Note: Descriptions are shown in the official language in which they were submitted.
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WASTEWATER TREATMENT SYSTEM FOR A MARINE VESSEL
Field of the Invention
The invention relates to the biological treatment of wastewater on a marine
vessel. More particularly, the invention relates to a wastewater treatment
system for a
marine vessel wherein the unit operations of the system include design
features for
preventing erratic movement of the wastewater due to sea-induced movement of
the
marine vessel.
Background of the Invention
Wastewater treatment systems on marine vessels are becoming increasingly
important due to tightening environmental regulations. These regulations
particularly
effect passenger carrying marine vessels, such as cruise ships, naval vessels,
ferries
and the like. The regulations set discharge criteria for "greywater" (laundry
wash
water, shower water, etc.) "blackwater" (toilets, oily wastewater, etc.) and
sometimes
"bilge water" (water due to infiltration, spills and plumbing leaks, typically
accumulated
in a lower portion of the vessel).
In land-based wastewater treatment systems, there are a great number of
process configurations for treating combined greywater and blackwater. Initial
solids
removal is normally accomplished using screens and/or sedimentation tanks to
remove large particulate matter from the wastewater. Primary or physical
treatment is
used to settle out solids, often in a large clarifier with a prolonged
residence time to
allow the particulate matter to settle. Flocculating agents are sometimes used
to aid in
the settling process. Biological treatment is often employed, with aerobic
treatment
using a suspended growth process (eg: activated sludge) being typical.
Dissolved air
flotation (DAF) is occasionally utilized to separate buoyant materials (eg:
oils and
greases) from the wastewater. DAF is normally conducted in a large tank with
air
introduced to the wastewater and dissolved under pressure prior to entering
the tank
so that a release of pressure in the tank causes micro-bubbles to form, which
enhance
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the buoyancy of the materials being separated. The materials are skimmed off
of the
top of the tank and the treated wastewater normally overflows a weir at the
top of the
tank.
The foregoing processes have a number of limitations when applied to marine
vessels. First, space is limited on a marine vessel, so a wastewater treatment
system
is required with minimal footprint. Clarifiers, activated sludge processes,
and
conventional overflow DAF tanks have too large of a footprint to be
practically
implemented on a marine vessel. Second, the overall retention time of the
system is
limited due to space, so treatment processes with fast kinetics are desirable.
Residence times on the order of days, typical for clarification, activated
sludge and
conventional DAF processes, are simply not feasible on a marine vessel. Third,
the
system must be able to deal with variability in flow-rate and shock-loading
due to
varying concentration of pollutants in the wastewater. The aforementioned
conventional processes are easily upset due to wide variations in flowrate and
concentration, which can lead to washout of microbial cultures (in the case of
suspended growth aerobic processes) and generally an inadequacy in meeting
treatment objectives. Fourth, sea-induced movement of the marine vessel causes
erratic movement of the wastewater, such as sloshing and potentially spillage,
which
both disrupts the treatment process and creates a potentially human health
hazard.
For these reasons and others, conventional processes cannot be employed on a
marine vessel and there is accordingly a need for improved treatment methods
and
equipment.
A shipboard wastewater treatment system is disclosed in United States Patent
6,361,695 to Husain, et al. This system utilizes a suspended growth biological
treatment process and a hollow fiber membrane. In addition to the
aforementioned
disadvantages of suspended growth processes for marine applications, membranes
have the disadvantage of requiring a high pressure drop and of rapidly fouling
in oily
wastewater, with a corresponding decrease in permeate and increased retentate
volume for further treatment.
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A shipboard fixed-bed bioreactor system is disclosed in United States Patent
5,807,485 to Caplan, et al. This system is directed to the treatment of bilge
water, not
mixed greywater and blackwater, particularly bilge water contaminated with
petroleum.
The preferred microbial culture contains petroleum degrading organisms, which
tend
to have kinetics that are too slow for rapid treatment of greywater and
blackwater. In
addition, no means is included within any of the units for limiting erratic
movement of
the wastewater due to sea-induced movement of the ship.
