Design of a sewage treatment works for a rural community. Authors: Edwards, Ekanem, Singh,...

7/30/2019 Design of a sewage treatment works for a rural community. Authors: Edwards, Ekanem, Singh, Sutherland.

1/58


2/58

Design of a sewage treatment works for a rural community 1ABOUT THE AUTHORS

The findings of this report, were researched, selected and compiled together, by the

students, the names of whom are listed on the front page.

Ross Edwards studied the preliminary treatment and sludge processing phases, which

involve the screening, grit removal, plus the sludge dewatering, stabilising, thickening and

conditioning processes. He wrote the text for Screening, and for most of Chapter 4. He also

produced the calculations for Appendices 2, 8 and 9.

Emmanuel Ekanem considered the options for tertiary treatment, and researched the

legislation, which governs the discharge of sewage effluent: into sensitive waters. He also

advised on the layout and contents of the report.

Rohit Sinyh worked on many of the engineering calculations for the grit removal, secondary

treatment, and the clarifiers. He produced the calculations in Appendices 3, 5, and 6 (thealkalinity).

Malcolm Sutherland investigated the likely flow-rates and composition of the raw

wastewater, the equalisation method, studied ihe options and principles of secondary

treatment involving (dc) nitrification, and enquired about the monitoring required. He also

compiled the report, wrote the text for the other chapters, and produced the calculations

for the other appendices.

All four authors contributed their knowledge to producing the Power point slides.


3/58

Design of a sewage treatment works for a rural community 2EXECUTIVE SUMMARY

This report contains a basic theoretical design of a sewage treatment works (STW), which is

to serve a rural community of 10,000 people (where the population rises to 12,000 during

the summer). The works is designed to discharge treated water to sensitive waters, and

should be in operation for the next 50 years. This is a combined sewer, which therefore

accepts storm-water as well as domestic wastewater.

The STW will contain the following: 2 storm-water tanks, an equalisation tank, dualscreening, 3 grit removal chambers, 2 anoxic tanks, 2 oxidation ditches, 2 secondary

clarifiers, a sludge thickener tank, a sludge stabilization tank, and a sludge filter belt press.

The STW is designed to treat a maximum influent of6DWF (around 18000m3/day), and

retain up to the same volume in the storm-water tanks for later treatment. Excess

wastewater in flows exceeding 12DWF (36000m3/day) will be passed via the storm-water

tanks, screened, and discharged; this excess wastewater is not expected to contain BOD, Nand P levels, which exceed EU guidelines.

The dual screens will be fitted within the 1.5m diameter sewer entering the STW plant, and

are designed to treat up to 12DWF. The grit chambers each treat up to 3DWF. Each of the

treatment plants downstream, are expected to treat up to 6DWF. The screenings and grit

will be collected for landfill disposal.

The anoxic tank is located upstream from the oxidation ditch. The ditch is designed to allow

nitrification to occur, whereby organic and ammoniacal nitrogen is converted to nitrate. This

wastewater is passed back to the anoxic tank, where the influent wastewater and recycled

sludge from the secondary clarifier help to eliminate oxygen in the water, and allow de-

nitrification to occur. The volume of the oxidation ditch and anoxic tank, are 3600m3 and

1300m3respectively. The volume of the secondary clarifier is approximately 1500m

3.

Lime and FeCl3(iron chloride) addition help to maintain the pH of the wastewater at around

pH7, and remove phosphorus. Around 430kg of lime and 210kg of FeCl3 is required. It is

calculated that around 1620kg of solids, in 173m3of surplus sludge, is produced each day.

The surplus sludge undergoes 4 treatment stages, before it can be collected for landfill


4/58

Design of a sewage treatment works for a rural community 3CONTENTS

Abbreviations Page 4

Introduction Page 5

1: Inputs and Legal requirements Pages 6 - 10

1:1: Inputs and Wastewater Composition1:2: Legal Requirements for the Discharge of Sewage Effluent into Sensitive Waters

