ÐÓ°ÉÔ­´´

Door frames and apple trees are not the first things that spring to
mind when the subject of sewage is raised at dinner parties. But these and
many other unexpected items, such as bricks and planks of wood, have turned
up at sewage works with the usual mixture of paper, rags, sanitary towels
and human waste that disappears down the loo. It’s not the big, solid items
that create hazards, however. It’s the things you can’t see-the microscopic
organisms and poisonous chemicals-that pose a threat to people, wildlife
and the environment if effluent is discharged into water courses without
thorough treatment.

In June 1988, the European Community’s environment council decided to
draw up a new directive on the treatment of municipal waste water. Its decision
was not before time: in 1984, the latest date for which full figures are
available, only 45 per cent of the organic contaminants in the community’s
municipal waste water was treated. The aim of the directive is to raise
substantially the cleanliness of the Continent’s waterways, coastlines and
estuaries, by forcing member states to improve standards of effluent treatment.
The first draft, issued in December 1989, stimulated wide debate; the council
hopes to agree the revised version when it meets next week in Brussels.

Municipal waste water is a potpourri of rainwater and effluent discharged
into sewers by households, industry and agriculture. Failing to treat it
properly can be catastrophic. Nutrients such as nitrogen and phosphorus
combine with metals, fats and toxic chemicals as well as bacteria and viruses
to soak up the oxygen dissolved in freshwater upon which fish and other
aquatic life depend for survival. The pollutants can also make bathing unsafe
and damage shellfisheries.

At present, most municipal waste water undergoes up to three tiers of
treatment at a sewage works, depending on the type of waste received, the
size of the community being served and the nature of the waters into which
the treated effluent is discharged. After removing solid objects, sewage
workers allow the liquid to settle in a tank, a process called primary treatment.
In small communities that rely only on primary treatment, the sludge is
often spread on agricultural land, and the water discharged into rivers,
streams, estuaries or coastal waters. In towns of 2000 inhabitants or more,
both the liquid and sludge usually undergo one or two further stages of
treatment .

The main requirement of the revised EC directive is that secondary treatment
should be the norm. The draft proposes tertiary treatment for discharges
to ‘sensitive’ water courses, such as those choked by nitrogenous and phosphorous
fertilisers and agricultural wastes. Other sensitive waters are defined
as those from which we draw drinking water but which have high levels of
nitrates, and those of special scientific importance. The directive will
force member states to identify ‘sensitive’ and ‘less sensitive’ areas and
to tailor treatment standards accordingly by the end of 1998. The draft
says that primary treatment may suffice in ‘less sensitive’ areas, such
as some coastal waters, where the oxygen content of the water is high and
the tides and salt in the water combine to detoxify the waste naturally.

Britain’s Department of the Environment maintains that to meet the requirements
of the directive on ‘sensitive’ coastal and estuarial discharges alone would
cost £1.5 billion in capital terms and £45 million a year
in running costs. This is over and above the £12 billion that the
newly privatised water industry in England and Wales has already earmarked
for improving the sewerage system over the next decade. Regional councils
in Scotland will face a huge bill to bring coastal discharges up to standard
because so many small communities in coastal locations rely solely on primary
treatment. The councils may be forced to seek financial help from Brussels.

Industry throughout Europe fears that higher costs of sewage treatment
will be passed on to business in the form of higher charges of up to 20
per cent. ‘British business currently spends around £400 million
each year on disposal of trade effluents to sewer. The increased costs .
. . will therefore be significant, particularly for smaller and medium sized
companies,’ says the Confederation of British Industry. The Union of Industrial
and Employers’ Confederations of Europe and Britain’s Chemical Industries
Association express similar misgivings.

Against this political, financial and regulatory backdrop, engineers
are seeking to develop new and ingenious methods of cleaning effluent. Among
the more promising approaches are the use of reed beds and peat filters
for cleaning domestic, industrial and agricultural waste waters. Biotechnology
companies want to use bacteria to break down contaminants unique to particular
waste streams and to purify underground water supplies . Researchers are
also investigating membranes, the fine sieves used in industrial processes,
as a means of ridding effluent of viruses, which can survive conventional
secondary and tertiary treatment.

