Waterford Tender Design

8/2/2019 Waterford Tender Design

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ASL

12

Process Design for the

Waterford city Waste

Water Treatment PlantBid for contract Nr. 5

Activated Sludge Limited



Activated Sludge limited

Process Design for the Waterford city Waste Water Treatment Plant

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EXECUTIVE SUMMARY Activated sludge limited (ASL) is grateful to the Waterford city council for the opportunity to

participate in the bidding process for the Waterford city waste water treatment plant (Contract Nr

5).

Activated sludge limited was incorporated in 1982 as small civil engineering consulting company. We

are pleased to note that we previously designed and build a number of works for Waterford city

council such roads, sewerage and the city council building which we completed ahead of schedule in

1991. Our works therefore speak for themselves.

Like Waterford city, we have grown in leaps and bounds. We are now a publicly traded company

quoted on the London stock exchange (LSE: ASL). We have interests and expertise in virtually all

fields of engineering.

ASL is the first company in the UK to extract struvite, a slow-release nitrogen and phosphorus based

fertilizer from sewage digestate liquor. This novel process is being piloted at the Birmingham water

treatment plant which we designed and built. The plant is already saving money on environmental

remediation and the government has just approved struvite for sale as a save environmentally

friendly fertilizer. Struvite recovery is built into this design we are submitting for the Waterford

scheme.

Using our patented technology, we are ahead of the competition in our efficiency at combining

primary and secondary sludge thickening with digestate sludge drying in the same facility. In this

way, we have saved our clients millions of pounds in operational costs.

Our general design principles lay emphasis on functionality, sustainability and optimum return on

investment.

We have assigned our director of engineering design, Dr Nigel Horan to oversee the implementation

of this project. Dr Horan is renowned expert on waste water engineering with a lot of publications to

his name. His appointment underscores the value we attach to this bid.

Given that as a company, we have grown with the city of Waterford, we look forward to another

opportunity to cement this long-term relationship we have had with Waterford city.





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Eng. Adamu Onakpa

BSc. MsE. Ceng.

Senior Process Design Engineer

Activated sludge limited





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ContentsEXECUTIVE SUMMARY ............................................................................................................................ 1

INTRODUCTION ....................................................................................................................................... 4

General Requirements from the Tender document ............................................................................... 5

INFLUENT ............................................................................................................................................ 5

Nutrient Removal .............................................................................................................................. 11

PERFORMANCE STANDARDS............................................................................................................. 11

PROCESS DESIGN REQUIREMENTS .................................................................................................. 12

PRELIMINARY TREATMENT ................................................................................................................... 18

INLET WORKS .................................................................................................................................... 18

PRIMARY TREATMENT .......................................................................................................................... 20

Tank shell design ............................................................................................................................... 20

Design of tank internals .................................................................................................................... 21

THE ACTIVATED SLUDGE PROCESS DESIGN .......................................................................................... 22

Design calculations............................................................................................................................ 23

SECONDARY TREATMENT TANKS .......................................................................................................... 27

Tank design calculations ................................................................................................................... 27

Design of tank internals .................................................................................................................... 28

SLUDGE ARISINGS ................................................................................................................................. 28

Sludge storage prior to digestion ...................................................................................................... 29

Sludge thickening and dewatering .................................................................................................... 29

MESOPHILIC ANAEROBIC DIGESTION ................................................................................................... 30

Design of the anaerobic digester ...................................................................................................... 30

Gas Generation ................................................................................................................................. 30

Gas storage ....................................................................................................................................... 31

CHP Power output............................................................................................................................. 31

Dewatering ........................................................................................................................................ 31

Nutrient recovery .............................................................................................................................. 32

CONCLUSION ......................................................................................................................................... 33

REFERENCES .......................................................................................................................................... 34





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INTRODUCTIONThis bid document is intended to communicate the technical process design for the water form

waste water plant. Each unit process is preceded by a brief explanation followed by calculations

used to arrive at each design specification.

It begins with a complete summary of design requirements and everything is extracted from the

tender document. Other parameters not contained in the tender are derived from generally

accepted engineering standard tables.

The plant is designed to treat projected incremental load input up to the 2025 with a maximum flow

rate of 1.687m3/s. the design BOD and TSS loads are 11,436Kg/d and 8005Kg/d respectively. Both

primary and secondary sludges are to be thicked to 6% dry solids. The dried sludge is to be 23% dry

solids.

The CHP plant has a designed power output of 4.1MW. A single element of the design is at the

minimum up to the tender requirement and to compy with stringent environmental regulations





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General Requirements from the Tender document

The following information provided in the Tender has been used to design the Plant as specified:

INFLUENT

The Works shall be designed for the projected domestic and commercial waste water inputs for the

Year 2025 with an allowance for industrial wastewater, infiltration and storm water.