A three zone dissolved air flotation clarifier with improved efficiency is
disclosed
in United States Patent 5,863,441 to Krofta. The clarifier has parallel
inclined baffles
and an overall U-shaped flow pattern; however, the baffles are not arranged to
create
a plurality of U-shaped flow paths. The clarifier is not for use in marine
applications
and does not include features for mitigating erratic movement of wastewater
due to
sea-induced movement of a marine vessel.
Although DAF units having baffles creating U-shaped flow paths are known for
land based applications, the baffles in these units are disposed only in a
central zone
of the unit and neither extend upwardly to a nominal waterline of the unit,
nor
downwardly to a bottom fluid distribution zone. Baffles extending to the
nominal
waterline are important to prevent erratic movement of the wastewater,
particularly
wastewater at the waterline, due to sea-induced movement of the marine vessel.
These prior art DAF units also do not have both a fluid inlet and a fluid
outlet at a
bottom thereof, nor do they have discharge openings for each U-shaped flow
path in a
sidewall of the unit. A plurality of sidewall discharge openings are desirable
in marine
applications to limit the footprint of the unit, which is otherwise increased
due to the
need for sumps, weirs, collectors, etc., and to limit erratic movement of the
wastewater
in the sumps. Prior art DAF units normally are open topped to permit ease of
maintenance and to permit introduced air to readily escape from the unit. In
comparison, for marine vessel applications the desire to control odours in the
enclosed
space where the DAF unit is located lead to the preferable use of closed tops
and
odour control means.
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Accordingly, there remains a need for an improved wastewater treatment
system for a marine vessel.
Summary of the Invention
According to an aspect of the invention, there is provided a wastewater
treatment system for a marine vessel comprising: a solids separator; an
aerobic
attached growth biological reactor; a flocculator comprising a plurality of
tubular
elements connected in serial fluid communication; and, a dissolved air
flotation unit
comprising a plurality of spaced apart baffles arranged to create a plurality
of inverted
U-shaped flow paths, each U-shaped flow path having wastewater flowing
upwardly
along a first side of the flow path and downwardly along a second side of the
flow path,
the baffles mitigating erratic movement of the wastewater due to sea-induced
movement of the marine vessel.
According to another aspect of the invention, there is provided a wastewater
treatment apparatus for use on a marine vessel, the apparatus comprising a
dissolved
air flotation unit comprising a plurality of spaced apart baffles arranged to
create a
plurality of inverted U-shaped flow paths, each U-shaped flow path having
wastewater
flowing upwardly along a first side of the flow path and downwardly along a
second
side of the flow path, the baffles mitigating erratic movement of the
wastewater due to
sea-induced movement of the marine vessel.
According to yet another aspect of the present invention, there is provided a
method of treating wastewater at sea on a marine vessel, the method
comprising:
separating particulate matter from the wastewater; treating the wastewater
aerobically
using an attached growth process; adding a flocculating agent to the
wastewater and
flocculating the wastewater in a flocculator comprising a plurality of tubular
elements
connected in serial fluid communication; and, separating flocculated material
from the
wastewater by dissolved air flotation in a dissolved air flotation unit
comprising a
plurality of spaced apart baffles arranged to create a plurality of inverted U-
shaped
flow paths having wastewater flowing upwardly along a first side of each flow
path and
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downwardly along a second side of each flow path, the baffles mitigating
erratic
movement of the wastewater due to sea-induced movement of the marine vessel.
The invention advantageously includes features for preventing erratic
movement of the wastewater due to sea-induced movement of the marine vessel.
In
5 particular, the tubular flocculator is employed in order to provide the
desired residence
time in a controlled, non-erratic manner. Also, the DAF unit include internal
baffles to
prevent erratic movement of the wastewater and the baffles may extend upwardly
to
about the nominal waterline within the DAF unit to further prevent erratic
movement at
the waterline. Other advantageous features include low residence time
requirements
due to high kinetic rates, particularly within the biological reactor, which
results in
space and footprint savings. The system may advantageously utilize closed
vessels
and odour control means where appropriate to reduce the potential for
unpalatable
and potentially hazardous gas emissions within the confined space of the
marine
vessel. The odour control means may include activated carbon adsorption or
external
venting.
