Landfill Leachate Treatment

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					    Landfill Leachate Treatment
                    Simple as One, Two, Three

                 Leachate treatment facility at Sita’s Arden Quarry landfill site

The treatment of landfill leachate needs to be efficient, reliable and as simple as possible.
Unfortunately many systems are over engineered or do not take into account the biology of
the process. In essence, the system must look after the bacteria; otherwise the bacteria will
not treat the effluent. In this article we will review some basic facts regarding innovative
technology in a leachate treatment system, which has been running for the last 18 months
with 100% compliance with the discharge consent.

The Concept.

Bacteria, protozoa, algae and multi cellular organisms will develop in the treatment system.
It is the activity of these organisms that is responsible for the treatment of the effluent. The
treatment system is only a life support mechanism to enable the organisms to perform at
their best. The principal group is the bacteria, and they have some basic requirements that
must be satisfied.
Bacteria measure around 0.5 to 5 microns in size and there are millions of different species
of bacteria which can assist with treating leachate. Some bacteria prefer high BOD, or
ammonia, and require oxygen for growth. Other bacteria are incapacitated by oxygen,
whereas some can adapt to aerobic and anaerobic conditions and change their metabolic
pathways depending on circumstances. Bacteria can also change to become adapted to an
environment and exotic “food sources” such as PCBs. Give bacteria the right conditions and
they can perform a wide range of treatment techniques such as assimilation, decomposition
and de-toxification of the effluent, all techniques should be encouraged in a stable leachate
treatment system

Bacteria prefer to live in colonies, which is called floc. By providing numerous different
environments in the treatment system it is possible to ensure that there is a wide species
diversity. The advantage of this is a stable biomass population, which will be capable of
processing leachate successfully under a wide range of conditions. How then do we achieve
this task?

Bacteria stability.

The bacteria floc is like a small fragile ball of cotton, the bacteria act like a colony that
supports each other. The larger the bacteria floc the more stable the colony and the better it
will perform. The colony requires food and usually oxygen, the food comes from the leachate
and the oxygen from the aeration system.

The aeration system must provide sufficient oxygen to ensure that the levels do not fall below
2mg/l. However best performance is achieved by the bacteria if the oxygen levels are
maintained above 5mg/l. The higher oxygen level will require more energy, however the
improved bacterial performance is justified by the higher dissolved oxygen concentration.

Techniques used to provide oxygen include, surface aerators, venturi injectors and air
diffusers. Surface aerators mechanically aerate the water; they will cause some damage to
the bacterial floc and they use a great deal of kinetic energy and are generally not as efficient
as other means of aeration.

Venturi injectors have a high oxygen transfer coefficient, but during the passage of the
bacteria through the injector, the bacteria are exposed to extreme shear forces and pressure
gradients, which breaks up the bacterial floc. If the injectors are using air and not pure
oxygen, then nitrogen gas in addition to the oxygen will be dissolved into the water. The
nitrogen will come back out of solution in the aeration tank, just like taking the top off a
lemonade bottle. Small bubbles of nitrogen will stick to the bacteria, and develop inside the
bacteria. The nitrogen bubbles give the bacteria buoyancy, which prevents them from
settling. Degassing of nitrogen inside the bacteria effectively gives the bacteria the "bends",
exactly the same condition divers experience from rapid decompression. The nitrogen will
increase the internal pressure within the bacteria and in some cases they can actually
explode. These conditions are hardly conducive to the development of a stable bacterial
biomass. Bacteria suffering from the “bends” are often characterised by poor settlement
characteristics and carry over into the final effluent.

Aeration, especially fine bubble diffused aeration, provides a high level of oxygen transfer,
and is very gentle with the bacteria. Large bacterial flocs up to 5mm in diameter will develop
and settlement velocity and clarification of the leachate is excellent.
There has been a move towards covering lagoons and tanks in order to prevent emission of
odours. However if the systems are properly monitored with dissolved oxygen probes, and
oxygen levels maintained above 2mg/l, then there will be no release of odours. By using
open tanks or lagoons further assistance in the treatment of leachate is provided by
photosynthetic algae.

