Hybrid wastewater treatment
Field of the invention
[0001] The invention relates to an improved wastewater treatment process using an
activated sludge reactor in a hybrid process including an aerobic granular biomass reactor.
Background
[0002] In common practice, wastewater treatment plants (WWTPs) include a biological
process step in which part of the wastewater containing solid matter, suspended and soluble
organics and nutrients, is treated by activated sludge (consisting of mainly micro
organisms). This process can take place either anaerobically or aerobically. The most
widely applied process for the aerobic treatment of wastewater is called the 'conventional
activated sludge' (CAS) process. It involves air or oxygen being introduced into a
biological treatment reactor which contains a mixture of screened and sometimes primary
treated sewage or industrial wastewater and purifying biomass, also referred to as 'activated
sludge'. The mixed liquor suspended solids (MLSS) develop into a biomass-containing floe,
which typically grows in suspended fluffy aggregates. The subsequent settling tank (usually
referred to as "final clarifier") is used to allow the biological floes to settle, thus separating
the purifying sludge from the treated water. The settled sludge is recycled towards the
biological process as 'return activated sludge' (RAS). To keep the biomass in the treatment
reactor at a desired level during biomass growth, periodically part of the RAS is wasted as
'waste activated sludge' (WAS).
[0003] The CAS process is applied in a variety of configurations, comprising one or
multiple tanks in parallel or sequential treatment train(s). Such tanks can for example be
operated as plug-flow reactor, as continuous stirred tank reactor (CSTR) or as sequencing
batch reactor (SBR). Although the CAS process is widely used, it has several important
drawbacks, like: poor settling sludge characteristics, limitation to low MLSS
concentrations, the tendency to develop floating sludge and a defined activated sludge
residence time. These drawbacks are briefly described hereafter.
Poor settling sludge characteristics
[0004] Due to its floc-like structure, the settling characteristics of activated sludge are
relatively poor, even when the plant is operating well. This results in the need for large
final clarifiers and accordingly high construction costs and large plant footprint. Many
improvements in the past therefore focused on achieving improved separation techniques.
One of them is the use of microfiltration to separate the activated sludge from the treated
water in a Membrane Bio Reactor (MBR). Another one is the addition of chemicals to
improve the biomass settling characteristics. In W096/14912 a method is described that
improves the settling properties of activated sludge by extracting gas and creating higher
biomass density. The method of selectively withdrawing poorly settling sludge is described
in EP1 627854.
Limitation to low MLSS concentrations
[0005] The CAS process is limited to a relatively low concentration of MLSS, typically 3-
5 g MLSS/L. Higher concentrations of MLSS lead to an unfavourable prolonged sludge
holdup in final clarifiers and, especially during conditions with higher than average
hydraulic flows, to potential sludge washout. State of the art measures to increase the level
of MLSS focus on the application of microfiltration for sludge / water separation
(Membrane Bio Reactors) instead of settling or the use of submerged carrier material to
enhance the biomass concentration, as for example described in WO03/068694.
Floating sludge
[0006] The CAS process coincides with a periodical occurrence of floating or very
difficult to settle 'bulking sludge', a phenomenon caused by an increased growth of
filamentous micro-organisms in the activated sludge floes. Typical counteracting measures
include chemical oxidation to destroy mainly the filamentous organisms or use of special
biomass selection reactors prior to the activated sludge in which the growth of filamentous
micro-organisms is reduced.
Defined activated sludge residence time
[0007] The CAS process for nutrient removal is typically designed with a defined
activated sludge residence time in the system of 5-15 days. This time period sets a limit to
the accumulation of favourable species of micro-organisms with low growth rates, which
cannot be maintained in the treatment system. Measures to extend the sludge residence time
include the Membrane Bio Reactor, the addition of submerged carrier material for attached
growth and the use of bio-augmentation. In these bio-augmentation processes, a specific
micro-organism population is cultivated and often immobilized in bio-augmentation
reactors. The reactors are fed with specific substrate or integrated waste side streams from
the wastewater treatment facility and then dosed to the CAS system, as described in e.g.
EP0562466. Another example of such an in-situ bio-augmentation process is described in
WO00/05177: it describes an external bio-augmentation reactor to enrich specific
organisms in the activated sludge matrix.
[0008] The drawbacks of typical CAS systems are overcome to a large extent by the
aerobic granular biomass (AGB) process and system as developed by Delft University of
Technology (WO2004/024638). In this process granular biomass with a typical size of 0.2-
3.0 mm is grown that has very different characteristics from the floes as grown in CAS. For
example the settling velocity of the applied granules is in the range of 5.0-50.0 m/h (in
comparison: typical for CAS would be 0.5-1.0 m/h). Sludge volume indices (SVI) for
aerobic granular biomass are 70 ml/g or lower and typically are comparable in value after 5
and 30 minutes of settling time. In addition, MLSS concentrations can be kept at levels 2-4
times higher than in CAS systems, resulting in approx. 2-4 times more 'purification power'.