Reed beds began to proliferate in the US in the early 1960s and have
been used increasingly in Europe over the past decade. In Britain, the water
companies have been conducting trials with them since 1984. Most existing
reed bed systems are only suitable for treating domestic effluent, but newer
systems can also treat concentrated industrial and agricultural effluents.
The principle behind the systems is simple. The reeds absorb oxygen from
the atmosphere, channel it down their stems and pass it on to aquatic bacteria.
These bacteria, which degrade waterborne effluent, depend on oxygen to survive.
According to Gareth Job, a postgraduate student at the University of Birmingham,
the common reed (Phragmites australis) extends fine hair roots into the
water and these provide the bacteria with a rich source of oxygen. The bacteria
destroy organic material in the effluent that would otherwise starve aquatic
life of oxygen. They also destroy nitrogenous material. Phosphates can be
adsorbed by the material that the reeds grow in.

Job has designed a system to treat highly smelly, decomposing run-off
from a pig farm, waste that is six to seven times as concentrated as ordinary
domestic effluent. The waste flows first through a series of so-called ‘vertical’
reed beds. These consist of layers of filtering material of increasing porosity,
from sand to rocks, on which the reeds grow. The effluent then trickles
down a conventional horizontal bed of limestone chippings, which also contains
reeds (see Figure ). The vertical beds clear the effluent of most organic
material and the horizontal filter, which receives the pre-treated water
directly from the vertical system, denitrifies the waste and removes phosphates
from it. According to Job, the treated effluent meets existing national
standards, which typically require a 150-fold or so reduction in the oxygen-depleting
content of ordinary domestic waste water.

Job’s pilot plant covers 150 square metres of land at Rugeley in Staffordshire
and could treat up to 14 cubic metres of waste a day, the amount expected
from a community of about 70 people. Moreover, the system requires no electrical
supply and virtually no maintenance, which make it 10 times cheaper to run
than conventional processes. The system also breaks up the sewage sludge
without generating the smell associated with conventional treatment works.
This is because leaves from the reeds collect on the surface of the beds
to form an odourless compost that consumes the methane and other gases that
are thought to make the drying sludge on conventional sewage farms smell
so strongly.

Job underlines his claim that the systems are environmentally friendly
by pointing out that reed warblers nested at Rugeley last year. The beds
also have industrial applications. ICI has just installed a £4 million
system at its Billingham complex, in northeast England, to detoxify phenolic
waste and British Steel has also commissioned a plant, he says.

Researchers at Birmingham are also developing reed bed systems that
clean up the waste liquor created when farmers ferment grass or maize to
generate animal feed, or silage. The effluent, which is around 250 times
more concentrated than domestic effluent, has an extremely high content
of oxygen-depleting nitrogenous and phosphorous material.

Another ‘environmentally friendly’ technology with bright prospects
is the use of peat to clean up effluent. Peat is a natural material formed
over the past 10 000 years through the decomposition of vegetation. Soily
particles within the peat abstract organic matter from effluent by forming
chemical complexes with the pollutants. Naturally occurring microbes can
also make the soil acidic so that it filters toxic metals by exchanging
ions with the effluent.

Not surprisingly, many of the key developments have occurred in Ireland
which has extensive reserves of peat. Bord na Mona, the Dublin-based semi-state
agency responsible for peat production and marketing in Ireland, has developed
a peat derivative that is suitable for treating domestic effluent and run-off
from septic tanks. ‘It’s the ultimate anti-pollutant,’ says Eddie O’Connor
an executive at the Irish agency. ‘It can absorb Escherichia coli (pathogenic
bacteria found in the human gut), heavy metals, oil and smells,’ he says.
One such system has already been installed at an arts and leisure centre
in Annaghmaerrig, County Monaghan. The centre’s sewage runs through the
peat and is discharged to a nearby lake, which also provides the centre’s
drinking water.

