Regelwerk - ATV-A 126e

8/9/2019 Regelwerk - ATV-A 126e

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ATV RULES AND STANDARDS

W A S T E W A T E R - W A S T E

ATV STANDARD

ATV-A 126

Principles for Wastewater Treatment in SewageTreatment Plants According to the Activated

Sludge Process with Joint Sludge Stabilisationwith Connection Values between 500 and 5000Total Number of Inhabitants and PopulationEquivalents

December 1993ISBN 3-934984-15-0

Marketing:

Gesellschaft zur Förderung der Abwassertechnik e.V. (GFA)

Theodor-Heuss-Allee 17

D-53773 Hennef

Postfach 11 65 . 53758 Hennef

Grundsätze für die Abwasserbehandlung in Kläranlagen

nach dem Belebungsverfahren mit gemeinsamer

Schlammstabilisierung bei Anschlusswerten

zwischen 500 und 5000 Einwohnerwerten


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The ATV Working Group 1.7.4 "Operation and Maintenance of Pumping Stations,Pressure Pipelines and Stormwater Tank Systems" has prepared the following Standard.

These principles have been prepared by the ATV Specialist Committee 2.10 „SmallSewage Treatment Plants“ to which the following members belong:

Dipl.-Ing. Albrecht, EssenDr.-Ing. Baumgart, EssenLBDir Bucksteeg, MünchenDipl.-Ing. Grosche, RadebeulRD Dr. Kollatsch, Halle/SDipl.-Ing. Maus, Arnsberg (Chairman)LBDir Schweizer, RottweilLRBDir Tiedke, MindenDipl.-Ing. Tuttahs, BochumLBDir Voss, Kiel

Dr.-Ing. Zerres, Stuttgart

The Standard presented here has been prepared within the frameworkof the ATV committee work, taking into account the ATV Standard A400 "Principles for the Preparation of Rules and Standards" in theRules and Standards Wastewater/Wastes, in the January. 1994.version. With regard to the application of the Rules and Standards,

Para. 1 of Point 5 of A 400 includes the following statement "TheRules and Standards are freely available to everyone. An obligation toapply them can result for reasons of legal regulations, contracts orother legal grounds. Whosoever applies them is responsible for thecorrect application in specific cases. Through the application of theRules and Standards no one avoids responsibility for his own actions.However, for the user, prima facie evidence shows that he has takenthe necessary care.

All rights, in particular those of translation into other languages, are reserved. No part ofthis Standard may be reproduced in any form by photocopy, microfilm or any otherprocess or transferred or translated into a language usable in machines, in particulardata processing machines, without the written approval of the publisher.

Gesellschaft zur Förderung der Abwassertechnik e.V. (GFA), Hennef 1993

Produced by: Rheinischer Landwirtschafts-Verlag G.m.b.H., Bonn


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Contents

Preface ............................................................................................................................5

1 Area of Application ...................................................................................................6

2 Processes.................................................................................................................7

3 Dimensioning............................................................................................................8

3.1 Loading Values .................................................................................................8

3.1.1 Wastewater Inflow......................................................................................8

3.1.2 Water Pollution...........................................................................................9

3.1.3 Sludge Yield ...............................................................................................9

3.2 Activated Sludge Section ................................................................................10

3.2.1 Aeration Tanks.........................................................................................10

3.3 Secondary Sedimentation ...............................................................................12

3.4 Sludge Storage Reservoir ...............................................................................15

4 Construction Principles...........................................................................................15

4.1 General ...........................................................................................................15

4.2 Primary Pumping Stations...............................................................................15

4.3 Flow Measurement..........................................................................................16

4.4 Stormwater Treatment ....................................................................................16

4.5 Mechanical Pre-treatment ...............................................................................17

4.5.1 Screens, Sieves .......................................................................................17

4.5.2 Grit Chamber ...........................................................................................17

4.5.3 Grease Trap.............................................................................................17

4.6 Aeration Tanks................................................................................................18

4.7 Secondary Sedimentation Tanks ....................................................................18

4.8 Operational Building and Ancillary Works .......................................................19

4.9 Other Structures for the Safeguarding of Operations......................................19


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5 Sludge Treatment and Disposal .............................................................................19

5.1 Sludge Yield....................................................................................................19

5.2 Sludge Consistency and Dewatering Capability..............................................20

5.3 Sludge Storage ...............................................................................................20

5.4 Sludge Dewatering..........................................................................................21

6 Operation ...............................................................................................................21

7 Symbols .................................................................................................................22


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Preface

The ATV Specialist Committee 2.10, from the start of its work and now for over twodecades, has followed the development and the practical application of activated sludgeplants with joint sludge stabilisation.

