13-jun-2006, STOWA

Background

Anaerobic sludge digestion has shown to improve the sludge de-watering characteristics and to produce a stabilised sludge. The Schwarting/Uhde process which combines mesophilic and thermophilic treatment steps has been developed to optimise the performance and operation of the digestion process.

The Schwarting / Uhde process was originally developed for the treatment of manure and biowaste and was applied in sewage sludge treatment on a full-scale for the first time in 1994.

Description and working principle

The Schwarting/Uhde process is an anaerobic digestion process, designed for the treatment of primary and waste activated sludge. The process is characterised by relatively short retention times and an increased reduction of the organic dry solids (oDS) content.

The Schwarting/Uhde process is usually designed as a two-stage process with two cylindrical tanks in series, which are both operated in upflow mode. However, it can also be operated as a single-stage mesophilic process, either to serve as a high-rate digestion process in an existing plant (extension option with low space requirements) or to be used as a sludge stabilisation process for smaller wastewater treatment plants.

Contrary to conventional anaerobic digestion, the digester contents are not completely mixed but the objective is to achieve a defined plug flow. Perforated plates in the cylindrical digesters are used to obtain an even flow distribution and to realise plug flow conditions. The mixing of the digester content is achieved by periodically raising and lowering the column of liquid in the tank, thus creating turbulence at the perforated plates. This patented impulse mixing system also enables the separation of the biogas from the liquid. Biogas is collected at the top of the digesters, whereas settled solids, which accumulate at the bottom of the tanks, are frequently removed via screw pumps. Heat loss from the digester tanks is compensated by a temperature controlled heat insulation.

Raw sludge (primary and waste activated sludge) is fed to the process at a dry solids (DS) content of about 4% to 10%.  After macerating the sludge is preheated by the digested sludge from the second stage and finally heated up to 37°C before being fed into the first digester, which operates in the mesophilic temperature range.  The retention time in the first digester is about 5 to 6 days, during which the organic solids content is reduced by about 40%.  Before entering the second digester the sludge is heated to 55°C.  The second digester operates in the thermophilic range and the retention time of the sludge is again 5 to 6 days, during which an overall organic solids reduction of 55% is achieved. The overall retention time is 10 to 12 days, compared to 20 days for conventional mesophilic digestion.

Design guidelines / Technical data

General process design

·         The high loading rate in combination with plug flow conditions results in a reduction of the required digester volume by one 40 to 60%. So far, digester size has been limited in full-scale applications to about 700 m³.

·         Retention times in the mesophilic (37°C) and the thermophilic digester (55°C) are 5 to 6 days each.

Performance

General performance

·         Within 10 to 12 days retention time, organic solids reductions of more than 55% can be achieved as compared to 40% by conventional anaerobic digestion with retention times of 20 to 30 days.

·         The thermophilic operation reduces the content of pathogenic and thus pasteurizes the sludge.

·         Heat recovery results in relatively low additional heat requirements.

·         The high loading rate in combination with plug flow conditions results in a reduction of the required digester volume by 40 to 60%. Improved dewaterability of the digested sludge.

Performance at specific installations

WWTP Mittleres Glemtal (60.000 p.e.)

Overall treatment design: the 1^st digester operates in mesophilic range (37°C); the 2^nd digester operates in thermophilic range (55°C).

Operational stability and maintenance

·         Maintenance is reduced since no moving parts are installed in the tanks.

·         The defined plug-flow is supposed to guarantee a reliable operation with varying loads. Sludge foaming is not a problem due to the plug flow conditions.

Capital and operating cost

WWTP Mittleres Glemtal (60.000 P.E.)

·         Investment cost of 4,5 Mio. euro (as of 1994);

·         Estimated savings in sludge disposal costs of 345.000 euro/year (as of 1996).

Reference installations

There are 4 WWTP's using the Schwarting/Uhde-process, all of them are situated in Germany.

Two-stage process

WWTP Mittleres Glemstal (D) (60.000 p.e.) (1994)

WWTP Ammersee (D) (120.000 p.e.) (1999)

Single-stage process (high-rate loading design)

WWTP Heidelberg (D) (345.000 p.e.) (to be commissioned 2001)

WWTP Tauberbischofsheim (D) (47.000 p.e.) (to be commissioned 2001)

Suppliers / Patents

The Schwarting/Uhde-process was developed by Schwarting Umwelt GmbH, Flensburg, in co-operation with the Fraunhofer-Institut für Grenzflächen- und Bioverfahrenstechnik (FhIGB), Stuttgart. The patent covers the partial intermixing system (impulse pump) in the digester tanks. In July 2001 Schwarting Umwelt GmbH has become part of the Farmatic Biotech Energy AG group.

Schwarting Umwelt GmbH

Lise-Meitner-Str. 2 - 24941 Flensburg - Germany

Tel: +49 461 9992 121  -  Fax: +49 461 9992 101  -  E-mail: info@schwarting-umwelt.de  -  Web: www.schwarting-umwelt.de

 Literature references

[1]   Kampner, R.: ”Optimierte Klärschlammfaulung: Hohe Abbaugrade innerhalb kurzer Verweilzeiten”.  WLB Wasser, Luft und Boden, 4/2000, pp. 40-43.

[2]   Merz, H.-U.; Schmickl, M.; Trösch, W.; Galander, C.: ”Zweistufiges mesophil-thermophiles Verfahren zur Schlammfaulung erstmals in großtechnischem Maßstab erprobt”.  Korrespondenz Abwasser, Vo. 46, No. 8, 1999, pp.1238-1243.

Graphics

Scharting/Uhde process: Mixing