IT’S summer. You’re by the sea or at the lake. Take a moment to appreciate
the clear, sparkling waters. They are probably the cleanest they have been this
century. Sewage outfalls that used to blight coastlines and sicken swimmers are
disappearing in the industrialised world as we demand more and more purification
before waste water is released.
But we are not producing any less of the prime ingredient in sewage. So where
is all the shit going, if not into water? See that sandwich in your hand? That’s
right. In Europe and North America, between one and two-thirds of
sludge—the biosolids from sewers—is now spread on farmland.
In principle this is fine. For centuries, human manure has been as highly
valued by farmers as any other. Modern Western sensibilities are more delicate,
but there’s no avoiding the fact that if we don’t want faeces in our rivers and
coastal waters we must find somewhere else to put them.
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And as sewage outflows get cleaner, there is more sludge to dispose of. By
2005, the amount produced in Europe will have increased by 50 per cent as
European Union water directives take effect. Agricultural use will have to grow
as other disposal routes close. In Britain, 55 per cent of a million tonnes of
sludge produced this year will be spread on farmland. By 2002, 61 per cent of
1.5 million tonnes will end up there.
So is it safe? Human faeces contain microbes that cause diseases in humans,
especially gut diseases. If these pathogens are getting into our food then it’s
not just the thought of excrement on food crops that could turn your stomach. In
the US, France, Germany and elsewhere, all sludge is treated before being used
as fertiliser. It undergoes various forms of heating, drying, fermentation or
other treatments that kill germs—to varying degrees—and make sludge
more chemically stable. Britain alone allows farmers to put raw sewage on their
land, though pressure from food retailers is likely to put a stop to this by
2002. But the question remains as to how best to treat it.
About a quarter of all sewage sludge spread on British farmland this year was
raw—little more than settled sewage with the plastic and detritus filtered
out. The water companies that produce it say this is safe, as long as users
follow voluntary guidelines based on European Union requirements. These prohibit
the spreading of sludge on fields where fruit or vegetables are already growing,
and say farmers must wait three weeks before harvesting fodder or letting
animals graze on sludged land, and ten months before harvesting crops that touch
the soil and are “normally eaten raw”—strawberries, for example. The EU
rules are the same for raw and treated sludge, except raw sludge must be
ploughed or injected into soil. Earlier this year, the British government told
an inquiry by the House of Commons Select Committee on the Environment that this
is safe.
The food industry disagrees. “Using raw sewage sludge to fertilise food crops
is simply not acceptable any more,” says Alec Kyriakides, chief microbiologist
at the British supermarket chain Sainsbury’s. For example, he says there is
little proof that Escherichia coli O157, a bacterial strain that can
cause fatal food poisoning, dies in soil during the waiting periods laid down in
the guidelines. It is known to survive in animal manure in fields for months
(This Week, 14 June 1997, p 12).
The same Commons inquiry heard that dumping
untreated human sewage and abattoir waste on farmland may put people at risk
from E. coli O157 infection. And there is evidence that ploughing raw
sewage into soil may actually help pathogens to survive by keeping them in dark,
damp conditions.
If the food industry won’t buy crops fertilised with raw sludge, farmers
won’t use sludge. And many water companies are counting on farmers as other
disposal routes start to close down. Sea dumping has been illegal in North
America since 1993 and will become illegal in the EU at the end of this year.
Landfill sites are filling up, and rotting sludge emits methane, a potent
greenhouse gas. It can be burned, but burning sludge can release air pollutants,
and because it takes energy to dry the sludge before it can be burnt, there is
no net energy generation. Incineration costs three to six times the current cost
of spreading sludge. So the water companies have just reached an agreement with
a consortium of British food companies, promising to give farmers only treated
sludge by 2002.
Safe sludge
How exactly it will be treated remains to be decided. “We have nothing
against recycling sewage sludge, as long as the companies can show it is safe,”
says Kyriakides. “So far, they haven’t.” To that end, the water companies have
also agreed to fund new research. To satisfy the food industry, this will have
to determine which treatments reduce the pathogens in sludge to safe
levels— although “safe” might be hard to define, as the water companies
now say even untreated sludge is safe.
A report on sewage by Britain’s Royal Commission on Environmental Pollution
in 1996, and the Commons inquiry this year, both decided that safe means killing
as many germs as possible. They called for all sludge used in agriculture to be
pasteurised, which involves heating at 70 °C for an hour. The Water Services
Association, the umbrella group for British water companies, rejects this,
saying it is expensive and unnecessary.
