|
Aeration
Oxygenation and Water Treatment |
The Treatment of Leachate
from
Landfill Sites
by
Dr.Howard T Dryden
Dryden Aquaculture Ltd 1997
Introduction
There are now over 6000 landfill sites in the UK, many of which will generate a
leachate which requires water treatment in order to protect the receiving environment be
it a river, loch or even the sea.
The principal means of treating the leachate water is by the action of aerobic bacteria
in an aeration lagoon. The key point of any water treatment system is therefore the
efficiency of the aeration system at mixing the water in order to keep the bacterial floc
in suspension, and being able to supply sufficient oxygen to the bacteria. Most systems
should be able meet these criteria but at a cost in terms of capital items and running
cost in terms of electrical consumption.
This paper details the Dryden Aqua systems for landfill sites which have a lower
capital cost than most surface aerators, and approximately 1/4 of the running costs. There
are also many other benefits to be gained from the Dryden system which will be explained
later in the paper.
Concept
The performance of the water treatment system depends upon the activity of the bacteria
and protozoans in the aeration lagoons. It is therefore important that the best possible
conditions are presented to the bacteria in order for them to perform their function.
There are several different groups of bacteria present in the water, however they will
all require oxygen and are classified as aerobic bacteria. There are two basic groups or
aerobic bacteria;
1. Heterotrophic bacteria
2. Autotrophic bacteria
The heterotrophic bacteria utilize organic matter as an energy source, and in this
process the BOD of the water is reduced. BOD refers to the Biochemical Oxidation Demand of
the water and is a measure of the amount of oxygen required by the heterotrophic bacteria
necessary to oxidise the organic matter present in the water. The test is normally based
on the BOD measured over a 5 day period at a temperature of 20 degrees centigrade. Most
landfill sites will be at temperature less that 20 degrees centigrade for the better part
of the year. The most critical times are actually the winter months when the water
temperatures could be as low as 4 degrees centigrade. The lower the temperature of the
water, the slower the biochemical activity of the bacteria and the more difficult it is to
reduce the BOD.
The autotrophic bacteria use inorganic carbon as their carbon source, and the group of
interest to us in water treatment are the autotrophic nitrifying bacteria that use
ammonium as an energy source and convert the ammonium to nitrate. Essentially ammonium is
toxic to aquatic organism and nitrate is relatively none toxic, although it will still
cause eutrophication.
The population density of heterotrophic bacteria and autotrophic nitrifying bacteria in
the aeration lagoon will depend of the quantity and quality of the raw leachate water. The
aeration system will provide sufficient oxygen for the bacteria and will insure that there
is adequate mixing in the lagoon to prevent dead or anaerobic areas. Anaerobic areas refer
to locations in the lagoon where the dissolved oxygen levels are too low to support
aerobic bacterial activity.
Stabilization of the water treatment process
In addition to insuring that there is plenty of oxygen in the water, it is also
important to insure that the water chemistry remains constant since a fluctuating water
chemistry will inhibit the activity of the bacteria.
Equally if the bacteria are given too much leachate to treat they can either be washed
out of the lagoon or there may be insufficient biomass of bacteria in the aeration lagoon
to treat the leachate. Also if insufficient leachate was added to the aeration lagoon, the
concentration of bacteria in the lagoon would decline to match the loading. If the amount
of leachate was then increased there may be insufficient bacteria present in the aeration
lagoon to treat the influx of leachate. Creating stable conditions and as near a constant
feed of leachate to the treatment aeration lagoon is therefore an important criteria. In
order to meet this requirement a collection or storage lagoon for leachate should be
constructed, a second ( or more) lagoon for the treatment of the leachate and a third
lagoon from which the treated leachate is discharged.
Leachate storage lagoon
The collection lagoon should have as large a capacity as possible, since it is the
function of this lagoon to act as a leachate flow buffer. The larger the lagoon, the
easier it is to maintain a more constant and consistent supply of raw leachate water for
the bacteria in the treatment lagoon.
Treatment lagoon
Leachate will be pumped or delivered to the treatment lagoon on a daily basis. The
treatment lagoon should have a capacity that will give a minimum of a 10 day residence
time. This time period is not required in terms of the length of time required to treat
the leachate, however it is essential as a water storage buffer such that when leachate is
added to the lagoon only 10% of the total volume of the lagoon will be taken up by the
fresh batch. Because of the relatively small quantity of raw leachate water to the total
volume it is unlikely that the water chemistry will change significantly with the addition
of the leachate. The chances of shock loading the bacteria are therefore minimised.