1:3: Predicted Energy and Capital Costs

1:4: Layout of the Sewage Treatment Plant

2: Preliminary Treatment Pages 11 - 15

2:1: Equalisation2:2: Storm water Tanks

2:3: Screening

2:4: Grit Removal

3: Secondary Treatment Pages 16 - 22

3:1: Principles of Secondary Treatment

3:2: Choosing and Appropriate Treatment

3:3: Designing the Secondary Treatment Plant

3:4: Supplied Oxygen Demand for the Aerobic Tank

3:5: Sludge Recycling and Re-circulation

3.6: The Secondary Clarifier

3:7: De-nitrification and the Anoxic Tank

3:8: Chemical Treatment

4: Sludge Treatment Pages 23 - 30

4:1: Characteristics of Sludge and Processing

4:2: Sludge Thickening


5/58

Design of a sewage treatment works for a rural community 4ABBREVIATIONS

BOD: Biochemical Oxygen Demand

BOD:the BOD measured over a 5-day incubation, and normally called BOD

C-BOD:the carbonaceous BOD

D.O.:Dissolved Oxygen (in wastewater)

DWF:Dry Weather Flow (m3/day) (domestic wastewater only)

MLSS: Mixed Liqour Suspended Solids

MLVSS:Mixed Liqour Volatile Suspended Solids

N-BOD: Nitrogenous BOD

Q: flow rate (m3per day)

Total BOD:the sum of C-BOD and N-BOD


6/58

Design of a sewage treatment works for a rural community 5INTRODUCTION

The purpose of sewage treatment, as defined by the Royal Commission on Sewage Disposal

(1889-1915), is to remove the pestilence and odours which ii produces, along with other

elements, compounds and organic matter which can degrade both water quality and its

ecology (Harrison, 2001).

The report is based an entirely theoretical design of a sewage treatment works (STW), which

is to serve a population of 10,000 residents, and an extra 2000 visiting tourists during thesummer The most important issue is that the plant is to discharge treated effluent into

waters, which are classified under EU regulations as sensitive. The aquatic habitat is

therefore vulnerable lo small nutrient inputs, especially N and P, which can lead to

eutrophication. Attention must therefore be given to removing nitrogen and phosphorus

from the wastewater.

The area of the proposed site is 150m x 300m (approx.), which is 45,000m

2

. It generallyconsists of broad topography, with a very gentle slope downhill to the edge of the sensitive

lake. A combined sewer overflow (CSO) sewer pipe of 1.5m diameter arrives at the site.

The main decisions in this report, include the selection of appropriate preliminary, primary,

secondary, chemical, and sludge treatment methods. The preliminary treatment has to be

very effective, as no primary sedimentation tanks are needed for the nitrifying secondary

treatment. This is the oxidation ditch treatment, which also requires anoxic tank treatment,

where nitrogen is converted from NO3to N2gas.

Phosphorus must also be removed, and there are biological and chemical treatment options

for achieving this. Sludge disposal and treatment must produce de-watered compost, which

is suitable for landfill disposal methane production must also be fully accounted for. Finally,

the influx of storm-water should be predicted, and this must be contained within the STW

site with few exceptions.

On a final note, as nearly all the values and calculations produced in this report are

theoretical, it is almost certain that the predicted tank sizes, settings and treatment

requirements will be slightly different for an active STW. Over the years, the population is

expected to increase, and the composition and volume of wastewater produced per person


7/58

Design of a sewage treatment works for a rural community 6CHAPTER 1: INPUTS AND LEGAL REQUIREMENTS

1.1: Inputs and wastewater composition

Wastewater from both storm water and domestic sources is a mixture of suspended and

dissolved materials. Suspended solids (SS) cause turbidity within the water, reducing

photosynthesis, and suffocating benthic life, especially in rivers. Sewage Treatment usually

involves preliminary, primary, secondary, and (more rarely) tertiary treatment. Domesticsewage is composed of about 1g/L of impurities in suspended, colloidal or dissolved form, as

shown in Figure 1:

Figure 1:a generalised diagram showing the major components of domestic wastewater (Harrison, 2001).

Contaminants in effluent, which can harm receiving waters, include carbonaceous BOD,

nitrogenous BOD, phosphorus, solids, heavy metals, organics, pathogens, and even thermal

pollution. The design on the treatment plant is mainly focused on removing both forms of


8/58

Design of a sewage treatment works for a rural community 7Total suspended solids in raw sewage occur at around 400mg/L in UK waters (Rendell, 1999;

Metcaff and Eddy, 2002). Typical P concentrations range between 5 and 15mg/L, and Total

N values (the sum of all organic and ammonium nitrogen), range between 40 and 80mg/L(Mudrack and Kunst, 1997).

It is essential to know how much wastewater the proposed works is expected to receive.

