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The wonderful world of reed beds by Louise Halestrap

Source: Clean Slate 29 Summer 1998

Since Prof. Kikuth first discovered that reedbeds could be used to treat sewage, the scope for reed bed use has gradually widened. They can now be used for almost all contaminated waters such as landfill leachate, agricultural slurries, mine leachate and even road run-off. By far the most popular reason for putting in a reedbed is still to treat sewage in inaccessible locations or in small quantities, e.g. single dwellings, which were previously untreated. They are also being used more and more as tertiary (final) treatment ‘tacked’ on the end of conventional sewage treatment plants to deal with the ever increasing amount of sewage produced.

Coming to the reedbeds from a composting and laboratory based background, I was aware of the theory behind them, but nothing could prepare me for the hands-on experience of their practical management.

Since the two reedbed sewage treatment systems were built at C.A.T., they have passed through three sets of hands: Jeremy Light, a biologist who designed and built them; Chris Weedon, a biochemist who did a PhD on them!; and me, a microbiologist.

When I was first introduced to the Eco-cabins’ reed beds I was pleasantly surprised. This was the place where the sewage of 30 people was treated? The reeds were blowing in the breeze and the beautiful, crystal-clear Eco-cabins’ pond was buzzing with wildlife, I thought ‘Mmm, I’m going to like it here.’ Then I was shown the main site system solids collection tank, where all the solids from up to 1000 visitors a day are separated. This was another story. A big green net in a 6 ft hole, full of straw and lumps (yes discernible lumps) of poo, you could even see bits of sweetcorn. What with the overpowering smell and the realisation that I was expected to physically remove the net by jumping onto this evil smelling 4 tonne globule of other peoples excrement, I nearly fainted. I started secretly planning my escape from the department. Chris and Peter thought it highly amusing that I was so squeamish, but were very tolerant and kind to me through these first few weeks - maybe they knew that shit grows on you. I can happily say that I no longer want to escape - far from it. I could even say that I have overcome my fears and have learned to love sewage.

Reed bed sewage treatment systems are incredibly robust. If designed properly, reed beds can be built very cheaply (like our Eco-cabins’ system) or as more expensive engineering projects (like our main system). The Eco-cabins’ system was only the fourth reed bed installation in the UK, so the designers didn’t have much to go on and were given a minimal budget (in case it all went wrong). The builders often had to make do with freebies which didn't quite fit the specification, yet amazingly it still works. This makes it an interesting system to examine further.

The water cleaning process begins at the settlement tank (1), which is designed to give primary treatment to the sewage and settle out the solids. It is made with brickwork and has a railway sleeper lid. The waste that is lighter than water will float and form a dark brown crust. This crust can be spooned out periodically with a special type of garden fork and then composted with straw or removed by tanker. The heavier solids settle to the bottom of the tank in a gravy-like slurry, and a pipe leads from this tank to a sludge bed allowing the tank to be emptied when necessary (about fortnightly) onto a sludge bed (4), a modified gravel filled reed bed. If this system were to be designed with the knowledge we now have, we would feed the effluent directly onto the first set of vertical flow reed beds to allow extra cleaning, but with the current system it passes through one meagre bed and into the secondary solids collection tank.

The liquid effluent from the settlement tank (see diagram) passes through two sets of vertical flow reedbeds (2 + 3) planted with Phragmites australis (the common or Norfolk reed) which is considered to carry oxygen to its roots and leaks oxygen for the micro-organisms living in the biofilm (living layer) around the root. The micro-organisms are then able to degrade aerobically the nitrogenous compounds in the sewage to nitrate (thus avoiding the build-up of ammonia which is toxic to most plants and animals - it also smells bad). The beds are fashioned from old glas- fibre cement cattle-feeding troughs filled with graded gravel and plants. They act like a bath with its plug out, allowing water to run over the surface, through the substrate and out the ‘plughole’. These were too shallow for optimum performance but they were free, so we installed twice as many to achieve the same amount of cleaning.

The liquid then passes into the secondary solids collection tank (5) (an adapted old 1500 litre orange juice container) where it occasionally gets recontaminated with the sludge bed effluent. All the dead microbes which have been breaking down effluent are naturally washed out of the vertical flow beds and into this container. It has never been emptied, but the floating solids (resembling very fine humus) never increase in volume as these dead bodies are recycled by a host of living organisms, from bacteria to worms and even slugs!

The second vertical flow bed (6) is also ingenious: it is made of ferrous cement, which is chicken wire coated in a layer of cement. This is unconventionally filled with ordinary edible plants living on an entirely hydroponic diet of NPK (nutrient)-rich effluent. The idea behind this is to take the nitrates out of the system by growing high nutrient-demanding crops such as cabbage and tomato. The crops are huge and the effluent results are promising. Incidentally, these crops aren’t eaten, but there is no reason why they shouldn’t be if well cooked.

The now fairly clean water goes through a brickwork horizontal flow bed (7) with yellow flag iris, water mint and marsh marigold. This bed is deliberately flooded to provide an anaerobic (oxygen depleted) environment, which allows anaerobic micro-organisms to turn the nitrate into nitrite and eventually nitrogen gas. It also provides a great environment for single celled predators such as protozoa and amoebae, which love to eat the other microbes including human pathogens like E. coli.

The near pure water then carries on its journey into the pond (8) - yes, that same crystal-clear pond mentioned earlier.

The pond is a beautiful wildlife haven, with true and false bulrush, yellow flag, bogbean, sedges, mimulus, watercress and many others to increase the biodiversity of the whole ecosystem. In spring hundreds of frogs, which are indicators for clean water come to spawn.

The pond acts as a buffer against fluctuations in load. Its efficiency was tested recently when vandals opened the settlement tank outlet valve, allowing thousands of litres of blackwater, sludge and even the crust onto the sludge bed. This was a system overload and if it wasn't for the pond, dirty water would have leached out onto the surrounding fields. The pond went very murky and smelt bad for two days, but the tadpoles didn’t die and soon after the pond regained its clarity. Imagine emptying a whole septic tank onto your lawn and expecting it not to smell!

There are several carp in the pond which act similarly to mine canaries in that if they float then we know that something is wrong. Happily we have never lost a single fish in this way, though we have lost some to the local heron.

The clean water in the pond meets drinking water standards for chemicals (such as nitrate) but there are still a few bacteria left. So although I wouldn’t drink it, it is probably a sight clearer than an ordinary fishpond, especially if you keep ducks on it.

In summary, it is possible to clean sewage and create a wildlife resource that won’t break the bank.

If you are a single household thinking about installing a reedbed or constructed wetland but don’t know where to start, C.A.T. Consultancy can design a simple plan including sizing and planting plans. We also provide a more in-depth design service for those with larger projects.

Relevant C.A.T. publications: Sewage Solutions and The Reed bed Tipsheet.


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