The design flows have been calculated as multiples of dry weather flow (DWF) using the formula

below:

DWF = (P x Q/1000) + C + E (m3/d)

Where:

P - Resident population

Q- Per capita sewage discharge including infiltration, litres/day

C-commercial flow rate, m3/day

E- Industrial waste water flow rate, m3/day

The estimated resident populations for the catchment area of the Waste Water Treatment

Plant is as follows:

Year 2005 51,640

Year 2025 67,905

The domestic flow arriving at the waste water treatment plant has been calculated using the

following criteria from the tender document.

Per capita sewage discharge 150 litres/h/d

Allowance for infiltration 50 litres/h/d

Commercial flow 24 litres/h/d (i.e. 16% of per capita above)

The industrial contribution comprises the existing estimated industrial flow of 5,424 m 3/d (which is

taken to be the loading for year 2005) and the future industrial loading of 12,254 m

3

/d (which istaken to be the design industrial loading.





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The estimated existing and design influent are summarised in Table 1.1 below Particular

Requirements for design, 2004)

Table 1.1

Contributing

Flows

Year 2005

Design DWF

Design

DWF

m3/d m

3/d

Domestic(P x Q) 10,328 13,581

Commercial (C) 1,240 1,630

Industrial (E) 5,424 12,254

Total 16,992 27,465

Flow will arrive at the works from five pumped sources feeding into the main gravity inlet sewer and

from a separate collection system servicing the Belview Area.

The breakdown of these estimated contributions is given in Table 1.2





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Table 1.2

Source

Feed

PumpingStation

2005 Estimate Design AllowanceInstalled

Pump

Capacity

(m3/s)

Industrial

Flow

(m3/d)

Total

DWF

(m3/d)

Industrial

Flow

(m3/d)

Total

DWF

(m3/d)

Main

Inlet

Sewer

Waterpark 4,454 11,040 5,098 13,758 0.922

Maypark 740 3,577 740 4,470 0.358

Rocklands 230 2,302 230 2,955 0.205

Slieverue 7 - 10 0.009

-

Beacon

Quay25 - 32 0.009

Gravity-

Belview

- - 3,370 3,370 0.068

Sub Total 5,424 16,951 9,438 24,595 1.577

Belview- 41 2,816 2,870 0.11

Area

Total 5,424 16,992 12,254 27,465 1.687

The estimated maximum rate of flow that could arrive at the Works through the main inlet sewer

based on installed pump capacities is 1.577m3/sec. It is expected that the domestic and future

industrial flow from the Belview area will arrive at the Works at a flow rate which could be up to 3 x

DWF for domestic flows and 2 DWF for industrial contributions and hence an allow for a further flow

of 0. 110m3/s from this source will be made. The total design peak flow to the Works is therefore

1.687m3/sec.





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All flows arriving at the plant shall receive preliminary treatment. The design flows which are to

receive preliminary, primary and secondary treatment are summarised in Table 1.3. Primary and

secondary treatment shall be provided for flows up to 3 times the design DWF. The minimum

capacity of the primary and secondary treatment facilities will be 0.954 m3/s which could occur on a

continuous basis. Flows received at the treatment plant in excess of 0.954 m3/s will be temporarily

diverted to storm water tanks.

Table 1.3: Summary of Design flow for Preliminary, Primary and Secondary Treatment

Flows to

Treatment

Design DWFDWF

Multiplier

Design

Peak Flow

m3/d m

3/s m

3/s

Flow to

Preliminary27,465 0.318 5.3 1.687

Flow to Primary

Treatment0.318 3 0.956

Flow to

Secondary

Treatment

0.318 3 0.956

Flow to Storm

Treatment

Upstream of

0.733

* Return liquors flow rates are not included

Waste Water Characteristics

Industrial Waste Water

The survey and analytical results shall be interpreted and the relevant characteristics of the

anticipated industrial loads and flows in the context of achieving the required performance

standards will be determined. Conclusions will also be drawn concerning the availability of nutrients

and trace elements in the waste water to provide for biological treatment. If the nutrients are

insufficient, provisions will be made accordingly.





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Domestic Wastewater:

The average domestic pollutant loads have been estimated based on per capita contributions of:

60 g BOD/capita/day

126g COD/capita/day,

70g SS/capita/day,

These have been applied to the design domestic population for the Year 2025 which is

67,905 persons.

Pollution Loading:

Table 1.4 below summarises the estimated pollution loading for 2005 and the Design Pollution

Loading. The loads given shall be considered as maximum 7 day moving average loads.

Table 1.4: Estimated Pollution Loading

Pollutant Unit

Estimated

Load

2005

Design Load *

BOD kg/day 6,254 11,436

COD kg/day 12,508 22,872

SS kg/day 4,378 8005(I)

(I) Based on percentage of industrial waste water with low SS.

* Return liquors and any contaminated site drainage loads are to be added to the above figures.





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Table 1.5 gives an estimated breakdown of the contribution from the main sectors

contributing to the organic loading to the waste water treatment plant.

