Device for purifying anaerobic biological wastewater

The invention relates to a device for purifying anaerobic biological wastewater, which wastewater is fed in through a feed pipe (4) and the purified wastewater and built-up gas are removed respectively by a discharge pipe (24) and a gas outlet pipe (18), said device comprising a main chamber (1), wherein wastewater, which is to be clarified, is fed to a lower area and at least one lifting pipe (9), which is used to remove the built-up gas and the purified wastewater, which contains sludge particles, emerges from an area which is higher than said lower area, said lifting pipe joining a return tank (2) in order to separate gas and purified wastewater, which contains sludge particles, and the outlet (11) thereof m the return tank (2) being higher than the inlet (10) thereof in the main chamber (1). The return tank (2) is connected to an auxiliary chamber (3), wherein purified wastewater, which contains sludge particles and which was lifted to the return tank (2) via the at least one lifting pipe (9), flows out from the return tank (2) to the auxiliary chamber (3). The purified wastewater flows off to the discharge pipe (24) from an upper region of the auxiliary chamber (3) and the auxiliary chamber (3) is connected in a lower region via a valve, preferably a non-return valve (15), or a pump to a lower region of the main chamber (1).

FIELD OF THE INVENTION

The invention relates to a device for purifying anaerobic biological wastewater, which wastewater is fed in through a feed pipe and the purified wastewater and the built-up gas are removed respectively by a discharge pipe and a gas outlet pipe, said device comprising a main chamber, wherein wastewater, which is to be clarified is fed to a lower area and at least one lifting pipe, which is used to remove the built-up gas and purified wastewater, which contains sludge particles, emerges from an area which is higher than said lower area, said lifting pipe joining a return tank in order to separate gas and purified wastewater, which contains sludge particles, and the outlet thereof in the return tank being higher than the inlet thereof in the main chamber.

BACKGROUND

Devices for purifying anaerobic biological wastewater are well known. Usually these consist of a single reactor chamber, into whose bottom area flows the wastewater, which is to be clarified. A sludge bed, which represents the biologically active zone for purifying the wastewater and through which the wastewater enters, is provided in a lower area of the reactor chamber. The wastewater is purified while flowing through the sludge bed, its organic content is reduced and biogas (essentially consisting of methane and carbon dioxide) is built-up. The built-up biogas rises through the sludge bed and the zone lying above it, is trapped by separator baffles and discharged to a gas outlet pipe. Above the separator baffles is a calming zone, whose purpose is to enable entrained biologically activated sludge particles to fall out and settle again. The purified wastewater is removed from the upper end of the calming zone.

Problems arise with this arrangement inter alia due to the accumulation of floating sludge below the separators. Also it is necessary to filter the exhausted air due to gases being released in the calming zone, since otherwise serious odour problems would arise. Furthermore uneven flows in the sludge bed can arise due to insufficient thorough mixing of the sludge bed, as a result of which the purifying efficiency of the plant is reduced.

Devices of the type initially specified are known from EP 0 170 332 A1 and EP 0 539 430 B1. Lifting pipes emerging from an area lying above the feed pipe for the wastewater (and above the sludge bed) and below the fluid level in the main chamber, to which rising gas trapped by separator baffles is fed, are provided here. Gas and wastewater, which has been purified but still contains sludge particles, are lifted through the at least one lifting pipe to a return tank by the pressure of the gas. Biogas and the entrained wastewater are separated in the return tank. The biogas is diverted to a gas outlet pipe and the wastewater together with the contained sludge particles flows back from the return tank through a return pipe to the main chamber, and to be precise to the bottom area of the same, where thorough mixing in the sludge bed takes place. Above the lifting pipes with the associated separator baffles is a calming zone and further separator baffles may be provided there if necessary. The purified wastewater flows out from the upper area of the main chamber. Also problems arise with this arrangement inter alia due to the accumulation of floating sludge in the separators and in the wastewater removal area, which entails high maintenance costs.

