Fig. 5 Squat by a wall. The smoke from the lime kiln, and from his pipe, clearly shows the direction of the wind, and the obvious reason why she is holding her nose. From Bewick (1790).
CLEANLINESS IS NEXT TO FASTIDIOUSNES – THE HUMAN OBSESSION WITH SEWAGE
I HAVE A personal history in sewage. When my botanist father, from whom I acquired my interest in natural history, bought his first house in south London in 1957, he looked around his local patch for parks and open green spaces to visit. The closest, and most interesting by far, was the South Norwood Sewage Works. Although not generally open to the public, he obviously got permission from Croydon Council, and in 1961 he published a paper in the London Naturalist on the 171 plant species he had found there (Jones, 1961). For the benefit of those readers unacquainted with the mechanics of sewage treatment he started his botanical article with a description of the site and (aided by the works’ chief chemist) how the various sedimentation and digestion tanks, biological sprinklers, sediment drying beds, sludge-spreading meadows and filtration fields worked.
When he died in 2014 I found half a dozen old books on sewage treatment, water purification and sanitary engineering still on his shelves, as seemingly out of place amongst the serried volumes on insects, snails, county floras, Darwin and rural economy as his collection of Biggles stories and the Harry Potter series. They were all standard sewerage textbooks from the first half of the 20th century, including the one written by a leading chemist of the day Samuel Rideal (1883–1929) in 1900, which my father listed in the references at the end of his South Norwood paper. In the book was a postcard from the editor of the London Naturalist commending him on the spate of articles my Dad had recently sent, and bemoaning the fact that his own recent marriage was taking up too much time to find space to write anything.
By the time my Dad was traipsing through the dense green stands of goose-foot, fat-hen, stinging nettles and tomato plants that dominated the South Norwood sludge meadows, sewage had been treated on the site for 90 years, the scientific culmination of a century of major change in the management of human effluent.
That it needed to be managed, anyone who has ever had a blocked drain will understand instinctively. It is no surprise that human excrement smells, and by varying degrees it is at best unpleasant, at worst downright disgusting. Our visceral disgust (making one’s gorge rise, a personal favourite expression) at our own biological output is nothing to do with modern hygienic sensibilities, or Victorian prudery even, it is a far more ancient trait – one that has evolved to protect us from disease.
In the far deeps of prehistoric human ancestry, there was no hygiene code to keep cooked and raw meats separate, no understanding of the processes by which provisions go off, no concept of hand-washing to prevent germs being spread from faeces to food, but the look and smell of potentially harmful bacterial breeding grounds left its mark in our revulsion. Humans are naturally repelled by the smell of faeces (our own at least), just as we are by rancid food, and the evolutionary explanation is that this core disgust (which can be visualised in the human brain by MRI scans) prevents the ingestion of noxious materials. Bluntly, any proto-humans who lacked this dung-associated disgust would soon succumb to food-poisoning, cholera, typhoid, polio or a bucketload of other unpleasant and frequently fatal diseases which would, in turn, prevent them passing on their naïve and incautious behaviour to any offspring. Death was ever the prime mover in natural selection. On the other hand, our antediluvian ancestors who avoided their own excrement, because of some instinctive repulsion, survived, flourished and passed on this successful aversion behaviour to their offspring, and eventually down to us.
Similar, but more subtle disgust can also be elicited by seeing sick people, vomit, corpses, bodily sores and wounds, pus, blood and gore, or creepy-crawlies – all potential health dangers for the unwary (Curtis et al. 2004). Disgust is a genuine physiological response in the human body, characterized by lowered blood pressure, increased sweating, nausea (sometimes gagging), stopping movement, involuntary shuddering, hair-raising and goose-bumps, exclamations of ‘yeuch!’ and grimacing. There is no doubt that there is an underlying innate biological disgust response in humans, but this is honed and developed by learning. This is just as well, because it’s no good having a disgust that cannot be overridden (unlearned); at least metaphorically, managing sewage is a hands-on activity.
