Computer-Aided Design of Sequencing Batch Reactors

Jeremy Dudley, David L Russell and Youri Amerlinck

Introduction

The activated sludge process was first conceived as a batch reactor, operating in draw-and-fill mode, around 1914. The work of Arden and Lockett in England pioneered the development of the activated sludge process. The first activated sludge system was a fill and draw reactor (our modern day SBR) which was built in Davyhulme, Manchester, UK. By 1917, the first continuous-flow plant was built in Worcester and Wittington, UK. At that time the literature never considered or discussed the problems associated with flow variations, bulking sludge, and the initial high oxygen demand when sewage was first introduced into the oxidation tank. The high oxygen demand and the flow variation led to the development of the conventional activated sludge plant as we know it today6.

The developers of the first activated sludge process did not consider the difficulties of operating the process. Without adequate automated control systems, the operation of an SBR can be a continual juggling act. In a multiple tank system, one tank is filling, while a second is reacting, a third decanting and a fourth is settling or resting. The first operators of the fill and draw plants may have been good candidates for early retirement because of the continual demand for attention to the plant. By 1920 the fill and draw system was replaced by a continuous process and the first plants were built in the US The first step aeration or tapered aeration plant was built in 1940 at Wards Island, New York. From that point on, until the mid-1970s (at least in the US) the batch process or SBR was largely ignored or consigned to the laboratory.

In all this development and change, the problems associated with bulking sludge were noted, but were poorly understood. At the time (Heukelekian and others in the 1940s6) considered bulking sludge to be an environmental disease of the activated sludge brought on by poor (high) C: N ratios, low dissolved oxygen (DO) levels in the aeration tanks, and increasing organic loads on the plants6. But sludge bulking began to become an operationally important issue.

In the mid-1970s Dr. Robert Irvine refocused attention on the Sequencing Batch Reactor. At that time, the SBR was the new hot topic in waste treatment1. The SBR was promoted as being a pure plug flow system that produced a crystal-clear effluent, and had no sludge bulking problems. Because it operated as a fill and draw tank it was ideal for small communities or where there was an intermittent flow at periods of low flow, just let the balancing tank fill. When full, it would discharge into the SBR. A typical cycle is shown in Figure 1.

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Figure 1- SBR cycle

The SBR process was then modified further instead of having a holding tank and an SBR, why not have two SBRs? The fill period could be extended; reliability was improved, as there were two duplicated items of equipment rather than a dedicated holding tank (unable to aerate or settle/decant) and the SBR tank. And still a good quality effluent was being produced. Following on from the success of SBRs other semi-batch systems were introduced, using compartmented reactors and allowing inflow during the settle periods. Part of this development was facilitated by breakthroughs in control systems technology. The development of inexpensive control systems which automated the sequencing of the SBR became the standard. SBRs moved to become, again, a mainstream process variant9.

Storms on the horizon

SBRs were going in everywhere. And then problems began to reappear. Sludge bulking re-appeared, despite the claims that SBRs guaranteed that bulking was a thing of the past. Effluent quality was no longer maintained. And there appeared to be little understanding of why this was the case.

The problem appears to be worse in the UK. This, in turn, appears to have been some US-based vendors selling SBRs without accommodating for the different effluent standards. Many US sites have a discharge permit based on a monthly average. Most UK sites have a two-tier consent: the first is that the effluent quality must be, 95% of the time, and allowing for the statistical issues associated with estimating 95% compliance, below a set value. And a second tier is then imposed, which must be met 100% of the time without fail. Some of the SBRs were designed without understanding the difference between meeting a monthly target a 50%-ile and a 95%-ile. Failures were bound to occur.

Existing design methods

There are several methods for designing SBR systems. Almost all of them are based upon the development of a steady state design, and then volumetric apportionment of the system as if it were an extended aeration system. The conventional approach to design is based upon an F/M (food to micro-organism) ratio, treatment cycle duration and hydraulic retention time. The Ten States Standards, one of the more widespread

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design standards in the US, does not consider SBRs. Common design standards are given in the following table.

