aerobic bioreactors for sewage treatment
4.3 IncrEASIng oxygEn trAnSFEr FroM A BuBBlE .1 Fine bubble diffusers and oxygen transferring .1 Fine bubble diffusers and oxygen transferring
technologies
Haney’s (1954) evaluation of bubble aeration effectively outlines the basic controlling parameters for subsurface aeration design are: (1) bubble size, (2) relative velocity and (3) residence time. By influencing these parameters it is possible to increase the OTR and the OTE from a rising bubble in a Wastewater Treatment
Tank, but in most cases the influence of the parameter requires additional energy and the modification may have an energy neutral or even energy negative impact.
E.G., By increasing the tank water depth from 3 m to 6 m a rising bubble has approximately twice the residence time in the water with which to transfer oxygen.
But for air to exit a diffuser plate underneath 6 m of water it must have pressure greater than 6 m hydrostatic head. This results in an increase of 80% in the theoretical energy required to achieve a two fold increase in OTE.
4.3.1.1 Smaller bubbles
Decreasing the bubble size which results in an increased gas-liquid surface area available for mass transfer (Table 4.1) has so far achieved a relatively good success at increasing the overall energy efficiency. This has resulted in fine bubble aeration becoming the standard aeration system recommended a head of coarse and medium bubble aeration, despite fine bubble aeration having a higher maintenance requirement.
table 4.1 Increase in bubble surface area available for mass transfer due to decreasing bubble size.
Increase in contact area Bubble diameter (mm) number of bubbles
per m3
total bubble surface area (m2)
15 5.66 × 105 400
10 1.91 × 106 600
3(FBDA) 7.07 × 107 2000
1(UFBD) 1.91 × 109 6000
Smaller than fine bubbles are ultrafine bubbles or micro bubbles, but concerns still exist regarding these technologies for the aeration of wastewater. Microbubbles can produce a much higher gas liquid interfacial surface area, but they also have a higher energy cost than typical gas distributors, the microbubble generators can allow downsizing of the aeration tank and shorten the overall residence time of the wastewater so that a reduction in the overall cost can be achieved (Terasaka et al. 2011). Despite having close to 100% Oxygen Transfer Efficiency it has been observed that microbubble generation is influenced by fouling which can occur on the porous membranes (Liu et al. 2013) and the aeration process can have adverse effects on the overall process, causing floatation and/or the break-up of activated sludge flocs. (Liu et al. 2012). Disadvantages of micro bubble aeration include higher capital cost, a higher head loss across the diffuser, increased air filtration requirements, and a tendency for the microbubble generation membrane to tear when over-pressurized.
Innovative energy efficient aerobic bioreactors 61
4.3.2 Increasing contact time
Along with smaller and smaller bubbles other approaches have been taken to increasing the oxygen transfer efficiency for bubble aerated systems, many of these have looked at providing more contact time between the introduced air and the liquid.
4.3.2.1 Diffusaire
With a view to achieving a higher Oxygen Transfer Efficiency with respect to the energy input Duiffusaire, an Israeli company have developed a system to increase the retention time of the bubble in an activated sludge tank. Air is added to the wastewater in a vertical tube placed in the Activated Sludge tank. The wastewater to be aerated is then pumped in such a manner as to create a downward flow in the tube and thereby increase the residence time of the bubble in the liquid, and allow more time for oxygen transfer. (Yousfan et al. 2011) The aerated liquid along with some entrapped air then escapes from the bottom of the tube back into the activated sludge tank. It is claimed that diffusaire proprietary technology can reduce the energy requirements for aeration by up to 50% (Diffusaire, Advanced Aeration Solutions).
4.3.2.2 Sorubin
Although somewhat similar in outer appearance to the diffusaire technology, OptusAir developed by a Sweedish company Sorub in creates a vortex in the centre of the vertically mounted tube through the use of an impellor placed at the bottom of the tube. The vortex entraps air from the surface and disperses this air liquid mixture from the bottom of the vertical tube into the wastewater treatment tank.