The activity of algae has been completely ignored in many effluent treatment systems. They
are excellent scavengers for ammonia, phenols, list 1 substances and heavy metals. Algal
based effluent treatment systems are in the early stages of development, however, by not
covering the tanks, it gives the opportunity for algae to develop. As a by-product, algae
produce oxygen which helps support the aerobic bacteria, yet another example why species
diversity increases system stability. Even when the aeration systems are turned off, the
oxygen levels can actually rise in the surface water of the treatment tanks due to algal

Environment niches

The bacteria need to live in colonies and there needs to be a high species diversity of
organisms for best performance by the system. If we take a number of tanks, all connected
in series in which the leachate flows from one tank to the next, the treatment of leachate can
be divided into the following stages:-

Step 1

Raw methanogenic leachate from the landfill site will have high levels of oxygen demand
(COD and to a lesser extent BOD) and ammonia. In an aerated aerobic tank, heterotrophic
bacteria, which utilise organic carbon as a food source, will develop. Given sufficient oxygen
and time they will break down much of the COD and all of the BOD to carbon dioxide and
water. They also utilise ammonia as a source of nitrogen for protein synthesis. Heavy metals
and persistent organics, including some pesticide residues, will also be taken in by the
bacteria and incorporated into their cell biomass.

When the bacteria have exhausted the food supply in the leachate, i.e. the readily oxidisable
content has been consumed, then they will become cannibalistic. They will feed on any
available oxidisable carbon source and this includes any dead bacteria in the tank. The so-
called endogenous respiration phase prevents the build up of sludge and some treatment
systems have been operated successfully since 1992, without the need for sludge removal.

Step 2

Some bacteria oxidise ammonia to nitrate. They are known as the nitrifyers and are
autotrophic bacteria, which utilise an inorganic carbon source for cell synthesis. They are
renowned for being temperamental, have a very slow growth rate and have great difficulty
competing with the heterotrophic, or COD reducing bacteria. They are essential for the
oxidation of ammonia, however, and are a vital component of the leachate treatment system.
They need to be protected. The autotrophic bacteria like to stick to a surface, so we help
them by including a floating plastic bacterial support media in the aeration tank. The plastic
biofiltration media increase the surface area within the tank for colonisation by bacteria to
several thousand square metres.

By operating at least two treatment tanks it is possible to crudely segregate the heterotrophic
bacteria from the autotrophic bacteria. Both types of bacteria will be in each tank but they
perform better in the tank that provides the conditions that they prefer. The net result of a
multiple tank system is that the treatment system will be more stable, adaptable and will have
a greater treatment efficiency by enabling a wider range of organisms to develop with greater
species diversity.

Step 3

Even with the introduction of biofiltration media in the tank, bacteria will be present in the final
effluent and can be lost from the treatment system. This has two major disadvantages:-

1.     The bacteria contribute to the suspended solids and may breach the consent
conditions of a discharge to surface water.

2.    The bacteria will result in a charge from the water company for suspended solids if the
discharge is a trade effluent consent.

It was mentioned previously that the autotrophic bacteria are slow growing and they are the
workforce for the oxidation of ammonia. To allow them to be lost from the treatment system
is undesirable and is a problem faced not only in leachate treatment but also sewage
treatment by the water industry.

The innovative solution at the Arden Quarry landfill site, operated by Sita, has been to filter
the leachate, using a tertiary treatment step, directly from the aeration tank to remove the
bacteria from the final effluent. The effluent is discharged to sewer after being held in a
balancing tank and the suspended solids content is typically less than 1mg/l. The bacteria
that are intercepted by the filter are recycled back to the aeration tanks every 6 hours to
ensure that the bacteria are still viable. By the use of this tertiary treatment technique the
essential nitrifying bacteria are retained in the treatment system.

The leachate at Arden Quarry is typical of many landfill sites. The ammonium levels range
from 500 to 1000mg/l, and the COD from 500 to 3000mg/l. The water volumes treated on a
daily basis range from 50 to 200m3/day.

Fig 1 shows the level of ammonium in the discharge from start-up of the treatment system,
over a period of 1 year. After start-up it took approximately 4 weeks for the bacteria to
achieve satisfactory results. The level of ammonium entering the system ranged from
500mg/l to 1000mg/l with the average around 800mg/l. The final effluent quality was
extremely stable and never exceeded 3mg/l after the system was conditioned.
Fig 1. Ammonium level in treated leachate

                                                   Discharge ammonium levels for landfill leachate
                                                                                                       influent = 500 to 1000mg/l


    ammonium mg/l





The COD levels range from 500 to 3000mg/l with an average of approximately 2500mg/l.
It is relatively easy for the bacteria to remove approximately 80% of the COD, however the
remaining component becomes increasingly difficult for the bacteria to digest. Fig2 below
shows that the bacteria were able to reduce the COD level by approximately 90%; the
remaining fraction is classified as hard COD. It is possible to remove this hard COD by
cracking the organic molecules using oxidation techniques followed by biological treatment,
removal levels better than 98% can then be achieved.
Fig 2. COD level in treated leachate

