The potential of eco-technological
wastewater treatment for improvement of the drinking water quality of Matagalpa, Nicaragua
Resumen y recomendaciones en Espagnol: El potencial del tratamiento eco-tecnológico de desagües para el mejoramiento de la calidad del agua potable de Matagalpa, Nicaragua
Joost Jacobi and Adriaan Mels (Lettinga Associates Foundation, Wageningen University)
download the pdf file of the M.Sc. Thesis of Joost Jacobi (740 kb)
Background of the investigation
Decreasing water availability and pollution of surface waters is a subject of major global concern. The situation in the middle-sized city Matagalpa (an estimated 160,000 inhabitants) in Nicaragua is a good example. This city is situated in the vulnerable watersheds of the Molino Norte and San Francisco. The basins have surface areas of 22.3 and 30.0 km² (a total of 5.230 ha) and have longitudes of 8 and 7 km respectively. An important part of the drinking water supply of the city Matagalpa is coming from these rivers of which is said that discharges have decreased in the last ten years. With a rapidly increasing population of the city of approximately 4% per year, the water demand is steadily rising and it is increasingly difficult to respond with an adequate drinking water supply.
Moreover, the river quality is a problem due to various activities in the mountainous upstream area of the watersheds. Coffee plantations (fincas) located of the cuencas Molino Norte and San Francisco catch most of their process water from the water streams for processing the coffee beans. After this process, polluted water is running towards the city Matagalpa. Also water intensive fern-nurseries are sources of pollution because of the various chemicals that are used in fern cultivation. Other pollution sources are animal and human excrements that, when washed down to the rivers, contribute to increased concentrations of coliform bacteria in the water and posing potential health risks.
Research objectives and approach
The objective of this investigation was to gain more knowledge about the past, present and future situations of the two watersheds with respect to water use and pollution. Through fieldwork, interviews with local stakeholders and literature research the water situation with respect to water quantity and quality and the driving forces influencing the water use activities was investigated. A second research activity was to explore the potential of implementing eco-engineered treatment technologies to treat coffee wastewater before discharge into the rivers.
Quantity and quality of the water sheds
Although there is a lack of figures, most stakeholders in the region acknowledge that the river discharges of Molino Norte and San Francisco are decreasing. Since 2003, extra water is pumped from a valley situated 20 kilometers southeast of the city. In the dry season (December until April), when the river discharges are rather low and the drinking water requirement of the city is higher than average, extra water is pumped from a nearby watershed (river Aranjuez) into the river Molino Norte.
Coffee production in the area is of great economic importance. In the two watersheds 15 large fincas are responsible for more than 90% of the total coffee production. After harvesting of the coffee berries, river water is used for the wet process, which takes place at the farm. After de-pulping the coffee berries, the coffee bean undergoes a fermentation process and is being washed afterwards. The (polluting) by-products of the process are: coffee pulp, mucilage (slimy layer surrounding the beans) and the wash water. The pH of the coffee wastewater drops to 4-5 and the concentration of organic material (COD) is very high (5-8 g/l). When stored the pH is further decreasing due to acidification. Coffee production is a seasonal activity (January April) and takes place at the end of the dry season in a period when the river discharges are at their lowest.
Besides the coffee wastewater, human and animal excrements are affecting the quality of the river water and result in pollution with human pathogens. During the coffee harvest season, labourers come to the rural areas and live at the various fincas. The rural population in the two watersheds is then increased by fifty percent. Although latrine-projects have been set up, most excrements end up in the fields. During the monsoon, the drinking water company reports high concentrations of coliform bacteria and suspended solids at the beginning of the rainy season, when heavy rains wash away soils, including the excrements.
When extra water is pumped from Aranjuez into the river Molino Norte, another source of pollution if affecting the river quality. Nearby the pumping station, water intensive fern-nurseries are located. The ferns are exported and are an attractive business from an economic point of view. Besides the high water demand of the ferns, a lot of pesticides are used. This year the quality of the water was poor and MARENA (ministry of environment and natural resources) doubted whether to pump this water from Aranjuez into the river Molino Norte.
Current measures for pollution prevention at coffee farmers
At a number of coffee farms, preventive actions have been taken to reduce the content of COD of the wastewater. Coffee pulp is no longer dumped in the river but turned into organic fertilizer after composting. At most farms, pilas (infiltration pits) can be found in which the wastewater can infiltrate and evaporate. As valuable land is needed for the installation of such pilas, they often have insufficient capacity. Moreover, due to the soil properties, the infiltration capacity is limited. At 8 larger farms anaerobic bioreactors (UASB) are installed to reduce the COD content of the wastewater of which 6 are in operation. There is one example at coffee farm La Hammonia where a UASB is used in combination with aerobic treatment and the use of a bio-filter (a pond with water plants) and which treated water is finally used for (sprinkler) irrigation.
Calculations on the COD load of the coffee wastewater to the rivers were made with data found in literature and up to date coffee production figures. Based on data of the river discharges during the dry season and the coffee production data it is estimated that coffee wastewater constitutes up to 8-10% of the total river water. Based on mass balances is can be shown that the wastewater load of coffee farms results in very high COD loads to the river, despite the current pollution prevention measures that have already been taken . The self-purification capacity of the rivers has not been taken into account in this calculation.
Considering the COD loads it is very unlikely that the (physical-chemical) drinking water treatment plant that is used to treat the river water before supply to the city can completely remove the organic pollution that is discharged during the coffee campaign (not taking into account the high loads of nutrients that are discharged). The organic material that remains in the drinking water forms a hygienic risk because of growth of bacteria in the piping network. Chlorine is used to minimize the risk but may result in the formation of chlorinated hydrocarbons.