The smell of faeces needs no description, we are all intimately familiar with our own and perhaps our children’s. The smell, though, is not from minute airborne particles of the dung itself, but comes from the gases and other volatile substances produced during digestion. As mentioned in chapter 1, the two most obvious gases are methane and hydrogen sulphide. Methane (CH4) is the most basic hydrocarbon, and the principal component of the natural gas that comes through the cooker hob. Hydrogen sulphide (H2S) is the smell of rotten eggs, and the usual offending substance produced in schoolboy prank stink-bombs. It is strong smelling and can be stomach turning, perhaps an echo from a time when eating raw eggs could be unhealthy to the foraging Australopithecus, if the hatch-by date of potential food items had been passed.
Both of these simple breakdown molecules are produced by the bacterial digestion of food, and although not part of normal human (or other vertebrate) metabolism, bacterial ancestry is such that their microbial progenitors were all busily multiplying and evolving when these two substances were (along with ammonia) major constituents of the Earth’s atmosphere a billion years ago and more. The gases are generously spiced with nasally challenging volatiles such as skatole and indole. These polycyclic (6-carbon and 5-carbon rings fused) organic compounds are also produced by bacteria, and from the breakdown of the amino acid tryptophan (one of the 22 essential amino acids that humans need in their diet to build proteins), and they stink. Other equally offensive strong-smelling scents (usually reminiscent of rancid meat) are produced by methanethiol (CH3SH), and various methylsulphides – (CH3)2S, (CH3)2S3 and (CH3)2S2 – from the breakdown of cysteine, a common sulphur-containing amino acid also needed to build proteins. These volatile substances are immediate danger signals produced by carrion.
Humans are no longer so reliant as other animals on their sense of smell, but we will turn our nose up at anything giving off these compounds. What started off as an instinctive evolved avoidance eventually became a much-debated philosophical enquiry into the nature of disease. The germ theory of disease transmission by which invisible microscopic organisms breed and multiply inside a human victim, then get transferred around in excrement, bodily fluids or sneezed droplets, only arrived in human understanding 150 years ago. But for several millennia before this, a vague association between disease and bad smells has bubbled through the history of medicine. Miasmas, lifting off the fetid lands of swamps, sewers, mudflats or any place heavily hung with the scents of decay, were blamed for all manner of epidemic and plague. Malaria, though transmitted by mosquitoes, was closely associated with the marshy swamps around cities, where the mosquito larvae fed in the sewage-rich effluent of the slow-moving waters – the very name comes from the Italian mala-aria ‘bad airs’.
Whatever the misunderstandings about disease, through all of recorded history the disposal of ordure, and distancing ourselves from its smell, has occupied our thoughts and, to some extent, our literature. In a sparsely populated Iron Age world, the easiest thing would be to take an iron spade, dig a hole and bury the waste. So Moses bade the Israelites:
Thou shalt have a place also without the camp, whither thou shalt go abroad; And thou shalt have a paddle upon thy weapon; and it shall be, when thou wilt ease thyself abroad, thou shalt dig therewith and shalt turn back and cover that which cometh from thee. (Deuteronomy 24:13,14)
For the next three and a half millennia this personal self-interring hygiene, followed by the latrine and the earth closet, would be standard practice. When my intrepid traveller friend Mark De Pienne set off on a Namibian safari a few years ago he was similarly instructed to take himself out of the camp with his folding shovel, to bury his deposit when the urge arose one evening. And I’m sure he would have achieved it had not his efforts been thwarted by the appearance of a large menacing snake at an inopportune moment. As he hightailed it (perhaps lowtailed it, I’m not sure) back to the tents, leaving an unfortunate trail behind him, he had other things on his mind than worrying whether his actions would offend the elders, or bring damnation, disease or large predators down upon the trip. When he later recounted his antics Mark never did tell me whether he went back to finish the burial task. But if he didn’t, it wouldn’t have mattered a great deal, one lost stool in the vastness of Africa.