Table 1.5

Estimated 2005

Loading(kg BOD/d)

Design Load(kg BOD/d)

Domestic 3098 4,074

Commercial 496 652

Industrial 3060 6,710

Total 6,254 11,436

Variation in loading

The Works shall be capable of treating the design peak flow rate arriving at the plant as given in

Table 1.3 and be capable of treating the Design Load for the various pollutants as given in Table 1.4.

The design shall allow for increases and decreases in the pollutant concentrations within the

catchments and for the variations in infiltration within the collection system.

Modular Expansion

Increased Loadings (+10%, +20%)

The design assumption to be made with respect to influent for the+10% and +20% increases in BOD,

COD and SS load is shown in Table 1.6 below. The loads given shall be considered as maximum 7 day

moving average loads.

Table 1.6

Load

Pollutant Unit +10% Load +20% Load

BOD Kg/day 12,580 13,723

COD Kg/day 25,160 27,446

Ss Kg/day 8,806 9,606





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Note I Return liquors and contaminated site drainage flows and loads are to be added to the above

figures.

Note 2 The increased loads could arise from any part of the catchment.

Nutrient Removal

The design influent flow to be assumed for the purposes of expanding the design for nutrient

removal (nitrogen and phosphorus) is given in Table 1.3 and 1.4. The total nitrogen concentration in

waste water shall be assumed to be 40mg/l (all as organic nitrogen and ammonia). The

concentration of phosphorus shall be assumed to be 6mg/litre.

PERFORMANCE STANDARDS

General

The Works shall meet the environmental standards with respect to final effluent quality, sludge

quality as well as satisfying odour and noise requirements. The Works shall meet the Works

performance Guarantees, and the performance standards specified for individual process units

in this tender.

Final Effluent Discharge Standards

The Works shall be capable of producing final effluent meeting the standards specified in

Table 1.7 below.

Table 1.7: Final Effluent Discharge Requirements

Parameter

Standard Compliance Criteria

Target Target

Target A TargetB

A B

BOD, mg/1 25 50 No more than three daily

samples per 60 days with

a value for any one

parameter or all

parameters to be greater

than the standard

No samples with a

value for any one

parameter to be

greater than the

standard

COD, mg/l 125 250

TSS, mg/l 35 87.5





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Compliance with the above standards shall be monitored by daily flow proportional sampling.

The above final effluent standard shall apply to the effluent stream from the secondary treatment

plant i.e. excluding storm tanks overflow.

Sludge Quality Standards

The sludge at the time of its use or disposal, shall meet the standards shown in Table 1.8.

Table 1.8: Sludge Quality Requirements (Digestion & Dewatering)

Parameter Standard Compliance

Faecal coliform

concentration 1 000 MPN/gds

Continuously less

than the

Dry solids

concentration23% (w/w)(IJ

Greater than the

standard

Retention time in

pasteurization

Min.2 hoursContinuously

greater than the

Temperature of

sludge during

pasteurization.

Min. 55°CContinuously

greater than

PROCESS DESIGN REQUIREMENTS

Degree of Treatment Required for Particular Flows

The following degree of treatment is required for influent flows:

All flows arriving at the Works shall receive preliminary treatment comprising grit removal andscreening.

Flows up to the design peak flow rate in Table 1.3 shall receive primary treatment and secondary

biological treatment.

Flows in excess of the design peak flow rate for primary and secondary treatment in Table

1.3 shall be diverted as storm water to holding tanks and, shall be returned to the primary

and secondary treatment processes whenever the flow to the Works drops below the





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capacity of the primary and secondary treatment stages.

The possibility that the wastewater arriving at the Works may be septic shall be taken into account

and provisions made in the design accordingly.

Preliminary Treatment

Screening

Screening equipment shall be installed within a building. The screens shall have maximum apertures

of 5 mm in either direction. The design velocity through the apertures shall not exceed 1.2 m/s.

Grit Removal

The grit separators shall be capable of removing at least 95% of particles with a specific gravity of

2.65 g/cm3

and with a diameter of 0.2 mm and greater.

Storm Water Treatment

The treatment plant shall include for the storage of storm water in holding tanks with a minimum

capacity of2, 640m3

representing 1 day storage at maximum overflow rate.

Primary Treatment

Primary settlement tanks shall be designed for flows up to the design flow stated in Table

1.3. Tanks shall have a minimum straight wall height of 2.5 m as measured from the top water level

to the top of the base and shall provide a minimum retention period of 1.5 hours at maximum flow

conditions.

The inlet pipe shall discharge within a stilling box or diffusion mechanism located at the centre of the

tank. Where stilling boxes are used these shall occupy a minimum liquid surface area 10% of the

liquid surface area of the tank

Secondary Treatment

Secondary biological treatment shall be provided for flows up to the design flow stated in Table 1.3.

Biological treatment systems shall be designed to achieve a treated effluent standard in accordance

with the requirements stated in Table 1.7.





