8th European Waste Water Management Conference
SEQUENCING BATCH REACTORS - PAST, PRESENT AND FUTURE
Smyth, M.1, Horan, N.J.2 1Aqua Enviro, 2The University of Leeds Email: [email protected]
Abstract
A sequencing batch reactor (SBR) is a variant of the activated sludge process: a suspended growth, variable-volume, wastewater treatment technology. SBRs were wisely adopted by the UK Water Industry in the mid to late 1990's and into the early 2000's (AMP 2 & AMP 3). However during this period it became apparent that some SBRs were prone high suspended solids losses during the decant phase of the cycle and on occasions the sludge blanket itself. Often this was a result of filamentous bulking, in particular caused by Microthrix parvicella. The process therefore failed to consistently deliver a compliant final effluent and as a result SBRs fell out of favour, and in some cases were even replaced in AMPs 3 & 4 with conventional activated sludge processes.
In the past 5 years there has been an increase in the uptake of SBRs and they look set to be the choice of technology for organic waste digestion plants treating dewatering liquor. In addition 2014 will see the completion of the £200million upgrade to Liverpool WwTW with 16 basins on 2 levels.
This papers looks to: review the causes of the problems witnessed in the 1990's which included: filamentous bulking and foaming); poor sludge age control, resulting in partial or unintentional nitrification; inadequate blower capacity and shortened cycle times in high flow conditions. evaluate how these factors have been accounted for in present day designs, consider whether design modifications will result in a trouble free future for SBRs.
Keywords
Microthrix, foaming and bulking, sludge age, nitrification inhibition, liquor treatment
Introduction
The activated sludge process is the preferred technology worldwide for large, domestic wastewater treatment plants. It can be configured to remove carbonaceous material, nitrogen and phosphorus. There have been many variants of the process since its discovery 100 years ago by Arden & Lockett (1914) and the Sequencing Batch Reactor (SBR), a suspended growth, variable volume wastewater treatment technology, is one example. SBRs became widespread in the UK Water Industry in the mid to late 1990's and into the early 2000's (AMP 2 & AMP 3). During this period however it became apparent that some SBRs were prone to losing high levels of suspended solids during the decant phase of the cycle and on occasions the sludge blanket itself. The process appeared to encourage the development and growth of a particular filamentous bacterium Microthrix parvicella, which 8th European Waste Water Management Conference
once established can in a very short period of time cause severe episodes of sludge foaming and bulking.
The process therefore failed to consistently deliver a compliant final effluent and consequently SBRs fell out of favour and in some cases were even replaced in AMPs 3 and 4 with conventional activated sludge processes. In the past five years there has been an increase in the uptake of SBRs and they look set to be the choice of technology for organic waste digestion plants treating dewatering liquor. In addition 2014 will see the completion of the £200million upgrade to Liverpool WwTW with 16 basins on 2 levels.
Process Description
A sequencing batch reactor (SBR) is a variant of the activated sludge process; a suspended growth, variable volume wastewater treatment technology where treatment takes place in a single tank (circular or rectangular) and therefore removes the need for an independent secondary sedimentation tank and recycle system (Gerardi, 2010). SBRs operate in cycles which, for predominantly domestic wastewaters, are typically 4 to 6 hours in length and can be configured for carbonaceous treatment, nitrogen and/or phosphorus removal (Wilderer et al. 2001). Usually a site will operate with at least 2 basins (e.g. Nairn WwTW) and up to as many as 16 (Liverpool WwTW).
Figure 1: Example cycle times
A basin on a four hour cycle will therefore have six cycles each day and in a four basin configuration 2 will always be filling and aerating, 1 will be in the settle phase and the 4th in Decant and Idle.