With the introduction of Appendix 1 of the „General Basic Administrative Regulations onthe Minimum Requirements for the Discharge of Wastewater into Lakes and Rivers“, theclassification sizes of sewage treatment plants were regraded and the respectivelyassociated minimum requirements were again tightened. ATV Standards A 126 and A131 must therefore be revised and newly matched to each other for limitations. Thus, thelimit of the area of application was reduced from the previous 10,000 to 5,000 totalnumber of inhabitants and population equivalents (IPE).

ATV Specialist Committee 2.6 has thoroughly reworked ATV Standard A 131„Dimensioning of Single-Stage Activated Sludge Plants with More Than 5000 Total

Number of Inhabitants and Population Equivalents“ with regard to deliberate nitrogen andphosphorus removal. The new version of this Standard was published in February 1991.

The new version of ATV Standard A 126 presented here „Principles for WastewaterTreatment in Sewage Treatment Plants According to the Activated Sludge Process withJoint Sludge Stabilisation with Connection Values between 500 and 5000 Total Numberof Inhabitants and Population Equivalents“ takes into account, above all in comparisonwith the area of application of ATV Standard A 131, different minimum requirements forplants of expansion sizes < 5000 IPE and the resultant process-technical concept.

The following, essential aspects mark this Standard:

- ATV Standard 126 regulates dimensioning, construction and operation of thecomplete sewage treatment plant. The regulations are kept deliberately simple andclear.

- As small sewage treatment plants are fundamentally more operationally sensitivethan large plants due to larger variations in loading and less staff, simpleconstructional design and robust mechanical and electrical equipment and simpleoperation have priority over precise process technology.

- Nitrogen removal is not generally demanded with sewage treatment plants < 5000

IPE. The run-offs of small sewage treatment plants contribute in sum to a minordegree only to the nitrogen loading of lakes and rivers. However, the operationaladvantage of denitrification should be used for the stabilisation of the treatmentprocess and for the reduction of the wastewater discharge. This statement does notcontradict the requirement, in individual cases, for specific (thoroughly optimised)nitrogen removal from pollution control areas.

- Without a thoroughly optimised denitrification a dimensioning parameter for thesludge loading corresponding with the previous dimensioning recommendations ofBDS = 0.05 kg/(kg.d), in deviation to that of ATV Standard A 131, can remain retainedin ATV Standard A 126. Operationally desired denitrification takes place with sewage

treatment plants < 5000 IPE, with regard to the requirement for simplicity inconstructional design, equipment and operation, for practical reasons simultaneously.


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If, in individual cases, a thoroughly optimised denitrification is necessary then thedimensioning principles of ATV Standard A 131 can be applied.

- Small sewage treatment plants are, in the main, newly constructed in comparisonwith the large ones for which mainly inventory expansion and rehabilitation are fitting.In the future, even more than the average number of plants of small developmentsize will be built, in particular for the improvement of wastewater treatment in ruralareas as, inter alia, a balanced water management, the ecological retention of smallflowing waters or economic reasons support this. Statistic data evaluations can - asproposed in ATV Standard A 131 - often not be furnished for the design of smallsewage treatment plants. The initial loads and the development values must then bedetermined via formulations from experience. Thus the application of simpledimensioning rules can also be practical.

With small sewage treatment plants, which are dimensioned, constructed and operatedaccording to the recommendations of this Standard, discharge values which meet the

current requirements of sewage treatment plant size classifications 1 and 2 according to Appendix 1 of the Basic Wastewater Administrative Regulations (Rahmen-AbwVwV) canbe met with careful operation. The practice oriented determinations should here serve sothat defective cheap solutions are avoided equally as exaggerated, luxury solutions;rather, the bandwidth for water quality managerially necessary, technically practical andeconomically suitable planning, construction and operation should be shown.

The November 1987 Edition of ATV Standard A 126 is replaced with the publication ofthis Standard.

1 Area of Application

This Standard applies for the planning, construction and operation of sewage treatmentplants according to the activated sludge process with joint sludge stabilisation withconnection values between 500 and 5000 total number of inhabitants and populationequivalents (IPE).

The given area of application of between 500 and 5000 IPE does not represent therespective upper and lower application limit for this process. Activated sludge plants with joint sludge stabilisation can also be suitable for connection values over 5000 IPE. Thelower limit of the area of application of 500 IPE is derived from the fact that, as a rule,

small plants cannot be fed with stormwater.

