The sewage treatment plant of Halle-Nord in Germany changed during the years 1994 –1997 from the old mechanical system to a new modern complete biological sewage treatment plant for about 300,000 inhabitants.
The activated sludge tank was built as four lines in cascade construction with an upstream denitrification, with two cascades for Bio-P (biological phosphorus elimination) for each line. After the launch in 1997 the plant functioned well without any problems until 1999. However, in September 1999, for the first time, an increase in filament formation was observed under the microscope. At the same time the sludge volume index (ISV) also increased systematically to 150 mL per g dry matter (150 mL/l SV) and even exceeded this value.
With the beginning of the warmer season and the increasing temperature of the wastewater, filamentous bacteria growth rose sharply and became a problem in the activation basins and in the final sedimentation tank. A huge amount of floating sludge accumulated in the sedimentation tanks and removal of the floating layer became a serious problem (Figure 1).
Figure 1.View of the final sedimentation basin with floating sludge
With enormously time-consuming, technical and chemical commitment, the situation was resolved without running beyond the regulated maximum values of the wastewater treatment plant.
The floating sludge was pumped out of the effluent from the activation basins and the surface of the final sedimentation tank with a special high-pressure flushing and suction vehicle for several days. The sludge had to be removed completely from the system and be disposed of. Strongly inhibiting chemicals were used to kill the filamentous bacteria in the activation tank and to avoid foaming in the digestion tower. The manpower as well as the usage of chemicals created enormous costs for the operation.
By microbial examination, the filamentous bacteria, Microthrix parvicella (Figure 2), was relatively easily found to be the causative organism of this disruption. In the following years, the observation of the development of the sludge volume index and the examination of the mud microscopic images was considered to be very important.
Figure 2.Fluorescence-marked Microthrix parviacella in activated sludge; Source: © Bayerisches Landesamt für Umwelt
For the wastewater technicians, there are two important methods for the observation of the growth of biomass and also the filamentous bacteria. The sludge volume index provides information about the composition of the activated sludge, and the microscopic mud image combined with special staining methods differentiates the species of the filamentous bacteria.
But all these methods do not allow early enough detection of the problematic bacteria population, and are less selective and very time consuming. Microscopic sludge examination is based on the subjective cognition of the observer and should always be performed by the same experienced person. Staining methods need certain biological skills and a dedicated laboratory space. However, that is not usually the case in a sewage plant.
To avoid floating sludge, it is important to keep filamentous bacteria, in this case Microthrix parvicella, in low concentration. Therefore, a rapid detection system is essential.
In the summer of 2005, a project was created in partnership with Scanbec from the Technology and Founder Center in Halle/Saale. In this partnership, a rapid test system was developed to detect Microthrix parvicella.
The HybriScan test system for wastewater analysis is based on the detection of microorganism-specific target molecules with a special catcher and detection probe, resulting in a sandwich hybridisation. The method is designed to detect only a certain group or species of problem microorganisms that is guaranteed by using sequence-specific probes for ribosomal RNA. The measured absorption from the addition of substrate that is converted into a blue dye is proportional to the concentration of Microthrix parvicella.
HybriScan Waste Water determines both the Microthrix parvicella and the total bacterial count. By the parallel determination of the total count, the ratio of Microthrix parvicella concentration to the total bacteria number of the sample can be calculated. From the knowledge of the bacterial count, external process-related influences, like rain and variable inflow, can be taken into consideration.
Since September 2006, the laboratory of the sewage plant in Halle/Nord has been using the HybriScan test as a parallel method to observe the sludge volume index (Figure 3).
Figure 3.Graph of the sludge volume index compared to the cell concentration of Microthrix parvicella
The existing data provided important information about the influence of different parameters that helped in avoiding disastrous situations (as in the year 2000) without using expensive chemicals.
It was found that the index monitored the increase in cell concentration of Microthrix parvicella. The index, however, was not accurate, and the changing population in the activated sludge basin was detected too late. This information is very important for determining the need of dosing inhibiting chemicals to kill Microthrix parvicella. With advance information on the growth of Microthrix parvicella, the precipitating agent could be used more efficiently, since the influence of filamentous bacteria can then be measured directly.
The graph shows a significant seasonal variation of the cell concentration of Microthrix parvicella. It was observed that the sludge volume index in the summer months (high temperatures) was swinging at a high level. It was assumed that this was related to the growth of Microthrix parvicella. With the HybriScan system this could be denied, as the graph shows a significant decrease of the Microthrix parvicella population. This observation is supported by the literature, which suggests that at higher temperatures Microthrix parvicella loses its growth advantages compared to other organisms in the biosolids.
For us as operators of the sewage plant, the HybriScan Method is an important tool to detect Microthrix parvicella and to keep a stable, well-performing composition of the activated sludge. We were able to see changes in the process parameters of the wastewater plant at an unusually early stage, and were able to implement action needed for maintaining a stable system. In comparison to other known methods, the HybriScan test kit is fast (performed detection in two hours), and specific to the organism providing semi-quantitative results.