days in the influent to thewastewater treatment plant plus a planned capac-ity reserve, is to be used for the classification intothe Size Class in accordance with Appendix 1 ofthe [German] Wastewater Ordinance and for thedetermination of the dimensioning capacity of theplant the assessment under water law. If the di-mensioning capacity is determined on the basis ofthe number of connected inhabitants, the inhabi-tant-specific BOD5 load for raw wastewater from

Table 1 is to be used.

In principle it applies that the sewer system andwastewater treatment plant are operated for thesame wastewater effluent and influent.

For dimensioning, the following important numeri-cal values are required from the influent to the bio-logical reactor, if applicablewith the inclusion of thereturn flows from sludge treatment (comp. 3.2):

Relevant organic load (Bd,BOD) for the calculationof the required trickling filter volume or the nec-

essary surface area of rotating biological con-tactors for wastewater treatment without nitrifi-cation as well as for the determination of thewaste sludge production.

Relevant organic load (Bd,BOD) and nitrogen load(Bd,TKN) for the calculation of the necessary trick-ling filter volume or the required surface area ofrotating biological contactors for wastewatertreatment with nitrification.

Relevant concentration of nitrogen (CN) and theassociated concentration of organic matter(CBOD) for the determination of the nitrate to bedenitrified with the dimensioning of trickling fil-ters for denitrification.

Relevant daily wastewater inflow Qd and maxi-mum inflow with dry weather QDWfor the dimen-sioning of trickling filters.

Maximum inflow with dry weather QDW,2h andmaximum dimensioning inflow QComb,In for thedesign of the secondary settling tank.

Daily loads can only be calculated on the basisofvolumetric- or flow-proportional 24 hour compos-ite samples and the related daily inflow. The rele-

vant loads are to be determined on the basis of

measurements on arbitrary days, i.e. with the in-clusion of wet weather days. Relevant are thoseloads which are undercut on 85 % of the days. Atleast 40 load values are to be included for the de-termination of the values. The relevant concentra-tions are to be determined using relevant loadsand the associated daily wastewater inflows.

If the daytime and weekly courses of the concen-trations and inflows of the wastewater deviate fromthe variations with predominantly domestically pro-duced wastewater, for example through the indus-trial wastewater component, then this is to betaken into account with the determination of thedimensioning quantities.

Arrangements should be made to balance peaks if

the daily curves of the nitrogen loads show up in 2-hourly atypically high loading peaks (greater than 2times the daily average), whereby the loading fromsludge treatment must also be taken into account.

If the data are insufficient or the expense for inves-tigation, for example with small plants, are in no re-lation to the use, loads and concentrations can bedetermined on the basis of connected inhabitantsplus industrial/commercial and other loads.

Details on the determination of relevant loads and

concentrations are to be taken from the StandardATV-DVWK-A 198E Dimensioning Principles forWastewater Facilities [4].

If the relevant loads have to be estimated basedon the connected inhabitants the values in Table 1can be used. The estimation of the associatedwastewater inflow is to be undertaken in accor-dance with the ATV-DVWK Standard [4]. Until thisstandard is published determination of the waste-water flow can be determined in accordance withStandard ATV-A 131E (1991). [Translators note:Standard ATV-DVWK-A 198 was published in April2003 and was translated into English in 2004].

Table 1: Inhabitant-specific loads in g/(I.d),

which are undercut on 85 % of thedays, without taking into accountthe sludge liquor

Parameter Raw waste-water

Retention time in the primarysettling stage with QDW

0.5 to 1.0 h 1.5 to 2.0 hBOD5 60 45 40TKN 11 10 10

P 1.8 1.6 1.6


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Deliberate investigations of wastewater and de-termination of loads over two to four weeks cannot,as a rule, be used directly for dimensioning, as onecannot be certain of having considered the rele-vant period of time. They are, however, practicalfor the supplementing of the existing database.The loading of internal return flows, for examplefrom sludge treatment should also be recordedwithin the scope of such investigations.

3.2 Loading from Sludge Liquor andExternal Sludge

Water from the thickening and dewatering of (an-aerobic) digested sludge contains ammonium inhigh concentrations. It can be assumed that 50 %

of the organic nitrogen introduced into the sludgedigester is released as ammonium nitrogen. Ifsludge liquor is produced for a few hours dailyonly, or on odd days weekly, an intermediate stor-age for dosed input is necessary.

Return loading with phosphorus and organic mat-ter (BOD5and COD) is, as a rule, small from dewa-tering of digested sludge. Therefore a return load-ing may not be added, for example, globally as apercentage to all loads from the wastewater.

As a rule, more or less anaerobic processes occurin sludge silos for aerobic stabilised sludge. Withthis, ammonium can be released and rerisolutionof phosphorus is possible, if excess biologicalphosphorus removal is applied. In order to mini-mise impairment of the biological treatment

- sludge liquor should be drawn off in small quan-tities

when dewatering the silo content filtrate or cen-trate should be collected in silos of a similar size

and be fed to the inlet over a long period of ti-me.

If external sludge (sludge from other wastewatertreatment plants, faecal sludge or similar) is dis-charged, then an intermediate storage can bepractical in order to make a dosed input possible.

Further information on the determination of sludgeliquor quantities and characteristics are to be takenfrom [7].

4 Pre-treatment

The wastewater flowing into the trickling filters androtating biological contactors must be as free aspossible of disturbing substances and settleablesolids in order to avoid blockages. Therefore a pre-treatment and primary settling of the inflowingwastewater before the biological reactor is indis-pensable. With denitrification trickling filters this isparticularly important as the removal of faults thereis very expensive. Normally primary settling tanks,possibly also fine sieves, are employed for this.

Depending on the treatment requirements the pri-mary settling tanks should be dimensioned differ-ently. With pure carbon removal and nitrification

(without denitrification) the retention time with dryweather should not be less than 1.5 to 2.0 hours.With pre-anoxic denitrification and a lack of an or-ganic carbon compound the retention time can bereduced from 0.5 to 1.0 hours with dry weather.

