Cost-effective sludge dewatering

The increasing costs of wastewater sludge disposal were a key focus in the development of the VTA mudinator^®. These costs are becoming increasingly problematic for small and medium-sized treatment plants. As legal regulations become stricter, the option of using wastewater sludge for agricultural purposes in fields will no longer be possible in the future.

So what are the alternatives?

Ultimately, the only remaining option for wastewater sludge disposal is thermal utilisation. As the cost of incinerating wastewater sludge has increased significantly, the disposal costs borne by treatment plants have also risen. In Germany, for example, prices exceeding €100 per tonne are now not uncommon. Treatment plants with a strong cost awareness, particularly in municipalities, are therefore increasingly striving to achieve the best possible dewatering rate.

Technical solution approaches for cost reduction

The VTA mudinator has already been presented in detail in Laubfrosch (issue 79). The new technology makes it possible to increase the rate of dewatering of the digested sludge by up to 5 percentage points. This means that the machine can be refinanced in as little as three years, depending on the size of the plant. The VTA mudinator^® has now been proven in practice and, since its introduction and market launch, numerous successful laboratory tests and trials at large plants have been carried out. It became apparent that – although a wide range of different dewatering aggregates were in use in the large plant trials – corresponding results were achieved on a laboratory scale. In short, the practical test delivers what the lab promises.

The VTA mudinator has been featured in detail in the Ağaç Kurbağası (Tree Frog) magazine (Issue 79). The new technology makes it possible to increase the dewatering rate of digested sludge by up to 5%. This means that, depending on the size of the plant, the machine can pay for itself in as little as three years. VTA mudinator^® has proven itself in practice so far, and numerous successful laboratory tests and trials in large plants have been conducted since its launch. Although a wide variety of dewatering aggregates were used in the large-scale plant trials, similar results were also achieved on a laboratory scale. In short, the practical test delivers what the lab promises.

Laboratory testing and large-scale plant trials

As in large-scale plant trials, a suitably scaled VTA mudinator^® is used in laboratory analyses. The aim is to optimise the sludge structure using ultrasound and thus improve load balancing in conjunction with the flocculant. This optimises the dewatering rate by creating more shear-resistant flocs. In a sample calculation based on a 50,000 PE plant, the payback period of the VTA mudinator^® can be easily demonstrated. With annual savings of €36,000, the VTA mudinator^® will pay for itself in just under 2.5 years, including investment, operating costs and depreciation.

This is followed by the presentation of two large-scale trials conducted at treatment plants that had previously been tested in the laboratory with VTA mudinator^® and VTA Biocitran^®. In this case, the acoustic irradiation time, energy input and polymer quantities were determined and applied at a 1:1 ratio in the large-scale trial.

In the future, it will no longer be possible to spread sewage sludge on fields; thermal utilisation will be the only remaining option for disposal.

Sewage treatment plant 1

In this example, the sewage sludge was dewatered using two identical screw presses. Feeding was carried out centrally via a sludge pipeline with a volume of approximately 3 m³/hour. After conditioning the polymer (430 l/h from a 0.28% solution), both dewatering units achieved a dry matter content of approximately 20%. For on-site trials, the VTA mudinator^® screw press 2 was placed in front of screw press 1, which remained unchanged. During the trial period, the untreated sludge could not achieve a consistent dewatering rate, even showing a decrease of approximately 1 point. In contrast, the sludge pre-treated with the VTA mudinator^® increased the dewatering rate by 1.5 points. The trial was conducted similarly at the laboratory facility. After finding the appropriate settings and parameters for the sludge to be treated, the laboratory test facility increased the dewatering rate by 2 points. Thus, the laboratory results were fully confirmed in the large-scale plant trials. In addition, significant time savings and increased efficiency were achieved through pre-selection.

Wastewater treatment plant 2

At the second treatment plant, 3.5 m³/h of digested sludge was fed into a screw press. The polymer addition was approximately 430 l/h in a 0.2% solution. During standard operation, an average solids discharge of 24.9% was achieved here. In laboratory tests, VTA chemists again determined the optimum settings. In this case, the laboratory plant (VTA mudinator^® together with VTA Biocitran^®) achieved a dewatering rate that was up to 4% (almost 29%) better. In the large-scale plant trial, both the VTA mudinator^® and the VTA Biocitran^® were tested (the product was introduced in issue 79 of the Tree Frog magazine). The combined use of the two innovations resulted in a solids discharge of 29.3%, corresponding to an increase of 4.4 points. To further substantiate the positive results, the TR(A) value or TR(DA,max) value of the sludge was determined in this trial. The aim was to demonstrate once again how much water could theoretically be removed from digested sludge using mechanical dewatering, both with and without the use of VTA mudinator^®.

Field report from the 80th issue of the VTA Group's scientific journal ‘Ağaç Kurbağası’ (Tree Frog).