|
Wastewater and Sewage Treatment
- the methodology, terminology and results
There are many ways of treating human waste so
as to render it safe to health and non-damaging
to the environment. Indeed, methods exist to recycle
it to form a valuable resource as a fertilizer
and land conditioner. And then there are numerous
ways to dispose of it that threaten human and
animal health and the eco-system in general. Here
is a brief summary of those means.
The necessity
It has long been known that contact with sewage
poses the greatest single known threat to health.
The ancient Chinese dynasties, the Egyptians under
the Pharaohs, the Saxons and the Romans all recognised
this, and took steps to prevent the faecal related
diseases which they well recognised could wipe
out entire communities. They knew nothing about
pathology, bacteria or viruses, but they were
fully aware of cause and effect, so practiced
simple but effective means of sewage disposal.
Dogs and other carnivores attempt to cover and
bury their own excreta, yet few of these creatures
have studied pathology. Such methods of safe disposal
come from inherited instinctive behaviour, because
species that followed this practice survived whilst
those who failed to do so became extinct. Every
nursing mother knows of faecal dangers. Children
are taught to thoroughly wash their hands after
visiting the toilet. You know the basic hygiene
required, I know it, and every little child knows
it, but it appears that our water companies and
the British government remain blissfully unaware,
as they continue to permit the dumping and pumping
of millions of gallons of untreated sewage into
the sea every day.
Sewage is a valuable product, which, on the proviso
that it is free from heavy metals and dangerous
industrial and chemical inputs, and has been sterilized
and properly treated so as to kill off infective
pathogens, can be used as a basis for many things
valuable to the community. Indeed, in most civilized
countries sewage is so dealt with, normally composted
and used for the land, which also reduces the
need for damaging artificial chemical fertilizers.
Here are some of the many means of treating sewage
- the good, the bad and the ugly.
Primary Treatment.
This is not actual treatment, although claimed
to be. In the past, many communities were decimated
due to washing in, bathing in and even drinking
the water into which their excreta had been disposed.
In the late 1940's a widespread epidemic of both
typhoid fever and polio hit Britain. Both are
'faecal-oral' diseases, e.g. infections spread
by ingestion or oral contact of the faeces by
hosts. Many people died as a result of swimming
in sewage-polluted waters, including Caroline,
the daughter of Tony Wakefield, who formed the
Coastal Anti-Pollution League, the very first
group to oppose the bad practice. Most of Britain's
sewage was going totally untreated to rivers and
to the sea, as it had been for many years. As
a result of the evidenced serious health threat
the government of the day set up a Royal Commission
enquiry under the Chairmanship of Sir Richard
Southwood. After studying the evidence they belatedly
reported in 1959, concluding their report with
the statement:-
"On the proviso that the bathing water is
not so polluted as to be aesthetically revolting,
there should be no danger to public health."
In other words, if you can't see it, you're safe.
But although seemingly banal and dismissive, it
was in fact partly true, as one would easily have
spotted the turds, condoms, sanitary towels and
other sewage outfall debris littering the bathing
water and the shoreline, and decide not to enter
unless one had called the Samaritans first. If
the obvious indicators of faecal pollution were
not obvious, then the chances are that it would
be reasonably safe to enter that water. Sadly
the Commission's ambiguous conclusion was seized
upon by the water authorities as an excuse to
continue to pump untreated sewage to the sea.
Anglian Water's following criteria for marine
sewage disposal stated:-
"Where effluent is discharged to the sea,
the impact on the receiving water cannot be assessed
in a manner used for works on inland waters. The
criteria for the performance is then related to
the visible effect of the discharge on the coastal
water or the foreshore. A sea outfall is judged
to be satisfactory if under normal operation it
does not cause solid matter to be deposited on
the shore …" . i.e. if you can't see
it, you're safe!
The net result was the 'Long Sea Outfall', in
which disposal the effluent is chopped up and
forced through a 6 to 8mm mesh to trap any items
that would betray the evidence of the outgoing
sewage. The soup is then pumped to the sea without
any treatment whatsoever to destroy the pathogens.
In fact the result is far worse than that of the
predecessor, as the turbidity density produced
by the sludge in the receiving water reduces light
penetration. It further demands the available
oxygen. Both of these are essential to enhance
the decay of the bacteria and viruses. The viruses
attach mainly to the sludge particles, and remain
infective for long periods.
