The water
purification marsh at Aröd 2:226: function and progress
Vattenreningskärret i Aröd 2:226; funktion och utveckling
1. English summary
Scientific summary: - In the autumn of 1990 through the spring of 1991, a water purification marsh was built on ½ hectare of arable land of a farm on the Swedish West Coast. The aim was to find an ecological and relatively inexpensive way to reduce the nutrient leaching from sewage water and manure from the barn as well as from the domestic wastewater. Today, the transport of organic material to estuaries is reduced and forms no problems. However, soluble nutrients are brought to watercourses and to the sea from wastewater purification plants and from sub-surface sand filters (Fig. 14). Organic material is consumed by benthic invertebrates in the estuaries and is thus transformed to fish and waterfowl food. But, large amounts of soluble nutrients, especially of nitrogen, cause an overgrowth of phytoplankton and see weeds, which bring on oxygen deficiency and dead bottoms in the sea. The primary aim of this project has been to put available knowledge in practice concerning the effect of nutrients on the sea, the functions of wetlands, the creation of an ecosystem, and the failure of traditional water purification methods. This preliminary report gives the results obtained so far.
The design of the water purification marsh is founded on some ecological principles. Shallow wetlands with effluents belong to the most productive ecosystems on earth, because light, heath, water and nutrients occur in optimal proportions, and since the first three factors are constant, increased supply of nutrients of right composition may increase production of plant biomass and, consequently, reduce the concentration of nutrients.
The concept of optimum sustained yield implies that the production of an organism is reduced by exploitation to about half of its carrying capacity, and a high yield of primary production is thus enhanced through grazing animals of both invertebrates and vertebrates which – together with theirs predators – are allowed to form food chains and food webs which, however, tend to be relatively simple but effective in early stages and especially in temporary waters.
Since production is highest in early successions, management efforts in a water purification marsh should favour the maintenance of short plants like algae, grasses and annuals that appear during the first growth season, thereby preventing mature or climax stages to develop, which reduce light and heat in the shallow water.
The above-ground ecosystem allowed in a water purification marsh imports energy circuits of both a grazing circuit and an organic detritus circuit (Fig. 13, bottom left), since both decomposition of dead materials of organic origin (Org.) and consumption of living plants or plant parts produced from released inorganic nutrients (N + P) are utilised in the food chains. In the traditional wastewater purifying plants and subsurface sand filters, the food chains usually stop after bacteria, fungi and protozoa, and released nutrients are not transformed to algae until they reach the sea (Fig. 13, bottom right).
In a water purification marsh, there is also a water circuit (Fig. 13, top, left). Increasing sun light and heat favour evaporation, which in turn increase nutrient concentration in the remaining water and, consequently, increase plant growth. Some of this water returns to the ground as dew, a circulation that is favourable to the local climate, especially during long and dry periods. In winter, formation of ice also concentrates the nutrients, which favours algae growth and nutrient reduction. Nutrient concentration from both evaporation and ice formation are favoured by a large water surface in relation to water volume, which imports another reason to create a very shallow marsh. Moreover, in slowly floating shallow water, dissolved oxygen is easily available from both the atmosphere and from photosynthetic sources. In subsurface sand filters, both domestic wastewater and ground water are rapidly brought to the sea in dark and cold underground culverts (Fig. 13, top, right).
The marsh is built through the construction of 10 levees on a sloping clay field (Fig. 1) at vertical intervals of 0,18 m, which implies that the first levee lies 1,62 m above the 10th. The water is slowly floating through a tube or over a threshold to the next levee, and the total length the water has to pass is about 690 m. however, at reduced flow of surface water during dry periods, the lower levees may dry out, but the upper levees are permanently flooded from domestic wastewater. In summer, the flow of sewage water from the barn is low.
So far, most measurements of soluble-nutrient reduction in the water purification marsh are performed of specific conductance (µS/cm), since the occurrence of soluble nutrients is of highest interest (Fig. 2, 4-7). These measurements, together with some measurements (Table 3) of phosphorus (PO4- -P and total-P) and nitrogen (NH4+ -N, NO3- -N, organic N and total-N), indicate that the rate of nutrient reduction is high when the nutrient concentration is high, and these conditions usually predominate in the first levees, where algae growth is most stable (i.e. Fig. 5: A on 21 November 1990).
Comparisons are made between the water purification marsh and sub-surface sand filters, which are claimed by the local authorities to counteract overfertilization in water courses and in the marine environment. The results (Table 4) indicate that the difference in conductivity between affluent (in) and effluent (out) water is insignificant during dry periods, when the water is not diluted by surface water or subsoil water. The expensive sand filters may reduce the organic content to some degree but the nitrogen content that is most dangerous to the marine environment is hardly affected.