Scientists of the research group ‘Microbiology and Ambiental Techniques (MITA)’ of the Universidad de Granada have studied the application of sludges generated by urban wastewater-treatment systems together with the application of the compost generated by urban solid-waste treatment systems in the replanting of highway slopes. The head of the study, Francisco Osorio Robles, a professor at the Department of Civil Engineering, explains that: “This is a new practice which reduces the environmental impact of road building and provides a solution for the problematic effects of wastes on our society.”
According to data published by the National Plan for Treatment Plant Sludges, around 800,000 tonnes of sludges were generated in Spain in 1997, and an estimated 1.5 million tonnes were expected for the end of 2005. According to this researcher, this amount will increase in the forthcoming years because of the growing number of wastewater-treatment plants built during recent years as well as those under construction, in compliance with the Council Directive 91/27/EEC of 21 May 1991. This regulation, concerning urban-wastewater treatment, establishes a time-table for the obligatory provision of collection and treatment systems for urban wastewater in accordance with the population. The final deadline was the end of 2005.
Reducing the environmental effects
All the taluses studied were specifically built for the research study in Gádor, Almería (S Spain). The area is arid and soil conditions are unfavourable, as is usually the case of road taluses. The researchers have worked in an area of more than 1000 m2 divided into 32 experimental plots 5 m high each. The dimensions of these plots in addition to the soil characteristics as well as the agricultural and building practices used, have reproduced real working conditions on a road in order to achieve sound and reliable results.
The variables analysed in the research included: the talus’ slope, sludge and compost contents, plant species sown (broom, thyme, esparto grass, and genista), as well as the seeding modality (hydroseeding, at random or manual).
There are few studies that describe the use of sludge as a material for the replanting of road taluses. However, this research shows that using such biosolids for restoring taluses is a way of increasing the plant cover, even by adding small amounts of seeds. Increased plant cover has a direct effect on reducing erosion, thereby decreasing the environmental impact of road building. Based on these results, the researchers suggest using wastewater sludges and compost for restoring road taluses.
Francisco Osorio points out that there are no specific regulations regarding this new application, but that the heavy-metal content and the microbiological parameters of each subproduct used in the study conform to agricultural regulations. In this sense, it poses no threat to public health.
Low cost
All treatments suggested in the study are feasible, both technically and financially. Therefore, the researchers state that this application could be included in future road-building projects. Today, part of the highway-building budget is allocated to restoring road taluses, usually by means of hydroseeding. This procedure costs an estimated 50,000 euros per hectare, but such a value could be reduced to 18,000 euros if low-cost hydroseeding with sludge was used and even to 2750 euros by using a treatment with sludges without seeds.
Dehydrated sludges were used for the study. The dehydration process in wastewater-treatment systems would be unnecessary if liquid sludges were used, lowering costs accordingly. However, transportation costs would increase, but handling would be easier and irrigation costs lower.
New market
Among the potential uses for sludges, the most practical is for fertilizing and for recycling soil nutrients and organic matter. Other less feasible uses include the energy valorisation and incineration of sludge deposits in a rubbish dump. In Spain, agriculture is a deeply rooted occupation, and the use of such products could be a prioritized solution, but only 40% of the sludges are used in agriculture. According to Francisco Osorio, although sludge and compost improve soil properties, beneficial results are noticeable over a longer term than are the effects of conventional inorganic fertilizers.
Reference
Prof. Francisco Osorio Robles.
Department of Civil Engineering
Phone. 958 249 463 Mobile phone. 620 050 142
E-mail: fosorio@ugr.es
Prof. Juan de Oña López.
Department of Civil Engineering
Phone.: 958 249 979
E-mail. jdona@ugr.es