The following summary gives an overview of the standard specifications and regulationsin which stipulations are made for activated sludge plants with joint sludge stabilisationfor connection values outside those applicable for ATV Standard A 126.

ATV Standard A 131

Dimensioning of Single-Stage Activated Sludge Plants with Connection Values over 5000Total Number of Inhabitants and Population Equivalents.


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ATV Standard A 122

Principles for the Dimensioning, Construction and Operation of Small Sewage TreatmentPlants with Aerobic Biological Treatment Stage for Connection Values between 50 and500 Total Number of Inhabitants and Population Equivalents.

DIN 4261, Part 2/Part 4

Small Sewage Treatment Plants with Wastewater Aeration - Application, Dimensioning,Execution and Test/Operation and Maintenance (Area of application up to 8 m3/ddomestic and industrial wastewater inflow; this corresponds to a connection value up tosome 50 inhabitants)

In addition the following standard specifications and regulations are still to be applied incertain cases of application:

ATV Standard A 109

Standards for the Connection of Autobahn Facilities to Sewage Treatment Plants

ATV Standard A 123

Treatment and Disposal of Sludge from Small Sewage Treatment Plants

ATV Standard A 129

Wastewater Disposal from Recreational and Tourist Installations

DIN 19520

Abwasser aus Krankenanstalten, Richtlinien für die Behandlung [Wastewater fromHospitals, Standards for Treatment]

In addition attention is drawn to:

DIN 19569

Principles for the design of structures technical equipment for sewage treatment plants;General principles.

2 Processes

The activated sludge process with joint sludge stabilisation is characterised in that thesludge loading in the aeration tanks is low and a sludge digestion can be dispensed with.From this there results:

- a good treatment performance,as very low BOD5 and COD run-off values are achieved;

- a large loading margin,


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as the loading variations, which are typical with small connection values, are pickedup in the large volume aeration tanks;

- a high operational safety,

as large buffer capacities as well as simplicity of process and the operation of the

overall plant confer a high degree of operational safety and process stability;

- a simple stabilisation of the sludge,

as there is no primary sedimentation tank, there is therefore no resultant raw sludgeto be treated and the surplus sludge removed from the aeration tanks is extensivelystabilised under aerobic conditions;

- possibilities for a reduction of nutrients,

as the ammonium compounds are nitrified and denitrification is also possible throughcontrol or limitation of the oxygen addition. Simultaneous precipitation can be

employed for phosphorus elimination.

Due to these characteristics activated sludge plants with joint sludge stabilisation offerfavourable preconditions for the treatment of wastewater from small communities,localities and settlements. Operational difficulties such as the formation of bulking andfloating sludge cannot, even with these plants, be completely excluded. These canappear in particular if commercial or industrial dischargers significantly influence the rawwastewater or cause large surge loads.

3 Dimensioning

3.1 Loading Values

3.1.1 Wastewater Inflow

If with sewage treatment plants of this size, in particular in the lower range, statisticallyassessable data on the amount of wastewater and its development are not available thefollowing is to be assumed for residential areas:

QD = 0.004 l/s per inhabitant, corresponding to 0.144 m3/h per inhabitant.

The portion of wastewater from commerce for local disposal is included in this inflow.

Sewer infiltration water is to be taken into account separately according to localconditions. The maximum dry weather inflow from communities with commerce andindustry to cover the outside requirement is to be calculated in individual cases and is:

Qdw = QDI + Qi in m3/h

QDI = QD + QC + QI in m3/h

The combined sewage flow that must be treated in a plant can is calculated as:

Qcs = n.QDI + Qi in m3/h


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As a rule for dimensioning purposes n = 2.

Qcs = 2.QDI + Qe in m3/h

The dimensioning of stormwater tanks with a direct discharge into the sewage treatmentplant is to be matched with the QD value relevant for the dimensioning of the sewagetreatment plant.

3.1.2 Water Pollution

The following inhabitant related loads are to be assumed for dimensioning:

BOD5 = 60 g/d*)COD = 120 g/dFilterable substances (DSo) = 70 g/dTKN = 11 g/d

P = 2.5 g/d

The loads from supernatant liquor from gravity thickeners are included in the above loaddetails. BOD5 and nutrient loads from commercial and industrial concerns which coverthe outside requirement are to be determined in individual cases.

3.1.3 Sludge Yield

From experience an excess sludge yield of 1.0 kg/kg, referred to the degraded BOD5,can be taken as a basis with domestic wastewater.