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Activated Sludge Process

Activated sludge systems shall be designed to maximize the settle-ability of the mixed liquor in

downstream clarifiers. Plug flow type systems are the preferred option but consideration will be

given to other systems. Selector tanks should be provided and systems shall be designed to achieve

a Stirred Specific Sludge Volume Index less than 120 ml/g at a mixed liquor suspended solids

concentration of3.5 g/l.

A minimum sludge age of 5 days shall be maintained at a temperature of 100C and the system shall

be designed for a mixed liquor temperature range of 100C - 20

0C.

Dead spots shall be eliminated by providing baffle walls and fillets to corners of tanks where

necessary. A minimum freeboard of 0.75 m shall be provided in all aeration tanks.

Aeration systems shall be capable of maintaining a dissolved oxygen concentration of 2 mg/litre

throughout the full volume of the aeration lank at all times. Suitable arrangements shall be provided

for draining aeration tanks.

Secondary Settlement Tanks

The upward flow method shall be used in the design of the tanks.

The upward flow velocity shall not exceed 0.9 m3/m

2/h with all tanks in operation and 1.2 m

3/m

2/h.

with one tank out of service for maintenance. The minimum hydraulic retention time shall be two

hours and the maximum solids loading rate shall not exceed 75 kg/m2/d.

In all cases the following shall apply:

Settlement tanks shall incorporate rotating bridge scrapers and scum removal systems

Scum removed from the surface of the tanks shall not be returned with the return sludge.

The inlet pipe shall discharge within a stilling box or diffusion mechanism located at the centre of the

tank. Where stilling boxes are used these shall occupy a minimum liquid surface area of 10% of the

liquid surface area of the tank. The Contractor's design shall include for a means of removing scum

from the stilling box.

Sludge return pumps shall be capable of returning flows in the range not narrower than 0.5 to 1.5

times the average inlet flow or within the range 0.3 to 0.7 m3/m

2/h as an underflow rate where the

solids flux design method is adopted.





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The side wall depth of the settlement tanks shall not be less than 2.5m

Aeration Systems

Aeration systems shall be designed to maximise oxygen transfer and to react to the changing oxygen

demands in biological treatment systems. Consideration will be given to the separation of aeration

and mixing mechanisms in aeration tanks where appropriate to maximise energy efficiency. Tapered

aeration in plug flow systems shall be provided.

Aeration systems which incorporate blowers shall comprise at least 3 blowers. Blowers shall

be capable of delivering maximum air requirements with the largest single unit out of service.

Duty blowers, diffusers and piping shall be capable of delivering at least 150 per cent of the

air requirements based on the maximum design loading. The maximum air flow in pipework

shall not exceed 15 m/s.

Provision of a system for condensate draining and flushing of the air system will be made.

Pressure tapping shall be provided to monitor pressure drop for each air valve.

Sludge Handling and Treatment

Primary Sludge

Primary sludge from the primary settlement tanks shall be thickened prior to treatment.

Thickening/consolidation tanks shall be designed such that when operating in series with the

sedimentation tanks the thickened sludge shall have a minimum dry solids content of 6%.

Tanks shall be covered and the air extracted for treatment. Tanks shall be designed to provide a

minimum hydraulic retention time of one day at maximum daily sludge flow rates.

Secondary Sludge

Sludge produced in secondary biological treatment processes shall be thickened to a minimum dry

solids content of 5% systems prior to digestion or other sludge treatment processes.

Sludge Storage

Sludge storage facilities shall be provided for Thickened primary sludge and secondary sludge as well

as digested sludge.





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Separate tanks for storage for 3 days production of thickened

primary and secondary sludge at maximum design loading will be provided in addition to a minimum

of 5 days storage capacity for digested sludge. All sludge storage facilities shall be covered and the

headspace will be extracted to the odour treatment plant.

Sludge Blending

Thickened primary and secondary biological sludge shall be combined and mixed in the sludge

blending/buffer tank from where the sludge shall be pumped to the sludge treatment processes.

Sludge Pasteurisation

Sludge pasteurization will be designed to meet the standards required for disposal of sludge to

agriculture. The system shall be designed to handle peak flows generated by the treatment plant.

Sludge Digestion

The sludge retention time shall be a minimum of 14 days. The digesters shall be equipped with an

automatic temperature control systems. In each digester, at least two temperature sensors shall be

installed. External heat exchangers shall be utilised.

The average daily temperature of the sludge in the digester(s) shall not be outside the range of 35±0.5°C. The instantaneous temperature shall be continuously in the range 35±2°C. The biogas

generated during digestion shall be directed to the gas holder. An enclosed flare stack shall also be

provided. All necessary safety devices shall be provided.

Sludge Dewatering

Mechanical thickening and dewatering plants, and associated pumps shall be arranged in at

least one duty stream and at least one additional stream of the same size as the duty stream.

The duty stream(s) shall be sized to handle at least the average daily sludge quantities over

not more than 20 hours per day. The additional stream shall be used to treat the peak loads

and serve as a standby during average loading.

Polyelectrolyte solution preparation and dosing systems shall be provided for all mechanical

devices used for sludge thickening and dewatering. Duty and standby dosing units shall be





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provided.