A distinction is to be made between devices for purifying wastewater and such for digesting sludge with a high organic content, which is biologically degradable, as occurring in agriculture or industry. Such sludge before treatment has 10-20% dry substance (after treatment correspondingly less, 5-6% for example). In the case of wastewater on the other hand the dry substance content is below 1%, normally in the range of 0.1%. Usually the devices for digesting sludge are not suitable for wastewater purification. Devices for digesting sludge are known for example from DE 32 11 888 A1, DE 82 11 869 U1 and U.S. Pat. No. 4,302,329 A.

The device known from DE 33 30 696 A1 is applicable both for digesting sludge and for purifying anaerobic wastewater. The main chamber, in which the main organic activity takes place, has lateral overflow outlets, which lead to a separating chamber surrounding the main chamber. On the outer edge of the separating chamber is an overflow channel, from which the discharge pipe for the treated substrate emerges. A current is created first downwards and then upwards by a partition wall, which is open at the bottom in the separating chamber. Organically activated sludge, which can be pumped back through a return pipe to the main chamber by means of a pump, settles at the bottom of the separating chamber. Also in the case of this device floating sludge accumulates in the main chamber and inside the separating chamber, as a result of which maintenance work is necessary. Furthermore retention of biologically activated sludge is not optimum. If an excessive amount of biologically activated sludge is shed, the biological activity in the main chamber however is reduced to an unacceptable degree.

SUMMARY

The object of the invention is to create a device of the type initially specified, wherein maintenance costs are low and the shedding of biologically activated sludge is effectively counteracted.

The auxiliary chamber, into which according to the invention the wastewater lifted to the return tank flows off, forms a calming zone for the wastewater, so that decantable biologically activated sludge can effectively settle. Floating sludge and other floating matter (particles containing oil for example) on the other hand can flow off together with the purified wastewater. The retained sludge, which has sunk to the bottom, can be returned again to the main chamber via the connection of the auxiliary chamber to the main chamber. A very effective, low-maintenance device is thus created.

A device according to the invention can be advantageously constructed without separators for separating gas. The calming zone conventionally found above the separators is not required in the case of a device according to the invention, since the biologically activated sludge settles in the auxiliary chamber. In an advantageous embodiment of the invention the main chamber is tightly sealed in the region above the inlet into the at least one lifting pipe and any pipe-work in this region joining the main chamber can be closed off by valves if necessary. In continuous operation therefore gas builds up in the region of the main chamber lying above the inlet of the at least one lifting pipe, as a result of which the fluid level stands in the region of the inlet of the at least one lifting pipe.

If the outlet for discharging the purified wastewater from the auxiliary chamber advantageously lies higher than the inlet into the at least one lifting pipe, preferably at least 2 meters higher, this leads to a pressure above atmospheric pressure of the gas lying in the upper region of the main chamber. This then creates a pump effect due to the gas flowing into the lifting pipe, as a consequence of which wastewater containing sludge particles is entrained.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The device in accordance with the invention, illustrated in the figures, has a reactor or main chamber1, into whose lower area the feed pipe4for the wastewater, which is to be clarified, joins. The wastewater in all cases is fed into the lower quarter of the main chamber1, a distance from the bottom of less than 1 meter being preferable. In the exemplary embodiment shown, the feed pipe4comprises several discharge openings21. For example the feed pipe could also be formed by several branches routed separately into the main chamber1.

A lower part of the main chamber1forms an active zone5, which comprises a sludge bed (=fluidised bed), in which biologically activated sludge is present. The active zone5can also be formed as a so-called hard bed, which is constructed by piling up a filter medium with a bio film grown thereon and layers of biologically activated sludge in-between.

The majority of the biological activity and thus the main wastewater purification take place in this active zone5. Organic and biologically degradable components contained in the wastewater are degraded while flowing through the active zone5. At this point gas known as biogas, which rises from the active zone5, is released.

Above the active zone5is a liquid and gas zone6, the fluid level7standing during steady-state (continuous) operation being indicated by a broken line. Biogas, which has risen through the liquid, builds up above the fluid level7.

The normally relatively sharp interface8between the active zone5and the liquid and gas zone6can alter, depending on the operating state, in the course of operation.