But away from nomad Africa the fertile crescent of the Near East produced agriculture, farming, settled civilisation, villages, towns and eventually cities. It was the concentration of the masses of squirming humanity into huddled proximity, and the burgeoning output of bodily waste, which produced an incentive for the first major advance in sewage processing; it was all about flushing it away, and it has stayed with us ever since.
Fig. 5 Squat by a wall. The smoke from the lime kiln, and from his pipe, clearly shows the direction of the wind, and the obvious reason why she is holding her nose. From Bewick (1790).
The fifth of Hercules’s twelve labours was to clear out in a single day a 30-year accumulation of manure produced by 1,000 divinely healthy and immortal cattle in the stables of King Augeas. No latrine digging here: Hercules simply diverted the course of the rivers Alpheus and Peneus to wash away the filth. You can almost hear Peisander, the ultimate compiler of the epic poem written about 600 BC, smirking, as he uses the plot device of the flushing water system, just as today’s novel writers might invoke a bit of cutting-edge modern technology to spice up a story. It’s difficult to know exactly when sewage piping, guttering or drainage channels were first used, but there are archaeological remains from at least 5,000 years ago.
Amongst the oldest claimed toilets in the world are the niches in the Orkney settlement of Skara Brae, inhabited around 3,000 BC. What may be primitive drains were found under the large cells set into the monumentally thick stone walls of the village houses. In the Indus Valley, brick toilets with wooden seats and a chute taking ordure into street drains or cesspits were available for the removal of the effluent of the affluent citizens around 2,600 BC. And by the 18th century BC water-flushed sewage systems attached to toilets are known from Minoan Crete, Pharaonic Egypt and elsewhere in the Middle East.
By the time of the Romans, toilet technology was well established, and the control of flowing water into and out of cities by aqueducts, canals and sluices was common throughout the civilised world. Bath-houses with semi-communal sitting toilets are known from all over the empire, and the Cloaca Maxima (‘great sewer’) through Rome was a marvel of ancient engineering. It may have started as a series of drainage ditches and an ancient Etruscan canal, but as the city developed from around 600 BC it was soon covered over to create a large arched tunnel under the Forum. It still drains rainwater and debris into the River Tiber in the centre of Rome today. Incidentally, the word ‘sewer’ comes, via medieval Latin seware, from the Roman exaquare (ex plus aqua = water), meaning a place where water is drained off. Cloaca (still used in the anatomical drainage sense in birds and reptiles) comes via the similar-sounding and possibly similarly derived cluere, Latin for cleanse.
The disposal of human sewage into drainage ditches and ultimately into rivers and other waterways still goes on, and on the face of it this is a sensible, necessary and environmentally acceptable management process – so long as it does not create pollution. This fine line, between civil engineers’ need to get rid of the stuff, and the local residents’ desire for clean water, has shifted many times over the millennia.
The Great Stink of July and August 1858 is usually quoted as a revolutionary event in the story of Victorian sewage treatment. The hot weather that summer exacerbated the smell of human faeces and industrial effluent baking and fermenting on the mudbanks of the Thames in London; it interrupted the workings of Parliament at Westminster and raised the spectre of miasmatic diseases striking down the populace of the capital. But this was a scenario that must have been acted out endlessly since the first latrine drained into the stream from which drinking water was drawn.
It’s all to do with diluting volumes of water. Modern raw sewage is 99.9% clean water, but this still means vast quantities of obnoxious solids and ammoniacal liquids in the flow. The Thames, though a relatively large river, had a slow flow of water down the shallow geological incline of its broad valley so sewage was only gently wafted on the current. And since it was a tidal waterway, the sewage was then pushed back upriver twice a day, with each tidal surge. The sewage was being flushed away from the houses, but not fast enough, nor far enough.
