Group of Environmental Biotechnology

Universidade de Santiago de Compostela

Universidade de Santiago de Compostela Campus Vida Cretus

Group of Environmental Biotechnology

Universidade de Santiago de Compostela

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1.2.1 Autotrophic denitrification in mainstream (Anammox processes)

Currently, there is a growing need for sustainable wastewater treatments able to fulfill the increasing restrictions of the regulations regarding effluent quality in terms of nitrogen concentrations. The application of the conventional nitrification/denitrification processes to remove ammonia from wastewater is appropriated to treat wastewater with balanced organic matter to nitrogen ratio (C/N). However, the cost of this technology increases when the wastewater to be treated presents low C/N ratio, since in these cases the addition of an external source of organic matter (methanol, acetic acid, etc.) is necessary to complete the denitrification step. This is the case of the wastewater produced in the anaerobic sludge digesters, landfill leachates, predigested effluents from the canning industry, etc. The anammox systems operate at temperatures similar to those of the mesophilic anaerobic digesters.

The anammox (Anaerobic Ammonium Oxidation) process has emerged recently as a promising alternative for the treatment of these kinds of wastewaters. In this process, ammonia and nitrite react to form nitrogen gas which is released in the gas phase. The anammox process has to be combined with a previous partial nitrification stage where half of the ammonia is oxidized to nitrite. This generated effluent in then used as feeding to the anammox system. Main advantages of the partial nitrification-anammox process compared to the nitrification/denitrification processes are: anammox is an autotrophic process that generates small amounts of sludge; the process saves aeration costs since only half of the ammonia has to be converted into nitrite for the process to take place and no organic carbon is required. This process can be applied to sewage systems using two different configurations: a two-units configuration (partial nitrification stage + anammox stage) or a single-unit configuration where micro aerobic conditions are maintained and processes take place simultaneously (Completely Autotrophic Nitrogen Removal Over Nitrite - CANON process).

In the Biogrup research works were developed since the year 2000 on the anammox process. This research was focused primarily on obtaining systems with large biomass retention capacities, to overcome the drawback of the slow growth of the microorganisms involved, and on the application of this process to the treatment of industrial effluents from anaerobic digesters using the two reactors configuration. The effects of different compounds frequently present in wastewater from industrial origin were also evaluated (salts, antibiotics, etc.) in order to expand its application.

Nowadays the application of anammox based processes in the wastewater treatment plants has given a step forward. The new goal consists of removing the nitrogen present in the main stream of the WWTPs to improve their energetic efficiencies which involves the operation of the anammox process under its optimal temperature conditions. This new approach aims to remove not only the nitrogen but also to provide a better use of the energy contained in the organic matter. The organic matter will be removed either by an anaerobic psychrophilic digester or an aerobic stage operated at low solids retention time followed by an anaerobic digestion of the generated sludge. Then ammonia coming from these units will be removed in an anammox based process in a single-unit system. Both strategies would allow operational costs reductions of about 68 and 28%, respectively. The bottlenecks of the anammox based processes application to the main stream of WWTPs are: the long start-up period, the need of a control strategy to maintain the process stability and the possible need of a postreatment to obtain the suitable effluent in terms of nitrogen content.

Another very interesting process for removing nitrogen from wastewater is the autotrophic denitrification, where reduced sulfur compounds (eg H2S) are used to convert the nitrate or nitrite into nitrogen gas. This process has a strong interest in the treatment of wastewater containing nitrogen and sulfur compounds such as the effluents from the petrochemical and canning industry. In this aspect, the investigations carried out in the group are focused in the definition of the conditions which enable the stable application of this process to industrial effluents.


Completed Projects

The aim of this project is to expand the field of application of the partial nitrification-anammox processes to different types of wastewater at low temperatures. Problems arising from the scale-up of the reactor are also studied in order to reduce the start-up time of these processes at industrial level. The specific objectives of the project are:

A. Treatment of high ammonia concentrated effluents: application to pig slurry. The production of pig slurry is a serious environmental problem that requires an economically feasible solution. The organic matter removal in these effluents can be reached in anaerobic systems at room temperature. However, this technology eliminates mainly organic matter and requires a post-treatment system for the removal of ammonia. A two stage system (partial nitrification-anammox) and a one stage system (CANON) will be applied, at laboratory scale, to treat pig slurry.

B. Treatment of wastewater with low ammonia concentration: Application to urban wastewater. Among the weaknesses of the activated sludge technology (commonly used in the WWTP) is its limited ability to remove nitrogen. This means that in some cases it is necessary to extend the reactor to comply with the legislation, with a high economic investment. A CANON biofilter for the post treatment of the urban wastewater is proposed as an option to improve the economic performance of the plant.

C. Scale up study: Application to urban wastewater. The effect of scale change on stability and efficiency of the system and in the physical properties of the biomass is going to be assessed. A CANON reactor at pilot plant scale (100 L) will be used to carry out the post-treat of urban wastewater. The results will allow the design of reliable industrial units which will benefit the implementation of this technology.

D. Application plan of the proposed technology in different sectors. The obtained results will be analyzed from the economic and environmental point of view in order to know the feasibility of the proposed technology. A mathematical model to predict and optimize the operating variables effect on the performance of the systems will be developed. Finally, the more suitable implementation strategies and operating conditions will be deduced.