Table 1 Typical SBR design figures

Sequencing Batch Reactor Design Basis Typical figures5

Parameter Low Loading Rate Medium-High Loading Rate Municipal Industrial Municipal Industrial F/M 0.05-0.1/day 0.05-0.1/day 0.15-0.4 0.15-0.6 Treatment cycle duration (hours)

4.8-6.0 4.8-48 4 4-24

Low water MLSS Mg/L

4000-4500 4000-6000 2000-2500 2000-4000

Hydraulic retention time (hours)

18-24 Variable 6 14 Variable

Iowa Criteria4

Nitrifying No Nitrifying F/M (domestic) 0.05 0.10 0.15-0.4 MLSS & MLVSS Calculate at low water and select an appropriate value.

WEF SBR design guidelines2

Flow, gal/d (m3/d)

Detention time (hours)

Cycle time (hours)

Design F/M ratio

SRT Reaction time

(hours) 2,000-

832,000 (7.5- 1352)

7.6-49 3-24 0.037-0.32 0.028-431 0.7-18

Settling time (hours)

Effluent BOD5 mg/l

Effluent SS

mg/l

Effluent NH3 mg/l

0.75-3 5-11 6-18 1.8-10

If there is any guidance from these examples it appears to be in the fact that there are extreme ranges of data over which one can design an SBR, and still obtain a reasonably good effluent. Additional information in the design manual goes on to provide indication of high treatment efficiency often above 95% and high system reliability with good effluent quality. Even Metcalf and Eddy1 use a modified MLSS basis for design of an SBR, and it is designed much in the same manner as an extended aeration plant is designed.

1 The range of data are quite extreme, and may be misleading. The actual values are 0.028, 43,

18.8, 0.067, and 15.45 days so that a designer could pick almost any value desired for an SRT.

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A common design approach is shown in Figure 2. Computer-aided modelling can help with much of the design, but not with some of the critical starting points: the design flows and loads and the choice of configuration. (Having chosen the configuration the modelling can help evaluate the design for meeting design objectives, and as an iterative procedure can assist in deciding which of competing alternatives would be the preferred option.)

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Definition of Input Data Effluent consent Dry weather and maximum storm flows Average organic and nitrogen loads

Process Configuration Plant with or without balancing tanks Filling strategy (continuous or intermittent)

Cycle Design Assumptions Duration of cycle times for fill, react, settle, decant, idle Adequate adjustment for storm flows

Tank Hydraulic Design Tank number, volume and plan area Sludge age/loading rate/SSVI Assessment of bulking risk

Equipment Capacities Aeration Waste activated sludge withdrawal Decanting

Verification of Performance Dynamic simulations (if required) Pilot tests (if required) Nitrogen balance

Figure 2 Typical steps in SBR design

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Many of the design methods in common use are based on steady-state assumptions. A common method is that presented in Randall et al7. Because of the simple design equations there is no assessment of the sludge settling: will the sludge blanket stay well below the water level during the decant cycle2

610/1 SVIMLSSfactorsafetydecantvolumeFillvolumeTotal

=

? The suggested approach is to calculate the fill volume as the daily volume (with a suitable peaking factor) divided by the number of cycles and SBRs (less one, since one is always outside the fill cycle). The SBR volume is then calculated as

This equation assumes that the SVI is a good assessment of the volume that the sludge will occupy at the end of settlement3

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. But the SVI test is affected by the settlement depth being in a 50 cm cylinder, rather than a 3 m or deeper tank. For a deep tank, or a long settling time, this may require a bigger SBR than is necessary. There are other equations, to assess if the SBR will meet the required ammonia effluent value the effluent BOD and suspended solids are assumed, because of the superficial handling of settlement. The recent IWA monograph on SBRs glosses over the design aspects, possibly because of the problems in developing a simple design method.