Furthermore, both the layered structure of granules in aerobic, anoxic and anaerobic zones
and the range in granule sizes result in a large distribution of sludge ages. This enables
specific and favourable micro-organisms with low growth rates to survive in the AGB
process.
[0009] However, one drawback of the AGB process is the fact that the granules need to be
grown in a discontinuously fed system, in sequencing batch reactors. It has been reported
that AGB can only develop and be maintained in batch-wise operations, during which slow
growing micro-organisms are selected at high feed concentrations followed by a famine
regime during non-feed conditions (see: WO2004/024638). Such conditions can by
definition not be established easily in continuously fed CAS systems.
[0010] Therefore, the technology cannot easily be used to retrofit continuously fed CAS
systems into systems aimed at growing AGB. Replacement of the widely used continuous
CAS systems would mean large capital disinvestment. Efforts to develop a continuously
fed AGB system have been reported in literature but so far prove not feasible at practical
conditions. Reference is made to a study on the formation and stability of aerobic granules
in a continuous system: (N. Morales, et al., Separation and Purification Technology,
volume 89, page 199-205, 2012). Efforts also have been made to replace the activated
sludge in continuous MBR systems with aerobic granular biomass in order to reduce
membrane fouling. It was investigated whether the activated sludge in the continuous MBR
systems could be replaced by granular biomass grown in cultivation reactors or grown in
granular biomass reactors. The results showed that it was not feasible to keep the aerobic
granules in the MBR system: the granules deteriorated quickly (Reference: Xiufen et al.,
Characteristics of Aerobic Biogranules from Membrane Bioreactor System, Journal of
Membrane Science, 287, page 294-299, 2006). As a consequence, in the current state-ofthe-
art, upgrading performance of existing CAS systems using aerobic granular biomass is
only possible by retrofitting CAS systems into sequencing batch operated AGB reactors.
[001 1] Even if in a hypothetic case granular biomass would be able to survive in CAS, the
size and settling characteristics of the granules are such that in many CAS the mixing
intensity is not sufficient and they will settle to the bottom and as such becoming inactive
for the treatment process.
[0012] An assumed drawback of batch operated systems like the AGB system is the
sensitivity to off-spec high hydraulic load fluctuations. This is because all operations take
place in one tank and the feed to one tank is discontinuous. This is different from CAS
systems equipped with large final clarifiers, which clarifiers can act as buffer tank to
prevent sludge loss. This drawback can be counteracted by installing feed buffer tanks or
adjusting feed patterns over the multiple AGB process tanks.
[0013] JP-A 2009-090161 discloses an aerobic wastewater treatment comprising a series
(not a parallel arrangement) of aeration tanks. Granular flaked sludge is produced in an
oscillating bed with carrier material in the first aerated tank and fed to the second tank. JP-A
2007-136368 discloses an aerobic wastewater treatment wherein sludge is granulated in a
contact tank, and sludge is then fed to a downstream reactor; surplus granular sludge from the
aerobic reactor is returned to the contact tank. WO 2007/029509 discloses an aerobic
wastewater treatment process with sludge return, using a partitioned aerated tank and
microorganisms immobilised on a carrier in the first compartment.
Summary of the Invention
[0014] It was surprisingly found that the deficiencies and drawbacks of the prior art
processes could be overcome by adding an AGB system to a CAS system and manipulating
the sludge flows from the AGB system. The resulting hybrid process tie-in considerably
improves the performance and flexibility of state-of-the-art wastewater treatment plants.
[0015] The invention thus comprises a novel process for biological treatment of waste
water in which the performance of CAS systems is improved. The addition of one or more
AGB reactors serves two purposes: 1) to treat part of the raw wastewater and by doing so
contribute to the overall treatment performance of the overall hybrid treatment plant and 2)
by doing so synergistically enhancing the performance of the existing CAS without adding
chemicals, without a full CAS system's retrofit to sequence batch operation, without using
degasification measures or using membranes, without using submerged biomass support
material and without bio-augmentation with cultivated special or immobilized microorganisms
produced by external substrate. Also, hydraulic load fluctuations can be
accommodated while maintaining effective waste treatment.
Brief description of the drawings
[0016] In the appended drawings:
Figure 1 schematically depicts a hybrid wastewater treatment process equipment of the
invention;
Figure 2 schematically depicts a variation of the hybrid wastewater treatment process of the
invention as operated in a start-up stage;
Figure 3 schematically depicts another variation of the hybrid wastewater treatment process of
the invention comprising a waste sludge processing unit;
Figure 4 schematically depicts a reactor to be used for the aerobic granular process of the
invention.
