Technical Field The present invention relates to an anaerobic reactor for the treatment of wet organic wastes such as black water from households, manure, sludge and slurries including a closed vessel or housing preferably provided with a detachable lid, further including an inlet for the wet organic waste, an outlet for purified water and an outlet for produced gas. The invention also includes a method of operating such reactor.

Background art

Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion. Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge.

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane and carbon dioxide, and traces of other ‘contaminant’ gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestible also produced can be used as fertilizer.

Anaerobic digestion can be performed as a batch process or a continuous process. In a batch system, biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process.

In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in - first out). Here, the end products are constantly or periodically removed, resulting in constant production of biogas. A single or multiple digester in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred tank reactors (CSTR), upflow anaerobic sludge blankets (UASB), expanded granular sludge bed (EGSB) and internal circulation reactors (1C reactor). Another abbreviation which is often seen is ABR, anaerobic baffle reactor, which was originally designed to take advantage of the UASB principle while being able to handle feeds with high particulate contents, but it has not been as efficient as intended. An example of an anaerobic reactor is shown in DE 3604415 A1. It includes a cylindrical, closed reactor vessel which is subdivided by further concentrically arranged cylinders into a plurality of stages, which, depending on the direction of flow, have overruns at the top end or overflow ducts at the bottom end. When biogas is produced from highly organically polluted waste waters, e.g., three stages are situated in the reactor through which flow passes successively. In the inner stage the hydrolysis takes place, in the middle stage acid formation takes place and in the outer stage the methane formation takes place. The untreated water is continuously introduced into the inner stage from below by a feed pipe. Pre-clarified water collects after passing through all stages in an annular duct and is taken off via a connection branch. It is fed into an annular outer space via feed branches, where degassing takes place. The gases produced from the outlet orifices of the reactor, are fed jointly for suitable further processing. The degassed liquid passes into a sedimentary device where the entrained sludge is separated off and returned from there to the reactor, while the clarified waste water collects in an annular duct and is taken off via an outlet pipe. This known process includes several stages where the wet organic waste to be treated passes over several thresholds by overflow arrangements. Such flow regime contributes to the build up of floating sludge that eventually will cause clogging of the reactor.

From WO2018135952A1 , it is known an apparatus and method for treatment of wet organic matter for producing biogas, comprising a closed reactor for anaerobic digestion of wet organic matter. The reactor comprises two vertical tubes, an outer tube defining a first chamber encircling an inner tube which is divided into a first region and a second region of a second reactor chamber by a vertical partitioning wall. The first reactor chamber comprises a particle retaining unit connecting the first and second reactor chamber. The anaerobic reactor exhibits a top discharge pipe for gas generated in either of the two chambers of the reactor. The purpose of the particle retaining unit is to hold back floating particles reaching the surface of the waste water in the reactor, forming a floating layer. By holding back the floating organic material the intention is to prolong the residence time of such material and thereby provide sufficient time to digest it.

However, tests with black water from toilets have later shown that the particle retaining unit is not functioning according to its intention with substrates having a high content of particulate matter. A thick layer of floating sludge is formed over time which is not digested and this layer eventually is clogging the water outlet of the reactor and needs to be removed.

Floating sludge refers to accumulated particulate matter at the interfacial between reactor liquor and headspace. Reason for this aggregation and accumulation of particulate matter at the liquor surface is a lower density than the density of the reactor liquor, which again is usually due to entrapment of gas bubbles. Critical accumulation of floating sludge occurs into a thicker layer mainly in reactors or reactor chambers having a phase separator in its outlet arrangement which prevents floating particulate matter to enter the effluent. Integrated phase separators as described in WO2018135952A1 show a high efficiency in biomass retention but result into a suboptimal interfacial geometry between liquor and headspace with potential dead zones that are prohibiting an efficient withdrawal of floating sludge via a single sucking port. Floating sludge needs to be removed from the top by opening the reactor lid, which is a cumbersome and expensive operation making the reactor useless and not suited for commercial exploitation.