With normal degradation of the organic pollutants fed in the disposable sludge yield of:

- not thickened (ca. 1.0 % DSM) ca. 5 l/(PE.d);- pre-thickened (ca. 2.5 % DSM) ca. 2 l/(PE.d);- stored (ca. 5.0 % DSM) ca. 1 l/PE.d).

With simultaneous treatment of stormwater the above DSM (dried solid matter) valuesincrease by up to 20 %; however, as a rule, the given specific sludge yields remain thesame due to the better thickening through the additional mineral contents from thesurface water inflow.

In cases in which a deliberate P-elimination is also demanded with sewage treatmentplants of this area of application, simultaneous precipitation is applied. If the SludgeVolume Index reduces through this then a higher dry solids content can be set. Theabsolute quantity of organic dry solids in activated sludge then remains almost even.Thus also no changes result for the sludge volumes. The dry mass of the sludgeincreases according to the precipitant applied.


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The dimensioning values are not altered (see Sect. 3.1.3) with the application ofprecipitant for P-elimination.

3.2.2 Aeration Installations

The necessary oxygen transfer capacity in wastewater is:

24

BOOC 5

BOD,DL ⋅=α in kg/h

Oxygen load OL ≥ 3.0 kg/kg

Hence

5

5

BOD,d

BOD,DB125.0

24

BOC ==α in kg/h

This value, with normal communal wastewater, covers the oxygen consumption bymicro-organisms for carbon and nitrogen oxidation purposes as, as a rule, the ratioTKN:BOD5 ÷ 1:5. As 4.6 grams of oxygen per gram of ammonia nitrogen is used for thecomplete nitrogen oxidation, oxygen shortage can occur if the ratio TKN:BOD5 rises tolevels above 1:3.5 (e.g. due to such industries as, inter alia, slaughterhouses). In such acase it is advisable to verify the additional oxygen demand for the nitrogen oxidation. Ifinflows of digested sewage are to be expected, an additional 2 mg/l of oxygen per 1 mg/lof hydrogen sulphide for oxidation. If the method of intermittent aeration is selected fordenitrification this is to be taken into consideration with the design and selection of the

aeration system. The increase in capacities should take place corresponding with theduration of the “off-times“ (increase of the oxygen loading value).

If the oxygen supply capacity in clear water under standard conditions is quoted, this

figure must be higher by a factor of 1/α than the calculated necessary oxygen supplycapacity in wastewater (oxygen transfer correction factor = 0.5 - 0.9 depending on thetype of aeration installation). In practice the following design values for oxygen yieldand/or oxygen transfer in sewage have proven their value, and they should only bedeviated from in very special cases:

Surface Aerator

Oxygen yield αOCN = 1.0 - 1.6 kg/kWh

Required motor output

NOC

OCN

α

α= kW


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Fine-Bubble Diffused Air Aeration

The air volume flow Q A which is needed to achieve the required oxygen transfer capacity

in wastewater (αOC), calculated dependant on the depth of air introduction hE is:

EO

Ahf

OCQ2⋅

α= in m3/h

Specific oxygen utilisation

- without separate recirculation 3O m/g108f 2 −= per m of depth of air introduction

- with separate recirculation 3O m/g1512f 2 −= per m of depth of air introduction

Plug-flow aerators are calculated for an airflow of maximum 10 Nm3/(h.m). With a

minimum admission of 3 Nm3

/(h.m) a control range of 3:1 results. Larger control rangesmust be covered by switching in and out aerator pipe trains.

Manufacturer's details are to be taken for the calculation of point aerators. Here attentionis to be paid that the reduction of the specific oxygen transfer compared with pure watertransfer performance is taken into account.

A value of V A ÷ 15 m/s should be applied for the air velocity in the pipelines.

The power density of the circulation installations with separate circulation, referred to thetank volume can, under favourable conditions, be under 2 Watt/m3.

In practice, depending on the shape and volume of the tank, the sludge content, and thecirculation system, values between 3 and 8 Watts/m3 result.

Guaranteed values for both the oxygen transfer capability, the energy absorption andalso the minimum flow rate at the tank bottom should be given by the manufacturer. Itthese deviate from the values already verified in practice then verification on the finishedstructure is recommended.

3.3 Secondary Sedimentation

From a process technology point of view, secondary sedimentation and aeration are tobe considered as a unit. It follows from the dimensioning regulations that the dry solidscontent of the aeration tank and the sludge volume index determine the size of thesecondary sedimentation tank. Savings with the aeration tank volumes, which are to bebalanced only by higher dry substance contents for the balance of the permitted sludgeloading, lead to larger secondary sedimentation tanks.