This was certainly not a modern phenomenon: around 1358 King Edward III was so revolted by the abominable fumes emanating from dung and filth accumulated on the banks of the Thames that he issued a royal charter demanding that no rubbish, at least, should be dumped into the river or its tributaries, and that it should all be carted out beyond the city walls. Semi-liquid excrement would, though, be sloshed or drained into London’s streams and ditches for centuries to come, and although many of these waterways, notably the Tyburn and Fleet, were eventually covered over, indeed built over to the point where their original course was lost, they continued to dump raw sewage into the main river until that fateful assault on the capital’s nasal senses in 1858. The inner London district Shoreditch is probably a corruption of Soersditch (sewer ditch), which likely drained into the Walbrook; this small river originally flowed through the centre of the Roman walled city, down to the Thames, but is also now covered over and lost.
After the Industrial Revolution, and the increasing movement of people from rural life into the cities, even the faster-flowing rivers of the West Riding of Yorkshire could not cope, and they became smelly and lifeless, little more than open sewers, with filthy water swilling over rancid deposits. In 1866 Charles Clay, an agricultural implement manufacturer from Wakefield, wrote to the Rivers Pollution Commissioners to complain about the miserable plight of the River Calder thereabouts; instead of ink, he wrote his testimonial by dipping his pen into the river water immediately below the town’s outflow sewer.
Wakefield is the lowest town on the Calder, which, by the time it arrived there, had also had a chance to receive poisonous outflows from Halifax (via the River Hebble), Huddersfield (via the Colne), Kirklees, Dewsbury and Hebden Bridge, either in the form of human sewage or from the heavy mill industries which used the water as a raw material, or for power.
Problems like this were found throughout the industrialising world. At around this same time Chicago was a small Great Lakes town of about 4,000 people. Its drinking water was extracted from Lake Michigan and its sewage dumped into the Chicago River, which flowed, somewhat unfortunately, back into Lake Michigan. The immense size of the lake meant that there was no problem for many years, but eventually the sewage outflow created a slick large enough to impinge on the drinking water source intakes. The intake tunnels were lengthened to reach into deeper, more distant water, but such was the burgeoning population of the late 19th-century city that its increasing sewage output eventually swamped this measure too. Chicago’s ingenious, but environmentally quite shocking, solution was to reverse the flow of the river. By cutting a canal and lock series through to the Des Plaines River to the west, water now flows up the Chicago River from the lake, rather than down into it, eventually taking the city’s waste into the distant Mississippi River, a gift for US citizens living on the other side of the great watershed.
Disposal by dilution was, until the arrival of industrial cities on the planet, relatively easy to achieve, either by draining sewage into large rivers, or into the sea. Despite some offended sensibilities, it was clear that discharge into water worked as a viable disposal, not just because the waste could be diluted to the point of insignificance, but because there was also a natural breakdown of any organic matter by fish, invertebrates and micro-organisms living in the water, which effectively neutralised any danger from the effluent.
But when the links between disease and excrement were firmly established, and burgeoning human populations became increasingly thirsty for clean piped drinking water, calculations had to be done to work out exactly how much sewage could be diluted into how much water before problems arose.
At first no one could agree. In one celebrated spat, Dr Charles Meymott Tidy, sanitary chemist and barrister, contended that 5% sewage could be purified by natural oxidation in the course of 10–12 miles down a brisk river over a gravelly bed. On the other hand Sir Edward Frankland, professor of chemistry and Fellow of the Royal Society, maintained that a stretch of 200 miles would not do it. Frankland’s views were later upheld by the Rivers Pollution Commissioners (of whom he was one), who agreed that there was no river in the United Kingdom long enough to eradicate sewage, even if it were put into it at the source. There was only one answer: sewage from towns or cities had to be removed, treated, broken down or somehow cleaned up. If the final run-off from this process were to find its way back into the rivers and streams from which drinking water was extracted, it was necessary for humans to remove most of what they had put into it.
