Thursday, 31 October 2013

Drinking water and the “miraculous” power of sand

In the developed world, we sometimes take drinking water for granted. Turn on the tap, or faucet, and out comes a seemingly endless supply of water that is safe to drink. Of course, the supply is not only used for drinking and cooking and, in London, < 5% is used for this purpose. 1 The rest of this high quality water we use for personal washing and bathing; cleaning all manner of domestic items, from dishes to surfaces; we use it to wash cars and in high-velocity cleaning jets; and, often in quantity, to water our gardens and yards. Just as we have little interest where waste water goes (we know that it goes into a sewerage system of some kind and is therefore removed from our dwellings), most of us are not aware of how our drinking water is treated. We know it comes from the Water Company pipe system and originally from rainfall on to land, and we are also aware that supplies need to be maintained, so know that reservoirs may be constructed to hold water that might otherwise be lost as rivers flow to the sea. In some areas, we know that supplies come from boreholes into an aquifer within permeable rock strata.

Whether water comes from an aquifer or from surface run-off, it must first be treated to ensure that it is safe and does not have a bad taste. A number of treatment methods are used by Water Companies, but a common one is the use of beds of sand through which water is allowed to trickle and I will only discuss this one method here.  Slow sand filters take up a lot of space and a works may consist of more than ten beds, each of surface area > 1000 m2. A bed consists of a concrete tank with porous bricks over its base, these being overlain with cobbles and then a thick layer of sand. Water is pumped into the tank above the level of the sand to a depth sufficient to create a head of pressure and it then percolates through the sand, cobbles and bricks, exiting from a pipe low down in the bed. Although slow sand filters may be used to “finish” water that has been pre-treated, they are effective at cleaning very dirty water and making it potable. During a visit to Japan, I visited the city of Ueda, where water of poor quality is drawn from a polluted river and fed to slow sand filter tanks that have masses of filamentous algae growing in them. Some of the organic matter is thus converted to large numbers of algal cells and these also charge the underlying sand with oxygen resulting from their photosynthesis. No-one would be tempted to drink the water in the bed, yet the quality of the filtrate is excellent, as I learned when trying a sample - it was crystal clear and sweet-tasting. So what is the “miracle” of the sand?

A slow sand filter bed under construction.

A filter bed at Ueda, Japan.

Beds of sand act as a physical filter for particles that cannot pass through the pores between grains, but how do the filter beds remove fine particles of organic matter, bacteria, and the like? A visit to a newly-laid slow sand filter gives us a clue to answering this question because water must pass continuously through the column of sand for days before the filter becomes effective. In this time, the sand grains become covered with microbial biofilm (directly analogous to the plaque we brush from our teeth) and this film is responsible for the removal of impurities, some of which become stuck on the matrix and some of which are taken up by the micro-organisms contained within the film (and which exude this sticky coating). Of course, pores must be kept open and this is achieved by a complex community of single-celled organisms that graze on the biofilm, while some of these organisms also capture fine particles from the water passing between sand grains. There are animals present too, with small worms of various kinds being abundant, feeding on organic matter and, in so doing, further ensuring that pores remain open. This complex community, that takes time to develop, keeps microbial activity in check and ensures that fresh adsorptive biofilms are developed continuously. It is a process that occurs within stable sand banks in natural water bodies and, in slow sand filters, we use it for our own purposes. In engineering a system for cleaning water for human use, we didn’t realise that we were adapting an ecosystem that has been in existence for hundreds of millions of years, a very considerable time before humans appeared.

One of the benefits of slow sand filters is that they can be used, on a small scale, by village communities, where water purification from polluted sources becomes possible, with obvious benefits for health. 2 As with large-scale filters, the continuous process does require maintenance. If the water is rich in particles and nutrients, it is likely that a schmutzdecke (“dirty layer”) will form at the sand surface and this results in blocking and inefficiency. This is significant when a city needs a constant supply. There are a number of solutions and these include covering beds to reduce biological activity in the water column as, even with pre-treated water, there can be a considerable build up of algae and other matter in beds open to the atmosphere. Open beds are colonised by very large numbers of dancing midge larvae, many of which live within silk tubes on the surface of the sand. They graze over the surface and thus keep pores clean, transforming the organic matter into faecal pellets that are well-bound and therefore do not obstruct the functioning of the bed. The larvae thus play an important role in the functioning of the filter and may be found in densities approaching 50,000 per square meter; an additional benefit being that the silk of the tubes produced by many larvae is highly adsorptive and thus adds to the cleaning process. There comes a time when the bed, whether large or small, will need to have the surface of the sand scraped off and removed. Re-filling the bed with water then allows filtration to proceed, as the underlying sand is well-conditioned, and re-colonisation by midges begins within minutes.

The cleaned surface of a sand filter - the upper part shows the schmutzdecke that eventually reduced the efficient performance of the filter. The sand in the lower part is conditioned with a rich microbial community together with many other types of organism.