Figure 2: 4 basin mode operating on 4 hour cycle times 8th European Waste Water Management Conference
There are a number of features which distinguish the different types of SBR systems available (often referred to by their acronyms: IDEAL, ICEAS, CASS, CAST, JetTech) in the market place, including the:
Cycle times, can be fixed times with fixed phases (figure 2 and 3), fixed times with fixed phases or both variable. Successful operation requires an automated control system (Bungay et al. 2007) or knowledge-based Intelligent Environmental Decision Support System (Sottara et al. 2014). Feeding regime which can be true batch process or can be permit during the decant phase at high flows (figure x). Configuration of a selector which can be external to the tank (captive contactor), internally configured and baffled Internal selector) or can exploit the whole basin by manipulating the fill regime (whole basin selector). Decant mechanism which can be fixed or floating (figure x), which should include a scum/foam guard to ensure that floating material is not entrained (Wisaam et al, 2007). Type of aeration which can range from fine bubble to jet aeration (figure x).
A feature of an SBR is that it is variable volume, the level in the SBR will at any point in time be between the bottom water level (BWL, typically in the region of ~4 metres) which is reached at the end of the decant period and a maximum top water level (TWL, typically ~6 metres). Whether or not the TWL is reached is dependent upon the incoming flow, but it cannot be exceeded otherwise untreated/partially treated wastewater and/or mixed liquor would be discharged.
For some plants if the flow treated and the level measured in the SBR looks set to exceed TWL the cycle time may be shortened (e.g. from 4 to 2-2.5 hours) and the option to introduce filling during the decant period introduced, thus permitting continuous fill. This change in cycle times generally occurs when either a basin is out of service (meaning that proportionally other basins must treat more flow) or during wet weather events when full flow to treatment conditions prevail. In the latter case the influent is very dilute and thus a shortened aeration period is not detrimental.
Figure 3: Storm Cycle
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Figure 4: Floating decanter system and jet aeration
Problems Encountered with SBR Technologies
SBRs became widespread in the UK Water Industry in the mid to late 1990's and into the early 2000's (AMP 2 and AMP 3) and were successfully marketed on their claimed simplicity, small footprint leading to reduced capital costs and especially their operational flexibility. In particular two solutions led the market, the CASS (Cyclic Activated Sludge System offered by Earth Tech Engineering, now AECOM) and the Jet Tech Omniflow SBR (US Filter). Although both technologies are fundamentally very similar, there are enough differences to make their design, construction and operation very different (Kirkwood, 2001).
However, a number of problems associated with the operation of sequencing batch reactors were seen in this period (table 1) but by far the most serious (in terms of environmental impact and frequency) was the loss of the sludge blanket during the decant phase leading to catastrophic consent failures on total suspended solids, BOD and COD, reviewed here. The major cause of this was filamentous sludge bulking and foaming.
Table 1: Problems encountered with SBRs Problem Contribute to foaming and bulking? Aeration systems prone to blocking due to Yes, ability to flush out toxic intermediary settlement in the air-off phases. Biofilm formation products from nitrification reduced risk very high. Air in the recirculation pipework due to air passing the isolation valve and being retained by No the head of water above the pipe. Faulty or leaking recirculation valves and leaking No valves on the decanter system Lack of IDSC (inlet distribution and sludge consolidation manifold) leading to short circuiting Yes, selector effect difficult to achieve and disruption of the sludge blanket. 8th European Waste Water Management Conference
Shortened cycle times during high flow Yes, leads to inadequate dissolved conditions oxygen provision Yes, ability to flush out toxic intermediary Aeration unable to ramp up to DO set points products from denitrification (nitrous and quickly enough nitric oxides) reduced Yes, nitrification enhances the likelihood Aerated sludge age measurement and control of a bulking & foaming event Load balancing between SBRs Yes, unintentional or partial nitrification Poor sludge dewaterability Yes, influences sludge age control
From the perspective of plant performance the major problems has been one of sludge foaming and bulking. This is a phenomenon in which the aeration basin is covered with a thick and stable foam and the sludge settles poorly during the settle phase. As a result it is very difficult to achieve the necessary solids consent.