Researchers at Queen’s University Belfast, meanwhile, are developing
a system that can use peat or lignite (brown coal), both of which are cheap
and plentiful, to strip toxic metals from industrial waste waters. Big chemicals
companies, including Du Pont of America and ICI of Britain, are funding
the work. The orthodox way to remove toxic metals is either to filter the
waste with activated carbon powder, which is expensive, or to add lime to
the waste, which creates a fluffy gel, or floc, that must be disposed of
in a landfill site. The newer system allows the metals to be recovered for
recycling and the peat or lignite to be reused.

Give peat a chance

The researchers, Pauline Brown and Orla Flynn, are measuring the ease
with which peat and lignite scavenge copper, lead, zinc, cadmium and mercury
from solutions containing 1000 parts per million of metal ions. This exceeds
vastly the amounts typical of industrial waste streams, which seldom contain
more than a few parts of metal per million of water. The existing directive
on industrial wastes restricts levels in discharged water to parts per billion
of metal; mercury and cadmium must be removed completely.

Using sphagnum peat moss from Derry and lignite from an open seam in
Crumlin, Brown and Flynn found that the materials can adsorb the ions in
as little as 30 minutes; the peat was marginally quicker than lignite, but
the lignite had a greater capacity. They say these preliminary results indicate
that peat and lignite are much more cost effective than activated carbons,
which cost between £500 and £10 000 per tonne and adsorb between
6 and 9 milligrams of metal ions per gram. They found that peat, which costs
about £35 per tonne, adsorbs between 10 and 55 milligrams of metal
ions per gram, while lignite, which costs about £20 per tonne, adsorbs
between 5 and 70 milligrams per gram.

Now they plan to experiment with other types of peat and lignite. They
want to investigate how the acidity and type of material affect the adsorption
of metal ions, and whether the system works as well with solutions containing
not just single types of metal but a mixture of them.

Another area exciting interest is the use of immobilised bacteria to
detoxify industrial waste. The organisms are ‘immobilised’ by being bound
to a solid medium, such as carbon powder. As effluent is passed through
this medium, the bacteria scavenge or degrade chemicals in the waste stream.
Monsanto, the American biotechnology company based in St Louis, Missouri,
has built a pilot system at its rubber chemicals production plant in east
St Louis. According to Bill Adams, who developed the system at Monsanto,
bacteria extract the key toxic by-products from the waste stream, including
methyl ethyl ketone, amyl isobutyl ketone, methyl isobutyl ketone, ethylbenzene
and various xylenes. He says that the bacteria, which are bound to activated
carbon powder, remove 90 per cent of the dissolved by-products.

Designer bacteria

Adams went to a local sewage plant to find the bacteria he needed. He
selected those that broke down the toxic by-products of rubber production
and that tolerated the high levels of salt of the waste stream. He ended
up with five species of pseudomonas bacteria. These microbes convert the
toxic waste into smaller, harmless compounds, or into carbon dioxide and
water.

The attraction of using immobilised bacteria is that specific organisms
can be selected to treat specific types of waste, says Adams. Moreover,
the technology confines the organisms in high densities so that they do
not lose any of their potency.

Researchers at the University of Bradford, led by Roger Bickley, take
a completely different approach to cleaning up waste water containing organic
pollutants. In conjunction with researchers from the University of Bath,
Lawrence Hogg and postgraduate colleagues at Bradford have devised a laboratory
scale reactor that uses ultraviolet light to turn organic waste, such as
phenol, chloroform and the herbicide Atrazine, into harmless by-products,
such as water, carbon dioxide and mineral salts. The system relies on titanium
dioxide dispersed throughout the waste solution to act as a catalyst. Bathed
in ultraviolet light, titanium dioxide causes the water molecules in solution
to break down into highly reactive chemical agents called hydroxyl radicals,
which destroy the organic compounds. Hogg says there is still some way to
go before the process can be scaled up for industry.

