WASTEWATER TREATMENT AND MICROBIOLOGIC HYGIENIZATION WITH CONSTRUCTED WETLANDS AND REED BEDS
DAESCHLEIN G., BELOW H., KRAMER A.
Institut für Hygiene der Ernst-Moritz-Arndt-Universität Greifswald
Abstract:
Exploiting the attenuation effects discovered in the waters of natural wetlands, the implementation of constructed wetlands has been developed for different kinds of wastewater treatment applications in the last fifty years. It has been shown that, depending on the constructive system, bacterial as well as chemical load can be eliminated or reduced to obtain water for at least irrigation purpose. The total bacterial load as measured in colony forming units per ml (CFU/ 100ml) can be reduced by 4-5 logsteps, the indicator parameter bacteria E. coli, coliforms and fecal enterococci by up to 4 and more log steps. The conditions of the local soil environment (pedosphere) as well of the root environment (rhizosphere) determine the interactions between host and graft bacteria as well as bacteria and rhizosphere metabolism and consequently the microbiological outflow and hygienic benefit of the plant. The killing of the bacteria is induced following the time-dependent adsorption to colloidal soil particles and thin water films. Other effects include sedimentation and toxic root excretions. The sufficient clearence of many facultative bacterial pathogens during the passage of the wetland plants nowadays is obvious, however the faith of some pathogens of particular hygienic importance like salmonella, vibrio cholerae and pathogenic protozoae as well as of multiresistant clones and their corresponding genes in the sustained natural attenuation process has to be clarified.Because of the effectiveness of the hygienization and sanitation of wastewater with high rates for the reduction of nitrogen and phosphorus as well as of pollutant cyanides, AOX, pentachlorphenol and heavy metals together with their low costs for investment and operations, constructed wetlands and in particular reed beds are the favored technology for decentralized wastewater treatment for the countryside, besides creating additional wet areas and protecting groundwater ressources.
I. INTRODUCTION
Worldwide more and more wastewater is produced by the more and less developed countries. In contrary to the less developed countries, the industrialized countries of the "first world" can invest in large wastewater treatment technology to gain a sufficient amount of recycled water of standardized quality. Nevertheless, these countries still have problems with punctual toxic industrial wastewater inflow and try to reduce it by interventions "at the source", whereas low income countries have to overcome problems with general intoxication of the water supplies, e.g. rivers, lakes, wetlands, lacking the investments for industrial renovelations and supportive "avoiding-strategies".
Developed countries use large wastewater treatment plants, particularly in urban environments and different decentralized treatment technologies in the countryside. The less developed countries also use conventional treatment plants in the city areas but often lack any treatment for wastewater in the countryside. Consequently in these regions hygienic water supply is rare, for drinking water as for use in agriculture and polluted and hygienically critical wastewater often directly is used for irrigation purpose. Enteral infections and chronical intoxications influencing communal health are direct consequences of the overall contamination of the water.
In the near future basic solutions of these questions are unexpectable, therefore alternatives for main deficitary treatment practice are imperative now. As in low income countries investments also for water engineering are low, it is useful to plan decentralized low investigative instead of conventional centralized high level treatment plants in the countryside. For these countries and especially in regions with arid zones, constructed wetlands and reed beds are the method of choice for rural and decentralized wastewater treatment with low investigative demands and less dependence on foreign specialized know-how. The aim of this decentralized alternative wastewater treatment consists in long time elimination power for bacterial as for organic and anorganic burden. The water has to be sufficiently cleared to obtain irrigation quality for agricultural purpose (reduction but not elimination of nitrogen and phosphorus intended).
II. HISTORY AND PRESENT OF CONSTRUCTED WETLANDS AND REED BEDS
Plants with cemented large scale ponds and biological and chemical degradation technique are constructed since abaout 100 years and are mainstream technique in centralized wastewater technology worldwide. However since the fifties of the past century also in the more developed countries the need for decentralized and less costly techniques led to investigations with alternative techniques taking in account the attenuating effects of natural treatments in constructed wetlands. The German investigators Seidel, Kickuth, B
örner and Löffler were the pioneers in the fields with the first scientific descriptions of cleaning proceedings of effective pilot reed bed plants. Following these years a great variety of constructed wetlands and reed beds were engineered all over the world and the principles of effective natural wastewater treatment scientifically accepted. More and more data upon elimination of organic and anorganic burden became available, the recent years underline the potencies in binding considerable amounts of water ans soil pollutants like heavy metals, PCB, AOX and cyanides. Concerning bacteriological data of a variety of pathogenic and facultative pathogenic bacteria scientific evidence of efficiency has still to be proved but, concerning the elimination of bacterial load of fecal human and animal origin remains undisputed.Elimination potency of reed beds in wastewater
1. Organic burden
Reed beds are highly capable to eliminate nitrogen and phosphate. The main elimination pathway for nitrogen is nitrification followed by denitrification. Both activities are oxygen-dependent. A potent oxygenisation considerably supports the elimination of nitrogen, what can be achieved by intermittent water supply and the use coupled beds. A reconduction of passed water to the inflow can ameliorate the denitrification. Bacterial metabolism in the rhizosphere is crucial for substantial nitrogen elimination. The position of the horizontal filter (better denitrification) bed after the vertical one (better nitrification) also enhances nitrogen reduction. Nitrification and denitrification take place simultaneously.
