|Titel||Anaerobe behandeling van huishoudelijk afvalwater in subtropische en gematigde klimaten|
|Looptijd||01 / 1997 - 05 / 2004|
|Leverancier gegevens||METIS Wageningen Universiteit en Researchcentrum|
|The purification of polluted wastewater can be seen as the paradigm of all cleaner technologies. It has a direct effect on the quality of the environment, reducing and sometimes eliminating some negative aspects of development and urbanization. The removal of organic material from wastewater can be done by biological methods using aerobic and/or anaerobic microorganisms. Aerobic microorganisms thrive in the presence of oxygen while the contrary is true for the anaerobic ones. In the anaerobic fermentative process the final products are gases, predominantly methane and carbon dioxide (biogas) (van Haandel and Lettinga, 1994). Some of the advantages of anaerobic treatment are the following:
High efficiency. Good removal efficiency can be achieved in the system even at high loading rates and low temperatures.
Simplicity and flexibility. The construction and operation of the reactor is relatively simple and can easily be applied to either a very large or a very small scale.
Low energy consumption. As far as no heating of the influent is needed to reach the working temperature, and all plant operations can be done by gravity, the energy consumption of the reactor is almost negligible. Moreover, energy is produced during the process in the form of methane.
Low sludge production. Compared to aerobic methods the sludge production is much lower, due to the slow growing rates of anaerobic bacteria. The sludge is well stabilized for final disposal and can be preserved for long periods of time without a significant reduction of activity, allowing its use as inoculum for the start-up of new reactors
Anaerobic treatment of wastewater was used in the first half of the century but the predominance of aerobic methods became overwhelming later (McCarty, 1981; van Haandel and Lettinga, 1994). The introduction in the late 70 s of the upflow anaerobic sludge bed reactor (UASB) (Lettinga et al, 1980) renewed the interest on anaerobic systems. The adequate retention of active sludge within the UASB reactor enables good treatment performance at high organic loading rates. It is often questioned why aerobic treatment of municipal wastewater is not replaced more rapidly by the economically more attractive and the conceptually more holistic direct anaerobic treatment, which is increasingly recognized as the core of a sustainable and appropriate wastewater treatment system for developing countries (Lettinga et al., 1987; Mergaert, 1992; van Buuren, 1996). In regions with a hot climate, the big potentials of anaerobic treatment for raw domestic sewage, notably of the UASB reactor concept were demonstrated (Lettinga et al., 1987; Vieira, 1986; Mergaert, 1992; Schellinkhout, 1994). Several full scale plants are already in operation (Draaijer, 1992; Schellinkhout, 1992; Vieira, 1992; Vieira, 1994). Inoculation with active biomass was shown not to be a prerequisite for start up (Louwe Kooijmans, 1986) and many reactors started up without being inoculated at all, either at pilot-plant scale (Barbosa, 1989), or full scale (Draaijer, 1992). However, there are still some limitations for the application of anaerobic treatment technology to domestic sewage in a wider set of climate conditions. Especially at low temperatures, accumulation of poor biodegradable suspended solids can affect the performance of the system. This situation can cause drops in the sludge methanogenic activity and the hydrolysis rate, reduction of the sludge retention time, deterioration of bacterial aggregates and formation of scum layers, leading to overloading of the reactor. Research on the treatment of low concentrated sewage at low to moderate temperature conditions in one phase UASB systems was made by de Man et al. (1986). At temperatures of 12-18° C, COD removal efficiencies of 45-75 % could be achieved at HRT s of 4-8 hours. However, at lower temperatures the accumulation of suspended solids becomes significant due to very slow hydrolysis. To overcome the problem of accumulation of suspended solids pretreatment of raw sewage for the removal of SS is recommended. Wang (1994) proposed the use of a two-step UASB (or UASB-EGSB) process. The first step is a relatively high loaded UASB system for the removal and hydrolysis of suspended COD. The second step is a normal UASB system designed to convert the dissolved COD produced in the first step to methane gas. Both the physical and the biological processes in the first step need further research. The removal of suspended solids will depend on factors like HRT and upflow velocity, gas release and sludge bed characteristics, but also on the characteristics of the suspended solids (mainly size and density). The particle size is also an important factor in the subsequent hydrolysis of the entrapped particles. Moreover, the hydrolysis rate will depend on the composition of the particles (fraction of lipids, proteins and carbohydrates). To model the physical and biological processes going on in an UASB reactor at different conditions, a comprehensive method for characterization of sewage has to be developed. The key factor determining the ultimate amount of hydrolysis and methanogenesis in an UASB system at certain temperature conditions is the Sludge Retention Time (SRT). Assessment of the relationship between SRT and hydrolysis for different temperature conditions is necessary to decide between one or two phase UASB-system for the treatment of different types (concentration and composition) of sewage. Some of the factors determining the SRT are the incoming of settleable and non settleable solids, the wash-out of sludge (or some specific fractions of the sludge), the entrapment and further conversion of SS in the sludge bed, sludge aggregation (thickening), biomass growth (on different fractions of biodegradable matter), biomass decay, etc. On the other hand, the role of temperature varies from factor to factor and can be decisive when trying to model the whole process. The study of the relevance of the different factors affecting the SRT and the determination of the optimal SRT at different temperatures and for different types of sewage is important to assess the most favorable conditions required to achieve maximum hydrolysis and methanogenesis in the anaerobic treatment system.