That water is cleaned by passage through beds of sand has been known since ancient times, so the engineers of the last few hundred years were following an established principle. In the first of the Great Plagues of Egypt we know that surface waters became undrinkable because something turned all water both red and toxic. I have argued that this redness resulted from the erosion of sediments that subsequently caused the widespread death and decomposition of aquatic organisms, and that this poisoned the usual sources of drinking water. 3 Interestingly, we read in Exodus Chapter 7 verse 24 that “....all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river.” 4 In a commentary, 5 we read further: “The Egyptians dug round about the river for water to drink, and it seems that the water obtained by this means was not bloody like that in the river...” and I suspect that they were digging down into sand deposits, through which water had passed and been purified by the microbial community coating the sand grains. It would not be regarded as a miracle, although it might seem to be such by some observers. Miracles usually have a rational explanation and we know just how “miraculous” water treatment using sand filters appears to be in providing us with the drinking water that is piped into our homes. If we don’t take the supply for granted, that is.

1 Roger S Wotton and Helen Evans (2005) London’s Water Supplies. pp 135-143 in London’s Environment: Prospects for a Sustainable World City (ed. Julian Hunt). London, Imperial College Press


Thursday, 17 October 2013

More on the Natural History of the Unmentionable

The Renforsen rapids on the Vindel River in Sweden are at their most tumultuous during snow melt in the mountains and surrounding lands. A large volume of water then passes downstream and Renforsen becomes highly turbulent, as can be seen in the photograph below. The water contains many fine mineral particles and fragments of organic matter, but also some most important aggregates.

In 2001, the late Bjรถrn Malmqvist and I, together with Yixin Zhang, took water samples at Renforsen and found that they contained faecal pellets that we recognised came from blackfly larvae living on, and within, the substratum of the river. These insects collect particles from the water passing over them and push the material collected through their tube-like gut in about 45 minutes. There is no mixing and less than 5% of the material taken in is digested or otherwise assimilated, with the final section of the gut compressing the gut contents to produce pellets. Larvae feed continuously, so each larva is like a machine for converting very many tiny particles into much larger faecal aggregates. As there are millions upon millions of larvae on the bed of the river, it would be expected that there will be huge numbers of faecal pellets in transport. That is what we found. 1

93.7, 47.5 and 69.2 tonnes dry mass of blackfly larval faecal pellets passed an imaginary line across the river at Renforsen each day during three successive summers. 1 Without the blackfly larvae producing aggregates, it is likely that much more material would have been carried downstream and, eventually, to the sea. Of course, this is the fate of some of the faecal pellets, but many are retained within the river as they fall through the water of the slower-flowing reaches. They thus have an important impact on the river community in transferring matter from the water column to the substratum, where most micro-organisms, algae and animals live. Our findings so impressed the producers of the Swedish children’s TV series Myror i brallan (“Ants in your pants”) that a section of one of their programmes in November 2011 was devoted to the story of this transformation, 2 with the presenter saying: “Every day, the amount of blackfly poo drifting down a river would fill a whole house.......”

If the numbers of pellets produced is impressive, so is their constitution, as most of them appear to be intact despite the highly turbulent flow. This is partly because they are small and are thus bounced around in the current, but also results from the way they are stuck together. In the photograph above, the blackfly faecal pellet from the Vindel River (about 0.5 mm across) is blue in colour and this was the result of staining with a dye for complex carbohydrates (exopolymers). It is clear that these binding agents are found throughout the aggregate, yet the exopolymers are not produced by the larva, but are collected from the water column or exuded by micro-organisms and algae that have been ingested. The secretions are “Nature’s glue” and are the principal binding agent of the faecal pellets of all animals, although some animals produce their own exopolymers in the form of mucus from cells in the gut wall.

Not only do pellets remain intact as they pass through rapids, but they stay as aggregates for weeks after they pass to the substratum, so their value to the microbial, plant and animal community is not only short term. They provide nutrients and also food for the many animals that eat them. It is a pity that faecal pellets and their fate has been so little studied, although attitudes to their importance in flowing fresh waters is changing. 3 Perhaps scientists are affected by the general approach to the subject? It is not what most people first think of when discussing the Natural History of streams and rivers and one can imagine this conversation as being typical:

“So, you’re an Aquatic Biologist”

“Yes. I’m so pleased to be able to have spent a lot of time doing something that I loved from childhood”

“What do you study? Whales and dolphins?”

“I’m afraid not. I have spent most of my career looking at rivers and streams”

“At salmon and trout?”

“Actually, I’m fascinated by the importance of faecal pellets produced by invertebrates”

“Faecal pellets? Oh... interesting...”

Of course, the exchange has not taken place, but it might have done, as it shows the popular liking for large organisms and distaste for excreta, as discussed in my previous blog post on this subject. Fortunately, the young viewers of Swedish TV were provided with an enlightened exception to this general rule and that is an example for us all.