The aeration system installed in the treatment lagoon will provide the oxygen required
by the bacteria. Details regarding sizing of the system are given later in the paper.
Discharge Lagoon
Once a day the aeration system is turned off in the main aeration lagoon for a period
of approximately three hours in order to allow the bacteria floc to settle. After a three
hour period the top 10% of the treated leachate water is delivered to the discharge
lagoon.
From the discharge lagoon the treated leachate is discharge to the receiving
environment over a 24 hour period.
There may be occasions when the treated leachate dose not quite comply with the
discharge criteria for the site in terms of BOD or ammonium, therefore additional
treatment might be necessary. The removal of ammonium from the leachate water by the
autotrophic nitrifying bacteria is generally a less stable process than the heterotrophic
bacterial digestion of the organic matter. Therefore occasionally the ammonium levels may
be higher than desired.
On sites where the ammonium levels are in excess of 300mg/l with a low BOD in the order
of 200mg/l, the majority of the ammonium will be treated by the nitrifying bacteria as
opposed to being assimilated by the heterotrophs. Under these conditions it is therefore
desirable to have some additional treatment of the water. The discharge lagoon provides a
means of achieving this task.
Since the treated leachate water is leaving the discharge lagoon on a continually basis
it is not possible to use a conventional extended aeration or activated sludge process at
this point in the process because the bacteria will be lost from the system. A floating
biofilter is therefore installed in the discharge lagoon to act as a support for the
nitrifying bacteria and thereby preventing them from being washed out of the discharge
lagoon. The bacteria also require oxygen therefore a number of diffusers are placed in the
discharge lagoon to insure that the leachate will flow through the biofilter and to insure
that there will be an adequate supply of oxygen for the bacteria.
As explained above, the nitrifying bacteria tend to be more sensitive to the
environmental conditions than the heterotrophic bacteria. The water chemistry is therefore
further stabilized to promote the growth of the nitrifying bacteria.
For nitrification, the pH of the water is very important, it is essential that the pH
is not allowed to fall below pH 7 otherwise nitrification will cease. In order to buffer
the pH of the water, a quantity of magnesium hydroxide pellets are added to the lagoon.
The magnesium will dissolve in the water in direct response to the pH of the water. One or
more tonnes of magnesium hydroxide may be required, however once there is a sufficient
quantity in the lagoon additional small quantities of magnesium need only be added at
periods of 6 months intervals. Equally since the bacteria require inorganic carbon,
addition bicarbonates may have to be added to the water in order to maintain the
alkalinity.
The nitrifying bacteria can be shock loaded by changing concentrations of ammonium. In
order to stabilize the ammonium concentrations and ammonium buffer (ammosorb) is added to
the lagoon. Several tonnes of the product may be required.
Ammosorb is very similar to sand in appearance, however it has an unusual property in
as much that it will selectively remove ammonium ions from freshwater. The product is used
simply by adding it to the discharge lagoon. The action of the air diffusers will insure
that the leachate water containing the ammonium is brought into contact with the ammosorb
on the bottom of the lagoon. As a plug of ammonium enters the discharge lagoon, the
equilibrium is shifted such that the ammosorb will absorb the ammonium from the water. The
ammosorb therefore prevents high levels of ammonium from developing in the discharge
lagoon. Since high levels of ammonium could shock load the nitrifying bacteria, the
ability to remove the peek loading is beneficial. As nitrification progresses in the
discharge lagoon the concentration of ammonium in solution decreases to a point such that
the equilibrium levels of ammonium are now in favour of the ammonium leaving the ammosorb
and going into solution. By this technique the nitrifying bacteria are protected from
shock loading, and when the ammonium ion levels are low ammonium leaches out of the
ammosorb to feed the nitrifying bacteria. The ammosorb therefore creates a very effective
ammonium buffering system and maximizes the performance of the nitrifying bacteria.
The water treatment system acts as a sequencing batch reactor, in order to increase the
the frequency of the batch process flocculation of the bacteria in enhanced using BioFloc.
We also find that the BioFloc promotes nitrification
The above completes the design concept of the leachate water treatment system. The
following section looks at the sizing of the aeration system for the main treatment lagoon
and concludes with a case study of a system which has been in operation for two years.
