Constructed Wetlands in Kenya 

By Dee Raymer 

Constructed wetlands (CWs) are a relatively new concept in tropical Africa, and of the few that exist none are community owned or managed. But Kenya now has six working CWs that recycle wastewater and return it clean to surface systems, create healthy aquatic ecosystems in the harshest of environments, yield significant amounts of biomass for mulch, fodder and compost, and provide thriving year-round wildlife and fish habitats. They offer a way forward for economical and environmentally sound wastewater management in Africa. 

In Kenya, the hopelessly over-optimistic UN slogan “Clean water for all by the year 2000” vanished quietly into the bad joke cupboard sometime during the mid 1990s. Throughout the parched millennium year, the majority of Kenyans would have been grateful for any water during the many prolonged periods when the taps ran dry.     

The better informed of the majority of Kenyans also know that, thanks to a deadly combination of environmental abuse and a thirty-fold population increase since 1900, Kenya has few options for reliable sources of clean water supply in the future. Galloping deforestation is reducing yields of hitherto reliable resources, and it is said that water from the huge Tana/Galana/Sabaki lifeline river network is no longer fit to drink, along its entire length.    

A senior officer with the Kenya Wildlife Service told me recently of his utter disbelief upon learning fifteen years ago that much of the water supply in Western cities is recycled. “The whole concept,” he said, “was totally unacceptable to me then – but we had not yet approached our present crisis point. Now I see all too clearly that one-time water use is impossible if we hope to have the use of any water in the future.”    

This viewpoint is slowly being forced on all of us now. In Nairobi’s eastlands, it is not an uncommon sight to see residents drawing water from the Nairobi River and its tributary, the Gitathuru – both scarcely better than open sewers nowadays.    

Flourishing market gardens are watered directly from these rivers, and from sewer mains that are sometimes deliberately fractured for the ‘regulated organic nutrients’ within. Can any Nairobian guarantee that they have never eaten any of that very healthy looking produce?     

Groundwater as well, while still assumed to be clean, is actually alarmingly polluted. The best test result I have seen for water from a rural Kenyan spring during the year 2000 was 35 E. coli per 100 ml (World Health Organisation potability criteria is zero), and the worst, 1800. This last fails to meet health standards even for drip irrigation. Some urban and peri-urban boreholes are now yielding water with E. coli counts similar to those in raw sewage from busy public toilets - it may look rather better, but drinking it untreated will see you in the doctor’s waiting room.    

Poor and often criminally irresponsible disposal of wastewater is causing a high countrywide incidence of water-borne diseases as we contaminate dwindling fresh water resources with our filthy discharges; poorer Kenyans, generally the most affected, cannot always afford the cost of treatment. (This brings to mind a particularly nasty tale from the Idi Amin era in Uganda when a visiting WHO official memorably declared it safer to drink Kampala’s sewage than the fruit juices sold on the streets - the typhoid count was lower.)    

We know that major problems exist at all levels and must look, sooner rather than later, to finding solutions. We MUST start cleaning up our carelessly-discharged wastewater for reuse rather than continuing to foul our environment with it.     

Conventional wastewater treatment relies on machinery and chemicals - long detention in unattractive and hugely expensive concrete ponds, using stirring and aeration machinery powered by electricity or fossil fuels, followed by chemical treatment to ensure compliance with public health criteria before reuse.    

But mechanical breakdowns, the frequent unavailability of spare parts (or the budget to buy them), power cuts, the human error factor and the need for large technical workforces bode ill for reliable and sustainable operation of such systems in a Third World context. History shows all too clearly that such high-tech and inappropriate solutions to wastewater management and treatment tend to stumble along erratically from aid package to aid package, punctuated by long periods of breakdown.    

This is not to say that the situation elsewhere is terribly more encouraging. I have yet to meet mains supply water in any major Western city that does not taste revolting. The health lobby has pointed accusingly for years at the two chemicals most commonly used in water treatment systems – the flocculent alum (which, incidentally, kills tadpoles) as a possible contributor to Alzheimer’s disease, and chlorine as a likely carcinogen. Chlorine certainly kills most life forms, and there is little reason to suppose that mankind escapes its effects unscathed. Whether or not we subscribe to the health lobby’s views is a matter of personal choice, but our collective health and immune systems are unquestionably under siege from modern technology’s chemical contributions to what we eat, drink and breathe.  