The minimum dry solids content following dewatering shall be 23%.

Dewatering systems shall be fully enclosed and located in a sludge dewatering building.

Solids Disposal

The disposal of all screenings, grit and sludge generated shall be carried out in accordance with the

requirements of the Waste Management Act 1996 and Implementary Regulations and in accordance

with any permits or licences obtained.





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PRELIMINARY TREATMENT

INLET WORKS

Approach Channels

The approach channels are designed to receive flows up to 5.3DWF from the incoming sewer as

requested by the tender document. A flow velocity range of 0.45 to 0.9m/s is chosen to prevent

both solids settlement at lower velocities and damage to screens at higher velocities (Horan, 2011a).

Screen specification with apertures of 3mm and matching the designed flow channel is obtained

from Degremond technologies online.

The design calculations for the approach channels are based on an adaptation of the formula A

5.3DWF = 27465M3/d = 1.687m3

/s (see table 1.3)

DWF = 0.318m3/s

At DWF,

Area =

= 0.707m2

At 5.3DWF,

Area = = 1.874m

2

Number of channels required = 2.651

This implies two channels will not be enough and three channels are therefore designed. The total

area of the three channels is

Flow Velocity = =

= 0.9m/s

To maintain flow velocity above 0.45,

= 0.471

The width required for the screen is given buy

= 1.052m





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Taking a depth to width ratio of 1.15, and let the width be 1m,

Depth = 1.15 x 1. 052m = 1.21m

Screens

Screen specification that fit the design of the channels is obtained from manufacturers. An inficro

screen (Degremond Technologies, 2011) climber screen with apertures of 3m will be installed. Such

a micro screen has the capacity to remove 95% of debris in the waste water

Grit removal

A hydrodynamic grit removal system will be installed. This is preferred because it is relatively

compact and maintains very high removal efficiencies over a wide range of flows (idswater, 2011).

This type of screen is highly efficient in separation, reduces odours, has low wear rate and can

remove all grit 0.1mm (140 mesh) and larger.

Storm Tanks Design

The storm tanks are designed to retain an excess of flow to full treatment of 2.3DWF (tender

requirement). Two tanks in series are designed with the first one receiving the first flush. The surface

loading is 10m/h, height to breadth ratio is greater than 2 and retention time is 2hrs (Horan, 2011b)

The designed volume of the storm tank is

V= Q x HRT = 2.3 DWF x 2/24 = 63169.5m3/d x 2/24d = 5263.2m

3

Tank area =

=(

)= 263.2m

2

As stated above, two tanks in series are used in the design, thus, the actual volume of each tank is

2631.6m3.

Taking L: W = 3.04, W = 6.58m, L= 3.04 x 6.58 = 20m

Tank depth =

=

= 10m





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Fig 1: layout of the preliminary treatment works.

PRIMARY TREATMENTThe primary treatment tanks designed are of the radial type. They are preferred because their

efficient scraping mechanism and equal flow divisions (Horan, 2011c). The tanks are designed for

BOD and TSS removal of 30% and 60% respectively.

The tanks are designed such that the influent is discharged towards the centre of the surface area. A

stilling basin which is 10% of the tank surface area in size is used to dissipate the velocity or energy

of the influent. The treated effluent flows over a v-notch weir while the settled sludge is withdrawn

from the tank bottom through a pipe.

Tank shell design

Design data

Surface loading rate at FFT = 90m3/m

2.d (Horan, 2011d)

HRT= 1.5 hrs. (Tender document)

Maximum allowable weir overflow rate = 450m3/m

2.d (ibid)

Flow to primary tank = 3DWF = 82395m3/d

Tank surface area=

= = 916m

2

Tank Volume = = 1.5d/24 82395m3/d = 5150m3

Inlet works

Storm Tanks

Formula “A”

weirInlet

sewer

Water course

> 6DWF

>3DWF

FFT Primary

sedimentation

tank





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For three tanks, volume required for each will be 1717 m3

with area of 305 m2

As shown in table 2.1 below (Horan, 2011e)

Table 2.1: Number of radial tanks and corresponding volumes

Tanks in

Service (m3)

One

spare

(m3)

Total

Volume

(m3)

Volume

saved

(m3)

1 at 5,150 5,150 10,300 -

2 at 2,575 2,575 7,725 2,575

3 at 1,717 1,717 6,868 3,432

4 at 1,287 1,287 6,435 3,865

Thus, three tanks each of volume 1717m3

are required. However designing for standard available

capacity for floor slope of 7.5 degrees yields the following:

Volume of conical portion = 160 m

3

(from table)

Volume/m of sidewall =346 m3

Assuming a side wall depth of 3.5m, implying volume of cylindrical portion = 3.5 x 346 = 1211 m3

Therefore total tank volume = 1211 m3

+160 m3

=1371 m3

And tank diameter = 21m

Three tanks each having a volume of 1371 m3

and area of 346 m2

are to be provided with a spare to

same specification. This is ideal for our volume requirement from the tender data.