Detailed description of the Invention
[0017] The invention thus provides a wastewater treatment process comprising subjecting
a part of a wastewater supply to an activated sludge process using floc-like aerobic
biomass, and feeding part of the wastewater to a granular biomass process using aerobic
granular biomass, wherein part of the biomass, i.e. the waste biomass and suspended solids,
issuing from the granular biomass process is fed to the activated sludge process.
[0018] The activated sludge process (CAS system) and the granular biomass process
(AGB reactors) are run in parallel, meaning that the main wastewater stream is split into a
stream subjected to the CAS system and a stream subjected to the AGB reactor(s), and the
split streams are not substantially intermixed during the treatment process, other than in
low amounts accompanying biomass transfer from the AGB system to the CAS system.
The parallel setup is described in more detail here below. The part of the biomass from the
granular biomass process that is fed to the activated sludge process, i.e. the suspended
solids, is especially the lighter part of the biomass, i.e. the part which has smaller particle
sizes and/or a lower settling velocity (in particular lower settling velocity) than the part that
is not fed to the CAS system, i.e. remains in the AGB reactor. Excess granular biomass
from the AGB reactors is preferably not fed to the CAS system, but will be processed or
reused outside the process. Excess sludge from the CAS system is preferably not fed to the
granular biomass process.
[0019] As used herein, aerobic granular biomass (AGB), to be used in the granular
biomass process, and floc-like aerobic biomass, to be used in the conventional activated
sludge process (CAS), are distinguished by one or more characteristics:
(1) sludge volume index (SV¾o), defined as the volume in millilitres occupied by 1 g of a
suspension after 30 min settling: the aerobic granular biomass has an SV¾o which is less
than 70 ml/g, preferably less than 60 ml/g, more preferably less than 50 ml/g, most
preferably less than 40 ml/g; while the floc-like aerobic biomass typically has an SV¾o of
more than 70 ml/g, particularly more than 80 ml/g, more in particular between 90 and 150
ml/g; a sludge biomass as used herein can thus be referred to as granular, if the SV o is less
than 70 ml/g, and as floc-like, if the SV¾o is more than 70 ml/g. In addition the
corresponding SVI after 5 minutes settling, referred to as SVI5 for aerobic granular biomass
is less than 150 ml/g, preferably less than 100 ml/g, more preferably less than 70 ml/g, most
preferably less than 60 ml/g; while the floc-like aerobic biomass has an SVI5 of more than
150 ml/g, typically more than 250 ml/g, A sludge biomass as used herein can thus
alternatively or additionally be referred to as granular, if the SVI5 is less than 150 ml/g, and
as floc-like, if the SVI5 is more than 250 ml/g.
(2) settling velocity, defined as the height of sludge settled per hour: the aerobic granular
biomass has a settling velocity of at least 3 m/h, preferably at least 4 m/h, more preferably
between 10 and 50 m/h, while the floc-like aerobic biomass has a settling velocity of less
than 3 m/h, particularly less than 2 m/h, more particularly between 0.5 and 1.5 m/h; a
sludge biomass as used herein can thus be referred to as granular, if the settling velocity is
more than 3 m/h, and as floc-like, if the settling velocity is less than 3 m/h.
(3) average particle size: the aerobic granular biomass comprises discrete particles that after
mechanical sieving in the laboratory under mild water wash has an average particle size of
at least 0.2 mm, preferably between 0.4 and 3 mm, while the floc-like aerobic biomass
agglomerates during mechanical sieving in the laboratory under mild water wash show an
average particle size of less than 0.2 mm, particularly less than 0.1 mm; a sludge biomass
as used herein can thus be referred to as granular, if the average particle size of the sludge
is more than 0.2 mm, and as floc-like, if the average particle size of the sludge is less than
0.2 mm.
[0020] The part of the biomass issuing from the granular biomass process that is fed to the
activated sludge process typically has characteristics which are intermediate between the
characteristics of the aerobic granular biomass and of the floc-like aerobic biomass as
defined above. Thus, the part of the biomass transferred from the AGB reactor(s) to the
CAS system will have a sludge volume index (SV¾o) as defined above between 40 and 90
ml/g, especially between 50 and 90 ml/g, and an SVI5 between 70 and 250 ml/g, especially
between 150 and 250 ml/g. Likewise, the part of the biomass transferred from the AGB
reactor(s) to the CAS system will have a settling velocity between 1.5 and 10 m/h,
especially between 3 and 10 m/h. In addition or alternatively, the part of the biomass trans
ferred from the AGB reactor(s) to the CAS system will have an average particle size
between 0.1 and 0.4 mm.