The experimental work will be conducted at Salta s wastewater treatment plant, in Salta Province, Argentina. Salta is located on a hilly landscape at 24° latitude south and the climate is defined as subtropical with a dry season. Mean ambient temperature is 16.5°C. Laboratory scale research and analysis will be performed at the National University of Salta (UNSa). Literature retrieval will take place mainly at Wageningen Agricultural University.
Experimental set up
The basic experimental methodology will consist of testing different reactor configurations (one or two-stage reactors) under the given set of conditions. The effect of different operational parameters like hydraulic retention time (HRT) and temperature will be investigated. According to a design developed at the Department of Environmental Technology (DET) of Wageningen Agricultural University (WAU) a two-stage reactor will be built and operated. Similar configurations are necessary to make the results comparable. The performance of this new configuration can then be compared to single-stage conventional UASB reactors. Reactor start-up will be performed using semi-digested sewage sludge as inoculum for the methanogenic step if no better material is available. During start-up and subsequent operation, 24 hours daily composite samples will be taken at different sampling points in the system. Raw or pre-settled sewage will be used according to the research needs.
As stated in the objectives, special emphasis will be given to elucidate the role of the SRT on the sludge activity. Hydrolytic and methanogenic activity of the sludge will be measured regularly. Based on presently used methodologies (DET, 1994), a test for hydrolytic activity will be used as a tool to assess the performance of the anaerobic system. Other operating variables such as flow rate, temperature, pH, gas production and gas composition, will be checked on site. Wastewater samples will be kept at 4° C. The analysis will be performed according to Standard Methods (APHA, 1989) or using HACH® micro-methods. The selection of these parameters, as well as the sampling frequency was decided in accordance to on-going projects at the DET. Laboratory techniques presently used at the DET (APHA, 1989; DET, 1994) will be followed.
The system under study will be simulated in a model that should be able to predict the best process configuration for a given set of environmental conditions and wastewater characteristics. Such a model could be a useful tool in the selection of a wastewater treatment technology for specific cases. This part of the project will be done in cooperation with specialists in the field of mathematical modeling and computer simulation.
The adequate progress of the project will be supervised in Argentina by Dr. Carlos Mario Cuevas, currently in charge of Industrial Microbiology at the National University of Salta (UNSa). Periodic progress reports and permanent contact with the staff of the Department of Environmental Technology at the WAU will guarantee the correct execution of the plan. Additional funds will be asked elsewhere to allow frequent travels to the Netherlands to search for literature and to discuss the state of the research.
The implementation of the project is highly plausible with the equipment presently available at the UNSa. A complete water analysis laboratory is already installed and in operation allowing the determination of all needed parameters. Apart from traditional equipment and glassware, ready-to-use HACH® micro-kits are available to perform quick on-site spectrophotometric determinations. Some bench-scale anaerobic reactors are also available to do specific research. A 500 l pilot scale UASB reactor was designed, built and is under operation since July 95 and can serve as a control to assess the performance of the new two-stage system. The laboratory personnel, mainly consisting of undergraduate students doing their final research, is already sufficiently trained to perform the required analysis. The research done so far was partially funded by the Research Council of the UNSa. Enough analytical and basic scientific knowledge is available within the University, currently offering careers in the field of Chemical and Sanitary Engineering as well as in Biology and Natural Resources Management.
Agreements have already been made with some official institutions in charge of public health and wastewater treatment in Salta Province. In that sense, authorization was given by the General Direction for Sanitary Works (DGOS) to install the pilot plant UASB reactor in the city s wastewater treatment plant. The research currently under way at the UNSa is being performed in cooperation with the Direction of Environmental Sanitation (DSA) and the Laboratory of Environmental Sanitation of the provincial Public Health Ministry (MSP). This close cooperation will favor a normal progress of the proposed research and will certainly ease the ulterior transfer of technology.
|Penvoerder||Sectie Milieutechnologie (WUR)|
|Financier||Wageningen University (WUR)|
|Financier||WOTRO Science for Global Development (NWO)|
|Samenwerking||Onderzoekschool Milieuwetenschappen - SENSE (VU)|
|D14442||Afvalwaterinzameling en afvalwaterzuivering|
|D14500||Chemische technologie, procestechnologie|
|D18120||Oppervlaktewater en grondwater|
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