Summary of the invention

The present invention provides an anaerobic reactor for the treatment of wet organic wastes as defined in the independent claim 1. Further, the invention provides a method of operating the anaerobic reactor according to the invention, as defined in the independent claim 9. The present invention is also directed to the use of the reactor and method of the present invention as stated in claims 13 and 14. Preferred embodiments are further defined in the dependent claims 2-8, method claims 10 - 12. Brief Description of Drawings

Fig. 1 shows a principle sketch in cross section of a reactor according to the invention, Fig. 2 shows a sketch of the same reactor from above, along section line

B - B in Fig. 1 ,

Fig. 3 shows the same reactor as in Figs. 1 and 2 under ordinary operation, and

Fig.4 further shows the reactor as shown in Figs. 1 and 2 under recirculating operation.

Detailed description of the invention

With the present invention, it is provided an anaerobic ABR type reactor where the problems related to build up of floating sludge is prevented by effective withdrawal and recirculation of the floating sludge to the inlet. Recirculation is actively performed with help of a pump. This recirculation pump provides a high turbulent passing zone so that entrapped gas bubbles are released, which prevents that sludge aggregates float up again. Efficient and regular withdrawal of floating sludge mitigates accumulation of floating sludge aggregates to a critical size and recirculation prolong the retention time for these particles and thereby completing the digestion of the biomass of the wastewater feed.

An example of an anaerobic reactor 20 according to the invention for the treatment of wet organic waste is shown in Figs. 1 and 2. It includes an outer casing or vessel 10 with a lid 11 forming a closed volume, but being provided with an outlet 12 for gas produced by the reactor. The vessel may be of any shape such as circular as shown in Fig. 2, oval, square, rectangular or polygonal. The reactor is divided by a partition wall 1 into two or more chambers (see use claims below). A first chamber I and a second chamber II, where the first chamber I at its lower end is provided with an inlet 8 for the organic waste feed and chamber II which at its upper end is provided with an outlet 7 for purified water. Chamber I is provided with a separation device such as a 3-phase separator 2 as shown in Fig. 1 , which is formed by v-shaped plates, and is integrated in the partition wall 1 below an overflow arrangement comprising overflows 6 and vertical ducts 13 for the flow of (partly digested) organic feed from the first chamber I to the second chamber II. Above the 3-phase separator 2, partly overlapping the separator, is further provided a slanted deflection plate 3 with a vertical upper part 19 which stretches across the vessel 10. This plate 3, 19 is designed to deflect upwardly floating sludge towards a circular or polygonal withdrawal funnel 14 at the opposite side of chamber I, in relation to the partition wall, and at the same time minimizing the area of liquor/headspace interfacial in chamber I and optimizing its geometry. The floating sludge withdrawal funnel 14 as such is a plate stretching across chamber I as shown in Fig. 2. A vertical suction duct 5 connected the withdrawal funnel 14 is provided to enable circulation of floating sludge as is further explained below. With the vertical part on the outside of the reactor, the duct 5 also functions as a liquid lock, preventing gas of escaping from the reactor.

The means for recirculation of floating sludge in chamber I include the slanted deflection plate 3 with the vertical upper part 19, the circular or polygonal withdrawal funnel 14 and the vertical suction duct 5.

By the arrangement of the integrated 3-phase separator 2 and deflection plate 3 as shown in Figs 1 and 2, there is a longitudinal slit 15 through which the partly digested wet organic waste passes from chamber I towards chamber II. The overflow(s) 6 at the upper part of the partition wall 1 (all together three openings as shown in Fig. 2) are each connected with a vertical duct 13 stretching towards the bottom of chamber II and thereby passing the partly digested waste to the bottom of the chamber II. In conjunction with each vertical duct 13 and overflow 6, there is provided a flanged opening 23 for inspection and cleaning/hosing of the inlet and duct in case of blockage. Even though Fig. 2 shows three overflows 6, vertical ducts 13 and flanged openings 23, the reactor according to the invention may be provided with one, two or more than three such overflows/ducts/openings.

