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Digested sludge degassing by vacuum treatment

Fig. 1: Model of a vacuum-degassing plant for digested sludge

The Problem

Digested gas is dissolved in sludge water due to the local temperature and pressure in the digester. At the bottom of a 20 m high tank there are 0.65 m carbon dioxide (CO2) and 0.05 m methane (CH4) per m sludge. Usually sludge is drawn off from the deepest point to prevent accumulation of sand and other sedimentable matter. At the overflow on top of the digester, in the downcomer to the thickener and during mechanical dewatering the dissolved gas becomes free step by step, because equilibrium is reached slowly. The gas consists - besides small quantities of typical Oduorants like ammonia and hydrogen sulphide and traces of others partly toxic components - mainly of CO2 and CH4, which are considered to be the main reason for the green house effect and should therefore not escape to atmosphere.
About that the releasing gas will complicate the dewatering of digested sludge; following thickeners are working insufficient because flotation occurs, there is not clear supernatant and the overflow will content great amounts of solids. The same stratification happens in the storage tanks ahead of dewatering machines. The dewatering itself especially in centrifuges will be hindered by the formation of finest gas bubbles.

The Solution

With the help of sludge degassing by vacuum treatment (EP 0289057) dissolved gas is removed to such an extent that the described disturbance don't happen any more. The appropriate choice of operating conditions and the constructive design of the degasser are decisive for the success. The released gas is collected and it is possible to use it together with digester gas.

The basic item of the degassing plant is the degassing vessel. Its upper part includes cascade installations, over which the sludge runs down in a thin film with a large permanent renewing surface. The bottom part is dimensioned to provide sufficient retention time for subsequent degassing and separation of very small bubbles.
Above the liquid level there is a foam layer, which collapses gradually in time. Separate stable foams may be controlled by diverse methods. A water ring pump provides the required vacuum in the degassing vessel.

The degassing plant should be installed preferable in a building near to the digester, so that it requires no precautions against the danger of freezing.

Fig. 2: Vacuum degassing plant with following thickener

Descriptions of process

Thickeners for digested sludge will operate well by use of a vacuum degassing plant, fig. 2. To reduce emissions digested sludge is drown off preferable directly from the digester tank, not by the overflow, e.g. valve V1 and V2 should be closed. Sludge level in the degassing vessel is controlled.

Best result are obtained by thickeners which are continuously run and which are equipped with a sludge level indicator. Is there no thickener but only a storage tank for the subsequent dewatering machines, flotation is also to suspect. The strong buoyancy due to gas bubbles is not always to be avoid, not even by heavy stirring. The concentration in the feed line is oscillating, an precise dosage of flocculants is impossible and the results of dewatering are bad. A possible arrangement in such case shows fig. 3. No storage tank is need, if the digestion tank is feed sufficient continuously.

Fig. 3: Vacuum degassing plant with following centrifuge


Fig. 4: Vacuum treated and untreated digested sludge

The effect of degassing depends on the digested sludge type and its history, therefore pilot tests are useful. Fig. 4 shows the typical sedimentation behaviour. Vacuum degassing does not improve size and structure of flocs. Therefore it is no compensation for conditioning!

Fig. 5: Sedimentation behaviour of degassed digested sludges from different sewage treatment plants

A reduction of the sludge volume after thickening of 30 % to 50 % is possible. Fig. 5 shows some typical sedimentation curves of samples from different sewage treatment plants. The dry substance content of the decantat lies under 0.5 %, generally between 0.2 % and 0.3 %. A thickening time longer then 48 hours normally does not give an additional improvement, because gas building starts again. Best results are realised with continuos operation of the thickener.

Volume reduction in the thickener effects every subsequent sludge treatment positively. Cost savings lead to amortisation within few years.

But also without thickening degassing of sludge has a positive effect, because emissions are reduced, flocculation is improved as well as dewatering in centrifuges and filters.

Technical data:

Vacuum: 0.2 - 0.4 bar abs.
Power consumption: approx. 0.2 kWh/m
Retention time thickener: 2 - 3 d

Fig. 10: Degassing-plant for 8 m/h at workshop

Pilot Plant

A lab test gives quick information about the effect of sludge degassing. For that a sludge sample being gently agitated in a water jet vacuum and subsequently left in a graduated cylinder for 1 to 3 days.

Fig. 6: Pilot plant for degassing of digested sludge

For semi-technical tests on the sewage treatment plant a pilot plant with a capacity up to 8 m/h is available. In general, this pilot plant can be easily connected to the existing facilities by the plant stuff with little effort.