The reasons for foaming and bulking in SBR systems operated in the UK are well but not fully understood. However one organism in particular, Microthrix parvicella, was routinely identified at sites experiencing loss of the sludge blanket. Microthrix is most frequently found where the (Eikelboom, 2002):
F/M <0.2/d Fats and grease present in wastewater. Low temperatures <15°C, so generally more of a problem in the winter and spring. Large anoxic zones and elevated nitrate levels both of which are associated with nutrient removal plants. Low DO concentrations can also be contributory.
Therefore a nitrifying plant operating in winter with inadequate or absent scum handling systems on the primary tanks and that struggles to reach >1.0 mg/l dissolved oxygen quickly after the anoxic zone, is a prime candidate for Microthrix proliferation.
Figure 5: Microthrix parvicella Gram stain, wet mount and Neisser stain (x100)
Microthrix is not unique to SBRs, but when compared to conventional activated sludge processes, it is not so flexible at dealing with the problems of bulking and foaming once they have occurred, because the SBR combines both biological treatment and solids separation in the same basin. In a conventional system the operator has a choice of where to manage the a foaming incident, it can be retained and managed in the aeration basin where it poses little threat to the consent, this choice is not open to the SBR operator.
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Figure 6: Foam contained in an oxidation ditch
When present Microthrix is effective at disrupting the floc structure of the sludge, it reduces the rate of separation of treated water from the mother liquid which leads to reduced settling velocities and SSVI3.5 (Stirred Sludge Volume Index) typically in the range 120-180 ml/g (Trumper et al., 2005). The presence of the organism in the mixed liquor also leads to a highly unstable sludge blanket-treated effluent interface that is easily disturbed by the motion and energy generated by the decanter. Furthermore the organism is hydrophobic and when a plant reaches 'tipping point' Microthrix has the ability to migrate to the surface of the basin and produce a thick, mousse like foam. Operators have reported that over the course of a weekend a plant can progress from having a 'small corner of foam' to in extreme cases exiting the basin itself.
Figure 7: Extreme foaming
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Figure 8: Foaming SBR
Microthrix is opportunistic and given the right conditions it can easily outcompete well settling floc-forming bacteria that designers and operators alike, aim to promote.
Plants that nitrify and denitrify encourage Microthrix and this is due to the way that the organism deals with the toxic intermediate products of denitrification (Gerardi, 2002), effectively performing a shunt reaction from nitrate to nitrogen gas.
Figure 9: The Microthrix shunt
Toxic intermediaries are flushed out in the aeration zone, if however there is a time lag in the provision of oxygen and achieving for example a minimum of 1 mg/l dissolved oxygen in these condition Microthrix has the advantage. This scenario is more likely to occur with SBRs where the cycle begins with an unaerated fill period or includes a fill option in the decant 8th European Waste Water Management Conference
phase. In conventional activated sludge plant design the part of the aeration lane following the anoxic zone is preferentially loaded with diffusers in anticipation of the high oxygen demand resulting from the combination of oxygen starved sludge and sewage. A plant that is nitrifying will operate with a longer sludge age (at the same temperature) than a plant designed for carbonaceous treatment alone.
Figure 10: Sludge age and nitrification
In theory, for a carbonaceous plant, maintaining the appropriate sludge age and avoiding nitrification should be fairly straightforward, however sludge age control for SBRs is more complex. In order to accurately calculate it and assess the likelihood of nitrification the following are measured the:
In-basin temperature. This is straightforward provided that a temperature monitor has been provided. Mass of MLSS in the SBR. As the level in the SBR varies throughout the cycle, the operator must either adjust the value recorded to BWL and use the BWL volume to calculate the mass or measure the level in the SBR at the time of sampling and calculate the active volume at that time. Where foam is present on the SBR non- routine sampling techniques must be employed to get a representative sample. Mass of sludge wasted from the process (Surplus/Waste Activated Sludge). SAS usually takes place at the end of the decant phase, there may be as little as a 10 minute window to sample every 4-6 hours. Mass of sludge lost in the final effluent. Impossible to determine if a plant is suffering from blanket loss and/or foam entrainment into the final effluent. Amount of time that positive dissolved oxygen is recorded in the cycle. Biomass actively convert carbonaceous material into new cell matter during those periods where a source of oxygen and electrons are available. On a four hour cycle with a two hour fill/aerate it is normal to assume that 50% of the time. However this estimate may be inaccurate, the actual period is difficult to determine when the DO set point is not reached quickly at the beginning of the cycle and during high flows when the cycle time shortens.
