AMONG the responsibilities of the water authorities, soon to pass into
private ownership, is the treatment and disposal of sewage effluents from
our cities. A large proportion of sewage effluent is dumped without treatment
in the North Sea either by ship or by outfall pipes, much of it along the
coasts of the Thames Estuary. But as concern grows over the pollution of
our coastal waters, there is increasing pressure to dispose of more of our
sewage waste on land.
This appears to be a sensible idea. What better way of dealing with
our sewage waste than to recycle it as an organic fertiliser in agriculture?
Unfortunately, sewage effluent often contains waste from industry which
contaminates the sludges with pollutants, particularly heavy metals that
are toxic if present in large concentrations.
The danger is serious only where domestic and industrial wastes are
mixed; sludges from sewage treatment plants in rural areas can usually be
used without concern. But most wastes come from large industrial conurbations,
and often contain significant contamination.
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Some 70 per cent of the 1 million dry tonnes of sewage sludge produced
each year in Britain is disposed of on land. Of this, 60 per cent is used
on farmland, the remainder going to landfill sites or land reclamation.
In 1977 the Department of Environment (DoE) published recommendations
aimed at controlling the amounts of potentially toxic metals added to farmland.
But the situation changed after a directive drawn up for the members of
the European Community passed into British law on 1 September 1989.
The DoE鈥檚 older limits were calculated in two ways: as the maximum amount
of metals allowed to be added in sludges over a 30-year period; and as the
concentrations of metals allowed to accumulate in soil. The new European
guidelines set limits on concentrations allowed in sludges to be used on
land, and annual rates for metals added to soil in sludge. They also laid
down limits on the concentrations of metals permissible in agricultural
soils (see Table 1).
The European guidelines are generally stricter, but the evidence on
which all of these 鈥榮afe鈥� guidelines are based is scientifically questionable;
in particular the Community accepted that it needed further information
before it could set a guideline for additions of chromium to soil.
There is little sense in regulating the amounts of metals that can be
added to soil over 30 years, as once there many metals will remain for thousands
of years. The limited solubility of heavy metals in soil could be interpreted
as a good sign for agriculture, since little of the metals will be taken
up by crops grown on lightly contaminated soils.
Of the potentially toxic elements in sewage sludge, zinc and cadmium
are most readily absorbed. Zinc is generally present in greater amounts
than other metals (see Table 2) and is toxic to plants at concentrations
that are too low to cause toxicity in animals. Zinc contamination is, therefore,
a problem for crop production; and since plants die if an excess of zinc
is absorbed they act as 鈥榖arriers鈥� preventing too much zinc from being consumed
by animals.
By contrast, concentrations of cadmium in plants can become dangerously
high for their use as food, without any obvious effects on growth. Also,
cadmium accumulates in the kidneys of mammals over long periods where it
can eventually disrupt them. Most concern over the build-up of heavy metals
in food chains is focused on cadmium.
Guidelines for environmental protection are based on evidence which
scientists use to infer upper concentrations of metals in soil that are
not directly toxic to most plants and are unlikely to be toxic to animals
or people. But there is growing evidence of damaging effects on microbial
processes in the soil at metal concentrations close to these guidelines,
which may cause serious problems for agricultural production in the future.
It was research done at the Woburn Experimental Farm which raised the
alarm on the toxic effects of heavy metals on microorganisms in soil. Thanks
to a long-term field experiment begun there in 1942, Woburn has both uncontaminated
plots of soil and plots contaminated with metals at concentrations close
to the limits laid down by the current guidelines.
Some leafy vegetables grew less well on metal-contaminated plots than
on uncontaminated plots, but no visible effects on growth were seen in other
crops. But when white clover was sown in 1984, there were alarming differences
in yield between the high- and low-metal plots. Yield was reduced by more
than 40 per cent where heavy metals had been added, and the plants were
yellow and stunted.
These effects were reproduced in glasshouse experiments where clover
was sown in soil taken from the field, but feeding the plants with nitrogen
eliminated the difference in growth. This suggested that the metals were
not poisoning the plants directly. So we looked next at the root systems.
Clover plants grown on uncontaminated soil had root systems with pink
nodules containing the nitrogen-fixing bacteria Rhizobium. Root systems
of the stunted plants grown on metal-contaminated soil were covered in small
white root nodules which could not fix atmospheric nitrogen. So the metals
were affecting the growth of plants by depriving them of nitrogen.
As reported in Soil Biology and Biochemistry (vol 21, p 841), large
concentrations of metals are directly toxic to Rhizobium and the small white
nodules are formed by the only strain of Rhizobium which can survive in
metal-contaminated soil. As this strain of Rhizobium can tolerate larger
concentrations of metals than most others, it may be possible to engineer
strains which can tolerate metals and fix nitrogen.