The US requires pasteurisation for top-grade sludge—including what the
National Parks Service puts on the White House lawn. “The Clintons are walking
around on poo,” says Alan Rubin, who is in charge of sludge at the US
Environmental Protection Agency (EPA). “But it’s very clean poo.” Germany
effectively pasteurises all sludge used in agriculture. But the US, like France
and other European countries, also allows sludge to be spread after less
rigorous treatments.
The most common technique—and the one that most British water companies
want to use—is called mesophilic anaerobic digestion. This means letting
the sludge stew in a closed container at between 20 °C and 40 °C for
four or five weeks, while bacteria break down reactive chemicals and produce
heat and methane. The methane is used to generate power at the sewage works, and
the digested sludge is chemically stable. Typically, says Rubin, this reduces
bacterial levels a thousand-fold, but still leaves plenty alive. The Royal
Commission concluded that the process is “not particularly effective” at
destroying viruses or the eggs of intestinal worms.
Italian researchers found that sludge treated by mesophilic anaerobic
digestion contained eggs from the intestinal worm Ascaris,
Salmonella bacteria and enteroviruses. A similar study in Western Australia
turned up Streptococcus bacteria, Salmonella and infective
oocysts of the parasite Giardia. All but the Giardia actually
multiplied in sludge stored for a year, although storage is used by some British
water companies to further treat digested sludge.
Most of these pathogens can be removed using a third treatment known as
thermophilic digestion. The sludge is left to stew for several days at around 60
°C, either anaerobically or aerobically. In the latter, the fermenting
sludge produces its own heat, and this technique is increasingly common in
continental Europe. But British water companies still use mesophilic digestion
almost exclusively, despite a recommendation made five years ago by their own
research lab, the Water Research Centre. It said that any sludge containing
oocysts of the parasite Cryptosporidium should be treated by
pasteurisation or thermophilic aerobic digestion before being spread on land.
Cryptosporidium is a protozoan that causes pain, fever and diarrhoea,
can be fatal, and is carried by up to 4.5 per cent of people in industrialised
countries. Hundreds fell ill with it in Britain this year. Animals are usually
blamed, but humans excrete it as well.
In May last year, Anglian Water, a privatised water company in southeast
England which spreads 40 per cent of its sludge raw, told customers to boil
drinking water after finding Cryptosporidium oocysts. If these remain
in treated water, there will be plenty in sludge. Anglian spokesman Graham
Frankland says that by 2000, the company will treat all its sludge with
mesophilic anaerobic digestion, plus quicklime (calcium oxide). This reacts with
water producing heat and raising the pH of sludge. The Royal Commission
said this “significantly reduces” bacteria, but “seems less effective” against
viruses.
Frankland says that no process short of oven-drying and turning sludge into
pellets kills all germs. These processes also destroy the soil-improving
qualities of sludge, says Frederic Cartegny of SEDE, a French company that
manages sludge application for farmers. Anglian plans to invest ÂŁ150
million in sludge treatment over the next two years, says Frankland, but “only
because of public perception,” he adds. “Untreated sludge is safe.” The proof,
says the Water Services Association is that “there has never been a single case
of human disease linked to the use of sewage sludge”.
The US National Research Council agrees. In 1996 it announced that “there
have been no reported outbreaks of infectious disease associated with. . .
adequately treated . . . sludge applied to agricultural land”. And that includes
sludge treated using mesophilic anaerobic digestion. The Environmental
Protection Agency also finds no difference between the health of families on
farms that use sludge, and farms that don’t. But as Ellen Harrison at Cornell
University in Ithaca, New York, points out, pathogens applied to land in sludge
are most likely to reach humans via groundwater, possibly far from the sludged
field. And the National Research Council admits it would be hard to see any
cases of common intestinal diseases caused by sludge, such as gastroenteritis,
against the high background incidence of infections spread by other routes.
Contaminated crops
Even uncommon diseases caused by sludge would be hard to trace. But we do
know that people have been infected with cholera and typhoid by crops watered
with sewage—where germs are less concentrated than in sludge. Last year in
the US, imported crops that had been irrigated with sewage, including salad and
berries, were implicated in gut upsets. And in 1985, the WHO revealed a link
between sewage irrigation and the intestinal worm Ascaris, which causes
pain and diarrhoea. In west Jerusalem, shifting boundaries and farming practices
led to the levels of Ascaris infection increasing when local vegetable
crops were watered with sewage, then decreasing when the sewage was turned
off.
James Smith of the EPA says Giardia, hookworm, Cryptosporidium,
and germs causing typhoid, cholera, hepatitis A, polio and
amoebic dysentery have all been found in sludge in the US. In Britain, there has
been a tendency to assume that serious gut pathogens are uncommon outside the
tropics, says Mick Brown of the pressure group Water Watch. “Water companies
have said they only need to treat sludge from an area with what they call a
cosmopolitan population,” he says. “They think rural English communities don’t
produce germs worth worrying about.”