[0021] In an advantageous embodiment, the activated sludge process of the hybrid
process of the invention is operated in a conventional, continuous mode, wherein effluent
from the activated sludge reactor is continuously fed to a clarifier, in which effluent is
separated into a clarified liquid and a sludge fraction. The clarified liquid is preferably
combined with the treated water issuing from the granular biomass process, where desired
with further post-treatments. Part of the non-dissolved material (i.e. the sludge fraction)
separated from the clarifier is returned to the activated sludge process and part may be
discharged or further treated as described below. Alternative embodiments, such as using a
sequencing batch reactor, without final clarifier, are also part of the invention.
[0022] The granular biomass process of the hybrid process of the invention is
advantageously operated batch-wise. The granular process can be operated by alternating
steps as also described in WO2004/024638 as follows: (a) adding wastewater to the aerobic
granular biomass in a reactor while discharging treated water from the reactor, (b)
supplying oxygen-containing gas, in particular air, to the added wastewater in the reactor,
while keeping the oxygen level in the wastewater in the reactor between 0.2 and 5 mg/1,
preferably between 0.4 and 4 mg/1, more preferably between 1 and 3 mg/1, (c) allowing the
granular biomass to settle, and (d) discharging part of the biomass (suspended solids:
MLSS) from the reactor; and then returning to step (a). At least part of this discharged
biomass is fed it to the activated sludge process. Step (d) of the process, i.e. the discharge
of part of the suspended solids, need not be included in each and every cycle of the process,
depending on the relative sludge requirements of the activated sludge process and the
granular process. For example, step (d) can be included in every second or third etc. cycle.
[0023] Instead of discharging treated water in step (a), i.e. at the same time as feeding raw
wastewater to the reactor, treated water can be discharged together with the discharge of the
part of the biomass from the reactor in step (d), i.e. prior to the feed step (a) which follows
step (d). In this case, the treated water and the biomass can be fed to the activated sludge
process. This is particularly useful when starting up the system.
[0024] An important feature of the present process is that the average particle size and/or
settling velocity of the biomass (suspended solids) that is removed from the aerobic
granular process and can be fed to the activated sludge process is lower than the average
particle size and/or settling velocity of the aerobic granular biomass remaining in the
aerobic granular reactor. However, the transferred biomass will have a larger particle size
and/or higher settling velocity than the average particle size and/or settling velocity of the
sludge in the activated sludge process, as explained above, and improves the performance
of the activated sludge process.
[0025] The granular biomass process is operated in an upflow mode, wherein the waste
water in step (a) is supplied from the bottom and upwardly displaces the treated water at the
top part of the reactor. The oxygen-containing gas is supplied in step (b) at the bottom of
the reactor, no earlier than after the supply of the fresh wastewater. In step (c), suspended
matter comprising partly granular biomass precursors, smaller granular biomass and bioagglomerates
with lower settling velocity is discharged at between 30 and 90% of the
height of the reactor measured from bottom to top, while larger surplus granular biomass,
can be periodically removed from the process from the bottom part of the reactor. Further
details can be seen in Figure 4 described below. Thus, two types of biomass can be
discharged from the aerobic granular process: firstly the suspended solids, i.e. the relatively
light, small-size and slowly settling part of granular biomass, which is frequently wasted at
at least 30% of the height of the reactor from the bottom, and secondly the heavy granular
biomass, which can be wasted at a lower frequency from the bottom of the reactor.
[0026] In a preferred process configuration shown in Figure 1, an AGB reactor (4) is
constructed and tied-in with the existing CAS reactor (3) is such a way that, in parallel to
the CAS reactor, the AGB reactor is fed with part (2) of the incoming raw or pretreated
wastewater (1) while the suspended material wasted from the aerobic granular biomass
reactors (10) is frequently fed to the CAS system (3+5) and gradually results in improved
capacity and purification capabilities of the CAS process. In Figure 1 (5) depicts the final
clarifier, continuously fed (6) by overflow from the CAS reactor, while (7) depicts the
sludge return flow, split into RAS (8) and WAS (9). The AGB effluent (12) is taken
directly to the effluent (13) from the final clarifier, for direct discharge or tertiary treatment.
Larger and fully grown surplus granules are periodically wasted from the AGB reactor (11).
The new AGB reactor(s) can be constructed by adding new tanks or by retrofitting part of
the existing CAS reactor(s) or compartments thereof or by retrofitting existing tanks or
clarifiers.