Millions of years before we became fixated on technology, nature had already worked this one out – she evolved the wetland and a highly complex ecology to go with it. Wetlands clean up dirty water biologically, without use of either chemicals or machinery, but it took us a long time to realize this.  

The first generation of wetlands used in dealing with wastewater, as employed in the United States, were natural swamps that simply provided handy dumping grounds for effluent. Marked improvement in quality was noted as wastewater proceeded through them, and researchers became interested in the reasons behind this.

At the Max Planck Institute in Germany in the late 1950s, invaluable pioneer research work was conducted by Dr Kathe Siedl, who investigated workings of the submerged gravel bed – which proved remarkably effective in breaking down very polluted water. The gravel bed became the first element in many designs for Constructed Wetlands.    

Nowadays, with heightened awareness of environmental issues, it is illegal, or at least actively discouraged in most countries, to discharge wastewater into natural wetlands. What is convenient and cheap for mankind is very damaging to an ecosystem that undergoes radical changes with heavily increased nutrient loadings.    

But Constructed Wetlands (CWs) are a different matter, being purpose-designed for each situation. Since there is no existing ecology in a new CW, life-forms that arrive as it matures will be those able to tolerate conditions in each element, from more pollution-tolerant at the start to less tolerant further along the system, where water is significantly cleaner. Thanks to the wealth of actively ongoing research into water-purifying processes effected by various aspects of the ecology, there are numerous design options.    

With the single exception of a riparian buffer installation at a commercial farm in Naivasha, which removes agro-chemicals from horticultural runoff before it enters the Lake (by passing it through some two kilometers of varied-depth lagoons and oxbow channels, intensively planted) other CWs operating in Kenya to date consist of four elements.    

Where sewage is involved, septic tanks of adequate capacity to handle daily flows are essential for the primary breakdown of solids, and for kitchen water additions, grease traps as well. Grease is remarkably persistent in an aquatic ecosystem and benefits none of the essential life forms. Now-liquid septic tank discharge is piped to the head of a gravel bed hydroponic section (GBH) for the first stage of wetland treatment.     

The GBH is simply a walled rectangle of dimensions determined by the daily volume of wastewater. Alternating baffle walls are built across it at regular intervals, to about two-thirds of its width, which force water into taking a serpentine flow-course through the GBH, maximising the flow-distance while eliminating stagnant areas.      

The base is thoroughly compacted and onto it is spread an even layer of substrate (graded crushed ballast) 60-80 cm deep depending on the type of wastewater being treated. Influent water is distributed across the full width of the channel formed by the edge and the first baffle wall, by a perforated spreader pipe.      

During the time a GBH takes to fill, the ubiquitous bacteria that feed on sewage and break it down begin to colonize all surfaces of the substrate. The addition of some water at filling stage from a mature GBH is very helpful in ‘seeding’ the system, by boosting populations of useful bacteria. Nature is no fool. Full biological maturity of a GBH, which may take a few months to achieve, is marked by a dramatic difference between influent and effluent water; opaque blackness has given way to clarity, although often with lingering greyness.       

As soon as the GBH is filled to the point of discharge, planting may begin. Only a fairly restricted range of species will tolerate the conditions – reeds, bullrushes and sedges being the chief contenders. These are planted into the substrate with their roots in contact with the water just beneath, and may need initial support until firmly rooted. Once established, they remove some 10% of pollutants as nutrients, but their major contribution to the workings of the GBH is the unique adaptation evolved by emergent swamp plants, to oxygenate their own root systems – which is why they can grow standing in water, whereas terrestrial plants in the same situation would drown.    

Since a CW is a biological system, the more life-forms working on the clean-up the better the results. Design and management of these systems must always be mindful of the importance of biodiversity. Bacteria colonizing the substrate surfaces, feeding on nutrients in the wastewater as it flows through will almost all, by definition, be anaerobic. By aerating their own rhizospheres, the plants now create conditions acceptable to aerobes and therefore greater treatment efficiency, with two classes of bacteria rather than one working on the problem.    

At the end of the GBH, a slotted subsurface collection pipe feeds water to a level control chamber (which enables adjustment of top water levels within the GBH), from where it is piped or channelled by gravity to the first in a series of three open ponds, or free water surface cells, referred to as SCs. The dimensions of SCs, which are soil based, are also determined by the daily volume of throughput.    

