How can the Mutag Biochip upgrade a pulp and paper WWTP?

COD degradation reactor (old) and Mutag BioChip™ reactor.
Colonised Mutag BioChip™s.
Pore system of the Mutag BioChip™ (cross section).
Blocked carrier (paper sewage water).
Mutag BioChip™s in parallel operation.
Mutag BioChip™s and conventional carriers in parallel operation (result).
Mutag BioChip™ high-load aeration system.
Mutag BioChip™ high-load reactor in operation.

Multi Umwelttechnologie AG has over 15 years experience with culture media for immobilising micro-organisms during the treatment of sewage water. During this time, we have used almost every known carrier in many different large plants, types of sewage water and areas of application. The knowledge that we have gained as a result, along with the comprehensive data material, enables us to make a well-founded assessment of the specific performance data of the in some cases extremely different carriers. Since Multi Umwelttechnologie AG was at no time linked to a particular supplier, we were able - on the basis of the operational experience gained - to undertake targeted optimisation either of the carrier itself or the process conditions. The emphasis here was on minimising operational problems arising from the weaknesses of "conventional" culture media that these days cannot be overlooked. The thorough implementation of our requirements for an optimum carrier led to one result: the Mutag BioChip™.  From our point of view, this is currently the "best available carrier" specifically for purifying types of sewage water that are difficult to treat.

1.       How the Mutag BioChip™ works

 The special way in which the Mutag BioChip™ works, together with its efficiency, are readily explained in relation to the characteristic fluid bed process conditions. A prerequisite for the biological transformation of sewage water contents in the fluid bed is the immobilisation of the micro-organisms on the surface of the carrier. The effectiveness of the biological transformation is determined here by the carrier's "active" surface. This requires the following demands to be placed on the carrier: Firstly, there must be sufficient protected surface to enable the micro-organisms to survive and multiply in these areas, and secondly it is necessary to realise maximum mass transfer (substrate, oxygen, metabolic products) between the micro-organisms and the sewage water. At first glance it would appear that fulfilling both these requirements simultaneously is contradictory it process terms, but this can be refuted below:

First, the error of maximising the volumetric surface (in m²/m³ carriers). It is of course possible to produce extremely porous carriers, but it is also a fact that these pores must be accessible to the micro-organisms as a potential colonisation surface. It is easy to see that this is scarcely possible for cavities inside a carrier. If, as is commonly the case, the transformation efficiency is correlated against the porosity, this represents a fatal distortion of the actual conditions.

This is different in the case of the Mutag Biochip™: Here, a relatively thin and largely open carrier provides an extremely large surface in which the micro-organisms can form colonies in protected pores, but at the same time still remain in intensive contact with the surrounding fluid (sewage water). Consequently, the micro-organisms can be optimally supplied with nutrients and the metabolic products are efficiently transported away, which at least partially explains the effectiveness and high degradation efficiency of the Mutag BioChip™. Expressed in figures: the active surface of the Mutag BioChip™ is more than 3,000 m²/m³ (Figures 1 and 2).

Next point: Limitation of the biological transformation by the "thick" biofilms due to siltation and "non-biological" impurities. Even if a high microbial population density can become established on a carrier, if the structure and / or geometry of the carrier is unsuitable, the mass transfer into the "deeper" layers of the biofilm is reduced. Consequently, the degradation efficiency of the immobilised biological system is continually reduced over the operating time. 

Figures 3 and 5 illustrate how this can appear for various carriers and carrier geometries. The consequences of a carrier blockage are not hard to imagine. However, the countermeasure is very simple. The special geometry of the Mutag BioChip™ enables the hydraulic shear forces acting on the surface to be intensified and a self-cleaning process to be initiated, which constantly renews the carrier's active surface. This effectively prevents limitation of the biological efficiency due to mass transfer resistances.

Problems with the distribution and mixing-in of the carrier are prevented by the parabolic shape. In order to optimise the mass transfer (as already addressed a number of times), the Mutag BioChip™ was shaped like a parabolic disc. Although this has an uncontrollable motion profile from a flow mechanics point of view, this has proven to be extremely positive in this case. In addition to the increased level of turbulence, which has the direct effect of increasing the mass transfer, the mobility of the individual carrier in the cluster is effectively increased. This results in a homogeneous distribution of the carrier within the entire reaction space, thus enabling the formation of "dead zones" to be effectively reduced.