The hydraulic feeding and the with this achievable thickening of the return sludge arerelevant for the dimensioning.

Therefore, in any case, the highest inflow independent of dry or rainy weather, is to beapplied. The maximum surface feeding results as:


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SVIDS

qq

AT

SV A

⋅= in m/h

The maximum sludge volume feed qsv is

- for hopper-bottom tanks ≤ 600 l/(m2

.

h)

- for circular tanks with scrapers ≤ 450 l/(m2 . h)

The required tank surface can be calculated as follows:

A

cs

A

DWSS

q

Qor

q

Q A = in m

2

The necessary tank depths are:

- for hopper-bottom tanks hf = n . r SS hc ≥ 1/3 hf

(Fig. 1) with wastewater treatment

hc ≥ 2.0 mwith combined wastewater treatment

hc ≥ 3.0 m

hi ≈ 1.2 hc

- for circular tanks htot ≥ 4.0 m

with scraper hmin ≥ 3.5 m

(Fig. 2)

Alternatively a secondary sedimentation tank dimensioning in accordance with ATVStandard A 131 is permitted.

The weir crest feed rate with maximum inflow may not exceed the value of 10 m3/(m.h).

For technical construction and economic reasons hopper-bottom tanks are made withdiameters up to about 10 - 12 m. Above this circular tanks with scrapers are used: thediameter of the central structure should be 1/5 to 1/6 of the diameter of the relevantdimensioning surface.

The transfer flow of the return sludge pumps is to be so designed that the return ratio

RV is, in general, 100 %, with Sludge Index values ≥ 120 mg/l up to 150 %, referred tothe dry weather inflow Qd. The return sludge feed must be controllable.


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Fig. 1: Hopper-bottom tank

Fig. 2 Circulator tank with scraper


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3.4 Sludge Storage Reservoir

Storage space for the following storage periods should, as far as possible, be madeavailable for the anticipated surplus sludge quantities:

with disposal ensured throughout the year ≥ 1 month

with agricultural utilisation to bridge a vegetation period or the winter ≥ 6 months

4 Construction Principles

4.1 General

As is general with sewage treatment plants a safe, extensively fault-free and simple

operation is to be sought with the planning and construction of small sewage treatmentplants, with which the maintenance tasks are kept to a minimum. With small sewagetreatment plants therefore attention is to be paid particularly to the fact that a wastewatertechnician cannot be permanently present. The desired simple and robust design willtherefore, as a rule, be adopted in the lower range. Even with small sewage treatmentplants the use of instrumentation and control techniques will increase. Installationsoffered today are cheaper, operationally safer and require less maintenance thanpreviously. They make it possible to integrate these plants into a broad control andmonitoring system.

4.2 Primary Pumping Stations

Small wastewater inflows with considerable fluctuations must be dealt with by smallsewage treatment plants and their pumping stations. When designing the pumps accountis to be taken that the daily peaks with dry weather inflow, referred to the daily average,are clearly higher than with larger plants. Storage space should be available to balancethe inflow variations. The volume of the inflow sewer can also be used as security.

Due to the small inflows often one pump would be sufficient to convey the total inflow.However, with the necessarily smaller machinery the solid matter carried by thewastewater leads clearly more frequently to blockages than with pumping equipment forlarger plants. These possible operational faults and the necessary maintenance and

repair tasks in any case require suitable installations and a reserve pump of the samesize. Operational safety takes precedence over cost and energy saving.

Archimedean screws are used for pressureless raising of wastewater in open flumes.With this even small dry weather inflows can be conveyed economically as theirefficiency only deteriorates significantly at less than 30% of rated output. A specialadvantage of Archimedean screw pumps is the fact that intermittent charging of thetreatment plant can be avoided.

When centrifugal pumps are used in pumping stations the units are more reliable andeasier to maintain if a dry-well installation method is used. As a more sophisticated

structure is necessary for them small treatment plants often have to rely on submersible


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pumps. Pumping stations with submersible pumps should, however, only be installed ifthere are clear construction costs advantages.

It may be sensible to use pneumatic pumping stations as primary pumping stations forsewage treatment plants if one has to pump over great distances. Odour and concretecorrosion problems due to digested sewage and the resultant development of hydrogensulphide are avoided due to the specific mode of operation of pneumatic pumps with aswitchable blown stage. Due to the generally worse efficiency one must frequentlyreckon with significantly higher electricity costs than with centrifugal pumps. Withconveyance using centrifugal pumps the odour and concrete corrosion problems aremanageable if the pressure line is temporarily charged with compressed air or ends in aventilated degassing shaft. The extracted exhaust air can be directed into the aerationtank when using compressed air aeration, otherwise via a compost filter.