With high wet weather inflow peaks the primarysettling tanks should be so dimensioned for a re-tention time shorter than 0.5 hours with wetweather inflow QComb. This applies above all forsmall wastewater treatment plants with a capacity

below 1,000 PT.

Sufficient sludge storage volume is also to betaken into account with small plants. This can, forexample, be arranged as separate tanks or in anImhoff tank combined with the primary settlingtank.

5 Trickling filters

5.1 Description of the Process

5.1.1 General

The treatment of wastewater in trickling filters asfixed bed reactors is effected by micro-organisms,which settle on the filter material as biofilm. In trick-ling filters the treatment process proceeds from topto bottom. In the various treatment zones there arerespectively biocoenoses of different compositioninvolved. Depending on the loading condition of

the trickling filter, the influence of nitrifying bacteria


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is, for example, first completely effective if the deg-radation of the organic loading is completed to alarge extent.

The following prerequisites are to be met for anassured efficiency of the trickling filter:

The filter material concerned must be flawless inconstitution and installation; filter materials mustcorrespond with DIN 19557.

Attention is to be paid to an even, surface-proportional distribution of the wastewater overthe trickling filter surface. The rotating distributormust be suitably designed for this. An as evenas possible complete wetting of the filter mate-rial surface with wastewater is to be ensured.Here a sufficient minimum surface loading and

fine distribution are significant. A sufficient flushing force for the removal ofwaste sludge is to be ensured, i.e. for the re-spective loading there is a minimum hydraulicload which, if necessary, is to be ensured usingreturn pumps.

An unhindered percolation of the wastewaterthrough the filter material must be avoided at allcosts.

It is recommended not to install differently struc-tured material in a trickling filter. To secure theremoval of sludge a sufficient transmissibility is

to be ensured in the vertical direction. The feed and return pumps are to be graded

according to the different inflows taking into ac-count the minimum surface loading rate. A con-tinuous feed is to be sought.

The air access from outside to the hollow floorof the trickling filter and into the filter material(exception denitrification trickling filter) must beensured via supply air openings. In order toavoid too heavy a cooling in winter the air open-ings should be constructed so that they are ca-

pable of being reduced. A too heavy cooling in locations with severe

winter climate is to be countered through insula-tion of the walls, an enclosed construction anddifferential pump operation in comparison withthe warm season; the forced ventilation recom-mended under circumstances in such a casecan, together with a treatment of exhaust air,contribute to the prevention of odour nuisanceswith very highly loaded trickling filters. As a rule,a treatment of exhaust air can, however, be dis-pensed with as long as the trickling filter is suffi-

ciently ventilated and the filter material in thisway itself acts as a filter.

5.1.2 Filter material

Most important component of the trickling filter arethe filler materials used which can be roughly di-vided into mineral material and material made fromplastic. With the selection of the filler material it isto be ensured that the wastewater sprayed overthe trickling filter and, with aerobically operatedtrickling filters, the air flowing through have overallfree access to the biofilm and that waste sludgecan be removed with the wastewater. A blockageof the hollow spaces can thus limit the treatmentefficiency or even reduce it completely to zero.

DIN 19557 differentiates between the theoreticalsurface, the effective surface (growth area) of thefilter material and the biologically active surface ofthe growth. The effective surface is the surface ofthe filter material wetted in operation. The theoreti-cal surface deviates from this. The ratio of effectivesurface to theoretical surface is defined by theutilisation factor. There is no doubt that the biologi-cally active surface of the growth would representthe correct reference parameter for the description

of the metabolic efficiency. The biologically activesurface is, as a rule, not to be determined. The di-mensioning details for trickling filters in this stan-dard therefore essentially concern the volumetricloading.

As a rule, lumps of rock or slag sized from 40 to 80mm, which are placed over the hollow base on asupporting layer sized from 80 to 150 mm, serveas mineral filler material for trickling filters. The re-quirements on characteristics, testing and installa-tion of the mineral filling are summarised in DIN

19557. The sizes 40 to 80 mm correspond withspecific theoretical surfaces of ca. 90 m2/m3and ahollow space share of ca. 50 %. Under operatingconditions ca. 2/3 of this can be assumed to bebiologically active.

Plastic filter materials have very different struc-tures. From this result in part considerable differ-ences between the theoretical, the effective andthe biologically active surface. Suppliers of plasticfiller materials should therefore present retraceable

calculations for the theoretical surface.


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So far as no reliable transferable experience isavailable for the filler and for the characteristics ofthe wastewater, trials should be carried out at leaston a semi-industrial scale.

5.2 Dimensioning

5.2.1 General Details on Dimensioning

Dependent on the level of treatment sought theBOD5volumetric loading and, in the case of nitrifi-cation, in addition the TKN volumetric loading inkg/(m3

d), are relevant for the dimensioning of the

trickling filter volume. The filler material planned forthe trickling filter contents, according to the permit-ted volumetric loading, results as

VTF,C= Bd,BOD,InB /BR,BOD [m3] (1)

In addition with nitrification:

VTF,N= Bd,TKN,InB/BR,TKN [m3] (2)

Thus the total volume is:

VTF= VTF,C+ VTF,N [m3] (3)

As a rule the BOD5concentration at the rotary dis-tributor CBOD,InB,RFis to be set at less than 150 mg/l

by return pump operation. For this, as also for apartial balance of large variations of the inflow, arecirculation ratio RRDW1 is sufficient with BOD5concentrations in the influent 400 mg/l. The trick-ling filter surface and the biological filler height re-sult as:

ATF= QDW (1+RRDW)/qA,TF [m2] (4)

hTF= VTF/ATF [m] (5)

Trickling filter filler heights of about 4 m for mineralfilled trickling filters have proved their worth. Withthe employment of plastic filler material with a highvertical transmissibility a larger filler height is rec-ommended.

The surface loading rate qA,TF with mineral filledtrickling filters, related to QDW (1+RRDW), shouldbe 0.4 m/h, with trickling filters with plastic filler ma-terial at least 0.8 m/h. Smaller filler heights up to aminimum of 2 m require a particularly even, finelydistributed filter dosing and careful selection of thefiller material, and enable a reduction of the sur-

face loading rate of up to 0.4 m/h. Plastic filler ma-

terial with good transverse distribution is to beused with smaller filler heights.