[0027] The ratio of the part of the wastewater fed to the granular biomass process and the
part of the wastewater fed to the activated sludge process can be controlled depending on
the quality of the wastewater. More typically the ratio of AGB flow and CAS flow is
selected between 5:95 and 75:25, particularly between 10:90 and 50:50. In this way, the
process configuration of the invention can be used to reduce one of the drawbacks of batchwise
fed AGB, being the challenge of handling large fluctuations in hydraulic load, with
larger ratios between storm and dry weather flows as occurring in areas with combined
sewer systems. For example, during storm water flow conditions, when the waste water is
abundant and relatively diluted, the majority of the hydraulic load can be fed to the
continuously fed CAS system and treated with the help of the final clarifier, while the
hydraulic load to the AGB system is only slightly increased. On the other hand, a relatively
high proportion of the wastewater can be fed to the granular biomass process in case of
lower volumes of relatively concentrated wastewater, such as may occur under dry weather
conditions. In particular circumstances the wastewater may be exclusively fed to the
activated sludge process or to the granular process. This process configuration can
significantly reduce the AGB tank volume or storm water buffer tank volume and saves on
overall construction cost.
[0028] In principle, any wastewater which is not excessively toxic to the biomass used
can be treated by the process of the invention. For example, the wastewater may contain
organic waste at a level of between 10 mg and 8 g expressed as COD, per 1, in particular
between 50 mg and 2 g COD per 1. Alternatively or in addition, the waste water may
contain total nitrogen (as ammonia and/or other nitrogen compounds) at a level of between
0.2 and 1000, particularly between 1 and 75 mg per 1(as nitrogen), which will result in at
least partial removal of the nitrogen as explained below. Also, the wastewater may contain
total phosphorus (as phosphate and/or other phosphorus compounds) between 0.05 and 500,
particularly between 1 and 15 mg per 1(as phosphorus).
[0029] The aforementioned process configuration can be favourably applied to increase
the overall capacity of WWTPs operating with CAS systems. In this process configuration
one or more new AGB reactors are constructed in a parallel treatment train, next to the
existing CAS systems. The existing CAS systems are fed with large part of the raw or
pretreated wastewater but a remaining part is treated by the AGB systems. By doing so, the
size and capacity of the AGB treatment train for the projected extension can be made
smaller because capacity and performance of the existing CAS systems is synergistically
increased. Meanwhile, the small footprint of the AGB system often allows it to be built as a
plant capacity extension next to the existing CAS system(s) on the same premises, which is
important when footprint for plant expansion is limited or costly.
[0030] Thus, the activated sludge process, i.e. the CAS system, can comprise two, three,
four, or more treatment trains run in parallel. The CAS effluent of the combined CAS
reactors may be fed to a single clarifier, or alternatively each CAS reactor may be provided
with its own clarifier. Preferably, each one of the multiple CAS reactors running in parallel
is fed with biomass from the aerobic granular process, although the biomass feed need not
be identical or continuous to each activated sludge reactor. When multiple CAS reactors are
used, the granular biomass process may comprise one treatment train, or alternatively
multiple granular treatment units. It is also conceivable that the process comprises a single
CAS system and two, three, or more aerobic granular trains.
[0031] The hybrid configuration of AGB and CAS in parallel has an additional advantage
in that an additional buffering tank is often needed to balance the discontinuous waste flow
from the AGB reactor to enable continuous sludge thickening and dewatering with reduced
equipment capacities. By applying the novel process configuration of this invention, all
wasted biomass and other suspended material from the AGB reactor can be discontinuously
fed into the parallel CAS system and further continuously be processed with the activated
sludge in the CAS sludge treatment facilities.
[0032] The unexpected advantages of the invention were tested and demonstrated. An
AGB reactor was constructed for replacing an existing CAS system and for accommodating
the required increased capacity and purification performance of the existing WWTP. The
AGB system was operated in parallel to the CAS system as shown in Figure 2 . The clean
effluent of the AGB system (12) was temporarily directed to the CAS system's final
clarifier. Waste from the AGB system (10), containing suspended matter comprising partly
granular biomass precursors, smaller granular biomass and bio-agglomerates, was also
temporarily wasted to the existing CAS system, which was operating in parallel to the
AGB. The waste material from the AGB system (4) was gradually transferred via ( 1 1-6-5-
7-8) to the CAS system (3+5). This was done as a temporary measure to compensate for the
reduced nutrient removal efficiency of the AGB system during start-up. It was then
surprisingly found that the performance and process stability of the CAS system improved
gradually but significantly as a result of this interaction with the AGB system. This
improvement further developed in time when discharge of waste material no longer
proceeded through (11) but directly to CAS system (3) and effluent was directly discharged
to (12) after final clarifier (5), as depicted in Figure 1.
[0033] As described above, waste sludge (suspended solids) material from the granular
biomass process is directed to the activated sludge process. Also liquid effluent from the
granular biomass process may be directed discontinuously to the activated sludge process.