4.3 Flow Measurement

Sewage treatment plants within this scope of application are to be fitted with a flowmeasurement device. It may be of a simple design and should also be suitable for themeasurement of small inflows. Siting of the measurement devices in the outlet bringsadvantages also due to the reduced danger of blockages. A secure access to themetering point must be guaranteed. Further information on the design and locating ofmeasurement devices is to be taken from DIN 19559.

4.4 Stormwater Treatment

Both combined and separate sewerage systems are also employed for stormwatertreatment in small catchment areas. The siting of stormwater tanks within the area of thesewage treatment plant is, from the operational point of view, the most favourable. In thiscase they should be so designed that, with operating faults, they can also serve ascollection tanks.

Due to the small inflows which can be accepted by sewage treatment plants in the lowerrange the control of the emptying of stormwater tanks and the danger of blockage of thesmall pipe cross-sections is a problem; the smaller the emptying quantity is, the moreuncertain is the control.

The throttle devices without outside energy which are available function according todifferent procedures. To be sought is a possibility for direct, smooth matching on site.The emptying gradient (base height difference) should be as small as possible; depositsmay not occur either before or after the throttle device.

Emptying via either pumps installed in the tank or via the primary pumping station hasshown itself to be advantageous. However, with centrifugal pumps the dependencebetween conveyance height (markedly different filling level in the stormwater tank orsuction space) and conveyance stream is to be taken into account and, under certaincircumstances, balanced using an appropriate control technique.


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4.5 Mechanical Pre-treatment

4.5.1 Screens, Sieves

For small plants one cannot do without automatically cleared screens/sieves as aprotective device to remove floating solids and textile materials. Fundamentally, ahydraulic verification is necessary for the selected device.

The accrued screenings/sievings must, in any case, be removed from the wastewater.Depositing in closable containers has proved effective. A comminution and return into thewastewater flow must not, in any case, occur. The installation of a press and dewateringfacility with disposal into a container or conveyance in a bagging installation, also thewashing of sievings, can prove useful.

Screens are employed with aperture widths of from 6 to 20 mm. With the larger aperturewidths operational problems can occur with advanced wastewater and sludge treatmentas well as with the agricultural utilisation of sewage sludge due to clogging andblockages. With the smaller aperture widths and with sieves the amount ofscreenings/sievings rises with increasing proportion of faeces, which can make disposalmore difficult.

At screens or sieves which cannot be overflowed an emergency overflow with a simple, ifrequired overflowable, manually cleanable rack screen (spaces up to approx. 100 mm) isnecessary in order to prevent a back-up in the sewer network and an overflow of thechannels with a failure of the automatic clearance device.

Screen/sieve installations are fundamentally, for reasons of frost protection and toprotect against the development of odours, small animals, birds, container back-up due

to rain and generally for the establishment of better working conditions, to be providedwith a superstructure. Care is to be taken for sufficient ventilation.

4.5.2 Grit Chamber

Ventilated grit chambers should be employed in the upper application range. Circular gritchambers represent a suitable solution for plants of the lower application range and asan alternative primarily with tight conditions. Sludge deposits can be partially reduced,using a lightly bubbling aeration or through a stirrer to even out the circular watermovements. With an aerated grit chamber and with a circular grit chamber the sand is

cleared by means of a mammoth pump or submerged wastewater pump.

A sand grader can be an advantage for better separation of mineral and organic particlesfor sewage treatment plants of the upper range with aerated grit chambers and circulargrit chambers.

4.5.3 Grease Trap

Many operational problems with floating solids on aeration and secondary sedimentationtanks can be avoided by installing a grease trap. This also offers the advantage with

accidents involving oil of drawing off the oil at a suitable point before entry into the


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aeration tank. With an aerated grit chamber or circular grit chamber normally the greasetrap is combined constructionally with these.

4.6 Aeration Tanks

The activated sludge stage is operated without primary treatment. Tank shape andaeration system are to be so selected that sludge depositing does not occur.

Compact construction methods are frequently offered which combine the activatedsludge stage and the secondary sedimentation, in particular with small sewage treatmentplants. Here, attention is to be paid that the locally conditioned peculiarities in the inflowconditions are taken sufficiently into account with the design of individual plantcomponents. A dispersed construction method offers the opportunity to design individualplant components as required. With compact construction methods this is to be verifiedin individual cases. With balanced tailoring to the local situation and the construction siteconditions, and taking into account all necessary ancillary equipment, the dispersedconstruction method need not be more expensive than a compact plant. The smaller tankvolumes make the increased employment of prefabricated construction elements,particularly with concrete and other basic construction elements, more easy.