In addition to the surface loading rate the design ofthe rotary distributor also has an effect on theflushing force F

F. For this the following relationship

applies

FF= qA,TF1000/(a n) [mm/arm] (6)

Values for FFof 4 to 8 mm have proved their worthin order to ensure a satisfactory sludge removal.The higher the trickling filter the stronger is the re-quired flushing force in order to be able to preventblockages due to heavy growth in the upper part ofthe trickling filter. Equally, for plastic filler materialswith increasing specific theoretical surface, in-

creasing values for FFare to be planned. Further-more, if the talk is of surface loading rate, here aloading rate by normally rotating rotary distributorswith a flushing force FF within this range is as-sumed.

In practical dimensioning the following procedurehas proved its worth:

a) Determination of the necessary trickling filtervolume VTF in m

3in accordance with Sections5.2.2 or 5.2.3 and 5.2.4 dependent on the

treatment target.

b) Calculation of the mean average concentrationat the rotary distributor without recirculationflow CBOD,InB= Bd,BOD,InB1,000/Qdin mg/l.

c) Determination of the required recirculation ratiofor the achievement of the desired mean con-centration CBOD,InB,RF at the rotary distributor(CBOD,InB,RF150 mg/l):RRDW(CBOD,InB/CBOD,InB,RF) - 1.

d) Determination of the maximum hydraulic filterloading of the trickling filter from the maximuminflow with dry weather to the trickling filter QDWin m3/h and the recirculation ratioQTF= QDW

(1+RRDW) in m

3/h.

e) Selection of a trickling filter filler height hTFin m.

f) Determination of the required surface of thetrickling filter ATF= VTF/ hTFin m

2.


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g) Examination of the surface loading rate of thetrickling filter with maximum inflow with dryweather including recirculation flowqA,TF = QDW (1+RRDW)/ATF in m/h. This sur-face loading rate should be at least 0.4 to 0.8m/h whereby, with trickling filters filled withplastic filler, the upper value is to be main-tained. If the given values are not met then thecalculation is to be repeated with modifiedheight or modified recirculation ratio.

h) Determination of the number of the rotary dis-tributor arms and the rate of rotation underconsideration of the flushing force FF.

i) It is to be ensured that, during the night, a con-tinuous operation of the rotary distributor is

guaranteed and the complete surface of thetrickling filter is evenly wetted.

5.2.2 Wastewater Treatment withoutNitrification

For the dimensioning of mineral filled trickling filtersand trickling filters with plastic filler material with aspecific theoretical surface of a minimum of 100m2/m3the following is recommended:

BOD5volumetric loading BR,BOD0.4 kg/(m3d)

With trickling filters with plastic filler material with aspecific theoretical surface of more than 100 m2/m3BOD5 volumetric loadings of more than 0.4kg/(m3d) are possible. These should, however, besubstantiated through trials (see Chapter 1.1) orreferences. Specific theoretical surfaces of morethan 150 m2/m3 and BOD5 volumetric loading ofmore than 0.6 kg/(m3d) are not effective for furtherimprovement of performance. It is pointed out that,

blockages can already occur with specific theoreti-cal surfaces of about 150 m2/m3.

With small wastewater treatment plants, due tomarked inflow or loading peaks, it is recommendedto reduce linearly the BOD5 volumetric loadingfrom 0.4 kg/(m3d) to 0.2 kg/(m3d) with capacitiesbetween 1,000 and 50 PT.

5.2.3 Wastewater Treatment withNitrification

With the dimensioning of trickling filters with nitrifi-cation the volumetric content planned for the filter

material is determined separately for the carbonremoval and for the nitrogen oxidation.

For the dimensioning of mineral filled trickling filtersand of trickling filters with plastic filler material witha specific theoretical surface of a minimum of 100m2/m3the following is recommended:

For carbon removal:

BOD5volumetric loading BR,BOD0.4 kg/(m3.d)

For nitrification:

TKN volumetric loading BR,TKN0.1 kg/(m3.d)

This value takes into account a nitrification alreadystarted in the carbon removal zone. The permittedvolumetric loading BR,TKN for the dimensioning isnot identical with the volumetric efficiency of deg-radation.

With trickling filters with plastic filler material, BOD5volumetric loading of more than 0.4 kg/(m3d) for carbon removal and TKN volumetricloading of more than 0.1 kg/(m3d) for nitrificationare possible. These should be substantiatedthrough trials (see Chapter 1.1) or references.Specific theoretical surfaces of more than 150m2/m3as well as BOD5-volumetric loading of morethan 0.6 kg/(m3d) and TKN volumetric loading ofmore than 0.15 kg/(m3d) are not effective for fur-ther improvement of performance.

With small wastewater treatment plants, due tomarked inflow or loading peaks, it is recommended

to reduce linearly the BOD5 volumetric loadingfrom 0.4 kg/(m3.d) to 0.2 kg/(m3d) and the TKNvolumetric loading from 0.1 kg/(m3d) to 0.05kg/(m3d) with capacities between 1,000 and 50PT.

Note: If nitrification takes place in a second trick-ling filter following extensive carbon removal in afirst stage with intermediate treatment, the follow-ing loading values are recommended: TKN volu-metric loading up to 0.1 kg/(m3d) with mineralfilled trickling filters and up to 0.2 kg/(m3d) withtrickling filters with plastic filler material. For this,


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plastic filler material with a specific theoretical sur-face of up to 200 m2/m3can be employed. For rea-sons of safety it is recommended that the nitrogenincorporation is not to be taken into account in thefirst stage.

Note on alkalinity: the remaining alkalinity in theeffluent from nitrifying trickling filters should, if nec-essary taking into account precipitants, not under-cut 0.5 mmol/l, in order to exclude possible inhibi-tion of the nitrification. This is to be ensured whennecessary through suitable measures. It is recom-mended that such measures are first carried outfollowing presentation of pertinent operational ex-perience.