There are no straightforward answers or figures to this problem, although Rendell (1999)

lists the following assumptions. Dry Weather Flow" (DWF) is the daily wastewater

production, under dry weather conditions. The calculation is as follows:

DWF = PG + E + (trade/industrial fl ow)

P = population (persons)G = volume produced per person per day (m

3/person/day)

E = infiltration of water in the pipe network (m3/day)

Trade/industrial flow in m3/day

This does not take into account, the storm water inputs, which are unpredictable over anyperiod of time. A theoretical approach taken by engineers in recent years is the "Formula A"

equation, which is used to calculate the peak flow of wastewater:

Peak fl ow = DWF + 1.36P + 2E

Thus the STW should be equipped to cope with all but the most severe storm overflows. The

infiltration will not be considered in the report, as this is a new pipe system (although it islikely to increase over the years). Rendell (1999), states that the average flow is around

3DWF.

The STW is designed to treat an influent of up to 6DWF per day, and a storm-water tank will

be provided, to accommodate up to 6DWF (this is normal practice, for STW plants

discharging to sensitive waters (Kiely, 1997)). It is designed to meet me needs of 12,000

people, and thus treat up to 18,000m3

of wastewater per day (2,400m3

at DWF x 6 x 120%).Multiplying values by 120% is used throughout the calculations, in order to allow some

variations in the flow-rate and organic load entering the plant.

With 10,000 local inhabitants, it can be assumed that there is no heavy industry. There will

be public services and small companies in the community but for this project only the


9/58

Design of a sewage treatment works for a rural community 8should be included unless it can be demonstrated that the removal will have no effect on (he

level of eutrophication.

...estuaries, bays and other coastal water's which are found to have a poor water exchange,

or which receive large quantities of nutrients.

Articles 4 and 5 of the same directive set out the minimum requirements of treated effluent

chemical standards for 5 main categories of pollutant (Table 1) (SEPA 2002):

Table 1: EU requirements for effluent discharges to sensitive waters (correct in 2002)

The predicted levels of these entering the STW are provided in Table 2and in Appendix 1

(Kiely, 1997):

Table 2: predicted concentrations of specified pollutants entering municipal sewage treatment works


10/58

Design of a sewage treatment works for a rural community 9

Table 3provides some suggested electricity consumption figures for activated wastewater

treatment plants with nitrification. These values are for a 5-mgd plant, which thereforetreats approximately 19,000m3 per day (no figures for a 4-mgd plant were obtainable):

Table 3

The net electricity use per unit volume of wastewater decreases with increasing size of the

wastewater intake by the STW, The values quoted here, should be observed cautiously;

converting these into financial costs does not consider other payments, such as

environmental levy (axes, VAT, maintenance, and labour costs.

1.4: Layout of the sewage treatment plant

The STW Site Map over-page (Figure 2) contains drawings of each treatment process unit

(e.g. sludge tank, equalisation tank). Those are not drawn perfectly to scale, although it can

be seen, that there is a substantial area of remaining space for future expansion.


11/58



12/58

Design of a sewage treatment works for a rural community 11CHAPTER 2: PRELIMINARY TREATMENT AND OVERFLOW FACILITIES

2.1: EQUALISATION

In general, there are peak flows in the mid-morning and early evening hours. With these

varying flows come varying concentrations of pollutants, particularly BOD (Figure 3). The

treatment plant is designed to treat wastewater set at a specific level, and thus the flow rate

within the STW premises needs to be controlled, by establishing it at an average hourly flow

rate (Kiely, 1997).

Figure 3: a typical depiction of varying wastewater flow rates Mid BOD levels throughout the day

A set volume of wastewater is transferred to an equalisation tank. Particularly if there is a

long dry weather spell, the wastewater will be more concentrated in pollutants. Peak daily

flows, if released into the STW, could result in poorly treated effluents. The volume of the 1

DWF-receiving equalisation tank is illustrated in Figure 4:


13/58

Design of a sewage treatment works for a rural community 12Qasim (1994), and Metcalf and Eddy (2002) provided graphs showing the average domestic

wastewater flow-rate throughout the day. However, the average input each day is 3DWF,

and storm-flow will alter the shape of the graph shown above. Metcalf and Eddy (2002)stated that using collected influent data, and using statistical methods of predicting the

average flow-rates, is needed, before an equalisation tank can be provided for a combined

sewer input. No predicted volume for the equalisation tank has been calculated as a result.

2.2: STORMWATER TANKS

SEPA expects a STW to have a 3 or 6-DWF capacity storm-water lank, and it is normal tor a

STW discharging to sensitive waters, to be able to retain up to 12DWF, and treat up to

6DWF of wastewater per day.