2.       Operational results

Of course, theoretical observations require verification of the forecast advantages within a confidence-building time frame. So far, we are able to fall back on three years worth of operational experience with the Mutag BioChip™, during which time it was in some cases possible to operate systems in parallel in order to compare Mutag BioChip™s vs. "conventional carriers". Unfortunately, not all operational results can be listed at this point, as this would understandably exceed the scope of this article. However, on the basis of the selected case examples, it is possible to meaningfully document the efficiency of the Mutag BioChip™. We consider it important to point out that the results listed here are of a fundamental nature, which enables them to be applied to other types of sewage water and applications.

In relation to the direct performance comparison, the extension of a high-load stage provided valuable results for treating the sewage water of a paper mill. In this case, the central task is to increase the system capacity from 25,000 kg COD/d to 50,000 kg COD/d, and at the same time to maintain paper production throughout the construction work. For this reason, the following procedure was chosen: Initially, the existing high-load reactor filled with conventional carrier continued in operation, with a second similar tank being set up in parallel. Following completion of the mechanical equipment (Figures 6 and 7), around 7% of the old reactor's carrier volume was put into the new high-load reactor in the form of Mutag BioChip™s for the purpose of initial orientation, after which the same volume of sewage water was fed to both high-load stages. On the basis of the positive findings here, the BioChip volume was increased in a second step to 11 vol. % of the carrier that is otherwise required. Figure 8 shows the results that were then obtained. As is clearly apparent, the Mutag BioChip™ reactor then attained the same degradation efficiency as the old reactor, which meant that it was not necessary to increase the BioChip volume further. The almost tenfold increase in the degradation efficiency of the Mutag-BioChip™ in direct comparison with the conventional carrier was clearly and impressively proven.

Based on these findings and the stable operating results of the new Mutag BioChip™ high-load stage, the old reactor was put out of operation and converted. Both stages are now equipped with the Mutag BioChip™ technology, and reliably deliver the required effluent values. With regard to the scale of the entire procedure, it is worth mentioning for the sake of completeness that in future, at least 1,000 m³/h of sewage water will be treated here, and that a performance increase of the low-rate activated sludge biology is also definitely planned.

As a second example and further evidence of the efficiency of the Mutag BioChip™ for nitrogen elimination also, its use in the nitrification stage for treating coking plant sewage water is described here. These are regarded not only as difficult to treat - in our experience they can be cleaned only with multiple biological stages. Particular demands are placed on the transformation rates here, and this applies specifically with regard to the reactor sizes that can be installed and controlled. Such plants with Mutag BioChip™s have been in continuous operation for more than two years. The largest have a nitrification capacity of around 100,000 population equivalent (PE) and 55,000 PE. The chip's superiority is apparent here also. Degradation rates of 4-5 kg NH4-N per m³ of carrier volume are constantly attained. And this is despite the fact that the Mutag reactors are smaller by a factor of 5 than the activated sludge tanks that are otherwise required.

 3.       Summary and outlook

Mutag BioChip™ has proved to be very adaptable as far as its use in a variety of media is concerned. This applies equally to the elimination of organic compounds (measured as CSB) and nitrogen compounds (e.g. ammonium).

The properties addressed here and the established operational results of the Mutag BioChip™ alone illustrate the superiority of this carrier compared with its conventional competitors. This applies equally to the associated system components (aeration, retention device), which optimally support the special advantages of the BioChip.

It goes without saying that as early as in the planning stages, the future-orientated provision of capacity reserves for sewage water treatment had been decided upon for the projects presented here, and the central task was for this to be achieved without structural modifications and "only" by replenishing the carrier as necessary. In terms of the process, there are limits here that in the first approximation can be reduced to the maximum possible carrier filling ratio. It is thus easy to understand that with the Mutag BioChip™ system, it is possible to achieve an almost tenfold increase in the extension reserve compared with conventional systems.

This is sure to be particularly good news for owners of existing plants: They are often faced with the problem that operational extensions are approved only if the pollutant load remains unchanged. By using the Mutag BioChip™ system, these plants can be upgraded relatively easily, quickly and economically. This is true even if they are a long way away, because the transport costs for the carrier are lower than usual by a factor of 10.