5.2.4 Wastewater Treatment with Nitrifi-cation and Denitrification

The following remarks apply both for mineral filledtrickling filters as well as trickling filters with plasticfiller material.

For procedural integration of denitrification withtrickling filter plants there are basically three possi-bilities:

simultaneous denitrification in the trickling filter

with recirculation of wastewater containing ni-trate

pre-anoxic denitrification in an anoxically oper-ateda) fixed bed reactor (e.g. trickling filter)b) activated sludge tanks with intermediate set-

tling tanks post denitrification process with addition of ex-

ternal carbon sources in aa) fixed bed reactorb) activated sludge tank

Attention is also drawn for these process tech-niques to two ATV Reports [neither available inEnglish]: Umgestaltung zweistufiger biologischerKlranlagen zur Stickstoffelimination [Conversionof two-stage biological wastewater treatmentplants for phosphorus removal] [5] and ATV Re-port Denitrifikation bei Tropfkrperanlagen [Deni-trification with trickling filter facilities] [6]. Below,only pre-anoxic denitrification itself is covered in ananoxically operated trickling filter.

To use existing trickling filters for targeted denitrifi-cation as a rule only small conversion measuresand an appropriate operation are required.

Through the prevention of the inflow of air (cover-ing of the trickling filter and prevention of the air in-flow through the outlet and lower air openings,usually already achievable through impounding ofthe outlet channels around the trickling filter) it ispossible to set anoxic conditions on the inside oftrickling filters, if recirculated effluent containing ni-trate of a downstream nitrifying treatment unit to-gether with the mechanically treated wastewater isapplied to the trickling filter.

An impounding of the trickling filter filler materialinvolves the danger of blockage and would, in

most cases, lead to static problems; it thereforeshould not take place. The partially treated effluentfrom upstream denitrifying trickling filters is fed viaan intermediate settling tank or directly to a subse-quent aerated nitrifying treatment unit. As a rulethese are trickling filters or activated sludge plants.

The following dimensioning values are given fordenitrification in trickling filters:

The achievable denitrification capacity is de-

pendent on the BOD5 volumetric loading andcan be determined using the values in Table 2.With this, the BOD5 removal in addition to theBOD5-loading is also dependent on whether theeffluent of the denitrification trickling filter followsan intermediate settling tank.

The daily average nitrate concentration to bedenitrified results as follows:

SNO3,D= CN,InB SorgN,ESST SNH4,ESST

SNO3,ESST XorgN,BM [mg/l] (7)

As influent nitrogen concentration (CN,InB) therelevant value determined for T = 12 C is to beapplied. If, during the year, at times of highertemperatures, higher CN,InB : CBOD,InBratios havebeen determined several load cases are, ifnecessary, to be considered.

The influent nitrate concentrations (SNO3,InB) is, ingeneral, negligibly small. With greater infiltrationrates (groundwater containing nitrate) or with in-flows from certain commercial and industrial plantsit can be necessary to take account of SNO3,InB in

CN,InB.


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At plants with anaerobic sludge digestion and me-chanical dewatering at the site, the nitrogen of thesludge liquor must be contained in the inflow con-centration (CN,InB) if no separate sludge liquortreatment takes place.

The concentration of organic nitrogen in the ef-fluent can be set as SorgN,ESST= 2 mg/l. With theinflow of certain commercial wastewater theconcentration can be higher. To be on the safeside, the ammonium content in the effluent fordimensioning is, as a rule, assumed asSNH4,ESST= 0. The nitrogen incorporated in thebiomass is taken into account simplified asXorgN,BM= 0.03 CBOD,InB.

The relevant effluent concentration of nitrate

nitrogen is to be applied as daily average. If, asin Germany, the monitoring takes place as grabor 2 hour composite samples, a significantlysmaller concentration than the monitoring value[effluent requirement for inorganic nitrogen(SinorgN,MV)]. It is practical to setSNO3,ESST = 0.8 to 0.6 SinorgN,MV,whereby the smaller value applies for plantswith greater variations in the influent load.

Surface loading rate qA,BF< 3 m/h. Too high oxygen transfer in the trickling filter is

to be avoided; therefore the recirculation ratio

should always be optimised and RRDW = 3, re-lated to QDW, should not be exceeded.

Table 2: Recommended values for the

dimensioning of the necessary

denitrification volume VTF,D

BOD5-volumetric

loading

Denitrificationcapacity

BOD5-removal BOD5-removal

without withintermediate

settlingintermediate

settling

kg/(m3.d) SNO3,D/CBOD,InB % %0.2 0.14 60 800.6 0.10 45 651.0 0.08 40 60

The values of Table 2 are valid for 12 C and anitrate concentration in the effluent of denitrificationtrickling filters 2 mg/l N. Intermediate values areto be interpolated.

The dimensioning of downstream treatment unitsfor nitrification and carbon oxidation can take place

taking into account previous treatment steps in ac-cordance with Sections 5.2.2 and 5.2.3 for trickling

filters and in accordance with ATV-DVWK Stan-dard ATV-DVWK-A 131E for activated sludgeplants.

With downstream trickling filters for nitrification therecirculation should be taken directly from the ef-fluent of the trickling filter to relieve the hydraulicload the settling tank.

It can be an advantage to feed the internal recircu-lation via the primary settling tank. With this, anadditional denitrification can be achieved, to alesser degree however and not capable of estima-tion. The additional hydraulic loading of the primarysettling tank is to be taken into account here.

With downstream nitrifying activated sludge plants

the recirculation should, in general, be taken fromthe effluent of the secondary settling tank. This is tobe taken into account with the dimensioning of thesecondary settling tank. With the employment of asuitable filler material (plastic), however, in principlea recirculation with activated sludge is also capableof being carried out after successfully executed pre-trials.

With the design and with the operation of tricklingfilters for denitrification attention is to be paid that,

following an opening of the trickling filter for thecleaning of the rotary distributor or similar, denitrifi-cation is not possible or not possible to the full ex-tent until the oxygen has again been fully depletedfollowing closure.