This, then, is the basic principle of sewage treatment – not just trying to disguise it by diluting it with vast amounts of water, but trying to remove the offending matter. Luckily, help was on hand.
The late 19th and early 20th centuries were a time of unprecedented scientific advancement and understanding of the physical and natural world. Great engineering works were being constructed all over the globe – monumental buildings, intercontinental ships, railways, tunnels, bridges and canals – cathedrals to industry all. Bacteria and other micro-organisms were identified as the culprits responsible for so many human diseases, putting the medieval notions of miasma and bad airs firmly into the museum display case of history, and paving the way for an understanding of hygiene, antiseptics, vaccinations and eventually antibiotics. The very atoms of the universe were being named, measured and ordered, and the quaint alchemical names for compounds such as ‘spirits of hartshorn’ and ‘fire damp’ were being displaced by the modern names we still know today (ammonia and methane, respectively), as their compositions were being derived, chemical reactions were being calculated and understood, and new molecules synthesised. All of these fields of human endeavour would be harnessed to get rid of the Great Stink, and smaller stinks across the world.
To know how to treat human excrement in sewage, the first lesson is to hark back to vaguely imagined beginnings, when all humans did the equivalent of Moses’ bidding and covered that which commeth from them. The lesson here is that human dung, like animal dung, as we’ll see later, is naturally broken down and reabsorbed by the environment, if given a chance. Soil is a far from neutral passive substrate, it is a dynamic biological system full of invertebrates and microbes, constantly breaking down and recycling organic matter from dead leaves, dead animals and animal waste. Below this the subsoil filters out particles and minute changes at the chemical level occur. Finally the bedrock is either porous like limestone, allowing the water to trickle through down to the aquifer from where fresh sparkling mineral water can be pumped or drawn, or it is impervious so that the water flows along the geological strata until emerging clean and fresh at some spring or river source. What sewage management actually does is to mimic this natural process, but on an enclosed and industrial scale.
There are variations, but the main theme is fairly straightforward. First waste water from houses and industry, and rain run-off from gutters and road surfaces, is collected in a series of increasingly larger pipes, drains, sewers and underground canals, where it is directed, by gravity, to the treatment plant. To get it further away from a city than simple downhill flowing will allow, the sewage can be pumped up to a higher level and taken on its way by another series of gently sloping pipes and conduits. The elegant architecture of Victorian pumping stations is still evident throughout much of urban Britain. Eventually, though, the water needs to stop and be cleaned, and although the design of sewage works may be rather more prosaic than prestigious, these are still obvious and distinct large industrial constructions in the landscape.
Here sewage removal and water flow control becomes sewage water purification. To start, large objects such as pieces of wood and other flotsam washed into the drains are caught in a giant sieve. This should also remove paper, nappies, wet-wipes, cotton-buds and any other non-digestibles flushed, rightly or wrongly, down the toilet. Large settlement tanks then allow for heavy solids to sink to the bottom; these include faeces, but also food items sloshed down the sink, together with grit and sand particles washed from roads in storm drainage. Every so often this sludge is dredged or pumped out, drained, dried and spread across fields as a solid manure.
Fig. 6 Every local authority aspired to have a sewage pump, it moved the stuff upwards and onwards. From Rideal (1900).
Fig. 7 When Joseph Rideal (1900) was pontificating about the purification of sewage, the rotary screen for removing large floating objects was the height of modern technology.
When commercial works like that at South Norwood were opened in the second half of the 19th century, there was usually some aspect of agriculture involved. Fields manured by sewage waste would be used to grow vegetables or other crops, and so the term ‘sewage farm’ came into widespread use. The process of heavy-duty solids removal was simple, efficient and little more than mechanical, but the remaining sewage water still contained dangerous levels of biological waste product and would fail any drinking water safety test. In the early days this water was also sprinkled onto the soil, across a series of irrigation meadows, but even quite large management plants could be overwhelmed by unexpected flows of water, especially after rains.