The fixed film aerobic bio-reactor may include a packed bed or trickling
filter. In
order to take advantage of the rapid mass transfer occurring with suspended
growth
processes in an effort to increase overall kinetic rates, a hybrid process may
be
utilized wherein a fixed film is grown on neutrally buoyant media having
properties
permitting fluidization. The media is preferably hollow so that the fixed film
can grow
on the inside of the media, thereby protecting the film from excessive
sloughing due to
fluid shear and abrasion with adjacent media. By fluidizing the media, mass
transfer
and overall kinetic rates are improved, which reduces the footprint and space
requirements for the biological process. Examples of suitable fixed film
fluidized bed
bioreactors and processes for their use are provided in US Patent 5,543,039
and US
Patent 6,126,829, which are both incorporated herein by reference for
jurisdictions that
permit this method. These exemplary reactors combine the advantages of both
fixed
film and fluidized bed operation in a single aerobic biological reactor.
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Further features of the invention will be described or will become apparent in
the course of the following detailed description.
Brief Description of the Drawings
In order that the invention may be more clearly understood, embodiments
thereof will now be described in detail by way of example, with reference to
the
accompanying drawings, in which:
Fig. 1 is a process flow diagram showing the inter-relationship between units
in
an embodiment of a wastewater treatment system according to the present
invention;
Fig. 2a is an end view of a tubular flocculator in accordance with an
embodiment of a wastewater treatment system according to the present
invention;
Fig. 2b is a side view of the tubular flocculator shown in Fig. 2a;
Fig. 3 is a schematic representation of a tubular flocculator showing a
serpentine flow path;
Fig. 4a is a side cross-sectional view of a DAF unit according to an
embodiment
of the present invention; and,
Fig. 4b is a top view of the DAF unit of Fig. 4a.
Description of Preferred Embodiments
Referring to Fig. 1, greywater and blackwater from the marine vessel is first
provided to an equalization and blending tank 1. A first transfer pump 2 is
then used
to direct the combined wastewater at a relatively constant flow rate through a
primary
screen 3 for performing solids separation. The separated solids are sent along
with
the sludge either to a handling and storage facility or to on-board
incineration with
optional de-watering and drying prior to combustion. A second transfer pump 15
is
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then used to provide sufficient pressure to the wastewater as it is
transferred for
aerobic biological treatment within a fluidized bed fixed film bioreactor 4.
The bioreactor 4 contains a plurality of neutrally buoyant inert structures
for
supporting the fixed film during fluidization. The neutrally bouyant inert
support
structures may comprise any suitable structure for supporting an attached
biological
film while being amenable to fluidization. For example, the structures may
comprise
cylinders, rings, saddles, or spheres. Preferably, the structures have a
hollow interior
for supporting a biological film, protecting the film from abrasion and
shearing while
permitting the passage of fluid past the film without plugging. The inert
support
structure may be made from any suitable neutrally buoyant material, for
example high
density polyethylene and may be formed using a mold or an extrusion process.
The
inert support structure is preferably tubular and preferably comprises
Hydroxyl-PACTM
media.
In operation, wastewater is admitted into the bioreactor through the inlet and
flows past the attached biological film on the fluidized support structures.
Organic
matter in the wastewater is consumed by the film in the production of
additional
biomass. Some of the produced biomass remains with the film and some of the
biomass is sloughed off due to the shearing action of the passing fluid.
Treated
wastewater exits the bioreactor in a treated effluent stream. A portion of the
treated
effluent stream may be recycled (not shown) in order to maintain fluidization
conditions
within the bioreactor. The degree of treatment that takes place in the
bioreactor is
dependent in part on the hydraulic residence time and the biomass residence
time.
These parameters may be selected to achieve the desired treatment objectives
for
organic matter and/or particulate matter content. For example, the biomass
residence
time may be extended to increase digestion and reduce the solids concentration
in the
treated effluent.