In the liquid and gas zone6below the fluid level7is purified wastewater, which however in particular still contains sludge particles. Sludge particles are entrained by the gas rising from the active zone5. Apart from such particles, which may settle in a calm environment, normally there is still a component of buoyant substances, particles of floating sludge or fat particles for example.

A lifting pipe9, which joins a return tank (return vessel)2extends from an area of the main chamber1, lying higher relative to the region where the wastewater is fed into the main chamber1, which is located above the active zone5. This lifting pipe9here leads upwards to the return tank2starting from the main chamber1, that is to say the inlet10into the lifting pipe9is deeper than the outlet11thereof in the return tank2.

The inlet10into the lifting pipe9advantageously lies at a distance below the upper boundary wall of the main chamber, which is less than a quarter of the height of the main chamber. A distance from the upper boundary wall of less than 15% of the height of the main chamber1is preferred.

The inlet10into the lifting pipe9in proximity to the lower end of the lifting pipe9comprises several slots, which in the exemplary embodiment shown represent window-like openings in the wall of the lifting pipe9. Several slots are provided at a distance from each other in the circumferential direction. Furthermore the lower end of the lifting pipe9is of open construction. Closed construction of the lower end is conceivable and possible.

Preferably the return tank2lies entirely above the main chamber1, as shown inFIG. 2.

Gas and purified wastewater, which contains sludge particles reaches the return tank2though the lifting pipe9, as yet to be described further below. Wastewater, which contains particles, and gas are separated in the return tank2. Gas passes out from the return tank2through a gas outlet13, which in the exemplary embodiment shown is formed by the open upper end of the return tank2, and enters the auxiliary chamber3.

The purified wastewater, which contains sludge particles, flows through the return pipe12leading from the return tank2to the auxiliary chamber3. The return pipe12here is routed from the return tank2preferably at a place, which is deeper than the outlet11of the lifting pipe9. The mouth14of the return pipe12preferably lies deeper than the place, at which it leaves the return tank2. In the exemplary embodiment shown this mouth14points downwards.

The auxiliary chamber3is connected in a lower area to the main chamber1with a valve, which is in the form of a non-return valve15in the partition wall22between the auxiliary chamber3and the main chamber1. Preferably this connection is less than 1 meter above the bottom of the auxiliary chamber3or the main chamber1.

The purified wastewater is removed from an upper region of the auxiliary chamber3, and to be precise in the exemplary embodiment shown there is an upwardly open discharge spout23, from which the discharge pipe24for the purified wastewater leads. The outlet25for the purified wastewater from the auxiliary chamber3is thus formed by the opening of the discharge spout23.

The fluid level16standing in the auxiliary chamber3during steady-state operation is represented by a broken line. In contrast to this the fluid level17in the return tank2may be slightly higher (due to hydrodynamic effects).

Biogas, which flows in from the return tank2builds up above the fluid levels16,17and can also build up to a lesser degree in the auxiliary chamber3itself. Instead of the auxiliary chamber3, which lies higher than the outlet25for the purified wastewater, there is an outlet29for the biogas to discharge this into the gas outlet pipe18. Preferably the built up biogas is converted into energy. A gas flare is conceivable and possible.

In the auxiliary chamber3decantable biologically activated sludge particles can settle and flow back again through the non-return valve15to the main chamber, so that biologically activated sludge is not wasted if possible. Floatable particles on the other hand are removed by the discharge pipe24, so that floating sludge is prevented from accumulating.

The auxiliary chamber3comprises an influx section19, which the return pipe12joins and an expansion section20lying above it, whose horizontal cross-sectional area is substantially larger, preferably more than three times as large as the horizontal cross-sectional area of the influx section19. A calmed zone is already present in the influx section19, whose purpose is to decant the sludge particles entrained by the wastewater, which has been fed-in. In contrast to this the upwardly directed current however can still entrain lighter, decantable sludge particles. Further calming takes places in the expansion section20, so that these sludge particles can also settle. Biologically activated sludge settling on the bottom of the expansion section20is returned by back flushing described further below. For executing such back flushing, a connecting pipe26, wherein an open- and closable stop valve is arranged, leaves the upper region of the auxiliary chamber3and joins the upper end of the main chamber1(it is also conceivable and possible for it to join the gas outlet pipe18, which leaves the upper region of the auxiliary chamber3).