Optusflow could enhance the effect of aeration and other treatment methods by up to 50%.
4.3.2.3 Sansox OY
The OxTubeBySansox (Finland), also increases the contact time and relatively velocity between the gas and liquid, but do so in a pipe. Using a specially developed static mixer to break up the flow pattern and create lots of eddies the OxTube minimises the boundary layer diffusional resistance between the liquid and gas bubble (Sansox, Oxytube).
4.4 BuBBlElESS AErAtIon–MEMBrAnE AErAtEd BIoFIlM rEActor
With increasing oxygen transfer efficiencies coming from smaller and smaller bubbles, the next step has been to develop aeration system without bubbles altogether.
This has resulted in the use of diffusive gas permeable membranes for aeration.
Although the initial goal for the development of this technology was to increase the oxygen transfer efficiency to the wastewater, the subsequent biofilm which formed on the gas permeable membrane surface, which was initially seen as barrier to oxygen transfer, was later discovered to take an active role in the wastewater treatment process and lead to the development of the Membrane Aerated Biofilm Reactor (MABR) (Timberlake et al. 1988). As a means to increase the oxygen transferred from the air to the wastewater, by placing the oxygen containing gas inside of amembrane, the residence time of the air in the wastewater is decoupled from the buoyancy forces which normally limit the contact time between the air and water. Therefore it is possible to dramatically increase the Oxygen Transfer Efficiency (OTE) up to a theoretical maximum of 100%. By controlling the flowrate and the partial pressure of Oxygen through the membranes the OTE and the Oxygen Transfer Rate (OTR) can be controlled. This allows for an additional level of control over the oxygen transfer process for an operator. Although a biofilm colonises the membrane surface preventing the transfer of oxygen to the wastewater the membrane supported biofilm takes an active part in the wastewater treatment and because the consumption of oxygen occurs locally to the membrane surface the active biofilm increases the rate of oxygen flux across the membrane (Shanahan & Semmens, 2006).
The unique counter diffusional concentration profile which is created through the membrane aeration, also results in the natural development of aspatially unique biofilm. This membrane aerated biofilm is a counter diffusional biofilm (Figure 4.1) as opposed to a conventional co-diffusional biofilm.
Figure 4.1 Schematic of a membrane aerated biofilm.
Along with increasing the OTE the gas pressure required to supply the oxygen to the MABR is dramatically reduced and subsequently the overall energy requirement. The structural integrity of the membranes themselves prevents the hydrostatic pressure closing off the lumen of the membrane and if non-porous membranes are used no flow of water back into the membrane lumen can occur.
Therefore the supplied gas does not have to overcome the hydrostatic head and the only pressure which the supplied gas has to overcome is the pressure drop due to the air flow within the membranes. It must also be noted that because oxygen is not being transferred to the wastewater there are no wastewater surface tension
Innovative energy efficient aerobic bioreactors 63 forces to be overcome, therefore the alpha factor does not play a role in the oxygen transfer in a MABR.
While university based research groups have been examining this technology for some time with one of the first papers identifying the wastewater treatment potential being published by Timberlake et al. 1988, until recently the technology had not progressed beyond the lab scale. During this time the MABR has been tested for different applications including high rate BOD removal, tertiary nitrification and simultaneous nitrification and denitrification. These have all been identified as areas where the MABR has a significant Advantage over conventional systems and due to the increased level of process control the MABR can be tailored for each of these processes or for a multiple of different processes sequentially.
Despite this concept being around for many years 2 major stumbling blocks have prevented it from being commercialised.
(1) The availability of cheap suitable membranes to exploit the cost saving which results from the reduced energy requirement.
(2) The ability to achieve effective long term stable performance, this has typically been most difficult in wastewaters with high BOD loading rates.