Sir Hugh Rossi, Chairman of the House of Commons
1989-90 Environmental Committee Enquiry into the
Pollution of Beaches likened Long Sea Outfalls
to the earlier British practice of building higher
chimneys that whilst reducing the level of local
pollution exported the problem to far wider areas
to decimate the trees and lakes in northern Europe.
By the time that the danger of Long Sea Outfalls
was recognised, despite the early warning and
advice of environmental and health organisations,
water companies had wasted many millions of water-ratepayers
money by destroying sewage treatment plants and
building many Long Sea Outfalls, only to find
that what they had been warned of, more pollution
dispersed over a far wider coastline, was true.
To conform to the requirements of the Urban Waste
Water Directive and to be able to comply with
the demands of the Bathing Waters and Shellfisheries
Directives, proper treatment of all marine outfalls
is essential.
Primary Treatment
This treatment removes the sludge, the solid content
of sewage. It may take the form of either filtration
or by settling the sludge by sinking it in large
holding tanks. The bacterial reduction is normally
90%, and the viral reduction even greater, as
the viruses mainly attach to the solids. As it
is mainly the sludge that demands the oxygen and
is also responsible for the turbidity, any following
secondary treatment then becomes effective. The
sludge removed by the primary process may be digested,
sterilized, or otherwise treated, and if free
of heavy metals or other toxic content, may be
safely used as a soil conditioner and fertilizer
or other uses. When dried by the heat sterilization
process it is known as 'Basic Sludge' and widely
used as compost. Sadly, certain Water Companies
provided the primary solids completely untreated
to farmers under the heading of 'Soil Conditioner'
so risking pathogen contamination of vegetables
and stock. This practice is now banned. But still
much of the sludge is wasted by disposal to landfill
tips.
Secondary Treatment
This involves aeration of the clear effluent produced
following primary treatment solids removal. The
liquid phase may be aerated in daylight in large
storage tanks, or it may be trickled or sprayed
over banks of stones open to the air and daylight.
Both are effective. The pathogen reduction is
a further 90%, giving now 99% total reduction
in infectivity. As the biological oxygen demand
is now substantially reduced and the transparency
to natural UV light penetration much enhanced,
if now placed into the sea, a further rapid die
off of organisms will come about. The Urban Waste
Water Directive, to be implemented by December
2000, requires that no sludge shall enter the
sea, i.e. primary treatment, and that secondary
treatment must be applied as a minimum.
Tertiary Treatment
This just means 'Third Degree Treatment', but
not that sort seen in the Hollywood gangster and
police movies. It may take the form of passing
the primary and secondary treated liquid phase
through clear quartz tubes exposed to intense
ultra-violet light. With fully transparent and
well-oxygenated liquid phase effluent, this can
totally destroy all known pathogens, and so be
employed to provide safe discharges to bathing
waters and to shellfish areas. Such has been used
for all of Jersey's outfalls since 1995, where
the direct output of the sea going sewage plants
produce pathogen levels far lower than in any
British bathing water. But 'Tertiary Treatment'
may also include chemically stripping off the
phosphate and nitrate levels found in the outfall.
The phosphates emanate from detergents and from
human urine in the effluent, whilst the nitrates
come from both sewage and agricultural sources.
Such nutrient stripping treatment may be essential
in waters where eutrophication is likely to produce
dangerous levels of toxic algae, such as brought
about the shellfish ban some five years ago. The
Urban Waste Water Directive requires that tertiary
treatment may be necessary in 'environmentally
sensitive areas'. Surely the entirety of the North
Sea is such? Denmark provided full tertiary treatment
to all of its sea outfalls in 1995. Britain has
but a tiny handful to this day.
Treatment Methodologies
There are many specific ways of treating sewage
to render it safe to human health and the environment.
Here follow the main.
Ultra-Violet Treatment
As stated under Tertiary Treatment, in UV treatment
the clear effluent resulting following primary
and secondary treatment is passed through transparent
tubes and irradiated with powerful UV light. If
fully efficient, the remaining viruses and bacteria
can be killed to give an output that could virtually
be drunk, even the undiluted output straight from
the effluent pipe being far less infective than
the sea of most of the British coastline. Such
treatment, if correctly run, gives bathing beaches
that easily meet the EC Guideline standard. UV
is already in use in many countries, but despite
promises to adopt for the Welsh and South Western
UK areas, few have yet to come about in practice.