[0034] Prior to start-up of the new AGB reactor the sludge volume indices (DSV½) in the
CAS system was 125-175 mL/g and this significantly dropped to 75-100 mL/g without any
changes made in the CAS system. As a result the biomass concentration in the CAS system
could be increased from 3-4 g MLSS/L to 4-5 g MLSS/L without affecting the level of
suspended solids in its effluent. Clearly, the waste biomass from the AGB system was
largely captured in the CAS system, to its advantage. Moreover, the total waste biomass
concentration from the CAS system increased from 8 g MLSS/L to 12 g MLSS/L in a
reduced hydraulic flow towards the sludge treatment facilities.
[0035] It was surprisingly found that the micro-organism population in the CAS system
had become more diverse and also featured significantly more specialized and slower
growing micro-organisms than before. The biomass of the CAS system still kept its floclike
structure but became more dense with inclusions of small waste granular biomass,
resulting in improved purification ability and settle ability. Also it was measured that the
concentration of biopolymers and extracellular polymer substances in the CAS system's
biomass had increased significantly. In addition to these findings, the existing high-loaded,
fully aerated CAS system showed a strong improved ability for denitrification. A
remarkable finding, because such high rate of denitrification would have been impossible
based on the prevailing aerobic conditions and sludge age in the CAS system. Tests showed
N0 N effluent concentration decreasing from 8-10 g N C 3 N/L to 3-4 g N C 3 N/L.
[0036] The particulate matter in the waste effluent of the AGB system will displace part
of the activated sludge in the CAS system. This AGB waste material (suspended solids)
contains fractions of tiny aerobic granules, granule precursors, bio-agglomerates and
granule fractions, resulting from breaking up of larger (and older) granules. As mentioned,
the AGB and parts thereof contain a highly diverse micro-organism population, including
specialized and favourable slow-growing micro-organisms. It was surprisingly found that
the physical characteristics of this particulate AGB waste material did not deteriorate in the
CAS system and the material also did not lose its denitrification capability as typical for
larger granules.
[0037] In another process configuration the synergistic effect of operating an AGB system
in hybrid configuration with a CAS system can be advantageously used to efficiently
remove nitrogenous components from wastewater. It makes use of the enhanced
capabilities of AGB systems to remove high levels of nitrogenous compounds from waste
water. In this process configuration, as depicted in Figure 3, the AGB system (4) is (partly)
fed by a side stream (16) of the CAS system (3+5) containing high levels of nitrogenous
compounds, for example originating from a waste sludge processing unit (14). Mostly such
side streams are small in volume but high in nutrient concentrations (nitrogen, phosphorus
compounds) in comparison with the influent (1), which can be treated by the AGB.
Examples of such side streams are: reject water from dewatering devices, decanting water
from digesters, water from anoxic selection tanks and mixtures of such streams with
influent. The effluent of the AGB system (12), together with its waste suspended material
and/or biomass (10), is directed to the CAS system. This hybrid AGB/CAS systems process
configuration proves another example of the positive effect of the addition of an AGB
system to a CAS system on the overall performance of a WWTP.
[0038] Surprisingly it was found that the invention also provides a cost-effective solution
to improve the capacity of biological removal of phosphorus (P) of an existing CAS system
equipped with chemical P removal. For conventional biological P removal in CAS systems,
anaerobic preconditioning of the activated sludge is required to get P release first before
increased P uptake can take place in the CAS system under subsequent anoxic / aerobic
conditions. As is known from WO 2004/024638, AGB systems have increased biological P
removal capabilities, related to the proliferation of Phosphate Accumulating Organisms
(PAOs) in the aerobic/anoxic layered granule. It is also known that due to pH profiles in the
granule, bio-catalysed phosphate precipitation can take place, further improving the P
removal capacity of the AGB system.
[0039] The invention can be used to add biological phosphate removal capacity to an
existing WWTP system and combine it with improved CAS biomass settling characteristics
as explained before. However, it was found that the overall biological P removal capacity
of the hybrid AGB/CAS systems was much larger than could be calculated based on the
sum of the two combined processes. A significant decrease in chemical dosing requirement
for P precipitation in the CAS was observed, resulting in a favourable much lower chemical
sludge production. It again showed that the AGB waste directed towards the CAS system
resulted in replacement or union of CAS biomass floes with small granular biomass from
the AGB system. This particulate matter still featured the good biological P removal
abilities under aerobic conditions in the CAS system. The invention allows additional
biological P removal capacity to be introduced in the CAS system without the elaborate
construction of separate anoxic and anaerobic compartments in the CAS system and with
chemical P removal in the CAS system becoming less important or even superfluous.
[0040] The invention can also be used to optimize the performance of a CAS system
treating wastewater mixtures including low molecular weight organics. Such compounds
often result in bulking sludge by filamentous micro-organisms, which are difficult to settle.