Aerobic conditions within the bioreactor 4 are maintained by the addition of
air
to the wastewater prior to entering the bioreactor. This air is normally
provided by the
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vessel's on-board compressed air supply (not shown) although air handling and
air
treatment equipment may also be provided to supply air of sufficient quality.
The fluidized bed fixed film bioreactor advantageously provides high rate
aerobic treatment, reducing the required residence time and associated
footprint, while
also having a greater overall resilience with respect to shock loading than
other
biological treatment processes. In addition, the efficacy of the fluidized bed
is
essentially unaltered due to erratic movements of the wastewater caused by sea-
induced movement of the vessel. Of course, those skilled in the art will
appreciate that
other types of aerobic biological treatment units may be used, although fixed
film
biological treatment processes are preferred both for their stability with
respect to
erratic wastewater movement and for their ability to deal with wide variations
in
contaminant concentration.
Following biological treatment, a third pump 5 is used to pump the wastewater
and a flocculating agent is added to the wastewater with a flocculant injector
6. A
static mixer may be used in conjunction with the flocculant injector 6 in
order to
uniformly distribute the flocculating agent within the wastewater following
injection.
The flocculating agent may comprise a polymeric flocculating agent or any
other
conventionally known flocculating agent suitable for wastewater treatment.
Preferrably, the flocculating agent is suitable for use with wastewater
containing oily
substances. An example of a preferred flocculating agent is a highly charged
cationic
emulsion polymer such as DrewflocTM 2468. After the flocculating agent is
added, the
wastewater enters the tubular flocculator 7.
With reference to Figs. 1-3, the tubular flocculator 7 comprises a flanged
inlet
30, a flanged outlet 31 and a plurality of horizontal tubular pipes 33. Each
pipe 33 is
interconnected with an adjacent pipe by means of elbows 34, 35, which are
arranged
to form a substantially U-shaped connector. The pipes 33 are arranged to
create a
serpentine flow path as fluid passes serially from one pipe to the next. The
serpentine
flow path is illustrated schematically in Fig. 3 and provides the desired
residence time
for floc formation within a relatively small footprint. In comparison with
conventional
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land-based wastewater treatment systems wherein flocculation is conducted
within a
large (typically open topped) vessel, the tubular flocculator advantageously
provides
the desired residence time in a manner that is not affected by sea-induced
movement
of the marine vessel. Since the water flows upwardly from the bottom of the
tubular
flocculator 7 to the top, the pipes 33 are full and there is no air-water
interface that
would otherwise be susceptible to erratic floc-disturbing movement. Wastewater
flowing within the pipes is therefore relatively unaffected by sea-induced
movements of
the marine vessel. Furthermore, due to the fluid velocity within the pipes 33,
the floc
stays in suspension within the tubular flocculator without a tendency to
settle. During
experimental testing, it was found that use of the tubular flocculator
enhanced
flocculation efficiency sufficiently that a coagulation pre-treatment stage
using a tri-
valent metal salt such as ferric chloride (FeC13) or alum (AI203) could be
advantageously eliminated without detrimental results on overall system
performance.
The tubular flocculator in combination with a suitable flocculating agent
therefore
advantageously allowed substantially all of the floc to be introduced to the
DAF unit 8
for subsequent separation from the wastewater.
DAF processes are generally known in the art and comprise the dissolution of a
gas within the wastewater whilst under pressure. The gas may be dissolved
either by
direct injection into the wastewater under suitable conditions or by mixing a
liquid
stream containing a high concentration of the dissolved gas with the
wastewater prior
to entry to the DAF unit 8. These gas dissolution operations are not shown in
Fig. 1,
but are well understood by persons skilled in the art. The gas may be either
an inert
gas or, as in the present invention, compressed air supplied from ship-board
compressed air sources or dedicated locally situated air compressors. The
wastewater is introduced under pressure into the DAF unit 8, where the
controlled
release of pressure from the fluid causes the gas to come out of solution in
the form of
micro-bubbles. These micro-bubbles agglomerate with the materials within the
wastewater desired to be separated by flotation (for example, light-weight
particulate
matter, oily substances, grease or fat globules, etc.) and enhance their
buoyancy,
thereby increasing the rate of flotation. The separated materials are normally
collected
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by a sludge scraper and the cleaned effluent normally exits in the prior art
via an
overflow weir located at an upper end of the separation zone.