This is where the copious research into the workings of wetland ecologies comes into its own. The clean-up processes are so many and complex that we may never know them all. But what we do know are the conditions that enable them to take place, and so SCs can be designed for maximum efficiency, with (again) serpentine flow-paths, anaerobic deeps, aerobic shallows, and riparian slopes that encourage colonisation by a wide variety of emergent and seepage-zone plants. All water is thoroughly turned and mixed between influent and effluent, while plants provide the baseline for any ecology, and are vital for a CW.    

Water entering SC1 is, for the first time, exposed to sunlight and air. Green algae rapidly makes an appearance. This is a positive sign – these microscopic plants are unable to survive in heavily polluted waters, and the oxygen they produce during daylight hours kick-starts the further breakdown of pollutants. The effect of this is noticeable – a sample of water taken from the GBH effluent still has a lingering tell-tale hint of its origins, but odour vanishes promptly with the appearance of green algae, which also provides an important early link in the food chain.    

CWs are a very new concept in tropical African countries (temperate Southern Africa is already using them). The first Kenyan installation in 1994, handling up to 80 cubic meters daily from a busy restaurant and a recreational complex, was believed to be the first of its kind in tropical Africa.    

Aquatic ecology professors and lecturers from several European universities state that significant biological maturity in a new CW occurs 3-5 years later, but in all of the Kenyan CWs it has taken less than a year. We can, to some extent, thank the climate for this, since they escape harsh winters.  

But I know of one installation done here to bought plans from elsewhere, that has developed no significant ecology. The difference lies in management. The engineer simply walked away from the completed ponds hoping, no doubt, that all else would occur spontaneously. It probably will, given enough time.    

Pioneer plantings in the SCs consist of a range of plants similar to that used in the GBH, but as conditions begin to modify, it is possible to add many rather more pollution-sensitive species. Experience soon teaches which are suitable, and once birds are attracted, introducing seed in the droppings, ‘volunteer’ plants begin to make their appearance.    

Frogs and aquatic invertebrates are amongst the first new arrivals to take up residence. Tadpoles feed on the algae and, since aquatic invertebrates are largely carnivorous, they form an early check on mosquito breeding. As vegetation establishes, new life-forms arrive in droves; the everything-eats-something-else factor brings checks and balances. The chief maintenance task is vegetation management – old vegetation falling in the water and rotting repollutes, while harvesting plants that are past their best (and which are composted for use as fertilizer elsewhere) leads to fresh regrowth with increased nutrient uptake.    

Water samples taken weekly from a number of points between GBH influent and SC3 final discharge show the improvements taking place sequentially, and from these it is possible to judge when fish may be introduced into SCs 2 and 3. Omnivorous fish tend to have a lighter, more beneficial impact on any one aspect of the ecology. Their presence attracts a further range of birds – the bird list for one Kenyan CW now contains some 153 species, all of which must be finding food or they would not waste their time there.    

So where do the pollutants go? One main way – up the food chain, as nutrients rather than pollutants. Kingfishers eat fish that eat daphnia that eat algae that eat the nutrients. They are also absorbed by plants, adsorbed onto base sediments or submerged roots, and transformed both chemically and biologically. Pathogens are killed by exposure to the sun’s ultra-violet rays, and there is some evidence that the roots of certain aquatic plants may exude disinfectant substances.    

There are now six fully operational CWs in Kenya, one treating sewage from the earlier mentioned restaurant and resort, and one at a country club, both within the greater Nairobi area. Another treats sewage from a new tourist camp in the Maasai Mara Game Reserve (the only one so far within the Protected Areas – more are long overdue). At Naivasha there is the agro-chemical buffer for the Lake, while another handles commercial laundry effluent. A CW at a Nandi Hills tea factory recycles agricultural waste and toilet effluent.    

All of these CWs comply with responsible standards for discharge and, between them, return just over 500,000 liters of cleaned water daily to various surface water systems for safe use by others. If a significant percentage of major water users, particularly industry, treated water and returned it thus, we might not have a water crisis.    