6 Rotating biologicalcontactors

6.1 Description of the Process

6.1.1 General

With the treatment of wastewater using rotatingbiological contactors the biomass required for thebiological wastewater treatment is firmly attachedto the rotating growth surfaces. Rotating biologicalcontactors are partially submerged and slowly ro-tate in a trough through which the wastewater

flows. The biofilm attached to the growth surfaces


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is subjected alternately to air and wastewater dur-ing rotation.

With the rotation of the rollers, following contactwith the wastewater on immersion, there is respec-tively an aeration phase above the water level. Theoxygen supply taken up with this must be sufficientto cover the depletion process during the sub-merged stage and for the maintenance of theaerobic conditions in the trough. This and themaintenance of a thin biofilm require a minimumrotation rate of the roller. The oxygen transferthrough the rotation is sufficient and is not a limit-ing factor for the nitrification, if at least 40 % of thediscs/roller surface is permanently out of the water.

The activated sludge, which is held suspended in

the water in the trough, plays only a small part inthe treatment performance and, from the technicalaspect of dimensioning, remains unconsidered.

Trough and biological contactor are to be so de-signed and/or the revolution rate is to be so se-lected, that a sufficient turbulence is guaranteed inorder that the settling of sludge in the biologicalcontactor and in the trough is prevented.

Biological contactors should be covered as prob-

lems with ice formation can occur in winter withopen biological contactors. The resultant gaseousmetabolic products also escape into the air spaceof the cover. In order to prevent a hazardous accu-mulation of the gaseous products of wastewatertreatment and always to provide sufficient atmos-pheric oxygen, an uninterrupted exchange of air inthe space above the rotation trickling filter is to beensured.

Favourable volumetric degradation performanc canbe achieved through the compact construction of

rotating biological contactors. Very high volumetricloadings and thus short retention times lead, how-ever, to only a slight equalisation of loading peaks.

Rotating biological contactors usually consist of 2to 4 sequentially arranged rollers in separatetroughs (cascade arrangement). The cascade typeof construction enables the realisation of varioussurface or volumetric loadings and carrier materialpacked to different densities. Then, on each roller,there is another growth to be found corresponding

with the degree of pollution of the wastewater. In

addition, cascades reduce the effects of loadingpeaks.

It can be an advantage to feed back from the efflu-ent of the last rotating biological contactor into theinfluent to the primary settling tank. With this anevening out of the hydraulic loading, a reduction ofthe danger of blockage and a reduction of peakloadings can be achieved. The additional hydraulicloading is to be taken into account.

With rotating biological contactors precautions areto be taken that the rollers, also after long idle pe-riods, can be taken into operation again withoutadditional measures.

6.1.2 Material and Types

Today, in addition to discs, rotating biological con-tactors made from profiled, wrapped foils as wellas made from corrugated or grid-shaped pipes areemployed. In any case attention is to be paid thatthe selected structure permits a secure removal ofwaste sludge, the oxygen transfer to the biofilm isguaranteed and, at the same time, the energy con-sumption of the system is kept as small as possi-ble.

Disc biological contactors

Disc biological contactors have discs mounted ona shaft with a non-structured, smooth or rough-ened surface which, following overgrowth by theactive biofilm, has a smooth appearance. Thediscs can also be made up from segments fitted tothe shaft by a girder construction. The discs nor-mally are flat, but can also have a shape whichsatisfies the demands of a flat disc, (e.g. roughlycorrugated discs). In particular the distance be-

tween discs may not be altered by the shape and,apart from the spacers, the distance may not haveany further bridging. The discs are separated fromeach other by spacers. The distance betweendiscs depends on the organic surface loading rateof the respective stage of a cascade. With disc bio-logical contactors the disc surface correspondsapproximately with the biologically active surface.

Roller biological contactors

In a roller body which is permeable to water there

are shaped, fixed or also movable filler material on


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whose surface the biomass grows. They can con-currently serve for oxygen transfer and the genera-tion of turbulence. With roller biological contactorsthe biologically active surface cannot be derivedfrom the theoretical surface (for this comp. Section5.1.2). The biologically active surface is, as a rule,smaller than the theoretical surface of the sfillermaterial. It is subjected to seasonal variations andoften first forms after longer operating periods. Aslong as no other data can be supplied a reductionof the permitted surface loading rate by 30 %, re-lated to the values for the permitted surface load-ing rate of disc biological contactors, is first as-sumed generally for the dimensioning inaccordance with Sections 6.2.2 and 6.2.3.

Depending on the degree of loading of the treat-

ment stage (cascade), materials with varyinglylarge surfaces can be employed. With high organicsurface loading rates or large specific theoreticalsurfaces there is a danger of blockage. This in-creases with unfavourable structural formation ofthe material which prejudices the removal of thesolids formed.

Suitable precautions are to be taken which preventblockage such as, for example, flushing facilities.

6.2 Dimensioning

6.2.1 General Details on Dimensioning

Concerning the comparability of the dimensioningproposals there is a similar problem with regard tothe biologically active surface as with trickling filterfiller material.

The dimensioning values presented below havebeen determined for disc biological contactors. Inthe meantime there are numerous other rotatingbiological contactors functioning satisfactorily, sothat the dimensioning rules are transferable if thetreatment performance can be derived from refer-ence plants operated over long periods.

In order to prevent the effects of peak loadings atrough volume of some 4 l per m2theoretical sur-face should not be undercut.

In the first place the following listed dimensions are

to be determined or specified for the dimensioning:

number of stages (roller, cascades) and specific theoretical surface of the roller material

used or the minimum separation of the discs.

Odour nuisances can occur with this process withhigh loadings, so that it is advisable to limit theBOD5 surface loading rate of the first stage withdomestic wastewater to a value of 40 g/(m2d).

From BOD5surface loading rates 20 g/(m2d) in

one stage, a minimum separation of discs of 18 mm is recommended for disc biological con-tactors. With this loading roller biological contac-tors are to be designed for specific theoretical sur-face of 100 m2/m3, unless it can be proved that ahigher surface is permanently available.

