Comparison of different methods of mechanical disintegration

 

Mechanical disintegration

Various disintegration machines are investigated in research projects. The most important ones are presented here.

Stirred Ball Mills (SBM) consist of a cylindrical grinding chamber of up to 1 m3 of volume which is almost completely filled with grinding beads. An agitator forces the beads into a rotational movement. The micro-organisms are disintegrated in between the beads by shear- and pressure-forces. For a continuous operation the beads are held back by a sieve while the suspension can flow through the grinding chamber.

High Pressure Homogenizers (HPH) consist basically of a multistep high-pressure-pump and a homogenizing valve. The pump compresses the suspension to pressures up to some hundred bar, realising a flow of up to some cubic meters per hour. The suspension passes through the homogenizing gap while the pressure drops below the vapour pressure of the fluid, and the fluid velocity increases up to 300 m/s. When the occurring cavitation bubbles implode, pressure gradients are induced into the fluid causing temperatures of some hundred degrees Celsius and pressure peaks of 500 bar locally.

Ultrasonic Homogenizers (UH) consist of three major components. A generator supplies a high frequent voltage of 20 to 40 kHz. A ceramic-crystal of piezo-electrical material transforms the electrical into mechanical impulses, which are transmitted by a sonotrode into the fluid. Cavitation bubbles are created by alternating overpressure and underpressure. Actually ultrasonic devices of some thousand Watt performance which are capable of serving a wastewater treatment plant are under construction.

The Mechanical Jet Smash Technique (MJS) pressurizes the sludge up to 50 bar and then releases the sludge through a nozzle. The accelerated sludge (30 to 100 m/s) smashes onto a plate where the disintegration takes place.

The High Performance Pulse Technique (HPP) is an electro-hydraulic method. The sludge is treated by high voltage of up to 10 kV. So a sudden disruption and release of organic substances takes place. The pulse period is only 10 ms, inducing shockwaves in the sludge which lead to disintegration.

The Lysat-Centrifugal-Technique (LC) uses a decanter equipped with a disintegration device located at the discharge of the dewatered sludge. Tools on either the rotor or the stator stress the sludge by shear forces.

In most of the investigations SBM, HPH and UH were used (Baier et al., 1997; Kunz et al., 1994; Müller, 1996). As a result of this research work the parameters for an energetically high efficient disintegration were obtained. Best results are achieved by using the SBM with small grinding beads and a moderate circumferential speed of the agitator, by the HPH at a pressure around 500 bar and the UH at a frequency of 20 kHz.

Concerning the use of MJS (Alshaabi, 1995; Winkler, 1997; Choi et al., 1997), HPP (Weise et al., 1998) and LC (Dohanyos et al., 1997; Otte-Witte, 1998) first results are presented. Recent investigations with the HPP-Technique show that a high degree of degradation can be obtained if the intensity of treatment is remarkably increased compared to the devices used in the U.S.A. At present the LC-machines only reach a low degree of disintegration at a low level of energy consumption.

The performance of the various disintegration methods can be compared with each other using the specific energy, which is defined as the amount of mechanical energy that stresses a certain amount of sludge. The degree of cell disruption can be measured using biochemical parameters like the rate of oxygen demand and the COD release.

Fig. 1.         Degree of disintegration as a function of specific energy for different disintegration methods

The results presented in Figure 1 are definitely not the final ones concerning the ultimate performance of the methods. All methods are still under investigation to improve their application for sewage sludge treatment. Therefore remarkable improvement of the performance can be expected. Besides the energy consumption there are other factors like investment-costs or the suitability of the machine for the practical application on a wastewater treatment plant which are of great influence on the selection of the method, where UH and LC are performing best.