Referring to Figs. 1 and 4, the DAF unit 8 according to the present invention
comprises a bottom inlet 40 through which flocculated wastewater is introduced
to a
5 bottom distribution chamber 41. The distribution chamber 41 is designed to
have a
minimal depth in order to reduce the overall size of the DAF unit and includes
a sloped
bottom portion to permit occasional drainage of the DAF unit through drain 42
as
needed for maintenance purposes. The wastewater from the distribution chamber
41
flows upwardly in parallel relation through a plurality of inverted U-shaped
flow
10 passages 43. Each U-shaped passage 43 is formed by a pair of spaced apart
baffles
44, 45 that are joined at their bottom edges to close a downward portion of
the U-
shaped flow passage. In each pair of spaced apart baffles, a first baffle 44
is lower in
height than a second baffle 45. This permits the wastewater to flow upwardly
over the
top of the baffle 44 and make a downward turn for entry into the downward leg
of the
U-shaped flow passage 43. The second baffle 45 extends upwardly to just below
the
nominal waterline 54 of the DAF unit 8. For example, the second baffle 45
extends
upwardly to about 10-15 cm below the nominal waterline 54 in order to provide
sufficient clearance for the flights 46 of sludge scraper 47 and to allow
normal water
level fluctuations due to variations in flow rate and/or sea-induced movement
of the
marine vessel. By extending to just below the nominal waterline 54, the second
baffle
45 causes the majority of the fluid in the U-shaped flow path 43 to transit
the turn at
the top of the first baffle 44 and thereby travel down the downward portion of
its
respective flow path 43. The height of the second baffle 45 also
advantageously
dampens any sea-induced erratic movement of the waterline 54, thereby
preventing
the floc within the wastewater from being disturbed and protecting the sludge
blanket
from being disrupted, which in turn would cause previously separated
particulate
matter to be re-introduced into the downward leg of the U-shaped flow path 43
with
detrimental results on overall performance of the DAF unit 8.
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In the DAF unit 8 of the present invention, a co-current flotation separation
occurs on the upward portion of the inverted U-shaped flow path 43. This co-
current
separation advantageously exacerbates the flotation of highly buoyant
materials. Less
buoyant materials are entrained in the wastewater and pass over the first
baffle 44 into
the downward portion of the inverted U-shaped flow path 43. A counter-current
flotation separation occurs on the downward portion of the U-shaped flow path
43
where less buoyant materials are provided with sufficient residence time that
their
micro-bubbles may agglomerate, thereby increasing the buoyancy of the
materials and
allowing them the opportunity to rise to the nominal waterline 54. The
combination of
co-current followed by counter-current separation advantageously increases the
efficacy of the flotation separation and allows the overall size and footprint
of the DAF
unit 8 to be reduced as compared with prior art flotation separation systems.
After passing through the downward portion of the inverted U-shaped flow path
43, the wastewater exits the DAF unit 8 through a plurality of exit orifices
48 within the
sides of the DAF unit. As can be seen with reference particularly to Fig. 4b,
the
wastewater exits the DAF unit 8 through side pipes 49 and enters collector
piping 50.
The collector piping 50 is then vertically inclined towards a weir inlet 51
that admits the
wastewater to an internal weir chamber 52. The internal weir chamber 52 only
has
fluid communication with the main separation portion of the DAF unit 8 through
the
collector piping 50. By adjusting the height of level-control weir 53, the
position of the
nominal waterline 54 within the DAF unit can be selected. Adjustment of the
position
of the waterline may be advantageous, for example, in conditions where a
significant
amount of separated particulate material is required to be removed by the
sludge
scraper 47. Wastewater overflowing the level-control weir 53 falls into
effluent sump
55 and exits the DAF unit 8 downwardly through effluent opening 60 in an
effort to
further reduce the overall footprint of the unit. A flotation pump 9 is used
to re-circulate
effluent from the sump 55 back to the inlet 40 of the DAF unit 8 and receives
an
injection of compressed air (not shown) in order to supply dissolved gas to
the DAF
unit for micro-bubble formation causing flotation separation.