The fill volume, in its turn, is calculated as

( ) daypercyclesofNumberSBRsofNumberfactorpeakingflowDailyvolumeFill

=1

The design is driven by the hydraulics, and the treated volume uses a peaking factor: should this be the ratio of average daily maximum to average flow? Average monthly maximum? Maximum allowed flow to treatment? The play-off is between risk of insufficient available volume against purchase cost. Without a design approach that allows the cycle times to be studied in greater depth, to see what scope there is for variation in the cycle times, the steady-state design approach exposes an unknown element of risk.

Another approach, available through a spreadsheet posted on the web, allows the engineer to become an integral part of an iterative method for designing SBRs: guess the biomass concentration in the SBR, and the nitrogen mass. Run the spreadsheet. Do the numbers that you guessed match those calculated in specified spreadsheet cells? If

2 One assumption for obtaining volume is to calculate the desired end point MLSS and make an

allowance for the volume based upon the assumed or measured growth rate and a 20% factor of safety for volume of the sludge blanket based upon the average MLSS.

3 This equation also shows that initially increasing the safety factor will result in requiring a larger SBR volume. But increasing the safety factor further will produce a negative volume readily identified when doing manual calculations, but a possible cause of confusion when such procedures are computerised.

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not, update the guessed values and rerun the spreadsheet. If you need to use more SBRs than the spreadsheet was intend to represent then you have to modify all the formulae, to allow for the changes introducing a fragility into the design approach that would be best avoided.

Dynamic models

The use of dynamic models allows inclusion of more realistic conditions. Typically SBR models, implemented in packages such as STOAT or WEST, include the following:

A choice of internationally-accepted detailed activated sludge models, such as the IWA models4 ASM 1, 2d and 3;

An internationally-accepted settling model, such as the Takacs model8;

Solving the reaction equations during the fill, react, settle and decant cycles, automatically recognising the effects of aerobic and anoxic periods;

Automatically resolving sludge yield through the models, requiring fewer assumptions about apparent sludge yield production from an SBR;

Allowing multiple SBRs to be represented, so that with a diurnal and weekly sewage profile the SBRs will receive different flows and loads at different times, and to each other.

Although the SVI is not a good estimate of the sludge volume in a deep tank it does correlate better against the sludge settling properties. The dynamic models are thus better set up to estimate the sludge blanket depth.

Typical layouts for a four SBR system are shown below. The first indicates what can be done in STOAT, and the second in WEST. The differences are superficial although the two programs may look different the underlying approaches and results are comparable.

This system was run with the following parameters:

Maximum volume: 1567.5 m3 Minimum volume: 1292.5 m3 Surface area: 275 m2 The operational settings were:

SSVI 100 ml/g Cycle time 4 h Aerobic fill 1 h React time 2 h Settle time 0. 5 h Decant time 0.5 h

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Figure 3 - Typical 4-basin layout in STOAT

Figure 4 - Typical layout in WEST

Seven days of sewage, with an hourly variation, was used. The flow, BOD and ammonia can be seen in the following figure, and summarised in Table 2.

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Figure 5 - Influent sewage profile

Table 2 - Sewage characteristics

Flow, m/h (MGD)

SS (mg/l) BOD (mg/l) NH3-N (mg/l)

Mean 74.6 (0.48) 88.4 182.3 29.6 Minimum 29.0 (0.18) 0.0 77.0 18.0 Maximum 137 (0.88) 121.0 253.0 37.0

Different models use slightly different sewage characterisation. BOD-based models typically ignore any organic nitrogen, assuming that this will be accommodated by the (unmodelled) effect of nitrogen uptake by the biomass. The BOD is split into two fractions, particulate and soluble and only the soluble BOD is available for oxidation and biomass growth. Particulate BOD must be converted into soluble BOD before it is available. COD-based models do comparable things; the COD may be treated as four fractions: degradable and nondegradable, soluble and particulate. They also usually require information about the organic nitrogen (directly, in the case of the ASM1 model, and less so for ASM2 and ASM3, as these use stoichiometric ratios against the COD). Where data is provided as BOD and ammonia then the COD-based models must have assumptions about the organic nitrogen and the ratio of COD to BOD and degradable COD to total COD. These ratios are nearly universal when treating domestic sewage and are documented in standard textbooks1 and the IWA report on the activated sludge models4.

