Aerobic granular system wins award for Croatia
Croatian company EcoEngineering was winner of the 2006 Eureka Lynx Award for outstanding technological achievement in May. Vice Soljan, director, and Veljko Matic, senior project manager, of EcoEngineering, and Professor Margareta Glancer from the Faculty of Food Technology and Biotechnology at the University of Zagreb describe a new system which uses specially designed mixtures of different bacteria, in granular form, to break down even the most toxic wastes.
From 2002-2004, during laboratory and pilot plant testing under aerobic conditions, granular biomass showed superior characteristics to conventional activated sludge used for municipal and industrial wastewater treatment. Tested on toxic, biologically hard biodegradable (pharmaceutical, chemical wastewater and landfill leachate) and high ammonia loaded wastewater, granules tolerated toxicity and maintained biological stability, where activated sludge flocs were destroyed and biological systems were stopped.
As the team worked on the bio-augmentation of selected micro-organisms, specific problems of adding microorganisms into existing systems were noticed. This fostered the development of a new process to overcome them.
Existing treatment plants have stabilised microbial populations, and augmentation of exogenous microorganisms usually ends with rejection of added microorganisms, washing them out of the system. The main reasons are, for example, different cultivation conditions (such as substrates and process and operational parameters) and preparation of the main system to accept externally added microorganisms.
In order to maintain micro-organisms in the system, the team first added mixed cultures, instead of one microorganism or one group of microorganisms (for example, nitrifying bacteria). Due to a co-metabolism effect, mixed cultures could carry out complete biological reactions (carbon or nitrogen removal) or carry out the reactions, which endogenous microorganisms could not.
That effect was even stronger when a mixture of autotrophic and heterotrophic bacteria was added. However, in certain cases, especially with industrial wastewater and unstable municipal plants, added mixed cultures could not stay in the system.
The team confronted the new problem by designing a biomass, which could stay in the system under extreme conditions. After one year of laboratory testing, two years of pilot plant testing and application in pharmaceutical industry, ARGUS was developed.
There are two main properties of granular biomass, which give it an advantage over microorganisms in activated sludge floc. The first is the higher settling properties of the granular biomass, which consequently requires small settlers enabling the fast recycling of active biomass.
The second is the fact that the granules maintain various strains of micro-organisms which are able to accept shock-loads and accomplish the biodegradation of complex compounds such as xenobiotics. Retention of those species of bacteria responsible for biodegradation of specific wastewater compounds is improved.
Micro-organisms which have low specific growth rate are consequently being washed out of the wastewater treatment systems, as are the autotrophic bacteria, which carry out the nitrification process. In order to apply ARGUS technology, in laboratory conditions, the right microbial mixture has to be prepared.
The mixture consists of microbial species, able to carry out target bioprocesses under requested process conditions and bacteria responsible for granulation. The process starts with granular biomass transfer and scale-up at an industrial location.
In most cases, for scale-up, temporary tanks (propagators) are needed. These are smaller in volume than the main stream reactors (2-5% of main system). Once the technology is applied, if there are no specific accidental situations or significant changes in production, the aerobic granules grow in the main system.
Otherwise, from time to time, an additional quantity of granular biomass has to be taken from the propagators and added to the main system. The simplified application process is shown in Figure 2.
The quantity of granular biomass and the microbial composition of granules depend on the existing treatment process and wastewater characteristics and are determined during the laboratory phase. According to industrial experience with improved existing wastewater treatment plants with ARGUS technology, ARGUS granular sludge removes ammonium nitrogen effectively (up to 99,9%) under high nitrogen load condition (0.05-0.08 kg N-NH4 / kg MLSS). Due to retrofit conditions in existing WWTP, granular biomass concentration was low (2.5-3 g/L), and no higher biomass concentration has been found necessary to accept higher organic load.
Although ARGUS has already been applied, investigated and proven, the concept is still in its research and development (R&D) phase. It has solved specific problems at complex and toxic pharmaceutical WWTPs in Lek-Novartis and Krka in Slovenia. ARGUS has also been successfully applied at a landfill site in The Netherlands.
Compared with other emerging technologies, following significant ARGUS advantages were noticed:
- Higher nitrification activity (double the nitrification rate in comparison with conventional autotrophic nitrification).
- Better nitrification activity in cold period (below 12°C) and stability during shock-loads of industrial pollutants.
- Less sludge produced (0.2-0.35kg/kg BOD5).
- Complete removal of industrial pollutants .
- Better sludge settling and lower Sludge Volume Index.