In an economic evaluation of the MABR (Casey et al. 2008), the two major economic factors for the commercialisation of the MABR were identified as membrane replacement cost and energy cost as these directly influence the increased capital cost associated with membranes in a tank and the operational saving. Today with more and more membrane providers coming to the market and increased membrane operational and production knowledge, membrane technology has now become commoditised and the cost of membranes has reduced significantly over the past 10 years. This coupled with the increased process knowledge, higher levels of biofilm understanding and the ability to have effective biofilm control has led to a number of companies commercialising the MABR concept.
Two different approaches have been taken to the scale-up of the membrane aerated biofilm concept.
4.4.1 Submerged membrane aerated biofilm reactors
Both Oxymem and GE Water and Process technologies have taken a more conventional approach towards the MABR by placing the gas permeable membranes into a wastewater treatment tank in place of other aeration devices.
This has been the approach taken by many research groups including those of Semmens (WERF, 2005) and Nerenberg (Downing, 2008) and is summarised in the reviews Syron and Casey (2008) and Martin and Nerenberg (2013).
This configuration of the MABR allows the wastewater to flow around the gas permeable membranes and the oxygen/air is then supplied via a pipe network to interior of the membranes. The biofilm grows on the outside of the membrane and can be removed or have its outer layers sloughed off by a change in shear force
usually carried out by through intermittent coarse bubbles scouring or a change in liquid flow direction. The mixing and oxygenation are independent in this system and can be controlled or modified according to the requirement or needs of the system. The configuration also allows for the use of oxygen enriched air or even pure oxygen to be used with up to 100% oxygen transfer efficiency (VOSS, 1994).
To make the systems very compact there is a tendency to provide as much surface area per unit volume although this has led to problems with biofilm over growth.
Both Oxymem and GE have chosen to uses dense (non-Porous) membranes which although having a higher initial resistance to mass transfer than hydrophobic microporous membrane, are not effected by long term operation (Semmens, 2005).
The GE MABR uses very fine bore polymethylpentane (PMP) membranes while Oxymem have developed a PolyDimethylSiloxane (PDMS) membrane for use in their MABR (Figure 4.2).
Figure 4.2 Photograph of gas permeable membranes covered with biofilm (provided courtesy of Oxymem.)
Innovative energy efficient aerobic bioreactors 65
A summary of the membranes used from both systems is given in Table 4.2.
table 4.2 Submerged MABR data taken from (1) Syron et al. (2014), (2) Stricker et al. (2011), (3) Adams et al. (2014).
Summary of oxyMEM and gE Zeelung results Membranes
oxyMem gE (Zeelung) gE membrane
Configuration Hollow fibre Micro bore hollow fibre
Air Pressure mBar 100 mBar 410 mBar 550 mBar
OTE >50% 31% >60%
Operational data has been presented by both of these companies at international conferences and it is very likely that larger scale systems will be installed in the coming years.
4.4.2 passively membrane aerated biofilm reactors
In a concept similar to a trickling filter or biotower two companies Emefcy and BioGill pump the wastewater to be treated to the top of their membrane aeration unit. The wastewater then flows down through the inside of the membranes with the oxygen/air surrounding the membranes and the oxygen in the air at atmospheric conditions diffusing through the membrane into the pollutant degrading biofilm inside the membrane. This approach requires no energy for an air blower or mixing, only pumping energy is required.
The Spiral Aerobic Biofilm Reactor or SABRE has been developed by Emefcy an Israli company. The technology uses a membrane envelope wound into a spiral
with a defined space between each coil so that air can flow all around the membrane.
The wastewater to be treated is then pumped into the top of the membrane envelope, and flows around the spiral to the end where it exits the membrane. The biofilm grows on the inside of the membrane and any sloughed or detached biomass leaves with the treated wastewater to be removed in a subsequent unit operation. Energy requirements of 0.02 kwh/m3 of water treated have been reported for this system (Spiral Aerobic Biofilm Reactor-Emefcy).