The simulation results can be seen for ammonia in Figure 6. Despite the simulation using a repeating seven-day period for four weeks there is no equivalent repeating pattern for the SBRs effluent. (The pattern almost repeats, but not quite.) This is caused

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by the effect of the SBRs receiving different volumes and loads with each cycle during the seven-day period, as can be seen by in Figure 7 the result is that there is no equivalent to a traditional plug flow system, which would show a repeating seven-day cycle in the effluent quality.

The average effluent ammonia is 0.8 mg/l, but the 95-percentile value is 1.4 mg/l. With a 1 mg/l effluent standard this would be an acceptable design in the US, but would fail in the UK4. The sampling requirements for SBRs are more difficult than for continuous-flow systems, because of the intermittent nature of the discharge. This can result in requirements for flow balancing on the discharge, and also for more frequent, automated, sampling of the effluent both issues that can reduce some of the claimed cost-savings attributed to SBR systems.

Figure 6 - Combined effluent: Ammonia

4 This example does not compare the ease of meeting effluent standards between the USA and the

UK, but rather brings out the differences in the understanding of the standards on the resulting design. The regulators would adjust the numerical requirements to ensure comparable environmental objectives.

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Figure 7 SBR tank volumes

Cycle times

The cycle times in SBRs are an additional operational parameter that can be adjusted to improve performance. Unlike continuous-flow systems, where changing the spatial variation of the treatment system requires expensive alterations, SBRs can readily achieve the same effect using temporal variations and these require only modifying a program in the PLC systems controlling the SBRs. The fill period usually has little scope for variation, because of the need to accept the incoming sewage flow, but the use of the idle time does allow for variations in the reaction, settling and decant times.

What happens as the times allowed for settling and decant vary? Clearly, decant with no settle will decant mixed liquor, and produce an unacceptable effluent. Equally clear is that a long settle, with a high-speed decant, and may cause turbulence that will lift the sludge blanket. Some of these effects can be represented in the models, as they include an induced upflow caused by the decant flow.

It is easy to plot the variation of the sludge blanket depth and the water level during the simulation. Figure 8 shows that the settle period, at 30 minutes, leaves plenty of time for the sludge blanket to descend well below the decant levels. Using this a control system can be set up that the normal operational period is 30 minutes settle followed by 30

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minutes decant, but in an emergency this can be accelerated to 10 minutes settle. The decanting period usually cannot be accelerated, as the mechanical movement of a weir or the diameter of a submerged pipe sets the discharge flowrate.

Figure 8 - Levels in a single cycle

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Figure 9 - Levels with a 10-minute settle

If the various pollutants are examined, as in Figure 10, additional information can be gathered. The dissolved oxygen control algorithm uses a PI controller with the simulation package defaults. There is a high DO during the fill period (modelled as an air flow with no attempt at DO control the air flow is used for mixing), with DO starting to fall off during the start of the reaction cycle. The minimum air flow is too high, so that the DO is maintained well above the desired set point of 2 mg/l, and this high DO means that there is continued reaction during the settle and decant phase, until the oxygen is consumed. Denitrification does not take place because of the exhaustion of the soluble BOD. It should be possible to reduce the air provided to this SBR design and still maintain the desired effluent quality.

The concentrations of pollutants are low, so that a shortened react time could be considered in an emergency. For one or two cycles, depending on the consent regime, it may be possible to dispense with oxidation completely and rely solely upon dilution as a means of getting the required volumes through the system.

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Figure 10 - Variation of pollutants in a cycle

Adjusting the fill cycle from aerobic to anoxic (unaerated) results in slightly higher ammonia, but in return a much lower nitrate. Although the fill period is unaerated there is initially a significant dissolved oxygen this is caused by the stratification of the SBR during the settling period, and the rapid settling of the sludge blanket so that much of the tank volume has insufficient biomass to deplete the oxygen When the tank contents are mixed, at the end of the settling cycle, then the oxygen within the bulk of the tank increases.