Such crops as were grown were unlikely ever to make much of a commercial return; they were a secondary product in the primary aim of purifying the water, and most ‘farms’ ran at a loss. Nevertheless cabbages were grown, and fodder plants for animals, including mangold-wurzel, lucerne and grass for grazing and hay. Wheat and potatoes could not cope with the quantities of water being filtered through the soil. Willows, grown for basketry, proved to be too brittle if grown in sewage. Eventually land on the farm would become ‘sewage-sick’, covered with a layer of slimy sediment and rife with algal growth.
To avoid waterlogging the sewage farm’s soil, new techniques for improving and increasing the bacterial breakdown in the water were developed. Fountains sprayed the bacteriologically active water into the air so that more oxygen would get dissolved in the water, speeding up the microbial digestion. This water was then drizzled down through beds or towers of sand, gravel and crushed rock to mimic natural filtration. This worked to a point, but filters were liable to become quickly clogged and often needed to be dug up and replaced. My Dad’s old textbooks are packed with calculations for flow and filtration rates.
Fig. 8 Kessel separator. Sewage management was the rocket science of the early 20th century. From Martin (1935).
Fig. 10 Fiddian distributors. Circular trickle beds soon became a familiar sight in the landscape, complete with very farm-like houses for the operatives. From Martin (1935).
Circular trickle beds became the norm, across much of the UK at least: large broad cylindrical brick-lined or concrete pits are loaded with small chunks of coke, rock, lava or slag (more recently ceramic, polyurethane foam or plastic) and a rotating gantry above sprinkles the water to percolate down through the medium – not flooding it, but trickling down through the intricate network of surfaces, where air spaces provide access to oxygen throughout the bed. A microbial slime develops on this substrate, its porosity vastly increasing the surface area on which the organisms grow, and it is in this huge three-dimensional well-oxygenated labyrinth that the organic materials in the water are metabolised allowing cleaned water to flow out from beneath.
Fig. 9 Lowcock filter. The trickle-down sewage system, worked by mimicking the natural filtration of water as it passed down through the soil. From Barwise (1904).
The cesspit is a much smaller single-dwelling version of this simple technique. The enclosed tank allows sedimentation and as digestion of the organic material progresses, watery material drains slowly through soakaway pipes, or porous brickwork, into the surrounding soil where natural bacterial action continues. Occasionally a build-up of solids needs to be dredged out, but otherwise the limited amount of sewage is gradually biodigested and water returned to the soil.
Cesspits and sewage farms are now relatively old technology, but they suffice, and new sewage treatment centres still work along the same basic principles of solids settlement and removal followed by biological digestion of the remaining tiny waterborne organic particles, and the bacteria themselves eaten by other waterborne organisms. In London the water authorities use aeration lanes, large rectangular tanks where air is pumped into the water to encourage the bacterial cleansing; instead of a wet rocky base, the bubbles oxygenate the water, allowing a bacterial soup to carry out the work – a sort of jacuzzi for germs. Sorry. There may be chemical treatments to precipitate specific industrial pollutants, but the business of removing human ordure is pretty down to earth.
Fig. 11 The modern aeration tank. From Martin (1935).
In a world where global warming, climate change and altering weather patterns seem set to descend upon us, clean water will become even more of a key environmental issue than it is now. The rain-heavy water-rich northern hemisphere (including the British Isles, renowned for its rainy weather) may soon find itself shaken out of its complacency as water becomes scarcer, and recycled purified sewage water may become the norm for many of us. Drinking our own waste-water may not sound very nice, but how can we even be sure that it is safe?
A TEST FOR PURITY, OR AT LEAST POTABILITY
What we need is a real number, something that can be measured, compared and used to justify that the water being returned to the waterways (or directly into drinking water reservoirs) is clean enough not to cause environmental or medical upset.