Another similar concept which pumps the wastewater to be treated to the top of an above ground membrane unit is the Biogill developed by Biogill Operations Pty ltd (Australia). Biogill utilises folded Nano Ceramic membranes and the wastewater flows downward inside the membranes with a biofilm developing on the inside of the membrane. The nanoceramic membranes chosen by Biogill are porous to water and some of the wastewater permeates through the membranes creating a wet membrane surface and an environment suitable for the growth of fungi when contributes to nutrient removal. The wastewater is then collected in a decant tank underneath the membrane unit where the detached biomass is allowed to settle out of the wastewater. Based on the design parmeters given in the Biogill Technical System and specification guide the energy required for treating the water is 0.17kWh/m3 at the scenario given in Table 4.3.
table 4.3 Energy requirements for biogill, using operational data available through www.biogill.com.
Biogill energy requirements
BOD 500 mg/l
Flow rate 8000 l/hr
Recirculation rate 15
Recirculation pump flowrate 120 m3/hr
Height 2.5 m
Pump efficiency 0.6
Pumping energy 1.36 kw
Energy required per unit volume 0.17 kw/m3
4.5 loW EnErgy AMMonIA rEMovAl 4.5.1 Ammonia removal
Biological nitrogen removal is an oxygen intensive process with 4.57 g of O2
required for every g of N-NH4 oxidised. Traditionally the pathway for complete nitrogen removal was through the complete oxidation of ammonia all the way to nitrate (NO3) and then denitrifying the nitrate through the addition of a carbon source and allowing heterotrophic bacteria to utilise the nitrate as a source of
Innovative energy efficient aerobic bioreactors 67 oxygen. The product of this two stage biological process was nitrogen gas which escaped back into the atmosphere. Depending on the exact configuration chosen this process generally required significantly more air, additional recirculation pumps, longer biomass retention and significantly larger tanks than the more traditional biological carbon removal processes. Overall the energy increase for the addition of nitrification was estimated at 69% in a case study by Menendez and Veatch, 2010.
Thanks to an increased understanding of the biological nitrogen removal process, scientists and engineers have been able to provide the suitable conditions for the each of the different bacterial groups which are involved with multiple stages of the nitrogen removal process and through the control of these conditions along with reaction time, the nitrogen removal process can be split up into its component steps.
4.5.2 Shortcut nitrification
The first step of ammonia removal is carried out by Ammonia Oxidising Bacteria (AOB), these bacteria oxidise ammonia to nitrite, while the second group of bacteria Nitrite Oxidising Bacteria (NOB) oxidise the nitrite to nitrate. To achieve complete nitrogen removal it is not necessary to oxidise ammonia all the way to nitrate, by controlling the process in such a way as to prevent the complete oxidation of ammonia to nitrate, the intermediate produced nitrite can be subsequently denitrified to N2 resulting in a saving of 1.17 gO2 per g of N-NH4. Shortcut nitrification, as it known, uses effective process control to reduce the growth rate of the NOB and minimise the amount of nitrite converted to nitrate through a better understanding of the growth rates of the two groups of bacteria required for the nitrification process and their kinetic parameters such as oxygen affinity (Ciudad et al. 2006). Trials have shown that process can be applied to wastewater with high ammonia concentrations for example, leachate (Akerman, 2005). The process has been scaled-up and is available commercially from Veolia under the name Anita-shunt.
4.5.3 Anammox
The identification of the Anaerobic Ammonia Oxidising group of bacteria (anammox) and the ability to construct full scale anaerobic ammonia oxidising reactors further reduces the energy required for nitrogen removal. This anaerobic ammonia oxidation process has been successfully scaled up by many companies including Paques and Gronmij. To date there are many different variants of the anammox technology including suspended culture (flocs) self-supporting biofilm (granules) and attached biofilm based systems (MBBR).
Although the anammox bacteria do not require Oxygen, some oxygen is required to produce the Nitrite which the anammox bacteria utilise for the ammonia oxidation. Therefore through the implementation of anammox the oxygen requirement for the aerobic treatment is significantly reduced.