In fact, nodules can be made to fix nitrogen even in metal-rich soil
by inoculating the soil with a normal strain of Rhizobium. Once the Rhizobium
bacteria are within the tissue of the root nodule, they are protected from
the metals in the soil. But if no clover plants are grown on the soil, even
for just a few weeks, the effective rhizobia will be killed.
Other research has shown that in the soil with a high concentration
of metals, the biomass of free-living microbes is halved, and blue-green
algae (cyanobacteria) are effectively eliminated.
What are the questions raised by these findings? The first need is to
pinpoint which of the cocktail of metals in the original London sewage sludge
used at Woburn is responsible for the toxic effects. Cadmium, zinc, copper
and nickel are probably the most toxic of the elements normally found in
large amounts in sewage sludge, as they are more soluble in the soil.
It is particularly alarming that these effects occur at concentrations
of metals that are close to the current guideline limits of the European
Community. As yet no one can say what are 鈥榮afe鈥� concentrations of metals
in the soil for the microbes.
Clearly the accumulation of cadmium in food chains is serious, as it
is toxic and readily absorbed from the soil by plants. The direct effects
of other metals toxic to animals, such as lead, are less of a worry, although
animals may absorb more by eating soil while grazing.
The underlying problem is that we are storing up trouble for generations
to come, since the metals will remain in the soil indefinitely. As agriculture
moves towards systems based less on fertilisers and other agrochemicals,
we are likely to become increasingly dependent on soil microbes to fix atmospheric
nitrogen and to maintain efficient cycling of nutrients from organic manures.
The obvious way to avoid these problems of pollution is to prevent contamination
of sewage sludges at source, by imposing more stringent controls on industry
to stop discharges of metals into the sewage system.
But this will not be cheap. How effective the new private water authorities
will be in minimising contamination of sewage sludges with heavy metals
remains to be seen.
* * *
Treatment puts the squeeze on sludge
HOW sewage is treated varies considerably between sewage works. The
main aim is to remove the solids and nutrients from the liquid, which can
then be safely released into watercourses. The remaining sludges are available
for use by farmers in either liquid form, which contains roughly 5 per cent
solids, or as 鈥榗ompressed cake鈥�, which contains about 25 per cent solid
material. The simplest form of sewage treatment is by sedimentation of the
solids in settlement tanks.
Heavy metals in sewage become bound to the organic matter which settles
out in sedimentation tanks. Although the initial concentration of metals
may be very small, the removal of water concentrates them in the sludge.
Sewage sludges are useful as fertilisers since they contain considerable
quantities of nitrogen and phosphorus, and only small amounts of potassium.
Liquid sludges are more useful as they retain the soluble nitrogen that
plants can use directly. This is lost from sludge when it is compressed
into cake. Cake sludge has obvious advantages, in that it costs less to
transport.
Liquid sludges are applied by spraying or by injection into the soil
using special machines. Where sludge is sprayed onto grass, no grazing is
allowed for up to six months, depending on the type of sludge; but animals
can be returned to graze only three weeks after sludge has been injected
into the soil. Cake sludge is usually applied by muck spreader, and may
then be incorporated into the surface layers of soil.
The main sources of some heavy metals which contaminate sewage sludges
are known. For instance, large concentrations of chromium are released by
leather tanneries, and cadmium and nickel come from factories producing
long-life batteries. Other metals are found in effluents from industrial
processes, including zinc and copper plating. There are chemical treatments
for recovering metals from effluents before they leave the factory. However,
the costs of such systems rise steeply as they become more efficient.
鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
1: Upper permissible values for concentrations of heavy metals in agricultural
soils with pH of 6 to 7 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
Element EEC Britian DoE Germany (1986) (1989)
(1977) (1982) (mg/kg dry matter) 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
Cd 1-3 3 3-5 3 Cu 50-140
135 N/A 100 Ni 30-75 75
N/A 50 Pb 50-300 300 50 100 Zn
150-300 300 N/A 300 Hg 1-1.5 1
1 2 Cr 鈥� 鈥� 600 100 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
DoE (1977) specified only EDTA-extractable Cu, Ni and Zn. 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
(Sources: Commission of the European Community; UK Statutory Instrument
No 1263; UK Department of the Environment; FRG Klarschlammverordnung.) 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
2: Average concentrations of metals in British sewage sludges (DoE, 1981)
and the EEC limit values for concentrations in sludge to be used in agriculture
鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�- Metal
Average concentration in EEC limit values British sludges (mg/kg
dry solids) 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
Zinc 1820 2500-4000 Copper
613 1000-1750 Nickel 188
300-400 Cadmium 29 20-40 Lead
550 750-1200 Chromium 744
鈥� 鈥斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌斺赌�-
Ken Giller is a lecturer in Soil Microbiology at Wye College, the Agriculture
School of the University of London. Steve McGrath is a principal scientific
officer at Rothamsted Experimental Station, Harpenden, part of the AFRC
Institute of Arable Crops Research.