The decreased drinking water quality during the coffee season is confirmed by a survey among inhabitants of Matagalpa. Frequent complaints about the bad smell and odour of the drinking water during the coffee season and irritated skin after washing were encountered. Many people add extra chlorine at home or use some kind of filtration technique to minimize this nuisance. Those who can afford it buy bottled water.
During the last years the legislation regarding coffee wastewater discharge has sharpened. Legislation set by MARENA requires treatment upto < 200 mg COD/l. The implementation of more adequate treatment facilities is however problematic, because most cases farmers lack finances to invest in treatment systems because of the decreased world market prices for coffee. The prices have decreased by more than 50% compared to the price level of 1999. Another bottleneck is that MARENA lacks the capacity to do sufficient controls.
Wastewater treatment by wetlands?
A part of the investigation focused on the question whether eco-engineered wetland systems can form a (partial) solution of the above-described water quality problem. The background of this idea forms the Waterharmonica project, a Dutch research and implementation programme sponsored by the Water Boards into the implementation of wetland systems as an integral part of wastewater treatment. The basic idea is that eco-engineered treatment systems may form an important link between a basic wastewater treatment and safe discharge in surface water and reuse.
The conclusion is that wetlands may form an interesting option for the improvement of the river water quality and thus the drinking water quality. In the case of decentralised operating systems at the coffee farms, implementing (more) treatment systems at the 15 larger coffee plantations will be most efficient. Especially implementing treatment at the five or six coffee farms that are located near the Molino Norte is of major interest, because of its importance in the drinking water supply (main source). The advantages of wetland systems are the relatively low capital investment and low maintenance. A disadvantage might form the land requirement. Pre-treatment with a bioreactor will be necessary, considering the very high COD and the low pH of the wastewater water. The bioreactors can be followed by eco-technological systems in the form of e.g. constructed wetlands or aquatic plant treatment. Wood or reed production in or after the wetland can make these systems more attractive and integrate them with other agricultural activities.
Another option is a more centralised system in the riverbanks before the drinking water intake or before the drinking water treatment. In this case constructed wetlands will form a treatment and buffer zone as well as for temporary water storage and possibly nature development. The latter is desirable as the river discharges strongly fluctuate during the year and a sufficient drinking water supply is in danger in the dry season. Well-managed plantations of suitable tree species can contribute to the increase of water holding capacity and the prevention of erosion.
This study was done in close cooperation with Projecto Cuencas Matagalpa and financed by STOWA, (dutch foundation for applied water research). We thank Edgard de Léon of PCM and the people of NOVIB for facilitating the investigation in Nicaragua. We thank the members of the supervising committee Theo Claassen, Sjef Ernes, Wim van der Hulst, Ruud Kampf, Bert Palsma, Gerard Rijs and Ton Schomaker, for their valuable inputs.
Figure: Location of the 15 major coffee plantations in the watersheds Molino Norte and San Francisco
These are technical recommendations to Projecto Cuencas Matagalpa, NOVIB and Aqua for All based on this study:
Despite the preventive measures that have already been taken it is clear that coffee production is still a major river polluter during the dry season and is seriously affecting the ultimate drinking water of the water that is withdrawn from the watersheds. The government has clear-cut goals to decrease the pollution load by the coffee farmers.
Reduction of pollution can be achieved by the introduction or further improvement of the wastewater treatment of the 15 larger coffee farmers. Especially implementing treatment at the five or six coffee farms that are located near the Molino Norte is of major interest, because of its importance in the drinking water supply (main source). These are four technical ways to reduce the pollution:
1. Introduction of decentralised constructed wetlands systems at the larger farmers: the advantages are the relatively low capital investment and low maintenance. A disadvantage might form the land requirement. Pre-treatment with a bioreactor such as UASB will be necessary, considering the very high COD and the low pH of the wastewater water. The bioreactors can be followed by eco-technological systems in the form of e.g. constructed wetlands or aquatic plant treatment. Wood or reed production in or after the constructed wetland can make these systems economically more attractive, integrate them with other agricultural activities and can contribute to reforestation.
2. Introduction of a centralised treatment. This could be done by connecting the larger farmers (especially the ones near Moline Norte) to a piping network and have centralised wastewater treatment in a combination of a UASB bioreactor and constructed wetlands. The advantage farmers need less operational and management capacity to treat the wastewater.
3. Treating of the river water in an constructed wetland before the drinking water intake
4. Improvement of the drinking water treatment plant, e.g. by introduction of an aerated reactor + activated carbon filtration
It is not possible at this moment to assess the most cost-effective solution. However solution 1 and 2 are the ones preferable from the point of pollution prevention .
Valuation of water
A major bottleneck is the low world market price for coffee; most of the farmers lack investment potential to fund their own treatment facilities. To our opinion is it interesting to develop of different financing mechanisms to support the farmers. The wastewater of coffee farmers is affecting the drinking water quality. The inhabitants of Matagalpa pay for the drinking water. Further improvement of the quality by centralised measures at the drinking water treatment plant are probably more costly than pollution prevention measures at the farmers (decentralised treatment or a piping systems). Why not use part of the drinking water fees to help the farmers to finance their treatment?
Change of production
Due to the low coffee prices 60% of the farmers (especially the smaller ones) consider changing their activities. What might be needed for these farmers is a programme addressing alternatives / challenging changes into other forms of agricultural production.
Other point of interest
The Mayor of Matagalpa has asked through Joost Jacobi if the wastewater treatment system of the city of Matagalpa can be improved. Our suggestions:
· The installation of a UASB before the current stabilization ponds would improve the capacity of the ponds by a factor 2.
· The stabilization ponds can be turned into constructed wetlands in which biomass like reed or trees are grown. This would be another source of fuel for the area.
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