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Aerobic Thermophilic Digestion
for stabilising and pasteurising raw sludge

Fig. 1: Three-stage plant on a sewage purification plant

The task

Spreading of liquid sewage sludge to agricultural used soil is a natural way for smaller sewage treatment plants. A simple and cost efficient system to stabilise and - if possible - pasteurise the sludge is necessary.

Anaerobic sludge digestion requires high investments and substantial operating and maintenance costs. Such process is economic only for large purification plants, in particular, if further expenditures for sludge pasteurising are required.

In Germany land application of sewage sludge is allowed only in accordance with the official sewage sludge regulation (AbfKlärV). In other countries, e.g. in Switzerland, also pasture-grounds can be used.

Our solution

The LIMUS-Biotherm-Process has been designed particularly for sludge treatment of smaller purification plants. Both digestion and safe hygienisation in one step is ensured. Thermophilic Aerobic Digestion is economically in investment, simply in operation and thus superior to other systems for a population between 5.000 and 50.000.

The process is suitable also for the pre-treatment of faecal sludges.

Principle of process

A substantial heat of reaction is released by the biological oxidation of the organic material - approx. 15.000 kJ/kg COD reduction. Theoretically heating up to almost 90°C would be possible with municipal mixed sludge having a degradable COD of 25 kg/m3, assuming complete reduction and without calorific losses.

With the LIMUS Biotherm-Process the energy content of raw sludge is used with good efficiency for its self-heating, so that temperatures up to 75°C are obtained. Since the biological processes are substantially faster under these thermophilic conditions than within the mesophilic range, the retention time necessary for sufficient stabilisation can be reduced to 6 days. Accordingly the reactor volume decreases to about 25 % with same reduction degree.

As in the case of anaerobic digestion, a complete reduction of the degradable organic substance would be possible only with infinite residence time. Also, anaerobic sludge treated up to the technical limit creates further - although small - quantities of gas. The same applies to aerobic thermophilic stabilised sludge as a function of the environment conditions.

The treated sludge can be usually after-thickened by decantation. For longer storage an easy aeration is recommended in order to keep the sludge fresh.

With very thin or already partly-stabilised sludge that contains few biologically degradable material simultaneous stabilisation and hygienisation can be achieved by means of additional measures:

Fig. 2: Aeration versions

Aeration and mixing

For the realisation of the process a high utilisation of supplied air is essential. At the same time sufficient circulation must be provided. Usual sewage aerators, which are optimised on a high oxygen yield, are unsuitable.

LIMUS uses self-sucking BIOPROP stirrers, either alone or in combination with slowly running mixers, see A in Fig. 2.

The new developed LIMUS jet-aerator represents an advancement, because air supply and circulation are decoupled, so that each of these two variables can be optimised individually, see B in Fig. 2. With this version slimmer reactors can be used.

In the waste air outlet at the head of the reactors mechanical foam breakers are arranged. A subsequent treatment of the exhaust air can be executed by means of bio-filters or bio-scrubbers. Another possibility is injecting the untreated exhaust air into an aeration basin.


Fig. 3: Hygienisation conditions

The connection of impact time and temperature on killing of pathogens is a straight line in single logarithmic representation. The line drawn in fig. 3 is based on literature data and own investigations and fulfils the specifications of ATV and German Federal Health Office.

Operational sequence

The process can be executed single or multi-stage. The tanks are closed, thermally insulated and corrosion proof.

A LIMUS Biotherm system is fed with sludge in regular intervals in such a way, that mixing of treated and untreated sludge is impossible.

Fig. 4: Two-stage system

The two-stage system according to fig. 4 is fed usually once daily. First the valve V3 is opened, so that sludge from the second reactor flows off; its volume corresponds to the volume of raw sludge to be treated. Afterwards V3 is closed and V2 opened; the same quantity flows from first into the second reactor. Finally, when V2 is closed and V1 opened, the first reactor is filled up with raw sludge.

Fig. 5: Single-stage system with heat recovery

In the single-stage system with heat exchanger of fig. 5 the sludge remains on a comparatively higher temperature the entire retention time. From the coat space of the heat exchanger and the reactor treated sludge is discharged three times daily. The reactor is filled again from the interior of the heat exchanger with preheated raw sludge.

The operational sequence is automatically controlled and requires supervision only.

Waste heat utilisation

When using well thickened mixed sludge or a heat exchangers upstream, the large temperature enables extraction of heat from the second reactor without endangering sludge stabilisation and hygienisation. This warmth can be used to heat buildings.

Technical data

Operating costs almost entirely result from electricity consumption for oxygen input, mixing and foam fighting. An average specific energy consumption of approx. 15 kWh/m³ can be expected with thickened mixed sludge of usual composition, solid contents of about 4 % and normal biological reducibility. Deviating characteristics lead to other values.

The following approximate values apply to average municipal mixed sludge: 

Retention time in the reactors:  > 6 days
Temperature in last stage: 55-65°C
Reduction of organic matter: 35-45 %
Electricity consumption: 12-20 kWh/m3

Fig. 6: Two-stage system with heat exchanger in a German purification plant