DoT: Sludge biosolids production – Thermal processes

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Sludge thickening and dewatering processes mentioned above are not sufficient for the sludge to be allowed for use e.g. in agriculture or for other useful purposes. These deposits still may include various types of pathogens and dangerous chemical compounds disqualifying them from further use. Several of the thermal sludge treatment processes are described below.

These apply mainly to the dewatered sludge:

Mesophilic anaerobic digestion with pre- or post- pasteurization

The characteristic of this process is that the anaerobic digestion process with mesophilic bacteria takes place at relatively low temperatures, because these bacteria develop best at a temperature of about 25- 300C and then they are most efficient. The technological regime must be strictly adhered. Temperature and retention time of treated sludge must not be exceeded. The pasteurization process however takes place at a temperature of about 700C and a at strictly defined, impassable retention time. Retention time of mesophilic bacteria digestion (hydrolysis) process is usually a minimum of 12 days. The resulting final sludge, after being analysed and commissioned, can be used in agriculture.

Thermophilic anaerobic digestion

requires additional heat input. Retention time for sludge treated this way is at least 48-72 hours, at 50-550C, including at least one hour sludge retention at a minimum temperature of 700C. Due to the high energy consumption, this process is relatively rarely used, although some treatment plants utilise it.

Thermophilic aerobic digestion

also requires an external energy source. The process is concluded in batches due to significant changes of sludge loading conditions in summer and winter. All sludge is treated at 500C for a minimum period of 7 days. A minimum volatile compounds level of at least 38% must be achieved in this process. Due to the need of an external energy source use to heat the process system, this method is not very popular.


one of the most popular methods of thermal sludge treatment. This process involves mixing dehydrated sludge with fillers containing an external carbon source (e.g. molasses). Such a mixture is collected on an embanked plot, arranged in piles or stored in special tanks. The sludge pasteurization process involves the activity of microorganisms, resulting in the treated sludge temperature increase. This temperature reaches about 550C and should be maintained for a time specified for each method. For example, if the sludge is kept on embanked plots, this period should last about 15 days with the sludge being circulated approx. 5 times in the meantime. In case of prisms, the storage period at a constant temperature of 550C is a minimum of 3 days. The advantage of the composting process lies in the low operating costs, but a strict process regime must be maintained to obtain a good product and to avoid troublesome odours. When used in agriculture, composted sludge can be mixed, under strict control, with green vegetation.

Alkaline stabilization

consists of adding hydrated or burnt lime to the dewatered sludge and mixing the two components. To achieve full pasteurisation, the pH should be raised above 12 and the sludge should be kept at 550C for a minimum of 3 hours or at 700C for a minimum of 1 hour. To achieve such temperatures, excess lime should be used and external heat source introduced. This method of sludge treatment is the cheapest – operating costs are low. However, it results is the increased volume of sludge, because of introducing a certain volume of calcium compounds.

Thermal drying of sludge

a very simple method, because the dewatered sludge subject to a direct or indirect drying process, using an external heat source, which can be, e.g. gas, fuel oil, steam, etc. The drying effect is fast and significant because of evaporating water directly from the sludge. Due to the relatively high temperature of the process, pasteurization is one hundred percent. Granulated sludge is obtained, which (if it meets the standards) can be used e.g. in agriculture or as an addition to the production of pellets used in the combustion processes. Thermal drying results in very high sludge dewatering level reaching over 90% of dry matter. Sludge volume is also significantly reduced. The weak point is the relatively high operating costs resulting from the use of an external heat source. Inflammation of the deposits may also occur.

Main parameter: Energy consumption

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