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Figure 11 - Pollutants with unaerated fill

Design MLSS

Because of the batch reactor of an SBR designing for the MLSS is difficult. The MLSS varies during the cycle because if the effects of biomass growth and the dilution of the incoming sewage. In addition, wasting during the settling and decanting phases is affected by the varying concentration of the settled sludge unlike a settler there is no mass balance around the settler to help decide the appropriate wastage flow rate.

The dynamic models again allow for an assessment of the wastage on the operating MLSS, and for the effects of the chosen MLSS on effluent quality. Here we present results for three scenarios, with MLSS values of approximately 2,000 mg/l, 1,100 mg/l and 400 mg/l.

The effluent BOD is around 4 mg/l whether the MLSS is 2,000 (Figure 12) or 1,1000 mg/l (Figure 13), rising to around 6 at 400 mg/l (Figure 14). Whether adequate flocculation could be guaranteed at an MLSS of 400 mg/l would be a design question, currently outside the area for computer models. There are conventional activated sludge plants that have operated at this low a level, although in the UK the common guidelines would recommend operating at 1,000 mg/l or higher, and preferably above 2,000 mg/l.

The effluent TSS does not vary much with changing MLSS (Figure 16 and Figure 14). Providing the sludge blanket can settle to well below the decant position then the

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effluent TSS is affected more by the solids that are left in suspension than by issues of MLSS or SSVI. The conventional Takacs model normally has the limiting effluent TSS values set as a constant fraction of the MLSS (typically, around 0.1-0.5% of the MLSS) which should result in an improved effluent quality as the MLSS drops; but at low concentrations the solids settling velocity decreases, so that effluent quality deteriorates.

The most common reason for operating at a high MLSS is to ensure nitrification with a reasonable aeration tank volume. In the simulations presented here the ammonia is around 0.2 mg/l at 1,100 mg/l MLSS (Figure 15), rising to around 2 mg/l at 400 mg/l (Figure 14). The nitrification performance appears to be excellent even at the low MLSS, and would suggest that there can be scope for further optimisation of the base design, to reduce the total volume (the decant volume cannot be adjusted, as this is set by the hydraulic load entering the plant: but the non-decanted volume is a design variable).

Figure 12 Effluent BOD at 2,000 mg/l MLSS

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Figure 13 Effluent BOD at 1,100 mg/l MLSS

Figure 14 Effluent BOD, NH3 and TSS at 400 mg/l MLSS

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Figure 15 Effluent NH3 at 1,100 mg/l MLSS

Figure 16 Effluent TSS at 1,100 mg/l MLSS

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Storm effects

One of the most important aspects of SBR design is how the system will behave during a storm. The steady-state design cannot represent a storm, and so storms are handled by the combination of safety factors and peaking factors. The real plant has to be designed with a control system that contains a panic mode for when all the tanks are full and still there is inflow to the site. Typical panic modes are to let the excess flow bypass treatment or to begin an emergency settle/decant cycle while permitting inflow.

With the dynamic model it is possible to properly study the effects of magnitude and duration for a storm, and see how the SBRs will perform. It is also possible to implement a representation of the control system to study how that control will perform ahead of commissioning5

An example of this is shown in

.

Figure 17. This shows that a short storm, where the flows get up to three times the average, if sustained for more than about two hours will result in the SBRs being unable to accept the storm flow. Further simulations can then be done to study the controller behaviour under such storm conditions and provide an operating envelope around the storm magnitude and duration that can be accepted.

Figure 17 - Storm bypass behaviour

5 SBR control systems are a vendors proprietary knowledge, and consequently none of the current

commercial simulator packages have a full representation of the control system for an SBR.

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Bulking revisited

The current generation of activated sludge models cannot represent bulking behaviour. Sludge settleability is a model given, chosen by the user. There are models that can estimate the ratio of filaments to nonfilaments, but these models have not been fully proven, and going from estimates of the filament to nonfilament ratio to an estimate of settling behaviour is outside the capability of even these models.

