Anaerobic Municipal Wastewater Treatment: Comparison and Assessment of Different Design Approaches for UASB-Reactors

MA-Thesis / Master von Felipe Teixeira de Carvalho

Introduction:

It is well known that freshwater is finite and an indispensable resource for any living organism on Earth. Inappropriately, during the last decades, anthropogenic activities expansion, in parallel with population growth, has been the main cause of the deterioration of water quality.

According to UNESCO the world’s population is growing nearby 80 million people each year, which suggests an increasing of freshwater demand of about 64 billion m³ a year. Likewise, the demographic estimations indicate that 90% of the 3 billion people, who are expected to be added to the world population in 2050, will be living in developing countries, mainly in regions that are already by this time in water stress.

However, in order to relate the increasing demand for water, not only the demographic aspect should be taken into account but also economic and social aspects must be considered. The economic expansion affects water since there is an increase in the number of consumers as well as modifications in their consumption habits, in a way that services are offered, goods are produced and transported. The social aspect points out to individual rather than collective actions mainly considering poverty, education, culture, lifestyle and consumption patterns. Obviously the demand and the importance for satisfactory sanitation conditions become indispensable. The World Health Organization (WHO) and The United Nations Children's Fund (UNICEF) report that 2.5 billion people still have a lack of access to improved sanitation, including 1.2 billion people who have no facilities at all. While in developed areas the sanitation coverage achieves 99%, in developing regions this number is around 53%. Furthermore, in Latin America and the Caribbean the coverage sanitation is approximately 79%. In Brazil, target area of this study, only 55.2% of the municipalities are covered by a sewage collection system.

In this manner, coverage sanitation does not mean necessarily that the wastewater is treated. Hence, the wastewater must be followed by a treatment system (removal of physical, chemical and biological compounds) in order to achieve pollution mitigation targets for the environmental quality and human health and welfare. According to UNESCO more than 80% of the domestic wastewater in developing countries is discharged untreated, polluting rivers, lakes and coastal areas. Therefore, a large number of technologies have been developed with the intention of achieving those standards.

In this scenario, UNESCO specifies that the high investment costs for wastewater treatment have been used as a justification for the developing countries. In this way, many literature publications as von Sperling and Chernicharo and Grau show a variety of available processes for wastewater treatment, allowing the selection of the most adequate solution, considering the technical and economic aspect.

Hence, the anaerobic treatment technology emerges as a great alternative due to its low cost of implementation and operation, minimal mechanisation and sustainability of the system as a whole. In addition, as developing countries are mainly situated between the tropics, where the climate is warm most of the time, the implementation of anaerobic systems is favoured, especially the usage of the upflow anaerobic sludge blanket (UASB) reactor. Consequently, the UASB reactor has been spread to many countries in Latin America as: Brazil, Mexico, Colombia, Cuba, Uruguay.

In recent years, with the increasing demands on wastewater treatment, some dimensioning procedures have become an important tool. The application of those approaches in wastewater issues tries to explain how the involved processes, as microbiology and biochemistry, work out with the intention of designing a tool for process understanding and optimisation. Furthermore, those procedures stand for reducing extensive and complex experimental data, observing the correlations between the performance of the plant and its main design and operating variables.

At the Institute of Sanitary Engineering and Waste Management (Institut für Siedlungswasserwirtschaft und Abfalltechnik - ISAH), Leibniz University of Hannover (Germany), extensive experiments with domestic wastewater and the usage of the UASB technology in lab and pilot scale were carried out. Consequently, those experiments together with the collected data were the basis for a developed (empirical) dimensioning approach of this type of reactor.