The elimination of phosphorus by soil-binding as by incorporation in phytogene and bacterial biomass also works oxygen-dependent and can reach a reduction of 400 – 1000 mg P/ m2a day.
2. Anorganic burden and pollutants
Anorganic burden as potassium, calcium and (to a less extend) sodium effectively can be eliminated in reed beds, the ions partly were utilized as growth factors in the plants.
Heavy metals like cadmium and lead were insolubilized as sulphides or hydroxides in large amounts pH-dependent in the soil and finally deposited in the rhizosphere and partially in the above aerial parts of the plants. This activity (phytoremediation) can be used to sanitize soil and wastewater from large quantities of heavy metals over long periods (years).
Besides heavy metals important amounts of AOX, PCP and cyanides can be eliminated with reed beds with increasing potencies by selecting adapted plants like Juncus effusus (eg. for elimination of PCP).
3. Microbiologic burden
The following mecanisms are important for the elimination of bacterial load in reed beds.
The mean elimination capacity of reed beds amounts to 3 to 5 logsteps of bacterial load, occasionally to 6 and more. Best results were achieved combining several beds, a single bed reaches 2-3 logsteps of elimination. Most data concern the reduction of the indicator organisms for fecal contamination, enterococci and coliform bacteria, as well as of total bacterial counts. In the inflow water, bacterial concentrations mostly achieve 106 – 108 CFU/ 100ml, and are reduced generally to 103 CFU/ 100ml.
Also for parasitic cysts of cryptosporidium parvum and gardia lamblia as well as for coliphages that represent enteroviruses in the wastewater, acceptable reduction capacities of 2-3 logsteps could be found in reed beds. Nevertheless, no abundant elimination data exist for many pathogens and facultative pathogens like mycobacteria, vibrio cholerae, yersinia enterocolitica, campylobacter spp., legionella pneumophila nor for the faith of relevant multiresistant bacteria and their corresponding genes like the vanA and mecA gene of Enterococcus faecium (VRE) and Staphylococcus aureus (MRSA).
Reed beds with 2 filterbeds are able to fulfill the requests of the German guidelines for irrigation water and the European guideline for bath water.
The effectiveness to bacterial reduction in reed beds is dependent on
Intended use of constructed wetlands and reed beds for wastewater treatment in
a. More developed countries
1) Decentralized plants in the countryside with agricultural utilization of irrigation water
2) Specialized plants for elimination of pollutants, eg. heavy metals, AOX. PCP cyanides
3) Outstanding residential estates without access to sewerage systems
4) Wastewater treatment of grounds and parks for irrigation water supply (instead of loss of water in centralized plants)
b.Less developed countries
Construction and types (Reed beds, Fig.1)
State of the art are multi-stage filter beds planted with reeds (Phragmites, Arundo, Typha, Juncus) with combining consecutive horizontal and vertical filter beds.
In Germany the key for filtersize dimension is 5 m2/ inhabitant for horizontal beds and 2,5 m2/inhabitant for vertical beds.
III. SAFETY AND RISKS AND SURVEILLANCE
Sensible point of the reed beds is the filter function of the pedosphere in the first 20 cm of the entry of wastewater (inflow). Blocking of the filter area (Colmation) may arrive as a consequence of massive stemming, massive hydraulic burden (no inflow intervalls), heavy organic load and in case of freezing of the beds. High temperatures of the water inflow up to 40°C do not diminish the reduction power.
For surveillance regular checks of the bacterial load of the inflow and outflows including total bacterial counts and amount of coliform bacteria (CFU/100ml) should be undertaken monthly. Together with repeated and regularly visual checks of the plants, the monitoring of the bacterial reduction power can detect trends and initiate precautions and interventions.
Heavy precepitations can lead to inflow dilution in the beds with consecutive decreasing of the reduction power.
Summary
Reed beds as effective and wastewater treatment plants are well suited for hygienization of domestic and industrial (little and medium-sized industries) wastewater in less as more developed countries. Plants with 2 or more coupled horizontal and vertical filter beds for 15 to 500 inhabitants are state of the art. Wastewater treatment combined with irrigation water supply is a well suited application for agricultural regions and especially arid zones in the countryside. A special application is sanitation of contaminated soils and wastewaters with heavy metals, AOX, PCPand cyanides.
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