Each of the 2 tanks is to accept up to 3DWF (7200m3). Sedimentation will occur, as the

wastewater must be retained, prior to being re-allocated to the STW. The percentage of

settleable solids being retained by the tank varies; one German design is reported tointercept up to 70%, although this depends on sampling data for the analysis of the SS

settling characteristics (Michalbach, 2000). A more realistic prediction is around 25% (Article

20, Watershed Protection Techniques, 2000).

Storm-water tanks at the Forfar STW are radial (akin to radial primary sedimentation tanks);

the tank at St Andrews STW is rectangular. Sludge removal can be conducted through a pipe

(sludge well) in the radial tank, whereas a scraper device (e.g. the Parkson Corp.SuperScraper, 2002) (B) can be used in rectangular designs (Figures 5, 6):

Fi 5 h di l di i k d l k


14/58

Design of a sewage treatment works for a rural community 13Grit removal

It is not mentioned how much BOD is removed through storm-water storage, but forprimary sedimentation, this seldom exceeds 30%*. Furthermore, the BOD concentration

will approach 25mg/L at 12DWF. No references were found, which stated the (%) solids

content of the outgoing sludge production (which will also contain grit). Should the storm-

water input exceed 12DWF, the excess needs to be screened prior to it being discharged.

Figure 7shows the tangential design, which is simpler to operate (Metcalf & Eddy, 2002).

The wastewater passes into the upper chamber, around it, and exits (blue arrows). The gritis kept behind the mesh in the chamber, and passes downwards (brown arrow).

Figure 7: The tangential fine-mesh screening"; removal chamber for excess storm water

2.3: SCREENING(see Appendix 2for calculations)

Bar screening is the coarse screen of choice for medium to large wastewater treatment

facilities. The screen, and cleansing provision, will be housed within a chamber located

directly behind the inlet sewer, and immediately ahead of the grit removal unit (Figure 8):


15/58

Design of a sewage treatment works for a rural community 14Chamber parameters

The chamber is required to house a dual screen assembly, each capable of handling peakdesign flow (12DWF). This ensures system redundancy in the event-of failure of either

assembly. Stop gates are proposed are proposed both up and downstream of each rack at

the entrance to and exit from each channel. The channel base is to be set 0.08m lower than

the sewer inlet, within the normal parameters (0.07-0.15m) (Qasim 1994). Headless and

velocity within the chamber can be controlled either internally via installation of a Parshall

Flume, for example - or externally - by control of wet well levels- Velocities should not

exceed 0.9ms-1

through the screen, nor drop below 0.4ms-1

. These parameters prevent flow-through and sedimentation of solids.

Assembly cleaning

A mechanical system will be employed in this design, within the regular screening chamber.

This is deemed cost and energy efficient for medium sized facilities (Metcalf and Eddy,

2002).

A continuous back-raking, back-return system is proposed due to the decreased potential

for failure as a result of obstruction, relative to that of a front-cleaning mechanism. This

system does have drawbacks. The submerged moving parts require the isolation and

drainage of the channel in order that maintenance is expedient. However, this is considered

and provided for in the dual-channel design employed. Screenings are to be elevated to the

operation floor and disposed of into a removal hopper.

Screening composition and quantity

Screening composition and quantities can vary widely. The amounts removed by the bar

screen in the proposed system are around 0.06m3/day (max 0.1 m

3/day) (Appendix 2).

Screenings recovered from the chamber produce odours, so the removal of screenings to

sanitary landfill will be daily, or as is dictated by the odour nuisance (Metcalf and Eddy,

2002).

A summary of the screening provisions is provided in Table 4:


16/58

Design of a sewage treatment works for a rural community 152.4: GRIT REMOVAL (see Appendix 3for calculations)

The provision of the grit removal chamber is essential, as primary sedimentation is notrequired for the oxidation ditch. It is there to remove heavy inorganic particles, which can

abrade mechanical equipment, and reduce the frequency of anaerobic digester cleaning due

to grit accumulation (Figure 9). Rectangular and square-horizontal grit removal chambers

have been used for many years, and this design has been selected. It is calculated that a

chamber, with a hydraulic retention time of 1 minute, designed to receive an influent

of3DWF, will have a length, width and depth, of 15m, 0.4m and 1m, respectively. Four of

these will be provided: two to treat the 6DWF (max.) of influent; and two redundantchambers.