The batch nature of treatment also means that basins operating on fixed cycle times will receive over the course of 24 hours varying loads. 8th European Waste Water Management Conference
Figure 11: Diurnal load profile (1)
For example a basin being filled in periods where the morning and evening peak loads are being experienced will receive a greater load than one that sees part or none of that period. In the example below for a 3-basin SBR operating on a 6-hour cycle, Basin 3 over the course of the day receives 85% of the load of basin 1 and 90% of basin 2. These differences result in different Food: Microorganism ratios and sludge ages across the plant as a whole. Those basins that receive proportionally less load are more susceptible to unintentional nitrification/denitrification and filamentous bulking and foaming.
Figure 12: Diurnal load profile (2)
Design and Operating Considerations
The main issue associated with a nitrifying-denitrifying SBR treating domestic sewage is 8th European Waste Water Management Conference
Microthrix parvicella and the resultant foam and poor settling characteristics to the activated sludge. It is not a question of if the organism will be present rather when will its presence become a threat to consent, a number of design and operation considerations should be allowed for:
The start of the cycle is crucial, enough blower provision must be in place to flush out the toxic intermediaries. The first 10 minutes of the aerate period (referred to as the 'Flash Mix') should see the DO achieved in excess of 1 mg/l as quickly as possible. Blowers should be oversized to achieve this, the rate of oxygen provision not solely based upon traditional calculation rules for peak loads. Naturally this will result in increased capital (and operational) costs in the form of additional diffusers, larger blowers and the need for variable speed blowers to meet the oxygen demand after the flash mix period. Incorporate a cycle time that is not fixed for 4 or 6 hours, rather selecting e.g. 4h15 minutes to ensure that individual basins see different load conditions over time. Waste activated sludge in the aerate period to permit accurate sludge age control. Increased buffering and thickening capacity downstream will be required. This strategy also enables operators to accurately control in-basin MLSS levels and therefore optimise mass flux values on a basin to basin basis. Utilise the entire decant period. The decanter should be at TWL at the start of the decant period and the software capable of determining the slowest rate of decant throughout the full period (rather than reaching a fixed BWL and an idle period following). This will minimise turbulence with in the basin and disturbance of the sludge blanket. If the SBR includes separated zones (e.g. CASS, Jet Tech design does not) ensure that the option for variable speed return activated sludge (RAS) is included and sufficient control mechanisms (e.g. aeration, redox, on-line TOC analysis) incorporated to encourage fully aerobic operation to promote floc forming bacteria (Trumper et al, 2005). Build in additional settlement capacity. Traditional activated sludge clarifiers are usually designed for a maximum SSVI3.5 of 120 ml/g/. Domestic wastewater SBR sites have seen values of 120 or 140 ml/g, the latter provides a greater margin of safety but increases the overall footprint and capital costs.
