Mohamed Rabie Mohamed Ghazy
Sustainable Sewage Sludge Management in egypt Based on Life Cycle Assessment
Abstract
All over the world, the need to develop and apply more sustainable sewage sludge handling systems is growing. These systems must focus primarily on the recovery of valuable compounds in sludge and at the same time must ensure the elimination of hazardous substances. Egypt is also confronted with this problem and the issue is even more critical due to the rapid increase in sludge quantities on account of the continuously growing population with a concomitant increase in the number of wastewater treatment plants (WWTPs). So far, little attention has been given to the produced sludge in practice. However, the generated sludge needs to be treated adequately and managed in an economically and environmentally acceptable way to prevent the negative impacts of its application or disposal. The main objectives of this study are to evaluate the current situation of sewage sludge management in Egypt and to determine the costs as well as the area of application of sludge treatment options in reference to Egyptian conditions. Selected treatment processes and scenarios were evaluated using the Life Cycle Assessment (LCA) methodology. A general concept for sewage sludge management was developed in order to support decision making for treatment, disposal or recycling options of sewage sludge in Egypt.
Since the availability of data on sewage sludge management in Egypt is limited, site visits and field studies were carried out at the main Egyptian WWTPs. Additionally a data survey was conducted using many sources such as administrative organizations, research institutions as well as the WWTPs themselves. Data were collected from WWTP managers by questionnaires during the field studies. Furthermore, samples of sewage sludge from a number of WWTPs were collected and analyzed intending to represent relevant sludge treatment options used in Egypt. The collected data were used to present a brief overview of the existing WWTPs, quantities of sludge production and characteristics. The characteristics of sewage sludge were discussed taking into regard the results of own sampling analysis as well as previous studies. Options for the beneficial use of sewage sludge, agronomic value of sludge and potential market demand under Egyptian conditions were also evaluated. The alternative treatment options for sewage sludge that have recently been developed in Egypt were outlined. To develop a conceptual model for sustainable sewage sludge management in Egypt, five alternative scenarios for sludge treatment and disposal were evaluated economically and environmentally. The scenarios were set up to develop different options of sludge treatment schemes that could be applied in Egypt based on the results of the evaluation of the current situation. These scenarios should be able to produce sludge that is inherently safe for the unrestricted use in agriculture.
An environmental impact assessment of the selected scenarios was conducted using LCA methodology, which is largely in accordance with the ISO 14040/14044 standards. In LCA, all resources used as well as pollutant emissions associated with the life cycle of each sludge treatment option were considered including extraction and processing of raw materials, manufacturing of chemicals, operation, transportation, recycling and final disposal. Life cycle inventory data for each treatment process including parameters describing energy and raw materials used, emissions to air, water and soil as well as waste generation were prepared. The data were compiled from real site investigations, simulation results, literature and qualified assumptions. All values regarding the consumption or production of matter and energy for each scenario were calculated using energy and material mass balances. The flows were normalized to a functional unit (FU) defined as the handling of one ton of dry solids. All environmentally relevant data that were assessed in the inventory analysis phase were evaluated in a life cycle impact assessment phase (problem-orientated approach) based on their potential contribution to ten environmental impact categories. The results of the environmental characterization indicators were aggregated and weighted using two weighting methods into an overall environmental score to facilitate comparison across the studied scenarios. An economic assessment of the selected scenarios for WWTPs with capacities of 5,000, 10,000, 20,000 and 50,000 m3/day were carried out based on the data of the operation and maintenance requirements and costs which were gathered from the main WWTPs in Egypt. The economic evaluation is based on current construction costs and unit costs for energy, labor and land in the Egyptian market.
The results showed that the reuse of sewage sludge in agriculture under the Egyptian conditions offers the most sustainable and economical route for sludge disposal because Egyptian farmers are prepared to pay for any source of organic fertilizers. A potentially high demand for treated sewage sludge exists in the Egyptian market. The current dry sludge production is estimated at 0.87 million tons dry solids per year, which represents only about 5 % of the requirements of reclaimed desert land. The treated sewage sludge has an economic potential of 19 million US$ per year, which is more than 40 % of the current annual operation and maintenance costs of Egyptian WWTPs.
The difference in total costs of sludge treatment processes in the Egyptian market compared to the international market can be explained largely by the significant difference in annual operation and maintenance costs resulting from low wages and energy prices. The application of anaerobic digestion technology in Egypt was more cost effective for WWTPs with capacities greater than 40,000 Population Equivalent (PE), whilst the mechanical dewatering processes were more cost effective for WWTP capacities greater than 210,000-270,000 PE. The economic evaluation of the studied scenarios revealed that the fifth scenario (aerobic digestion, natural dewatering, composting) was the best choice for WWTP capacities up to 5,000 m3/day, whereas, the second scenario (anaerobic digestion, natural dewatering, composting) was more suitable for WWTP capacities from 8,000 to 20,000 m3/day. Moreover, the forth scenario (anaerobic digestion, mechanical dewatering, composting) was more affordable for WWTP capacities greater than 20,000 m3/day and the first scenario (anaerobic digestion, combined sewage storage lagoons and air-drying) for WWTP capacities greater than 50,000 m3/day.
The results of the environmental assessment showed that the application of aerobic digestion for sewage sludge stabilization in Egypt caused higher negative impacts for all environmental categories, while the application of anaerobic digestion resulted in an improvement in all categories. The application of windrow composting using rice straw as a bulking agent led to an improvement in several environmental categories as well. The final environmental ranking of the different scenarios revealed the first scenario to be the environmentally preferable one, while the third scenario turned out to be the worst. From the results of the economic and environmental assessment a comprehensive approach for sewage sludge management in Egypt was developed.