Sizing the Aeration System
The size of the aeration system is taken as being the amount of air that is required to
be diffused through the water in order to always maintain dissolved oxygen concentrations
in excess of 2mg/l for the aerobic bacteria.
The size of the aeration system is also a function of the efficiency of oxygen transfer
and this is related to the dissolved oxygen concentration in the water. The higher the
dissolved oxygen concentration the more difficult it is to get the oxygen into the water
and the lower the aeration efficiency. However the loading on the aeration system may not
be a constant therefore the system should be sized on the basis of the maximum loading
that it will receive.
The details below relate to the sizing of an aeration system required to reduce the BOD
of the waste water by approximately 95% in 24 hours. BOD reductions of better than 99% are
possible. Please note that it is not the aeration system that is reducing the BOD but
rather the activity of the aerobic bacteria in the activated sludge. The aeration system
is being used as a means of keeping the bacteria in suspension and to supply 100% of their
oxygen requirements.
Table 1. Guide for sizing the aeration system
BOD per day Volume of air *Kw rating No
kg cubic metres/hr of blower of diffusers
50 150 3.5 15
100 250 6.8 25
200 400 11 40
300 550 16.3 55
400 700 18 70
500 850 24 85
* Approximate kw/hr power absorbed by blower diffusing air into a 3 metre deep aeration
tank
Please note that the above table should only be used to give you a guidline in the
sizing of the aeration system. Several assumptions were made in calculating the above
table, the main assumptions are given below;
- Maximum BOD of the water is less that 5,000mg/l
- Water depth is 3 metres.
- Water temperatures not greater than 20 degrees centigrade
- Figures based on aeration efficiency in leachate water
- Ring main is approximately 100m in length
- Blower situated next to the lagoon
Efficiency of the aeration system
Observed Oxygen Transfers * range 2 to 7 kg O2 /kw / hr
There are many factors which will effect the efficiency of oxygen transfer, however in
relation to other types of aeration and mixing system there are some basic fundamentals,
the most important of which is the conservation of energy.
Any system which throws the water up into the air is using a great deal of kinetic
energy to achieve this task. Certainly oxygen will be dissolved into the water as the
water travels through the air, however the energy used in lifting the water is no longer
available for putting oxygen directly into the water.
It is impossible for any aeration device operating at atmospheric pressure to raise the
dissolved oxygen level over 100% saturation. Since aeration efficiency approaches zero at
100% oxygen saturation, it is important that the aeration device moves the water in the
aeration lagoon in order to bring water with a low dissolved oxygen content in contact
with the surface of the lagoon and the with the aeration device, equally important, the
aerator must move the water with a high oxygen content away from the aerator. All aerators
must therefore be capable of moving large quantities of water very efficiently. This
necessitates either a multiple number of small aerators in an aeration tank or several
very large aerators. With large aerators short circuiting of the water flow distribution
occurs, therefore the best option is a multiple number of smaller units. Which ever option
is used, the more mechanical devises there are in an system, the more energy is lost
through friction and mechanical aspects. Also with mechanical devices there is likely to
be more maintenance and specialist equipment required to keep the system running.
With the diffused aeration system, there is only one or two central air blowers which
can deliver air to any part of the facility. The air blowers are the only mechanical
device in the system and these are virtually maintenance free, (basically a service
comprises of one oil change once per year and check belt tension occasionally). A multiple
number of diffusers are used therefore the diffusers can cover the base of the lagoon and
thereby insure thorough mixing and aeration of the water.
Diffuser efficiency factors
The main aspects are given below
1. Bubble size & water quality
2. Water depth
The size of the bubble produced by the Dryden diffuser is very fine (approximately
1/8" to 1/32"), The smaller the bubble the greater the surface area of bubble in
contact with the water, hence the higher the oxygen transfer efficiencies. Smaller bubbles
also have a greater friction across the water air bubble interface, thus the smaller the
air bubble the slower it will travel and the longer it will remain in contact with the
water.
Air diffusers are one of the most efficient means of moving and mixing water by
employing the air lift effect. The air lift effect is a function of the friction between
the water, and the air bubbles as they rise up through the water. The air bubbles on their
passage to the surface actually drag as opposed to push the water. Air diffusers can move
a tremendous amount of water, for example at a depth of about 3 metres each cubic metre of
air passed through a diffuser will move in the region of 50 to 75 cubic metres of water.