In 1912, the Royal Commission on Sewage Disposal came up with the figure which is now the international ‘20:30 standard’. The numbers refer to a biochemical oxygen demand (BOD) not exceeding 20 mg/litre, and suspended solids not exceeding 30 mg/litre. The suspended solids measure is pretty self-explanatory – it’s the amount of fine particulate matter still floating in the water. It doesn’t take much imagination to guess what these are particles of, though this will also include silt, dust and other benign materials. The BOD is slightly more complex – it’s an indirect measure of how much organic (i.e. mostly faecal) matter is still in the water by calculating the biological activity from naturally occurring bacteria, as they break it down completely. It is calculated in a litre of test water incubated at 20°C for 5 days. If there is little organic matter, it only requires 20 mg of oxygen (about a tablespoon) per litre of water for the natural bacteria to harmlessly digest it away. A heavier organic load requires more bacterial breakdown to clean it, hence more oxygen per litre over the test period. Untreated sewage may have a BOD of 600 mg/litre. A pristine clear freshwater river or stream will have a BOD less than 1 mg/litre. The Royal Commission’s 20 mg/litre has stood for over a century, and although some people still flinch at the idea of any organic material from sewage finding its way back into drinking water, this is still regarded as a ‘safe’ value.
Fig. 12 Sewage treatment was more than just physical removal, it was about measurement and chemical analysis of water before it was allowed to be discharged back into the environment. From Barwise (1904).
Back in South Norwood, the purified water from the sewage farm, presumably adhering to or below that all-important 20:30 measure, was eventually discharged into the delightfully named Chaffinch Brook, and then found its way via the River Beck, the Pool River and, the Ravensbourne into the River Thames at Deptford Creek. I’ve waded in the water of the Creek, and fallen over in it; apart from getting a bit muddy, it never did me any harm.
The sewage farm was decommissioned in 1962, and it was always a surprise to me that it had not been immediately developed for housing; it became the new South Norwood Country Park instead. I was later told by my grandmother, who lived hard by, that this was because of dangerous levels of lead in the soil, and that the local authorities could not risk home-owners eating vegetables grown in it. Nigh on a century of flooding the fields with water from houses supplied by pre-copper pipe lead plumbing had created its own environmental problem for later generations – a sad irony in view of the sewage farm’s original agricultural purpose.
Today, human excrement is taken away from us at the push of a button or the crank of a handle, and our noses rarely get to complain about being in close proximity to it. But we are not alone in shunning our own bodily waste, or at least viewing it very suspiciously.
Walk through a rich cattle-grazing meadow in summer and it is immediately clear that the grass is not growing evenly. This is nothing to do with the irregular chewing of wayward animals, or an uneven soil layer trampled underneath, or the varied assortment of wild flowers preferentially eaten or distasteful to the tongue, sprouting in the grass. The field seems dotted with slightly taller, slightly greener, slightly lusher tufts of grass, growing seemingly at random across the otherwise evenly cropped sward. Each of these tussocks represents the place where a cow pat fell last autumn, or earlier this spring. There may be some minor increase in grass growth from nutrients being recycled and absorbed through the plant roots, but the major cause of these more prominent tufts is that the cows avoid eating the grass near where their own dung has fallen. Horses act similarly.
The precise reasons and mechanisms of this avoidance remain speculative. There may be a similar disease-avoidance evolutionary mechanism to that at work in humans. Cow dung, especially in wild, or feral breeds, has a heavy parasite load in the form of intestinal worms and flukes, as well as bacteria. Not eating grass near the dung might help avoid reinfection. Oddly, cows will eat grass growing from horse droppings, and horses will eat grass growing out of cow pats. Different cow- and horse-specific parasites may mean that ingesting the ‘wrong’ worm cysts may not present much of a health risk. Cows and horses show no constraint when it comes to eating grass where they have urinated. There is no evolutionary pressure here, since urine does not contain parasites, or indeed bacteria, so there’s no biological need to avoid it.