Design of tank internals

The stilling basin designed to be 10% of the tanks surface area as required by the tender document.

In addition, a scum removing mechanism is attached to the stilling basin. The outlet weir which

collects settled sewage prior to distribution is located 0.5m from the tank wall.

Area of the stilling basin = 346 x 10%

=346m2

x 0.1 = 34.6m2





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Outlet weir length for the three tanks = 3π (21-1) m = 188.5m

The weir loading rate =

=

= 437.2m3/m

2.d

Surface loading is high and should be reduced by providing a cantilevered double sided weir.

However, this is still within the recommended weir loading rate of 100 to 450m3/m

2.d (Horan,

2011c).

Figure 2: A simple line diagram of the primary sedimentation tank (adapted from Horan, 2011d). The

secondary sedimentation tank has a similar design but without the scum removal mechanism

THE ACTIVATED SLUDGE PROCESS DESIGNThe activated sludge process basically involves the oxidation of organic matter (as BOD) by bacteria

in the presence of oxygen. The end products are largely new bacterial cells, carbon dioxide as well as

relatively smaller quantities of nitrogen and phosphorus compounds.

Once the bacteria multiply in huge quantities, the settle to form a floc called the activated sludge.

For a brand new treatment plant, the reactor has to be seeded with bacteria from external sources

and allowed to grow for a few weeks to a few months. For existing plants, the recycle flow from the

secondary tanks return some active sludge to seed the influent coming from the primary tank into

the reactor. Manual or mechanized systems keep the reactor chamber supplied with air; allowing

bacteria to breakdown the organic matter. A good balance between the substrate in the wastewater

6

2

7

8

5

1

2

3

1- Inflow 2- Stilling basin 3- Scum removal 4- Scraper

5- Sludge withdrawal 6- Scraper drive (fixed bridge mounted)

7- Peripheral overflow weir 8- Flow slope





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and the microorganism must be maintained for proper working of the system. A certain amount of

the microorganisms must be wasted to achieve this (Lin, 2001)

Design calculations

The primary settlement tanks remove 30% and 60% BOD and TSS respectively. Thus, the BOD and

TSS load to the aeration tanks are:

BOD load = 11436 Kg/d x 70% = 8005.2Kg/d

TSS load = 8005Kg/d x 40% = 3202Kg/d

Therefore,

TSS: BOD = 3202Kg/d: 8005.2Kg/d = 0.4

Population equivalent PE = =

= 111,833

With PE greater than 20,000, a sludge age of 12 days is adopted and from table 2.3 below (Horan,

2011f), the yield is worked out as 0.68

Table 2.0: Relationship between TSS: BOD, sludge age and yield

TSS:BOD SLUDGE AGE

4 6 8 10 15

0.4 0.74 0.7 0.67 0.64 0.59

0.6 0.86 0.82 0.79 0.76 0.71

0.8 0.98 0.94 0.91 0.88 0.83

1 1.1 1.06 1.03 1 0.95

1.2 1.22 1.18 1.15 1.12 1.07

The food: micro-organism ratio F/M is calculated from the relationship;

Where, = sludge age, Y = microbial yield.





24

Page 24 of 34

= 0.12/d

The reactor volume is

Where V= Reactor volume, QS = BOD load, X = MLSS

V =

= 18596m3

Selector design

COD loading = 2 x BOD loading

COD loading to reactor = 2 x 8005.2Kg/d x 70/100 = 16010.4Kg/d

Solids loading =

Using floc loading of 150g COD/Kg MLSS (Horan, 2011h)

Solids loading =

= 106,736Kg /d

RAS flow rate =

= = 10.6736m

3/d

Total flow to selector = RAS flow + influent flow

= 10.6736m3/d +82395m3/d = 93068.6m3/d

Calculation is made of a volume that yields a retention time of 15 minutes (ibid)

V = HRT x Q =

= 969.5m3

Area=volume/reactor depth= 969.5/6=161.6m2







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SOTR = N0=

Where

N0 = Oxygen transfer into clean water (Kg/d)

N = Oxygen transfer into sewage (Kg/d)

Cs = Saturated oxygen concentration

α = Correction factor for the effect of waste water on KLa

β = Correction factor for the effect of waste water on Cs

TABLE 3.0: Aeration pockets

Height (m) 6 6 6 6 6

c (mg/L) 0.2 1 1.5 2 2.5

T (oc) 20 20 20 20 20

α 0.4 0.5 0.7 0.8 0.9

β 0.85 0.85 0.85 0.85 0.85

OTR (kg/d) 3152 788 525 525 263

SOTR(kg/d) 9460 2060 1038 965 458

Diffuser (nr) 438 95 48 45 21

Diffuser performance = 0.15Kg O2/diffuser

Number of diffusers = SOTR/Diffuser performance





27

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SECONDARY TREATMENT TANKSThe secondary tanks are designed according to the tender requirement include making provision for

flows up to the design flow rate and using modular construction for ease of expansion. Structurally,

secondary sedimentation tanks are constructed the same way as primary tanks. They are however

designed to have larger areas to cope with the huge amount of BOD and TSS removal needed.