The inlet 8 of the reactor formed by a pipe stub 16 is equipped with a horizontal plate 9 to ensure horizontal distribution of the wet organic waste feed into chamber I. Further chamber I as well as chamber II are provided with outlets 17 and 18 in the bottom to extract precipitate that is produced during the digestion process.

The anaerobic reactor 20 as described above is working in two different modes, 1) an operating mode and 2) a recirculation mode as described below and with reference to Figs. 3 and 4: Wet organic waste such as black water is fed in form of feed pulses, typically lasting between 10 - 100 seconds, to the reactor, preferably by a pumping means 21 from a feed buffer tank (not shown), through the reactor inlet 8 and is distributed along the bottom of chamber I. Under normal operation, mode 1) as illustrated by the arrows in Fig. 3, the wet organic waste passes slowly upwards in chamber I, being slowly digested and then passing the slit 15 via the separator 2 and further through the overflows 6 and still further down through the ducts 13 and into chamber II at its bottom. Flere, the partly digested wet organic waste continues the digestion process towards the outlet 7 of the chamber and is finally led out through the outlet 7, preferably mostly as purified water. Flowever, any particles passing the outlet 7, are separated in a downstream trap or collector/filter (not further shown) and is returned to the inlet 8 of the reactor by pumping means (not shown). Under the anaerobic process taking place in chambers I and II, gas that is produced is evacuated through the gas outlet 12 (shown in Fig. 1), while digested particles with higher density precipitate at the bottom of the chambers and are extracted/ tapped at regular, required intervals. On the other hand, lighter particles with lower density are separated by the 3-phase separator 2, and are further deflected to the surface towards the withdrawal funnel 14.

As to the recirculation mode of operation, mode 2), illustrated by the arrows as shown in Fig. 4, withdrawal and recirculation of floating sludge is obtained by means of an externally provided pump 22 connected to the suction duct 5 through a piping loop (not further shown) and further connected to the inlet 8 of the reactor 20. The sloped plate 3 deflects floating sludge towards the withdrawal funnel 14 from where it is returned with help of a pump to the reactor inlet 8 and distributed along the bottom of the chamber I by the horizontal plate 9. By this pumping activity, forced circulation is obtained via a high turbulent pass way comprising withdraw funnel 14, suction duct 8 and the recirculation pump 22, returning floating sludge forming aggregates to the bottom of chamber I. The high turbulent pass way enforces a release of gas entrapped bubbles so that a majority of recirculated aggregates will not float up again. The recirculation further enhances residence time of the returned floating sludge aggregates in the most active reactor zone at the lower third of chamber I. The pump may be of any type providing sufficient turbulence to release entrapped gas bubbles, but preferably, the applicants own Vacuumarator® liquid ring screw pump with macerator is well suited since it effectively breaks down the floating sludge aggregates which enhances release of entrapped gas bubbles.

The recirculation operation is preferably controlled by a PLC based on an algorithm calculating the instant detected load of the reactor. Analogous to feed, recirculation takes place in form of pulses and needs to be optimized for each reactor based on different factors such as reactor size, the type and consistence of the wet organic waste and the instant load. If instant load of the reactor is high since a high frequency of feed pulses provides a frequent mixing of the sludge blanked in the lower third of chamber I, which mitigates entrapment of gas bubbles into sludge aggregates to a certain extent, so that a less frequent recirculation is needed. However, preferably the floating sludge recirculation will be performed as pulsations lasting between 10 - 100 seconds and at intervals of 10 - 90 minutes between feeding pulses. In extreme situations, when there is no wet organic waste fed to the reactor, which may be the case for black water installations outside the tourist season, only the recirculation, mode 2) takes place until all organic material is digested.

Even though the recirculation operation preferably may be controlled by a PLC based on the instant load detected, as described above, the reactor may be operated based on other data control means and further using some kind of detector such as ultra sound, radar or possibly photo detector to reveal build-up of floating sludge in chamber I and then trigger the recirculation mode operation based on such detection.

Depending on different parameters and factors such as capacity requirements, type of wet organic waste and purification requirements, the use of a reactor according to the invention may include two or more reactors connected in series or in parallel, or the chambers of the reactors may be connected in series or parallel. Having described embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.