However, there is other laboratory work (Dennis and Irvine, 1979), backed up by the empirical understanding of bulking causes, which indicates that bulking behaviour in SBRs is exacerbated by having long fill periods. A short, sharp, fill is more likely to produce a good-settling sludge. Dispensing with a balancing tank and switching to a system of SBRs may well be the cause of some of the recent occurrences of bulking behaviour.

That plug flow behaviour promotes a good-settling sludge is well-known, and has been since the 1970s, as seen in the following diagram10.

0.001 0.01 0.1 1.0 100

100

200

300

Stir

red

Spe

cific

Vol

ume

Inde

x S

SV

I 3.5(

ml/g

)

Plug Flow Complete Mixing

Figure 18 Variation of SSVI3.5 with degree of plug-flow behaviour

Although the models cannot predict the likelihood of bulking they can assess the robustness of the design to bulking, by carrying out simulations with several assumed SVI values6 Figure 19. The effect of this variation is shown on the sludge blanket in and 6 The simulation programs use the SSVI3.5 rather than the SVI this is a stirred sludge volume

index in which the results are reported at a reference concentration of 3.5 g/l MLSS. The result of using a defined test cylinder, stirring conditions and reporting concentration has been that the correlation between SSVI3.5 and the sludge settling parameters is improved other settleability measures, such as the SVI test.

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the effluent solids in Figure 20. Typical design values are that SSVI = 100 is a good-settling sludge; 120 is a typical sludge; 150 is a bulking sludge; and 200 is an extreme value. The figures show that the this design would be acceptable for a bulking sludge, but that if the sludge showed unusually poor settleability then the sludge storage volume would be insufficient.

Figure 19 - Sludge blanket variation with SSVI

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Figure 20 - Effluent solids variation with SSVI

Aeration capacity

Calculating the aeration requirements for SBR systems is more difficult than for continuous-flow plant, especially when there are multiple tanks. A common approach for a continuous flow system is to evaluate the oxygen demand under average load and then to apply a peaking factor to estimate the maximum daily requirement.

For an SBR let us take the numbers used above: 1 hour fill, 2 hour react, 4 hour cycle, and 4 SBRs in total. Each SBR has to degrade in 2 hours 1 hours worth of incoming load. So each SBR needs half the aerator capacity of a continuous flow system. But if each SBR has a dedicated blower then, with four SBRs, we need to install twice as much aeration capacity as for a continuous flow system and we know that this excess capacity will be under-utilised.

However, each SBR also behaves like a plug flow tank, where the oxygen demand will be much higher at the start of the react phase than at the end. In a continuous flow system we compensate for this by providing less aeration capacity at the tail end of the plant. In an SBR we need a control system that can provide the turndown, but where the aerator power has to be sized for the peal demand at the start of the cycle. If we assume that the early part of the react cycle has a peak of 1.5 over the average for the cycle we then conclude that the SBR system, using the simple approach of dedicated

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blowers, will need something like three times the installed capacity of a continuous flow system.

To improve on the provision of the aeration capacity requires allowing for two factors. The first is that the SBRs do not all operate with the same oxygen demand at the same time, so that by looking at the demand across all the SBRs there is scope to optimise the aeration capacity. This is most easily done with a computer model, where the demand can be summed for all the SBRs. The second factor, which current process simulators do not handle, is that the water depth in the SBRs may be different, implying that the air distribution control will need to handle the effect of these different discharge heads on the blower performance.

The typical profile, for one SBR, for our simulations is given in the following figures. These show the oxygen demand during a two-day period (Figure 21) and the oxygen demand converted to a cumulative frequency response (Figure 22). They show the difference between looking at single SBR and the system of all four SBRs.