A selector tank as first step in the biological treatment process is often used to minimize
this problem. In such selector tanks these components are selectively party biodegraded. It
was found that when such wastewater or part of the wastewater was treated in the AGB
system operating in parallel to the CAS system, part of the low weight organics were
biodegraded by the AGB in anaerobic zones of the granules. This results in lower energy
consumption for aeration and the production of biogas, which could be captured and
utilized. Especially it was highly remarkable that anaerobic degradation of lower alcohol
compounds such as methanol and ethanol was measured, since such compounds in
traditional anaerobic reactors are hardly converted at all. Further it was noticed that this
remarkable anaerobic treatment ability was transferred from the AGB to the CAS system in
the hybrid CAS/AGB set-up. In conclusion: another configuration of the invention is
operating an AGB system in parallel to a CAS system in order to improve the sludge
settling characteristics while at the same time reducing the required aeration capacity in the
overall WWTP.
[0041] The operation of the aerobic granular process is schematically illustrated in Figure
4, showing an aerobic granular reactor 4 . The reactor is operated in an up-flow mode
comprising a lower bed 40 containing the larger granular biomass, and an upper part 4 1
containing suspended matter comprising partly granular biomass precursors, smaller
granular biomass and bio-agglomerates. Wastewater 2, and optionally side stream 16, is
introduced at the bottom through inlet means 42. Air is introduced through inlet 43 at the
bottom with distribution means (not shown), and spent air is leaves the reactor at the top.
Cleaned effluent 12 leaves the reactor through overflow and exit 45. Excess material 10,
having an average particle size which is lower than the average particle size of the granular
biomass in the reactor, can be discharged through exit 46, which is located somewhere
between 30 and 90 of the (liquid) height of the reactor. Larger excess granular biomass 1 1
can be removed through outlet 44. Inlet 42 and 43 and outlets 44, 45 and 46 are preferably
provided with a valve for controlling the inflow and outflow of the various streams. In
particular air supply and distribution means 43 is provided with a flow regulator controlled
by the oxygen level in the reactor content so as maintain an oxygen concentration in the
reactor content within the required limits, i.e. 0.2-5 mg/1, for producing optimum granular
biomass characteristics.
[0042] In a further advantageous process, the hybrid configuration of CAS and AGB
systems is applied to target granulation in the AGB rather than waste water treatment. The
produced surplus of grown granules can be harvested as valuable waste biomass and sold as
seed material for new AGB systems.
[0043] The invention further comprises equipment for implementing the hybrid process
configuration with AGB and CAS systems as described. Such equipment advantageously
comprises an activated sludge reactor (3) with a liquid inlet, a liquid outlet, a gas inlet, a
granular biomass reactor (4) with a liquid inlet (42) at the bottom of the reactor and a liquid
outlet (45) at the top of the reactor and an outlet (46) at at least one third of the height of the
reactor (4), a gas inlet (43) at the bottom of the reactor, a liquid line connecting an outlet of
the granular biomass reactor (4) with an inlet of the activated sludge reactor (3), and
preferably a separator (5) connected with a liquid outlet of the activated sludge reactor (3),
the separator having a sludge outlet and a clarified liquid outlet, and further comprising a
control valve for regulating the relative liquid flows to the liquid inlet of the activated
sludge reactor (3) and the liquid inlet of the granular biomass reactor (4). The equipment
may comprise multiple activated sludge reactors (3) and/or multiple granular biomass
reactors (4) arranged in parallel.
Claims
1. A wastewater treatment process comprising subjecting wastewater to an activated
sludge process using floc-like aerobic biomass, characterized in that part of the
wastewater is fed to a granular biomass process operated in parallel to said activated
sludge process and_using aerobic granular biomass, and part of the biomass issuing
from the granular biomass process is fed to the activated sludge process, wherein
said part of the biomass fed to the activated sludge process has a lower settling
velocity than the part of the biomass of the granular biomass process that is not fed
to the activated sludge process.
2 . A process according to claim 1, wherein the aerobic granular biomass is
characterised by one or more of the following: a sludge volume index, defined as
the volume in millilitres occupied by 1 g of a suspension after 30 min settling,
which is less than 70 ml/g, preferably less than 50 ml/g; a settling velocity of at least
3 m/h, preferably between 10 and 50 m/h; and an average particle size of at least 0.2
mm, preferably between 0.4 and 3 mm.
3 . A process according to claim 1 or 2, wherein the floc-like aerobic biomass is
characterised by one or more of the following: a sludge volume index, defined as
the volume in millilitres occupied by 1 g of a suspension after 30 min settling, of
more than 70 ml/g, more in particular between 90 and 150 ml/g; a settling velocity
of less than 3 m/h, more particularly between 0.5 and 1.5 m/h; and an average
particle size of less than 0.2 mm, particularly less than 0.1 mm.