The quantities of grit being removed will vary. This is performed using a conveyor with

scrapers, from which the grit is placed in a large refuse container, which can then be

collected for disposal at a landfill. Metcalf and Eddy (2002) quoted values of 0.004 to 0.2m3

grit per mega-litre of waste-water (for 2.4ML at 1DWF, the maximum grit collected would

be 0.6m3

).

Figure 9: a schematic diagram of the grit removal chamber and the conveyor used to collect the grit


17/58


18/58

Design of a sewage treatment works for a rural community 17The Oxidation ditch layout(see Appendix 4for calculations)

The oxidation ditch is different by comparison with a conventional activated sludgetreatment (where DO levels are lower, around 1mg/L at 20C) (Arundel, 2000). An example

(in Mequito, Nevada) is provided in Figure 10. Rendell (1999) stated that the MLSS must be

around 3500mg/L. The two-stage biological nitrogen removal model is represented in

Figure 11.

Figure 10: an oxidation treatment system in the United States

Figure 11: the de-nitrification process whereby anoxic conditions are created by combining BOD-rich raw


19/58


Figure 12: note that the BOD at 1DWF is set at 350mg/L, to account for unexpectedly high inputs. This setting

applies to the calculations in Appendices 5, 6, and 8 through 10

The Total-BOD is different to the BOD, (C-BOD) value, which does not account for

nitrification (N-BOD) (this occurs after 6 days incubation (Figure 13) (Metcaff and Eddy,

2002)). The Total-BOD = 756mg/L, and the calculation is given in Appendix 4.


20/58

Design of a sewage treatment works for a rural community 19Rendell (1999), in calculating the volume, simply uses a BOD5value, which does not include

the nitrifying component. The volume of the tank is calculated to be around 3600m3.

Providing a second (redundant) oxidation tank is a normal feature (Metcaff and Eddy, 2002).The biomass production is predicted to be around 350kg VSS/day (Appendix 4), and the

MLSS concentration is set at 3500mg/L (Rendell, 1999).

If a depth of 7.5m is allowed, and the width/length ratio is 1:10, then the dimensions would

be (approx.) 7.5m x 70m (length) x 7m (width).

3.4: SUPPLIED OXYGEN DEMAND FOR THE AEROBIC TANK

There are 2 ways of calculating the oxygen demand, and both results are given in Appendix

4. The more detailed method, prescribed from Metcalf & Eddy (2002), is used here. This is

based on both the C-BOD, and N-BOD:

It is predicted mat around 50 tonnes of air will be required for the oxidation ditch. Four

types of aerators can be used to provide this, and these are calculated to have different

oxygen transfer efficiencies and power consumption levels (Table 5) (Rendell, 1999):

Table 5: oxidation ditch aerators


21/58

Design of a sewage treatment works for a rural community 20For both me oxidation ditch, and for extended aeration, the recommended recycle ratio (r)

values will lie between 0.75 and 1.0 (also see Figure 14). The surplus sludge being sent to

the anaerobic tank will be around 173m3

/day, with a solids mass of around 1 tonnes (thisincludes precipitates from Fe salts used in P removal). The sludge mass (Appendix 7) is

significantly larger than the biomass produced (Appendix 5). This may owe to the use of

empirical values and calculations under Rendell's (1999) method. The result in Appendix 7

is still used for the calculations in the sludge treatment, although this may be an over-

estimation.

Figure 14: the pipe network within secondary treatment which involves recycling sludge

3.6: THE SECONDARY CLARIFIERS

The final clarifier fulfils a similar purpose to that of a primary sedimentation tank. The depth

must be greater than 3 metres. Ideally, about 4 m should be allowed, for the 0.7 to 1.0m

of sludge thickening, plus the 1m layer of buffering, and the 2 m layer of effluent

clarification. The hydraulic retention time should not fall below 2 hours (resulting in

inefficient settling of solids), nor should it rise far above this value (as septicity may develop)

(Kiely, 1997).