With BOD5 surface loading < 20 g/(m2d) in onestage, a disc separation of 15 mm and a specifictheoretical surface of 150 m2/m3 is recom-mended.

It can be an advantage to install an intermediatesettling tank between the units for carbon removaland nitrification. In this way the waste sludge canbe separated which increases the performance ofthe nitrification units. In such a nitrification unit theseparation of the discs can be reduced to 10 mm

and the specific theoretical surface of the rollersincreased to a maximum of 200 m2/m3.

Subsequently the BOD5surface loading rate BA, tobe determined dependent on the treatment target,is relevant for the determination of the requiredtheoretical surface.

The necessary theoretical surface ARC is deter-mined as follows:

Bd,BOD,InB1000

ARC,C= ___________________[m2] (8)BA,BOD

In addition with nitrification:

Bd,TKN,InB1000ARC,N=

___________________[m2] (9)BA,TKN

The overall necessary theoretical surface is calcu-lated from the BOD5daily load flowing into the re-actor and the TKN daily load flowing into the reac-


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tor as well as from the permitted surface loadingrates BA,BODand BA,TKN:

ARC= ARC,C+ ARC,N [m2] (10)

6.2.2 Wastewater Treatment withoutNitrification

2 to 4 stages are to be assumed for the dimension-ing of the theoretical surface. With this the follow-ing dimensioning values are recommended:

For disc biological contactors:

2 stage cascades:BA,BOD 8 g/(m

2d)

3 and 4 stage cascades:

BA,BOD10 g/(m2 d)

With small wastewater treatment plants, due to themarked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5surface load-ing rate from 8 respectively 10 g/(m2d) to4 g/(m2d) between capacities of 1,000 and 50 PT.

For other rotating biological contactors:

2 stage cascades:BA,BOD5.6 g/(m

2d)

3 and 4 stage cascades:BA,BOD7 g/(m

2d)

With small wastewater treatment plants, due to themarked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5surface load-ing rate to 3 g/(m2 d) between connection capaci-ties of 1,000 and 50 PT.

6.2.3 Wastewater Treatment with Nitrifi-cation

If nitrification is necessary then a three or fourstage cascade plant is advisable for the dimen-sioning of the theoretical surface. The followingdimensioning values are recommended:

For disc biological contactors:

3 stage cascades:BA,BOD8 g/(m

2d) and BA,TKN1.6 g/(m

2d)

4 stage cascades:

BA,BOD10 g/(m

2

d) and BA,TKN2 g/(m

2

d)

With small wastewater treatment plants, due to themarked inflow and loading peaks, it is recom-mended to reduce linearly the BOD5surface load-ing rate from 8 respectively 10 g/(m2d) to4 g/(m2d) and the TKN surface loading rate to1.6 respectively 2 g/(m2d) to 1.2 g/(m2d) betweencapacities of 1,000 and 50 PT.

For other rotating biological contactors:

3 stage cascades:BA,BOD5.6 g/(m

2d) und BA,TKN1.1 g/(m

2d)

4 stage cascades:BA,BOD7 g/(m

2d) and BA,TKN1.4 g/(m

2d)

With small wastewater treatment plants, due to themarked inflow and loading peaks, it is recom-

mended to reduce linearly the BOD5surface load-ing rate to 3 g/(m2d) and the TKN surface loadingrate from 1.1 respectively 1.4 g/(m2d) to0.85 g/(m2d) between capacities of 1,000 and50 PT.

If, for the individual growth materials, it is verifiedthat the specific biologically active surface is per-manently more than 70 % of the specific theoreti-cal surface, the dimensioning values can be raisedcorrespondingly to a maximum of the values validfor disc biological contactors.

The values for BA,TKN take into account a nitrifica-tion already started in the carbon removal zone.The permitted surface loading rate BA,TKN for thedimensioning is not identical with the rate ofnitrification.

A biological denitrification as with trickling filters isalso possible with rotating biological contactors.Technical testing is, however, still outstanding.

Note on alkalinity: the remaining alkalinity in the ef-

fluent of nitrifying rotating biological contactorsshould, if necessary taking into account precipi-tants, not undercut 0.5 mmol/l in order to excludepossible inhibition of the nitrification. If required thisis to be ensured through suitable measures. It isrecommended that such measures are imple-mented following availability of relevant operationalexperience.


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7 Phosphorus Removal

With fixed bed systems the P-removal can beachieved reliably only through chemical precipita-tion. For this the addition of precipitating chemicalsin the effluent of reactors before the secondary set-tling and/or preliminary precipitation is advisable. Ifnecessary the optimum dosing point is to be de-termined through trials. With preliminary precipita-tion an undersupply of the fixed bed biology withphosphorus due to the precipitation is to be pre-vented. ATV Standard ATV-A 202 [currently notavailable in English] is to be observed.

Due to the slight sensitivity to low pH values,measures to raise the alkalinity as a rule are not

necessary with trickling filters and rotating biologi-cal contactors. In particular, with the application ofacidic precipitants, however, attention is to be paidthat a residual alkalinity is retained in the effluentof the secondary settling tank. Possibly, the addi-tion of alkaline precipitant or basic neutralisationagent is necessary

8 Waste Sludge

Production

The size of the possible range of variation withwaste sludge production is, based on biology, de-pendent on the type of wastewater, the loadingand the hydraulic conditions. The biological wastesludge production, related to the loading valuesrecommended here, in the absence of measuredresults with eliminated BOD5 incl. stormwatertreatment, can be assumed to be 0.75 kg SS perkg. The calculation of the precipitation sludge pro-duced can take place in accordance with ATVStandard ATV-A 202. A simultaneous sludge stabi-lisation is not possible as primary and secondarysludge is produced separately.

9 Secondary SettlingTanks

9.1 General

Secondary settling tanks of trickling filters and ro-tating biological contactors have the task of sepa-rating the waste sludge, removed from the biologi-cal reactor with the treated wastewater, from thewastewater. In comparison with activated sludgeplants the secondary settling tanks of trickling fil-ters and rotating biological contactors are fed withsignificantly smaller quantities of sludge with set-tling characteristics which are normally withoutproblem. Sedimentation of the individual small par-ticles here is of decisive significance for the reten-tion performance.