Preferably there is also a connection between the main chamber1and the auxiliary chamber3, which joins the main chamber1with the auxiliary chamber3in a region, which lies above half the height of the main chamber1and above the active zone1, but below the inlet10into the lifting pipe9. This connection could be formed by a pipe. In the exemplary embodiment shown it is formed by an opening28in the partition wall22between the main chamber1and the auxiliary chamber3. The cross-section of this opening28is smaller than the cross-section of the opened non-return valve15.

When the plant is started up there is still no biogas initially and the fluid level7in the main chamber1stands at the upper end of the main chamber1. The stop valve27is closed. The main chamber1is therefore tightly sealed in the region above the inlet10into the lifting pipe9. The fluid level7falls due to the formation of biogas, which rises, as the result of which liquid flows initially through the opening28out of the main chamber1to the auxiliary chamber3. As soon as the fluid level7has reached the inlet into the lifting pipe9, gas flows into the lifting pipe9. Here this gas is under a pressure, which depending on the liquid column corresponds to the height difference between the fluid level16in the auxiliary chamber3and the fluid level7in the main chamber1. This gas flowing into the lifting pipe9therefore entrains liquid and also any particles contained therein. This gas lifting effect thus acts as a kind of pump, whereby liquid and particles contained therein are also pumped to the return tank2(pumps, which work on this principle, are known as “air-lift” or “injector pumps” for example).

Due to the pump effect of the gas flowing through the lifting pipe9(depending on the operating state) more liquid can be taken via the return tank2to the auxiliary chamber3than wastewater through the feed pipe4of the main chamber1. In steady-state continuous operation a quantity of purified wastewater corresponding to the supply of wastewater is removed through the discharge pipe24. Surplus liquid therefore flows back through the non-return valve15and to a lesser degree through the opening28(since its cross-sectional opening is smaller) to the main chamber1, thereby entraining sludge particles that have settled in the auxiliary chamber, which in this way again flow back to the main chamber1through the non-return valve15.

It would also be conceivable and possible, in order to connect the lower area of the auxiliary chamber3to the main chamber1, to provide a pipe equipped with a pump. If sludge, which has settled in the auxiliary chamber3, is to be returned to the main chamber1, this pump is switched on.

For executing back flushing the stop valve27is opened. The fluid level16in the auxiliary chamber3falls as a consequence and the fluid level7in the main chamber rises, until these are at the same level. Thereby a corresponding quantity of liquid flows through the non-return valve15of the auxiliary chamber3to the main chamber1. Thorough mixing of the sludge bed in the main chamber1is thus achieved. Furthermore sludge, which has settled in the auxiliary chamber3, is returned to the main chamber1. Sludge, which has settled in the expansion section20, is back-flushed to the influx section19of the auxiliary chamber3.

The connection between the main chamber1and the auxiliary chamber3, formed by the opening28in the exemplary embodiment shown, could also be dispensed with. After back flushing however no gas would flow initially through the gas outlet pipe18. Due to the presence of the opening28the flow of gas is thus more balanced.

In place of the upwardly open construction of the return tank2extending above the height of the outlet25for the purified wastewater, the gas outlet13of the return tank2could also lead to a pipe, through which gas is directly fed to the gas outlet pipe18.

The non-return valve15instead of its arrangement in the partition wall22could also be provided in a connecting pipe joining the lower area of the auxiliary chamber3with the lower area of the main chamber1. In lieu of a non-return valve15an open- and closable stop valve could also be provided in the partition wall22or in a connecting pipe between the auxiliary chamber3and the main chamber1.

Also several lifting pipes9could be provided, which all join the same vessels forming the return tank2or into different subsidiary vessels, which together form the return tank2.

It would also be conceivable and possible for the auxiliary chamber3to comprise several single chambers.