Alongside some of these more exotic approaches, there have been numerous
refinements of the traditional technologies and processes for detoxifying
waste water. Researchers at Loughborough University of Technology have discovered
that activated carbon scavenges radioactive isotopes much more effectively
from the effluent streams of nuclear facilities if its surface has been
oxidised with nitric acid. According to Mike Streat, a researcher in the
university’s Department of Chemical Engineering, the oxidised carbon bears
weakly acidic chemical groups that readily swap ions with those isotopes
of metals such as strontium, caesium and cobalt.

Last but not least is a refinement in membrane technology that cleanses
effluent of almost all major contaminants, from bacteria to heavy metals,
without the need to add any chemicals. Rodney Squires, managing director
of Crossflow Microfiltration, the company that licenses the technology,
says the system would be ideal for tertiary treatment of sewage because
the pores are fine enough to sift out most of the viruses that slip through
conventional sand filters.

The inability of conventional secondary and tertiary treatments to intercept
viruses was identified by the National Rivers Authority, Britain’s pollution
watchdog, as a major problem that needs to be resolved. Anglian, Northumbrian
and Yorkshire water companies are evaluating the system alongside conventional
filters. Cross-flow microfiltration, as the process is known, is also a
speedier way of filtering solutions, says Squires. A flow of fluid across
the membranes, as well as through them, prevents them from clogging up.
‘You are effectively sweeping up as you go,’ he adds. Squires says the system
can cope with 8 million litres of waste water a day and peak loads of three
times as much-sufficient to serve communities of up to 40 000 people.

Engineers are rising to the challenge of meeting higher standards of
effluent treatment. In Britain, companies have formed the Effluent Processing
Club to pool research and ideas and to provide one another with technical
support. One of the first tasks of the club, which is run jointly by the
Water Research Centre at Medmenham in Berkshire and AEA Technology at Harwell
in Oxfordshire, is to help industry to grapple with the provisions of the
European directive on municipal waste water.

Further reading: Papers from the annual research meeting of the Institution
of Chemical Engineers, held at Cambridge in January

* * *

1: The conventional treatment for cleaning up waste

When municipal waste water arrives at a sewage works, it undergoes preliminary
screening to remove all the large solids-from door frames and apple trees
to toilet paper and sanitary towels. It is normally raked mechanically except
in smaller works-usually in rural or coastal areas-where people remove the
solids manually. In a second preliminary stage, grit is removed by allowing
it to settle.

What remains is fluid containing suspended organic material in clumps
no more than about 1 centimetre across. This fluid is delivered to a settlement
tank for primary treatment, where a very smelly, decomposing sludge forms.

After the liquid is drawn off from the sludge, which is treated separately
or discharged, it is treated biologically. The dense liquid is pumped into
percolating or trickling filters for secondary treatment. These filters
are usually concrete receptacles about 10 metres across and 1 metre deep.
They contain a bed of stone fragments, up to a few centimetres thick. Bacteria
that break down the organic material in the liquid grow naturally as slimy
films on the stone bed. The liquid is cleaned as it trickles over this bacterial
slime.

Sewage works in large towns operate a slightly more sophisticated filter
system for secondary treatment, known as the activated sludge process. Bacteria
grow in a tank of fluid drawn off from the primary treatment works. Air
is bubbled through the tank to sustain the microbes. A so-called ‘floc’
of bacteria forms and this is transferred, along with the liquid, to a settling
tank where the bacteria settle and form a sludge.

By the time secondary treatment is complete, either by use of trickling
filters or the activated sludge process, the water is generally deemed fit
for discharge back into the aquatic environment, between 10 and 12 hours
after it entered the sewage works. Sometimes, a further stage of cleansing
is necessary, called tertiary treatment. This usually involves passing the
liquid from the secondary treatment stage through a fine sand filter.