Therefore, the aim of this study is to compare this developed design approach to another existent approach and to assess the applicability and limitations of this approach in Brazil, since if compared to Germany, warm countries normally result in higher temperature inside the reactor, thus higher activity of biomass. Additionally, the following subtasks are to be considered:

Determination of the state-of-the-art of science and technology regarding anaerobic municipal wastewater treatment with UASB reactors in Brazil, Compilation of data about operation characteristics of existing municipal UASB reactors in Brazil, Application of the design approach provided by ISAH and the presented design approaches in subtask a using the collected data in subtask b, Discussion of the computed results in subtask c and assessment of the applicability and limitations of the used design approach.

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Chapter 7, Conclusion and recommendations:

The present study attempted to give an overview of the UASB usage in developing tropical countries, for instance Brazil. In contrast to its application in Europe, it was demonstrated that the UASB technology in Brazil is mostly applied to treating domestic wastewater.

Additionally, it was shown throughout literature research that the UASB technology in Brazil is capable of achieving extraordinary rates of COD removal, with mean of 68% and maximum of more than 90%, which is considered extremely high.

In order to dimension an UASB reactor, firstly the von Sperling and Chernicharo approach was introduced and then, the main focus of this study, the approach designed by Urban at ISAH in the Leibniz University of Hannover (Germany). In such manner, it was found out that the central difference between both approaches is related to the loading rate. While in the von Sperling and Chernicharo approach the hydraulics of the reactor are more substantial, in the Urban approach the entire calculation is ruled by the SLR.

Furthermore, von Sperling and Chernicharo call attention to the point that if the system is fed with low-strength wastewaters, such as domestic wastewater, the hydraulic aspect becomes more significant than the organic load itself; however, this statement requires deeper investigation. In von Sperling and Chernicharo the concentration of COD in the outflow of the reactor is only estimated as a function of the HRT whereas in Urban the hydrolysis, the biomass growth and the methane formation are also considered.

The next step was to investigate the Urban approach with Brazilian data obtained from literature. In this manner, the Urban approach showed serious limitation. The characteristics of the CODhom removal registered by the UASB reactors in Brazil, when applied as cleaning target to the Urban approach, proved to be incompatible with the procedure. This divergence was detected in the SLR calculation, which is estimated as a function of temperature and CODhom removal (cleaning target). Thus, if the SLR assessment, which is the central point of the procedure, is miscarried, all the other steps will be also miscalculated.

Moreover, one reason for this incompatibility might be the method used to establish the Urban approach, which was shaped fundamentally under empirical conditions, using only one source of wastewater. Therefore, it is improbable that the system holds similar comportment in any circumstance. It is a challenge to make such generalization of the process, since wastewater has several compounds and parameters that might affect the anaerobic process, for instance microbiology, environmental facts or even the reactor construction aspects.

Another cause for the mismatch between the Urban approach and the Brazilian values might consist in the presence of a mechanical treatment before the UASB reactor. Urban used wastewater mechanically pre-treated through screen, grit chamber and primary clarifier. However, in the Brazilian literature values were used only screen and grit chamber.

The methodology adopted in this study has to be considered as well for the inconclusive results in the analysis of the Urban approach. Since the data were acquired throughout literature research some doubts concerning the reliability of the data might arise.

Perhaps, the techniques used in order to measure the COD might be in discordance as well. In Germany, as applied by Urban, the COD is measured following the norm DIN 4045. In Brazil, the technical norm NBR 10357 is used for that purpose, or even, many studies base on the Standard Methods for the Examination of Water and Wastewater (Standard Methods Committee). However, a comparison between these methods is not included in the scope of this study. In this manner, the incompatibility between the Urban approach and the Brazilian rates of CODhom removal is in a such way unclear.

To put it in a nutshell, the incompatibility between the Urban approach and the literature values as well the methodology adopted in this study were more or less responsible for the inconclusive results. Perhaps, a more accurate way in order to obtain the data would be through direct measurements, for instance to monitor one or several WWTPs during a long period of time. Additionally, with the intention of improving the Urban approach and trying to establish a procedure which could be wider applied, the finest way would be also to monitor several WWTPs in distinct parts of the globe, during a period of one or two years.