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The sludge scraper 47 comprises an endless belt 56 which passes over a drive
pulley and a pair of idler pulleys. The idler pulley nearest the anterior end
of the DAF
unit 8 is elevated with respect to the other pulleys in order to create an
upwardly
inclined section of the endless belt 56 corresponding in angle to a sludge
ramp 57.
Attached substantially perpendicularly to the endless belt 56 are scraper
flights 46. As
the endless belt 56 rotates in a clockwise fashion about the pulleys, the
flights 46
engage the sludge blanket floating at the nominal waterline 54 and act upon it
to move
separated particulate matter towards the anterior end of the DAF unit 8 and up
the
inclined sludge ramp 57. The sludge then falls off the end of the sludge ramp
57 and
is deposited within a sludge sump 58 that is periodically emptied through a
downwardly oriented sludge exit opening 59. The sludge exit opening 59 is
oriented
downwardly in an effort to reduce the overall footprint of the DAF unit 8.
Sludge is
mixed with other particulate material produced during the process, such as
screen
tailings from the primary screen 3, and either held within a storage tank for
subsequent
disposal or optionally dried and incinerated.
Referring again to Fig. 1, the effluent of the DAF unit 8 is provided to a
polishing
filter 10 by means of a fourth transfer pump 11. The polishing filter 10 may
comprise
either a media-based depth filter, a fixed element filter, a filter cloth
based plate and
frame filter, or any other suitable polishing filter type. In one embodiment,
the
polishing filter 10 comprises at least one rotating disc comprising a
plurality of pie-
shaped elements covered in a filter cloth. Rotation of the disc permits
periodic
backwashing of the pie-shaped elements and/or removal of the filter cloth. A
backwash pump 12 is provided for this purpose. The polishing filter 10 may
optionally
be augmented by membrane filtration as a tertiary polishing step, depending on
effluent treatment objectives and the intended end use of the treated
wastewater.
However, membrane filters are generally not preferred for the secondary
polishing
filter 10 due to maintenance and reliability concerns, the associated pressure
drop at
the overall system flow rates, and the overall cost of membrane filters for
shipboard
applications.
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Following polishing filtration, the wastewater enters an ultraviolet (UV)
disinfection unit 13 for inactivation of bacteria, viruses, algae and
parasites prior to
eventual disposal. In the process configuration shown, UV irradiation is
employed
following polishing and prior to storage of the wastewater in an effluent
holding tank
14. In emptying the holding tank 14, the flow of wastewater through the system
is
stopped and the tank is emptied through the UV disinfection unit 13. This
process
configuration permits the wastewater to be disinfected a first time prior to
storage,
reducing the toxicity of the effluent in the event of a spill and also
maintaining overall
storage tank cleanliness, and also permits the wastewater in the storage tank
to be
disinfected a second time immediately prior to disposal in the event that any
re-growth
or photo re-activation has occurred during storage. Chemical disinfectants may
optionally be used to augment or replace the UV disinfection unit 13, albeit
less
desirably due to safety concerns associated with the handling or on-site
generation of
these disinfectants.
A number of the unit operations shown in Fig. 1 require venting due to
fluctuations in water level or the addition of gases during operation. In
order to
mitigate health and safety concerns, theses unit operations are normally
vented to an
exterior of the marine vessel (for example, through the ship's ventilation
system) and
the unit operations are desirably operated under a slight negative pressure to
prevent
fugitive emissions. Optional odour control means, such as activated carbon,
may be
employed in conjunction with gas venting to minimize odours prior to discharge
and
enhance operator safety within the confines of the wastewater treatment room.
Other advantages which are inherent to the structure are obvious to one
skilled
in the art. The embodiments are described herein illustratively and are not
meant to
limit the scope of the invention as claimed. Variations of the foregoing
embodiments
will be evident to a person of ordinary skill and are intended by the inventor
to be
encompassed by the following claims.