4 . A process according to any one of claims 1-3, wherein the part of the biomass
issuing from the granular biomass process that is fed to the activated sludge is
characterised by one or more of the following: a sludge volume index, defined as
the volume in millilitres occupied by 1 g of a suspension after 30 min settling,
which is between 40 and 90 ml/g, preferably between 50 and 90 ml/g; a settling
velocity of between 1.5 and 10 m/h, preferably between 3 and 10 m/h; and an
average particle size between 0.1 and 0.4 mm.
5 . A process according to any one of claims 1-4, wherein the aerobic granular biomass
is characterised by a sludge volume index, defined as the volume in millilitres
occupied by 1 g of a suspension after 5 min settling, being less than 150 ml/g,
preferably less than 100 ml/g.
6 . A process according to any one of claims 1-5, wherein the floc-like aerobic biomass
is characterised by a sludge volume index, defined as the volume in millilitres
occupied by 1 g of a suspension after 5 min settling, being more than 250 ml/g.
7 . A process according to any one of claims 1-6, wherein the granular biomass process
is operated by the consecutive steps of (a) adding wastewater to the aerobic granular
biomass in a reactor while discharging treated water from the reactor, (b) supplying
oxygen-containing gas, in particular air, to the reactor, while keeping the oxygen
level in the wastewater in the reactor between 0.2 and 5 mg/1, (c) allowing the
granular biomass to settle and (d) drawing off part of the biomass from the reactor
and at least partly feeding it to the activated sludge process, wherein the average
particle size of the biomass that is drawn off is lower than the average particle size
of the biomass remaining in the reactor.
8 . A process according to any one of claims 1-7, wherein the granular biomass process
is operated by the consecutive steps of (a) adding wastewater to the aerobic granular
biomass in a reactor, (b) supplying oxygen-containing gas, in particular air, to the
reactor, while keeping the oxygen level in the wastewater in the reactor between 0.2
and 5 mg/1, (c) allowing the granular biomass to settle and (d) discharging treated
water from the reactor including drawing off part of the biomass from the reactor
and at least partly feeding it to the activated sludge process, wherein the average
particle size of the biomass that is drawn off is lower than the average particle size
of the biomass remaining in the reactor.
9 . A process according to claim 7, wherein granular biomass process is operated in an
up-flow mode, wherein the wastewater in step (a) is supplied from the bottom and
displaces the treated water, which is discharged in the same step at the top of the
reactor, the oxygen-containing gas in step (b) is supplied at the bottom of the reactor
and in step (d) the biomass having the lower particle size is drawn off at between 30
and 90% of the height of the reactor from bottom to top.
10. A process according to any one of claims 1-9, wherein the ratio of the part of the
wastewater fed to the granular biomass process and the part of the wastewater fed to
the activated sludge process can be controlled depending on the quality of the
wastewater supply and is selected between 5:95 and 75:25, particularly between
10:90 and 50:50.
11. A process according to any one of claims 1-10, wherein the wastewater contains
organic waste at a level of between 10 mg and 8 g, expressed as COD, per 1, and/or
total nitrogen (as ammonia and/or other nitrogen compound) at a level of between
0.2 and 1000 mg per 1, particularly between 1 and 75 mg per 1, and/or total
phosphorus (as phosphate and/or other phosphorus compounds) between 0.05 and
500 mg per 1, particularly between 1 and 15 mg per 1.
12. A process according to any one of claims 1-1 1, characterized in that the activated
sludge process comprises two or more treatment trains.
13. A process according to any one of claims 1-12, characterized in that the granular
biomass process comprises one treatment train.
14. A process according to any one of claims 1-13, characterized in that the granular
biomass process comprises two or more treatment trains.
15. A process according to any one of claims 1-14, characterized in that at least part of a
side stream process, derived from the activated sludge process and containing
nutrient levels higher than the initial wastewater, is returned to the granular biomass
process.
An installation for carrying out the process according to any one of the preceding
claims, comprising an activated sludge reactor (3) with a liquid inlet, a liquid outlet,
a gas inlet, a granular biomass reactor (4) with a liquid inlet at the bottom of the
reactor and one or more liquid outlets at at least one third of the height of the reactor
(4), a gas inlet at the bottom of the reactor, a liquid line connecting an outlet of the
granular biomass reactor (4) with an inlet of the activated sludge reactor (3), and a
separator (5) connected with a liquid outlet of the activated sludge reactor (3),
having a sludge outlet and a clarified liquid outlet, and further comprising a device
for regulating the relative liquid flows to the liquid inlet of the activated sludge
reactor (3) and the liquid inlet of the granular biomass reactor (4).