Due to the small particle concentration, coagula-tion and precipitation with mixed liquor suspendedsolids contents of ca. 30 to 100 mg/l SS in the ef-fluent of single-stage trickling filters and rotatingbiological contactors have no relevant effect. Theyare to be taken into account only with very exten-sive requirements on solids removal.

Particle concentration and the coagulation effect

can be increased significantly through sludge re-circulation or addition of precipitants in the influentof the secondary settling setting tankstage or intoan upstream precipitation chamber.

Through this the retention of solids can be im-proved (see also ATV Advisory Leaflet ATV-M 274Einsatz organischer Polymere in der Abwasserre-inigung [Application of Organic Polymers inwastewater Treatment - currently not available inEnglish]).


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9.2 Dimensioning of the SecondarySettling Tank of Single-stageTrickling filters and Rotatingbiological contactors

If trickling filters and rotating biological contactorsare employed with normal municipal conditions assingle-stage plants for biological pre-treatment ornitrification, then the secondary settling tank canbe dimensioned simply according to pure hydraulicaspects with the surface loading rate qAand flowtime tSST.

The surface loading rate of the secondary settlingtank is made up from the quotients of the maxi-mum hourly inflow to the secondary settling tankQSST including all return flows which flow into thesecondary settling tank, and the surface area ofthe secondary settling tank ASST. The wet weathercase is also to be taken into account and, in mostcases, decisive for dimensioning.

Thus, with the surface loading rate

qA,SSTQSST/ ASST [m3/(m2h) or m/h] (11)

the respectively greater value is to be applied for

QSST= QDW (1+RRDW) or QSST= Qm (1 + RRm)

Through the performance of the (trickling filter-) feedpumps and an appropriate regulation (e.g. floatvalve or recirculation pumps), it is to be ensuredthat the recirculation ratio does not exceed the se-lected dimensioning value.

With trickling filters or rotating biological contactorsthe surface loading rate of the secondary settlingtank may not exceed 0.80 m/h if effluent limitingvalues of SSe< 20 mg/l are to be maintained [2].

The required tank surface area results as

ASST,nec= QSST/ qA,SST, perm [m2] (12)

The flow time can be defined as theoretical flowtime in the secondary settling tank as

tSST= VSST/QSST [h] (13)

It should not be less than 2.5 h.

The required tank volume thus results as

VSST,nec= tSSTQSST [m3] (14)

The minimum depth of water hSSTis 2.0 m (in circu-lar tanks 2/3 of the radius).

If flocculation is carried out through dosing ofphosphate precipitants or polymers into the influentto the secondary settling tank, the surface loadingrate can be increased to 1.00 m/h, if the secondarysettling tank maintains a minimum depth of waterof hSST 2.50 m (in circular tanks 2/3 of the ra-dius).

In the case of intermediate settling tanks or withshort retention times using downstream ponds,surface loading rates of 1.5 to 2.0 m/h can be se-lected with appropriately reduced flow times.

With constraints which lead to increased loading of

the trickling filter or rotating biological contactorand unfavourable conditions for the settling proc-ess in the secondary settling tank (e.g. preliminarysettling with tSST< 0.75 h with Qm, higher combinedwastewater inflow greater than 2.2 QDW, small re-circulation ratio with QDW), the permitted surfaceloading rate of the secondary settling tank shouldbe reduced by up to 20 %.

With existing secondary settling tanks the dimen-sioning values can be determined through full-

scale loading trials.

9.3 Notes on Tank Shape and Design

The structural aspects which effect the dimension-ing or which are assumed for the dimensioning aredealt with within the scope of this standard. Furtherplanning aspects of construction and design, forexample due to space and underground condi-tions, progress of construction, traffic safety orsimilar are not listed here explicitly; for this see the

ATV Manual [2].

The tank shape is not decisive for the settling effi-ciency and sludge collection in secondary settlingtanks after trickling filters and rotating biologicalcontactors. Even with vertical flow secondary set-tling tanks no better effluent values can beachieved than with secondary settling tanks, solong as with these the minimum retention time ismaintained. The reason for this lies in that, due tothe small particle concentrations no floc filter can

be installed. With trough-shaped tanks with steep


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slopes (at least 60), however, no mechanicalsludge removal is necessary.

Compared with rectangular tanks circular tanks of-fer the advantage of a smaller weir overflow rateand are frequently somewhat more cost effective,on the other hand, however, there is the greatersensitivity to wind and the greater space require-ment. As a continuous sludge recirculation is notnecessary with trickling filters and rotating biologi-cal contactors, at most simple sludge scrapers sur-fice even with rectangular tanks.

With rectangular tanks the ratio of the depth of thetank to the length of the tank should be ca. 1:15 to1:25. For the width of the tank values up to 7.0 mhave proved themselves in practice.

An as even as possible distribution of the inflowover the cross-section of the flow is to be sought.The weir overflow rate qWOmust be smaller than15 m3/(m h).

As only the waste sludge is to be separated out inthe secondary settling stage tank trickling filtersand with rotating biological contactors, cloth filtersor microstrainers can be considered in place of thesecondary settling tanks. References and appro-

priate functional verifications are to be requested,operating safety and maintenance expense are tobe observed.

So far as further treatment stages are addeddownstream the employment of lamella separatorsis also possible to reduce the space requirement, ifthe increased maintenance expense is accepted.With regard to the surface loading rate these are tobe dimensioned precisely as for secondary settlingtanks. For this see the ATV Manual [2].

10 Costs andEnvironmental Effects

With this Standard planners and examiners receivea differentiated working basis for the dimensioningof trickling filters and rotating biological contactors.From this they can, from the process technical as-pect, develop the most sustainable and most eco-nomical solution with regards to the required envi-

ronmental protection.

The requirements on the quality of the water to bedischarged into surface waters are not establishedin this Standard; they are legally prescribed either([German] Wastewater Ordinance) or are specifiedby the authorities. This Standard is aimed at a se-cure and economical observance of these specifi-cations.