The sewage from aeration chamber flows into the secondary sedimentation tank and remains there

for 2hrs. During this period, the sludge settles and thickens into various zones at the bottom of the

tank. Part of the thickened sludge is returned to the activated sludge plant to seed the incoming flow

from the primary thank with bacteria. The balance of the thickened sludge corresponding to the

microbial growth is withdrawn and sent to the anaerobic digestion plant for further processing.

Liquid effluents from the secondary thanks typically have nearly all their BOD removed in the tank.

They are tested to ensure they have met consent standards before being discharged to the

environment.

Tank design calculations

The tank area is given by

Where: A = tank area

SSVI = sludge specific volume index = 3500ml/g

Q u = underflow rate = 1144.3m3/h

Q s =3DWF = 3433.1m3/h

Inserting values in the above equation gives

A = 7563.8m2

The total flow rate to the tank is

Q s + Q u = 3433.1m3/h + 1144.3m

3/h = 4577.4 m

3/h





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Surface loading = =

= 0.61m

3/m

2h

Tank volume = Q x HRT = 4577.5m3/h x 2hrs = 9155m

3

Three tanks in service are designed with one spare. Thus the respective volume and area of each

tank are

Volume of each tank = 9155m3/3 = 3051.3m

3

Area of each tank = 7563.8 m2/3 = 2521.3m

2

Other calculations of tank parameters shown below were done using standard tables (Horan,

2011d). A floor slope of 100

and sidewall depth of 3.4 meters were used.

Tank diameter = 30m

Conical section = 653m3

Circular portion = 3.4m x 707m3

=3056.8m3

Design of tank internals

Stilling basin is designed with an area equal to 10% 0f tank surface area (tender requirement).

Stilling basin for each tank = 2521.3m2

x 10/100 = 252.13m2

Outlet weir is 0.5m from the circumference of the tank. Therefore,

Weir length = 3(D-1) πm = 3 x 9 x π = 73.3m

Weir overflow rate =

= 401.9m2/d

This is between the generally recommended range of 100-450m2/d (Horan, 2011). It is however still

high and will be reduced using a double-sided weir.

SLUDGE ARISINGS

The primary tanks removed 60% TSS equivalent to 4803Kg/d at 1.5% solids (i.e. 15Kg/m3).

Amount of sludge removed = 0.6 x 8005 = 4803Kg/d





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Volume of sludge removed = = 320.2m

3/d

The amount of sludge produced by the secondary tank is = Yield x BOD load

= 0.68 x 8005.2Kg/d

= 5443.3Kg/d

SSVI = 120ml/g. This implies 1Kg of the sludge will occupy 0.12m3

of volume. Therefore;

Volume of sludge produced = 0.12m3/Kg x 5443.3Kg/d = 653.2 m

3/d

Total daily sludge production = 320.2m3

/d +

653.2 m3

/d

=

973.4m3

/d

Sludge storage prior to digestion

A minimum of three days storage capacity for both primary and secondary thickened sludge based

on maximum design loading capacity is provided.

For the primary thickened sludge, storage volume = flow x HRT

320.2m3/d x 3d = 960.6m

3

Storage capacity provided = 1000m3

For the secondary thickened sludge, storage volume = 653.2 m3/d x 3d = 1959.6 m

3

Storage capacity provided = 2000m3

Sludge thickening and dewatering

Primary and secondary sludge are thickened separately. The centrifuge thickening system is used

because it is highly efficient at managing large quantities of sludge and dewaters the digested from

the anaerobic digestion process. Centrifugal thickeners are also amenable to modular

construction(Europy, 2011)

Primary sludges are thickened to a dry solids content of 6% with a VSS concentration of 75% while

secondary sludge are thickened to a dry solids content of 6% with a VSS concentration of 80%.





30

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MESOPHILIC ANAEROBIC DIGESTIONAnaerobic digestion involves complex biochemical processes in which micro-organisms breakdown

organic matter in the absence of air. Anaerobic digestion that takes place at 35-370C is called

mesophilic anaerobic digestion (Vindis, 2009).

Design of the anaerobic digester

The sludge flow to the digester is = 973.4m3/d

Average flow to digester = 1.3 x 973.4m3/d = 1265.42 m

3/d

The retention time is 14 days (tender document)

Digester volume = Flow x HRT = 1265.42m3/d x 14d = 17715.88m

3

Taking a side depth of 9m (with 0.5m additional as free board)

Tank area = 17716m3/ 9m

=1968.4m

2

The design provides for two digesters each of volume 17716m3/

/2 = 8858m3

Gas Generation

The primary tank generates:

TSS = 320.2m3/d x 60Kg/m

3= 19212Kg/d

VSS = 19212Kg/d x 75/100 = 14409Kg/d

Ash = 19212Kg/d - 14409Kg/d = 4803Kg/d

The secondary tank generates:

TSS = 653.2m3/d x 60Kg/m

3= 39192Kg/d

VSS = 39192Kg/d x 80/100 = 31353.6Kg/d

Ash = 7838 Kg/d

Combined TSS = 39192Kg/d + 19212Kg/d = 58404Kg/d





31

Page 31 of 34

Combined VSS = 31353.6Kg/d + 14409Kg/d = 45762.6Kg/d

Combined ash = 7838.7Kg/d + 4803Kg/d = 12667.4Kg/d

48% of the VSS is destroyed in the digester.