Oxygen transfer and water circulation systems determine the constructional shape of anaeration tank. It is important both for tanks and power units to be matched to one anotheras well as to save energy with a simple and effective oxygen regulation. Aeration takesplace via pipe, point, disc and plate aerators. Additional power units for the circulationmake an intermittent operation of the oxygen transfer installation; thus the denitrificationis encouraged. However, with this the stabilisation is fundamentally to be ensured.

As a rule newly built sewage treatment plants are not fully loaded when taken intoservice as a certain provisioning for the future is necessary and normal. The desiredsludge loading can, however, be set and, with staged regulation, the oxygen supply canbe operated with energy saving through appropriate reduction of the sludge content inthe aeration tank.

Aeration and circulation installations must also be removable even with full tanks insofaras only one tank is available. The aeration installations should be so combined that canbe raised completely from the tank.

4.7 Secondary Sedimentation Tanks

Deep hopper-bottomed tanks with vertical flow are particularly suitable for small sewagetreatment plants as mobile sludge removal facilities are not necessary. The design of theoutflow must be such that an extensively even removal from the top surface can beachieved. They must be maintenance friendly and accessible for cleaning tasks. Scumboards or equivalent facilities combined with reliably functioning installations, removablein winter, are to be provided for the removal of floating sludge. A good degassing of theinflow to the secondary sedimentation tank encourages the settling effect.

The bottom scraper must be removable from a filled tank insofar as only one tank isavailable.


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4.8 Operational Building and Ancillary Works

An operational buildings also necessary in plants in the lower application range. A controlbox for low voltage distribution, a store for holding plant specific spare parts with workbench and working area to perform monitoring functions or set up in-house monitoring,and sanitary facilities are to be planned as minimum equipment required. Attention is tobe paid that there is a sufficient number of wash-down hose connections throughout theplant. A telephone connection is to be planned.

Even sewage treatment plants of the above area of application must be capable ofapproach by heavy vehicles for the removal of sludge or larger repairs. Carriagewaysmust have a minimum curve radius of 12 m and a minimum width of 3.5 m. Pavementsmust be at least 0.6 m wide and must have at least a loosely compacted surface. Withpaved paths, slabs must be placed close to one another.

An easily approachable point for the taking of samples is to be constructed within thearea of the run-off from the sewage treatment plant.

4.9 Other Structures for the Safeguarding of Operations

Due to the disproportionate costs it will generally not be possible to avoid completelyinterruptions resulting from power failure, by providing reversible power supply or astand-by power unit, or to avoid a mechanical breakdown by keeping complete units ofequipment in store. A parallel arrangement of individual process stages can be practicalwith plants in the upper area of application. In individual cases consideration must begiven to the question as to what extent by-passes are necessary to deal with emergencysituations. The possibility of providing power using mobile generators is at least also tobe planned. Mechanical installations must not only be easily exchangeable but also areto be arranged in an easily accessible manner. Fault indicators are to be provided.

Screens/sieves are fundamentally to be provided with cover for protection against frost.Heating of individual components is, from experience, not sufficient. In areas with longperiods of frost grit chambers should also be included in the cover, at least the sanddischarge device should be protected against freezing up. Centrifugal aerators must beprotected against becoming out of balance due to freezing spray with the aid of acovering. Roller type aerators can have a heated cover. Aeration tanks with compressedair aeration are, as a rule, less endangered by frost. The same applies with secondarysedimentation for hopper tanks as they penetrate deep into the ground. In open sludge

silos the circulation facility is to be operated more when there is a danger of frost.

5 Sludge Treatment and Disposal

5.1 Sludge Yield

In activated sludge plants with joint sludge stabilisation sludge occurs only as surplussludge. In special cases such as, for example, in places with erosion endangered outerareas or heavy traffic loading due to agricultural vehicles, considerable increments mayalso have to be applied, even with grit chambers that function well, to the values given in

Sect. 3.1.3 due to the washing away of the finest mineral components with the surfacewater. In sewage treatment plants with additional phosphorus elimination using


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With closed sludge silos attention is to be paid to digester gas which forms. In somecases measures to protect against explosions are necessary. Odour nuisances can alsocome from open sludge basins which, for example, can be prevented by additional waterimpoundment.