This method is quite one of the very best methods
possible, and is much to be recommended. Unfortunately
is does not sterilize the sludge, as the UV is
unable to penetrate the turbidity.
Natural Treatments
In east and southeast Europe, and indeed in most
non-western countries, sewage is regarded as a
resource rather than a problem. It is composted
with animal waste, straw, seaweed, or any natural
organic material to provide a totally safe valuable
moisture retaining agricultural product. Pathogen
destruction is brought about by the heating provided
in the composting cycle. On the proviso that no
toxic chemical or industrial contents are present,
the product makes a remarkably good fertilizer.
Wye College in Kent have a rapid method of composting
the primary recovered sludge with waste straw
and seaweed recovered from the beaches to provide
a superb potting compost used by many plant providers.
It is an excellent alternative to peat.
For small non-rural communities with the required
land space available, the sewage can be trickled
through reed-beds, which take out most of the
organics and nutrients whilst the oxygen in the
air and the ultra-violet light from the sun kills
the pathogens. Although excellent, such is not
really viable for large urban communities with
considerable loadings and high land costs.
Individual 'dry' toilets work well as pollution
preventers, as indeed do the cesspools still used
in many country areas. It could be said that the
flush toilet, thought once claimed to be the very
height of hygiene, has in fact been the basic
cause of most of the faecal pollution and eutrophication
of our rivers and seas that we have today.
Filtration
The 'Memcor' micro filtration process, whilst
not addressing the primary sludge problem, is
an excellent process for purifying the liquid
effluent before disposal to river, estuary or
sea. The liquid phase is passed through a very
fine electronically created membrane with pores
so fine that they filter out all the sludge, bacteria
and viruses. Backpressure cycles are incorporated
to prevent clogging. Similar systems are used
on the MIR Space station, where all the wastewater
goes in one end to provide fresh water at the
output. Such a system provides excellent recycling
and complete safety. It is much to be recommended.
Oxy-ozo-synthesis
Here the sewage is acidified and injected with
ozone created by passing oxygen through an electric
arc. It not only sterilizes the sludge, but bleaches
it too, permitting its re-use as insulation material,
for newsprint, as packaging or as fuel, which
sells at £50 per ton. The supernatant liquid
too can be ozonised in the outgoing pipeline to
render it free of all pathogens prior to discharge
to water. An excellent process although possibly
more expensive than some. It was successfully
used for the treatment of New York's and New Jersey's
sewage until the Union bosses objected to the
loss of employment of those who took the sludge
out in barges to dump in the Atlantic Ocean!
Chlorination
Chlorine treatment, as with ICI's 'Coastguard'
system, whilst effectively destroying the pathogens,
is not to be recommended. The large number of
organochlorines and organic bromines formed can
be highly toxic to the marine environment as they
are both mutagenic and carcinogenic. Another method
involving chlorination is by the addition of sodium
hypochlorite into the sewage. The same environmental
hazard results as with direct chlorination.
Electrolysis
This is simply passing a high electrical current
through the effluent, producing by electrolysis
hydrogen, oxygen and heat. This method certainly
sterilizes very well, but partly so because of
the chlorine produced from saline water. It is
not to be advised.
Peracetic Acid
As used by the 'Oxymaster' system, this process
relies on the addition of acetic acid and hydrogen
peroxide to the sewage. Whilst this effectively
kills off the coliform indicator bacteria, it
may be seen as merely cosmetic, as it is far less
effective against other bacterial pathogens and
viruses. Thus whilst bathing waters may be brought
to meet the EC Directive standards by its use,
the most infective pathogens may remain infective.
It can also oxidise the urea in urine to give
rise to free chlorine.
Other Methods of Treatment
Yes, there is many more, and no doubt many more
still will be many more evolving in the future.
But this treatise is not intended to be exhaustive
nor even comprehensive. It is merely to show that
human effluent can be treated to render it not
only benign but also actually beneficial to mankind
and the environment. Whatever treatment is selected,
it just has to be superior to dumping such a hazardous
material into the marine or estuarine environment
to give health problems to people, marine mammals
and all sea creatures upon which we depend.
|