                                                                   COD discharged for landfill leachate
                                                                                        influent = 500 to 3000mg/l
   COD mg/l



















Tertiary Treatment

Effluent treatment for the final removal of suspended solids is not new. A variety of clarifiers
and sand filtration systems are available. The inherent problem with sand filtration has been
irreversible biofouling of the media by the bacteria that have been intercepted. Simple
backflushing is not sufficient to remove the accumulated solids and even with air scouring
and the addition of cleaning chemicals the performance of the sand filter deteriorates,
sometimes after a matter of weeks.

To protect the sand filter, good settlement is required and the target maximum solids
concentration in the feed effluent is usually less than 50mg/l. At Arden Quarry the feed to the
tertiary treatment system is taken directly from the tanks whilst they are being aerated. The
solids loading can be up to 1500mg/l. This would be suicidal for conventional sand filtration
AFM filtration

AFM (Active Filter Media) is the filtration media used at Arden Quarry. It is similar in
appearance to sand and is used in a standard pressure sand filter. AFM is manufactured
using recovered green and brown glass bottles, which have been processed to create a
filtration media that is resistant to biofouling. Examination under the electron microscope
reveals that AFM has a very smooth surface compared to sand and it is this smooth surface,
which prevents bacteria sticking to the surface of the filter media. Simple backflushing is
sufficient to restore the filter to full capacity even with a solids loading in the raw effluent feed
some 30 times greater than would be acceptable in a conventional sand filter. In addition to
being able to remove much smaller particles from water than sand, AFM also has an overall
negative charge at its surface, which can attract heavy metals and persistent organic

Table 1 shows that the leachate at Arden Quarry has prescribed substances at very low
concentrations at, or about, the limit of detection. The level of prescribed substances is at its
highest in tank 3, where they have been concentrated by bioaccumulation during many
months of treatment. Significantly the concentration is below the limit of detection in the final
Table 1       Removal and Assimilation of Prescribed Substances

                                      Raw          Tank 3      Final
                                    leachate                  effluent
                 Tributyltin        <0.0398        0.058      <0.0398
                 Tributyltin          0.069        <0.05       <0.05
                 Mercury              0.097        0.160       <0.09
                 Mercury              <0.1         0.105       <0.1
                                        Results in µg/l

The AFM filter at Arden Quarry is only a small unit taking up six square metres of space. It
will, however, process 250m3 of effluent per day. Larger leachate volumes can be processed
by using multiple filters in parallel. Since July 2002 it has intercepted bacteria from the
aeration tanks and has virtually eliminated the discharge of bacteria and other organisms as
suspended solids. The bacteria have been salvaged and recycled back into the treatment
systems. The system has proved to be very robust and has been compliant with the trade
effluent consent and has removed prescribed substances to below the limit of detection.

The discharge from Arden Quarry is authorised under a trade effluent consent issued by
United Utilities under the Water Industry Act 1991. The receiving sewage treatment works is
up to treatment capacity and non-compliance with the trade effluent consent could have
serious consequences for both Sita and United Utilities.

Brian Ellor, Trade Effluent Policy Manager for United Utilities, comments “The trade effluent
discharge from Arden Quarry is received at a United Utilities sewage treatment works which
discharges into sensitive and high-quality controlled waters. The leachate treatment system
has been operational since July 2002 and we have experienced no problems so far. The
successful use of a tertiary treatment technique, which can process final effluent directly from
an extended aeration system, is highly innovative. We at United Utilities are looking at AFM
for the removal of certain contaminants from some of our sewage treatment works final
effluent. The fact that AFM is a recovered material is another environmental bonus for the

A series of demonstration projects are being undertaken with AFM as part of a detailed R&D
programme. The research is supported by Life Environment and WRAP (Waste and
Resources Action Programme). Andy Dawe, responsible for WRAP’s Glass Programme
says, “The results which have been achieved using AFM in landfill leachate treatment,
sewage treatment, swimming pools, industrial process water treatment and even drinking
water filtration have been universally successful. This high quality filter media has the
potential to radically reduce the incidence of effluent non-compliance. This is an excellent
example of where recycling not only reduces landfill, but also helps to ensure that the
environmental impact of the landfill is itself reduced.

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