Given that hT= 2 hours for the full (6DWF) intake:

Volume of clarifier => 18000 m3/day (24hrs 2hrs)


22/58

Design of a sewage treatment works for a rural community 213.7: DENITRIFICATION AND THE ANOXIC TANK

In the oxidation ditch, the high MLSS (Mixed Layer Suspended Solid) concentrations and(F/M) ratio allows nitrification to occur, which involves two bacterial organisms:

Nitrosomonas

55NH4++ 76O2+ 109HCO3

- C5H7O2N (biomass) + 54NO2

-+ 57H2O + 104H2CO3

Nitrobacter

400NO2-+ NH4

++ 4H2CO3+ HCO3

-+ 195O2 C5H7O2N (biomass) + 3H2O + 400NO3

-

For denitrification to proceed, the water must be completely deprived of free oxygen (i.e. it

becomes anoxic), in order for nitrate to be converted to nitrogen gas:

NO3(aq) NO2(aq) NO (aq) N2O (aq) N3(g)

If this is performed in a separate channel which precedes the aerobic reactor tank, then a

hydraulic retention lime of about an hour may be adequate (Rendell, 1999). The dissolved

oxygen (DO) must be less than 0 5ppm; the oxidation ditch DO is min. 1.5ppm. For an MLSS

of between 2000 and 4000mg/L, the anoxic tank volume must be around 40% and 20% of

the aerobic tank volume, respectively (Barnes et al, 1983). Qasim (1999) states that an h, of

1.5 hours is adequate, thus:

720m3x 1.5hours = 1125m

3(x 120% = 1350m

3)

This is around 37.5% of the volume of the oxidation ditch, within Bames' recommendation.

If the depth = 7.5m, the length and width of the tank could born be around 13.5m. As the

water is septic, strong odours may be produced, and so this process will be housed inside,

unlike the oxidation ditch (Metcalf& Eddy, 2002).

3.8: CHEMICAL TREATMENT(see Appendix 6for calculations)

Inefficient de-nitrification


23/58

Design of a sewage treatment works for a rural community 22complicated, and possibly unsuccessful, approach (Arundel, 2000). A simpler method is to

treat the secondary effluent using inorganic salts.

Aluminum and iron salts are often used. Fe is less toxic to the microorganisms, and can be

applied as FeSO4 or FeCl3. FeSO4 addition requires careful lime addition, and this can be

complicated and inaccurate. FeCl3is also cheaper, as it does not require a nearby industrial

source. Less FeCl3 is required (209kg/day), in comparison to FeSO4 (360kg/day). FeCl3 is

therefore chosen, which will produce around 140kg of Fe(OH)3precipitate (Rendell, 1999;

Metcalf and Eddy, 2002).

Alkalinity treatment: lime addition

As seen on page 17, the chemical reactions involving Nitrosomonasand Nitrobacter, include

the reaction with carbonate, which leads to increased acidity. This effect must be counter-

balanced with the addition of CaCO3, or else the secondary treatment ecosystems will

collapse (Metcaff and Eddy, 2002). The required addition to the oxidation tank is around

356.4kg/day (see Appendix 6).


24/58

Design of a sewage treatment works for a rural community 23CHAPTER 4: SLUDGE TREATMENT AND DISPOSAL

In municipal WWTP, sludge is a treatment by-product derived mainly from primary

sediments and secondary clarification, Pre-treatment products - including screenings and

removed grits - are not handled as sludge. The nature of sludge is dependent upon the

stream designed and implemented- However, a generic characteristic of sludge is the high

water content. This, coupled with the nature of the solids associated, creates the

requirement for adequate processing.

4.1: CHARACTERISTICS OF SLVDGE AND PROCESSING

Sludge consists of a suspension of organic and inorganic solid materials (at 1-5%), held

within a liquid phase, primarily of water, but also containing a plethora of aqueous

materials. It is precisely this diversity of content, which causes the high level of system

complexity and financial commitment involved in sludge processing. The chemicalcomposition of sludge is described in the tableover-page (sourced from Qasim, 1994).

Wastewater sludge types dictate the content and character. The three dominant types are

primary (extracted from the primary clarifiers), waste activated (from biological treatment)

and combined sludge (both primary and WAS). The basic blueprint for sludge processing

involves four stages:

(a) Thickening

(b) Digestion/stabilisation

(c) Dehydration

(d) Disposal

Sludge management commands that odour production and release must also be accounted

for, and dealt with appropriately. For example, the design proposal commands the presenceof odour scrubbers at any point of emission (eg. unit housing dewatering press and

screening filter).


25/58



26/58

Design of a sewage treatment works for a rural community 25Continuous Flow Gravity Thickening is the method, which has been preferred in this design.