Domestic Wastewater Treatment by SBRs in 2014 and Beyond
Liverpool WwTW marks the first large capital investment in SBR technology for domestic wastewater treatment in the British Isles for over a decade. The £200 million project see the infilling of Wellington Dock and build of the SBRs (£145m), upgrade to the existing outfall (£11m) and improvements on the existing site (£45m) (waterprojectsonline.com, 2013). SBR technology was selected for the site due to financial and space constraints. Awareness and mitigation of the issues with SBRs is evident in the design approach and final solution with the final design being informed by onsite pilot plant data and Biowin Modelling and CFD, neither of which were available to designers in AMP2:
Nitrification - whilst the site is not required to nitrify it has been designed for this purpose (and to denitrify) with a 12 day aerated sludge age due to concerns over treatability of the wastewater (high industrial fraction) at sludge ages of 4-6 days (carbonaceous) (Black, 2014). Microthrix parvicella would therefore be expected to be present in the biomass. Measures to control the organism include those previously listed: 8th European Waste Water Management Conference
o increased blower capacity to flush out toxic intermediaries by increasing the air flow provision rate from 11,800 to 16,000 m3/hr and the diffuser density from 5.6 to 9.6%.
Figure 13: Biowin modelling of dissolved oxygen (Black, 2014)
wasting activated sludge in the aerate cycle thus permitting better control over in basin MLSS and sludge age. the option to recycle activated sludge. optimising decanter control philosophy.
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The CFD modelling has been employed to simulate a range of worse case scenarios taking into account poor performance of primary settlement tanks, a maximum SSVI of 120 ml/g and piles within the basin and it is perhaps these two factors which (in the authors’ view) have the greatest unknown. Microthrix disrupts the floc structure of the biomass, the rheology of the mixed liquor and the stability of the sludge blanket-treated effluent interface whilst more conservative design select a design SSVI of 140 ml/g.
The settlement characteristics are further complicated as the cycle incorporates a continuous fill-decant, which will disturb the sludge blanket. The reason for the fill-decant is that Liverpool WwTW is a 1.5 DWF works as opposed to the conventional 3 DWF. The SBRs will therefore operate more frequently at FTFT than a 3 DWF works. This factor does however mean that a shortened or altered 'storm' cycle is not required, which helps with ease of operation. Whilst a continuous fill means that each basin will receive equal flow and load over the day the same the oxygen requirement at the beginning of the fill/aerate cycle will vary, being greater where peak load conditions have been experienced in the fill/settle and fill/decant cycles. The propensity for a slow ramp up in dissolved oxygen profiles is exacerbated in this situation and hence the risk of bulking.
Other areas of CFD modelled uncertainty in the authors’ view include the potential for saline intrusion, which can result in density currents plus the presence of high levels of dissolved solids and sulphate; insufficient alkalinity for full nitrification; and the high proportion of industrial COD in the influent. Combined these factors increase the likelihood of bio-fouling of the aeration system and diffusers in the non-aerate phases which in turn result in reduced rates of oxygen provision and transfer, increasing the bulking and foaming risk.
Figure 14: DO ramp up profiles
Liquor Treatment by SBRs in 2014 and beyond
In recent years there has been an increase in the uptake of SBRs for liquor treatment in the organic waste digestion sector. This move is being driven by the costs associated with the 8th European Waste Water Management Conference
transport of whole digestate, limited availability of landbank and to help achieve the requirements of PAS110 in terms of the residual biogas potential limit.
Whilst the basic principles of sequencing batch reactors apply in that treatment takes place in a single tank or set of tanks the nature of the influent is vastly different and as a result cycle times are much longer and the problems encountered different. SBRs treating dewatering liquor do not suffer from the Microthrix problem since this organism is almost always associated with low strength domestic wastewaters in nitrifying plants (Eikelboom, 2000).
In fact these SBRs rarely suffer from filamentous bulking although their performance is frequently affected by high levels of suspended solids passed forward to the basins from the dewatering process. Whilst a centrifuge or belt press operating in the wastewater industry would be expected to remove >98% of the solids fed to the unit in this industry 90% is more common. In practice therefore a system fed with 3% dry solids (30,000 mg/l) would pass forward 3,000 mg/l to the SBR.