11 Relevant Regulations,Directives and StandardSpecifications

[Translators note: those references available in English are shown assuch. Otherwise a courtesy translation is provided in square brackets.]

Abwasserverordnung [(German) WastewaterOrdinance]

Ordinance on the requirement on the discharge ofwastewater into surface waters (AbwV). Bundes-gesetzblatt 1999, Part 1, No. 6 dated 18.02.1999

ATV-DVWK Standards

ATV-A 122EPrinciples for Dimensioning, Construction and Op-eration of Small Sewage Treatment Plants with

Aerobic Biological Purification Stage for Connec-tion Values between 50 and 500 Inhabitants andPopulation Equivalents, Issue 6/91

ATV-A 129Abwasserbeseitigung aus Erholungs- und Frem-denverkehrseinrichtungen [Wastewater DisposalFrom Recreation and Tourist Facilities], Issue5/1979

ATV-DVWK-A 131EDimensioning of Single-Stage Activated Sludge

Plants, 5/2000

ATV-A 257EPrinciples for the Dimensioning of Wastewater La-goons and Intermediate Trickling Filters or Biologi-cal Contactors, Issue 10/1989

ATV-A 202Verfahren zur Elimination von Phosphor aus Ab-wasser [Processes for the Removal of Phosphorusfrom Wastewater], Issue 10/1992


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ATV-M 271Personalbedarf fr den Betrieb kommunalerKlranlagen [Personnel Requirement for the Op-eration of Municipal Sewage Treatment Plants], Is-sue 9/1998

ATV-M 274Einsatz organischer Polymere in der Abwasserre-inigung [Employment of Organic Polymers inWastewater Treatment], Issue 11/1999

Standard specifications

EN 1085Wastewater treatment - Vocabulary

DIN 4045

Wastewater engineering - Vocabulary

DIN 4261-2Small sewage treatment plants Plants with sew-age aeration; application, design; construction andtesting

DIN 18202Dimensional tolerances in building construction -Buildings

DIN 19553

Klranlagen Tropfkrper mit Drehsprenger,Hauptmae [Sewage treatment plants Tricklingfilters with rotating sprinklers, main dimensions]

DIN 19557-1Sewage treatment plants mineral filter media forpercolating filters requirements, testing, delivery,placing

DIN 19557-2Klranlagen Fllstoffe aus Kunststoff fr Tropf-

krper Anforderungen, Prfungen [Sewagetreatment plants filter materials for trickling filters requirements, tests]

DIN 19558berfallwehr mit Tauchwand, getauchte Ablaufroh-re in Becken; Baugrundstze, Hauptmae, An-wendungsbeispiele [Overflow weir with scumbo-ard, submerged outflow pipes in tanks;construction principles, main dimensions, e-xamples of application]

DIN 19569-1Principles for the design of structures and technicalequipment for sewage treatment plants; generalprinciples

DIN 19569-2Principles for the design of structures and equip-ment for sewage treatment plants; installations forseparating and thickening solids

DIN 19569-3Baugrundstze fr Bauwerke und technische Aus-rstung; Besondere Baugrundstze fr Einrichtun-gen zur aeroben biologischen Abwasserreinigung[Principles for the design of structures and techni-cal equipment for sewage treatment plants; specialconstruction principles for installations for aerobic

biological wastewater treatment]

Literature

[1] ATV (Publ.):ATV-Handbuch Biologische und weiterge-hende Abwasserreinigung [ATV Manual Bio-logical and advanced wastewater treatment].

4th Edition, Berlin: Ernst & Sohn, 1997[2] ATV (Publ.):

ATV-Handbuch Mechanische Abwasserreini-gung [ATV Manual Mechanical wastewatertreatment] 4th Edition, Berlin: Ernst & Sohn,1997

[3] ATV ReportMehrstufige biologische Klranlagen [Multi-stage biological wastewater treatment plants]Korrespondenz Abwasser 2/1989, p. 181-189

[4] Standard ATV-DVWK-A 198E

Standardisation and Derivation ofDimensioning Values for Wastewaterfacilities, 2003

[5] ATV ReportUmgestaltung zweistufiger biologischerKlranlagen zur Stickstoffelimination[Conversion of two-stage biologicalwastewater treatment plants for phosphorusremoval] Korrespondenz Abwasser 1/1994,p. 95-100


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[6] ATV ReportDenitrifikation bei Tropfkrperanlagen [De-nitrification with trickling filter facilities] Kor-respondenz Abwasser 11/1994, p. 2077-2081

[7] ATV-DVWK ReportRckbelastung aus der Schlammbehandlung Menge und Beschaffenheit der Rcklufe[Return loading from sludge treatment quantity and characteristics of the returnflows] KA-Wasserwirtschaft, Abwasser, Abfall8/2000, p. 1181-1187

The above Standard ATV-DVWK-A 281E replaces the draft of ATV Standard ATV-A 135 and contains up-dated approaches to dimensioning for trickling filters and rotating biological contactors with secondary set-tling.

The advantages of the treatment of wastewater in fixed bed reactors in general lie in the small energy con-sumption and the simple and stable method of operation. With the trickling filter process the wastewater issprinkled over the filler material. With this the necessary oxygen is taken up passively. An active aerationusing energy is, as a rule, not required. On the other hand, with rotating biological contactors, the disks orrollers up to a half submerged in a wastewater trough, are rotated about their longitudinal axis using en-ergy. Aeration also takes place passively during the emerged phase.

Trickling filter and rotating biological contactor facilities enable the colonisation with micro-organisms whichhave long generation times. Thus even compounds which are difficult to degrade can be eliminated withsmall loading.

The standard in addition contains details fort he dimensioning of trickling filters with denitrification. It shouldbe emphasised, with the dimensioning of secondary settling tanks for trickling filters and rotating biologicalcontactors that, based on the results of new investigations, the necessary tank surface has been increasedand the tank depth reduced.

With trickling filters the dimensioning depends on the filler material used. The Standard shows how to takeinto account adequately the characteristics of the various obtainable filler materials.


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