The sludge left over from the primary and secondary stages of treatment
is transferred to a ‘digester’, a tank from which air is excluded. In this
anaerobic environment, naturally occurring bacteria, suited to airless conditions,
convert the organic material into a ‘biogas’, a mixture of methane and hydrogen,
which escapes to the atmosphere or is used to generate power (‘All gas and
garbage’, New ÐÓ°ÉÔ­´´, 3 June 1989).

The treated sludge is sometimes suitable for spreading on agricultural
land, either as a liquid or as a dried, solid fertiliser. This is not the
case in industrial areas, however, where discharges to sewers may contain
toxic chemicals and poisonous heavy metals. At present Britain empties one
quarter of this ‘toxic’ sludge (250 000 dry tonnes a year) into the North
Sea, a process which the government has agreed to phase out by 1998 to allay
fears that the sludge harms marine life.

Instead of being spread on agricultural land, the sludge can be incinerated
or dumped in landfills, though neither disposal method is without its environmental
problems. Heavy metals and pathogenic bacteria may leach out of landfills
through soil into surface or ground waters, says the National Rivers Authority.
Incineration can produce a solid ash contaminated with heavy metals. The
detoxification of sludge is therefore one area where new technologies are
urgently required.

* * *

2: Controlling contamination in the underground reservoirs

The task of keeping surface waters clean is only half the problem. Much
of the water supply in Europe comes from aquifers, which are prone to serious
contamination by chemical and metallic compounds. These pollutants reach
the underground reservoirs by percolating through soil and rock formations.
Nitrates from agricultural run-off are a particular problem, as are chlorinated
organic solvents and disinfectants from industry.

In Britain, half the drinking water comes from aquifers, and one-fifth
of the country’s total water requirement. At present, the usual way of decontaminating
aquifers is either to treat the water just before it is used or to pump
contaminated water to the surface, treat it, then return it to the aquifer.
However, both these techniques are energy intensive and they can create
their own environmental problems-for example, by damaging aquifers through
intensive pumping operations. Also, many of the contaminants remain lodged
in microscopic cavities in the walls of the aquifer, and so escape decontamination
processes.

One approach is to treat the aquifers in situ with bacteria that degrade
the toxic chemicals. Keith Halden, a postgraduate at the Department of Chemical
Engineering at the University of Cambridge, is evaluating the scope for
degrading chlorinated hydrocarbons in aquifers with a group of bacteria
called the methanotrophs. These microbes survive by digesting methane and
to do so produce an enzyme called methane mono-oxygenase that can break
down most chlorinated organics. The enzyme converts them into smaller organic
compounds that other organisms break down.

Halden’s experiments have focused on a soilborne strain called Methylosinus
trichosporium OB3B. Early studies showed that copper, which can be present
in fairly high concentrations in aquifers, inhibits the pollution-degrading
effect of this strain. However, further work showed that M. trichosporium
is only sensitive to copper in the early stages of growth, so Halden proposes
growing the bacteria above ground in copper-free solutions before injecting
the mature cells into the aquifer with some nutrients. Another problem is
that the bacteria require more oxygen than is normally available in aquifers.
Halden is evaluating means of oxygenating soils and aquifers, perhaps by
pumping in hydrogen peroxide or ozone. This may kill native organisms which
would otherwise compete with M. trichosporium for nutrients.

Halden says the rate of degradation is high, particularly in the case
of trichloroethylene, one of the most problematic and ubiquitous contaminants.
Halden cites studies showing that in 40 per cent of the boreholes near Birmingham,
levels of this solvent exceeded 30 micrograms per litre. The EC recommended
limit is 1 microgram per litre.

Another advantage of the bacteria is that they stay alive and active
for 10 to 20 days. To improve the potential of the technique, Halden is
evaluating the ability of the bacteria to cope with the prevailing conditions
of temperature and acidity in aquifers. He is also hoping to see how well
bacteria penetrate the tiny pores where contaminants collect.