The reason for the reduced performance is because food waste is a predominantly an organic material and after homogenisation and dilution contains almost no settleable solids (unlike a primary or secondary sludge). It is largely a colloidal suspension of organic material with the fibres associated with the feedstock. During the digestion process the particle size is further reduced due to hydrolysis, and the fibrous material swells and largely resists biodegradation. The anaerobic biomass does not produce exopolysaccharide and therefore whole digestate is uncharged and thus requires larger poly doses and potentially an additional source of cations (e.g. iron) to aid flocculation. Thus food waste digestate comprises a mixture of anaerobic biomass and swollen fibrous material. The fibrous material is loath to shed water and due to its large size will block conventional filter media, making this a material that is difficult to filter. (Baddelely et al., 2014)
Figure 15: The sludge particle
As a result the SBR contains a large but difficult to quantify amount of digested solids, which makes sludge age control challenging and on occasions impossible. Furthermore digestate produced from food waste digesters (where the measured hydraulic retention time is <40 days and organic loading rate >3kg.VS/m3/d) usually contain much higher levels of volatile fatty acids and ammonia-N (relative to sewage sludge digesters), modelling of the levels to be anticipated and/or upfront bench scale trials are essential to determine the carbonaceous and nitrogen loads as well as the dewaterability potential.
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Irrespective of the readily biodegradable (RBCOD) fraction the concentration of ammonia-N nitrogen to be treated by the SBR will be in the range 1,000-5,000 mg/l. At these levels additional challenges are posed when aiming to achieve nitrification and denitrification, if required.
Figure 16: Nitrification-Denitrification
In order to achieve nitrification 7.14g of alkalinity per gram of ammonia-N oxidised to nitrate is required. Supplementary dosing, usually in the form of sodium hydroxide, may well be required, where this is the case pH control becomes critical as with increasing pH the proportion of free ammonia is increased.
Figure 17: Free ammonia
Free ammonia is toxic both to Nitrosomonas (ammonia to nitrite) and also Nitrobacter (nitrite to nitrate) with inhibition for the former from 10-30 mg/l and as low as 0.1-1 mg/l for Nitrobacter. At pH 9.3 free ammonia and ammonium are in equilibrium.
Figure 18: Equilibrium curve for the effect of pH on free ammonia concentration (Gerardi, 2002)
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Accumulation of nitrite within the activated sludge process can also lead to inhibition of Nitrobacter, typically in the range 150-200 mg/l. Temperature can also be an issue. For plants treating liquors over 30oC both Nitrobacter and Nitrosomonas will become partially inhibited, quantifying the inhibition is challenging and required laboratory/ bench scale investigation. In order to account for these factors a purpose designed balancing tank where pH, temperature and load can be balanced are required. Incorporating denitrification into the SBR cycle time will also aid with pH control and reduce the alkalinity requirement, however an additional source of carbon will be required in the form of for instance, . molasses.
Conclusions
SBRs treating domestic wastewater are prone to filamentous sludge bulking and foaming, the organism responsible is usually Microthrix parvicella. Where present the design SSVI can be exceeded resulting loss of the sludge blanket, the organism also disrupts floc structure which leads to an unstable sludge blanket-treated effluent interface.
SBR cycle times encourage the proliferation of Microthrix especially where the process is configured to nitrify and denitrify. Unlike floc forming bacteria the organism is able to cope with the toxic intermediate products of denitrification by performing a shunt from nitrate to nitrogen gas.
Accurate sludge age control is a necessity in attempting to control Microthrix and this is not easily achieved in SBRs where different basins receive varying flows and loads over the course of the day and/or during storm/shortened cycle times.
Further design considerations to minimise filament levels include oversizing blowers to rapidly flush out toxic intermediate products during the fill/aerate period, adopting a cycle time that manages catchment specific load conditions, wasting mixed liquor during the aerate phase to control sludge age accurately and adopting a conservative design SSVI3.5 at the design stage.
SBRs are becoming more popular for the treatment of liquors from dewatered food waste digestate. Whilst sites do not appear to suffer from filamentous bulking on the whole, food waste digestate dewaters poorly, leading to large influxes of digestate solids into the SBR which complicates sludge age control, reduces potential throughput and compromises final effluent quality.
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