1 Malmqvist, B., Wotton, R.S. and Zhang, Y. (2001) Suspension feeders transform massive amounts of seston in large northern rivers. Oikos 92: 35-43.

My thanks to Dr Brendan McKie for alerting me to the Myror i brallan programme.

Monday, 14 October 2013

The Natural History of the Unmentionable

A Natural History of the Unmentionable is the sub-title of Nicola Davies’ book Poo, 1 written for a young audience, but also an interesting read for adults. Nicola has a degree in Zoology from Cambridge University and her book carries this quote on the dust cover:

However you look at it, poo is probably the most useful stuff on Earth. It comes in all shapes and sizes and every animal has its own special sort. Find out what it’s for, where it goes, what we can learn from it and lots more in this lively and fascinatingly informative natural history book. You’ll never think of poo in the same way again!

I’m sure readers of her book never will think of poo in the same way.

We are conditioned from an early age to think that faecal matter is unpleasant and offensive and I, too, feel this way about human and pet excrement, especially as it is a means of spreading harmful bacteria and parasites. Although very young children can gain delight in their excreta, and stools were pored over by generations of physicians for clues of illness or state of mind, I’m very much of the view that the WC is essential in removing the stuff as quickly as possible. However, we seem to carry over our distaste of human and pet excreta to those produced by all animals, despite the considerable importance of this matter in the cycling of nutrients. Interestingly, faeces are also little studied by scientists, who conduct many investigations of feeding in animals but, like all of us, tend then to lose interest in what happens to the material that results from feeding, other than that which is absorbed and used for growth. There are, however, exceptions to this general rule.

We know that spreading faeces on land improves the growth of crops, and farmers are not put off by muck, as they know its value. There are also many examples of using excreta from livestock in gardening, especially to supply nutrients for growing roses or rhubarb. It is not just the rich supply of nutrients, but the way that these are released over time, as the muck becomes broken down and rain percolates the rich supply of nitrogen etc. into the soil. We use the same approach when applying compost produced either from kitchen waste, lawn mowings, trimmings from garden plants, etc. Have you ever wondered what happens when you add material to your compost heap? We know it decomposes, but what are the changes it undergoes?

The first step in breakdown is provided by fungi and bacteria, often associated with the material we add, but found in abundance in compost heaps. Fungi break down resistant materials such as cellulose and invade plant tissues, as well as growing over their surface; while bacteria become attached, often forming films, and these micro-organisms continue the break down of the plant remains. Animals like earthworms and many other types of invertebrates, some of them tiny, are very numerous in our compost heaps, where they feed on the decaying remains. The main source of nutrients for these animals is not plant matter but the attached micro-organisms and these are digested, leaving materials that are little affected by digestion. These are excreted as faecal pellets and masses - in huge numbers. The mature compost heap can thus be said to consist of largely of the faeces of various animals and plant material that is difficult to degrade. The latter provides a means of improving the texture of soils, especially clays, while the former is the source of nutrients. Faecal masses, such as those produced by earthworms, break apart quickly, but the pellets produced by other animals remain intact for much longer.  Pellets are thus a natural and small-sized analogue of the pelletised fertilisers sold in garden shops.

What goes on in our compost heap is, of course, a microcosm of what occurs more widely in Nature. When going on walks, we notice the droppings of farm animals like cows and sheep and the droppings of rabbits and deer, each having a characteristic form. Trackers following game recognise the faeces of their quarry and we use the appearance of droppings to monitor the numbers of otters, for example, noting whether they have been excreted recently. We are also familiar with bird droppings which, as solid and liquid excreta are mixed before being expelled (unlike many animals), are not usually formed into pellets. The same, of course, can be said for cows, but their loose droppings result from their particular gut structure, with incomplete removal of moisture and no effective means of compression before expulsion. What we don’t see are all the faecal pellets of invertebrates, as these are small and also blend in with the surroundings, so microscopic examination of soil surfaces is required before we see the extent of pelletisation. With the naked eye, we may notice the frass produced by larvae that bore into fruits and we may see the droppings of insects that live in silk enclosures, but that’s about it. The pelletisation is not only of advantage to the animal, in allowing the uptake of nutrient-rich fluids by compression of material in the hind-gut. Forming the waste into pellets ensures that micro-organisms surviving passage through the gut are packed tightly together with the substrates on which they act, with the additional colonisation by both bacteria and fungi once the pellets are excreted. Their action allows the slow release of nutrients that are then available to the plant community, just as in the application of garden compost.

This is just a small insight into the importance of faecal pellets and masses and there are many examples given in Poo 1 (faecal matter from aquatic animals will also feature in a future blog post). I agree with Nicola Davies that we should not be upset by mention of the stuff, but recognise that it is an essential feature of Natural History - and of natural ecosystems.

1 Nicola Davies (2004) Poo: A Natural History of the Unmentionable. London, Walker Books. With illustrations by Neal Layton.