There is nothing wrong with the grass growing out of the dung, and if it is cut and presented to the cattle away from the pat site, they gobble it down. It is the smell of their own dung that averts their noses, and takes their scissor-cutting teeth elsewhere in the field.
Human aversion to dung is not confined to our own waste; we tend to shun all excreta. There are still regular complaints when farmers go muck-spreading, as the aroma of cow slurry goes wafting across the countryside. In the heyday of horse-drawn transport, concerns were raised about the vast quantities of horse dung dropped daily onto the streets of towns and cities. Concerns centred around disease spread and plagues of flies, rather than the smell. Horse dung is one of the least offensive in this regard. George Cheyne (1671–1743), physician, philosopher and mathematician, summed it up neatly in his Philosophical Principles of Religion (1715) as he sought to demonstrate the great wisdom of the Creator, who knew that horses would frequently be around humans: ‘[T]he cleanness, beauty, strength and swiftness of the horse, whose breath, foam [sweat], and ev’n excrements are sweet, and thereby so well fitted for our use and service!’
Perhaps the least offensive dung of any large mammal is that dropped by elephants. These huge animals eat so much herbage, and pass it so quickly that there is some truth in the notion that their excrement is barely more than just processed vegetation. According to trackers, a good way to tell how fresh the dung might be (i.e. how recently the elephant passed this way) is to thrust your hand right into the pile to see how warm it is.
Other dungs we find less or more offensive for some not altogether clear aesthetic reasons. As a general rule, herbivore dung seems relatively easy on the human nose; omnivore dung surprisingly and perhaps unnervingly is very familiar to us (since we too are omnivores), and carnivore dung is especially unpleasant. Anyone who has accidentally traipsed dog dung into the house knows the powerfully evil smell it leaves.
My own personal bugbear is fox dung. When we moved into our present house, my two daughters were aged 3 years and 18 months old, respectively. Whilst we busied ourselves unpacking cardboard boxes of books and crockery, they explored the new garden. The 3-year-old paddled up and down the uneven lawn on a small tricycle, but when she came indoors it very soon transpired that she had stepped in a large semi-liquid ooze of oily grey fox dung out there, and she’d got it all over her shoes. By the time I had cleaned her up I had it all the way up to my elbows. I have been waging war against incontinent vulpines ever since.
Fox dung is especially fragrant, gaggingly so, not because it might spread disease, but to prevent the spread of foxes. It is a powerful scent marker, laid down by the owner of a territory to warn other foxes to keep away. This accounts for foxes’ repugnant habit of laying their droppings in obscenely prominent positions – on my front-door mat, at the front gate, on a crisp wrapper dropped in the street, on an upturned paint can left by the farmer at the side of the barn. It is deliberately left in the open, at a prominent spot, so that no one and nothing can miss it.
Other animals make similar statements. Rabbits and hares leave their crottels (also croteys, or crotisings) on ant hills and tree stumps. These small pellets are not at all strong smelling to the human nose, but male and female scents are apparently left in the urine which is also added. Badgers, living in social groups of up to a dozen in a large burrow, combine their massed faeces into a latrine – a series of small oblong holes dug nearby, and constantly replenished and reinforced with their dark, thick, tarry dung (faints, fuants, or archaically werderobe). Again, the purpose seems to be to explain to any wandering badgers that this is a home patch, occupied by a gang of well-fed and well-organised owners, who have staked their claim here.
Otters leave their spraints on small scraped piles of sand or silt at the edges of lakes or rivers, or at prominent places along the stream bank. Despite a diet of fish and other mostly waterside animals, these droppings are not repulsively strong smelling, and although they may be slightly fishy, they also have a sweet musky smell, often reminiscent of violets, or Earl Grey or jasmine tea. Again, their purpose is not to offend or intrigue humans sniffing along the riverbank, but to communicate to other otters in the vicinity subtle messages about fertility, territoriality and body size – the otter equivalent of macho posturing.