Gravity thickening is executed within circular sedimentation dishes, which allow the influent

solids to stratify contents of the thickener. This is illustrated in Figure 15, with aphotographed example in Figure 16:

Figure 15:concentration profile of municipal wastewater sludge gravity-thickener (Qasim, 1994)

Figure 16: sludge thickener (in the foreground)

The system is based upon the principle of sedimentation pressure, within the thickening

layer, forcing the water and gas from the inter-aggregate spaces. This area - the "sludge

blanket" - must be maintained accurately using a SVR, value (ideally 0.5 to 2d) to determine


27/58



28/58

Design of a sewage treatment works for a rural community 27DAF is less economically viable for such a small plant. It is capable of higher removal rates

and accuracy than gravity thickening. However, for the design proposed, these are not

necessary and are coupled with aeration systems that are, by comparison, highmaintenance financially. Centrifugal thickening again incurs high power and maintenance

expenses. Thus, it is frequently limited to places where no other option is practicable (i.e.

limited space).

4.3: SLUDGE STABILIZATION

Sludge stabilisation is the process by which pathogens, material putrification and odours areall attenuated by either physical, or chemical, or biological methods. The final disposal

procedure for the treated sludge dictates the degree of stabilisation if any - required. At a

medium-sized STW, a process of anaerobic biological digestion has become the treatment of

choice and as such has been adopted for this design (example in Figure 16).

Figure 16: anaerobic sludge digestion tanks

Anaerobic stabilisation

Anaerobic digestion is a process of organic matter utilisation by a combination of anaerobic

D i f k f l i 28


29/58

Design of a sewage treatment works for a rural community 28Anaerobic digestion falls into one of two categories - "high rate" or "standard rate" - based

on the temperature at which the reaction is executed- Standard rate systems are unheated

and unmixed, resulting in 30-60d digestion periods. High rate systems are fully mixed andheated - usually by methane-derived energy - to temperatures within the thermophilic

range of methane-producing bacteria (45-65C). This results in efficiency and dehydration

benefits and so was used for this project:

Figure 17: the high-rate sludge digestion apparatus (Qasim, 1994)

The factors dictating anaerobic operation, i.e. bioreactor capacity, heating, mixing, reactor

gas harvesting and use, supernatant quality and sludge are calculated in Appendix 8.

4.4: SLUDGE CONDITIONING

Further dehydration is needed. Conditioning procedures involve physical or chemical

processes. This stage can also be used to fundamentally change the sludge according to the

downstream requirements - for example disinfection and odour treatment. Chemical

conditioning processes are used in conjunction with mechanical filtration systems. This can

be subdivided into organic and inorganic chemical dosing.

Organic chemicals used are hydrophilic, long chain polymers, which displace sludge- boundwater, remove charge and precipitate aggregate formation. This is the method of

conditioning selected for the project design, as only small quantities are needed.

D i f t t t k f l it 29


30/58

Design of a sewage treatment works for a rural community 29gravity. The true dehydration process is a result of pressure exerted by roller compression of

the sludge. The parameters for belt filter sludge processing are displayed in the scanned

tables over-page (sourced from Qasim, 1994).

The benefits of such a system are the drier cake that is formed, in comparison with other

filtration methods.

Centrifugal systems, although similar to vacuum filtration in performance, were discounted

due to excessive maintenance costs and solids presence (in supernatant). Drying systems

were also discounted, due to the land requirement and climatic considerations.

The dewatered sludge, subsequent to the dehydration process, is suitable for short/medium

term storage prior to disposal to sanitary landfill site.

Figure 18: filter belt press used in a STW in Montana (sourced fromwww.ci.helena.mt.us,2002)

Sludge processing: summary tables

http://www.ci.helena.mt.us/http://www.ci.helena.mt.us/http://www.ci.helena.mt.us/http://www.ci.helena.mt.us/


31/58




32/58

Design of a sewage treatment works for a rural community 1Flow Monitoring

Monitors must be established to measure (1) influent, (2) effluent, (3) storm tank (3DWF)overflow weir discharge, and (4} the combined storm overflow weir (6DWF) discharge.

Annual reports must be submitted to SEPA, giving all flow data records; these must contain

values for DWF, average WF, maximum storm flow-rates, and the duration of overflow rates

from the combined (6DWF) and on-line (3DWF) storm-water tanks.

Unusual Conditions

The performance of the STW will not be reviewed for periods, during which there are:

1 very low temperatures (effluent


33/58

Design of a sewage treatment works for a rural community 2CONCLUSION

As stated in the introduction, the calculations and assumptions in this report are entirely

empirical, and not all issues in relation to designing a sewage treatment plant have been

thoroughly discussed. For example, temperature and heat transfer in the treatment tanks,

disinfection, and the efficiency of the treatment methods, has not been fully considered.

