Triple A: The revolution in the primary clarifier

The wastewater treatment plant in Strass in the Zillertal is located at the junction of three important Tyrolean Alpine valleys. Wastewater from the Achental, Inntal and Zillertal valleys flow together here, bringing with it ever greater challenges. Due to the increasing peaks in winter tourism, the capacity of the Achental-Inntal-Zillertal Wastewater Association (AIZ) plant had to be increased from 167,000 population equivalents (PE) to 250,000 PE. And: this optimization was to be implemented without building new basins. Instead, the operators relied on increased efficiency and innovative technology: the "Triple A Process" was the solution. This process stands for "Alternating Activated Adsorption" (AAA) and describes a revolutionary aeration procedure in the first treatment stage, developed by Bernhard Wett. By utilizing the previously unused potential in the primary clarifier, it is possible to significantly increase the plant capacity without structural expansion, to further increase the gas yield and to reduce the energy consumption of the entire plant. 

This efficiency gain was achieved at the AIZ simply by retrofitting existing intermediate clarifiers with the latest, forward-looking technology. "When it comes to innovative treatment technology, the AIZ has been a showcase plant for years," says Gerald Glaninger, sales manager and procurator at AQUACONSULT ANLAGENBAU GmbH. The aeration technology of the Lower Austrian company is used in all clarification stages of the AIZ. It now also plays an essential role in the Triple A process. "The process will revolutionize primary treatment," Glaninger is certain. But how does it work in detail? 

AAA in four phases

In the AAA basin of the AIZ, a total of four phases take place within a cycle lasting about one hour. The solids settle first in the unaerated phase at the bottom of the basin. In the AAA process, this layer is used as a natural filter. Over a period of about half an hour, fresh wastewater is fed into the reactor basin from below. The sludge blanket above prevents mixing with the already pre-purified water near the surface. During the inflow, which lasts about half an hour, the pre-cleaned water near the surface is displaced and flows into the second treatment stage. On the one hand, the sludge layer acts as a barrier; on the other hand, it also acts as a filter and a site for massive COD degradation by bacteria. In the process, an EPS matrix (extracellular polymeric substance) is formed. These are long-chain compounds formed by microorganisms. EPS is often also referred to as biofilm and helps the cells to bond with neighboring cells. The cleaning or filtering effect is so high that it agglomerates 60 percent of the organic matter contained in the wastewater and also binds nitrogen vigorously. Classical primary clarifiers usually achieve just 30 percent. Another advantage lies in the technical implementation. The process does not require any complex clearing mechanics to get the sludge out of the basin. The injected air is sufficient.

AEROSTRIPS installed on two height levels for the first time

The inflowing wastewater passes through an alternating mixing as well as settling and filtering process in the existing intermediate clarifiers. "The finely injected air through our AEROSTRIPS takes on a crucial role in achieving the activation of biosorption," says Gerald Glaninger. The air supplies bacteria with oxygen and promotes the formation of sludge flocs; it additionally takes over the complete mixing of the entire basin contents. In the subsequent settling phase, the sludge flakes sinking to the bottom form a solid layer, the so-called "floc filter".

A particular challenge was the funnel-like basin geometry and the system-related internals. To avoid negative effects, zones with distances of more than two meters between the diffusers had to be avoided. However, this was easily achieved thanks to the slim design of the strip aerators, which are also available in different lengths. "In order to make the best possible use of the full water depth in all pool areas for air intake, our AEROSTRIPS were also distributed for the first time on two height levels, in an inner and an outer ring at the bottom of the circular pools," Glaninger describes. "A new, efficient solution for this process technology, which is unique in Austria in this size." 

Behind "AAA" at the Strass wastewater treatment plant are also a total of three "A" companies that joined to implement the new process: ARAconsult, responsible for planning and process engineering – AERZEN as supplier for highly efficient turbo blowers – and AQUACONSULT for the diffuser systems. 

Increased efficiency and cost savings

The AEROSTRIP strip diffusers hold an essential contribution in this innovative process and for the plant operators it was clear to implement the new project again with AQUACONSULT's diffuser technology. "We had already had very good experiences with our existing basins. Also the fast and flexible installation, as well as the easy maintenance are important features of the AEROSTRIPS", says Martin Hell, laboratory manager and deputy operations manager at the AIZ. The energy balance already looks very good after just a few weeks, but more precise information will be available in the course of the year. 

In any case, the cost savings in the investment are enormous with the Triple-A process compared to a conventional rebuild. The entire rebuild cost the wastewater association 1.8 million euros. "Alternatively, with a conventional conversion, we would have had to expand the low-load stage by an additional activation line with 5,000 cubic meters," calculates AIZ operations manager Christian Fimml. The total costs including plant technology would have been 4.3 million euros. This results in a saving of 2.5 million euros. 

Conclusion: The new process is interesting for wastewater associations and municipalities that are about to expand their plants. The investment pays off very quickly – because even if the demand for electrical energy increases in the pretreatment stage, the less is needed in the subsequent activation stage. The less organic matter arrives in the activation, the lower the oxygen demand will be.