The smoothing effect of looking at all four SBRs is clearly evident; the continued peaks in the oxygen supply are as a result of the uncontrolled aerated fill cycle, and could be further optimised. The demand frequency curve shows that there is always a need for aeration in at least one SBR, and that the turndown required across the whole system is around 3:1 (50:15 kg O2 h), compared to a single SBR with a range of 30:0 kg O2/h.

Figure 21 Typical oxygen demand variation with time

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Figure 22 Typical oxygen demand variation frequency

Applications

Programs such as STOAT and WEST have been used by operating companies as part of their preliminary design, to ensure that SBRs will be acceptable to meet the required effluent standards. This usage has also provided a benchmark against which vendor designs can be assessed too small a recommended volume may indicate that the proposed design will have many problems meeting the acceptance tests, while too large a volume implies that a cheaper offering should be available. When used this way successful tenders have come close to the design recommendations from these programs, providing greater confidence for the operating companies that their capital investment will meet the regulators requirements on both environmental and financial criteria.

Conclusions

SBRs can offer flexible treatment systems, but recent operational problems have cast a cloud over their continued acceptance. Designs have been based on empirical methods adopted from steady-state models, with many fudge factors to handle the dynamic nature of SBRs. Vendor design methods are not usually disclose the vendors do accept the effluent guarantees and may be based on steady-state or dynamic models, but appear to be commonly spreadsheet-based methods, using greatly simplified activated sludge models. The commercial simulators, while not including the proprietary knowledge of the vendors, provide a framework for analysing many of the known design requirements for SBR systems, and can assist in identifying potential design short-comings.

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References

1. Metcalf and Eddy, 1991, Wastewater Engineering: Treatment, Disposal and Reuse, McGraw Hill, 3rd Edition

2. RW Dennis and RL Irvine, 1979, Effect of fill: react ratio on sequencing batch biological reactors, Journal WPCF 51(2) 55-263

3. WEF, Design of Municipal Wastewater Treatment Plants Volume 2, 4th Ed., Water Environment Federation, Alexandria, VA

4. M Henze, W Gujer, T Mio and M van Loosdrecht, 200, Activated sludge models ASM1, ASM2, ASM2d and ASM3, IWA Publishing, London, UK, ISBN 1 900222 24 8

5. TL. Kirschenmana and S Hameed, 200, A Regulatory Guide to Sequencing Batch Reactors, Iowa Department of Natural Resources, presented at the Second International Symposium of Sequencing Batch Reactor Technology, July 10-12, 2000, Narbonne, France

6. K Mikkelson, 1995, AquaSBR Design Manual, Aqua Aerobic Systems, Rockford Il, USA

7. D Orhon and N Artan, 1994, Modeling of Activated Sludge Systems, Technomic Press, Lancaster, PA, USA

8. CW Randall, JL Barnard and HD Stensel, 1992, Design and retrofit of wastewater treatment plants for biological nutrient removal, Technomic Publishing, Lancaster, PA, USA

9. I Takacs, GG Patry and D Nolasco, 1991, "A dynamic model of the clarification-thickening process", Water Research 25(10) 1263-1271

10. PA Wilderer, RL Irvine and MC Goronszy, 2001, Sequencing batch reactor technology, IWA Publishing, London, UK, ISBN 1 900222 21 3

11. B Chambers and E Tomlinson, 1982, Bulking of Activated Sludge, Ellis Horwood Publishers, Chichester, UK

Jeremy Dudley, PhD, is a chartered engineer at WRc plc (Swindon, UK; Dudley@wrcplc.co.uk) and one of the principal developers of the STOAT modelling engine. David L Russell, PE, is a chemical engineer and president of Global Environmental Operations inc. (Lilburn, GA; dlr@mindspring.com), which distributes WEST software. Youri Amerlinck is a Process Consultant for HEMMIS, NV in Kortrijk, Belgium (YA@hemmis.be). Hemmis makes WEST software.

Computer-Aided Design of Sequencing Batch ReactorsIntroductionStorms on the horizonExisting design methodsDynamic modelsCycle timesDesign MLSSStorm effectsBulking revisitedAeration capacityApplicationsConclusionsReferences