Disinfection tends to apply to STWs in Europe, which discharge to waters being used for

recreational purposes. Although this report assumes that this is not the case, the use of the

lake could change in the near future.

One important fact to consider is that, on average, the removal efficiency of BOD, N and P

will never be complete in the secondary treatment. Although BOD removal is usually above

90% in oxidation ditches, N and P removal is less successful, with proportions rarely above

50% (Rendell, 1999). Both oxidation ditches may therefore be required to treat the nitrogen.

The secondary treatment is also vulnerable to temperature and climate. Wheretemperatures fall below 5''C, the micro-organisms may even become dormant.

Sedimentation in the clarifier is also subject to temperature, whereby the hydraulic

retention time increases with decreasing temperature (Metcalf and Eddy, 2002).

Disposal of sludge and grit to landfill is an increasingly expensive option. An alternative may

be agricultural land spreading, although this is also a controversial issue, due to the

presence of heavy metals and pathogens in the sludge.

On a final note, not only are the predicted treatment tank volumes, wastewater

characteristics, and other calculated values approximate; the findings in this report provide

only a limited insight into the many factors, which will determine the efficiency and

requirements of the STW as a whole. It can be said however, that the types of treatment

selected are appropriate, and the proposed site provides enough space for the STW, and for

gradual future expansion.



34/58

g f g f y

REFERENCES

Arundel, J. Sewage and Industrial Effluent Treatment. Ch.4: Biological Treatment Methods,

ppIOO-103, 2000 Blackwell Science Ltd., U.K.

Bames, D., Forster. C.F, Johnstone, D.M. Oxidation Ditches in Wastewater Treatment. Ch.6:

Aeration Methods, ppl84. 1983 Pitman Books Lid,. U.K.

Ebbing, D.D. General Chemistry (5thEd.). CD,3: Chemical Reactions: An Introduction, pp96.Copyright 1996 by Houghton Miffin Company

EC Directive 98/15/EEC amending Directive 91/271/EEC: Urban Wastewater Treatment.

Water Quality in the European Union (website), 2002

Harrison, R.M. Pollution - Causes. Effects and Control (4th

Ed.). Ch.5: Sewage and Sewage

Sludge Treatment, pp 13-119. 2001 The Royal Scoiety of Chemistry.

Kiely, G. Environmental Engineering. Ch.12: Wastewater Engineering, pp516-529, 531-533,

538-539, 541-544, 546-551. Copyright 1997 Irwin/McGraw-Hill International (UK) Ltd.

Metcalf & Eddy, Inc. Wastewater Engineering(4th

Ed.). C'h.2: Constituents in Wastewater,

pp54-55, 88. Ch-3: Analysis and Selection of Wastewater Flowrates and Constituent

Loadings, ppl86. Ch.5: Physical Unit Operations, pp315-327, 385-387, 394- 395. Ch.6:Chemical; Unit Processes, pp494-497. Ch.7: Fundamentals of Biological Treatment, pp611-

626. Ch.8: Suspended Growih Biological Treatment Processes, pp710-718, 738-739, 748-

754, 710-718. Ch.14: Treatment, Re-use and Disposal of Solids and Biosolids, pp1491-1493.

Ch.15: Issues related to Treatment-Plant Performance, ppl 652. 2002 McGraw-Hill

Companies Inc.

Mudrack, Kunst. Biology of Sewage Treatment and Water Pollution Control; pp12, 15.

Parkson Corporation. The SuperScraper Bottom Sludge Scraper.

(Website no longer available)

d



35/58

USEPA. Process Design Manual for Sludge Treatment and Disposal. USEPA Technology

Transfer EPA-625 (1974. 1979).

Veissman, W. (Jr.), Hammer, M.J. Water Supply and Pollution Control(5th

Ed.). Ch.8: Water

Quality, pp281-288. 1993 Harper Collins College Publishers. U.S.A.

Water Environment Federation, 1997. Energy Conservation in Waste-water Treatment

Facilities - Manual of Practice(MFD-2). Ch. 1: Energy-Efficient Wastewater Treatment, p9.



36/58



37/58



38/58



39/58



40/58



41/58



42/58



43/58



44/58



45/58



46/58



47/58



48/58



49/58



50/58



51/58



52/58



53/58



54/58



55/58



56/58



57/58


58/58

Date post:	14-Apr-2018
Category:	Documents
Upload:	dr-malcolm-sutherland
View:	217 times
Download:	0 times

Design of a sewage treatment works for a rural community. Authors: Edwards, Ekanem, Singh,...

Documents