But Smith points out that hospital
toilets flush into ordinary sewers. Healthy travellers can carry cholera and
typhoid, and Cryptosporidium and E. coli O157 are solidly
British. Earlier this year, people from two villages in Scotland claimed that
sludge used to reclaim nearby mining land not only smelled “indescribable”, but
had also caused a rash of local illnesses, including scarlet fever and
meningitis. Their claims remain just that—it is difficult to prove any
association.
The water companies insist that even if pathogens persist in sludge, they die
during the 10 months before crops can be harvested from sludged fields. The US,
more cautiously, won’t let the public walk on sludged land for a year. Smith
says the eggs of some parasitic worms can persist in soil for seven
years—and just one causes infection. Bacteria can persist for a year, he
says, viruses for six months.
But those may be underestimates. John de Louvois of the Public Health
Laboratory Service told the Commons inquiry that most survival studies were
carried out before microbiologists could reliably detect pathogens in soil. The
few studies performed with sensitive, recent techniques show that some pathogens
last longer than had been thought. Enteroviruses, he says, can survive “many
hundreds of days” in soil. “All the major bacterial pathogens associated with
sewage,” he says, are now known to lurk in soil at levels below the detection
capabilities of the studies used to establish current safety
guidelines—but they can still multiply and cause infection.
David Kay, professor of environmental science at the University of Leeds,
says the scientific literature contains “almost no good information to tell us
how rapidly viruses die” in sludged soil, and little data even for well-studied
coliform and streptococcus bacteria. Caution, and two British inquiries, suggest
we should pasteurise the muck. Over the next three years, the water companies
will try hard to convince us with their research that sewage—even if laden
with germs—is safe on fields.
SEWAGE is a valuable resource, according to Gary Gardner of the Worldwatch
Institute, a think-tank in Washington DC. OECD citizens excrete enough to
replace 8 per cent of their current consumption of chemical
fertiliser—which is made using fossil fuel. On the other hand, sludge
produces no net atmospheric CO2. “We have problems with sewage mostly
because it is treated as a waste, not a resource,” he says. Only by keeping
faeces separate from the rest of our waste can we realise its full
potential.
Excreta are usually flushed down the same pipe as industrial and household
wastes, so chemical contaminants end up in the sludge spread on fields. Heavy
metals are the main hazard, particularly in the US where the permissible levels
are mostly higher than those in the European Union.
The EU sets a range of limits on heavy metals in sludge. The lower ones are
based on the rate at which metals are cleared from soil by leaching and plant
uptake, and thus should prevent metals accumulating in soil. Even at the top EU
limits, it should take 78 years before any metal reaches its legal maximum in
farmland. But in the US, scientists at the Environmental Protection Agency took
a different approach. It decided how much of each metal was safe in food, and
how much could be applied to land before that level was reached in crop or
livestock. This policy is wide open to potential miscalculation.
David Bouldin of Cornell University in Ithaca, New York, says the EPA
underestimated how much metals plants take up. Others say it underestimated how
much earth is ingested by grazing animals. The Cornell team calculates that some
EPA limits will allow metals to accumulate in soil over a few years to levels
that require remediation under other US legislation.
But it might be impossible to put lower limits on US sludge. “In the 1970s,
we encouraged industries to hook up to the sewer system instead of dumping
effluent,” says Alan Rubin of the EPA. Europe has tried to keep industrial waste
out of the sewers. The main source of zinc in British sewage, for example, is
household goods such as dandruff shampoo.
Gardner admits that separating domestic and industrial wastewater from
excrement streams will be difficult for industrialised countries that already
have mixed sewage systems. “Developing countries that haven’t invested in sewers
yet could avoid the mistake of combining human and industrial wastes,” he says.
Only a tenth of households in the developing world are on sewer lines.
Gardner suggests that they could make best use of a valuable commodity by
installing systems such as one developed in Mexico, called by its Spanish
acronym SIRDO. It collects the wastes of up to a thousand people in tanks, and
lets them ferment at around 70 °C into harmless fertiliser. SIRDO tanks are
managed by apartment caretakers in Mexico City. In villages where tanks are used
to keep sewage out of wastewater, intestinal diseases have been cut
twenty-five-fold.
Even in industrialised countries, abandoning the sewer for high-tech versions
of the anaerobic digesters common in Chinese farmyards could be economically
interesting, says Gardner. Large apartment blocks, he suggests, could install
sewage tanks in the basement to ferment excreta, use the methane for fuel, and
sell the residue as clean fertiliser.