Therefore one 3 metre long diffuser, passing 10 cubic metres of air per hour will move in
the order of 500 to 750 cubic metres of water per hour. A 40 diffuser system will be
moving 20,000 to 30,000 cubic metres of water per hour for an energy expenditure of
12kw/hrs. These figures are quite tremendous and no surface devise or water pump could
match the amount of water moved by the diffusers. This mass movement of water is a key
feature of the diffusers and is one of the reasons for the high efficiency of the process.
Water Depth
As the water depth increases the contact time between the rising bubbles and the amount
of water moved by the bubbles increases. The transfer efficiency of oxygen from the air
bubbles to the water therefore improves as the water depth increases. However as the depth
of the water increases the air pressure required to compensate for the hydrostatic water
pressure increases and the amount energy required to push the air down to the diffuser
increases. The most suitable depth range for the aeration system presented in this report
is 2 to 5 metres employing positive displacement roots type blowers as the motive force.
Electrical
All of the electrical connections are to the motor on the air blower, the air blower
should be enclosed in a suitable shed or acoustic enclosure. The blower may be located
next to your power supply or suitable location on the site. There are therefore no
electrical motors, submerged in the water, floating on the surface, or suspended above the
water. This makes for an electrically very simple and safe installation of the electrical
equipment.
Aerosol
Because the diffused aeration system is not thrashing the surface of the water or
throwing water into the air, it does not generate a significant water aerosol. This is an
important aspect if the water being treated contains potentially pathogenic organisms or
noxious substances.
Case Study
Presented here are the results covering the start up of a diffused aeration water
treatment system for the Argyll & Bute District Councils landfill water treatment
system in Dunoon, Scotland.
The data presented below is for the first 120 days after start-up of the system, start
up date was November the 5th, thus the water temperature was only 7 degrees centigrade and
dropping. Low water temperatures slow down bacterial activity and reduce bacterial
performance.
On start-up the aeration system the water was anoxic with no aerobic bacteria. The
lagoon was not seeded with bacteria, instead we allowed the aerobic bacteria to develop
naturally in the water. The system could be seeded with activated sludge from a water
treatment plant. However unless the general water chemistry of the water treatment plant
is similar to the leachate water, problems can occur. For example if the leachate water
has a high salt concentration and the activated sludge has come from a freshwater
installation, on addition of the bacteria to the leachate water the bacteria may
experience osmotic shock and could be killed by the leachate water. All that this exercise
would accomplish would be to increase the organic loading on the new system. However if
you can create the conditions that would be acceptable to the bacteria, then bacteria that
are adapted to the leachate water will rapidly develop in the system. This was the
procedure adopted in this case study and the results for the first 120 days are presented.
The diffused aeration system employed delivers 400 cubic metres of air per hour
distributed through 40 diffusers. The BOD of the leachate ranges from 500mg/l to 1000mg/l
with ammonium concentrations in the order of 100mg/l. The volume of leachate treated on
this site is a variable due to the geographical location of the site. The site experiences
an average rainfall in excess of 2000mm per year and can generate up to 2000 cubic metres
of leachate per day.
The average flow of leachate treated by the system is 200 cubic metres per day, however
during periods of sustained heavy rain the site has treated in excess of 1000 cubic metres
per day for periods of up to 1 week. Problems are experienced under these circumstances
due to wash out of bacteria from the system. However that fact that the system has managed
to continue under these extreme circumstances is a sign that the process has a high in
built stability.
The above graphs for BOD and ammonium show that after a period of approximately 30 days
the treatment system was reducing the ammonium levels and the BOD levels by at least 90%.
During this period of time the system was receiving an average of 200 cubic metres of
leachate each day.
The system has now been in operation for almost two years, presently the ammonium
levels are rising and the BOD concentrations falling in the raw leachate water.
Nitrification is well established and the system is regularly producing final effluent
concentrations of ammonium less than 1 mg/l and BOD figures less than 10mg/l.
Summary
The paper presents a strategy for the treatment of leachate water using a three lagoon
system with extended diffused aeration employing activated sludge. The final stage of the
process employs a fixed film biological filter incorporating ammonium buffers and pH
buffers.
The extended aeration process using the Dryden diffused aeration system has now been in
use for 10 years. The key features are that the system is very effective in treating
leachate water as demonstrated in the case study. The system has also been proved to be
extremely reliable and easy to maintain. Finally because of the high electrical efficiency
of the system the running costs can be up to 4 times less than conventional surface
aerators. On the basis of electrical savings alone the capital cost of the system can
normally be recovered in less than 1 year.