5.4 Sludge Dewatering

If wet sludge cannot be utilised in agriculture, as a rule, mechanical dewatering isnecessary. The choice of the dewatering process depends ion the requirements ofsewage sludge disposal. If the sludge has to be stored in a dump the necessaryproperties for this in accordance with the rules of the dump operator are to be achieved.

With the dimensioning of the wastewater treatment stage the treatment of the yieldedfiltrate water must also be taken into account in all cases as it does not occurcontinuously, despite the selection of the smallest dewatering unit, due to the inherentover-capacity of this installation. This applies particularly when sludge is dewatered inmobile dewatering facilities at long intervals and the filtrate water is yielded in surges inlarge quantities. In order not to load non-adapted activated sludge in surges in this way,possibilities for intermediate storage and dosed input into the aeration tank are to beprovided.

Should sludge be transported to a large sewage treatment plant for further treatmentthen technical feasibility and costs of a preliminary dewatering, e.g. using a centrifuge,can be investigated.

For plants in the lower design capacity natural dewatering of sludge on sufficientlydimensioned drying sites can certainly also be taken into consideration.

6 Operation

Sewage treatment plants require a regular, working daily monitoring and maintenance.This requires appropriately trained sewage treatment plant personnel who observe theindividual operational processes and, if required, intervene. The treatment performancemust be checked regularly through in-house examinations. Holiday and illness deputationmust be ensured.

Practical experience has shown that, with a sewage treatment plant with a designcapacity of 5000 IPE without ancillary plants (e.g. pumping stations or stormwater tanksin the sewer network) some eight hours per working day are to estimated for operation,maintenance and in-house monitoring. For plants of the upper area of application onefull-time member of staff and at least a number of hours from an additional assistant arenecessary for this. With sewage treatment plants at the lower limit of the area ofapplication one has to reckon with an average working daily time requirement of threehours.

Plants must be so designed that, independent of the design capacity, routine tasks canbe completed by one man in two hours per day. The additional working time is thenavailable for larger scale maintenance tasks and small repairs. The association of

personnel, in particular between sewage treatment plants of the lower limit of the area ofapplication to larger neighbouring sewage treatment plants in which trained personnel for


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such tasks are available is recommended. These personnel are then also available iftasks are to be carried out which require at least two persons, for example for reasons ofaccident prevention. Creation and equipping of a laboratory site/laboratory and the scopeof in-house monitoring depend on the relevant regulations of the Federal State and thelocal conditions.

7 Symbols

ADS d Sludge age

ASS m2 Surface of secondary sedimentation tank

Bd.BOD5 kg/d BOD5 load per day

BDS kg/(kg.d) BOD5 sludge loading

BV.BOD5 kg/(m3.d) BOD5 volumetric loading

DS AT kg/m3 Dry matter content in activated sludge tank

f O2 g/(m3.m) Specific oxygen utilisation

hc m Depth of cylindrical top above hopper of hopper-bottomed tank

hE m Depth of air introduction; height of rise of compressed air in thewater

hf m Depth of funnel of hopper-bottomed tank

hi m Depth of inlet in hopper-bottomed tank

hmin m Minimum depth of secondary sedimentation tank

htot m Total depth of secondary sedimentation tank

IPE IPE Inhabitant and population equivalents

N kW Motor output

OL kg/kg Oxygen load, quotient from oxygen transfer capacity-and-BOD5 volumetric loading

OC kg/h Oxygen transfer capability in clear water under normalconditions

OCN kg/kWh Oxygen yield in clear water

q A m/h Surface loading

qSV l/(m2.h) Sludge volume feed rate

Q m

3

/h Daily wastewater dischargeQi m

3/h Sewer infiltration water discharge

Q A m3/h Air volume flow

QC m3/h Commercial wastewater discharge

Qcs m3/h Combined wastewater discharge

QD m3/h Domestic wastewater discharge

QDI m3/h Domestic and industrial wastewater discharge

Qdw m3/h Dry weather discharge

QI m3/h Industrial effluent flow rate

Qr m3/h Stormwater discharge into the contaminated water sewer of a

separate sewer system


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QSS m3/h Daily yield of surplus sludge

r SS m Radius of secondary sedimentation tank

SSB kg/kg Specific surplus sludge production, referred to Bd.BOD5

SVI ml/g Sludge Volume Index

TKN Kjeldahl nitrogen concentration (ammonia-N and organic N)V AT m

3 Useful capacity of activated sludge tank

α Oxygen supply factor

αOC kg/h Oxygen transfer capability in wastewater

αOCN kg/kWh Oxygen yield in wastewater

Regelwerk - ATV-A 126e

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