VSS Destroyed = 45762.6Kg/d x 48/100 = 21966Kg/d

Gas yield

1kg of VSS destroyed yields 0.45m3

of methane. Total volume of methane produced is therefore

VSS destroyed x 0.45m3

= 21966 x 0.45m3

= 9884.7m3

Gas storage

Two inflatable membranes gas storage shall be provided having volumes of 5000m3.

CHP Power output

1Nm3

of methane yields 9.97kWh (Horan, Nigel, 20). Energy produced daily is therefore:

9.97kWh/1Nm3

x 9884.7m3

= 98550.5kWh

Power =

Dewatering

The digestate is thickened to a minimum of 23% dry solid as the tender requires. The sludge

thickening centrifuge is used for the dewatering process thus keeping operational cost low.





32

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Nutrient recovery

The digestate from the MAD process is very rich in nutrients especially nitrogen and phosphorus.

They can be applied directly to agricultural lands within the limit of local regulations. The digested

liquor is also rich enough to precipitate a magnesium ammonium phosphate called struvite

(Mg.NH4.6H2O). Struvite has been found to be a good slow-release fertilizer and in the face of global

phosphorus shortage (Horan Nigel 22), struvite production will contribute to the economy of the

waste water plant operations.

The Ostara pearl® process (Horan Nigel 22) of manufacturing struvite from digestate liquor is built

into this design as part of future expansion plans. Up to 90% phosphorus and 40% ammonia load can

be removed from the digestate liquor using this method (Ostara technologies).





33

Page 33 of 34

CONCLUSION

Units operations Flow (m3 /d) BOD (kg/d) TSS (kg/d)

1. Screen 145564.5 11,436 8005

2. water course >145564.5 11,436 8005

3. Grit chamber 145564.5 11,436 8005

4. Primary Sedimentation tank 82,395 8005.2 3202

5. Storm tank >63169.5

6. Returned Activated Sludge 27,465

7. Secondary sedimentation tank 82395 988.7 1400.78. Sludge thickener 973.4

9. Blender/storage 973.4

10. Digester 973.4

11. Digestate store 973.4

12. Belt filter press

13. Biogas Storage

14. CHP

14

5

67

9 8

10

11

14

1213

2

3





34

REFERENCESHoran N. (2011) Approach channels and screens (CIVE 5532 Lecture note), school of civil engineering,

University of Leeds.

Degremond Technologies. (2011). Infilco climber screen. Available at www.degremont-

technologies.com/IMG/pdf/CLIMBER_SCREEN_US_Infilco.pdf (accessed on 12/1/12)

Idswater. (2011). PISTA Grit Removal System. Available at

http://www.idswater.com/water/us/grit_removal/1037/products.html (accessed on

12/1/12)

Horan N. (2011) Dealing with drainage (CIVE 5532 Lecture note), school of civil engineering,


Horan N. (2011) Grit Removal (CIVE 5532 Lecture note), school of civil engineering, University of

Leeds.

Horan N. (2011) Design of radial flow tanks (CIVE 5532 Lecture note), school of civil engineering,


Horan N. (2011) Primary Sedimentation Tanks (CIVE 5532 Lecture note), school of civil engineering,


Lin, S (2001) Water and Wastewater Calculation Manual , London, Mc Graw-Hill.

Horan N. (2011) Estimating Reactor Volumes (CIVE 5532 Lecture note), school of civil engineering,


Horan N. (2011) Selectors and Anoxic Zones (CIVE 5532 Lecture note), school of civil engineering,


Horan N. (2011) Reactor Aspect Ratio (CIVE 5532 Lecture note), school of civil engineering,


Euroby. (2011). Centrifugal Thickening. Available at

http://www.euroby.com/cent_thickening.htm?gclid=CIHy2PGd860CFUoifAodjRZ9sA (accessed on

16/1/2012)

Vindis et al. (2011). The impact of anaerobic digestion on biogas production. Available at

http://www.journalamme.org/papers vol36 2/36210.pdf (accessed on 20/1/2012)

http://www.degremont-technologies.com/IMG/pdf/CLIMBER_SCREEN_US_Infilco.pdf



http://www.idswater.com/water/us/grit_removal/1037/products.html

http://www.journalamme.org/papers_vol36_2/36210.pdf



http://www.idswater.com/water/us/grit_removal/1037/products.html



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