Technical Field

This invention relates to waste-water treatment plant that utilises a rotating biological contactor (or RBC) to provide a large area biomass support surface on which biological break-down of pollutants entrained in waste water can occur. Discussion of Prior Art

It is known to provide many hundreds of square metres of biomass support surface in a waste-water treatment plant, by securing packs of profiled plastics sheeting around a central support shaft to make a cylindrical rotor, to partially submerge the rotor in waste water to be treated and to rotate the rotor so that areas thereof are sequentially submerged (to pick up biomass from the waste water) and exposed to the atmosphere (to permit the desired enzyme action on the biomass to proceed at an advantageous rate).

An RBC for a large waste-water treatment plant could be over 5 metres in diameter and over 8 metres in length and when fully loaded with wet biomass could weigh in excess of 60,000 Kg. The support of the shaft of a rotor of this size in a waste-water tank poses problems.

It has recently been proposed in US-A-4522714 (Thissen) to make the shaft of an RBC as a closed hollow volume and of a size such that it will impart significant buoyancy to the rotor and then to support the shaft in the tank via sub-shafts, projecting from opposite ends of the shaft, in end bearings mounted in the tank. The bearings for the sub-shafts can be mounted on frames resting on the bottom of the waste-water tank adjacent to opposite ends thereof or the bearings can be suspended from hangers depending from tank wall-mounted brackets.

The provision of sub-shafts increases the manufactur¬ ing cost of the rotor and inevitably provides weakened

regions of the shaft at which fatigue cracks can form leading to premature fracture of the shaft. Summary of the Invention

According to this invention the shaft of an RBC is supported from below within the tank of a waste-water 5 treatment plant by at least two rolling members at each end of the shaft, the at least two rolling members at each end defining a cradle into which the shaft end can be lowered to rotatably rest.

The-, rolling members could be an arcuate array of 10 balls but preferably cylindrical rollers are used.

Conveniently there are three axially parallel rollers defining each cradle and suitably the rotating axes of the three rollers and the rotating axis of the shaft define the corners of a quadrilateral which is symmetrical

15. about the diagonal thereof which connects the axis of the shaft with the rotating axis of the lowest roller. The semi-angle between the said diagonal and each side of the quadrilateral that passes through the shaft axis, conveniently lies between 20° and 70° and preferably

20 between 30° and 60°. Four or five rollers can also be used to define each cradle.

Preferably the cradle of rolling members at each end of the shaft is disposed so that the upper surface of the shaft is at or close to the normal free surface

25 of waste water in the tank, however the percentage immer¬ sion of the shaft may vary from 65 to 100%, depending upon the process application. Suitably the shaft is a closed hollow volume so that the weight of water it displaces exceeds the weight of the shaft and also assists

30 in providing additional buoyancy to an extent that it substantially reduces the effective weight of the rotor when loaded, with biomass and when immersed in water.

Each cradle of rolling members can be supported

from the bottom of the tank or it can be hung in the respective end of the tank from an exposed upper surface of an adjacent tank wall.

In addition to the provision of rolling members to support the residual weight of the rotor in the tank, further rolling members may be provided to ensure axial positioning of the rotor shaft in the tank.

Where rollers are used for the rolling members, the diameters of the rollers making up each cradle are suitably between one half and one tenth of the diameter of the shaft and the length of each roller is suitably between 100 and 300 mm.

The shaft can be between 500 and 1500 mm in diameter and the rotor can be submerged to between 65 and 100% of the available area of the biomass support surface.

The rotor can be driven by air bubbled into the tank below the rotor (as described in US-A-3886074 ), by one or more of the rolling members making up each cradle being driven, or by a motorised drive member (e.g. a chain, toothed or pegged wheel) engaging the periphery of the rotor or a drive flange attached to the shaft. Brief Description of the Drawings

The invention will now be further described, by way of example, with reference to the accompanying draw¬ ings, in which:

Figure 1 is a side elevation of the RBC of a waste- water treatment plant supported in a tank in accordance with the invention in different ways on the left- and right-hand sides, and

Figure 2 is a sectional view on the line II-II in Figure 1.

Description of Preferred Embodiments

An RBC 1 comprises a hollow shaft 2 surrounded,

apart from over two end regions 3 and 4, with an annular array 5 of packs of plactics sheeting profiled, to pro¬ vide a multitude of channels for water and air flow to the interior of the packs and to provide a total biomass support area in excess of 20,000 square metres. The packs of plastics sheeting can be mounted in radially extending supporting frames in the manner des¬ cribed in the speci ication of European Patent Application 85309188.2 filed 17th December 1985.

Each end 3 or 4 of the shaft 2, is supported in a cradle of rollers 6 freely journalled from a suitable roller support means 7. On the right-hand side of Figure 1, the support means 7 is a frame 8 resting on the base 9 of a tank 10 containing the RBC 1 and on the left- hand side the support means (here designated 7 ¹ ) depends from an upper wall 11 of the tank 10.

Since the shaft 2 merely rests in the cradle of rollers 6 (see Figure 2 ) it can readily be removed for servicing/repair by lifting it off the cradles at each end. Any roller 6 that becomes defective can be removed and replaced, by temporarily taking the load of the shaft off the cradle at the end where a replacement roller is required while effecting roller replacement within the tank (usually with a reduced water level therein). The normal water level is shown at 12 in Figures 1 and 2.

Figure 1 also shows two alternative ways of rotating the RBC 1. Firstly, in the bottom of the tank 10, air nozzles 13 can be located, through which air from a blower 14 issues to be trapped on the ascending side of the rotor and thus generate a turning moment. Secondly, on one end of the shaft 2 a toothed flange 15 is mounted which engages a rotating peg 16 driven by a motor 17. ^" Clearly other drive mechanisms can be evolved.

Figure 2 shows, in dotted lines, two . additional rollers 6 ¹ which make a cradle of five rollers in all. This may be desirable with the heaviest rotors.

To prevent axial movement of the shaft in the tank one or more of the rollers at each end can be flanged ^* so that the end of the shaft bears against the roller flange(s). Alternatively a further roller can be provided (e.g. with its axis vertical) at each end to bear against the respective end of the shaft.

The invention appears to be particularly applicable to rotors with diameters between two and six metres and axial lengths of up to eight and a half metres.

To oversome possible problems caused by shaft deflec¬ tion if a rotor loaded with biomass is left in a drained tank some means may be provided to prevent excessive de¬ flection of the shaft between its end supports.

Thus, for example, one or more cradles can be pro¬ vided to rest on the bottom of the tank between adjacent arrays 5, the upper edge of each cradle having a surface which does not contact the shaft 2 under normal loading conditions of the latter. An alternative to a cradle supporting from below would be a chain or hawser looped below the shaft at one or more points between the end rollers.

To lift the RBC from the tank, it may be desirable to provide lifting lugs at each end and at least one similar lug uniformly spaced-apart between the end lugs. The lifting lugs can then be engaged by lifting bars or cables depending from a spreader bar when lifting of the RBC is required.

A treatment plant according to this invention can

be used for sewage treatment and also for carbonaceous BOD removal, nitrification and denitrification appli¬ cations.