All of the existing CWs in Kenya were started with private sector initiative and investment. Public sector officials and others from large organizations supposedly concerned with health and environment have seen them and appear to be dumbfounded. That nature can effect better and cheaper purification than concrete and machinery, while creating an attractive habitat for birds, wildlife and many small aquatic creatures whose natural habitats are under increasing threat, seems to be beyond their acceptance. I suspect that they think it is all done by mirrors…    

The big horticultural producers have discovered, perhaps to their initial surprise, that CWs bring them tangible economic benefits; environment impact inspectors from their major overseas customers are enthralled by them, and approval thus earned reflects favourably in securing the sale of their produce.    

Two new CWs, one each at Timau and Kericho, are on the eve of commissioning, pending completion of the pipework. Funding is being sought for another at the Kenya Wildlife Service’s headquarters in Nairobi, so that 55,000 liters of treated water can benefit wildlife in Nairobi National Park. One is proposed for a new school now under construction in Arusha, Tanzania – which may well be a first for that country. Recovered water will be used for toilets, irrigation and washing. In due course, I imagine that water from many CWs may have to be used for all purposes; a much healthier option than that offered by chemical treatment, to ensure compliance with public health guidelines, could then be effected by in-line UV radiation.     

How much land does a CW require and what does it cost? A rough rule of thumb calculation of size is 4-5 square meters of processing area (divided between the four elements) for each person using the system. The largest CW in Kenya so far, with a 1,200 person equivalent, occupies just over half a hectare.    

Costs vary considerably, depending on geology, degree of site slope, method of excavation, length of pipe runs, whether the work is done in-house or by a contractor, and the local cost of materials, particularly ballast for the GBH. On one CW handling around 30 cubic meters daily, and hand-dug on a level site, excavation costs in 1997 were Kshs 220,000/- ($US 2,750, at present rates) If machine dug, there would have been little change from three times that sum. Pipe runs were minimal, but blockwork and GBH substrate added another 150,000/- ($US 1,875).    

CWs come in all sizes, from municipal schemes (Lakeland, Florida in the US: pop. 79,000, and Lalling, France: pop. 15,000) to tiny domestic ones. In the Gloucestershire village of Camphill, England, each household treats its own wastewater.    

Obvious constraints on very large schemes are land areas available and the cost of substrate. Some towns and cities, in the interests of reducing requirements for both, have developed hybrid systems, using activated sludge plants or primary anaerobic ponds followed by mechanical aeration, with SCs used for polishing thereafter.    

As community-based concept, CWs have almost limitless potential in Africa. A system could be hand dug with pooled community labour, providing year-round growth potential for food crops in designated seepage zone areas. Napier grass as supplementary stock feed for the dry weather would thrive, thatching and handcraft materials could be harvested regularly, and aquaculture practiced in SCs 2 and 3 as a source of protein for the community. Water of reliable quality would be available to all, perhaps under the strict control of a village committee. Health would be safeguarded, as a healthy CW outperforms conventional wastewater treatment systems.    

As with solar power, there is initial capital outlay, but operational costs are modest; one or two staff, trained on the job, manage routine work satisfactorily on existing CWs, but casual labourers may need to be employed to help with seasonal grass cutting or harvesting. In a community system, if closely controlled, sheep and goats could surely do much of the mowing.    

An aspect often overlooked is a CW’s educational potential – student groups from all academic levels are infectiously thrilled on CW tours. Municipalities that have CWs report a keen community pride in them, and many double as bird and wildlife sanctuaries. They offer many recreational possibilities as well as being extremely pretty; used in housing schemes, the SCs could be incorporated into public open spaces as hard-working ornamental components. With CWs, wastewater treatment can come out of hiding and be admired, a major shift from our conventional out-of-sight-out-of-mind attitude.    

We have here the option of a positive and eco-friendly way forward in dealing with a situation that needs urgent attention. The private sector, where it has taken the option, is convinced. Several of the schemes now underway represent repeat business from those who, in a manner of speaking, took the first plunge. Commercial companies are usually more agile in their approach to problems than monolithic public institutions, where inertia and the status quo too often substitute for dynamic, informed decision-making.     

CWs are better suited to the tropics than to the temperate latitudes where they were evolved. They can, and should be, replicated all over tropical Africa in a wide variety of applications. New problems call for new and creative solutions, and others would do well to follow the private sector’s trailblazing. 


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